Green Pulp and Paper Industry: Biotechnology for Ecofriendly Processing 9783110592412, 9783110591842

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Amit Kumar, Puneet Pathak, Dharm Dutt (Eds.) Green Pulp and Paper Industry

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Green Pulp and Paper Industry Biotechnology for Ecofriendly Processing Edited by Amit Kumar, Puneet Pathak, Dharm Dutt

Editors Dr. Amit Kumar Department of Biotechnology College of Natural & Computational Sciences Debre Markos University Debre Markos Ethiopia Dr. Puneet Pathak Nanotechnology & Advanced Biomaterials Group, Avantha Centre for Industrial Resarch & Development Thapar Institute of Engineering & Technology Campus, Patiala, Punjab 147004, India Prof. Dharm Dutt Department of Paper Technology, Indian Institute of Technology, Roorkee Saharanpur Campus, Saharanpur-247001, India

ISBN 978-3-11-059184-2 e-ISBN (PDF) 978-3-11-059241-2 e-ISBN (EPUB) 978-3-11-059187-3 Library of Congress Control Number: 2021937999 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2021 Walter de Gruyter GmbH, Berlin/Boston Cover image: Amit Kumar Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Contents List of contributing authors Editors Biography

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XIII

Sunita Chauhan and Badri Lal Meena 1 Introduction to pulp and paper industry: Global scenario 1 1.1 Early writing materials and true paper 1 1.2 Fiber, the fundamental unit of paper 3 1.2.1 Different types of papermaking fibers 4 1.2.2 Composition of papermaking fibers 5 1.3 Basic process of papermaking 11 1.3.1 Mechanical pulping 12 1.3.2 Chemical pulping 12 1.4 Environmental concerns of papermaking 15 1.5 Ways to address the environmental concerns of paper industry 16 1.6 Global status of the paper industry and challenges ahead 18 1.7 Indian paper industry 18 1.8 Changing trends of paper industry and future projections 21 1.8.1 Efforts of minimum impact and effluent closures 21 1.8.2 Shrinking demands of graphic paper but increase in demands of packaging grades 22 1.8.3 Impact of the ban on “Single Use Plastics” 22 1.8.4 Substantial transformation of the paper industry 23 1.8.5 Paper industry in the post COVID-19 era 24 1.9 Challenges of paper industry for the next decade 24 1.10 Conclusion 25 References 25 Puneet Pathak and Chhavi Sharma 2 Processes and problems of pulp and paper industry: an overview 2.1 Introduction 31 2.2 Papermaking process 32 2.2.1 From wood/ agro-residues 33 2.2.2 From recycled fibers 41 2.3 Problems faced by the paper industry 42 2.3.1 Raw material 42 2.3.2 Yield 45 2.3.3 Pitch Problem 45 2.3.4 Pollutants 45

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2.3.5 2.3.6 2.3.7 2.3.8 2.3.9 2.4 2.5

Contents

Energy 47 Water 48 Wastewater 48 Emissions 49 Solid waste 50 Green chemicals to mitigate the problems Conclusions 54 References 55

51

Amit Kumar, Mukesh Yadav and Workinesh Tiruneh 3 Debarking, pitch removal and retting: Role of microbes and their enzymes 59 3.1 Introduction 59 3.2 Debarking 60 3.2.1 Enzymatic debarking 61 3.3 Pitch removal 61 3.3.1 Pitch 61 3.3.2 Pitch-related problems in pulp and paper processing 3.3.3 Methods of pitch removal 62 3.4 Retting 71 3.4.1 Enzymatic retting 72 3.5 Limitations and future prospective 73 3.6 Conclusion 73 References 74

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Amit Kumar, Archana Gautam and Dharm Dutt 4 Bio-pulping: An energy saving and environment-friendly approach 79 4.1 Introduction 79 4.2 WRF for biopulping 80 4.3 Biopulping process 82 4.4 Factors affecting biopulping 82 4.5 Reduction in pulping chemicals 84 4.6 Effect of biopulping on kappa number, lignin content and brightness 84 4.7 Effect of biopulping on pulp yield and viscosity 85 4.8 Effect of biopulping on strength properties of pulp 86 4.9 Mechanism for energy saving and strength properties improvement during biopulping 86 4.10 Limitations of biopulping 88 4.11 Conclusion 89 References 89

Contents

VII

Amit Kumar 5 Biobleaching: An eco-friendly approach to reduce chemical consumption and pollutants generation 93 5.1 Introduction 93 94 5.2 Bleaching processes: Cl2, ECF and TCF bleaching 5.3 Bleaching and associated environmental issues 95 5.4 Biobleaching 96 5.4.1 Xylanase biobleaching 97 5.4.2 Pectinases and mannanases biobleaching 100 5.4.3 Ligninolytic enzymes biobleaching 106 5.4.4 Role of enzyme dose and reaction conditions in biobleaching 110 5.4.5 Effect of enzyme pretreatment on kappa number, ISO brightness, brightness ceiling, reducing sugars and chromophores in pulp filtrate 112 5.4.6 Reduction in chemical consumption and pollutants generation 114 5.4.7 Effect of biobleaching physical strength properties of paper 116 5.5 Bleaching by microbial treatment 117 5.6 Limitations and future perspectives of biobleaching 118 5.7 Conclusion 118 References 119 Varun Kumar, Puneet Pathak, Nirmal Sudhir Kumar Harsh and Nishi Kant Bhardwaj 6 Biodeinking: an eco-friendly alternative for chemicals based recycled fiber processing 129 6.1 Introduction 129 6.2 Waste paper: an excellent raw material 130 6.3 Deinking of waste papers 131 6.4 Conventional deinking 132 6.5 Biodeinking 133 6.6 Microorganisms used for the production of the deinking enzymes 133 6.7 Mechanism of biodeinking 135 6.8 Effectiveness of various enzymes towards the deinking process 138 6.9 Factors affecting biodeinking 145 6.10 Advantages 146 6.11 Limitations and future recommendations 146 6.12 Conclusions 147 References 147

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Contents

Amit Kumar, Chhotu Ram and Adebabay Tazeb 7 Enzyme-assisted pulp refining: an energy saving approach 155 7.1 Introduction 155 7.2 Mechanical refining and its effect on paper properties 156 7.3 Enzyme-assisted refining 159 7.3.1 Enzymatic modification of fiber and its effect on pulp and paper properties 159 7.3.2 Requirement of controlled enzymatic treatment 161 7.3.3 Fiber length and other pulp & paper properties 162 7.3.4 Energy saving during enzyme-assisted refining 163 7.4 Enzyme-assisted refining of recycled fiber 168 7.5 Enzyme-assisted refining studies at mill scale 169 7.6 Limitations and future prospective 170 7.7 Conclusion 170 References 171 Amit Kumar 8 Dissolving pulp production: Cellulases and xylanases for the enhancement of cellulose accessibility and reactivity 175 8.1 Introduction 175 8.2 Methods of dissolving pulp production 176 8.2.1 Acid sulphite pulping method 176 8.2.2 Pre-hydrolysis kraft pulping 177 8.2.3 Upgradation of paper grade pulp 177 8.3 Dissolving pulp reactivity improvement 178 8.3.1 Mechanical treatment 179 8.3.2 Chemical treatments 179 8.3.3 Ultrasonic treatment 180 8.3.4 Enzymatic treatment for reactivity improvement 180 8.4 Applications of dissolving pulp 187 8.5 Conclusion 188 References 188 Puneet Pathak, Varun Kumar, Nishi Kant Bhardwaj and Chhavi Sharma 9 Slime control in paper mill using biological agents as biocides 9.1 Introduction 193 9.2 Slime 194 9.3 Sources of slime 196 9.4 The problems in paper mills 198 9.5 Methods for slime detection of in the paper industry 200 9.5.1 Slime collection boards 200 9.5.2 Identification of the contaminated points 200

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Contents

9.5.3 9.5.4 9.5.5 9.5.6 9.6 9.6.1 9.6.2 9.6.3 9.6.4 9.6.5 9.7

Standard plate count method 200 Dip Sticks Method 201 Luminescence 201 Bio-Lert Method 201 Control of slime in paper mill 202 Conventional methods 202 Use of enzymes 204 Biodispersants 208 Bacteriophage 209 Inhibitors for biofilm formation 210 Conclusion and future prospects 211 References 212

Chhotu Ram, Pushpa Rani, Kibrom Alebel Gebru and Mebrhit G Mariam Abrha 10 Pulp and paper industry wastewater treatment: use of microbes and their enzymes 217 10.1 Introduction 217 10.2 Pulp and paper mill process 218 10.2.1 Paper mill effluent characteristics 220 10.2.2 Paper mill wastewater treatment 221 10.3 Microbiological treatment 223 10.3.1 Bacterial treatment 224 10.3.2 Fungi treatment 227 10.3.3 Enzymatic treatment 230 10.3.4 Anaerobic treatment 230 10.4 Conclusion 231 References 231 Index

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List of contributing authors Puneet Pathak Nanotechnology & Advanced Biomaterials Avantha Centre for Industrial Research and Development Paper mill campus Yamuna Nagar, Haryana 135001 India [email protected] Chhavi Sharma Directorate Avantha Centre for Industrial Research and Development Yamuna, Nagar, Haryana India [email protected] Sunita Chauhan Scientist Kumarappa National Handmade Paper Institute (KNHPI), Ramsinghpura, Sikarpura Road Near Sanga Automobiles Maruti Workshop, Sanganer, Jaipur, Rajasthan 302029 India, [email protected] Badri Lal Meena Secretary and Director Kumarappa National Handmade Paper Institute (KNHPI), Jaipur, Rajasthan, India [email protected] Amit Kumar Department of Biotechnology College of Natural and Computational Sciences Debre Markos University (Ethiopia) Debre Markos, Gojjam Ethiopia [email protected]

https://doi.org/10.1515/9783110592412-203

Mukesh Yadav Department of Biotechnology Maharishi Markandeshwar (Deemed to be University) Mullana, Ambala India [email protected] Workinesh Tiruneh Department of Animal Science College of Agriculture and Natural Resources Debre Markos University Debre Markos, Ethiopia [email protected] Archana Gautam Department of Paper Technology Indian Institute of Technology Roorkee, Saharanpur Campus Uttar Pradesh India [email protected] Dharm Dutt Department of Paper Technology Indian Institute of Technology Roorkee, Saharanpur Campus Uttar Pradesh India [email protected] Adebabay Tazeb Department of Biotechnology College of Natural and Computational Sciences Debre Markos University Debre Markos, Ethiopia [email protected]

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List of contributing authors

Chhotu Ram Department of Chemical Engineering College of Engineering and Technology Adigrat University Adigrat, Ethiopia [email protected]

Pushpa Rani Department of Environmental Science & Engineering Guru Jambheshwar University of Science and Technology Hisar Haryana, India

Varun Kumar Nanotechnology & Advanced Biomaterials Avantha Centre for Industrial Research & Development Paper mill campus, Yamuna Nagar Haryana 135001, India [email protected]

Kibrom Alebel Gebru Department of Chemical Engineering College of Engineering and Technology Adigrat University Adigrat, Ethiopia

Nirmal Sudhir Kumar Harsh Forest Pathology Division Forest Research Institute Dehradun, Dehradun, Uttarakhand 248006 India [email protected] Nishi Kant Bhardwaj Directorate Avantha Centre for Industrial Research & Development, Yamuna Nagar 135001 Haryana, India [email protected]

Mebrhit G Mariam Abrha Department of Chemical Engineering College of Engineering and Technology Adigrat University Adigrat, Ethiopia

Editors Biography Dr. Amit Kumar is currently working as Assistant Professor at Department of Biotechnology, College of Natural and Computational Sciences, Debre Markos University (Ethiopia). He completed his Doctorate in Biotechnology from Indian Institute of Technology Roorkee (India). He is extensively involved in research on industrial enzymes, pulp & paper biotechnology, biofuels production, and environmental biotechnology. He has published 30 research and review articles in various reputed international journals. He has also published two books entitled “Microbial Enzymes and Additives for the Food Industry” and “Nanobiotechnology for Green Environment”. He has guided several graduate and post-graduate research projects. Dr. Puneet Pathak is currently working with Avantha Centre for Industrial Research & Development (ACIRD), Patiala, Punjab (India) as Research Scientist in the Nanotechnology and Advanced Biomaterials Group since May 2012. He has completed his PhD (Industrial Microbiology) from Indian Institute of Technology Roorkee, Roorkee, Post-graduation (Microbiology) from Chaudhary Charan Singh University, Meerut and PG Diploma (Biotechnology) from UP Technical University, Lucknow. He has about 9 years research experience. His research interests are industrial enzymes, recycling (deinking), fermentation, biotechnological applications in pulp & paper industry, nanocellulose, biorefinery, biofuels, valorization of lignocellulosic biomass and wastewater treatment. He has published 19 research papers in reputed peer reviewed international journals, 2 book chapters and attended 15 national and international conferences in India.

Dr. Dharm Dutt is a Professor at the Department of Paper Technology, Indian Institute of Technology Roorkee (India). He published about 180 research papers in national and international journals and supervised 19 PhD thesis. He is honoured as a star performer seven times by the IIT Roorkee. He presented research papers in Rome (Italy), Katmandu (Nepal), Paris (France), Berlin (Germany), Jyvaskyla (Finland), Kyiv (Ukraine), London (UK), Indonesia, Milan (Italy), Stockholm (Sweden), Valencia (Spain) and Lausanne (Switzerland). He published two books namely ‘Xylanases of Coprinellus disseminatus for Pulp and Paper Industry: Production, Characterization and Application in Biobleaching of Wheat Straw Pulp’ and Cellulose Production and Biodeinking of Sorted Office Paper’. He has handled R&D and consultancy projects of worth Rs. more than 3.5 crore.

https://doi.org/10.1515/9783110592412-204

Sunita Chauhan and Badri Lal Meena

1 Introduction to pulp and paper industry: Global scenario Abstract: In the today’s context of COVID-19, when a strong need of disposable items without compromising the environmental attributes is being felt, “Paper” is becoming a very mesmerizing and quintessential object. We find paper in our lives so omnipresent and enduring that we take it granted without understanding its complex nature, manufacturing/development process and the significant role, it has actually played in the history of civilization. It is so important that the Per Capita Consumption of paper for a country reflects the development status of its society. Therefore, this article attempts to introduce paper with its historical development, manufacturing process, environmental concerns, ways to address the environmental concerns, global and national status of pulp and paper industry along with the challenges ahead with a coverage of the potential role it is bound to play in future through an overall transformation of the industry itself. Keywords: challenges, future, handmade paper, industry, paper, pollution, status, transformation

1.1 Early writing materials and true paper The term “paper” has its origin from the ancient Greek word “Papyrus” which takes us to 3700 BC when the papyrus plant was first used as a writing surface. It was actually prepared by coating and pasting together the slices cut from the stalks of Papyrus (Cyperus papyrus), an African plant in much the same way as a carpenter builds up the sheets of laminated wood [1, 2]. Papyrus was not actually a paper in the true sense yet it was the first writing material in the history that had many of its properties similar to the today’s paper. Invented by the Egyptians, Papyrus was in common use till the initial years of second-century AD [3]. In modern times, there are reports where the traditional ancient Egyptian technique had been applied for the production of papyrus-like paper sheets from materials belonging to different plants including sedges, grasses and rushes [4]. During the period of evolution when Papyrus was not in existence, the man had originally started recording various events through drawing or writing in sands and upon the walls of caves. However, he had later on started using the materials like wood, metal, stone, ceramics, leaves, barks, cloth, etc., for inscription. Other This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Chauhan, S., Meena, B. L. Introduction to pulp and paper industry: Global Scenario Physical Sciences Reviews [Online] 2021, 6 DOI: 10.1515/psr-2020-0014 https://doi.org/10.1515/9783110592412-001

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1 Introduction to pulp and paper industry: Global scenario

types of early writing surfaces were parchment or vellum. Parchment, which is named after the ancient city of Pergamum in Asia Minor, also antedates true paper as a writing medium. Although it is thought to have been in use as early as 1500 BC, yet it was the King of Pergamum (197–159 BC) who is credited for its invention. Parchment used to be made from the split skin of sheep whereas for manufacturing vellum, skin was not split into layers but it used to be prepared by a lengthy exposure of the calf/goat/lamb skin to lime followed by scrapping with a rounded knife and rubbed smooth with pumice stone. Similarly, “rice paper” is the term used for the translucent papers made in Asia. Just like Papyrus, it was also a substance unrelated to true paper. The thin sheets of rice paper were cut spirally from the inner pith of Kungshu (Tetrapanx papyriferum), formerly Fatsia papyrifera, the plant growing in the hills of Taiwan region [5]. The natives of Central and South America were similarly using the beaten barks of hemp, fig and mulberry plants. Hun, tapa, amity and amate are some of the other regional variations of such paper-like writing materials. Parchment or skins of animals commonly used in other countries were not acceptable in India as they were considered ritually impure. Hindus used to write upon the well-beaten cloth and the tender inner bark of the birch tree, the bhurja patra [2, 6–9]. Historically, Cai Lun, a dignitary serving the imperial Chinese court is credited for the invention of paper. Reportedly, he started producing sheets of paper from scraps of old rags, tree bark and fishing nets in 105 AD Chinese deliberately hid the secret of paper making till the sixth century but then their invention was brought to Japan by Buddhist monk Dam Jing. The Japanese immediately learnt the technique and started making paper at their own using the pulp derived from mulberry bark. But the startling fact to be noted here is that the recent researches have provided ample evidences to show that the paper was being made in India as back as 250 BC. However, that paper could not be accepted by the Indian society at that time because it was not treated pious for writing religious books [10, 11]. The history of papermaking in India has been reported [12] wherein the earliest reference of a writing material resembling the paper is mentioned as “Sindoshi” from the historical works of the Greek, Strabo. Nearchos (325 BC), who was an admiral of Alexander, the Great had referred to “Sindoshi” which was described as a well beaten cloth or well beaten linen both of which indicated a process similar to that of papermaking. Unfortunately, since there is no actual material evidence available, its evidence may be considered as a matter of speculation. However, this reference is a strong indication that paper and its knowledge existed in the Indus region during those early years of civilization. Papermaking in its simplest and most general form may be defined as the process of assembling individual fibers together to make a sheet or web and the nearest approach to this known to exist in pre-historic times is the nest of the wasp, the paper like nature of which is due to wood fibers from partly decayed vegetation [1]. A thin sheet must be made from fiber to be regarded as a true paper. For this, the fiber needs to be macerated to get each of the individual filaments as a separate unit. Such macerated fibers intermixed with water are then lifted from the water in

1.2 Fiber, the fundamental unit of paper

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the form of a thin stratum with the use of a sieve-like screen. The water drains through the small openings of the screen and thus leaves upon the screen’s surface, a sheet of matted fiber. This thin layer of intertwined fiber is actually a sheet of paper. This was the manner in which the first paper was made. Even today, the most ponderous and the most efficient papermaking machine utilize precisely the same principle. Thus the actual fiber formation of paper has not changed even after a long period of more than two thousand years [2, 6].

1.2 Fiber, the fundamental unit of paper Paper is nothing but cellulose. A true paper may be defined as the thin felted material formed on flat, porous molds from macerated vegetable fiber. So fiber, more precisely the cellulosic fiber is the basic fundamental unit of paper [2]. For the pulp and papermaker, fiber is generally an elongated or tubular, slender and actually very small plant cell whose diameter is quite thin and microscopic i.e. less than 0.1 mm (100 µm) although its length may vary in the unaltered state from about 1 mm to well over 120 mm. The common papermaking fibers generally have length/diameter (L/D) ratio in the range of about 50–200:1. Plant fibers are the elongated, dead and hollow cells which mainly function in conduction besides providing mechanical support to the plant parts where they are located. They can occur as isolated cells, clusters or in large masses or “tissues” depending upon the plant type or part [13]. Since papermaking fibers are actually the structural cells of plants, paper can therefore be made, in principle, from the huge range of plant sources. Thus, paper can be made from any fibrous material like cotton/sugar cane/bamboo, but the vast majority is made from trees. The species of the tree used determines the type of paper produced [14]. The availability, crop yield per hectare and quality of the fiber are the principal factors that limit the sources of papermaking. A Simple Additive Weighting (SAW) multiple criteria decision making design has been proposed for the sustainable raw material selection of papermaking [15]. As paper is obtained from fibers which were actually the cells of land plants, before chemical and mechanical treatment, paper does not have a definite chemical composition but it is largely pre-determined by the fiber source. Unlike most chemical raw materials, the papermaking fibers are actually produced biosynthetically as plant cells. A papermaker therefore has a very little control over fiber shape and chemical composition. The morphology, structure and chemical composition of paper making fibers is very important to understand. They have a profound effect upon the subsequent chemistry of the papermaking process and also upon the physical as well as the mechanical properties of the end product. Wood can be used with intelligence only if we understand wood [16, 17].

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1 Introduction to pulp and paper industry: Global scenario

1.2.1 Different types of papermaking fibers The papermaking fibers can be obtained from woody trees or the nonwoody plants. The woody sources include hard woods or soft woods whereas the nonwoods include agro residues or the bast fibers /leaf fibers. Apart from these, the recycled fibers (RCFs)/the secondary fibers are another category of papermaking fibers. This category includes waste/recovered paper/old currency notes/mixed office waste/ magazine waste/news print waste or the cotton rags and animal dung of herbivores (Table 1.1) Woody raw materials are obtained from generally the two categories of trees and accordingly termed as Hardwoods and Softwoods. Major differences between these two categories are shown in Table 1.2. Table 1.1: Diversified sources of papermaking fibers [5, 18]. Woody materials

Nonwoody Materials

Recycled fibers or secondary fibers

Hard woods

Agro-residues

Pre-consumer waste

– Eucalyptus (Eucalyptus globules/ Eucalyptus grandis) – Aspen (Populas tremula /P. sieboldi) – Birch or Paper Birch (Betula papyrifera) – Maple (Acer campestre/A. platanoides) – Oak (Quercus pedunculata) – Beech (Fagus grandifolia) – Balsam (Abies balsamea)

– Wheat straw (Triticum vulgare) – Rice straw (Oryza sativa) – Bagasse (Saccharum officianarum) – Sabai grass (Eulaliopsis binata) – Bodha grass (Chloroxylon colorathus) – Lemon grass (Cymbopogon ciatrus)

– Paper trims/ waste generated at the papermaking mills/units

Soft Woods

Bast Fibers

Post-consumer waste

– Pine (Pinus silvestris, P. strobus) – Hemlock(Tesuga canadensis) – Cypress (Texodium disticum) – Spruce (Picera tremula)

– Sunn Hemp (Crotolaria juncea) – Flax/Linen (Linum usitatissimum) – Ankra (Calotropis procera)

– Mixed Office Waste – Magazine Waste – Newsprint Waste – Shredded Currency Waste

1.2 Fiber, the fundamental unit of paper

5

Table 1.1 (continued ) Woody materials

Nonwoody Materials

– Douglas fir (Pseudostuga menziesii) – Larch (Laryx occidentalis)

– Shogun (Daphne papyracea) – Hemp (Canabis sativa) – Jute (Corchorus capsularis/ C. olitorius) – Bhimal (Grewia oppositifolia) – Kenaf (Hibisus canabinus) – Ramie (Boehmeria nivea) – Paper mulberry (Broussentia papyrifera) – Silk Mulberry/Shatut (Moru alba)

Recycled fibers or secondary fibers

Leaf Fibers

Dung of herbivores

– Banana (Musa sapientum/M. paradisiaca) – Pine apple (Ananas comosus) – Sisal (Agave sisalana) – Water Hyacinth (Eichornia crassipes) – Abaca/Manila Hemp (Musa textilis)

– Rhino Dung – Cow Dung – Elephant Dung (Pachyderm)

1.2.2 Composition of papermaking fibers Chemical composition of any plant gives an idea about its feasibility to be used as a raw material for papermaking. Fibers being the most important constituent of plants, their composition and amount are reflected in the properties of their cell walls [23]. The lignocellulosic materials from wood and nonwood plants consist of cellulose, lignin, hemicelluloses, extractive and some inorganic matter. Chemical composition of the raw material is important to decide the techno-economic viability, pulping method and the strength of paper produced by it [24]. Plant cell walls bear the shapes differing from spherical to cylindrical and sizes ranging from under 1 mm to several

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1 Introduction to pulp and paper industry: Global scenario

Table 1.2: Differences between soft woods and hardwoods [13, 19–22]. Parameters

Hard woods

Soft woods

Origin/source Hardwoods are obtained from the angiospermic trees. Angiosperms are characterized by broad leaves and covered seeds.

Softwoods are obtained from gymnospermic/coniferous trees. Gymnosperms are characterized by needles, cones and naked seeds.

Habitat

Hardwood species are much more numerous because they occur in tropical as well as temperate regions.

Softwoods grow primarily in the temperate zones.

Evolution stage and structural relevance

Hardwoods are more evolved plants. Their fiber provides mechanical support only while the food is conducted through vessels. They have % fibers, % vessels and % parenchyma.

Softwoods are more primitive plants. Their fiber is used for providing mechanical support as well as to conduct food (sap). They have % fiber and % parenchyma.

Growth rate

Hardwoods have a slower growth rate.

Softwoods have a faster rate of growth.

Shedding of leaves

Hardwoods generally shed their leaves Softwoods generally retain their during the autumn and winter seasons and needles throughout the year and are therefore regarded as deciduous trees. therefore regarded as evergreen trees.

Fire resistance

Hard woods are more resistant to fire.

Soft woods are less resistant to fire.

Cost

Hardwood is costlier than the softwood.

Softwood is less expensive than the hardwood.

Fiber

There are four general types of normal hardwood fibers, each having a separate technical name. One or more of these can be found in the same species. The fibers can be divided into two broad categoriestracheids and true fibers. Tracheids include “vascular tracheids” and “vasicentric tracheids” both of which are found in only a few woods and only to a very limited extent. True fibers include fiber tracheids and /or libriform fibers. They are found in all species and are major component of hardwood pulps. Hardwood fibers are short and thick walled. .–. mm long, – mg/ m coarseness

Conifer fiber is known technically as a “longitudinal tracheid”, as its long axis is parallel to the stem/branch/ root. Tracheids have closed ends (imperforate) and its wall has numerous small openings, “pits” to allow intercellular communication. Structurally, soft woods are excellent source of pulp fiber due to the greater length and slenderness of its fibers. Softwood fibers are long with relatively wide lumens. .– mm long, – mg/ m coarseness

1.2 Fiber, the fundamental unit of paper

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Table 1.2 (continued ) Parameters

Hard woods

Soft woods

Microscopy

Hardwoods have vessel elements that transport water throughout the tree. So, the vessel elements appear as pores under the microscope.

In the softwoods, water transport occurs through medullary rays and tracheids. So, softwoods don’t have visible pores under the microscope.

Composition

Hardwoods generally contain cellulose ( ± %), hemicelluloses ( ± %), lignin ( ± %), Extractives ( ± %)

Softwoods generally contain cellulose ( ± %), hemicelluloses ( ± %), lignin ( ± %), Extractives ( ± %)

Lignin

Lignin content in hardwoods range from –%

Lignin contents of softwoods range between –%

Specific gravity

Fiber wall thickness is a major determinant of gross wood SG, but the volume of wood occupied by the vessel system is also important. Both of these factors together with fiber cell volume, combine to produce a broader range of SG’s between different hardwood species than exists among softwoods. Hardwoods show less consistency than softwoods in trends for changing SG in both the radial and axial directions (i.e. with tree age and tree height).

Is used to give a general idea of how much wood fiber or wood substance can be obtained per unit volume of a given type of pulpwood. The potential behavior of pulp fibers in papermaking and in the final product can often be correlated to wood specific gravity. Pulp fiber coarseness (wt/unit length) varies directly with wood SG and has a strong influence on the behavior of resulting paper products. The proportion of early wood and latewood (fibers) within annual growth increments, together with fiber wall thickness, are important variables governing the SG of coniferous pulp woods.

Density

Most of the hardwoods bear higher density than most of the softwoods. High-density woods are actually harder, more difficult to chip and necessitate adjustments in chipper operation/maintenance. Therefore, hardwoods need more energy for chipping and produce more variable chips. High SG woods yield stiff, rod-like fibers which drain water more easily (free pulp).

Most softwoods bear a lower density than most of the hardwoods. They produce thin walled, easily collapsed (less free) fibers and therefore behave differently from hard woods to mechanical refining.

Resistance to decay

Hardwoods have a higher content of xylans. Softwoods contain more glucomannans They are generally resistant to decay when than hardwoods. Softwoods are not as used for exterior work. resistant as hardwoods.

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1 Introduction to pulp and paper industry: Global scenario

Table 1.2 (continued ) Parameters

Hard woods

Soft woods

Ease of Use

Most hardwoods are sturdier as they have high density. Due to such denseness, the wood generally splits if we hammer a nail into it. So, it is always better to drill screw/bolts to fasten hardwoods together.

Softwoods are easier to cut. Structural lumber is soft and light, accepts nails easily without splitting. Therefore, it is a good alternative for general construction purposes.

Paper Character

The paper produced from hardwood pulp bears a strong absorptivity, high opaqueness, high thickness and high stiffness. So it is more suitable for the manufacture of printing paper.

The paper produced from softwood pulp bears a good flexibility, high folding strength, good tensile strength and printability. It is also used as a “reinforcement material” for other types of paper

Applications

Although hardwood pulp is worse as compared to the softwood pulp but it still belongs to a good paper pulp. Hardwood pulp is generally mixed with softwood pulp for manufacturing coated paper, lightweight coated paper, offset paper, etc. Hardwoods are generally used in highquality furniture, decks, flooring, and construction that need to last for a longer time.

Softwood pulp is an excellent material for papermaking. It is generally used for making text paper, coated paper, map paper, offset paper and other industrial grades. About % of the total timber comes from softwood. Softwoods have a wider range of applications and are used in building components (e.g., windows, doors), furniture, mediumdensity fiberboard (MDF), paper, Christmas trees, etc.

centimeters. In higher plants, two types of functional cell walls can be distinguished. One is the primary cell wall which surrounds the growing cell and other is the secondary cell wall, which is appeared on ceasing of the cell growth. The cell wall is actually a complex composite material containing the structural as well as the nonstructural components. Although lignin and proteins play an important part but these components are mainly polysaccharides. Usually, the structural component is partly crystalline and exists in the form of micro fibrils. The chemical treatment which is used for converting the wood/nonwood fiber into pulp, dictates largely the chemical composition of paper. It remains very much similar to that of the native wood when the pulp has received little or no chemical treatment (e.g. newsprint pulp). However, the composition may be different in the papers prepared from chemically delignified pulp. Chemical treatments on one hand reduce the percentage of lignin, hemicellulose and extractives, whereas on the other hand percentage of cellulose is increased. Chemistry of each of these individual components of the papermaking fibers is as below:

1.2 Fiber, the fundamental unit of paper

9

1.2.2.1 Cellulose Cellulose is the primary structural component of the cell wall as well as of the paper. Chemically, it is a semi-crystalline microfibrillar linear polysachharide of β-1, 4-linked D-glucopyranose. Like most polysachharides, it is polydisperse with a high molecular weight. Degree of Polymerization (DP) indicates the number of glucose units in a cellulose molecule. The DP is generally more than 10,000 in native wood but less than 1,000 for highly bleached kraft pulp. Cellulose being the major constituent of papermaking is expected to be in high quality and its quality depends on the raw material and pulping method used. The individualized fibers of pulp are much different from those of wood from which they originated. One of the most important changes is the great increase in surface area per unit of dry mass, i.e. specific surface area. Studies have shown that the specific surface area of never-dried pulp fibers can be more than 100 square meters per gram [25]. As compared to the relatively stiff fibers, flexible, ribbon like fibers tend to form stronger inter-fiber bonding [26]. Alpha cellulose is the purest form of cellulose. Therefore, a high percentage of alpha cellulose in paper provides a stable, permanent material [19]. 1.2.2.2 Hemicelluloses Hemicelluloses constitute about 15–30% of dry wood but have shorter chain of polysaccharides (DP of only 50–300) compared to cellulose [27]. The main function of hemicelluloses is to increase fiber-to-fiber bonding but at a higher amount, tends to lower the strength properties of paper [27]. The hemicelluloses are a group of nonstructural, low-molecular weight and heterogeneous polysaccharides. They are not related to cellulose and are formed through a separate biosynthetic route. Although their name seems to imply, but hemicelluloses are not the biosynthetic precursors of cellulose. Hemicellulose content is not as important as the cellulose contents in pulp but still it brings an important contribution to pulp quality and therefore its prospective loss raises some concerns [28]. Hemicelluloses can enhance pulp beatability, because their abundant end groups are more accessible to water molecules as compared to those of cellulose [1]. Hemicellulose in chemical pulps serves as an inter-fiber binding agent and improves the physical strength properties of the paper produced [28]. Loss of hemicellulose from pulp has negative effects on the pulp and paper properties [29]. This is because their loss decreases the number of free hydroxyl groups on the fiber surface thereby reducing the strength of hydrogen bond between the fibers [1]. The loss of hemicellulose also decreases the fibril surface area accessible to water molecules as well as the fiber surface charge, which causes changes in the fiber swelling and flexibility [30]. Due to their noncrystalline hydrophilic nature, the hemicelluloses actually contribute towards the swelling of the pulp and hence conformability of the wet fibers during sheet formation. The hemicelluloses (viz. glucuronoxylans) which are solubilized during the early stages of

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1 Introduction to pulp and paper industry: Global scenario

alkali pulping get re-precipitated onto the fiber surfaces and most likely back into the cell wall during the later stages of pulping. This is partly due to a decrease in their solubility caused by the consumption of alkali during the pulping process and partly due to the structural modifications. Removal of the uronic acid groups actually makes the polysachharides less soluble in alkali. The average value of hemicellulose that constitutes good quality paper is a function of the raw material, quantity of the hemicelluloses in the raw material and the method of pulping used. 1.2.2.3 Lignin Is an aromatic polymer having an extremely complex structure. Almost all of its properties are undesirable for papermaking applications. Therefore, it is desirable to remove lignin. Lignin not only causes the paper to become brittle but it is also oxidized photo-chemically to form colored byproducts. Such byproducts are responsible for the yellowing and discoloration of the bleached paper sheets. Lignin content of different woods ranges between 25–35% in softwoods and 18–25% in hardwoods [27] whereas the nonwood fibers contain around 5–23% lignin [31]. The ideal pulping process therefore dissolves lignin completely without causing any loss or degradation to the carbohydrate component. Several advances have been made towards its removal during the pulping processes. The ease of delignification of a raw material can be estimated from its lignin content [32]. Various types of pulping methods including Soda/Sulfite/Kraft pulping are used in papermaking. The biotechnological routes involving Biopulping or enzymatic pulping can also play significant roles in delignification of the papermaking fibers. 1.2.2.4 Resins and extractives A small proportion (usually less than 5%) of wood consists of the components which are extractable by organic solvents. The proportion of these extractives varies in hardwoods and softwoods and also between the species. They vary highly in their chemical compositions and may include alkanes, fatty alcohols, acids (both saturated and unsaturated), glycerol esters, waxes, resin acids, terpene and phenolic components. It is the pulping process used that decides the proportion of resins and extractives which remains in pulp and paper. The acidic components like resin and fatty acids can be removed by converting them to their soluble carboxylate salt forms with alkali. But, they are not so readily solubilized in acidic pulping. High extractive content lowers pulp yield, affects the brightness of unbleached pulp and increases the demand of pulping and bleaching chemicals [33]. Presence of extractives in woody materials causes an increase in the consumption of pulping chemical whereas a reduction in the pulp yields. Therefore, the raw materials with little or no extractive contents are preferred [34].

1.3 Basic process of papermaking

11

1.2.2.5 Inorganic content Ash content refers to the inorganic constituent of lignocellulosic material. It is the residue left after the combustion of organic matter at a temperature of 525 ± 25 °C [35]. The ash content mainly consists of the metal salts such as silicates, carbonates, oxalates and phosphates of potassium, magnesium, calcium, iron and manganese as well as silicon. These inorganic components are generally deposited in the cell walls, libriform fibers and luminar of parenchyma cells and also in the resin canals and ray cells [36]. High ash content creates problem during refining and recovery of the cooking liquors [34]. Presence of high silica content may complicate the chemical recovery process. Similarly high contents of Nitrogen in the spent liquor may result into the generation of NOx in the chemical recovery furnace whereas potassium in the fiber can combine with chlorine to form KCl thereby causing a corrosive effect on metal parts of the furnace and the boiler [19].

1.3 Basic process of papermaking Basic process of papermaking varies for the cellulosic RCFs and for the lignocellulosic woody or non-woody fibers. Being cellulosic in nature, recycled or secondary fibers are directly processed in hydrapulper with deinking (after shredding/chopping into uniform size) followed by beating/refining, papermaking, drying, calendaring and cutting. Whereas in the case of lignocellulosic materials, paper is actually made in two important steps. Firstly, extraction of cellulose fibers from the variety of sources and its conversion into pulp is done through delignification i.e. pulping. Pulp is then combined with water and taken over the papermaking machine where it is flattened, dried and cut into sheets and rolls. While using the cellulosic secondary fibers viz. cotton rags or textile industry wastes as a raw material (as in the case of handmade papermaking), the elegant and enduring paper suitable for the documents of archival use can be produced without the utilization of pulping chemicals. During the paper manufacturing process, from any lignocellulosic plant material, three components namely bark, fiber and lignin need to be separated. The fibers of woody trees, which are held together by lignin, are protected by the bark. Therefore, the fiber is extracted using the chemical or mechanical process. To make paper, individual wood fibers have to be separated from each other (defiberized) either mechanically by grinding the lignin (ground wood process) or chemically by dissolving it. A combination of both the processes is employed in chemi-mechanical or semichemical pulping. The actual procedures used by different paper mills are different but still a generalized method can be understood for mechanical pulping and chemical pulping process [37, 38].

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1 Introduction to pulp and paper industry: Global scenario

1.3.1 Mechanical pulping Debarking i.e. the removal of bark from the logs is done first and this excess material removed serves as a source of biomass energy in the paper mill. The debarked logs are then ground up through Refiner which has a rotating disc and a fixed steel plate. Heat and chemicals are also added generally. In the process of mechanical pulping, whole as well as partial fibers are created. Mechanical pulping process uses much more energy than that can be generated from the bark removed. But the process is still very cost effective due to the pulping yields obtained as high as 95% without producing any waste material. Mechanical pulping is the lignin saving process and the lignin that remains in the pulp imparts a grey yellow color to the paper. Paper produced from the mechanical pulp is regarded as the “ground wood paper”. News print papers are an example.

1.3.2 Chemical pulping Chemical pulping utilizes the wood chips instead of using huge logs. The chips are placed into big machine termed Digestor along with water and chemicals. The pressurized digestion at high temperatures results into the disintegration of wood chips thereby leaving wood fibers and generating black liquor as a waste material. After removing the black liquor, fiber is cleaned with water and sometimes bleached to ensure purity. After recovering inorganics (chemicals), majority of the black liquor can be re-used whereas the remainder of the liquid (natural biomass) is converted into energy. Such functions typically operate in a “closed loop” system. In most of the cases, the power generated is more than the needed and therefore, an environmental friendly power source is thereby created for the local communities. A Chemical pulping process generally removes more lignin and results essentially into individual wood fibers, which can be converted into many products, from linerboard (the walls of corrugated boxes) to tissue paper or magazine stock. Papers made from chemical pulp are usually brighter, smoother, and of a higher quality than their mechanically pulped counterparts. The next step of papermaking includes use of Paper machines which comprise of four primary sections. They are as below: – In the Wet End Section, pulp is first mixed with water, fillers and other additives and then it is pumped onto a belt. The belt is typically made of a mesh which allows all the fibers to go in a specific direction. “Grain direction” of the paper is actually decided by the orientation of fibers on this belt. – In the Wet Press Section, pulp is moved from the mesh belt to the felt belt. In the felt belt, pulp is moved through a series of high pressure rollers which are especially designed to push the liquid into the felt. With the rotation of the felt, pulp is passed through its own drying station so as to remove the moisture.

1.3 Basic process of papermaking

13

– In the Dryer Section, pulp starts to take the shape of a paper. This part of the machine contains a series of heated rollers, which enable a weaving of the web of paper. – In the last part of the machine i.e. Calendar Section, rollers are mounted opposite to each other. These rollers create pressure on the paper to impart a smooth finish. The paper smoothens more on using more number of such rollers. The less expensive papers prepared from mechanical pulps can be added with the gloss through super calendaring. For adding brightness to the higher quality stocks of chemical pulps, coatings are preferred. Mostly, China clay is added between the sections of “wet-press” and the “drying” to make the papers glossy. The dried pulp is also baled sometimes to be sold as the market pulp for its further conversion to the final products at other facilities. There is an important category of paper called “Handmade paper”. It is defined as a sheet of paper or board produced by hand in the sheet making process. The important to be noted here is that Khadi & Village Industries Commission (KVIC) has also included the paper and boards made on the Cylinder Mold Vat (CMV) up to a definite maximum deckle width of 102 cm, under the handmade paper category. In the present context when the entire world has developed a great charm for environmental friendliness, the handmade paper is becoming significant. This is because the processing of handmade paper is much more natural in terms of the chemical-utilization and the raw material usage. The handmade paper also enjoys important features of endurance through centuries and greater recyclability potential, which make it superior to the machine-made paper. In addition, the beauty of deckle-edge and strength isotropy due to the absence of grain is enough to fascinate anyone towards handmade paper [39]. The institutions like Kumarappa National Handmade Paper Institute (KNHPI), Jaipur and Handmade Paper Institute (HMPI), Pune are working for the handmade paper sector in India. The other countries like China, Thailand, Philippines, and Nepal are also engaged in handmade papermaking. Handmade paper is manufactured from the following three types of raw materials: – Traditionally used Cotton hosiery waste/cotton rags/tailor cuttings/textile industry waste – Waste paper of different types (Mixed office waste-MOW, Magazine waste-MW, News print waste-NPW, Shredded currency waste of RBI-SCW, etc.) – Natural fiber (Bast fibers/leaf fibers/agro-residues/Cow or Elephant dung, etc). The process of handmade papermaking utilizes the batch procedures at small scales. The process flow diagram of making handmade paper from the above three types of raw materials is given in Figure 1.1 and Figure 1.3 separately.

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1 Introduction to pulp and paper industry: Global scenario

Figure 1.1: Process of handmade papermaking from cotton rags [39].

Figure 1.2: Process of handmade papermaking from natural fibers [39].

1.4 Environmental concerns of papermaking

15

Figure 1.3: Process of handmade papermaking from waste paper [39].

1.4 Environmental concerns of papermaking Pulp and paper industry is one of the most polluting industries of the world [40, 41]. The whole process of papermaking is not only energy intensive but also hugely water intensive in terms of the fresh water utilization [42]. The amount of water consumption varies with the production process used for making paper and it can be as high as 60 m3/ton of paper produced [40]. Papermaking is actually a very thirsty business as fresh water is consumed at each and every step right from the cultivation of trees/ crops to pulping, bleaching and other stages of paper formation [43, 44]. There is not only an enormous consumption of fresh water but the paper industry also generates huge amounts of waste waters. The wastewaters generated have high concentration of various chemicals viz. sodium hydroxide, sodium carbonate, sodium sulfide, chlorine dioxide, calcium oxide, hydrochloric acid, etc [41]. Some of the major concerns of paper industry include high organic contents (20–110 kg COD/air dried ton paper), dark brown color, Adsorbable Organic Halides (AOX) and toxic pollutants [37]. The hugeness of water consumption and wastewater generation is not the only environmental problem of pulp and paper industry but the solid wastes including sludge of the wastewater treatment plants and air emissions are also among the severe problems caused by the industry A significant amount of solid wastes involving lime mud, lime slaker grits, green liquor dregs, boiler and furnace ash, scrubber sludges, wood processing residuals and wastewater treatment sludges, etc., is also

16

1 Introduction to pulp and paper industry: Global scenario

generated from different paper mills. Disposal of such solid wastes causes environmental problems due to the high organic content, partitioning of chlorinated organics, pathogens, ash and trace amounts of heavy metal contents [45]. Air emissions which vary with the type of mill as well as with the stage of papermaking also need serious attention. On one hand, Sulfite mills release emissions from recovery furnaces, burns and also sulfur oxides (SOx), Kraft mills on the other hand have problems of reduced sulfur gases. Odor problem occurs due to the Volatile Organic Carbons (VOCs) from wood-chips digestion, spent liquor evaporation and bleaching operations. Similarly, combustion process releases nitrogen oxides (NOx) and SOx [46]. All such pollutants including air emissions, liquid effluents, solid wastes, etc., have been described [37] and listed quantitatively along with their characteristics [47]. One more striking point to know here is that 27 trees of Eucalyptus are usually required (based on 45% yield, 50% moisture, and 6’ girth 30’ ht.) for producing one MT of Paper. Paper industry also contributes to the global warming problems by causing huge level of deforestation. The production as well as the use of paper is associated with so much of the adverse effects on environment that a collective term of Paper Pollution is generally used. The Life Cycle Assessment (LCA) of paper has revealed that the disposal of huge amounts of discarded paper products also generates environmental problems. When the paper is decomposed in landfills, it creates methane. Paper products are estimated to constitute up to 25% of all landfill waste [44, 48]. The amount of methane created by Paper in landfills is up to 69 times greater than that produced by fossil fuel electricity production and it also has 23 times the heat trapping power of carbon dioxide [49–51]. The tissue products like toilet paper, paper towels and facial tissue are although cheap and convenient in use but they actually cost a lot to the planet, Earth. Their everyday consumption actually strengthens or facilitates the “trees to toilet” pipeline. This is because of the uprooting of the centuries old trees for conversion into tissue pulp which is then rolled into perforated sheets or stuffed into boxes and flushed into toilets or thrown away into the bins. Such things lead to the destructive consequences for indigenous people, treasured wildlife and global climate. USA alone consumes 20% of the global tissue production [52]. Effect of various toxic pollutants of pulp and paper mills on water and soil quality has also been reported [53]. Thus the paper use is such an ecological concern that it has prompted several intervening actions all over the world.

1.5 Ways to address the environmental concerns of paper industry The global pulp and paper industry has a massive impact not only on the earth’s resources but also upon its inhabitants and the climate. It actually depends upon our actions whether the paper industry can be directed towards a progress or degradation further. Therefore the agencies like Environmental Paper Network (EPA), Confederation of European Paper Industries (CEPI), Ministry of Environment and

1.5 Ways to address the environmental concerns of paper industry

17

Forest (MoEF), Govt. of India, etc., are sincerely putting up their efforts for introducing appropriate actions so that the paper industry may be moved further towards the progress. Various proactive initiatives are considered as powerful tools to improve the environmental footprints of the process of paper manufacturing. Accordingly certain environmental and sustainability indicators have been proposed to measure the progress [54]. EPA (which is a global alliance of civil society organizations working together to achieve the Global Paper Vision) has surveyed and studied the social and environmental risks as well as the opportunities being faced by global pulp and paper industry. Consequently, an elaborative report titled “State of the Global Paper Industry Report (2018)” was released. Besides providing a brief of how the world’s pulp and paper industry is performing today relative to the goals of the Global Paper Vision, the said report has also identified several key features to accomplish the mission [43]. Addressing the entirety of the paper life-cycle the Global Paper Vision calls upon the global paper industry, consumers, retailers, governments, investors and Non-Governmental Organizations (NGOs) to commit to the following actions on an immediate basis: – Reducing the consumption of paper globally – Promoting a fair access to paper for each and every person – Maximizing RCFRCF content – Ensuring social responsibility – Responsible attitude in sourcing fiber – Reduction of Greenhouse gas emissions – Utilization of cleaner production technologies – Ensuring transparency and integrity Similarly, CEPI (2014) has also suggested measures to bring resource efficiency in the pulp and paper industry. In contrast to the linear model of Taking-Making-Disposing, a circular economy has been proposed [55]. Circular economy which takes insights from the living systems is actually a framework which considers that our systems should work like organisms that process nutrients for feeding back into the cycle. Based on renewable, recyclable raw materials, second generation biofuels can be produced to replace fossil fuels thereby moving towards the bio-economy. To address the recyclability limitation of the old paper, influx of new wood fibers can be done from renewable, sustainably-managed forests, thereby living the circular economy. Overall, following methods have been proposed to address the environmental concerns of the paper industry in a very interesting manner so that a positive future may be shaped [14, 37, 55–57]: – Taking fiber from the socially managed forests or from the agro-residues and keeping resource fiber use to a minimum. – Maximizing the utilization of wood for each and every of its component. Although varies from species to species, but the wood generally contains 45% fibers, 30% binding materials and 25% sugars and other materials. So, apart from making

18

– – – –

– – – –

1 Introduction to pulp and paper industry: Global scenario

paper and paper products, wood can be used in different ways to generate a vast range of sub-products in many industrial sectors. The unwanted material, lignin from the wood can be used as an additive for everything from concrete and textile dyes to batteries and fishery products. Recycling of the used papers and paper products. Using water carefully to minimize its intake and to maximize its recycling. Using recycling residues or by streams from paper production to generate energy or as a source of minerals from recycled sludge ash or as new feedstock for the low quality paper grades. Sufficient steps should also be taken to conserve energy and to control spills, leaks and odor. Biogas generation from the waste fibrous residues, effective bio-treatments of the waste effluents, etc. Overall improvement of the industry environmental practices. Industrial symbiosis of the paper industries by partnering with the public and private sector. Paperteries du Rhin owned by the Kunert group, France is a good example to follow.

1.6 Global status of the paper industry and challenges ahead The pulp and paper industry is one of the largest industries in the world, which is contributing to the world’s economy in a growing manner. North American, Northern European and East Asian companies are the dominant players. Latin America and Australasia also have considerable number of pulp and paper industries. World production of paper and paperboard has crossed the value of 400 million tons [58]. The global production and consumption of pulp by different regions is given in Figure 1.4 and Figure 1.5. The percentage wise use of different fibers for papermaking in various regions of the world is reported as 55% recovered fiber, 42% virgin fiber and 3% other fibers [59]. The Per Capita Consumption (amount of paper consumed in kg per person per year) of paper is very important parameter as it is considered as a yardstick of the development of any society. Per capita consumption of paper by region is shown in Figure 1.6 and Table 1.3 shows the values of various countries. Many countries have more than double the global average of 55 kg.

1.7 Indian paper industry The Indian paper industry has around 3.7% contribution in the world’s paper production. At present, there are more than 600 paper mills in the country which have an installed capacity of around 22 million tonnes with an average capacity utilization of 80% [60]. Having registered a Compound Annual Growth Rate (CAGR) of 8% during

1.7 Indian paper industry

19

Figure 1.4: Global pulp production by regions (Baffani, 2018; FAO, Yearbook 2015 Forest products, http://www.fao.org/3/a-i5542m.pdf.)

Figure 1.5: Total paper consumption by region (x1000T) (Source: FAO, 2016; Hagith, 2018).

the period from 2011 to 2016 vs 1% for its global peers, India is reported to be one of the major fastest growing paper markets in the world. Percentage use of the different types of raw materials in the country is recorded as 88% RCF, 23% wood based fiber

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1 Introduction to pulp and paper industry: Global scenario

Figure 1.6: Per Capita consumption of paper by region (Source: FAO, 2016 and Haggith, 2018).

Table 1.3: Per Capita consumption of paper [60]. S. no.

Name of the country

          

Germany USA Japan Korea Italy UK China Mexico Brazil India Global Average

Per capita consumption in kg           

and 9% agro based fiber [61]. The Indian paper industry produces 53% packaging grade, 38% waste paper grade and 8% news print grade [61]. Writing/printing paper and paperboard are the two largest and most profitable segments of the Indian paper industry. The market share of the three largest players in India are 4.1% (ITC), 2.6% (JK Paper) and 2.4% (TNPL). Although India is a fast growing market in the world, it is highly fragmented with the top 3 players accounting for only about 9.1% market share against 68% in the USA, 72% in Indonesia, and 21% in China. Dynamics of the Indian paper industry is shown in Table 1.4. The current status of the industry is ideal for a strategic consolidation according to which the viable stressed assets of the sick players may be acquired. At the same time, the healthy players may gain market share through inorganic expansions. As is obvious from the experiences of the other

1.8 Changing trends of paper industry and future projections

21

Table 1.4: Dynamics of Indian paper industry [60]. Products

Volume (mn tons, FY) Global India

Projected CAGR (–)

Industrial segment

Major players in the segment

Global

India

India’s Major players in the segment segmental market share

.%

Newsprints

.

. −.%

Writing and printing paper

.

.

−.

.

Writing and Printing paper Uncoated

.

. −.%

.%

Paperboardsvirgin

.

.

. .%

.

.%

% ITC, TNPL % Astron, RuAstron, Ruchira, Genus Paper, South India Paper, Shree Ajit Pulp and Paperchira, Genus Paper

Paperboardsrecycled .

.

.%

.%

Tissue paper

.

.

.%

.%

Specialty paper like Cigarette Paper, Decor Papers

.

.

−.%

.%

Industrial Paper incl. Kraft Paper

% NR Agarwal, Shree Rama Newsprint % Ballarpur Industries, JKPL % TNPL,WCPM,IP APPM, JKPL, Emami Paper, Seshasayee % ITC, TNPL, WCPM, Emami Paper, JKPL

% Century textiles, Ballarpur Industries, Orient Paper .% ITC,WCPM

markets, this process is supposed to be gradual but eventual, and the winners are bound to create reasonable value for their shareholders [60].

1.8 Changing trends of paper industry and future projections 1.8.1 Efforts of minimum impact and effluent closures Recently, several economic driving forces have emerged in connection with the environmental impact. Due to the several economic benefits, various companies are examining effluent closure or at least minimum-impact possibilities. Accordingly,

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1 Introduction to pulp and paper industry: Global scenario

many mills are trying to achieve minimum impact or closure of the bleach plant effluent. This might prove to be a fairly significant marketing tool for the pulp and paper industry in near future [62].

1.8.2 Shrinking demands of graphic paper but increase in demands of packaging grades Traditionally, paper is recognized mainly with the reading and writing uses but packaging application has presently become so important that it accounts for over 55% of all the global paper use (Figure 1.7). The popularity of new media and paperless reading has led to a contraction in demands of printing & writing paper as well as the newsprint grade. But as a whole, the paper and forest-products industry is growing, even though at a slower pace than before. This is because the other products are filling up the gap created by shrinkage in the demands of writing/printing grades. The growing internet and e-commerce business model has resulted into a rapid expansion of the express delivery business, and therefore the demand of packaging grades is escalating progressively [62].

Figure 1.7: Global consumption of different paper categories (Source: Haggith, 2018).

1.8.3 Impact of the ban on “Single Use Plastics” Single-use plastics or the disposable plastics commonly found in environment include the items like cigarette butts, plastic drinking bottles, plastic bottle caps, food wrappers, plastic grocery bags, plastic lids, straws and stirrers, other types of plastic

1.8 Changing trends of paper industry and future projections

23

bags, and take-away containers. This is the adverse outcome of the “Throwaway Culture” that treats plastic as a disposable material rather than as a valuable resource to be harnessed. Such plastic waste materials cause a huge economic harm. In the Asia-Pacific region alone, plastic litter is creating an additional expenditure of $1.3 billion per year to its tourism, fishing and shipping industries. Similarly, Europe is spending about €630 million per year in cleaning plastic waste from coasts and beaches. Studies suggest that at least $13 billion per year is the economic damage caused by plastic to the world’s marine ecosystem. The disposed plastic bags have even escaped to the remote areas, like the Pacific Ocean. Thus, they are not only posing a threat to aquatic life, but also to the landfills and the agricultural lands. Consequently, a massive environmental degradation of the so called civilized global community is bound to occur [63, 64]. Lately, large turtles of the endangered species were reported to get suffocated due to mistaken swallowing of the plastic sheets mixed with seaweeds [65, 66]. Therefore, many countries including India have enforced ban on single use plastics. This ban can have a great impact on the paper industry as paper can be considered safer to dispose than the plastic products. Scottish research has demonstrated that a tariff imposed on plastic bags can result in an increase of the paper bag consumption. It has been suggested that the natural fibers of paper and the recyclability potential of paper can be useful [67, 68].

1.8.4 Substantial transformation of the paper industry An effect of the increased per capita internet adoption on the global paper products has been forecasted to the years 2030 and 2026. With the help of the global forests products model [68, 69] Two scenarios have been examined anticipating the full internet adoption by 2100 (GFPM 2100) and more rapidly by 2050 (GFPM, 2050) [68]. Accordingly, the projections of global newsprint consumption were found to be 34.2–37.1 million tones lower than in the 2010 RPA report [70] and 2010 FAO report [71]. Similarly the projections of global printing and writing paper consumptions were found to be 76.7.–87.1 million tones lower than such studies done previously. It has been forecasted that the real price of writing printing grades will follow a downward trajectory into the year 2030 [72]. But this does not mean that the paper and forest-products industry is going to disappear. On the contrary, the industry is changing, morphing, and developing. It has been argued that the paper industry is just passing through the most generous change or transformation, it has experienced in several decades. The decline recorded in the forecast is supposed to be balanced by the increase in demand of the packaging and industrial grades as well as the hygiene products. Thus, the demand of fiber-based products is bound to increase globally wherein some segments may grow faster than the others [73]. With the help of a Delphi study [74] it has been reported that the European pulp and paper industry is facing the transition phase

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1 Introduction to pulp and paper industry: Global scenario

to a bio-economy. According to the projected 2030 scenario, the industry is going to produce more diversified products with a focus on the higher value addition targeting specifically the consumers having degree of higher environmental awareness.

1.8.5 Paper industry in the post COVID-19 era Use of paper is also bound to increase in the post COVID-19 era due to the popularization of the use of disposable items (paper cups, paper plates, napkins, tissue papers, glasses, etc.) [75] as well as growing popularity of online shopping to avoid crowded malls and maintain social distancing as a measure of protection from Corona. Frank (2020) has also reported COVID-19 impact analysis on pulp and paper, market business. Moreover, the biodegradable packaging with paper is bound to increase with the imposition of bans on “Single Use Polythenes” (disposable plastics) in many countries including India [76].

1.9 Challenges of paper industry for the next decade The paper industry needs structural changes by conversion of graphic paper companies into the ones manufacturing packaging and specialty grades. Moreover, the industry also needs innovations in distribution and the supply chain management so as to re-emerge from several years in the doldrums. Driven largely by the demographic shifts and the consumer trends of convenience and sustainability, consumer packaging and tissue is going to shoot roughly at par with the Gross Domestic Product (GDP). In the specific context of the recent concerns over plastic packaging waste, innovation seems to be a critical success factor. This could provide the opportunities as well as the challenges for fiber based consumer packaging. Transport and Industrial packaging also have vast opportunities for innovation and value creation intersecting the sustainability requirement, e-commerce and technology integration with an emphasis upon how to handle last-mile deliveries. The paper and forest-products industry is often labeled a “traditional” industry. But on having a look into the union of technological changes, demographic changes, and resource concerns being anticipated for the next decade, the industry should be ready to adopt changes that are, in character, as well as in pace and vastly different from what has been seen before i.e. anything but traditional. Therefore, it has been proposed [73] that following three broad themes have to be addressed through 2020 and beyond: – How to manage the short-to-medium-term turbulence of different grades of paper – Capabilities to find the next level of cost optimization – In the fundamentally changing business scenario, how to find out value-creating growth roles for the forest products

References

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1.10 Conclusion Paper is more than an industrial and commercial commodity today. The welfare of any nation cannot be achieved without rising in the consumption of all kinds of cultural and industrial grades of paper. Although paper industry is reported to be one of the most polluting industries of the world yet there are ways to address all the environmental concerns. With the suitable proactive measures and sufficient steps in the right direction, paper industry is sure to improve its environmental performance. The paper industry is moving into an interesting decade with so much of challenges that it can really undergo a transformation while keeping pace with the changing market demands and a balance between the economic, environmental and social objectives.

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Puneet Pathak and Chhavi Sharma

2 Processes and problems of pulp and paper industry: an overview Abstract: The pulp and paper industry is a highly energy-intensive and waterconsuming industry. This industry is known for the utilization of a wide range of raw materials, containing cellulose fibers (generally wood, recycled paper, and agricultural residues), for the production of various grades of paper. There are several processes involved in the conversion of raw materials to the paper product such as raw material preparation, pulping, pulp washing and screening, bleaching, stock preparation, papermaking and chemical recovery. All the processes are facing issues regarding process efficiency, product quality, energy & water consumption, and cost and environment. There is a need for further improvement and upgrading the technologies but the scale of operations, technological obsolesce and cost of implementing new technologies are some of the major issues. The main thrust areas of pulp and paper processing require major interventions in the adoption of green and clean technologies. Keywords: Pulp and paper industry, pulping, bleaching, energy & water consumption, environmental issues, enzymes

2.1 Introduction Pulp and paper industry is considered as among the most polluting industries consuming water (for the production of one ton of product) and energy in huge amount (fifth largest consumer, accounts for 4% of all the world’s energy use). The process of paper uses more water to produce 1 ton of product than perhaps any other industry. Globally, this industry is facing some challenges in terms of energy requirements and wastewater. Conventional technologies employed in this industry consume raw materials, chemicals, energy, and water, which is resulted in the generation of effluents and solid wastes in huge quantities. The capital and energy-intensive processes of the pulp and paper industry are based on old technologies of several decades ago and it is expected that the flexible mills of the future will consume less capital, energy, and water due to stricter environmental regulations. Over the past few decades, it is evident that basic research for the development of new processes and products is

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Pathak, P., Charma, C. Processes and problems of pulp and paper industry: an overview Physical Sciences Reviews [Online] 2021, 6 DOI: 10.1515/psr-2019-0042 https://doi.org/10.1515/9783110592412-002

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increased with time, these newly developed processes and products also bring new control problems with them. To maintain the industry viable, research activities in different pulp and paper processes are being continued for improving the process and product devolvement. All over the world, the different types of approaches and technologies are being explored rapidly to overcome the various problems faced by this water- and energy-intensive industry. Worldwide there are three major categories of the mills (1) Wood-based mills: producing bleached and unbleached pulp or paper, newsprint paper and rayon grade pulp; (2) Agro-residue based mills: producing bleached and unbleached pulp or paper (with and without chemical recovery system); (3) Recycled fiber-based mills: producing writing & printing grade, newsprint, unbleached Kraft paper, board (with and without deinking). Based on pulp and paper manufacturing facilities, there are two types of pulp and paper mills (1) Integrated mill having common systems for on-site manufacturing of pulp and papermaking (2) Non-integrated mills having no facility for pulping therefore always require pulp from outside of the mill with fewer expenses on land and energy. Integrated mills have common supporting systems (for steam and electricity generation, wastewater treatment, etc.) and reduced transportation costs (Bajpai 2018). In the paper industry, there are several global developments after the discovery of papermaking to remain in continuous improvement such as invention of the first continuous web paper machine (1798), development of the deinking process (patented in 1801), availability of first commercial Fourdrinier paper machine (1807), drying cylinder (patented in 1820) and First kraft pulping process (1865). The various developments such as the incorporation of kraft pulping, bleaching process, chemical recovery system, and commercialization of digesters and secondary headboxes were taken place between the years 1900 and 1950. After the year 1950 to the year 2000, several other processes and machinery were developed and commercialized like the use of + wire width paper machines, composite felts, new wet end chemistry, hybrid deformers, high yielding pulping, Z (ozone), D (chlorine dioxide), P (hydrogen peroxide) sequences and many more [1].

2.2 Papermaking process Papermaking from raw materials involves different processes which are as follows (Figure 2.1):

2.2 Papermaking process

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Figure 2.1: Schematic diagram showing different pulp and paper processes in an integrated pulp and paper mill (processes are shown inside the circle and basic requirements are shown inside the rectangle).

2.2.1 From wood/ agro-residues 2.2.1.1 Raw material preparation The major raw materials, required for pulp and paper manufacturing, are wood (hardwood and softwood), agricultural residues (bamboo, bagasse, reeds, cereal straw, grass, jute, flax, sisal, etc.) and recycled paper (for recycled based mills) (old newsprints, old magazines, mixed office waste papers, old corrugated containers, coated papers, etc.) [2, 3]. 2.2.1.2 Debarking and chipping Debarking is the process to remove the outer layer of the wood known as bark, which contains mainly tannins, resin acids, etc. The debarked portion is generally utilized for fuel or used in soil enrichment. Softwoods contain a higher amount of resin acid in comparison to hardwoods while agro-residues may not have resin acids. Chipping is the process to break the woody material in small size chips to provide a higher surface area, which facilitates the entry of cooking chemicals during the pulping process. During the passing of the chips through vibrating screens,

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2 Processes and problems of pulp and paper industry: an overview

oversized chips and undersized chips and dust are discarded. The chips of desired sizes were collected in huge bins as accepts for further chemical processing [4, 5]. In the absence of special pulping facilities, this rejected oversized chips are recycled back to “re-chipper” by a conveyor belt, and the fines are generally burned along with the bark waste (unless special pulping facilities are available) [5–7]. 2.2.1.3 Pulping Pulping is the process, which converts the wood chips into fibrous form (i.e. pulp). The pulping can be carried out by different methods such as mechanical and chemical [3, 8] (Figure 2.2).

Figure 2.2: Comparison of different types of mechanical and chemical pulping in terms of pulp color, pulp yield and uses.

2.2.1.3.1 Mechanical pulping Mechanical pulping of wood (softwood or hardwood) is done using a large amount of electrical energy. Fiber present in the wood is mechanically separated by the forceful entry of debarked logs between huge rotating discs of steel having teeth along with the passage of hot water. This mechanical action tears off the wood away from each other or presses the debarked logs touching the grindstones. The mechanical action by the grinder and refiners increases the temperature which helps in the softening of the lignin and breaking of bonds between the fibers [3, 8]. In thermomechanical pulping (TMP), high-temperature is provided by steaming before refining to soften the interfiber lignin and this action also removed the outermost layers of the fibers to facilitate interfiber bonding between exposed cellulosic surfaces. Due to good bonding between cellulosic fibers, TMP pulps are generally stronger than groundwood pulps. In the chemithermomechanical pulping (CTMP), the chemicals such as hydrogen sulfite are applied before the refining process to partial sulfonate lignin present in middle lamella. Due to better swelling properties and the lower glass transition temperature of lignin, high strength pulp fibers are easily separated during subsequent refining. In semi-chemical or chemi-mechanical pulping (CMP), before mechanical refining, the wood chips are pretreated with mild sodium hydroxide by impregnation for the partial removal of resin and lignin from the fiber showing pulp

2.2 Papermaking process

35

yields of from 65% to 85% (Average ~ 75%) for hardwoods. In the neutral sulfite semi-chemical (NSSC) pulping before refining action (disk refiner), chips are partially treated with chemicals using a buffered sodium sulfite solution to sulfonate mainly the lignin present in middle lamella for its partial dissolution. This makes the fiber weak and assists in fibre separation during mechanical refining. Due to good strength and stiffness, unbleached paper products like corrugating medium, grease-proof papers, and bond papers are prepared using NSSC pulp [9]. Mechanical pulps are weaker but about 50% cheaper than chemical pulps and account for 20% of all virgin fiber materials. The high yield (up to 90–95%) mechanical pulping process consumes extensive electrical energy (about 1300–2900 kWh/ton of pulp) [9]. The requirement of wood quantity (per ton of product) during mechanical pulping is less than the chemical pulping. Most of the electrical energy consumed by the refiner is converted into steam which is later used for the paper drying. During mechanical pulping, the average consumption of total water is about 5–50 m3/t airdried pulp produced. High yielding mechanical pulping produces poor quality paper i.e. low strength properties, low brightness, and high color reversion [7]. 2.2.1.3.2 Chemical pulping Lignin, present in lignocellulosic raw materials, has the glue-like property holding the wood cell wall components together. During chemical pulping, wood chips are cooked under high pressure and high temperature at extreme pH using chemicals to separate the cellulose fibers, for removal or solubilization of the most of lignin and partial hemicellulose present in the wood. Thus, the obtained pulp is rich in cellulose having lower hemicellulose and lignin [6, 10]. Chemical pulping is a low yield process (about 40–55%) producing pulp of high strength properties but it also requires costly waste treatment and chemical recovery systems. 2.2.1.3.2.1 Soda pulping In agro residue-based mills, soda pulping is preferred for getting the pulp from wheat straw, rice straw, bagasse, etc. These agro-residues are cooked using sodium hydroxide (caustic soda) at about 150–160°C for delignification to separate cellulosic fibers present in the raw materials. After washing this pulp is processed for bleaching to make a bleached pulp. The pulping chemicals are recovered from the black liquor to further their reuse in the pulping process (if the mill has a chemical recovery system). 2.2.1.3.2.2 Kraft sulfate pulping Kraft sulfate process is mostly used for the production of pulp due to its versatility. Wood chips are cooked at 165–170°C in addition to sodium hydroxide and sodium sulfide to dissolve major fractions of lignin and wood resins from the wood to separate

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2 Processes and problems of pulp and paper industry: an overview

cellulosic fibers to produce the pulp. The pulp produced has strong and long cellulosic fibers with low lignin content. This pulp is then washed to remove the black liquor. In a separate chemical recovery process, about 90–95% of the pulping chemicals are recovered for its reuse in pulping operation in a closed-loop system [9]. 2.2.1.3.2.3 Sulfite pulping The sulfite pulping, aqueous sulfur dioxide and a base of calcium, sodium, magnesium, or ammonium are used depending on the availability of chemical and energy recovery systems and usage of water. Both magnesium and sodium bases are easily recovered by chemical recovery. The lignosulfonates generated in the cooking liquor can be used as a raw material for producing different chemical products. Sulfite pulps are weaker, brighter and easy to bleach than pulp obtained after kraft pulping [11]. The sulfite pulping process is generally not suitable for treating the softwoods comprising resinous materials, hardwoods comprising tannin, and any raw material with bark material. 2.2.1.4 Retting Fibers can also be extracted from the stems of flax by the retting process. In the retting process, fiber bundles present in the cuticle layer of the epidermis and the woody core cells are separated as smaller fiber bundles and finally as fibers. The components, responsible for the binding of tissues together, are partially degraded for separating the cellulosic fibrous portion from the non-fibrous portion due to microbial pectinase enzyme activity during flax-retting [12]. 2.2.1.5 Bleaching The undesirable residual lignin provides the brown color, which causes a reduction in final brightness. Therefore, bleaching processes are employed to convert the low- brightness brown pulp into the high-brightness white pulp. The final brightness and color are dictated by the product standards. Depending on the raw material, pulping process, and required final target brightness, different bleaching sequences may be applied using various types of bleaching agents such as chlorine (Cl2), chlorine dioxide (ClO2), hydrogen peroxide (H2O2), caustic, oxygen, ozone, hypochlorite, and sodium bi-sulfite alone or in combination (Figure 2.3). ISO brightness levels of pulps can range from about 15% ISO for unbleached kraft to about 93% ISO for fully bleached sulfite pulps. During this process, residual lignin, phenolics, and resin acids are converted into respective chlorinated compounds that are finally transformed into highly toxic xenobiotics. These compounds are considered as carcinogens, mutagens, persistent, bio-accumulative, which finally disturb the aquatic ecosystem [Bajpai 2018, 4].

2.2 Papermaking process

37

Figure 2.3: Functions (F), advantages (A) and disadvantages (D) of different bleaching chemicals (the letter shown in bracket denotes the symbols of oxidant) [Based on 7, 3, and Reeve 1996].

2.2.1.5.1 Chlorine bleaching The chlorine bleaching is generally applied to remove 5–10% of the residual lignin. In this bleaching, other chlorine-containing chemicals (like chlorine dioxide or hypochlorite) are used in sequence for pulp brightening [9]. 2.2.1.5.2 Elemental chlorine-free (ECF) bleaching To avoid the use of elemental chlorine due to its environmental concerns, many large mills are using ECF bleaching technology using oxygen delignification (ODL) sequenced by ClO2 and additional bleaching chemicals to get the higher brightness. A typical ECF sequence generally includes chlorine dioxide, sodium hydroxide, oxygen, and hydrogen peroxide-containing bleaching sequences, such as DEOPDEPD. This is considered as the best available technology, for avoiding the formation of dioxin and other persistent bioaccumulative toxic compounds. Generally, ECF bleaching

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2 Processes and problems of pulp and paper industry: an overview

is preferred by the Kraft pulp mills over total chlorine-free (TCF) bleaching due to the deterioration of pulp quality in the later case because of ozone or peracids to attain high pulp brightness (Bajpai, 2018). 2.2.1.5.3 Total chlorine-free (TCF) bleaching In TCF bleaching, chlorine and chlorine-containing chemicals are not applied as bleaching chemicals. Therefore, ODL is generally used in combination with ozone or hydrogen peroxide as bleaching chemicals to enhance the brightness of the pulp. The bleaching efficiency of the process can be further enhanced by using enzymes (Xylanase, laccase, lignin peroxidase, etc.). Chelating agents (like ethylene diamine tetra acetic acid-EDTA) is also used to prevent the decomposition of hydrogen peroxide by binding with the metal ions present in the pulp. TCF bleaching can be applied to sulfite pulps owing to their good bleachability. 2.2.1.5.4 Hydrogen peroxide brightening Environmentally benign and expensive hydrogen peroxide is applied for the bleaching of pulp having lignin in high amounts (such as mechanical pulps) by altering the chemical structure (chromophoric groups) of lignin by oxidation. The oxidized lignin is not removed but it remains with the pulp. 2.2.1.6 Refining Typical wood fiber is mainly composed of an outer primary layer (P) and secondary layers (S1 and S2) (Figure 2.4). The inner layer S2 is a cellulose-rich layer. During the refining process, the peeling off of the undesired P and S1 layer occurs that modifies the fiber. Thereby, the inner S2 layer is exposed that provides more hydroxyl groups for hydration of fiber resulting in better swelling and higher surface area of fibers than unrefined fibers. Owing to the increase in surface area, more contact among fibers occurs by interfiber bonding such as hydrogen bonding, Vander-Waals interaction, or molecular entanglement, which makes the sheet of a complex fiber network, i.e. paper [13, 14]. During refining, fiber cutting/ shortening, fibrillation, fines development, and their partial solubilization also occur. All these effects influence different properties of the pulp as well as paper [9, 15–17] 2.2.1.7 Papermaking In the papermaking process, the cleaned bleached pulp is formed into wet paper sheets in wet-end operations (Figure 5). These wet sheets are dried and various surface treatments are given to the paper in the dry-end operations. For papermaking, traditional Fourdrinier machines, twin-wire machines, or gap formers and hybrid former machines are being used [18, 19]. There are three main sections in the Fourdrinier papermaking machine (1) the forming section, (2) the press section, and (3) the drier

2.2 Papermaking process

39

Figure 2.4: Representation of transverse section of fiber showing wall layers in a typical wood fiber.

Figure 2.5: Schematic diagram of different processes involved in papermaking.

section. A pulp slurry of about 0.5–1.0% pulp consistency is pumped into a box (headbox). This pulp slurry is directed to flow out through a slot onto a moving wire belt. On the wire, the water portion of the slurry is removed by draining and vacuum suction, leaving the wet and weak paper web containing fibers. This wet paper undergoes different treatments like pressing, heating, and drying. Finally, a continuous roll is ready for further finishing or modifications as desired or required [5, 9]. Different types of chemicals such as filler, sizing agents, coloring agent, wetstrength additives, etc. are added to the washed pulp (pulp stock) before the papermaking section to produce paper with desired specifications. These chemicals are as follows:

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2 Processes and problems of pulp and paper industry: an overview

2.2.1.7.1 Fillers Fillers (clay, calcium carbonate, titanium dioxide, etc.) are generally loading materials used mainly to reduce the cost of the paper. The fillers also used to increase the opacity of the paper, to aid a good finish on calendering, and to improve printing qualities by reducing show-through and strikethrough of the ink. After using the fillers in the paper, weight is increased more in comparison to the bulk of the paper; thus strength properties are adversely affected [9]. 2.2.1.7.2 Sizing chemicals Sizing agents are chemicals used in the process of sizing. The process of sizing can be accomplished in two principal ways: (1) by internal sizing at the wet end of the papermaking operation to control the absorption and penetration of liquid such as water and ink into the paper, paperboard, and sheet material rosin, alkyl- ketene dimer (AKD), alkyl succinic anhydride (ASA), starch, etc. (2) by surface sizing to prevent bleeding and feathering of the ink done on the paper machine by running the paper web through a size vat or a size press or on a stand-alone unit using pigments, starches, animal glue, glycerine, etc [20,21,22,23]. 2.2.1.7.3 Coating The coating of base paper is also done to achieve the desired functional properties of coated paper including resistance to the absorption and penetration of liquids, oils, gases, and chemicals; better printability; improved wear, surface (like smoothness) adhesion properties, etc. Coatings can be applied using aqueous (water-soluble binders like casein, starch, protein, acrylics, and polyvinyl acetates; used for commodity papers), solvent (binders insoluble in water; for specialty papers), high solids (coating of monomer polymerized by UV curing; for chemical, gas, or liquid-resistant specialized paper), or extrusion (molten film of wax or polymer; for chemical, gas, or liquid-resistant specialized paper) [7]. 2.2.1.7.4 Other additives used Biocides/ slimicides for slime control, antifoaming agents to control foaming problem, chemical aids for filler retention, pitch control agents, and wet-strength agents are also used as additives during papermaking [24]. 2.2.1.8 Chemical recovery process The main aim of the chemical recovery process is the recovery of the chemicals from the spent cooking liquor, recovery of heat energy from the burning of recovered lignin & other organic materials from the black liquor, and minimization of air and water pollution. During chemical recovery, dilute black liquor is concentrated in multiple effect evaporators to concentrate the black liquor of about 50% solids. It

2.2 Papermaking process

41

was further concentrated to get “heavy black liquor” of about 65% solids. Sodium sulfate is added s make-up chemical for the loss of soda. Black liquor is incinerated in a recovery furnace to get the smelt. This smelt received from the recovery furnace is dissolved to get green liquor, which is reacted with slaked lime to get white liquor by causticising process. Lime mud is produced as a by-product, which is burnt to recover lime for reuse in the causticizing process [25].

2.2.2 From recycled fibers Paper, the primary product of the industry, has been recycled for many years, but recently recycling has begun to receive greater attention. Deinking is a process for detaching and removing printing inks from the fibers of recovered printed materials to be recycled to improve the optical characteristics of pulp and paper. Deinking involves the dislodgement of ink particles from the fiber surface (ink detachment) and separating dispersed ink from fiber suspension by flotation and/or washing (ink removal) [Pathak et al. 2010, 26, 27] (Figure 2.6).

Figure 2.6: Schematic diagram showing different processes involved in the conventional deinking process.

Unsatisfactory separation results in poor- quality paper having low brightness and dirt specks. Larger ink particles > 50 µm are visible to the naked eye as black or colored spots in the paper. The efficiency of this method also depends on the technique and printing conditions, the kind of ink, and the kind of printing substrate [28]. The conventional deinking process requires large amounts of chemicals such as sodium hydroxide, sodium silicate, hydrogen peroxide, and surfactants [Pathak et al. 2010, 26, 27] (Figure 2.7).

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2 Processes and problems of pulp and paper industry: an overview

Figure 2.7: Chemicals used for conventional chemical deinking and their functions [27].

2.3 Problems faced by the paper industry Besides using well-established technologies, there are always possibilities of the origin of new types of problems in the pulp and paper industry related to raw material, pulp quality, material & resources utilization, paper quality, chemicals recovery and various environmental issues due to diverse variation in raw materials, processes and products, (Figure 2.8). To solve various modern technological practices are being adapted in developed countries (Figure 9).

2.3.1 Raw material Raw material shortage due to their alternate uses in wood or fibrous products creates competition between the paper industry and other industries. The cost of wood as a raw material can be minimized by the efficient use of wood and this requires to improve the efficiency of various processes of the pulp and paper industry. Moreover, the increasing uses of recovered or recycled fibers are also putting the pressure on the growth in virgin pulp volumes. The countries having problem of fiber-deficiency, poor availability of quality raw material at reasonable prices, unfavorable Government policies (no permission for industrial plantations), the low recovery rate of waste papers due to lack of an effective collection mechanism are importing pulp, wastepaper and pulpwood, causing reduced

2.3 Problems faced by the paper industry

43

Figure 2.8: Various problems related to products and processes in the pulp and paper industry.

profitability and capacity addition. The Government’s permission to use degraded forest land for industrial plantation may provide space to face this challenge resulting in green-cover areas in the country. Additionally, improved waste paper collection mechanisms will boost the utilization of waste paper as an alternative fibre source. These positive steps will finally benefit paper traders, farmers, State/ Central Governments and customers/ consumers by increasing the profit/ revenue/ income, easily making and availability of pulpwood to paper mills to reduce dependence on imported fibre or waste papers. Besides recycled fibres, researchers are devoting their time and money to explore alternate raw material to obtain virgin fires from annual-growth grasses and plants such as straw, flax, hemp, and other grasses (https://www.pulpandpaperca nada.com/rejuvenation-challenges-and-opportunities-1000128308/). The pulp from flax has already been prepared on Canada’s west coast. China’s Jincheng Paper Co. is preparing about 200,000-metric-ton-per-year reed pulp mill in Quebec. Samoa Pacific Cellulose LLC of California has run mill trial of bleached reed (Arundo donax) pulp commonly growing in Southern California. The sugar industry is utilizing large quantities of bagasse just as fuel. If these mills are encouraged to use coal-fired boilers so that this valuable fibre can be available to the paper industry as raw material to solve the problem of the paper industry to a great extent.

2 Processes and problems of pulp and paper industry: an overview

Figure 2.9: Modern technological practices in Developed Countries.

44

2.3 Problems faced by the paper industry

45

2.3.2 Yield 100% utilization of the wood or any other raw material is not possible due to concerns with the product quality. The processes producing high process yield are compromising with product quality and low yield processes develop a better quality product with environmental concerns. Therefore, papermakers are trying to develop and adapt the processes or technologies which are expected to give business advantage with environmental benefits [2, 3, 9].

2.3.3 Pitch Problem Pitch is responsible for causing many serious problems in the production process. The pulping process releases these resinous materials from the raw material. Due to their sticky nature, these lipophilic compounds stick to tile and metallic rolls and wires parts of papermaking machines and dryer section, causes stains on the felts and canvas, paper spotting, and paper web breakage on the machine. All these problems affect the production process and product quality. It also contributes toxicity to aquatic life (like fish) due to the persistence and toxic nature of resin acids. These lipophilic wood extractives cause acute toxicity to wastewater from pulp mills [6]. Therefore, the reduction of these compounds can be done by pretreatment of raw material to prevent their liberation in mills lacking secondary treatment in effluent treatment plants.

2.3.4 Pollutants The world’s most common pulping process, bleached Kraft (accounts for more than 50% of global pulp production), it is still dependent on older energy or water-consuming technology and toxic and environmental polluting pulp bleaching process. The use of chlorine-containing compounds in the conventional bleaching process creates environmental problems due to the generation of toxic compounds. Therefore, new bleaching processes are being developed using new less environmentally harmful chemicals requiring cycling of thickening and dilution, energy, and capital. The bleaching process at high concentrations will require high shear forces to chemicals with the pulp suspensions in comparatively small reactors and minimal time delay as observed with the big mixing tanks. Organochlorine compounds are generated during the bleaching process due to the reaction of chlorine with the residual lignin present in the unbleached pulp. In most of the developed and few developing countries, the limits of AOX content in finally discharged wastewater from the pulp mills into the receiving water bodies were decided by the respective Governments. To minimize the AOX content from chemical pulp bleaching operations as per the regulations, there is a need for process modification and

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2 Processes and problems of pulp and paper industry: an overview

investment in major capital items. Such as extended delignification of the brownstock in modified continuous or batch digesters and oxygen delignification are being used to remove maximum lignin from the pulp before chlorination. The maximum lignin removal resulted in lower residual lignin in pulp demanding lower consumption of chlorine to reduce AOX generation. These types of process modification or installation of new technologies are not affordable for smaller units. Due to usage of large amounts of various chemicals, such as sodium hydroxide, sodium carbonate, sodium sulfide, bi-sulfites, elemental chlorine or chlorine dioxide, calcium oxide, hydrochloric acid, etc. during papermaking processes, different types of organic and inorganic salts and toxic pollutants are generated in the effluents in large quantities which comes in the environment directly or indirectly. Major pollutants released from the pulp and paper industry are summarized in Figure 2.10.

Figure 2.10: Major pollutants released from the pulp and paper industry.

2.3.4.1 Organic pollutants and suspended solids The main causing agents for organic pollution in effluents are organic pollutants and suspended solids such as rejected fibers, fiber fines, starch, hemicellulose, and organic acids. This resulted in high BOD/COD concentration resulting in oxygen depletion available to flora and fauna after wastewater discharge. These organic solids are also responsible for toxic compounds like resin, fatty acids, and heavy metals present in the mill wastewater resulting in long-term effects over a wide area due to bioaccumulation and transfer through the food chain. 2.3.4.2 Organochlorine compounds During the bleaching process, different types of carcinogenic organochlorine compounds, like chlorinated derivatives of phenols, dibenzo-p-dioxins/ furans, chlorinated

2.3 Problems faced by the paper industry

47

derivatives of acids, chloroform and carbon tetrachloride and other neutral compounds are produced causing an environmental problem. Chloroform is produced during the hypochlorite stage. Some other micropollutants present in the bleaching effluent also contain chlorinated benzenes, phenols, epoxy stearic acid, and dichloromethane which are also suspected as carcinogens (Bajpai, 2018). 2.3.4.3 Chlorophenolic compounds The toxic chlorophenolic compounds, produced during elemental chlorine bleaching, are persistent and bioaccumulative and they tend to transform themselves into other compounds such as tri-chlorophenol and pentachlorophenol (Bajpai, 2018). 2.3.4.4 Dioxins and furans Dioxins (Polychlorinated dibenzo-dioxins) are also extremely toxic, due to its persistent and carcinogenic nature. Furans (Polychlorinated dibenzofurans) are chemically similar to dioxins but of less magnitude. These environmentally hazardous compounds are present in wastewater treatment sludge (Bajpai, 2018).

2.3.5 Energy The pulp and paper industry is an energy-intensive process industry, in which energy contributes about 18–20% of the manufacturing cost. The high specific energy consumption during the production of mechanical pulp will be a matter of concern in the future. TMP processes will be influenced to great extent due to the requirement of higher energy than groundwood processes. For the development of required pulp properties, beating and refining require substantial energy. The consumption of electrical energy has increased many folds with the pace of development, and now energy is a scarce and costly commodity. Thus, energy conservation has become a necessity in the paper industry, and any treatment of pulp that significantly decreases the energy requirement for refining will have a significant beneficial effect on the overall energy input. Total energy usage is mainly tracked by the total renewable energy per ton of product and total nonrenewable energy per ton of product. The conventionally used bark/ wood waste and lignin obtained from pulping liquor are being utilized as renewable fuel which is more than 50% of the total demand for nonrenewable fossil fuel in most of the integrated or nonintegrated mills. Reduction in water usage, recovery of high-level heat from digesters, improvement in insulation to avoid heat loss and the use of integrated systems to recover low-level heat are some other steps that can also help in energy conservation [9, 29–31].

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2 Processes and problems of pulp and paper industry: an overview

2.3.6 Water The pulp and paper industry is one of the largest consumers of water requiring an average of 50–60 m3 of water for the production of one metric ton of product (i.e. pulp or paper). All the processes of paper manufacturing require a significant amount of water. In recent decades, the paper industry is under pressure to reduce water consumption due to limited resources and the cost of freshwater. Due to environmental issues and regulations, the cost involved in wastewater treatment to reuse is also high due to energy extensive processes. All these factors applied pressure on this industry for reducing its water footprint and increasing water efficiency. The water demand can be reduced by innovations in the process such as bleaching of pulp at high consistency and hot-stock screening.

2.3.7 Wastewater The pulp and paper industry is a very water-intensive industry. The generation of wastewater and the characteristics of pulp and paper mill effluent depend upon the type of manufacturing process adopted. Hence, the treatments of the wastewaters from different mills become complicated because no two paper mills discharge identical effluents due to different combinations of unit processes involved in the manufacturing of pulp and paper. The wastewater of the paper industry contains various types of chemicals such as thiols, sulfur dioxide, sulfite, sulfides, bleaching chemicals i.e. hydrogen peroxide, chlorine dioxide, caustic soda, raw materials debris i.e. fibers and resins, and whitening agents i.e. kaolin, calcium carbonate, talc, and titanium dioxide. The presence of these chemicals creates environmental concern. The industry regularly monitors the emissions of these compounds from the wastewater.to water bodies by tracking the total wastewater flow per unit of production, total suspended solids (TDS), chemical oxygen demand (COD), biochemical oxygen demand (BOD), and color. The amount of different chlorinated organic compounds like dioxins and furans is measured in terms of the weight of absorbable organic halides (AOX) per ton of pulp. Lignin and its derivatives present in pulp and paper industry wastewater are mainly responsible for the high COD and brown color of the wastewater. The wastewater in the pulp and paper industry is usually treated by aerobic biological methods (mainly activated sludge process) which reduce the chemical COD and the BOD up to a certain level but are not efficient in degrading lignin derivatives and other phenolic compounds hence the color removal. Wastewater from pulp and paper mills constitutes a major source of aquatic pollution since it contains high organic substances causing high BOD and COD, extractives (resin acids), chlorinated organics (measured as adsorbable organic halides, AOX), suspended solids, metals, fatty acids, tannins, lignin, and its derivatives, etc. Lignin and its derivatives can form highly toxic and recalcitrant

2.3 Problems faced by the paper industry

49

compounds and are responsible for the high BOD and COD. During biological treatment, using microbial cultures, the biodegradable molecules are decomposed to low molecular weight end products and results in the reduction of BOD and COD. The hard to biodegradable lignin and its derivatives remain in treated wastewater and are responsible for color and organochlorine compounds The effluent may be toxic to aquatic organisms and exhibits strong mutagenic effects and physiological impairment based on the level of pollutants. Varieties of responses were reported in fish populations living downstream of bleached Kraft pulp mills. The treated wastewater from pulp and paper mills is also being used for irrigation but reluctance due to the presence of color [32–35].

2.3.8 Emissions Air emissions are one of the important environmental concerns faced by the pulp and paper industry. The major gases in the emissions are carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), volatile organic compounds, and particulate matters. The odor problem is mainly caused by sulfur emissions by reduced sulfur compounds like hydrogen sulfide (H2S), the mercaptans, including methanethiol (CH3SH, alias methyl mercaptan, or methyl sulfhydrate), methyl sulfide (CH3SCH3, alias dimethylsulfide), dimethyl disulfide (CH3SSCH3) and other compounds. These compounds are formed during kraft pulping (Figure 2.11) due to the reaction of methoxy groups present in lignin with the sulfides via nucleophilic substitution reactions. These reduced sulfur compounds can be smelled at a concentration of a few parts per billion and cause serious health hazards. Sulfur dioxide and sulfur trioxide (to a much smaller extent), are formed by oxidation of sulfur compounds in the recovery boiler, lime kiln, and smelt dissolving tank. Burning of sulfur-containing fuels in power boilers are also responsible for this emission and can be avoided by the use of fuels having low sulfur content. The majority of the oxides of nitrogen originate under the oxidative conditions of the lime kiln, Kraft recovery furnace and any combustion process at high-temperature Non-condensible gases (NCG) consist of TRS and smaller amounts of other volatile organic chemicals such as methanol, terpenes, phenols, and hydrocarbons liberated from the wood. Liberated organic chemicals may directly be pollutants or may form pollutants as they undergo photochemical reactions (formation of smog). Particulate matter (Sodium sulfate, sodium carbonate, sodium chloride, and other salts of sizes about 0.1 to 10 µm) originates primarily in the kraft recovery furnace, the smelt dissolving tank, and the lime kiln [9].

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2 Processes and problems of pulp and paper industry: an overview

Figure 2.11: Different types of solid wastes generation at pulp and paper mills.

2.3.9 Solid waste Solid waste arises at different stages of pulp and paper manufacturing stages. The types and quantities of solid waste differ in every mill based on the consumption of raw material, internal and external control measures taken, house-keeping, collection practices, and recycling/ utilization of generated waste. Various types of solid waste generated at different stages of pulp and papermaking are shown in Figure 2.11. Although the solid wastes generated at various stages of pulp and paper manufacture are unavoidable, however, with better in-plant control measures and good house-keeping and general consciousness at all levels, the quantum of solid waste

2.4 Green chemicals to mitigate the problems

51

generated can be reduced to a great extent. The solid waste generated can be (a) reduced by using preventive measures in some of the cases, (b) reusable and then recycled inside the plant or other systems where they can be utilized as raw material, (c) if find no use, it should be disposed of with care not to damage the environment or cause a public health hazard. The pulp and paper industry is a rising industry all over the world taking steps to increase their production rate. The generation of solid wastes will also be increased in the same proportion due to the increase in the production rate and to increase process efficiencies in the future. Most of the pulp and paper mills already have processes applied to internally treat the wastes which reduce the generation of solid waste. Such as bark residues from debarking and sludge are incinerated in the boiler to generate energy and leaving behind ash as waste. Despite the adaptation of new technologies and methods for solid waste management in the pulp and paper mills, the remaining or unutilized solid waste fractions are sent to landfills. The management of residues from pulp and paper production will progress in the future. The composition of the majority of the solid waste materials from the pulp and paper industry is not harmful. Therefore, these wastes are being used in alternate uses by several other industries permitting their usage can be significantly simplified without harming the environment, such as the primary sludge is sold to the vendors for board making. The use of many solid waste or residues is restricted due to current complex legislations so Legislation should be simplified to support this development. There are always differences in the amount and compositions of waste sludge generated from secondary fiber and virgin fiber-based mills due to the quality of raw materials used. Recycled fiber sludge contains higher rejects due to unrecyclable filler materials, coatings, and short paper fibers in the waste papers like office waste, coated papers, etc. washed out during the fiber-cleaning process. The composition of sludge varies from mill to mill, but it contains about 50% solids and 50% water. Solids generally contain 50% fiber and 50% inorganic material. The amount of sulfur is high in Kraft pulp mill sludge. Great variations occur within both plant types, depending on the processes and raw materials. Paperboard production mills generate lesser sludge than the printing and writing paper mills.

2.4 Green chemicals to mitigate the problems The basics of green chemistry are the designing of the process for the maximum conversion of the raw material in the product using environmentally safe chemicals and designing of energy-efficient processes. Chemical products should not persist in the environment after its end use and should degrade/ break down into harmless degradation products. The innovations in enzyme technology have shown a vast impact on almost every sector of industrial activity that ranges from a technical field to food, feed, and healthcare activity. This is attributed to enzymes’ biodegradable

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nature and cost-effectiveness that the enzymatic processes have rapidly become better financial and ecological alternatives to chemical processes. The pulp and paper industry is also using enzymes as green chemicals due to their application in various pulp and paper processes. Today, the main targets of the pulp and paper industry are to deliver improved product quality cost-effectively using eco-friendly techniques. The interest of papermakers for using target-specific enzymes was triggered shortly after the realization of the natural polymeric nature of paper consisting of cellulose, hemicelluloses, and lignin. Since then, extensive investigations for employing enzymes in various processes of pulp and paper manufacture were initiated [4]. Historically, the first enzyme to be used in pulp and paper manufacture was cellulase for facilitating fiber beating in 1959 by Bolaski and Gallatin [36]. In a study by Demuner et al., focused on technology prospecting in enzymes for their application in the pulp and paper sector, the development of enzymes was concentrated to only a few biotech enterprises with the domination of Novo Nordisk and Novozymes. Novozymes has been consistently increasing the investments in research and development (R&D) and increasing its sales in enzymes for the pulp and paper industry. Among different types of enzymes used for the pulp and paper industry, the most commonly used and important enzymes are cellulase, xylanase, laccase, and lipase (Figure 12a). The most related objectives of enzyme among different applications in pulp and paper processes are bleaching boosting (mostly using xylanases), enhancement of delignification from the pulp (using laccases), fiber modification (using cellulase), and pitch control (using lipase). Besides, great efforts have been made to increase the thermostability of these four enzymes and to turn xylanase and cellulase more alkali tolerant (Figure 2.12) [37].

Figure 2.12: (a) Key enzymes extracted in the patents. (b) Major objectives of development and applications of enzymes, as extracted in the patents on the enzyme for pulp and paper in the Derwent database from 1963 to July 2010. [Adapted from 37].

There may be variation in the methods/ chemicals applied in each papermaking process from wood and recycled fiber. Figure 2.13 shows different processes in

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53

papermaking from wood and the status of enzymes application at the mill, pilot, and laboratory scale.

Figure 2.13: Schematic diagram showing different processes in papermaking and status of enzymes application at the mill, pilot, and laboratory scale [4].

The functions and status of different enzymes used in the pulp and paper industry are summarized in Table 2.1. Table 2.1: Functions and status of different enzymes used in the pulp and paper industry [4]. Enzymes

Substrate Functions

Cellulase

Cellulose

Xylanase

Xylan

Application in the paper industry

Partial hydrolysis of cellulose Refining/fiber modification, vessel picking The release of ink from the Deinking fiber surface Hydrolysis of the colloidal Drainage improvement material in paper mill drainage Degradation of redeposited xylan and lignin carbohydrate complexes

Status/ Scale (Mill/ Pilot/ Lab) Mill Mill Mill

Bleach boosting

Mill

Deinking of newsprint and magazines Production of dissolving grade pulp Biopulping Removal of shives Drainage Refining Debarking

Mill Pilot Pilot Lab Mill Mill Lab

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2 Processes and problems of pulp and paper industry: an overview

Table 2.1 (continued ) Enzymes

Substrate Functions

Application in the paper industry

Status/ Scale (Mill/ Pilot/ Lab)

Bleach boosting

Mill

Biopulping

Pilot

Degradation of lignin in the presence of mediators (transition metal complexes)

Bleaching, effluent treatment

Pilot

MnLignin peroxidase

Degradation of lignin in the presence of additives

Bleaching, effluent treatment

Lab

Lipase

Fat/ oil

The hydrolysis of triglyceride

Pitch control, contaminant control, deinking of oilbased ink

Mill

Amylase

Starch

Hydrolysis α-, or α-, Surface sizing, starchand/ or α-, bonds of starch coating, deinking, drainage improvement, slime control

Mill

Esterase

Macrostickies

Breaks ester bonds

Stickies control

Mill

Protease

Protein

Hydrolysis of cell wall proteins

Biofilm removal

Mill

Pectinase

Pectin

Hydrolysis of cambial layer

Refining Energy saving in debarking, decreased cationic demand

Mill Lab

Mannanase Glucomannan

Laccase

Lignin

Removal of glucomannan

2.5 Conclusions Usually, due to the availability of finance, larger mills can integrate new technologies to overcome various issues such as different raw materials, variation in capacities and quality of various end products. Large mills are shifting towards oxygen delignification, ozone bleaching to adopt elemental chlorine-free bleaching and total chlorine-free bleaching. Mills are also operating the process in a closed-loop to reduce the water consumption and effluent pollution load significantly. The medium and small agro and recycle waste paper-based mills are yet to adopt some of the existing or emerging advanced technologies to achieve the desired efficiency and improved environmental protection. Due to financial limitations, smaller mills are not easily adopting the available modern technologies and still relying on the old technologies or processes resulting in increased pollution load in the effluent. The success of these

References

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developments and innovations in any industry also depends mostly motivated by process efficiency, product quality, and environmental concerns. Due to the utilization of diverse varieties of raw materials and different operational technologies sometimes create obstacles to adapt easily to the various necessary improvements in different operations. The problems faced by the industry can be overcome by using more efficient green chemicals and green processes to increase productivity, product quality, and economics. Worldwide, this paper industry is also distinguished to adopt the innovation and continuously trying to develop new processes using green chemicals such as the use of enzymes for different processes, recycling of waste papers, etc. Environmentally safe green technologies in paper manufacturing will be helpful to improve economic, environmental concerns and social development. Development of technologies for real-time monitoring of hazardous compounds (before/ during/ after formation) in the process and then control of precursor compounds or formed compounds is the need of the future. Ideal waste disposal strategy should also be developed to mitigate the problem of solid waste generation.

References [1]

Ogunwusi AA, Ibrahim HD. Advances in pulp and paper technology and the implication for the paper industry in Nigeria. Ind Eng Lett. 2014;4:3–11. [2] Gerald K. Raw material for pulp. In: Sixta H, editor. Handbook of pulp. KgaA, Germany: WileyVCH Verlag GmbH & Co, 2006:21–68. [3] Gullichsen. Fiber line operations. In: Gullichsen J, Fogelholm CJ, editors. Chemical pulping— papermaking science and technology. Helsinki, Finland: Fapet Oy, 2000:A19. [4] Pathak P, Kaur P, Bhardwaj NK. Chapter-6 Microbial enzymes for pulp and paper industry: prospects and developments. In: Shukla P, editor. Microbial biotechnology-an Interdisciplinary approach. Taylor &Francis Group: Boca Raton, FL 33487-2742, CRC Press, 2016:163–240. [5] Smook GA. Handbook for pulp and paper technologists. 2nd ed. Vancouver: Angus Wilde Publications, 1992. [6] Bajpai P. Brief description of the pulp and papermaking process. In: Biotechnology for pulp and paper processing. Singapore: Springer, 2018:9–26. [7] Bajpai P. Chapter 2 overview of pulp and papermaking processes. In: Environmentally friendly production of pulp and paper. John Wiley & Sons. 2010:8–45. [8] Casey JP. Mechanical and chemi-mechanical pulping: a perspective. Tappi J. 1983;66:95–6. [9] Biermann CJ. Handbook of pulping and papermaking. San Diego: Academic Press, 1996. [10] Eriksson KE, Cavaco-Paulo A. Enzyme applications in fiber processing. In: ACS symposium enzyme applications in fiber processing, San Francisco, California: American Chemical Society, Distributed by Oxford University Press, 1998. [11] Sixta H. Introduction. handbook of pulp. Sixta H, editor. KgaA, Germany: Wiley-VCH Verlag GmbH & Co, 2006;2–19. [12] Sharma HS. Screening of polysaccharide-degrading enzymes for retting flax stem. Int Biodeterior. 1987;23:181–6.

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[13] Baker CF. Advances in the practicalities of refining. scientific and technical advances in refining and mechanical pulping. In: 8th pira international refining conference. Barcelona, Spain: Pira International, 2005. [14] Baker CF. Refining Technology. Baker C, editor. Leatherhead, UK: Pira International, 2000;197. [15] Lumiainen J. Refining of chemical pulp. Papermaking science and technology, papermaking part 1: stock preparation and wet end. Vol. 8. Helsinki, Finland: Fapet Oy, 2000 1:86–122. [16] Sánchez C. Lignocellulosic residues: biodegradation and bioconversion by fungi. Biotechnol Adv. 2009;27:185–94. [17] Singh R, Bhardwaj NK. Enzymatic refining of pulps: an overview. Ippta J. 2010;22:109–15. [18] Atkins J. The forming section: beyond the fourdrinier. Solutions. 2005;88:28–30. [19] Buck RJ. Fourdrinier: principles and practices. In: TAPPI papermakers conference, Atlanta, GA, USA, 2006:24–8. [20] Luukkonen K, Malmstroem O, Zetter C. New and innovative internal sizing strategies for the sizing of PC containing fine paper. In: Papermakers conference TAPPI PRESS. 1995:435–435. [21] Neimo L. Internal sizing of paper. In: Neimo L, editor. Papermaking chemistry. Helsinki, Finland: Tappi Press, Fapet Oy, 2000:150. [22] Roberts JC. A review of advances in internal sizing of paper. the Fundamentals of paper making materials. In: Baker CF editors. Transactions, 11th fundamental research symposium. Vol. 1, Cambridge, 1997:209. [23] Roberts JC. Neutral and alkaline sizing. In: Paper chemistry. London, UK: Chapman & Hall, 1996:140–60. [24] Krogerus B. Papermaking additives. papermaking chemistry: papermaking science and technology book 4. In: Alen R editor, Finnish paper engineers’ association/Paperi ja Puu Oy. 2nd ed. Helsinki, Finland, 2007:255. [25] Vakkilainen EK. Chemical recovery. Papermaking science and technology book 6B. Gullichsen J, Paulapuro H, editors. Helsinki, Finland: Fapet Oy, 2000;7. [26] Pathak P, Bhardwaj NK, Singh AK. Optimization of chemical and enzymatic deinking of photocopier waste paper. BioResour. 2011;6:447–63. [27] Pathak P, Bhardwaj NK. Fungal enzymes application for recycling of waste papers. current advances in fungal biotechnology. Curr Biotechnol. 2018;7:151–67. [28] Bajpai P, Bajpai PK. Deinking with enzymes: a review. Tappi J. 1998;81:111–17. [29] Annergren G, Lundqvist F. Continuous kraft cooking: research and applications. Stockholm, Sweden: STFI Packforsk, 2008:82. [30] Marcoccia B, Prough JR, Engstrom J, Gullichsen J. Chapter 6 continuous cooking applications, papermaking science and technology 6. chemical pulping. In: Gullichsen J, Fogelholm C-J, editors. TAPPI and the finnish paper engineers’ association, book A. Helsinki, Finland: Fapet Oy; [Atlanta, Ga.]: [TAPPI Press], 2000:A512–A570. [31] McDonald S. Advances in kraft pulping. Pap Age. 1997;113:14–16. [32] Ahmad M, Taylor CR, Pink D, Burton K, Eastwood D, Bending GD, et al. Development of novel assays for lignin degradation: comparative analysis of bacterial and fungal lignin degraders. Mol Biosyst. 2010;6:815–21. [33] Bugg TD, Ahmad M, Hardiman EM, Singh R. The emerging role for bacteria in lignin degradation and bio-product formation. Curr Opin Biotechnol. 2011;22:394–400. [34] Masai E, Katayama Y, Fukuda M. Genetic and biochemical investigations on bacterial catabolic pathways for lignin-derived aromatic compounds. Biosci Biotechnol Biochem. 2007;23:0612070214-.

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[35] Ramachandra M, Crawford DL, Hertel G. Characterization of an extracellular lignin peroxidase of the lignocellulolytic actinomycete Streptomyces viridosporus. Appl Environ Microbiol. 1988;54:3057–63. [36] Kirk TK, Jeffries TW Roles for microbial enzymes in pulp and paper processing. In: ACS symposium series. 655. Washington, DC: American Chemical Society, 1996:2–14. [37] Demuner BJ, Pereira Junior N, Antunes A. Technology prospecting on enzymes for the pulp and paper industry. J Technol Manage Innovation. 2011;6:148–58. [38] Pathak P, Bhardwaj NK, Singh AK. Enzymatic deinking of office waste paper: an overview. IPPTA. 2010;22:83–8. [39] Reeve DW. Introduction to the principles and practice of pulp bleaching. Pulp bleaching: principles and practice, Dence CW, Reeve DW, editors. Atlanta: Tappi Press, 1996;1–24.

Amit Kumar, Mukesh Yadav and Workinesh Tiruneh

3 Debarking, pitch removal and retting: Role of microbes and their enzymes Abstract: Microbial enzymes are green and clean alternatives for several processes in the pulp and paper industry. Enzyme treatment decreases the energy requirement and minimizes the wood losses during drum debarking. Lipophilic wood extractives are known as pitch. Pitch deposition adversely affects the pulp quality and increases equipment maintenance and operating costs during paper manufacturing. Several chemical additives have been used to remove pitch deposits. Natural seasoning of wood is used to minimize pitch content in wood, but it has some disadvantages including yield losses and decreased brightness. Controlled seasoning with white-rot fungi or albino strains of sapstain fungi is an effective tool for degradation and removal of wood extractives. Enzymes including lipase, laccase, sterol esterase, and lipooxygenase have also been used to minimize pitch-related problems. Enzymatic retting has been proved an eco-friendly and economical solution for chemical degumming and traditional retting. Keywords: pitch control, natural seasoning, sapstain fungi, white-rot fungi, wood extractives

3.1 Introduction Enzyme technology is gaining global attention due to its specific and focussed performance. It provides environment friendly alternates for chemical processes [1, 2]. The removal of bark is the first step in the processing of wood for papermaking. Mechanical methods are used for debarking of wood logs. The use of enzymes for debarking assists in the mechanical debarking process by reducing energy demand [3, 4]. Lipophilic extractives such as resin acids, fatty acids, triglycerides, steryl esters, and sterols are commonly known as pitch. Pitch deposition during the papermaking process adversely affects the quality of paper [5]. Several chemicals such as alum, talc, ionic and non-ionic dispersants, cationic polymers, and other chemical additives have been used for dispersion and adsorption of pitch deposits during pulping and papermaking processes [6]. Wood logs are stored in woodyard for microbial action in natural conditions that are known as natural seasoning. Prolonged seasoning results loss of colour and pulp yield. Controlled seasoning with short This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, A., Yadav, M., Tiruneh, W. Debarking, pitch removal and retting: Role of microbes and their enzymes Physical Sciences Reviews [Online] 2020, 5. DOI: 10.1515/psr-2019-0048 https://doi.org/10.1515/9783110592412-003

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duration of time and selected fungi has been proved an effective approach for removal of extractive. Several sapstain and white-rot fungi (WRF) have been tested for removal of wood extractives from different softwoods and hardwoods. Bast fibres like flax, hemp, and nettle are sources for the valuable fibre that can be used for different products such as textile and high-quality papers. Separation of these fibres from other stem tissues is facilitated by a process known as retting [7]. Retting is a fermentation process carried out by bacteria (e.g. Clostridium etc.) and fungi (e.g. Aspergillus, Penicillum etc.) [7, 8].

3.2 Debarking The bark is the outermost layer of tree trunks and branches that protects tree from its environment. It refers to all the tissue outside of the vascular cambium. Cambium is the border between wood and bark (Figure 3.1). Bark is an unwanted part of plants that have negative impact on paper quality. Bark consumes chemicals during pulping and bleaching. Moreover, it also results in specks in the finished final product. Therefore, the bark is removed from the wood before the papermaking process. During debarking, the bark is removed together with the cambium layer [9, 10]. Mainly, debarking is carried out by mechanical method in debarking drums. Extensive debarking is required for high quality of chemical and mechanical pulp. Small residual bark can cause darkening of pulp. Extended treatment in debarking drums results in increased energy consumption and loss of raw material [3].

Figure 3.1: Tree stem showing bark, cambium and wood.

3.3 Pitch removal

61

3.2.1 Enzymatic debarking Pectinases, xylanases, cellulases, and proteases are capable of weakening of bonds between wood and bark or hydrolysing polymers available in cambium. Enzymatic debarking results precise detachment of bark at cambium would reduce energy demand and wood or pulp losses. Pectinases have been proved key enzyme for debarking of wood due to high content of pectin. Xylanases also play a role in debarking due to high hemicelluloses content in the phloem of cambium. Infiltration of enzyme molecules in cambium of whole logs is the one of limitation of enzymatic barking [3, 4]. Most of the studies performed so far have used enzyme preparation having polygalacturonase or xylanase as the main activity. However, other enzymes like cellulases still require more research for their role and optimal enzyme mixture suitable for debarking [11]. During wood logs storage moisture content plays an important role for removal of bark. Higher moisture content supports microbial activity that facilitates the bark removal. Excessive water may create anaerobic conditions that inhibits microbial activity and makes the bark removal difficult [12, 13]. Enzymatic treatment can be performed by immerging the log in enzyme treatment solution, or flushing and/or spraying the logs with enzyme treatment solution. Enzymatic treatment facilitates the mechanical debarking and bark is removed easily. As a result, the required energy during mechanical debarking will be reduced. Furthermore, Enzyme pretreatment of logs reduces the wood losses during traditional mechanical debarking [9]. Ratto et al. [3] studied the effect of pectinases and xylanase pretreatment on energy reduction during debarking of spruce wood. The commercial enzyme preparation “Pectinex Ultra SPL” (having polygalacturonase, poly methoxygalacturonase lyase, xylanase and endoglucanase) showed 50% reduction in energy requirement at enzyme dose of 185 nkat/mL (as polygalacturonase activity) of in the soaking solution. The energy reduction was minimized up to 80% at enzyme dose of 900 nkat/mL for 24-h treatment. Partially purified polygalacturonase having minimum xylanase and endoglucanase activity, showed only 13% reduction in energy requirement. Moreover, enzyme preparation having xylanase only reduced the energy requirement up to 18% during debarking of spruce wood [3].

3.3 Pitch removal 3.3.1 Pitch Wood is composed of cellulose, hemicelluloses, lignin, and wood extractives. Wood extractives are a group of heterogeneous compounds that are soluble in organic solvents or water. Lipophilic wood extractives are known as pitch. Some hydrophilic extractives such as lignans are also present in the wood. According to Technical Association of Pulp and Paper Industry (TAPPI), lipophilic wood extractives

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3 Debarking, pitch removal and retting: Role of microbes and their enzymes

include alkanes, fatty alcohols, free fatty acids, triglycerides, resin acids, steryl esters, sterols, conjugated sterols, and waxes. Besides lipophilic wood extractives, some degradation products such as tannins and small lignin fragments have been regarded as components of pitch [14–16]. Pitch contributes 2–8% of wood composition depending upon species and time of the year [17].

3.3.2 Pitch-related problems in pulp and paper processing Pitch deposition is a serious problem during pulp and paper manufacturing. During pulping and refining process, lipophilic extractives are released by parenchyma cell and softwood resin canals that form colloidal pith particles. These colloidal particles merge together to form larger droplets of pith and deposits in pulp and parts of machine forming “pith deposits” [18, 19]. Pitch deposition increases the probability of defects in the final products that adversely affects the quality of the final product. During paper manufacturing, heavy pith deposition reduces the efficiency of the washer, increase the dirt count leading to reduced brightness. Pitch deposition also results in spots and holes in the finished paper, sheet break and technical shutdown. Pitch deposition also increase the consumption of bleaching chemicals. It also increases equipment maintenance and operating costs during paper manufacturing. The economic losses associated with pitch deposition are due to loss of money as a result of contaminated pulp and the cost of pitch control additives [6, 14, 18, 20]. Furthermore, some of the wood extractives may be detrimental for environment when released in the wastewater. It is particularly important for modern environmentally sound bleaching processes such as total chlorine-free bleaching (TCF). During TCF, resin acids, fatty acids and steroids survive during pulping and are the primary source of toxicity while these compounds are destroyed by chlorine compounds such as chlorine dioxide during conventional bleaching methods [18, 20, 21]. Pitch content and pitch-related problems vary according to the plant species. Pines such as loblolly, slash, and red pines contain the pitch that creates serious problems. Hardwood pitch especially tropical hardwood species and eucalyptus can also be detrimental [17].

3.3.3 Methods of pitch removal As discussed in the previous section, pitch deposition creates several problems in pulp and paper processing; therefore their removal is necessary. There are several traditional methods that have been utilized to manage the problem of pitch deposition. Several chemicals such as alum, talc, ionic and non-ionic dispersants, cationic polymers, and other chemical additives have been used for dispersion and adsorption of pitch deposits during pulping and papermaking processes [6]. In addition to

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63

chemical additives some biological approaches such as seasoning of wood have also been used to minimize the pitch-related problems. 3.3.3.1 Natural seasoning The storage of wood logs or chips in woodyard for the natural growth of microorganisms before pulping is known as seasoning of wood. When trees are harvested bark, foliage and sapwood have living cells. Living ray parenchyma cells respire and release heat that provides suitable conditions for bacterial and fungal growth. Starch and simple sugars of ray parenchyma and subsequently the wood extractives are used as nutrients by microorganisms for growth and energy. The wood extractives are decreased by the hydrolytic and oxidative attack by plant enzymes and by the action of wood colonizing microorganisms. Therefore, pitch content is minimized during storage of wood [17, 22]. Prolonged growth of fungi results in decrease in pulp brightness and yield losses due to the uncontrolled action of microorganisms. Wood logs generally require up to 12 months, while the wood chips seasoning needs approximately two months [23, 24]. The reduction of wood extractives is desirable with minimal yield and brightness losses for the pulp and paper industry. Therefore, overlong storage of wood or chips is avoided and controlled seasoning is performed for a relatively short duration using selected fungi [22, 25]. 3.3.3.2 Using sapstain fungi for pitch control Controlled seasoning might be an alternate for natural seasoning that shows adverse effect on colour and yield of pulp. In controlled seasoning, a selected microorganism is inoculated on the wood logs or wood chips to degrade extractives and suppress the growth of other microorganisms. Sapstain fungi have been used effectively for pitch removal from wood logs or wood chips [25]. Sapstain fungi belong to division ascomycetes that colonize wood through parenchyma cells and resin canals that results in discoloration of sapwood tissue due to the presence of melanin-like pigment in fungal hyphae. Sapstain fungi can penetrate deeply in to sapwood, parenchyma cells and resin canals but tracheid cell walls are not degraded. Therefore, mechanical properties of wood are not affected adversely. During fungal colonization several carbon and nitrogen sources available in parenchyma cells, resin canals, and other wood tissues are used as nutrients. Simple carbohydrates, fatty acids and triglycerides are the main carbon sources that are utilized by colonized fungi. Moreover, sapstain fungi can have the ability to colonize non-sterile wood chips rapidly [22, 23, 25–27]. Sapstain fungi include species of Ophiostoma, Ceratocystis, Leptographium or Sphaeropsis. Research on the degradation of wood extractives by fungal treatment is focussed on Ophiostoma piliferum that is a darkcoloured ascomycete [6]. After fungal treatment wood may stain black or blue primarily due to melanin or melanin like compound found fungal hyphae [28]. Research has

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been done on the generation of albino strains of O. piliferum and these albino strains have been commercialized for wood extractives degradation. The albino fungal strains have been obtained by classic mating approaches (followed by single ascospore isolation). The albino strains of O. piliferum have been utilized to avoid the wood staining during fungal colonization [6]. Albino strain of O. piliferum marketed as “CartapipTM” does not result discoloration of wood. CartapipTM was developed by Sandoz Chemicals/Clariant Corporation and it has been used by pulp and paper industry for last two decades [6, 29]. Several sapstain fungi have been reported for the degradation of extractives (Table 3.1). Dorado et al. [23] studied the wood mass losses and found mass losses less than 5% after 8 weeks of treatment. The mass losses after fungal treatment can be due to wood extractives degradation mainly. These results show selective degradation of lignin by sapstain fungi and structural components including cellulose, hemicelluloses and lignin were not degraded. Therefore, pulp yield and strength properties might not be adversely affected by sapstain fungal treatment. Several studies reported that sapstain fungi are effective for both softwood and hardwood sources. Triglycerides were effectively removed by sapstain fungi and triglycerides are associated with pitch problems during softwood pulping. Therefore, sapstain fungi might be effective for pitch reduction in softwood species [23, 29, 30]. 3.3.3.3 Using white-rot fungi for pitch control White-rot fungi have the ability to break down and mineralise lignin that is the most resistant component of lignocellulosic biomass. These fungi can produce two types of extracellular enzyme system when growing on lignocellulosic materials. First extracellular enzyme system is the hydrolytic system that synthesizes cellulases and hemicellulases to degrade holocellulose. Second is oxidative ligninolytic enzyme system that hydrolyzes lignin and opens phenyl rings. The ligninolytic system composed of lignin peroxidase (Lip), Manganese peroxidase and (MnP) and laccase [37, 38]. White-rot fungi degrade lignin with two modes of action that are selective and non-selective decay. In selective degradation lignin and hemicelluloses fractions are hydrolysed while cellulose remains unaffected. In non-selective decay, all components are degraded in approximately equal amount [39–41]. Several white-rot fungi including Phanerochaete chrysosporium, Bjerkandera sp., Funalia trogii, Trametes versicolor, Pleurotus pulmonarius, Ceriporiopsis subvermispora, Phlebiopsis gigantea, etc. have been also reported for degradation of wood lipophilic extractives (Table 3.1). The degradation of wood extractives with white-rot fungi could be beneficial compared to other existing biological methods. In addition to wood extractives degradation, white-rot fungi are capable of selective lignin degradation that is known as biopulping. Biopulping results in significant energy saving during mechanical puling and improves the strength properties of paper [30, 42, 43]. Triglycerides and fatty acids are degraded by a variety of fungi, but resin acids, free and esterified

I:  weeks T:  °C M: % Bjerkandera sp., F. trogii

Sapstrain Ophiostoma ainoae, fungi Ceratocystis allantospora

WRF

P. chrysosporium BKM-F 

WRF

Sapwood and heartwood from Scots pine (Softwood)

Ophiostoma piliferum NRRL , (CartapipTM )Colourless strain, O. piliferum NRRL , (CartapipTM )- Wild type dark blue staining strain

I:  weeks T:  °C M: NA

% Loblolly pine and % Virginia pine wood chips (Softwood)

Fungi

Sapstrain & it’s albino strain

Treatment Type of conditions fungi

Raw material (type of wood)

Table 3.1: Pitch control by fungal treatment.

– Bjerkandera sp. found most efficient for degradation of extractives in both heartwood and sapwood

– Sapstain fungi were poorly effective against highly concentrated extractives mainly composed of resin acids in heartwood

– Sterols and resin acids were extensively degraded by WRF while these components not or poorly degraded by sapstain fungi

– Triglycerides, long chain fatty acids, steryl esters, and waxes in sapwood were almost removed by all fungi

– WRF removed higher amount of sapwood extractives compared to sapstain fungi

– Dicholoromethane extractives were decreased by % compared to control

– Fatty acids, resin acids and other extractives were degraded effectively by both CartapipTM  and CartapipTM  fungal products

– ISO brightness was improved by % by CartapipTM  (albino strain) while it was decreased by % using CartapipTM  (wild type strain) treatment compared to control

– Diethyl ether extractives were decreased by % by CartapipTM  and CartapipTM  compared to control (without any fungal treatment at time zero)

Effect on wood extractives

(continued )

[]

[]

References

3.3 Pitch removal

65

C. conicola, C. deltoideospora, Ophiostoma megalobrunneum, Pestalotiopsis crassiuscula, Phialophora mustea, Alternaria sp. O. piliferum (CartapipTM)

Sapstrain fungi & albino strains

WRF

I:  weeks T:  °C M: %

I: – weeks T:  °C M: %

Scots pine sapwood (Softwood)

(Pinus sylvestris) (Softwood)

Bjerkandera sp. T. versicolor

Fungi

Treatment Type of conditions fungi

Raw material (type of wood)

Table 3.1 (continued )

– –% of sterols were degraded after  week

– –% of free fatty acids were eliminated in  week

– % of triglycerides were degraded in  weeks

– Triglycerides were degraded by both fungi rapidly

– T. versicolor & Bjerkandera sp. removed  and % of total resin content, respectively, after  weeks of treatment

– C. deltoideospora and Ophiostoma piliferum treatment resulted – fold decrease in effluent toxicity

– Sapstain fungi degraded wood extractives selectively and showed their inability to decompose the structural components of wood i.e. cellulose, hemicelluloses and lignin

– Ophiostoma piliferum and Pestalotiopsis crassiuscula were observed most effective strains and degraded  and % of total extractives, respectively

Effect on wood extractives

[]

[]

References

66 3 Debarking, pitch removal and retting: Role of microbes and their enzymes

WRF

WRF

Sapstrain Ophiostoma querci fungi

I: – weeks T:  °C M: %

I:  weeks T:  °C

I:  weeks T:  °C M: %

Norway spruce

Eucalyptus globulus (Hardwood)

Eucalyptus Camaldulensis (Hardwood)

C. subvermispora P. chrysosporium

– Six strains of Ophiostoma querci was found to reduce lipophilic extractives by more than % – Strain C-was capable to reduce about % of all lipophilic extractives

– Acute toxicity in black liquors was reduced significantly

– Fungal pretreatment decreased free and esterified sitosterol up to % in pulps and liquors

– Trametes versicolor treatment contributed to a less toxic effluent and improved biodegradiblity

– The pretreatment resulted slightly poorer optical properties

– -week treatment removed resin acids and triglycerides by % and %, respectively

– For Loblolly pine, -% of resin was removed by C. subvermispora and O. piliferum after  weeks of treatment

– –% of resin was removed by all fungi after  weeks treatment

I = Incubation or treatment time, T = Temperature, M = Moisture content or air relative humidity

Bjerkandera adustra, Pleurotus pulmonarius C. subvermispora

Bjerkandera sp. BOS T. versicolor

Sapstrain O. piliferum fungi

WRF

– Degraded steryl esters and waxes effectively

Aspergillus luchuensis Gliocladium sp.

I: – weeks T:  °C M: %

– Degraded steryl esters and waxes effectively

P. chrysosporium

– Steryl esters and waxes were not removed by Cartapip TM efficiently

WRF

– Cartapip TM decreased steryl esters/waxes by  to % while –% of aspen triglycerides and -% lodgepole pine triglycerides

Cartapip TM (O. piliferum)

Albino strain

Loblolly pine & Spruce (Softwood)

Aspen & lodgepole I:  weeks pine T:  °C (Softwood)

[]

[]

[]

[]

[]

3.3 Pitch removal

67

68

3 Debarking, pitch removal and retting: Role of microbes and their enzymes

sterols and waxes are difficult to degrade by microbial action. Several sapstain fungi are found to remove the main components of softwood extractives but some white-rot fungi gave better results in terms of resin acid degradation compared to sapstain fungi [18, 30]. Martinez et al. [44] carried out extensive fungal screening for removal of Eucalyptus globulus extractives, a total of 33 basidiomycetes, 21 ascomycetes, and 19 conidial fungi were tested for wood extractives. White-rot fungi including Poria subvermispora, Phebia radiate, and B. adusta removed 75%–100% of free and esterified sitosterol [44]. 3.3.3.4 Using enzymes for pitch removal The enzymatic pitch control technology was first developed by Jujo Paper Co. (later Nippon Paper) in conjugation with the Japanese office of Novozymes. During paper manufacturing, enzymes including lipase, laccase, sterol esterase, and lipooxygenase have been used for minimizing the pitch-related problems [45]. Lipases are triacylglycerol hydrolases (E.C. 3.1.1.3) that have the ability to hydrolyse fatty acid ester bonds in aqueous medium and synthesize them in non-aqueous conditions [46, 47]. Several microbial lipases have been used to remove pitch during paper manufacturing (Table 3.2). Lipases minimize the pitch-related problems by hydrolysing the triglycerides component of pitch [54, 55] (Figure 3.2). A commercial lipase (Resinase®) developed by Novozymes has been successfully used for treatment of red pine mechanical pulps for several years in Japan. However, resinase has not been found any impact on steryl esters that also play a significant role in pitch deposition [56–58]. Steryl esterases (EC 3.1.1.13) are a subclass of esterases that mainly catalyse the hydrolysis of steryl esters. Some steryl esterases also have ability to hydrolyze or synthesize other substrates having ester linkages such as triacylglycerols [59]. Kontkanen et al. [56] studied the hydrolysis of steryl esters and triacylglycerols in mixed wood extractives by steryl esterase and reported 58%–80% of triacylglycerols were hydrolysed in absence of Polidocanol while in the presence of Polidocanol triacylglycerols were removed almost completely. With the addition of 1% of Polidocanol with steryl esterase almost 100% of steryl esters were degraded while steryl esters were not degraded without the addition of Polidocanol [56]. The use of laccases alone and in combination with redox mediators has been reported. Laccase mediator system acts selectively on lignin but its role has been described in extractives removal also [60, 61]. Molina et al. [62] treated different model lipids with Pycnoporus cinnabarinus laccase in the presence of mediator, 1-hydroxybenzotriazoles and analysed by gas chromatography. After 2 h laccase-mediator system treatment, 60-100% of oleic acid, linoleic acid, abietic acid, sitosterol, cholesteryl palmitate, oleate, linoleate and trilinolein were decreased while in the absence of mediator 20-40% of these unsaturated lipids were decreased.

Lipase

Laccase

Softwood thermomechanical pulp

Lipoxygenase

T. versicolor

Laccase

Eucalyptus globulus kraft pulp

Black spruce & Jack pine thermomechanical pulp

Streptomyces lividans

Lipase

Mixed softwood & hardwood pulp

Sigma-Aldrich (Oakville, ON)

Pycnoporus cinnabarinus

Novo Nordisk A/S

Sterol esterase Ophiostoma piceae

Eucalyptus globulus (Hardwood)

Source of enzyme

Enzyme

Raw material

Table 3.2: Enzymatic methods of pitch control.

[]

– Laccase treatment was applied during TCF bleaching

– Lipoxygenase reduced lipophilic extractives by more than % after  h of pulp treatment

– Lipoxygenase treatment of TMP reduced % of total extractives – Lipoxygenase showed specificity towards fatty acids and their esters

– Process water treatment with laccase resulted in  and % removal of fatty and resin acids, respectively

– From process water, the combination of surfactant and lipase reduced resin acids, sterols, and triglycerides by ,  and %, –respectively

– Pulp brightness was also improved due to simultaneous removal of lignin

– Only some intermediate products from sitosterol oxidation remained after laccase treatment

(continued )

[]

[]

[]

– % of triglycerides were hydrolysed within a short time duration

– Laccase mediator treatment completely removed main lipophilic compounds (free sitosterol, sitosterol esters, and glucosides)

[]

References

– Sterol esters were decreased by %

Effect on wood extractives

3.3 Pitch removal

69

Enzyme

Laccase

Laccase

Chitosanimmobilized lipase

Raw material

Fatty & resin acids

Eucalyptus globulus kraft pulp

Whitewater from peroxide-bleached softwood

Table 3.2 (continued )

–

Myceliophthora thermophila

T. hirsute,T. villosa

Source of enzyme

– .% of pitch particles in whitewater were reduced by immobilized lipase treatment

– Immobilized lipase showed good operational stability

– Laccase treatment with mediator (syringaldehyde) decreased free sterols, sterol glycosides and sterol esters by ,  and %, –respectively

– Laccase treatment alone was able to remove free sterols and sterol esters by  and %, respectively

– Brightness was improved by . and . points by laccase treatment with syringaldehyde and methyl syringate as mediator, respectively

– After laccase treatment (without mediator), brightness was improved by . points

– T. hirsute & T. villosa removed  and % of conjugated resin acids, respectively

– T. hirsute laccase reduced linoleic and pinolenic acid by  & %, respectively

– Both laccases were able to modify the fatty and resin acids to some extent

Effect on wood extractives

[]

[]

[]

References

70 3 Debarking, pitch removal and retting: Role of microbes and their enzymes

3.4 Retting

71

Figure 3.2: Action of lipase on triglycerides (a component of pitch).

3.3.3.5 Using bacteria for wood extractives removal Wood extractives removal by fungal and enzymatic treatment has been studied extensively. There are very few reports on the extractive removal by bacterial treatment. It is well established that fungi are more effective for lignin degradation, although some modification of lignin and lignin model compounds by bacteria has been reported [15, 63]. Bacteria are also found to decrease the amount of wood extractives in pulp & paper effluents, groundwood pulp and wood chips [15, 64, 65]. Kallioinen et al. [15] isolated bacterial strains from spruce wood and tested their ability for extractives degradation from Norway spruce wood chips. The most efficient strain was found Pseudomonas marginalis E-991,379 that reduced 67% of total lipophilic extractives after 2 weeks of treatment as compared with reference treatment. Triglycerides, steryl esters and resin acids were reduced by 90, 66 and 50% with the same strain [15].

3.4 Retting Retting is the extraction of fibre from non-fibrous tissue. It is the biochemical process that breaks non-fibrous polymeric compounds such as pectin, hemicelluloses, and other mucilaginous compounds available in epidermis and cuticle. These compounds hold adjacent fibre bundles together therefore, the removal of these non-fibrous substances expose the fibre bundles. There are four types of traditional retting processes that are dew or field, pool, stream and water retting [1, 66, 67]. Water retting produces the high quality of fibres compared to other traditional methods. However, there are some environmental disadvantages of water retting. It produces strong odour and releases environmentally unacceptable fermentation waste. Furthermore, it requires

72

3 Debarking, pitch removal and retting: Role of microbes and their enzymes

high labour cost that makes it less attractive. Therefore, alternative methods for traditional retting are required and focus has been on enzymatic retting [1, 66].

3.4.1 Enzymatic retting Fibres having non-cellulosic gummy material consist of pectin and hemicelluloses. The plant gum present on fibres must be removed for their utilization in the textile industry or papermaking. Chemical method of degumming utilizes hot alkaline solutions that are pollution intensive process and requires high consumption of energy. Therefore, enzymatic retting is eco-friendly and economical solution for chemical degumming and traditional retting. The combination of pectinases and xylanases can serve as an effective tool for removal of pectin and hemicelluloses [68–70]. Pectinases efficiently assist in degumming and retting of jute [71], flax [72], sunn hemp [73], ramie [70], kenaf [1] fibres by degrading pectin located in the middle lamella and primary cell wall of plants [69]. Enzymatic processing of fibre is more controllable and requires shorter treatment time. Enzymatic retting also minimizes the generation of effluents. Enzymatic retting produces high quality of fibres with consistent quality and varying freeness [1, 74, 75]. Moreover, Kapoor et al. [69] performed the chemical and enzymatic degumming of ramie and sunnhemp and concluded that neither enzymatic nor chemical treatment is sufficient for degumming of these bast fibres. A mild chemical treatment before enzymatic treatment achieved better results. Sharma and Satyanarayana [70] studied enzymatic retting of ramie fibres with thermoalkali-stable pectinases from Bacillus pumilus dcsr1 and reported improved tensile strength and Young’s Modulus of the fibres after combined treatment with sodium hydroxide (0.04%) followed by pectinases (300 U/g dry fibre). Indian handmade paper industry has shown significant growth in last decade due to improved demand at the national and international arena. Traditional raw materials for production handmade paper include hosiery and cotton rags and do not require any chemical during pulping maintaining eco-friendly status. The demand and prices of these raw materials are continuously increasing. Therefore, the sustainability of the handmade paper industry needs alternate fibre sources that can be available continuously. The use of alternate lignocellulosic material such as bast or leaf fibre may provide superior quality of pulp. Several fibre sources such as jute, flax, sunn hemp, ramie, kenaf, ankra, and banana stem, etc. have been tested for fibre extraction. Chemical methods or traditional retting are not environmental friendly process for fibre production from these lignocellulosic materials. Enzymatic retting, bio-refining, biopulping have potential to makes the handmade paper production ecologically compatible [66].

3.6 Conclusion

73

3.5 Limitations and future prospective Pitch deposition is a serious problem during pulp and paper manufacturing. Several biological approaches have been used to overcome this problem in pulp and paper industry. Natural seasoning is traditional biological approach to minimize the pitch deposition. But, natural seasoning have certain limitations. Generally, a prolonged storage of wood logs in woodyard is required for natural growth of microorganisms. This overlong growth and uncontrolled action of microorganisms decreases the pulp yield and brightness. Controlled seasoning with selected microorganism might be an alternate for natural seasoning. Sapstain fungi found effective for pitch removal from wood logs or wood chips but they may stain the wood with black or blue colour due to synthesis of melanin or melanin like compounds in hyphae. Although albino strains of a few sapstain fungi have been developed for pitch removal purposes. Further research on generation of albino strains of different sapstain fungi and their use for pitch removal may provide the more biological options for pitch removal. Th use of white-rot fungi for pitch removal provides double benefits in terms of wood extractives degradation as well as selective lignin degradation (biopulping). Several white-rot fungi have been successfully tested for pitch removal and they have more potential to be explored in future. The role microbial enzymes in debarking has not been studied extensively. Few investigations are available on the use of pectinases and xylanases. However, role of other enzymes like cellulases for debarking still needs more research. Infiltration of enzyme molecules in cambium of whole logs is the one of main limitation of enzymatic barking. Further research is required for improvement of infiltration of enzyme molecules in cambium. Enzyme may be applied after preliminary incomplete mechanical debarking and after enzyme treatment debarking may continue until complete debarking.

3.6 Conclusion Microorganisms and their enzymes play significant role debarking, pitch removal and retting. Enzymatic treatment of wood logs facilitates the mechanical debarking process and bark is removed from wood easily. Pitch deposition problems have been minimized by several chemical additives and natural seasoning. Controlled seasoning with selected microorganism and controlled microbial activity has shown several advantages over natural seasoning. Microbial lipases by hydrolysing the triglycerides component of pitch have also been found to control pitch-related problems. Steryl esterases and laccase have also been tested for pitch removal. Enzymatic retting is an environmentally approach for extraction of fibre from non-fibrous tissue. It minimizes the generation of effluents and produces high quality of fibres with consistent quality.

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Bionotes Dr. Amit Kumar is currently working as Assistant Professor at Department of Biotechnology, College of Natural and Computational Sciences, Debre Markos University (Ethiopia). He completed his Doctorate in Biotechnology from Indian Institute of Technology, Roorkee (India). He is extensively involved in research on industrial enzymes, pulp & paper biotechnology, biofuels production, and environmental Biotechnology. He has published several research and review articles in various reputed international journals. Dr. Mukesh Yadav is working as Assistant Professor at Department of Biotechnology, Maharishi Markandeshwar (Deemed to be University), Mullana-Ambala, India. He is working in area of microbial enzymes, microbial biotechnology and microbial metabolites of industrial importance. He has published research and review articles in various reputed national and international journals. He is life member of several professional scientific societies and contributing to the development of society. Mrs. WorkineshTiruneh is a Lecturer in the Department of Animal Science, College of Agriculture and Natural Resources, Debre Markos University, Ethiopia. She had studied her MSc in Animal Production from Hawassa University (Ethiopia) and her BSc in Animal Science from Jimma University, College of Agriculture and Veterinary Medicine (Ethiopia). Her research interest areas are poultry nutrition and breeding, dairy cattle nutrition integrated agriculture and microbial enzymes.

Amit Kumar, Archana Gautam and Dharm Dutt

4 Bio-pulping: An energy saving and environmentfriendly approach Abstract: Pretreatment of wood or other raw material with white-rot fungi (WRF) prior to pulping is known as biopulping. Lignin and hemicelluloses are removed selectively during early growth of WRF that produces enriched cellulose, known as selective delignification. Biopulping is considered as environment-friendly and cost-effective approach for delignification of lignocellulosic raw materials. The delignification efficiency of WRF during biopulping is directly related to ligninolytic enzymes production that is influence by several factors such as fungal strain, nature of raw material, oxygen availability, moisture content, pH, temperature, source of nitrogen, presence of Mn++ and Cu++ ions. The WRF, especially Ceriporiopsis subvermispora, Trametes versicolor and Phanerochaete chrysosporium, have been used dominantly for the purpose of biopulping. It is an energy saving process that also improves brightness of pulp and strength properties including tensile index, burst index and folding endurance of paper. Significant decrease in kappa number has also been attained by fungal pretreatment of raw materials. Biological pretreatment of raw material also reduces the requirement of pulping chemicals. Keywords: biopulping, white-rot fungi, refining energy, pulp brightness, strength properties

4.1 Introduction Pulping is the process of formation of fibrous raw material for papermaking that is carried out by breaking of bonds in woody raw material or other raw materials such as non-wood, grasses and agro-residues. There are three methods of pulping that are mechanical, chemical, and combination of chemical and mechanical pulping. Lignin is an undesired component of wood that is supposed to remove as much as possible during the pulping. In chemical pulping, raw materials are cooked with appropriate chemicals in aqueous solution at elevated temperature and pressure to solubilize the lignin. Cellulosic fibrous material is obtained after removal of lignin from lignocellulosic raw material. Three types of chemical pulping methods, including soda, kraft and sulfite pulping are used for solubilization of lignin. Sodium hydroxide, sodium hydroxide with sodium sulfide and sulfurous acid are used to dissolve the This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, A., Gautam, A., Dutt, D. Bio-pulping: An energy saving and environment-friendly approach Physical Sciences Reviews [Online] 2020, 5. DOI: 10.1515/psr-2019-0043 https://doi.org/10.1515/9783110592412-004

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4 Bio-pulping: An energy saving and environment-friendly approach

lignin during soda, kraft and sulfite-pulping processes, respectively [1, 2]. The objective of pulping is to eliminate lignin and avoid the cellulose decomposition. Chemical pulping is the main step for generation of pollutants in the entire process of papermaking. Biopulping can be the potential alternative for traditional methods of pulping. Biopulping is considered as environment-friendly and cost-effective approach for delignification of lignocellulosic raw materials. Pretreatment of lignocellulosic raw material with white-rot fungi (WRF) prior to pulping is known as biopulping. During pretreatment of raw material, fungus is grown on raw material with additive nutrients. When fungus grows on the raw material, it produces lignin degrading enzymes such as lignin peroxidase, laccase and manganese peroxidase [1, 3, 4]. Lignin and hemicelluloses are removed by the combined action of hemicellulases and lignin-degrading enzymes that is the prerequisite of pulping process.

4.2 WRF for biopulping Filamentous fungi are the effective degraders of wood. Based on the mode of degradation of biomass they are classified in to three groups, including WRF, brown-rot fungi and soft-rot fungi. WRF can synthesize cellulases, hemicellulases and lignindegrading enzymes therefore, they are able to degrade all components of wood i. e. cellulose, hemicelluloses and lignin, respectively [5]. However, different WRF may produce these enzymes in different proportions. Therefore, WRF that is able to produce large amount of lignin-degrading enzymes with negligible amount of cellulase are preferable for biopulping. The degradation of wood by WRF leaves the white and fibrous wood due to bleaching effect by oxidation and loss of lignin. WRF decompose lignin of middle lamella sufficiently that results separation of intact cells into fibers. Lignin and hemicelluloses are removed selectively during early growth of fungi. This produces enriched cellulose which is known as selective delignification. WRF produces extracellular oxidases, including manganese peroxidases (MnP), lignin peroxidases (LiP) and laccases that are involved degradation of lignin in wood [6–8]. Lignin peroxidase is involved in the degradation of lignin. It can oxidize both phenolic and non-phenolic compounds. Manganese peroxidase has lower redox potential and it does not oxidize non-phenolic lignin compounds. Laccases are the multicopper oxidase enzymes that show the ability to catalyze oxidation of various phenolic and non-phenolic compounds in the lignin [9–11]. Lignin degradation by WRF is resulted by two types of mode of action, including selective and non-selective decays. Selective lignin degradation by WRF depends on lignocellulose species, cultivation time and other factors [6, 12]. Lignin and hemicelluloses fractions are removed while cellulose fraction remains unaffected during selective delignification. In non-selective delignification, approximately equal amounts of fractions of all components of lignocellulosic materials are degraded [6, 13]. Several

4.2 WRF for biopulping

81

WRF, including Ceriporiopsis subvermispora, Corioulus versicolor, Ganoderma colossum, Phanerochates chrysosporium, Phlebiopsis gigantean, Physisporinus rivulosus, Trametes versicolor, Schizophyllum commune, Pleurotus ostreatus have been tested for biopulping [1, 4, 14–20]. Biopulping of several raw materials by different WRF have been shown in Table 4.1. Table 4.1: Biopulping of different raw materials. Fungus involved

Raw material

Duration of Reference the treatment

Phlebiopsis gigantean Ceriporiopsis subvermispora Trametes versicolor Ceriporiopsis subvermispora Schizophyllum commune ARC Ceriporiopsis subvermispora Ceriporiopsis subvermispora Ceriporiopsis subvermispora Ganoderma colossum Phanerochaete chrysosporium KCCM  Ceriporiopsis subvermispora Trametes versicolor Ceriporiopsis subvermispora SS- Phellinus sp., Deadalea sp., Trametes versicolor, Pycnoporus coccineus Phanerochaete chrysosporium KCCM  Pleurotus ostreatus & Trametes versicolor Ceriporiopsis subvermispora & Lentinula edodes Penicillum oxalicum (manganese peroxidase) Pleurotus ostreatus Phanerochaete chrysosporium

Pine logs Jute fiber Eucalyptus tereticornis Sugarcane straw Eulaliopsis binata Loblolly pine chips Brutia pine chips Mixed hardwood chips Silver leaf oak Pinus densiflora & Populus alba x glandulosa wood blocks

 weeks  weeks – weeks  days  days  weeks  weeks  weeks  weeks  days

[] [] [] [] [] [] [] [] [] []

Eucalyptus grandis wood chips Acacia mangium wood chips

– days  days

[] []

Hybrid poplar chips

 days

[]

Betung bamboo

 days

[]

Wheat straw & oak wood chips

 weeks

[]

Sugarcane bagasse

–

[]

Pineapple leaf fiber Red oak wood chips

– weeks  days

[] []

For commercial utilization of fungus in biopulping, a suitable fungus should have following characteristics: (1) It should have relatively fast growth rate. (2) It should not produce cellulases that will adversely affect the cellulosic fibers. (3) It should have the ability to grow on wide range of raw materials including softwoods, hardwoods, agro-residues and grasses.

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4 Bio-pulping: An energy saving and environment-friendly approach

(4) The growth of molds on substrate piles may results health problems in workers, therefore it should be safe (GRAS status). (5) Some fungi can produce the pigments that can adversely affect the brightness of pulp; therefore the fungus should not produce any pigment Spore-producing fungi are preferable due to easy and better inoculation of wood chips [3].

4.3 Biopulping process Size reduction is the first step of biopulping that involves the making of chips from the wood or chopping of grasses and agro-residues. These raw materials are contaminated with microbial cells and spores that may also grow biopulping fungus. Their growth would affect the growth of biopulping fungus. These undesirable microorganisms may produce cellulases that will be detrimental for cellulosic fibers. Steam sterilization is a preferable method of decontamination. In second step, desirable fungus is inoculated on the raw substrate and incubated at suitable temperature depending on nature of fungus (Figure 4.1). The duration of treatment may be 1–12 weeks depending upon the nature of substrate, type of fungus and size of the substrate (Table 4.1) [1, 31].

4.4 Factors affecting biopulping The efficiency of WRF during biopulping is directly related to production of ligninolytic enzymes that is influence by several factors such as fungal strain, nature of raw material, oxygen availability, moisture content, pH, temperature, source of nitrogen, presence of Mn++ and Cu++ ions. Some of species of WRF produce all ligninolytic enzymes and others can partially produce ligninolytic enzymes. The fungal strains producing all ligninolytic enzymes might be more efficient for biopulping. During biopulping, selective degradation of lignin also depends upon the nature of substrate [6]. Hatakka [32] showed selective degradation of lignin from wheat straw by Pleurotus sp. while the same fungus was able to cause delignification of hardwood birch and softwood pine. Nitrogen concentration and type of nitrogen source affects the production of ligninolytic enzymes by WRF as well as biopulping. However, the effect of nitrogen sources may vary among the species and strains of WRF [6]. The effect of inorganic nutrients including Mn++ and Cu++ on ligninolytic enzymes production has also been studied. The presence or absence of Mn++ regulates the production of MnP and LiP by several WRF. Production of extracellular LiP predominates without addition of Mn++, whereas the production of MnP

4.4 Factors affecting biopulping

83

Figure 4.1: An overview of biopulping process.

dominates with the addition of Mn++. This regulatory effect of Mn++ has been found several WRF such as Phanerochaete chrysosporium, Phlebia sp., Lentinula edodes and Phellinus pini [6, 33]. Cu++ has been found effective inducer of laccase production. Several reports have indicated that supplementation of Cu++ improve the production of laccase by WRF while few reports also describes the negative effect on the production of laccase. Improvement in laccase production by the addition of Cu++ has been shown by WRF including Pleurotus pulmonarius, P. ostreatus [34–36]. Different cultural parameters such as cultivation temperature, moisture content, pH and oxygen availability also influence the ligninolytic enzymes, synthesized by WRF. Temperature is a fungal-dependent parameter and most of WRF are mesophilic in nature. Several WRF such as Ceriporiopsis subvermispora, Phanerochates chrysosporium, Phlebiopsis gigantea, Phlebia tremellosa, P. brevispora, Pleurotus ostreatus, Schizophyllum commune, Trametes versicolor have shown better delignification ability at temperature ranging 20–30 °C [4, 14–16, 20, 22, 26, 37, 38]. Moisture content during growth of fungi is also a critical parameter that affects the enzyme production as well as delignification of lignocellulosic biomass. Delignification is an oxidative process that is improved by the replacement of air with oxygen during the growth of WRF on straw and wood. The increase in oxygen concentration in atmosphere not only

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4 Bio-pulping: An energy saving and environment-friendly approach

stimulates the degradation of lignin but also the non-lignin components of the substrate [38, 39].

4.5 Reduction in pulping chemicals Biopulping is an environment-friendly approach due to utilization of reduced quantity of pulping chemicals. Biological pretreatment of raw material reduces the requirement of pulping chemicals. During fungal treatment, aryl-ether linkages in the lignin are cleaved and the reduction of aryl-ether linkages indicates extensive lignin depolymerization. This might be one possible reason for the reduction in active alkali requirement to produce chemical pulp with a target kappa number [40, 41]. Another possible reason for reduction of active alkali might be the removal of extractives during fungal treatment. When fungus grows on raw material, it utilizes the extractives as nutrient and removes substantial amounts of wood extractives. During pulping some amount of pulping chemicals is consumed by extractives, resins and acetyl groups in the initial phase of chemical pulping. Resin canals become unobstructed due to removal of extractives that facilitates liquor penetration and reducing active alkali consumption by non-lignin components [40, 42, 43]. Yadav et al. [5] studied the effect of biopulping mixed hardwood chips on effluent load and reported significant reduction in COD (chemical oxygen demand) and BOD (biological oxygen demand) values of final effluent. The reduction in AOX (adsorbable organic halides) contents and color was also observed.

4.6 Effect of biopulping on kappa number, lignin content and brightness Most of WRF have ability to disintegrate and mineralize lignin. The colonization of WRF on substrate results lignin degradation and leaves intact cellulosic fibers. Biopulping decreases the pulping time, kappa number and bleaching chemical consumption of pulp due to degradation of lignin [44]. Biopulping with WRF also results weight losses of substrates due to significant degradation of lignin and hemicelluloses. Lignin content losses can be explained by degradation of β-O-aryl ether linkages of lignin by fungus [42]. A weight loss of 3.91 to 9.25 % for Acacia mangium wood chips was observed by biopulping by Phellinus sp. [25]. During early stage of decay, higher lignin degradation has been reported by researchers. Biological treatment of agro-residue such as rice, wheat and barley straw with C. subvermispora decreased kappa number by 34 %, 21 % and 19 %, respectively, as compared to control [45]. Levin et al. (2007) studied biopulping of loblolly pine (Pinus taeda) chips with Pycnoporus sanguineus and reported 11 % reduction in lignin content after 14 days of treatment [46]. Lignin content reduction during biopulping by different WRF has been shown in Table 4.2.

4.7 Effect of biopulping on pulp yield and viscosity

85

Table 4.2: Lignin losses during biopulping. White-rot fungus

Raw material

Lignin losses (%)

T. versicolor P. chrysosporium C. subvermispora P. chrysosporium C. subvermispora P. sanguineus T. versicolor C. subvermispora Marasmius sp.

Acacia mangium wood chips Pine wood Pine wood Poplar wood Poplar wood Loblolly pine chips Oil palm trunk Eucalyptus nitens Oil palm empty fruit bunches

. % after  days of treatment . % after  days of treatment . % after  days of treatment . % after  days of treatment . % after  days of treatment  % after  days of treatment . % after  days of treatment . % after  days of treatment . % after  days of treatment

References [] [] [] [] [] [] [] [] []

Positive and negative effects of biopulping on pulp brightness have been reported by different researchers. The degradation of lignin during fungal treatment might be the possible reason for improved brightness while small pigmentation by some fungi fungal strains results decreased pulp brightness. Several researchers found increased pulp brightness after fungal treatment of pulp [4, 23] while another group of researchers have reported decreased pulp brightness [42, 49, 50]. Gautam et al. [4] studied the pretreatment of Eulaliopsis binata by S. commune and reported an improvement of 4.1 % (ISO) compared to soda pulping. Copur and Tozluoglu [23] performed bio-kraft pulping of brutia pine chips by C. subvermispora and found 25.6 % higher brightness compared to kraft pulp. Jong et al. [49] studied the ability of several white-rot and brown rot-fungi to decolorize the mechanical pulp of Douglas fir. Pulp treated with Phanerochaete chrysosporium showed slight improvement in brightness (ISO) while C. subvermispora, D. squalens, H. fasciculare, T.versicolar and G. trabeum showed significant decrease in brightness of bleached and unbleached pulp after 7 days fungal treatment [49].

4.7 Effect of biopulping on pulp yield and viscosity Several investigators have showed deceased pulp yield while some researchers have reported improved pulp yield after fungal treatment of raw materials. Similarly, declined or improved pulp viscosity has also been reported by different group of researchers for different raw material with different fungal strains. Improvements in pulp yield and viscosity have been observed after fungal pretreatment of raw materials. Mardones et al. [40] performed fungal pretreatment of Eucalyptus nitens wood chips with C. subvermispora and studied the effect of fungal pretreatment on pulp viscosity and pulp yield. Pulp viscosity and pulp yield for E. nitens pulp were improved by 1.5 by 55.26 %, respectively, at kappa number of 16. The improvements in pulp yield and viscosity might be due to the utilization of reduced amount of active

86

4 Bio-pulping: An energy saving and environment-friendly approach

alkali to reach a certain kappa number. The high active alkali concentrations may induce the peeling reactions and polysaccharide degradation during kraft pulping [40, 51]. Costa et al [52] performed delignification of C. subvermispora treated sugarcane bagasse by soda anthraquinone pulping and reported 2 % decrease in pulp yield with slight improvement in viscosity after 120 min of cooking time. The relation of kappa number with viscosity should also be considered during viscosity analysis. Costa et al. [52] found improvement in viscosity/kappa number ratio from 0.30 to 0.35 after 120 days of treatment. Gulsoy and Eroglu [42] performed the biokraft pulping of European black pine with C. subvermispora and found slight decrease in pulp viscosity after 20 and 80 days of treatment time compared to control. Singhal et al. [53] carried out biopulping of bagasse with cryptococus albidus and reported increased viscosity/kappa number ratio.

4.8 Effect of biopulping on strength properties of pulp Biological pretreatment of raw materials also results improved tensile index, burst index and folding endurance of pulp (Table 4.3) [3]. Several researchers reported improved in tensile strength, burst index and double fold with declined tear index. Conversely, some investigators also found improvement in tear index after biopulping [4, 42, 54]. Gautam et al. [4] studied the biological pretreatment of Eulaliopsis binata with Schizophyllum commune ARC-11 before soda pulping and reported improvement in tensile index, burst index and double fold numbers by 24.94 %, 14.03 % and 48.45 %, respectively, compared to soda pulp. While tear index of bio-soda pulp was decreased by 12.86 % compared to soda pulp. Kang et al. [26] treated poplar wood chips with Phanerochaete chrysosporium KCCM 34740 for 10 days before kraft pulping and found improvement in tensile index, burst index and folding endurance by 27.50 %, 19.67 % and 51.74 %, respectively, compared to kraft pulping (control). Tear index of fungal treated poplar wood chips was decreased by 14.34 % compared to control.

4.9 Mechanism for energy saving and strength properties improvement during biopulping Physical strength properties during refining are improved by the addition of NaOH. NaOH saponifies esters that increases the carboxylic acid content of the pulp that attracts water molecules in the pulp by the osmotic pressure of their counter ions resulting in refining energy saving and improved hydrogen bonding among cellulosic fibers [55, 56]. During biopulping, strength properties are improved by the production of oxalic acid by WRF. Hunt et al. [55] explained the mechanism for improvement of strength properties during biopulping. WRF produce oxalic acid during fungal

4.9 Mechanism for energy saving and strength properties

87

treatment that oxalic acid reacts with –OH group on cellulosic fiber to create ester linkage. Since, oxalic acid is a dicarboxylic acid, therefore one carboxylic acid remains free. This free carboxylic acid groups increase the influx of water molecules in the fungal treated raw material that decreases the requirement for refining energy and improves the hydrogen bonding among cellulosic fibers. Improved hydrogen bonding results better physical strength properties [3, 55]. Cooking of substrates with oxalic acid has shown similar results as biopulping [57]. Biological pretreatment of wood chips and other raw material is found to decrease energy requirement for during pulping. If mechanical pulping is performed after biological pretreatment of wood chips, less energy is required during mechanical pulping. Reduced cooking time has also been observed when biological pretreatment is followed by sulfite and kraft pulping [46]. Mechanical refining is required to produce desired fiber properties for better quality of paper. It requires large amount of energy and is performed in beaters or refiners [58]. Biological pretreatment of raw material before pulping reduces the requirement of refining energy. Growth of WRF on wood or other raw materials results cell wall thinning, fragmentation, swelling and relaxing of normally rigid cell wall. Another possible reason for easy refining for fungal pretreated pulp may be the improvement in holocellulose/lignin ratio. After fungal pretreatment of raw material, an easy response for refining is expected compared to control (without fungal pretreatment) [42, 54]. Refining energy can be decreased due to easier beating (less beating time) of fungal pretreated pulp. Villalba et al. [54] performed biopulping of Loblolly pine chips with C. subvermispora for 4 weeks and observed significant declined in freeness (CSF) values for biokraft pulp compared to kraft pulp. The decreased freeness indicates less beating time (beating energy) requirement for desirable freeness level after fungal pretreatment. Yadav et al. [5] studied the effect of biological pretreatment of mixed hardwood chips by C. subvermispora and reported 11.7 % less of cooking time for biologically pretreated chips compared to control with equal active alkali charge and same cooking conditions. Costa et al. [52] also performed fungal pretreatment of sugarcane bagasse for 30 days with C. subvermispora and observed a reduction of 28 % in cooking time for kraft pulping compared to control (without fungal pretreatment). Therefore, biological pretreatment of raw material resulted reduction in energy requirement during cooking processes. Gulsoy and Eroglu [42] carried out the biokraft pulping of European black pine with C. subvermispora and found significant decrease in refining time. The beating time for untreated kraft pulp was 39 and 46 min to reach 35 and 50° SR. respectively. After 100 days pretreatment of wood chips with C. subvermispora, the beating time was observed 27 and 34 min to reach 35 and 50° SR, respectively. Therefore, the beating time was decreased by 30.8 % and 26.1 % to reach 35 and 50° SR, respectively [42]. Vicentim et al. [59] pretreated Eucalyptus grandis chips with C. subvermispora for 14 days and reported the saving of 27–38 % of beating time to get a target of 28° SR. Messner and Srebotnik [60] described energy saving of 37 % and 47 % for loblolly pine and aspen wood chips, respectively, after 4

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4 Bio-pulping: An energy saving and environment-friendly approach

weeks of treatment by C. subvermispora. Fischer et al. [61] performed the biological pulping of loblolly pine chips with C. subvermispora, followed by mechanical pulping and found 30 % reduction in refining energy requirement compared to untreated chips. Wall et al. [62] carried out pretreatment of aspen wood chips by Phanerochaete chrysosporium at temperature 39 °C for 4 weeks before mechanical pulping and reported 33 % saving in refining energy compared to control (autoclaved and untreated).

4.10 Limitations of biopulping Biopulping is environment friendly and energy saving process for pulp manufacturing. The application of biopulping at large scale also has some limitations. Sometimes, fungal treatment is found to decrease the brightness of pulp after long treatment time [63]. In most of the cases, 2–8 weeks of duration of treatment has been found suitable for pulping (Table 4.1). This long treatment time is one of the main disadvantages of biopulping. The continuous supply of inoculum of WRF is required for industrial application of biopulping. Therefore, inoculum preparation is an additional work and expense for biopulping [43]. The successful colonization of raw material by WRF needs the asepsis. Therefore, degree of asepsis should be controlled during biopulping [3, 64]. Table 4.3: Effect of biopulping on strength properties. White-rot fungus

Raw material

Effect of fungal treatment on CSF & strength properties

C. subvermispora

(i) Rice straw (ii) Wheat straw (iii) Barley straw Oil palm trunk

Tensile strength and burst factor were improved by  % and  %, respectively Tensile strength and burst factor were improved by  % and  %, respectively Tensile strength and burst factor were improved by . % and  %, respectively Canadian standard freeness was decreased from  to  and strength properties were increased slightly Tensile, tear and burst index were improved slightly Tensile index was improved while tear index was lowered after  weeks of treatment After  days treatment tensile and burst index were improved while tear index was lowered. After – days treatment tensile, tear and burst index were lowered

T. versicolar

C. subvermispora C. subvermispora C. subvermispora

Eucalyptus nitens Loblolly pine chips European black pine

References []

[]

[] [] []

References

89

Table 4.3 (continued ) White-rot fungus

Raw material

Effect of fungal treatment on CSF & strength properties

References

Phanerochaete chrysosporium KCCM C. subvermispora

Hybrid poplar

Canadian standard freeness was dropped by  mL compared to control

[]

Mixed hardwood chips

Tensile index, tear index, burst index, breaking length and folding endurance were improved significantly

[]

4.11 Conclusion The treatment of wood chips and other raw materials with WRF have shown potential for mechanical as well as chemical pulping. Biopulping has been proved as energy saving process and environment friendly approach during paper manufacturing. The colonization of WRF on raw materials has been found effective for delignification for chemical of mechanical pulping. Biopulping has also been resulted improved strength properties and brightness of paper.

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[29] Jha H, Patil M. Biopulping of sugarcane bagasse using manganese peroxidase from Penicillium oxalicum isolate-1. Rom Biotechnol Lett. 2011;16:6809–19. [30] Oriaran TP, Labosky Jr. P, Blankenhorn PR. Kraft pulp and papermaking properties of Phanerochaete chrysosporium degraded red oak. Wood Fiber Sci. 1991;23:316–27. [31] Akhtar M, Horn E, Lentz M, Scott G, Sykes M, Myers G, et al. Toward commercialization of biopulping. Pap Age, 1999. [32] Hatakka AI. Pretreatment of wheat straw by white-rot fungi for enzymic saccharification of cellulose. Eur J Appl Microbiol Biotechnol. 1983;18:350–7. [33] Bonnarme P, Jeffries TW. Mn(II) regulation of lignin peroxidases and manganesedependent peroxidases from lignin-degrading white rot fungi. Appl Environ Microbiol. 1990;56:210–17. [34] Palmieri G, Giardina P, Bianco C, Fontanella B, Sannia G. Copper induction of laccase isoenzymes in the ligninolytic fungus Pleurotus ostreatus. Appl Environ Microbiol. 2000;66:920–4. [35] Baldrian P, Gabriel J. Variability of laccase activity in the white-rot basidiomycetePleurotus ostreatus. Folia Microbiol (Praha). 2002;47:385–90. [36] Stajić M, Persky L, Hadar Y, Friesem D, Duletić-Laušević S, Wasser SP, et al. Effect of copper and manganese ions on activities of laccase and peroxidases in three Pleurotus species grown on agricultural wastes. Appl Biochem Biotechnol. 2006;128:87–96. [37] Zadražil F, Brunnert H. Investigation of physical parameters important for the solid state fermentation of straw by white rot fungi. Eur J Appl Microbiol Biotechnol. 1981;11:183–8. [38] Reid ID. Solid-state fermentations for biological delignification. Enzyme Microb Technol. 1989;11:786–803. [39] Zadražil F, Galletti GC, Piccaglia R, Chiavari G, Francioso O. Influence of oxygen and carbon dioxide on cell wall degradation by white-rot fungi. Anim Feed Sci Technol. 1991;32:137–42. [40] Mardones L, Gomide JL, Freer J, Ferraz A, Rodríguez J. Kraft pulping of Eucalyptus nitens wood chips biotreated by Ceriporiopsis subvermispora. J Chem Technol Biotechnol. 2006;81:608–13. [41] Gellerstedt G, Lindfors E-L. Structural changes in lignin during kraft pulping. Holzforschung. 1984;38:151–8. [42] Koray Gulsoy S, Eroglu H. Biokraft pulping of European black pine with Ceriporiopsis subvermispora. Int Biodeterior Biodegradation. 2011;65:644–8. [43] Bajpai P. Biopulping. In: Bajpai P, editor. Biotechnology for pulp and paper processing. Springer International Publishing, Singapore 2018. p. 67–92. [44] Garmaroody ER, Resalati H, Fardim P, Hosseini SZ, Rahnama K, Saraeeyan AR, et al. The effects of fungi pre-treatment of poplar chips on the kraft fiber properties. Bioresour Technol. 2011;102:4165–70. [45] Yaghoubi K, Pazouki M, Shojaosadati SA. Variable optimization for biopulping of agricultural residues by Ceriporiopsis subvermispora. Bioresour Technol. 2008;99:4321–8. [46] Levin L, Villalba L, Da Re V, Forchiassin F, Papinutti L. Comparative studies of loblolly pine biodegradation and enzyme production by Argentinean white rot fungi focused on biopulping processes. Process Biochem. 2007;42:995–1002. [47] Singh P, Sulaiman O, Hashim R, Peng LC, Singh RP. Evaluating biopulping as an alternative application on oil palm trunk using the white-rot fungus Trametes versicolor. Int Biodeterior Biodegradation. 2013;82:96–103. [48] Risdianto H, Sugesty S. Pretreatment of Marasmius sp. on biopulping of oil palm empty fruit bunches. Mod Appl Sci. 2015;9:1. [49] de Jong E, Chandra RP, Saddler JN. Effects of a fungal treatment on the brightiness and strength properties of a mechanical pulp from Douglas-fir. Bioresour Technol. 1997;61:61–8.

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[50] Oriaran TP, Labosky P, Jr. Blankenhorn PR kraft pulp and papermaking properties of Phanerochaete chrysosporium-degraded aspen. Tappi J. 1990;73:147–52. [51] Bierman C. Essentials of pulping and papermaking. New York: Academic Press; 1993. [52] Costa SM, Gonçalves AR, Esposito E. Ceriporiopsis subvermispora used in delignification of sugarcane bagasse prior to soda/anthraquinone pulping. Appl Biochem Biotechnol. 2005;122:695–706. [53] Singhal A, Jaiswal PK, Thakur IS. Biopulping of bagasse by Cryptococcus albidus under partially sterilized conditions. Int Biodeterior Biodegradation. 2015;97:143–50. [54] Villalba LL, Scott GM, Schroeder LR. Modification of Loblolly pine chips with Ceriporiopsis subvermispora part 2: kraft pulping of treated chips. J Wood Chem Technol. 2006;26:349–62. [55] Hunt C, Kenealy W, Horn E, Houtman C, Service F, Wi M. A biopulping mechanism : creation of acid groups on fiber. Holzforschung 2004;58:434–9. [56] Lars-Åke H, Marie B, Myat H. Effect of electrolyte concentration and pH on the beatability of unbleached kraft pulps. Nord Pulp Pap Res J. 2000;15:194. [57] Akhtar M, Swaney R, Horn E, Lentz M, Scott G, Black C, et al. Method for producing pulp, worldwide patent filing, wo 02/075043, Wisconsin Alumni Research Foundation, 2002. [58] Singh R, Bhardwaj NK. Enzymatic refining of pulps: an overview. IPPTA. 2010;22:109–15. [59] Vicentim MP, Faria R de A, Ferraz A. High-yield kraft pulping of Eucalyptus grandis hill ex maiden biotreated by Ceriporiopsis subvermispora under two different culture conditions. Holzforschung. 2009;63:408–13. [60] Messner K, Srebotnik E. Biopulping: an overview of developments in an environmentally safe paper-making technology. FEMS Microbiol Rev. 1994;13:351–64. [61] Fischer K of resin content in wood chips during experimental biological pulping processes, Akhtar M, Blanchette RA, Burnes TA, Messner K, Kirk TK. Reduction of resin content in wood chips during experimental biological pulping processes. Holzforschung. 2009;48:285–90. [62] Wall MB, Cameron DC, Lightfoot EN. Biopulping process design and kinetics. Biotechnol Adv. 1993;11:645–62. [63] Sykes M. Bleaching and brightness stability of aspen biomechanical pulps. Tappi J. 1993;76 121–6. [64] Ferraz A, Guerra A, Mendonca R, Vicentim MP, Aguiar A, Masarin F, et al. Mill evaluation of wood chips biotreated on a 50 ton biopulping pilot plant and advances on understanding biopulping mechanisms. Tenth international congress on biotechnology in pulp and paper industry, 10–15 June, 2007, United States Book of Abstracts, Madison, WI, 2007:23–4.

Amit Kumar

5 Biobleaching: An eco-friendly approach to reduce chemical consumption and pollutants generation Abstract: The pulp and paper industry is known to be a large contributor to environmental pollution due to the huge consumption of chemicals and energy. Several chemicals including H2SO4, Cl2, ClO2, NaOH, and H2O2 are used during the bleaching process. These chemicals react with lignin and carbohydrates to generate a substantial amount of pollutants in bleach effluents. Environmental pressure has compelled the pulp and paper industry to reduce pollutant generation from the bleaching section. Enzymes have emerged as simple, economical, and eco-friendly alternatives for bleaching of pulp. The pretreatment of pulp with enzymes is termed as biobleaching or pre-bleaching. Different microbial enzymes such as xylanases, pectinases, laccases, manganese peroxidases (MnP), and lignin peroxidases are used for biobleaching. Xylanases depolymerize the hemicelluloses precipitated on pulp fiber surfaces and improves the efficiency of bleaching chemicals. Xylanase treatment also increases the pulp fibrillation and reduces the beating time of the pulp. Pectinases hydrolyze pectin available in the pulp fibers and improve the papermaking process. Laccase treatment is found more effective along with mediator molecules (as a laccase-mediator system). Biobleaching of pulp results in the superior quality of pulp along with lower consumption of chlorine-based chemicals and lower generation of adsorbable organic halidesadsorbable organic halides (AOX. An enzyme pretreatment reduces the kappa number of pulp and improves ISO brightness significantly. Better physical strength properties and pulp viscosity have also been observed during biobleaching of pulp. Keywords: Xylanases, ligninolytic enzymes, laccase-mediator system, AOX, ECF & TCF, kappa number, ISO brightness

5.1 Introduction The bleaching section in the pulp and paper industry is the most crucial area from an environmental perspective. Pulping results in fibrous material that consists of cellulose mainly. Cellulose and hemicelluloses are inherently white and do not contribute to colour. Although the bulk of lignin is removed during the chemical pulping process, chromophoric groups available in residual lignin in pulp fibers mainly This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, A. Biobleaching: An eco-friendly approach to reduce chemical consumption and pollutants generation Physical Sciences Reviews [Online] 2020, 5. DOI: 10.1515/psr-2019-0044 https://doi.org/10.1515/9783110592412-005

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contribute to the colour of pulp. Unbleached pulp contains approximately 5% of lignin per unit of weight that must be removed to produce white pulp. This residual lignin is tightly associated with secondary wall layers of pulp fibers and removed during multi-stages bleaching sequences [1–3]. Bleached pulp has superior quality of brightness, softness, and cleanliness as compared to the unbleached pulp. It is essential for the manufacturing of many types of products such as writing and printing paper, tissue paper, sanitary paper, and absorbent products. Bleaching of chemical pulps is a multi-stage sequential process that is with two or more chemicals to achieve high pulp brightness [4]. Highly toxic and organically bound compounds, generally classified as adsorbable organic halidesadsorbable organic halides (AOX are generated during traditional bleaching based on chlorine-containing chemicals. The environmental protection regulations have been made stricter throughout the world to limit the effluent discharge in the environment. Due to growing environmental concerns and legislative pressures, the pulp and paper industry is forced to modify its current pulping, bleaching, and effluent treatment technologies [1, 5].

5.2 Bleaching processes: Cl2, ECF and TCF bleaching Till the end of the twentieth century, chlorine and chlorine-based chemicals were used for bleaching of pulp irrespective of their origin from softwood or hardwood. But now most of the pulp and paper mills are using chlorine dioxide (ClO2) as elemental chlorine-free (ECF) bleaching chemical for the production of high quality of pulp [6]. Conventional chemical bleaching of pulp that used to be initially carried out through Cl2, was later replaced by ECF and total chlorine-free bleaching (TCF). The introduction of ECF and TCF bleaching sequences brought a major technological revolution in the pulp and paper industry. ECF and TCF resulted in the production of environmentally benign paper and mitigation of environmental emissions. ECF bleaching utilizes ClO2 and sodium hypochlorite (NaOCl) as a bleaching agent [2, 3, 7]. In ECF bleaching, Cl2 is replaced with ClO2 that reduces the generation of chlorinated organic materials in bleach effluent. ClO2 is a more powerful oxidizing agent as compared to Cl2 and it preserves cellulose and attacks lignin more selectively to produce brighter and stronger pulp [2, 8]. TCF bleaching utilizes nonchlorine oxidative bleaching chemicals such as hydrogen peroxide (H2O2), oxygen (O2), and ozone (O3) [3, 7]. H2O2 is an effective chlorine-free oxidative reagent that is commonly used in all modern TCF bleaching sequences [9]. O2 delignification is practiced in several pulp and paper mills worldwide before ECF and TCF bleaching sequences. During O2 delignification, the alkaline suspension of brown stock pulp is exposed to O2 under pressurized conditions for the removal of a considerable fraction of residual lignin [10]. O3 is another oxidative bleaching agent that is used for residual lignin removal during bleaching. It

5.3 Bleaching and associated environmental issues

95

reacts with lignin and carbohydrates, thereby resulting in decreased pulp viscosity and fiber strength [11]. O3 is reported to be more selective towards lignin compared to carbohydrates [12]. The O3 treatment stage is part of ECF and TCF bleaching sequences at a commercial scale. It has been proved an efficient and competitive bleaching chemical in terms of delignification capability, cost, and environmental impact [13].

5.3 Bleaching and associated environmental issues Pulp bleaching aims to remove residual lignin remained within pulp fiber after the pulping process and to enhance the brightness of pulp to the satisfactory level [14]. The removal of lignin in a single step is not feasible; therefore, to remove lignin at the desired level, bleaching chemicals are used consecutively with intermediate washing and extraction stages. Several oxidants such as chlorine, ClO2, sodium hypochlorite, H2O2, O2, and O3 are used for bleaching different types of pulps. Sodium hydroxide is an alkali that is used in the extraction stage of pulp. A reducing bleaching chemical viz., sodium hydrosulfite (sodium dithionite) is generally used for the brightening of mechanical pulps. It converts chromophoric groups of lignin into leucochromophoric groups [2, 3, 15–17]. The chemicals such as H2SO4, Cl2, ClO2, NaOH, and H2O2 used for bleaching react with lignin and carbohydrates and produce a significant amount of pollutants that are released into bleach effluents [14]. During the process of chlorine bleaching, 10% of chlorine used gets bound to residual lignin and generates chlorinated organic compounds. The rest of chlorine (about 90%) ends up as common salt. Lignin, phenols, and resin acids present in pulp fiber get chlorinated and transformed into highly toxic xenobiotic compounds. These chlorinated organics are known as AOX. These compounds include chlorinated lignins, dioxins, furans, resin acids, and phenol. Some of these compounds are toxic, mutagenic, persistent, and bio-accumulating due to their lipophilic nature [5, 17, 18]. The nature and amount of AOX formation depend on the type of bleaching chemicals used and the nature of residual lignin in the pulp [14]. The chlorination stage of bleaching always produces 2, 3,7, 8-TCDD (tetrachlorodibenzo-p-dioxin), 2, 3, 7, 8-TCDF (tetrachlorodibenzofuran), and 1, 2, 7, 8TCDF. Several AOX are harmful to health and for the environment. Dioxins and furans can change blood chemistry and results in liver damage, skin disorders, lung lesions, and tumours [19, 20]. The use of such chemically treated paper for different purposes such as baby diapers and packaging of edible products like bread and biscuits, sweetmeat and crystallized fruits, and tea bags is of great concern [21]. The bleaching process also contributes to colour, Chemical Oxygen Demand (COD), and inorganic chlorine compounds such as chlorate, ClO3 ,̄ and Volatile Organic Compounds (VOCs) in the bleach wastewater. The VOCs include acetone, methylene chloride, carbon disulfide, chloroform, chloromethane, trichloroethane, etc [22].

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

5.4 Biobleaching The environmental issues and growing market demand for superior pulp and paper products force the development of green processing for a better quality of products along with the reduced generation of pollutants during bleaching [14]. Intensive research has been carried out to reduce the consumption of chlorine compounds to solve environmental problems. The pretreatment of pulp with enzymes such as xylanase or laccase is known as biobleaching or pre-bleaching. It has emerged as a viable option to minimize the consumption of chlorine compounds [23]. Enzymes provide an easy, simple and economical bleaching approach that results in several benefits such as higher brightness value with lower kappa number, reduced chemical consumption, improved pulp and paper properties, and lower capital investment [24, 25] (Figure 1). Viikari et al [26] first time reported the concept of enzymes assisted pulp bleaching. Initially, It was thought that limited hydrolysis of hemicellulose by hemicellulases may improve the extraction of lignin from kraft pulp during its subsequent chemical bleaching. Now, it has been well proved that pretreatment of pulp with enzymes such as xylanases and ligninolytic enzymes significantly improve the delignification and brightness of pulp without causing any loss in its viscosity and strength [27, 28]. Xylanases and ligninolytic enzymes such as laccase, LiP, and MnPMnP have been utilized for biobleaching of pulp. Enzymatic pretreatment of pulp decreases the bleaching chemicals required to attain the desired brightness as compared to the untreated control pulps [24, 29].

Figure 5.1: Benefits of biobleaching.

5.4 Biobleaching

97

5.4.1 Xylanase biobleaching Xylanases have been used in pulp and paper industry for different processes including biobleaching of pulp, deinking of recycled paper, enzyme-assisted refining and enzymatic debarking [17, 30, 31]. Xylanases are well known for the bleach boosting effect during bleaching [32]. Endo-β-xylanase has been proved to be the main enzyme for bleaching of pulp, although xylanase along with different hemicellulolytic enzymes enhances the effectiveness of enzymatic treatment [24]. Xylanase treatment reduces chemical consumption during bleaching as well as increases the pulp quality. The action of the chemical cannot remove the lignin completely from the fiber and some part of lignin reprecipitates on to the fiber surfaces. Therefore, the resultant pulp becomes brown due to the presence of residual lignin and its derivatives [29]. Xylanases hydrolyze the relocated and precipitated hemicelluloses on the surface of pulp fiber and open up more space for bleaching chemicals by attacking the lignincarbohydrate complex. The increase in permeability of fiber surfaces enhances the penetration of bleaching chemicals to the pulp and thereby improves the extraction of lignin during subsequent chemical/conventional bleaching stages [17, 29, 32, 33]. Besides lignin removal, xylanase action increases the pulp fibrillation and reduces the beating time of the pulp [34]. Xylanases are produced by a variety of microorganisms such as fungi, bacteria, and actinomycetes. Xylanases from different sources have variations in their characteristics. Generally, fungi excrete extracellular xylanase in higher quantities [32]. But fungi require long incubation time for xylanase production that is not desirable for industrial purposes. Bacteria show some advantages over fungi based pulp bleaching. Bacteria are fast growing as compared to fungi and need shorter incubation time to produce maximum xylanase activity. Bacteria also produce alkali and thermostable xylanases that is prerequisite for biobleaching purpose [35]. Xylanases have also been produced by different sources and utilized for biobleaching of pulp (Table 5.1). Table 5.1: Microbial sources of xylanases for pulp biobleaching. Microbial source of xylanase

Type of pulp for biobleaching

Reference

Bacteria Arthrobacter sp. MTCC  Bacillus amyloliquifacien Bacillus circulans AB Bacillus coagulans Bacillus halodurans B. halodurans C- Bacillus licheniformis Bacillus pumilus AJK

Kraft pulp Kraft pulp Eucalyptus kraft pulp Nonwoody soda pulp Eucalyptus kraft pulp Wheat straw soda-AQ pulp Eucalyptus kraft pulp Plywood waste soda pulp

[] [] [] [] [] [] [] []

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

Table 5.1 (continued ) Microbial source of xylanase

Type of pulp for biobleaching

Reference

Bacteria B. pumilus SYA B. pumilus SV-S B. pumilus CBMAI  Bacillus stearothermophilus SDX Paenibacillus sp. S- Paenibacillus campinasensis BL Staphylococcus sp. SG-

Oil palm empty fruit bunches pulp Mixed kraft pulp Eucalytus grandis kraft pulp Wheat straw pulp Eucalyptus globulus kraft pulp Hardwood kraft pulp Eucalyptus & Khar grass kraft pulp

[] [] [] [] [] [] []

Eucalyptus kraft pulp Eulaliopsis binata ethanol-soda pulp Eucalytus grandis pulp Recycled pulp Wheat straw soda pulp Bamboo pulp Kraft pulp Eucalytus grandis pulp Eucalyptus kraft pulp Sugarcane bagasse soda pulp Wheat straw soda AQ-pulp

[] []

Fungi Aspergillus flavus A. flavus ARC  A. flavus, A. niger Aspergillus niger A. niger An- Aspergillus nidulans A. nidulans KK Aspergillus japonicus var. aculeatus Chaetomium cellulolyticum Coprinellus disseminates SW- NTCC- C. disseminates SH- NTCC & SH- NTCC Humicola grisea var. thermoidea Penicillum crustosum FP Penicillum meleagrinum var. viridiflavum Penicillum corylophilum Penicillum janczewskii Schizophyllum commune ARC Trichoderma reesei QM Trichoderma viride VKF T. viride Trichoderma longibrachiatum Trichoderma harzianum Thermomyces lanuginosus SSBP T. lanuginosus T. lanuginosus MC

Eucalytus oxygen-bleached kraft pulp Eucalytus grandis kraft pulp Bamboosa tulda pulp Eucalytus kraft pulp Eucalytus kraft pulp Eulaliopsis binata ethanol-soda pulp Eucalyptus kraft pulp Newspaper pulp Eucalytus kraft pulp Eucalytus kraft pulp Unbleached waste paper pulp Nonwood pulp Wheat straw pulp Bagasse pulp

[] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] [] []

5.4 Biobleaching

99

Table 5.1 (continued ) Microbial source of xylanase

Type of pulp for biobleaching

Reference

Bacteria Actinomycetes Sreptomyces chartreusis Sreptomyces cyaneus SN Sreptomyces griseorubens LH- Sreptomyces sp. Sreptomyces roseiscleroticus

Wheat straw pulp Wheat straw rich soda pulp Eucalyptus kraft pulp Eucalyptus kraft pulp Hardwood kraft pulp

[] [] [] [] []

Although xylanases are effective in bleaching of pulp, some features such as the presence of cellulase in xylanase preparation, thermostability, and alkaline stability of xylanases are very crucial for bleaching. Moreover, low-molecular-weight xylanases provide an extra advantage of easy penetration into xylan on the pulp fiber surfaces [29]. The presence of cellulase in xylanase enzyme preparation hydrolyzes the cellulose and adversely affects the pulp and paper properties. Cellulase action during biobleaching affects the viscosity and physical strength properties negatively. Therefore, cellulase-free xylanases are essential for biobleaching of pulp [17]. The preparation of cellulase-free xylanases can be obtained through three approaches mainly. The first approach to obtain cellulase-free xylanases is the purification of xylanases to remove cellulase contamination. Some researchers have purified the xylanase and utilized for bleaching of pulp but removal of cellulases by purification or enzymatic inactivation of cellulases may not be feasible due to higher cost. Taneja et al. [53] partially purified the alkaline xylanase from Aspergillus nidulans KK99 and utilized for biobleaching of kraft pulp. The optimum bleach boosting effect was observed with an enzyme dose of 1.0 IU/g of dry pulp at pH 8.0 and temperature 55 °C for 3 h reaction time [53]. Kumar et al. [37] also studied the application of purified xylanase for prebleaching of hardwood kraft pulp. Bacillus amyloliquefaciens xylanase was purified and applied with a dose of 20 IU/g OD pulp for 3 h at 60 ° C. The kappa number was reduced by 18.3 and 23.8% with xylanase (20 IU/g OD pulp) alone and xylanase-sorbitol treatment, respectively [37]. The second strategy for the production of cellulase-free enzymes is the isolation of microorganisms from the natural habitat that fails to produce cellulase and produce xylanase in a fair amount. This might be the most appropriate approach for the production of cellulase-free xylanase. Several researchers have reported different microbial strains that can produce cellulase-free xylanases (Table 5.1). The third strategy for the production of cellulase-free xylanase is the development of microbial strains by genetic modification. The genetically engineered microbial strains can be used for cellulase-free xylanase production [34, 36].

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

5.4.2 Pectinases and mannanases biobleaching Apart from xylanases, some other enzymes such as pectinases and mannanases have also been tested for biobleaching. Pectinases are the enzymes that hydrolyze pectin available in pulp fibers. Approximatley 1–4% of pectin is present in hardwoods, softwoods, and bamboo; therefore, pectinases might be effective for bleaching of pulps manufactured by these substrates [75–77]. The US patent on the pectinase treatment of wood pulp discloses the fact that the aqueous phase of alkaline-treated pulp contains a considerable amount of pectins. Therefore, pectinase treatment in the aqueous phase of pulp results in the degradation of harmful pectins and improves paper-making process [78, 79]. Kaur et al. [75] applied cellulasefree xylano-pectinolytic enzymes for biobleaching of a mixture of mixed hardwood and bamboo pulp and found 8.5% reduction in kappa number, showing remarkable delignification by the combination of xylanases and pectinase. During the subsequent bleaching process, active chlorine consumption was reduced by 25% without any decrease in brightness. Physical strength properties and viscosity of pulp were also improved significantly [75]. Dhiman et al. [77] compared biobleaching of kraft pulp by xylanase alone and xylanase-pectinase combination in terms of chlorine consumption and physical strength properties. Xylanase alone and xylanase-pectinase combination resulted in 15 and 20% reduction in chlorine consumption, respectively. Xylanase-pectinase combination was also found more effective for physical strength properties improvement as compared to xylanase alone [77]. Some of more studies focussed on biobleaching by the combination of xylanases and pectinases have been reviewed in Table 5.2. Hemicelluloses are heteropolymers that are composed of hetero-1, 4-β-D-xylans and hetero-1, 4-β-D-mannans. Mannan degrading enzymes include endo-1, 4- βmannanase, exo- β-mannosidase, α-galactosidase, and acetyl mannan esterase. Endo-1, 4- β- mannanases are endo acting hydrolases that attack internal glycosidic linkage of mannan backbone chain to release short β-1,4-manno oligosaccharides [87, 88]. Endo-β-mannanases have been used for biobleaching of pulp and proved more effective in combination with xylanases. Endo-β-mannanases along with xylanases improve the brightness, decrease the kappa number, and reduce the environmental pollution load as compared to the traditional bleaching process [89,90,91]. A study performed on biobleaching of kraft pulp by mannanase and xylanase reported improvement in ISO brightness by 0.85 and 1.63 points through mannanase-xylanase combination and xylanase alone, respectively, compared to control [90]. Chauhan et al. [91] studied the production cellulase-free and thermo-alkali-stable β-mannanase by Bacillus nealsonii PN-11 and used for biobleaching of softwood pulp. Optimum bleaching effect was observed at a treatment temperature of 60 °C, pH 8.4, and reaction time of 60 min with enzyme dosages of 40 and 20 IU/g OD pulp of mannanase and xylanase, respectively. Under optimized conditions, mannanase pretreatment resulted in kappa number reduction by 14.50%, ISO brightness improvement by 2.72%,

.%

T:  °C I:  h pH: .

Xylanases ( U/g)

B. stearothermophilus SDX

.%

T:  °C I:  h pH: .

Xylanases ( U/g)

B. pumilus SV-S

–

Xylanases ( U/g)

B. circulans AB

Wheat straw pulp (%)

Eucalyptus kraft pulp (%)

Eucalyptus kraft pulp (%) T:  °C I:  h

Xylanases (. U/g)

Streptomyces sp. QG--

%

. and .%

T:  °C I:  h pH: .

C. disseminates SH- Xylanases  NTCC & SH- ( IU/g) NTCC

– Cl consumption reduced by % without any decrease in ISO brightness

– Cl consumption reduced by .% without any decrease in ISO brightness

– Cl consumption reduced by % without any decrease in ISO brightness

–

– AOX reduced by . and .% at Cl charge of .%

Treatment Kappa Reduction in chemical conditions number consumption and (T & I)* reduction pollutants generation

Wheat straw T:  °C soda-AQ pulp I:  h (&%) pH: .

Enzyme Type of pulp (dose, per (consistency, gram dry %) pulp)

Conventional bleaching

Source of enzyme

– Tensile strength, breaking length, burst factor, tear factor and pulp viscosity improved by ., ., ., . and .%, respectively

(continued )

[]

[]

[]

– Enzyme treatment improved pulp viscosity up to .–. cp. compared to control (. cp.) – Tensile strength, breaking length, burst factor and tear factor improved by ., ., . and .%, respectively

[]

[]

Reference

– Tensile strength and burst factor improved by  and %, respectively – Brightness (ISO) improved by % at same chlorine dose

– Brightness (ISO) improved by . and .% – Pulp viscosity improved by . and .% compared to chemical bleaching

Effect on pulp and paper properties

Table 5.2: Effect of biobleaching on kappa number, chemical consumption, pollutants generation, and pulp & paper properties.

5.4 Biobleaching

101

Xylanases ( U/g)

Xylanases ( U/g)

S. cyaneus SN

Paenibacillus sp. S-

Kraft pulp (%)

Nonwoody kraft pulp (%)

Commercial enzyme Xylanase (Sandoz) ( IU/g)

Penicillum corylophilum & Trichoderma longibrachiatum

.%

T:  °C I:  h pH: .–

. and .%

%

T:  °C I:  h pH: . T:  °C I:  h pH: .

–

%

T:  °C I: . h pH: .

– ClO consumption decreased by %

–

– Cl/ClO consumption reduced by % to obtain same ISO brightness

– Hypochlorite consumption reduced by % to give comparable ISO brightness

– Cl and ClO consumption reduced by  and %, respectively, to obtain same ISO brightness

Treatment Kappa Reduction in chemical conditions number consumption and (T & I)* reduction pollutants generation

Eucalyptus T:  °C globulus kraft I:  h pulp (%) pH: 

Wheat straw rich soda pulp (%)

Xylanases ( IU/g)

Elemental chlorine free bleaching

Pectinase ( U/g) Xylanases ( U/g)

B. subtilis B. pumilus

Kraft pulp (%)

Enzyme Type of pulp (dose, per (consistency, gram dry %) pulp)

Conventional bleaching

Source of enzyme

Table 5.2 (continued )

– Brightness (ISO) improved upto . points compared to control

– Pulp viscosity improved by . and .% – Brightness (ISO) improved by . and . points

– Enzyme treatment improved pulp viscosity to  cps compared to control (. cps)

– Tear and burst index were improved slightly

– Whiteness and fluorescence were improved by  and %, respectively, while yellowness was reduced by %

Effect on pulp and paper properties

[]

[]

[]

[]

[]

Reference

102 5 Biobleaching: An eco-friendly approach to reduce chemical consumption

–

.%

%

%

T:  °C I:  h pH: .

T:  °C I:  h pH: . T:  °C I:  h pH:  T:  °C I:  h pH: .

Eucalyptus LMS: kraft pulp Laccase (%) ( U/g) HBT (.% w/w)

Wheat straw pulp (%)

Kraft pulp (%)

Xylanase ( IU/g)

Xylanase ( IU/g), Laccase ( IU/g)

Xylanase ( U/g)

Thermomyces lanuginosus

Talaromyces thermophilus

B. pumilus ASH

Ganoderma lucidum RCK

.%

T:  °C I:  min pH: .

Xylanase ( IU/g) Pectinase (. IU/g)

B. pumilus AJK

.%

Mixed kraft pulp (%)

SAQ pulp (%)

Eucalyptus kraft pulp (%)

T:  °C I:  min pH: .

Xylanase ( IU/g)

T. reesei QM

– ClO consumption decreased by %

– NaOCl consumption reduced % to obtain same brightness

– NaOCl consumption reduced by .% to obtain same brightness (%)

– ClO consumption decreased by % – AOX generation reduced by %

– Cl and ClO consumption decreased by  and .%, respectively

– ClO consumption decreased by %

– Enzyme pretreatment resulted in % improvement in ISO brightness – Pulp viscosity, tensile strength, breaking length, burst factor, and tear factor significantly improved

– Pulp viscosity was slightly increased with laccase treatment

– Tensile and burst index reduced while tear index increased

– ISO brightness level maintained same as control (chemical bleaching)

– Pulp viscosity gain by .% with % less chorine – Breaking length, burst factor, and tear factor improved by ., . and .%, respectively

– Brightness (ISO) improved by . points compared to control

(continued )

[]

[]

[]

[]

[]

[]

5.4 Biobleaching

103

Enzyme Type of pulp (dose, per (consistency, gram dry %) pulp)

Xylanase ( U/g)

Bacillus sp. XTR-

Trametes villosa

LMS: Laccase ( U/g dry pulp) HBT (.%)

Total chlorine free bleaching

Kraft pulp (%)

Xylanase ( U/g)

Arthrobacter sp. MTCC 

Kenaf sodaAQ pulp (%)

Wood kraft pulp (%)

Wheat straw pulp (%)

Xylanase ( U/g)

Streptomyces rameus L

Conventional bleaching

Source of enzyme

Table 5.2 (continued )

.%

T:  °C I:  h pH: .

–

%

T:  °C I:  h pH: .

T:  °C I:  h pH: .

%

T:  °C I:  h pH: .

– ISO brightness increased by . points – Tensile and burst index were improved by . and .%, respectively

Effect on pulp and paper properties

–

– NaOCl consumption reduced .% to attain same level of ISO brightness as control

– ISO brightness reached .% compared to conventional TCF, .% – Delignification improved by .%

– Enzyme pretreatment resulted in .% improvement in ISO brightness – Burst index and tensile strength slightly improved

– NaOCl consumption – Pulp viscosity was maintained reduced % to without decreasing ISO brightness

– .% increase in residual chlorine

Treatment Kappa Reduction in chemical conditions number consumption and (T & I)* reduction pollutants generation

[]

[]

[]

[]

Reference

104 5 Biobleaching: An eco-friendly approach to reduce chemical consumption

Bamboo kraft T:  °C pulp I:  h pH: .

.%

–

%

–

– Bleaching chemical consumption reduced by .

– Ozone consumption reduced by %

* T: Temperature during enzyme treatment, I: Incubation time for enzyme treatment

P. meleagrinum var. Xylanase viridiflavum ( U/g)

T:  °C I:  h pH: .– .

Xylanase

Streptomyces rutgersensis UTMC 

Mechanical pulp (.–%)

Eucalyptus T:  °C globulus kraft I:  h pulp (%) pH: .–.

Xylanase ( U/ kg)

Pulpzyme HC (Novo Nordisk, A/S)

– Enzyme-peroxide treatment improved ISO brightness and pulp viscosity by . and .%, respectively compared to peroxide treatment only

– ISO brightness increased by % after  h treatment

– ISO brightness increased by  points at same ozone consumption

[]

[]

[]

5.4 Biobleaching

105

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

and an increase in viscosity by 2.72%. Xylanase treatment resulted in kappa number reduction by 22.34%, ISO brightness improvement by 14.75%, and an increase in viscosity by 2.40%. When biobleaching was carried out by both mannanase and xylanase, kappa number reduced by 32.23%, and ISO brightness and viscosity were improved by 16.39 and 3.20%, respectively [91]. Gubtz et al. [89] et al. also performed biobleaching of softwood pulp by Sclerotium rolfsii mannanase and xylanase. ISO brightness was improved by 1.9 and 2.8% with mannanase alone and mannanase-xylanases combination, respectively [89].

5.4.3 Ligninolytic enzymes biobleaching Ligninolytic enzymes are also found effective for biobleaching of different kinds of pulp. The ligninolytic enzymes studied till now are identified as extracellular and nonspecific enzymes that are involved in oxidative reactions, whenever bonds between the basic units and the aromatic structure of lignin are broken [24]. Bacteria and fungi produce extracellular oxidative enzymes that degrade the lignin. A small group of basidiomycetes termed as white-rot fungi (WRF) has especially developed the ability to breakdown and mineralize lignin by producing oxidative enzymes. WRF efficiently degrade the lignin within the plant cells using extracellular ligninolytic enzymes. Ligninases directly and specifically attack lignin to oxidize it and make it water-soluble. Oxidoreductases involve in lignin degradation are peroxidases and laccases. Peroxidases perform the oxidation of different types of molecules utilizing H2O2 as the oxidant [92, 93]. Laccases, LiPLiP, and MnP are the key enzymes in the oxidative enzyme system of WRF that have been used for biobleaching of pulp [94–96]. 5.4.3.1 Lignin peroxidases biobleaching LiP is known as a key enzyme for lignin degradation. LiPs are glycosylated heme proteins that oxidize both phenolic and nonphenolic compounds. They need H2O2 for the oxidation of lignin-related aromatic structures. During LiP catalysis, veratryl alcohol (VA) cation radical is generated that works as a free radical mediator for electron transfer between LiP and its substrate. VA is a secondary metabolite produced by WRF [97–99]. Colonia et al. [97] utilized Aspergillus sp. LPB-5 for biopulping of oil palm empty fruit bunches (OPEFB) and the same fungal strain produced xylanase (54.32 U/g substrate) and LiP (13.41 U/g substrate) that were used for biobleaching of OPEFB pulp. Biopulping and biobleaching with an enzyme having xylanase and LiP in combination resulted in 36.80 and 26.27% reduction in lignin and hemicelluloses, respectively. Alkaline treatment and biobleaching combination removed 81.97 and 93.89% of hemicelluloses and lignin, respectively [97]. Ozer et al. [100] performed the pine kraft bleaching with a ligninolytic enzyme having glutathione-s-transferase (GST),

5.4 Biobleaching

107

LiP, and laccase produced by Bacillus subtilis WB800. ABTS for laccase and VA for LiP were used as intermediate to increase enzyme activity, while no such intermediate was needed for GST. ISO brightness of pine kraft pulp was improved from 51.78% to 66.45%, while it improved from 53.89% to 64.67% for waste paper pulp [100]. Carvalho et al. [101] studied the biobleaching of hardwood kraft pulp with LiP by Phanerochaete chrysosporium and reported significant improvement in pulp selectivity under optimized conditions such as enzyme dose (2 U/g of pulp), H2O2 addition rate (10 ppm/h), and treatment time of 60 min at 30 °C [101]. The effectiveness of LiP for biobleaching is significantly affected by the presence or absence of VA and the concentration of H2O2. VA stabilizes the LiP under excess of H2O2 and acts as a mediator in the electron transfer process between enzyme and substrate. Generally, it has been applied in a range of 2.5–4 mM [101–103]. 5.4.3.2 Manganese peroxidases biobleaching MnP is also a glycosylated heme protein that catalyzes H2O2-dependent oxidation of Mn2+ into Mn3+ which in turn oxidizes lignin. Mn3+ is stabilized by organic acid to generate Mn3+ organic acid complex which acts as a low-molecular-weight redox-mediator. This complex is able to diffuse and break the aromatic rings of lignin. MnP catalyzes the oxidation of phenolic lignin subunits only. However, it has been reported that in the presence of glutathione, MnP oxidizes nonphenolic lignin model compounds including veratryl, anisyl, and benzyl alcohols [104–107]. Kaneko et al. [108] utilized purified MnP for biobleaching of oxygen–alkaline-treated hardwood kraft pulp (OKP) and in the presence of Tween-20 showed an improvement in ISO brightness by 5 points, while in the absence of Tween-20 ISO brightness improvement was not significant. Tween-20 may prevent the mechanical inactivation of MnP due to agitation or inhibit the absorption of enzyme onto the pulp that results in the inactivation of enzymes [108, 109]. In another study, the adsorption of LiP and MnP on unbleached kraft pulp was also reported [103]. Kondo et al. [109] also studied the role of surfactants on bleaching of hardwood kraft pulp by MnP from P. sordid YK-624 and reported ISO brightness improvement by 10 points in the presence of MnSO4, Tween-80, sodium malonate with continuous addition of H2O2. When pulp was treated with MnP in the absence of Tween-80, ISO brightness was improved by 4 points only. This showed the significance of surfactants in MnP bleaching. It was also suggested that surfactants also disperse hydrophobic degraded lignin in a water solution that prevents repolymerization of lignin [109]. Bermek et al. [106] performed experiments to improve the bleaching effect of MnP using mediators including unsaturated fatty acids, thio-containing compounds and various other organic compounds. Thio-containing compounds failed to improve pulp bleaching effect by MnP, while some unsaturated fatty acids, linoleic acid, and linolenic acid showed a better pulp bleaching effect compared to Tween-80. MnP pulp bleaching capability was higher with combination of Tween-80 and a carboxylic acid than that of Tween-80 alone [106].

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

Masarin et al. [110] studied lignin degradation in Eucalyptus grandis milled wood and kraft pulp by MnP from basidiomycetes Ceriporiopsis subvermispora. MnP-based bleaching of eucalyptus kraft pulp showed a reduction of 3.7 kg/ton of ClO2 compared to control with an ISO brightness improvement from 88 to 89 points compared to control (without enzyme treatment). MnP-treated pulp also exhibited superior brightness stability, the brightness reversion was lower (1.5% ISO) compared to control (2.4% ISO). MnP-based bleached pulp contained more carboxylic acids before bleaching. However, after bleaching, carboxylic acids were decreased compared to control that indicated the removal of MnP-oxidized lignin during bleaching. MnP treatment of nonrefined pulp exhibited a slightly higher drainage (17 °SR) compared to 20 °SR for the control pulp [110]. This increase in the drainage of pulp indicates reduced requirements of energy during the drying process at the industrial scale [110, 111]. Paice et al. [112] treated kraft pulp with Trametes versicolor that produced laccase and MnP. This study proved that MnP causes demethylation and delignification of kraft pulp in presence of H2O2. Purified MnP treatment alone also resulted in most of demthylation and delignification [112]. Immobilized MnP has been tested for biobleaching of pulp. MnP from P. chrysosporium was immobilized in FSM-16, a folded-sheet mesoporous material that has the pore size approximately equal to the diameter of the enzyme molecule. The immobilized enzyme showed higher thermal stability, tolerance to H2O2, and retained more than 80% of its activity after 10 days of continuous reaction. A two-stage reactor for enzyme treatment and pulp bleaching at temperatures 39 and 70 °C, respectively, was utilized that showed 88% improvement in brightness [113]. 5.4.3.3 Laccases biobleaching Laccases are multicopper oxidases that catalyze the polymerization-depolymerization reactions and perform the oxidation of phenols and aromatic or aliphatic amines to the corresponding reactive radicals using oxygen as electron acceptor [114,115,116]. Laccases oxidize phenols and aromatic amines such as methoxyphenols, phenols, polyphenols, anilines, aryl diamines, hydroxyindole, benzenethiols and some cyanide complexes of metals [117, 118]. Laccases have shown the potential to perform biobleaching of pulp by specific lignin oxidation and removal. The use of laccases at the industrial scale has some limitations such as availability of efficient and stable enzymes for industrial conditions and the requirement of mediator molecules [119]. The low redox potential of laccases hinders the direct oxidation reaction of complex compounds with high potential (E ° > 1.5 V). Laccase has a low affinity for its substrate due to lower diffusion of enzyme in the lignocellulose complex. However, when a mediator molecule is used, it transfers electrons between the active site of laccase and substrate and enables the oxidation of these complex compounds. Mediator molecules have several functional groups including HRNOH, NOH, and NO that are capable of accepting and giving electrons. The process in which reactions are performed by laccase and specific mediator molecule simultaneously is known as

5.4 Biobleaching

109

a laccase-mediator system (LMS) [120–122]. LMS has shown the potential for biobleaching of pulp due to direct action on lignin. But, mediator molecules are costly and increase the cost of the process. A mediator is a low-molecular-weight compound that is able to diffuse into pulp fibers [123]. The combination of laccases with mediator compounds increases the range of compounds to be oxidized to nonphenolic substrates and also improves their oxidation power [114, 124]. Laccases oxidize mediator molecule to generate an active radical that have great oxidation power and it is small enough to diffuse where enzymes cannot go through i.e. the wood cells. LMS is more effective for lignin degradation and it is an environmentally safe approach. Furthermore, lipophilic extractives and hexenuronic acids are also reduced by laccase-mediator treatment, the reduction in lipophilic extractives and hexenuronic acids in pulp minimizes the chemical consumption during subsequent chemical bleaching stages [114, 125–127]. Laccase-mediators are of two types i.e. synthetic and natural mediators. The most common synthetic mediators include 2, 2ʹ-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), 1- hydroxybenzotriazole (HBT), violuric acid (VA). HBT is proved to be one of the most effective mediators for delignification of pulp. But the disadvantage of this mediator is its high cost and possible toxicity. Such problems might be overcome by the use of natural mediators and phenolic compounds derived from the lignin that already have been tested for dye decolourization and pulp bleaching with promising results [7, 114, 125, 128]. The natural mediators include syringaldehyde, methyl syringate and pcoumaric acid. Black liquor of kraft pulping might be a source of some natural mediators, since they are generated from lignin. The use of such mediators decreases the cost of mediators [114, 129]. The main disadvantage of natural mediator is their effectiveness in pulp bleaching compared to synthetic mediators. Moldes et al. [119] tested some natural and synthetic mediators for pulp bleaching and reported syringaldehyde as the most promising natural mediator, even though its efficiency was lower than synthetic mediators [119]. Ozer et al. [120] utilized feruloyl esterase to release hydroxycinnamic acids (HA) from lignin subunits that acts as a natural mediator for laccase bleaching. TCF bleaching of eucalyptus kraft pulp was performed after treatment with LMS having ABTS (synthetic mediator) and HA (natural mediator). Feruloyl esterase, laccase-ABTS system, and feruloyl esterase + laccase reduced kappa number of kraft pulp by indicating 9, 18, and 30% delignification rate, respectively. Similarly, the highest ISO brightness improvement was also observed with sequential treatment by feruloyl esterase and laccase [120]. Sondhi et al. [117] carried out the production of thermo-alkali stable laccase from Bacillus tequilensis SN4 tested for biobleaching of softwood pulp. Laccase treatment reduced kappa number by 28% and ISO brightness improvement by 7.6% without mediator. LMS having N-hydroxybenzotriazole as mediator decreased kappa number by 47% and improved brightness by 12% compared to control [117]. Simultaneous action of xylanases and laccase may be a potential strategy to achieve a higher degree of delignification. Xylanases acts on hemicelluloses to expose

110

5 Biobleaching: An eco-friendly approach to reduce chemical consumption

the lignin and lignin is removed by the action of laccases. This results in an improved degree of delignification [93]. Xu et al. [123] performed biobleaching of wheat straw soda-AQ pulp by a combination of xylanase and laccase. For the synergetic effect of xylanase and laccase, the xylanase-laccase-extraction (XLE) sequence was found most effective compared with laccase-xylanase (LE) and laccase-extraction-xylanase (LEX) sequences. To attain a target ISO brightness level of 80%, XL pretreatment saved 28.6% of H2O2 requirement and improved the viscosity by 6.7% compared to TCF chemical bleaching. XL pretreatment also reduced refining energy consumption. XL treated TCF bleached pulp needed 1800 to 2000 PFI revolutions to reach 40 ° SR while TCF bleached pulp required 2800–3000 revolutions of PFI mill [123]. Dwivedi et al. [93] carried out co-cultivation of Penicillium oxalicum SAUE-3.510 and Pleurotus ostreatus MTCC 1804 under solid-state fermentation to produced xylanase and laccase that were used for biobleaching of mixed wood pulp. Enzyme treatment reduced kappa number by 21% while other parameters such as ISO brightness, yellowness, and viscosity of pulp were improved by 8, 3 and 5%, respectively, at enzyme dose (8IU g−1 of the mixed enzyme, xylanase/laccase = 22:1) and consistency of 10% [93]. Sharma et al. [130] evaluated the effectiveness of xylanase and laccase treatment for eucalyptus kraft pulp at the pilot scale experiment. Sequential treatment of xylanase followed by laccase minimized ClO2 requirement by 35%, significantly higher compared to xylanase (15%) and laccase (25%) treatments separately at a laboratory scale. At the pilot scale, sequential treatment of xylanase and laccase reduced AOX level and post colour number by 34 and 50%, respectively, while tear index improved by 15.71% [130].

5.4.4 Role of enzyme dose and reaction conditions in biobleaching The biobleaching efficiency depends on the type of enzyme, enzyme dose, reaction time, and the type of pulp. To attain the maximum effect of enzyme pretreatment on biobleaching, these conditions should be optimized. Several researchers have studied the effects of xylanase dose on the bleaching efficiency. It has been well proved that higher xylanase dosages might not improve the bleaching effect based on kappa number assay and ISO brightness, suggesting an appropriate dosage saves the costly enzyme preparation. Moreover, the higher xylanase dosage may decrease the bonding among fibers that may affect the physical strength properties of paper [62, 63, 131]. Xylanase pretreatment improved bleaching efficiency at enzyme dosages 2.5 to 40 IU/g of dry pulp depending on the source of enzyme and type of pulp (Table 5.2). Similarly, the determination of optimum treatment time (reaction time) is also necessary to save time for bleaching. The reaction time in the range of 30 to 360 min has been tested for xylanases from different sources and the pulp produced through different pulping methods (Table 5.2) [63, 132].

5.4 Biobleaching

111

The temperature during enzyme pretreatment is also a key factor for the effectiveness of enzyme. Unbleached pulp immediately after cooking is known as brownstock. Residual black liquor absorbed in brownstock has to be washed out by several stages with minimum temperature over 80 °C prior to the step of bleaching. The pH of this brownstock also remains in the alkaline range. Therefore, enzyme pretreatment before bleaching is desirable at a higher temperature and alkaline pH [40, 46]. Biotechnological operations at elevated temperature are also advantageous because it lowers the risk of contamination, improves the rate of reaction, reduces viscosity, and results in higher product yield. Therefore, bleaching is performed at higher temperatures by thermostable enzymes [27, 133]. The cooling of incoming pulp for optimum enzyme activity is an additional step that increases the operational cost of biobleaching [38]. As shown in Table 5.2, biobleaching has been performed at temperature 55 to 70 °C by thermotolerant xylanases from bacteria including Arthrobacter sp., B. circulans, B. pumilus, B. stearothermophilus, B. subtilis, S. cyaneus etc. Several fungal strains such as Aspergillus japonicas var aculeatus [54], Coprinellus disseminates SW-1 NTCC-1165 [21], A. niger [50] A. flavus [50], A. nidulans KK99 [53], Schizophyllum commune ARC11 [62], produced xylanases that were applied for biobleaching at 55 °C. T. lanuginosus SSBP xylanase was used at 60 °C for bleaching of nonwood pulps [67]. T. lanuginosus xylanase was applicable for biobleaching at 65 °C [68]. Talaromyces thermophilus was reported for thermotolerant xylanase production and xylanase treatment of pulp carried out at 70 °C [82]. Biobleaching of kadam kraft-AQ pulp by xylanases from disseminatus MLK-01 and C. disseminatus MLK-07 was performed at temperature 75 and 65 °C, respectively [132]. During enzymatic treatment of pulp, pH is another crucial parameter. Unbleached pulp after cooking and washing has an alkaline pH range. During biobleaching, the enzymes active at pH above 8.0 are considered suitable for several bleaching processes [38, 46]. Generally, most of the fungal enzymes are optimally active at acidic pH while bacterial enzymes show optimum activity in neutral to alkaline range. The application of acidic to neutral enzymes for pulp treatment requires the neutralization of pulp with acid to pH 6.0–7.0 for optimum activity of the enzyme. Neutralization pulp with acid is an additional step that will increase the process cost [38]. At the industrial scale, it is important to minimize the cost of the process, therefore, the use of enzymes having a broad range of operational pH and temperature is desirable. This makes large scale operations simple and costeffective [77, 134]. Most biobleaching studies with fungal enzymes have been performed at the pH range of 5.0 to 8.0 as indicated by Table 5.3. Most bacterial xylanases are reported for bleach-boosting efficiently at a pH range of 7.0 to 9.5 (Table 5.3).

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5 Biobleaching: An eco-friendly approach to reduce chemical consumption

Table 5.3: pH during pulp treatment by different fungal, bacterial and Actinomycetes xylanases. Fungi

T. reesei QM [] A. niger, Aspergillus niveus, Aspergillus ochraceus [] P. janczewskii [] P. meleagrinum var. viridiflavum [] T. viride [] Aspergillus caespitosus [] C. disseminates SH- NTCC [] T. lanuginosus SSBP [] A. flavus []

pH during pulp treatment . . . . . –. . . .

Acrophialophora nainiana Humicola grisea var. thermoidea [] T. lanuginosus [] T. lanuginosus MC []

.

P. corylophilum & T. longibrachiatum [] T. thermophilus [] C. disseminates MLK [] C. disseminates MLK [] A. nidulans KK []

. . . . .

A. niger []

.

. .

Bacteria and actinomycetes

B. pumilus ASH [] Streptomyces rameus L [] Bacillus sp. BP- [] B. coagulans []

pH during pulp treatment . . .  & .

Bacillus sp. XTR- [] S. griseorubens LH- [] Paenibacillus sp. S- []

. . .

B. pumilus AJK [] Geobacillus thermoleovorans [] Streptomyces sp. QG-- []

. .

B. pumilus SV-S [] Arthrobacter sp. MTCC  [] B. stearothermophilus SDX [] B. licheniformis [] B. subtilis, B. pumilus [] B. amyloliquifacien [] Paenibacillus campinasensis BL [] S. cyaneus SN [] B. stearothermophilus SDX []

. .

.

. . . . . .– .–.

5.4.5 Effect of enzyme pretreatment on kappa number, ISO brightness, brightness ceiling, reducing sugars and chromophores in pulp filtrate The effectiveness of enzyme treatment during bleaching is measured in terms of kappa number reduction, improvement in brightness, the release of reducing sugars, post colour number, and the release of chromophores in pulp filtrates. Kappa number is the indication of residual lignin or bleach ability of pulp and it decides bleach chemical requirement during pulp bleaching. Xylanase pretreatment of pulp results in partial degradation of xylan that facilitates the removal of lignin and chromophores associated with xylan. Therefore, xylanase pretreatment of pulp decreases

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kappa number and improves ISO brightness of pulp [46, 140]. Brightness of pulp increases due to oxidation and extraction of lignin-derived chromophores [47]. Hexenuronic acid (HexA) content of pulp also contributes to the kappa number of pulp. HexA interferes with the standard method of kappa number determination. The amount of KMnO4 consumed in acidic conditions provides an indirect measurement of lignin content in the pulp. KMnO4 reacts with a carbon-carbon double bond under acidic environment. Therefore, it can also react with HexA and results in false results regarding the quantification of lignin [141, 142]. The contribution of HexA for kappa number varies according to the type of wood. 10 µmol of HexA content corresponds to 1.05 kappa number (KN) units for birch kraft pulp [143], 0.9 KN units for Pinus radiata kraft pulp [144], 1.05 KN units for Eucalyptus globulus kraft pulp [141]. HexA can react with most electrophilic bleaching chemicals including Cl2, ClO2, O3, and per-acids while it reacts neither with O2 nor H2O2 in alkaline media. For this reason, ECF pulp contains less HexA than TCF pulp [141, 145]. Xylanases and laccases treatment reduces kappa number and HexA content of the pulp. Laccase treatment reduces the HexA content in the pulp due to oxidation of their double bonds by radicals formed during enzymatic stage [84, 141]. The effect of LMS and combination of LMS and xylanase was studied on HexA content of kenaf pulp. Laccase-HBT and (laccase-HBT)-xylanase treatment removed HexA content by 14 and 21%, respectively [84]. Enzymatic treatment increases the ISO brightness, the release of reducing sugars, and the release of chromophores in the bleach effluents. DNS (3, 5-dinitrosalicylic acid) method is commonly used for the estimation of the concentration of reducing sugars in enzyme pretreated filtrates. The quantity of the released chromophores is determined by measuring the filtrate absorbance at 237, 280, and 465 nm. Absorbance at a wavelength of 237 nm indicates the release of phenolic compounds or chromophores in pulp filtrate. The wavelength at 280 nm indicates the presence of lignin in the released coloured compounds. The release of hydrophobic compounds is analyzed by estimating the pulp filtrate absorbance at 465 nm. The release of chromophores by enzymatic treatment might be correlated with kappa number reduction. Xylanases attack lignin-carbohydrate complex and their action on precipitated hemicelluloses within pulp fiber releases the reducing sugars. Therefore, during xylanase biobleaching kappa number reduction, releases of reducing sugars, and chromophoric groups generation are interrelated phenomenon [17, 36, 62, 146]. Xylanase pretratment and extraction changes the reactivity of pulp by enabling a higher ClO2 substitution to achieve a target brightness and improves brightness ceiling of fully bleached pulps. The enzymatic treatment benefits are greater at higher brightness targets, especially near the brightness ceiling [147, 148]. Brightness reversion is another important parameter for fully bleached pulp. Brightness reversion of bleached pulp is undesirable process and it is measured in terms of post colour number (PC number). The higher carbonyl group content of indicates lower brightness stability and higher PC number [149]. Gangwar et al. [150] studied effect of xylanase pretreatment on ECF

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and chlorine-based bleaching of eucalyptus pulp. The xylanase pretreatment improved pulp brightness by 1.6 points while AOX and PC number were reduced by 32% and 48%, respectively, at xylanase dose of 0.5 kg/t of pulp. Biobleaching of kraft pulp by Aspergillus flavus xylanase showed the kappa number reduction by 35.93% with an enzyme dose of 20IU/g dry pulp. The absorbance at 237 nm was reached up to 0.229 from 0.057 while absorbance at 465 nm increased to 0.167 compared to control (0.035) [36]. Gautam et al. [62] performed the prebleaching of Eulaliopsis binata ethanol-soda pulp by Schizophyllum commune ARC-11 xylanases at enzyme dosages ranging from 4 to 14 IU/g of OD pulp. The maximum reduction in kappa number (14.51%) was observed at an enzyme dose of 10 IU/g of OD pulp compared to control. Further increase in enzyme did not increase the kappa number significantly. Similarly, maximum ISO brightness improvement (2.9 points) was recorded at enzyme dose of 10 IU/g of OD pulp. The same enzyme dose released reducing sugars up to 2.52 mg/g of pulp and increased up to 3.56 mg/g of pulp. The absorbances at wavelength 237, 280, and 465 nm were found to increase at enzyme dose of 10 IU/g of pulp while further increase in enzyme dosage did not affect the absorbance pattern significantly [62]. Another study [58] on biobleaching of Eucalyptus kraft pulp showed a reduction in kappa number by 35.03% using xylanase (25 U/g dry pulp) from Penicillum crustosum FP11. Xylanase treatment resulted in the release of reducing sugars up to 5.58 mg/g dry pulp compared to control (2.48 mg/g dry pulp. The amount of chromophores in terms of absorbance at 237 and 465 nm reached up to 0.215 and 0.122 compared to respective controls, 0.052 and 0.033 [58].

5.4.6 Reduction in chemical consumption and pollutants generation During the chemical pulping process, xylan chains are released and precipitate on the pulp fiber surfaces. This re-precipitated xylan acts as a barrier to bleaching chemicals and increases chemical consumption during subsequent bleaching steps [29, 151]. Xylanase treatment partly removes this re-precipitated xylan and opens more space for bleaching chemicals to enter the fiber. Xylanase treatment also decreases the hexenuronic acid (HexA) content in the pulp [33, 138]. HexA is well known to increase the kappa number of pulp and consumption of bleaching chemicals [136]. Xylanases hydrolyze re-precipitated xylan and reduce HexA, therefore xylanases treatment reduces the bleaching chemical consumption. The pretreatment of enzymes decreases the requirement of bleaching chemicals to obtain the same level of brightness and improved physical strength properties. The reduction in chemical consumption minimizes the generation of organochlorine compounds in bleach effluent [75]. Biobleaching of eucalyptus pulp with A. niger xylanases led to a reduction in chlorine charge by 20% with an acceptable brightness level [139]. Xylanase pretreatment of wheat straw pulp before ECF bleaching resulted in a 10% reduction in chlorine consumption while maintaining the ISO brightness and kappa

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number at the same level [40]. Table 5.2 shows that pretreatment of pulp with enzymes including xylanases, laccases, pectinases, and their combinations reduces the consumption of chlorine-based compounds from 10 to 42% during subsequent chemical bleaching. During TCF, biobleaching of wheat straw soda-AQ pulp was carried out by xylanase-laccase (XL) combination and XL pretreatment saved 28.6% of H2O2 requirement to obtain a target ISO brightness level of 80% [123]. The pollution generation by chemical bleaching or biobleaching sequences is estimated in terms of chemical oxygen demand (COD), biological oxygen demand (BOD), colour, and AOX generation. During bleaching studies, effluents from each bleaching sequence are collected and combined for the determination of COD, BOD, and AOX generation. Reduction in AOX generation by xylanases treatment is one of the most important benefits of biobleaching. Some 4-O-methylglucuronic acid groups available in hemicelluloses are known to be converted into corresponding unsaturated HexA during alkaline cooking of wood. It has been well proved that the generation of AOX during the pulp bleaching process has a close relationship with the HexA content of pulp [152–155]. Several researchers have indicated that HexA will consume ClO2 during the bleaching process. It is primarily the in-situ generated hypochlorous acid that reacts with HexA to form AOX [156, 157]. Therefore, the removal of HexA by xylanases action reduces the AOX generation. Moreover, when re-precipitated xylan and HexA are removed from the pulp fiber by xylanase treatment, the lignin could easily react with ClO2, and AOX generation decreases at the same dose of ClO2 [152]. During xylanase-aided ClO2 bleaching, xylanase treatment reduced HexA that resulted in a 21.4–26.6% reduction in AOX compared to control. ClO2 consumption was also reduced by 12.5–22% to obtain the same brightness [152]. Lal et al. [132] studied the biobleaching of kadam kraft-AQ pulp by xylanases from two different strains of C. disseminatus. At 4% chlorine demand, C. disseminatus MLK-01 and C. disseminatus MLK-07 reduced AOX generation by 19.51 and 42.77%, respectively, with an increase in COD and colour due to removal of lignin carbohydrates complexes [132]. Several studies on laccase pretreatment of pulp showed reduced chlorine consumption in subsequent chemical bleaching and reduced AOX generation in bleach effluent. Laccases assist the removal of lignin from pulp and hence reduce ClO2 consumption in the subsequent bleaching process. The mitigation in the AOX level can be explained by a reduction in the quantity of total chlorine atoms applied when laccases are used for before chemical bleaching [81]. Biobleaching of eucalyptus kraft pulp by laccase from G. lucidum RCK 2011 showed a 25% reduction in ClO2 consumption to achieve the same ISO brightness as chemical bleaching (control). Laccasebased bleaching of eucalyptus kraft pulp reduced AOX generation in bleach effluents by 20% at reduced ClO2 demand [81]. Xylanase pretreatment of pulp before chemical bleaching increases COD, BOD, and colour of bleach effluent [21, 33, 132, 158]. BOD of bleach filtrate was increased two times after xylanase pretreatment of softwood pulp. Similarly, COD and total organic carbon (TOC) in bleach filtrate were also increased. But, bleach effluents

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generated by xylanase biobleaching were more biodegradable, as indicated by a higher ratio of BOD to COD [159]. Singh et al. [21] studied bleach effluent characteristics after bio-conventional bleaching of wheat straw soda-AQ pulp and reported 10.91 and 38.0% increase in COD and colour, respectively. Borges et al. [158] performed xylanase prebleaching of Eucalyptus kraft pulp and analyzed the quality of effluent. Enzymatic prebleaching stage resulted in a substantial increase in COD and the colour of bleach filtrate. At enzyme dose of 25 g adt−1 and pH 7.0, the COD level was three times higher than control while colour was increased more than two times compared to control. Moreover, the COD level reached five times higher than control at an enzyme dose of 200 g adt−1 [158]. Xylanase action weakens carbohydrates bonds in pulp and hydrolyzes xylan. Because of this, lignin concentration and hydrolyzed xylan increase in the effluent, leading to increase in COD, BOD and colour [21].

5.4.7 Effect of biobleaching physical strength properties of paper The cellulose chain length is indicated in terms of pulp viscosity [36]. Xylanase pretreatment before chemical bleaching either maintains the pulp viscosity at the same level or improves pulp viscosity compared to chemical bleaching. The improvement in viscosity might be due to the removal of a lower degree of polymerized xylan and enrichment of high molecular weight polysaccharide. Xylan with a lower degree of polymerization than cellulose is expected to lower the viscosity of holocellulose [21, 28, 160]. Xylanases assists in the removal of lignin-associated hemicelluloses with minimum damage to cellulose [34]. Several researchers observed improvement in viscosity of different kinds of pulp by xylanases treatment [21, 25, 38, 45, 46, 161] (Table 5.2). The presence of endoglucanase in xylanase enzyme preparation can degrade cellulose chains to decrease the viscosity and physical strength properties of pulp. Therefore, cellulase-free xylanases are for essential biobleaching purposes to maintain the physical strength properties of pulp [38]. The selection of enzyme preparation, enzyme dose, and reaction time are closely related to physical strength properties such as tensile index, burst index, double-fold number, and tear index. One group of researchers showed that xylanase pretreatment of pulp improves the tear index and reduces tensile and burst index [21, 52, 68]. The tear strength of a sheet is a complicated combination of fiber average length, fiber strength, and fiber bonding. Tensile and burst index are influenced by both fiber average length and level of fiber bonding [21, 52]. Xylanase treatment removes xylan (low molecular weight) from pulp and increases the average molecular weight of the polymer system [21]. Xylanase pretreated wheat straw pulp shows longer fiber average length and lower fine content compared to control. Both of these factors seem to be disadvantageous for fiber bonding [52]. Therefore, tear index improves and other properties

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117

such as burst index and tensile index that depends upon hydrogen bonding decreases due to depolymerisation of xylan [21]. The reprecipitated xylan is thought to have a positive impact on interfiber bonding. Thus, the removal of hemicelluloses is not expected to cause an improvement in bonding properties of paper [68]. Another group of researchers reported improvement in most physical strength properties including tensile index, burst index, breaking length, and tear index during xylanase biobleaching [43, 45, 78]. Beg et al. [73] and Ninawe et al. [71] also observed improvement in tensile and burst strength of enzyme pretreated pulp. Several reports showing the effect of enzyme treatment on physical strength properties have been listed in Table 5.2.

5.5 Bleaching by microbial treatment Although biobleaching of pulp is feasible by potent biocatalysts like xylanases and ligninases, at commercial-scale production and application of these enzymes is a cumbersome process for the long run. Moreover, enzymes such as laccase require mediators to improve its efficiency and also suffer from stability problems. Therefore, a cost-effective strategy of direct biobleaching by growing microorganisms on pulp under mild treatment conditions has been tested [35, 162, 163]. Several researchers have performed direct microbial treatment for biobleaching of pulp. During microbial treatment, microorganism grows in pulp suspension and produces enzymes such as xylanases, LiPs, MnPs, and laccases. These enzymes act on hemicelluloses and lignin and show biobleaching effects as described in previous sections. For microbial growth, pulp suspension acts as a substrate, and additional nutrients such as carbon sources and nitrogen sources are also added for the maximization of enzyme production. Several studies have reported the pulp treatment by white-rot fungi and bacteria for biobleaching purposes [35, 146]. Lignin degradation by different fungal species or strains depends on the monomer composition of this complex structure. The search for new fungi with different enzymatic capabilities is going on from diverse habitats that vary both quantitatively and qualitatively in substrate affinity. Such fungal strain may be more effective for biopulping and biobleaching purposes [95, 164]. White-rot fungi such as P. chrysosporium [165, 166], C. subvermispora [167], Coprinellus disseminates [146], T. versicolor [168], Irpex lacteus [95], Phlebiopsis sp [95]., Thelephora sp [169]. and bacteria like Bacillus halodurans [35] have been used for direct biobleaching of pulp. Lal et al. [146] performed biobleaching of Anthocephalus cadamba kraft pulp through direct fungal treatment by FEQP (F-fungal treatment, E-alkaline extraction, Q-chelating stage, P-peroxide stage) sequence. F-stage mitigated kappa number by 55% and improved ISO brightness and viscosity by 17.3 and 7.63%, respectively. Kappa number reduction and brightness improvement were 22.1. and 6.3% higher in F-stage compared to enzymatic pretreatment [146]. Sharma et al. [35] applied xylanase producing Bacillus halodurans FNP135 for biobleaching of eucalyptus kraft pulp through solid-state fermentation (SSF) and submerged fermentation (SmF). SmF was

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found more effective and resulted in 35% reduction in kappa number along with 5.8, 8.7, 13.7, 20.7, and 8.6% improvement in ISO brightness, breaking length, burst factor, tear factor and pulp viscosity, respectively [35]. Selvam et al. [169] tested the potential of white-rot fungus, Thelephora sp. for delignification and bleaching of E. grandis wood chips and hard wood kraft pulp. Fungal pretreatment resulted in 25% reduction in kappa number and 2.0 ISO points improvement in brightness along with 50% reduction in chemical consumption [169].

5.6 Limitations and future perspectives of biobleaching The use of enzymes before chemical bleaching is an eco-friendly approach that minimizes the consumption of chlorine-based compounds and reduces the generation of pollutants. Byproducts generated from biochemical reactions of microorganisms are generally nonhazardous. Even though enzymes have shown potential for bleaching of pulp but their industrial applications still have some challenges. Enzyme treatment conditions such as temperature and pH are the limiting factors. At the industrial scale, to make the bleaching process simple and cost-effective, enzymes should be active in a broad range of pH and temperature. Although several bacterial and fungal xylanases active in alkaline pH range and higher temperatures are available, still there is a requirement for further innovative approaches for the screening of such enzymes producing strains [33, 77]. Although xylanase bleaching is commercialized all over the world but due to its limited benefits, laccase-xylanase aided bleaching is gaining more attention. However, a lot of research on enzyme production and process optimization is required to make it applicable at the industrial scale. Laccases not only proved effective for biobleaching but also have shown some limitations such as availability of efficient and stable enzymes for industrial conditions [119]. Laccases also have a low affinity for its substrate due to lower diffusion of the enzyme into pulp fibers. Although the use of synthetic mediator molecules resolves these problems, the cost of mediator molecules is very high. Some natural mediators have been tested to replace the synthetic mediator molecules but their efficiency is not comparable with synthetic mediator molecules. Therefore, further research is required to find natural, efficient, and cost-effective mediator molecules that can be used with laccases.

5.7 Conclusion Conventional chlorine-based bleaching of pulp generates highly toxic and organically bound compounds known as AOX. Some of them are harmful not only to human health but also for the environment. Biobleaching has shown tremendous potential to minimize the consumption of chlorine-based compounds during bleaching. Xylanase, pectinase, and ligninolytic enzymes have been proved safe biobleaching agents that

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achieve a higher brightness (ISO), lower kappa number, and improved paper properties. The generation of pollutants is also reduced by pre-treatment of pulp with enzymes.

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[143] Vuorinen T, Fagerström P, Buchert J, Tenkanen M, Teleman A. Selective hydrolysis of hexenuronic acid groups and its application in ECF and TCF bleaching of kraft pulps. J Pulp Pap Sci. 1999;25:155–62. [144] Allison RW, Timonen O, McGrouther KG, Suckling ID. Hexenuronic acid in kraft pulps from radiata pine. Appita J. 1999;52:448–52. [145] Chai X-S, Zhu J, Luo Q, Yoon S-H. The fate of hexenuronic acid groups during kraft pulping of hardwoods. J Pulp Paper Sci. 2001;27:1–5. [146] Lal M, Dutt D, Kumar A, Gautam A. Bio-bleaching of Anthocephalus cadamba kraft pulp through direct fungal treatment by FEQP sequence. Br Biotechnol J. 2015;8:1–13. [147] Manji AH. Extended usage of xylanase enzyme to enhance the bleaching of softwood kraft pulp. Tappi J. 2006;5:23–6. [148] Bajpai P. Biobleaching. Bajpai P. In: Biotechnology for pulp and paper processing, USA: Springer, 2018:159–213. [149] Zhou Z, Jääskeläinen A-S, Adorjan I, Potthast A, Kosma P, Vuorinen T. Brightness reversion of eucalyptus kraft pulp: effect of carbonyl groups generated by hypochlorous acid oxidation. Holzforschung. 2011;65:289–94. [150] Gangwar AK, Prakash NT, Prakash R. An eco-friendly approach: incorporating a xylanase stage at various places in ECF and chlorine-based bleaching of eucalyptus pulp. BioResources. 2016;11:5381–8. [151] Boruah P, Sarmah P, Das PK, Goswami T. Exploring the lignolytic potential of a new laccase producing strain Kocuria sp. PBS-1 and its application in bamboo pulp bleaching. Int Biodeterior Biodegradation. 2019;143:104726. [152] Nie S, Wang S, Qin C, Yao S, Ebonka JF, Song X, Li K. Removal of hexenuronic acid by xylanase to reduce adsorbable organic halides formation in chlorine dioxide bleaching of bagasse pulp. Bioresour Technol. 2015;196:413–7. [153] Björklund M, Germgard U, Jour P, Forsström A. AOX formation in ECF bleaching at different kappa numbers-influence of oxygen delignification and hexenuronic acid content. In: Pulping conference. 2002:20–4. [154] Björklund M, Germgard U, Basta J. Formation of AOX and OCI in ECF bleaching of birch pulp. TAPPI J. 2004;3:7–12. [155] Dai Y, Song X, Gao C, He S, Nie S, Qin C. Xylanase-aided chlorine dioxide bleaching of bagasse pulp to reduce AOX formation. BioResources. 2016;11:3204–14. [156] Torngren A, Ragnar M. Hexenuronic acid reactions in chlorine dioxide bleaching-aspects on in situ formation of molecular chlorine. Nord Pulp Pap Res J. 2002;17:179–82. [157] Ventorim G, Colodette J, Gomes A, da Silva L. Kinetics of lignin and HexA reactions with chlorine dioxide, ozone, and sulfuric acid. Wood Fiber Sci. 2008;40:190–201. [158] Borges MT, Silva CM, Colodette JL, Alves LB, Rodrigues GR, Lana LC, Tesser F. Effect of eucalyptus kraft pulp enzyme bleaching on effluent quality and bio-treatability. Pulp Pap Canada. 2010;111:T 187. [159] Senior DJ, Hamilton J. Use of xylanases for the reduction of AOX in kraft pulp bleaching. In: CPPA environmental conference. Quebec, Canada, 1991:310–4. [160] Kantelinen A, Hortling B, Sundquist J, Linko M, Viikari L. Proposed mechanism of the enzymatic bleaching of kraft pulp with xylanases. Holzforschung. 1993;47:318–24. [161] Choudhury B, Chauhan S, Singh SN, Ghosh P. Production of xylanase of Bacillus coagulans and its bleaching potential. World J Microbiol Biotechnol. 2006;22:283–8. [162] Singh G, Ahuja N, Batish M, Capalash N, Sharma P. Biobleaching of wheat straw-rich soda pulp with alkalophilic laccase from γ-proteobacterium JB: optimization of process parameters using response surface methodology. Bioresour Technol. 2008;99:7472–9.

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[163] Sharma P, Goel R, Capalash N. Bacterial laccases. World J Microbiol Biotechnol. 2007;23:823–32. [164] Daâssi D, Zouari-Mechichi H, Belbahri L, Barriuso J, Martínez MJ, Nasri M, Mechichi T. Phylogenetic and metabolic diversity of Tunisian forest wood-degrading fungi: a wealth of novelties and opportunities for biotechnology. 3 Biotech. 2016;6:46. [165] Kirk TK, Yang HH. Partial delignification of unbleached kraft pulp with ligninolytic fungi. Biotechnol Lett. 1979;1:347–52. [166] Nezamoleslami A, Suzuki K, Nishida T, Ueno T. Biobleaching of kenaf bast fiber, soda–AQ pulp using white-rot fungus. Tappi J. 1998;81:179–83. [167] Christov LP, Akhtar M, Prior BA. Impact of xylanase and fungal pretreatment on alkali solubility and brightness of dissolving pulp. Holzforschung. 1996;50:579–82. [168] Addleman K, Archibald F. Kraft Pulp Bleaching and Delignification by Dikaryons and Monokaryons of Trametes versicolor. Appl Environ Microbiol. 1993;59:266–73. [169] Selvam K, Shanmuga Priya M, Arungandhi K. Pretreatment of wood chips and pulps with Thelephora sp. to reduce chemical consumption in paper industries. Int J ChemTech Res. 2011;3:471–6.

Varun Kumar, Puneet Pathak, Nirmal Sudhir Kumar Harsh and Nishi Kant Bhardwaj

6 Biodeinking: an eco-friendly alternative for chemicals based recycled fiber processing Abstract: Recycling of recovered paper is an inevitable process for saving resources and the environment. Due to strict forest conservation regulations and limitations of the agro-forestry sector, the paper industry is facing the woody fiber crisis for decades. The recycling of waste paper for its utilization as a source of cellulosic fibers for papermaking is a resource-saving and eco-friendly approach and is a need of time. Deinking is an important stage in the recycling of recovered paper. In the conventional deinking process, chemicals have been used for removal of inks and other impurities from waste paper pulp slurry with some certain drawbacks like deinking inefficiency, fiber damage and generation of chemicals and fiber-rich effluent. The application of enzymes for deinking purposes is known as biodeinking and is considered as the potent and environmentally friendly deinking approach. The present write-up provides comprehensive information on various aspects of biodeinking. Keywords: biodeinking, waste papers, cellulase, xylanase, lipase, amylase, cutinase

6.1 Introduction The pulp and paper industry is confronting a severe shortage of cellulosic raw materials throughout the world. Recovered or waste paper has been proved as an admirable source of cellulosic fibers. The use of different types of waste papers for papermaking is increasing each year but the efficient utilization of waste papers is a need of present scenario. There are several obstacles in the proficient utilization of waste paper such as inks, stickies, gums, stapler pins and various types of coating and fillers on the paper. Among these contaminants, removal of ink is considered as a difficult task due to a variety of inks and their bonding with the paper surfaces [1]. The elimination of ink specks from the surface of pulp fiber is called deinking and two types of deinking technologies e.g. conventional deinking and biodeinking, are being used. Conventional deinking or chemical deinking is efficient to remove some types of inks such as water-based inks and lipid-based inks but incapable

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, V., Pathak, P., Harsh, N. S. K., Bhardwaj, N. K. Biodeinking: an eco-friendly alternative for chemicals based recycled fiber processing Physical Sciences Reviews [Online] 2021, 6. DOI: 10.1515/ psr-2019-0045 https://doi.org/10.1515/9783110592412-006

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of the effective removal of the fused inks (toners). In addition to this, chemical deinking also has some other drawbacks such as pulp yield loss, strength loss of paper and high chemical oxygen demand (COD) and biological oxygen demand (BOD) load of process water. Biodeinking deals with microbial enzymes and these enzymes are novel solutions to overcome these problems. They are capable to remove hard to deink toners from waste papers, maintain optical and strength properties of paper and reduced COD and BOD load of process water.

6.2 Waste paper: an excellent raw material As the shortage of woody fibers and due to certain constraints in application of non-woody bamboo and agro-residues and strict environmental policies, waste papers are considered as a significant source of cellulosic fibers for pulp and paper production and re-utilization of these waste papers for pulp and paper making is called “waste paper recycling”. Waste papers are also known as recovered, recycled or secondary fiber. Cardboard packaging containers (Kraft and corrugated grades), mixed office waste (MOW) papers, old newspapers (ONP) and old magazines (OMG) are the main types of waste papers and all these have different utilization degrees. The utilization rates of cardboard packaging containers and ONP are 93.9% and 92.8%, respectively, followed by MOW (12.8%) [2]. Industries and business houses are the major producers of paper wastes and have a 52% contribution to the total waste paper collection. Around 38% of total waste papers are generated by household applications sector and unsold OMG and ONP are produced around 10% of the total waste papers generation [3, 4]. Waste papers are a good source of cellulose-rich fibers and it contains 60–70% cellulose, 10–20% hemicelluloses and 5–10% lignin [5]. According to a report of the year 2018 of the United Nations’ Food and Agricultural Organization, approximately 235 million tons of waste papers were consumed globally in 2017. Japan has the maximum waste paper recovery rate (80%) followed by Europe (over 70%) and America (70%) in the year 2015 (State of The Global Paper Industry 2018 – Report). At the global level, the average waste paper recycling rate is around 57.9% with a maximum (85%) in Australia and lowest (61%) in Africa [6, State of The Global Paper Industry 2018 – Report]. According to the Confederation of European Paper Industries (CEPI), there are major five grades of waste papers namely low qualities, medium qualities, high qualities, Kraft qualities and special qualities. Among all grades, special qualities grade is difficult to recycle causing some specific limitations to the process. Waste papers are one of the major components responsible for the generation of solid waste affecting the environment and social life. The recycling of waste papers has certain benefits for preserving wood resources, saving energy and maintaining the environment. Recycling of one-ton waste paper saves at least 30,000

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liters of water and 3,000–4,000 kWh of electrical energy along with 95%reduction in air pollution. It also saves around 2.3 m3 volume used for landfills and reduces the disposal problem associated with waste paper [1]. Thus, the recycling of waste paper is also benefitted to reduce costs involved in capital items, operation of processes, raw material, energy and chemicals, along with reduced pollution load in terms of lower BOD, effluent volume, odor and CO2 emissions. Reduction of solid waste also helps to improve municipal landfill life, to reduce disposal fees for landfilling [7–11].

6.3 Deinking of waste papers Although waste papers are a significant source of alternate raw material for the paper industry and its utilization is expected to increase in the future, its use as a raw material has certain limitations due to the presence of various types of contamination in the waste papers. These contaminants include inks, stickies, gums, stapler pins, various types of coating and fillers on the paper. The removal of these contaminations is the main obstacle in the reuse of the waste papers. Among these contaminants, it is difficult to remove inks due to a variety of inks and their bonding with the paper surface [1]. Various types of inks having different compositions are transferred to paper by absorption, fusion with the cellulosic fiber during printing and writing. After drying and cooling, this ink makes bonds to cellulose fiber. During the deinking process, these ink particles are separated from fibre surface by the pulping process and then removed by washing or flotation steps to get the brighter pulp with minimum or no contaminants. The removal efficiency of ink particles also depends on the types of ink, ink composition, printing process, fiber type, ink thickness, absorption or penetration depth, ink particle size and printing age [8, 12–21]. Similar to a laundry process, the washing process also requires a surfactant for dispersing and washing the ink particles detached from the separated fibers. Surfactant helps in the formation of hydrophilic bonds between ink particles and water [22, 23]. Flotation is a process involving bubble–particle adhesion (or attachment) dependent on the surface chemistry, hydrodynamics and various process parameters of the flotation cell [24, 25]. The efficiency of the floatation process is also dependent on the hydrophobicity of ink particles, the bubble–particle size ratio and turbulence generated in the pulp suspension [26]. The flotation process is selective towards ink particle removal. It also maintains higher pulp yield, and easy in the handling of water & residues [27]. Traditionally, chemicals are being used for ink removal in the deinking process; this is called conventional deinking. Nowadays enzymes are emerging as a powerful tool for the deinking process and this is termed as biodeinking.

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6.4 Conventional deinking Conventional deinking is accomplished by the conversion of the waste paper to a pulp in alkaline conditions containing deinking chemicals (caustic soda, sodium silicate and surfactant) for ink removal. The mechanical shear forces during pulping in alkaline pH of pulp suspension are responsible for the partial removal of ink from the fiber surface due to the fragmentation of ink particles in different sizes [28]. In conventional deinking technology, chemicals are efficient for ink removal but some kinds of inks such as flexographic, UV, varnish, electro and toner inks are hard to deink by conventional chemicals [29]. Uncoated papers printed with copier and laser-printed toner are difficult to remove by the conventional alkaline deinking process because these ink contain thermoplastic binders that polymerize and thermally fused to the surface of the printed page and because they are nylonbased polymers that do not disperse, usually remain as large flat rigid particles that are very difficult to separate in the separation stage from fiber stocks. This is unfortunate because office copy paper is made of high-value bleached chemical pulp [19, 30, 31]. Fiber swelling caused by the chemicals also avoids the partial removal of toners and the release of fines from the fiber surface. Removal of fines is essential for detachment of toner particles because they remain attached to them. For removal of toners, a huge amount of chemicals, water and heating energy required and finally process is not cost-effective [19, 32–34]. Sodium hydroxide causes yellowness in pulp and a sufficient amount of bleaching chemicals required. It also causes fiber swelling which increases water uptake by fiber and hindrance in drainage. The chemicals adversely affect the strength properties of fibers. Tear index decreases after the chemical treatment of recycled fiber [35–38]. Large sticky contaminants are fragmented in small particles by the chemicals and washed out with the process water, thus increasing COD in process water and due to alkalinity acid consumption is increased for the neutralization [39]. Additional chemicals and multiple flotation steps make the process capital and energy-intensive. Nowadays recycling paper mills are also facing a problem of “Summer effect” primarily using ONP. In the summer months, there is a significant loss in the pulp brightness and it is called “Summer effect”. In this effect, the oxidation and polymerization occur in the ink particles of the waste papers at elevated temperature during transportation in closed containers [40]. The polymerized and oxidized inks are hard to fragmentation by chemical (alkaline) deinking, difficult to remove by the flotation process and the final pulp has decreased brightness and cleanliness [41, 42]. It is established that the use of chemicals in the deinking process consumes more energy and water and leads to environmental pollution due to the potential toxicity of chemicals.

6.6 Microorganisms used for the production of the deinking enzymes

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6.5 Biodeinking Biotechnology has capabilities to provide very efficient and environmentally friendly ways to overcome the problems related to waste paper deinking. Enzymes are a novel solution to overcome these problems with minimum impact on the environment. The use of enzymes instead of chemicals to remove ink from the waste paper fibers is called biodeinking or enzymatic deinking. The cellulases, lipases, xylanases, laccases and amylases in deinking of recovered papers are the highest studied enzymes and have the potential for application in the pulp and paper industry [17, 41, 43–49]. Cellulase enzymes are found to be more effective towards the deinking of MOW and non-impact printed waste paper [17, 43, 50]. Enzymatic deinking is reported to improve the ink removal, stickies removal, drainage and runnability. Enzyme treatments also decrease the pollution load by decreasing COD and BOD of process water and wastewater. It also avoids alkaline pH as required in conventional deinking helping to decrease the chemical costs [51–53]. Enzymes play a significant role in the improvement of pulp and paper properties. They provide better pulp properties like freeness and better strength and optical properties of the paper than the conventional deinking. Thus, biodeinking is used as an alternative to the use of chemical products at the disintegration stage. This technology uses an almost neutral medium and is comparatively more suitable for the maintenance of the environment [18].

6.6 Microorganisms used for the production of the deinking enzymes Microbes such as bacteria and fungi are the versatile groups of organisms and well known for the production of intracellular and extracellular enzymes. In biodeinking generally, extracellular enzymes used and fungi are the excellent producers of extracellular enzymes. A summary of microorganisms used for the production of deinking enzymes is presented in Table 6.1.

Table 6.1: Summary of the microorganisms used for deinking enzyme production. Microorganism

Major enzymes

Reference

Aspergillus niger Aspergillus sp. AMA Aspergillus terreus

Cellulase and hemicellulase Multiple endoglucanases Multiple endoglucanases

[, ] [] []

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Table 6.1 (continued ) Microorganism

Major enzymes

Reference

Aspergillus L Aspergillus terreus CCM  Cellulomonas fimi Clostridium thermocellum Coprinus cinereus  Fomes lividus Fusarium sp. Gloeophyllum sepiarium Gloeophyllum trabeum Humicola insolens Humicola insolens Myceliophthora fergusii Orpinomyces PC- Phanerochaete chrysosporium Pseudomonas aeruginosa Rhizopus oryzae Sclerotium rolfsii Thelephora sp Thermomyces lanuginosus Trametes versicolor Trichoderma reesei Trichoderma pseudokoningiiS Trichoderma viride CCM  Trichoderma sps. Trichoderma viride Vibrio alginolyticus

Cellulases Glycanases Endoglucanases Cellobiohydrolases Alkalophilic enzymes Cellulases, Hemicellulase Alkaline active cellulases Endoglucanases Endoglucanases Cellulase complex Endoglucanases Multiple endoglucanases EndoglucanasesCel B and Cel E Lignolytic enzymes Lipase Lipase Mannanase Cellulases, Hemicellulases Xylanase Cellulases, Hemicellulases Mixture of cellulases and xylanases Cellulases

[] [] [] [] [] [] [] [] [] Jeffries et al. [, , ] [, , ] [] Geng et al. [) [] [] [] [, ] [] [, ] [] [] []

Glycanases Cellulases Cellulases Direct use of bacteria

[] [] [] []

Table 6.2: Summary of some related research on biodeinking of different types of waste papers. Type of waste papers

Treatment

Optical properties

Physical properties

Reference

Old newspaper

Xylanase followed by laccase mediator system

Brightness (%]: . ERIC (ppm): .

Tensile index – TI [N.m/g): .

[]

Old newspaper

Xylanase and laccase mediator system

Brightness (%]: . ERIC (ppm): 

TI [N.m/g): .

[]

6.7 Mechanism of biodeinking

135

Table 6.2 (continued ) Type of waste papers

Treatment

Optical properties

Physical properties

Inkjet-printedpaper

Amylase and lipase

Brightness (%]: . ERIC (ppm): 

TI (N.m/g): . Burst index – BI (kPa.m/g): . Tear index – TrI [mN.m/g): .

[]

Laser printed paper

Hemicellulase and cellulase

Brightness (%]: 

TI (N.m/g):  BI (kPa m/g): . TrI [mN.m/g): .

[]

Old newspaper

Hemicellulase and cellulase

Brightness (%]: 

TI (N.m/g):  BI (kPa.m/g): . TrI[mN.m/g): .

[]

Mixed office waste paper

Cellulase and xylanase

Brightness (%]: . ERIC (ppm): 

TI (N.m/g): . BF (kPa.m/g): . TrI [mN.m/g): .

[]

Photocopier wastepaper

Crude cellulase and xylanase

Brightness (%]: 

TI (N.m/g): . BI (kPa.m/g): . TrI [mN.m/g): .

[]

Old newspaper

Laccase and xylanase

Brightness (%]: .

TI (N.m/g): . BI (kPa.m/g): . Tear Factor – TF [mN.m/g): .

[]

Old newspaper

Cellulase and xylanase

Brightness (%]:  ERIC [ppm): .

–

[]

Old newspaper

Cellulase-xylanase system Xylanase and pectinase

Brightness (%]: .

TF [mN.m/g): .

[]

Brightness (%]: . ERIC (ppm): 

TI (N.m/g): . BI (kPa.m/g):  TF [mN.m/g): 

[]

Old newspaper

Reference

6.7 Mechanism of biodeinking Enzymes are proteinaceous macromolecules that are specific towards reaction with substrates by the attachment through random diffusion and specific molecular interaction. Ionic charges, hydrophobicity and hydrophilicity of enzyme assist its attachment to the specific substrates. Binding domains and catalytic sites facilitate the degradation of polymeric chains present in the substrate [77].

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Cellulolytic system of microorganisms has three types of components based on their specific substrate. Endo-1,4-ß-glucanases (EG), Exo-1,4-β-glucanase (Exo-1,4β-D-glucan, cellobiohydrolases (CBH) and 1,4-β–glucosidases. Endo- 1, 4-ß-glucanases cleave randomly at 1,4-β-linkages within the cellulose chain. Exo-1,4-β-glucanase release cellobiose from free chain ends (non-reducing ends) of cellulose and the third enzyme, 1,4-β-glucosidases are responsible for the hydrolysis of cellobiose to its monomeric unit glucose, and cellobionic acid is hydrolyzed to glucose and gluconolactone [78]. Cellulases cause hydrolysis, superficial degradation and depolymerization of cellulose, resulting in removal of ink from fibers [42, 43, 51, 79]. During the deinking process, the random action of the endoglucanase component splits the cellulosic fibers at numerous amorphous sites and as a result, several non-reducing ends of the chains are generated and loosening of fibers facilitates the release of ink particles. Short fibers are also released from the surface of cellulose fibers due to the action of cellulases (endoglucanase) at the frazzled portions. This endoglucanase has also a short fiber-forming activity to remove residual fibers from the toner surfaces and make them hydrophobic to improve flotation efficiency [41, 43]. Hemicellulase system contains two main components e.g. endo 1, 4-β-D-xylanase and endo 1, 4-β-D-mannanase which take part in depolymerization of hemicelluloses. Hemicellulase destroys the complex lignin-cellulose by hydrolyzing to xylan to loosen the fibers and release short fibers to aid in deinking [38, 42]. Lipases are known to hydrolyze different types of esters, especially long-chain triacylglycerols, to produce free fatty acids, di-and mono-acylglycerols, and glycerol as degradation products by catalyzing the reverse reaction, esterification, inter-esterification, trans-esterification acidolysis, alcoholysis, aminolysis, oximolysis and thiotransesterification in anhydrous organic solvents [80], biphasic systems [81] and micellar solution [82, 83] with chiral specificity. Lipases are used to degrade oil-based carriers in ink used in newsprint printing and to break down pigments and resins present in toner [12, 49]. Laccase (benzenediol: O2 oxidoreductase) is an extracellular glycoprotein enzyme with an average molecular weight of ≈ 70,000 and mostly expressed by whiterot fungi. Laccase is a multi-copper (generally four copper ions) oxidase responsible for the degradation of lignin present in the lignocellulosic materials. Laccase is a phenol oxidase with broad specificities for aromatic compounds containing amine and hydroxyl groups. It oxidizes phenol and phenolic substructures into phenoxy radicals by one electron abstraction that can lead to either depolymerization or repolymerization of the substrate [84, 85]. Laccase also shows some potential for effective deinking of ONP waste by selectively removing surface lignin [64]. Amylases are starch-hydrolyzing enzymes, which hydrolyze starch molecules to give diverse products including dextrins, and smaller polymers composed of glucose units [86]. The α-amylase family comprises a group of enzymes that all act on

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137

the substrate containing glucose residues linked through an α-1-1, α-1-4, α-1-6, glycosidic bonds [87]. They hydrolyze the starch present in paper coating of coated waste papers and thus facilitate ink removal from fiber surface [12]. The possible mechanisms of the different enzymes for the deinking of waste papers by endo 1, 4β-D- glucanase, amylase, lipase is shown in Figure 6.1 [88]

Figure 6.1: Schematic diagram showing possible mechanisms of the enzymes for the biodeinking of waste papers by (1) endo 1, 4-β-D- glucanase; (2) amylase; [3) lipase [adapted from 88].

Cutinase (3.1.1.74] enzyme of α/β hydrolase family is produced fungi and several bacteria. This enzyme causes the hydrolysis of the ester bonds present in cutin of plant surfaces. In recent years, the cutinase from different fungi like Alternaria brassicicola, A. niger, Fusarium solani pisi, F. oxysporum, Monilinia fructicola, Sirococcus conigenus,

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6 Biodeinking: an eco-friendly alternative for chemical deinking

Trichoderma reesei, and Thielavia terrestris were studied [89]. Cutinase enzymes generally do not display interfacial activation or display little. Cutinases work on watersoluble esters like p-nitrophenyl esters and insoluble triglycerides, preferably shortchain substrates. Cutinases have the capacity to degrade aliphatic and aromatic polyesters [90] like polyethylene terephthalate (PET) [91] and polyester polyethylene furanoate (PEF) [92] by Thermobifida cellulosilytica hydrolyzes; polycaprolactone (PCL) and polybutylene succinate (PBS) by Thielavia terrestris [93]; polybutylene succinate (PBS) [94], polylactic acid (PLA) [95] or deacetylation of polyvinyl acetate (PVAc) [96] by F. solani, A. oryzae, A. brassicicola, and Humicola insolens. Cutinases are also reported to catalyze the esterification (between acids & alcohols) and transesterification reactions (between fatty salts and alcohols) [97]. Various type of adhesives used in papermaking have ester bonds containing polyvinyl acetate and polyacrylate as binders in synthetic toner or ink [98]. The cutinase and lipase enzymes both have similar types of catalytic activities. Lipase causes hydrolysis of vegetable oil-based inks converting water-insoluble triglycerides into di- and monoglycerides and glycerol. Cutinases also degrade polyester binders, triglycerides and esters which can be helpful in the deinking of waste papers (Figure 6.2) [89, 93]. Cutinase has better stability than lipase in combination with other enzymes [89].

Figure 6.2: Deinking mechanism diagram of cutinase [Based on 89].

Enzymes are multirole biological agents and they are not only facilitating ink removal from waste paper fibers but also positively affect the various properties of pulp and paper. The enzyme treatment enhanced the pulp drainability [46], fiber swelling [99], freeness of the pulp [66, 100] and optical and strength properties of the paper [43, 44, 46, 59, 66, 101]. Better paper machine runnability also reported by [102], in the case of enzymatically deinked pulp, without a drop in product quality with improved strength properties of paper.

6.8 Effectiveness of various enzymes towards the deinking process Firstly [44], used the enzyme for the deinking of ONP with cellulase showing similar pulp brightness to conventional deinking using alkaline peroxide. They also reported that commercial cellulase preparations reduced particle size of printing inks. Prasad

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et al [50].,evaluated cellulases and hemicellulases mixtures at acidic pH 5.5 for the deinking of ONP having letterpress and color offset inks. Studies done with letterpress and offset paper indicated that hemicellulase (xylanase) treatment for letterpress papers resulted in gaining highest pulp brightness. Cellulase treatment resulted in the lowest residual ink area. Cellulase and hemicellulase mixture was effective to achieve the best pulp brightness for colored offset papers. Similar enzymes were used for the deinking of flexographic printed ONP. Ligninolytic enzymes from white-rot fungus Phanerochaete chrysosporium and with other lignin-degrading fungi are also useful for the deinking of lignin-rich mechanical pulp such as ONP [103] [50]. treated photocopied and lasers printed papers with a pure alkaline cellulase showing improvement of 4 ISO units pulp brightness and 94% reduction in residual ink area (direct count) than the control (no enzyme). Fiber length distribution was affected using enzyme. Hemicellulases from Aspergillus niger and cellulases from Trichoderma viride were also effective for the deinking of ONP [54]. Pulp brightness was increasing with increase in doses of enzyme and reaction time. Pre-soaking [before pulping) of waste papers with enzyme helps to reduce the size of ink particles, but extended soaking showed negative effect on flotation effectiveness and brightness. Jeffries et al. [43] also reported that cellulase and xylanase enzymes [alone or in mixture] were more effective in the pulper at optimum conditions over chemical deinking of xerographic and laser-printed papers. They found decreased BOD of process water after enzyme treatment and increased pulp brightness and pulp drainability. According to [42], lipases and esterases are effective to remove oil-carrier-based inks and cellulases, hemicellulases and ligninolytic enzymes have the ability to alter fiber surfaces or bonds in the surrounding area of ink particles facilitating removal of ink particles by washing or flotation processes [42] [61]. reported unchanged strength properties and fiber length distribution after biodeinking. Yang et al [100]. reported the enzymatic deinking of MOW (comprising laser-printed, xerographic-printed and UV-coated papers) and ONPs/ OMGs using mixtures of enzymes followed by flotation. In comparison to the chemical treatment, higher pulp brightness and 94% lower dirt count (i.e. visible dirt) of deinked MOW [90% laser-printed, 3% colored and 7% of other papers) was observed after enzyme treatment. It was shown that the use of enzymes can also remove or reduce the deinking chemicals. Ow et al. [33] have reported higher pulp brightness in enzyme treated MOW [containing 90% laser-printed papers, 3% colored papers and 7% other papers] over chemically deinked MOW. Heise et al. [51] used low doses of commercial enzyme with a surfactant showing increased ink removal, improved drainage, comparable strength properties and lower pollution load (oxygen demands and toxicity] in comparison to control. The crude alkaline-active cellulase having carboxymethyl cellulase (CMCase) and filter paper cellulase (FPase) activities produced by two wood-decaying basidiomycetous fungi have also the ability to deink MOW having 70% coating of toners. In the presence of surfactant, ink removal efficiency of enzymes was generally increased [31].

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The deinking of laser-printed papers was performed using purified endoglucanases Gloeophyllum sepiarium (EGS) and G. trabeum (EGT), xylanase of Thermomyces lanuginosus (X) and mannanase of Sclerotium rolfsii (M) individually and in combinations [66]. The enzyme effect was improved in magnetic deinking showing 94% toner removal using EGS and X in combination. The use of pure EGT and EGS demonstrated the effectiveness of endoglucanases in deinking. The pulp yield (97.2%) of combined enzymatic–magnetic deinking was higher than the conventional flotation deinking [89%) using only tap water without any chemicals [102]., studied the effect of different parameters for the bacterial alpha-amylase like enzyme dose, reaction time, and addition of flotation aid to improve the flotation deinking of copy papers. After the enzymatic deinking, pulp brightness was similar to the original copy papers. About 70% reduction in the number of toner particles was observed after one cycle of flotation deinking without adversely affecting the pulp properties and paper strength properties. Rutledge-Cropsey et al. [104] investigated the effect of enzyme based deinking on paper machine runnability, pulp drainage and wet-web strength. The enzyme-treated pulp showed better machine runnability than the control pulp due to enhanced drainage and wet-web strength [102]. Treatment of cellulase, xylanase and lipase in combination with neutral surfactant on soybean oil-based ink printed paper resulted in reduced dirt counts and residual ink areas [60]. Enzyme dose of about 3 IU/g pulp was sufficient to deink MOW, ONP and vegetable oil-based ink printed papers. High dose of xylanase was suggested over cellulase for the ink removal from various types of waste papers [63]., reported that the use of two pure endoglucanases [Cel E and Cel B) led to a similar distribution of ink particle sizes and most of the residual ink particles were of 20 µm or smaller. Particles with a size of 20 µm or smaller accounted for more than half of the dirt count when both enzymes were used. These results show that the flotation process is not very effective for removing ink particles smaller than 20 µm. Cel B resulted to have more ink particles than Cel E having sizes of 20 µm and smaller. Cel E has a higher deinking effect than Cel B due to its efficiency to remove ink particles smaller than 20 µm. Viesturs et al. [49] suggested that majority of inks are attached to the coatings and fillers [mainly of CaCO3] present on laser-printed alkaline papers. This CaCO3 is dissolved by stock acidification during enzyme treatment before flotation helps in effective detachment of ink particles and dispersion of toner specks ensuring high deinking effectiveness. Elegir et al. [105] reported up a reduction up to 96% coverage area of the toner particles in a neutral enzyme-enhanced deinking of xerographic office waste using commercial alpha-amylase and cellulase with surfactant. Amylase enzyme facilitates the removal of small ink particles in flotation deinking and showed a good synergism with cellulase. Oksanen et al. [106] treated recycled pulps with purified Trichoderma reesei cellulases [endoglucanases EG I and EG II] and hemicellulases. Even low amount of EG I and EG II showed significant

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improvement in pulp drainage rate and EG II was more active at a particular level of carbohydrate solubilization. The efficiency of endoglucanase treatment towards pulp drainage was found to be increased after mixing of hemicellulase. Pala et al. [107] investigated the effect of several enzymes on old paperboard containers for secondary fiber upgradation. Non-hydrolytic cellulose-binding domains [CBD1 and CBD2] were used for deinking in place of whole enzyme to see effect of the enzyme dose & reaction time on the fiber-length distribution and pulp & paper properties. CBD1 showed improved pulp drainage rate and comparable tensile and burst indexes with respect to control due to the binding of CBD to the surface of fiber to modify the its surface/ interfacial properties. CBD in excess amount reduced final pulp characteristics due to the mechanical peeling effect and fines generation from the fibers surface. No soluble sugars were detected after enzyme treatment indicating fiber modification without enzymatic hydrolysis. Change in the density or permeability parameters were also unaffected. Park and Park [108] applied modified cellulase with the copolymer [derivatives of polyethylene oxide (PEO]] and maleic anhydride (MA) for the recycling of MOW. For the preparation of modified cellulase, amino groups of the cellulase were chemically reacted with functional groups of MA. In MOW recycling, modified cellulase improved pulp freeness, optical and strength properties in comparison to the conventional chemical deinking. This improvement was also higher than the improvement observed using native cellulase specially tensile strength and internal bond. Two extracellular alkali stable 1,4-β-d-glucan-4-glucanohydrolase (EC 3.2.1.4) fractions EndoA and EndoB obtained from alkalo-tolerant Fusarium strain was observed to be effective for the deinking of MOW in terms of increased pulp brightness and reduced ink counts [41]. Marques et al. [46] used two cellulase enzymes from Aspergillus terreus CCMI 498 and Trichoderma viride CCMI 84 for deinking of MOW resulting in improvement of the pulp properties and strength properties of paper. Kenealy and Jeffries [99] reported that fibers of enzyme-treated pulp underwent peeling process giving rise to flacks and filaments of material detached from fiber surfaces due to xylan hydrolysis. Enzyme treatments increased the fiber swelling and facilitated refining which in turn resulted in better physical properties. Pelach et al. [18] reported that the optical properties of enzymatic deinked paper improved slightly on increasing enzyme amount and the contact time. They also observed the positive effect of increased consistency on ink detachment. Bolanča and Bolanča [29] have shown the comparison between two enzyme preparations from Trichoderma namely Pergalase A-40 (endoglucanase I and II) and Indi Age super L [endoglucanase III). They found that endoglucanase III was more efficient for the deinking of waste papers in industrial conditions of deinking e.g.1.0 to 1.5 h of reaction time, 35˚C-50˚C temperature and a pH of around 4.5. Bolanča and Bolanča [29] investigated the enzymatic deinking process of digital printed papers [with liquid and powder toner] using enzyme of the fungus Humicola insolens in combination with non-ionic ethoxylated

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fatty acids. The results showed increased brightness, improved dirt removal with a reduction in mechanical properties of handsheets prepared after enzyme treatment. Enzymatic deinking was also suggested as competitive alternative to chemical deinking for MOW and photocopy prints by [101]. The selection of suitable enzyme preparations for each paper grade is a limitation for deinking process. Pala et al. [37] also showed the better results of enzyme treatment (based on enzyme preparations used] over the chemical treatment in terms of improved strength properties [tensile, burst and tear indexes) and pulp drainability. Lee et al. [45] studied enzymatic deinking of laser-printed waste papers on a laboratory scale using cellulase and hemicellulase of Aspergillus niger. Maximum deinking efficiency [about 73%] good optical and physical properties were obtained by the enzyme combination having both cellulase and hemicellulase. Soni et al. [48] isolated twenty thermophilic/ thermotolerant fungal strains and screened for endoglucanases, β-glucosidase, FPase and xylanases to assess their deinking efficiency. Out of 20 isolates, Aspergillus sp. AMA, A. terreus AN1, and Myceliophthora fergusii T4I were found to be effective to deink composite waste papers (70% OMG and 30% xerox copier/ laser prints] with improved brightness, tensile strength and tear index of recycled paper sheets. Performance and efficiency of ONP deinking by combining cellulase or hemicellulase with the laccase-violuric acid system [LVS) were investigated by [69]. Experimental results showed a positive effect of the laccase-violuric acid system on enzymatic deinking of lignin-rich waste papers. Pathak et al. [17] reported that the use of enzymes for the deinking of photocopier waste papers can improve the deinking efficiency (+ 24.6%] and pulp freeness (+ 21.6%) with a reduction in drainage time [−11.5%) than chemical deinking. Improvement in optical and physical properties was also observed. First time, Wang et al. [89] reported the application of cutinase for the neutral deinking of the MOW to replace traditional chemicals. The results of cutinases and amylases combined with cardanolpolyoxyethylene ether and other surfactants (CPE/AEO-9/AES] in a mass ratio of 1:1:1 showed improvement in the deinked pulp quality of MOW using 0.2% of surfactants, 10 U g−1of cutinase, 5 U g−1 of amylase at 50°C for 30 min. This treatment showed 92.24% brightness with a deinking efficiency of 92.63%, 24.83 N.mg−1 tensile index, and 9.48 mNm2g−1 tear index [89]. A cutinase from M. thermophila (MtCUT) expressed in the heterologous host P. pastoris KM71H also showed potential in the deinking process by hydrolyzing PCL at 30°C to reduce energy consumption [93]. Nathan et al. [109]evaluated the impact of enzymatic deinking of ONP using cellulase, xylanase, laccase and lipolytic enzymes. The individual (cellulase/ xylanase) and in combination (cellulase/xylanase with lipase) treatment of paper pulp could enhance the paper brightness to about 32.86% and 28.67%, respectively. Chromophores from phenolic and hydrophobic compounds were also released during the deinking process. The amounts of heavy metals in paper were reduced in pulp due to the removal of heavy metals with the ink particles. The effluent generated was demonstrated as safe for agricultural applications due to low phytotoxicity [109].

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Mohandass and Raghukumar [47] used directly a marine bacterium, Vibrio alginolyticus for the deinking and decolorization of the dislodged ink particles from the inkjet-printed paper pulp within 72 h by growing the bacterium in 3–6% pulp consistency suspended in seawater. Immobilized bacterial cells in sodium alginate beads were also able to decolorize this pulp within 72 h. The cell-free culture supernatant (nutrient broth supplemented with starch or Tween 80) effectively deinked and decolorized the pulp within 72 h at 30°C. After dialysis of culture supernatant through membrane [cut-off 10 kDa) reduced the decolorizing efficiency by 35–40%. It was suggested that amylase and lipase facilitated dislodgement of the ink particles from the inkjet-printed paper pulp. Induction of the formation of low molecular weight free radicals in the culture medium by the bacteria might be possible reason for the pulp decolorization. Use of V. alginolyticus cells or its culture supernatant showed an advantage over other enzyme-based techniques that dislodged ink particles are simultaneously decolorized, thus saving of time and water used for flotation to remove ink particles. Gupta et al. [73], applied co-produced laccase and xylanase enzymes in a single medium by Bacillus for the deinking of ONP. After the combined application of both the enzymes, the significant increment was observed in breaking length (34.8%], tear factor (2.4%), burst factor (2.77%), brightness (11.8%) and whiteness (390%) of deinked paper handsheets. The results of the combined application of enzymes were found better than the single enzyme. In the case of only laccase application the improvement in breaking length, burst factor, brightness, whiteness and tear factor was 4.9%, 0.69%, 2.2%, 123.52%, respectively. The reduction in tear factor was 14%. In the case of only xylanase application for deinking the improvement in breaking length, burst factor, brightness, and whiteness were 11.4%, 1.3%, 1.4%, 75.25% respectively. The decrement in tear factor was observed at 21.6%. Thus, the joint application of both enzymes showed better physical and optical properties than a single enzyme. The crude enzyme (containing cellulase and xylanase) obtained from Trichoderma harzianum PPDDN10 NFCCI 2925 showed efficiency in the deinking of the photocopier wastepaper [72]. The enzymatic deinked pulp was compared with pulp deinked with conventional chemicals. The application of the enzyme showed higher deinking efficiency (23.6%) and pulp freeness (21.6%) than conventionally deinked pulp. The paper handsheets made from pulp deinked with crude enzyme showed improved physical [tensile index (6.7%), burst index (13.4%), folding endurance (10.3%)] and optical properties [brightness [3.2%)] with decreased tear index by 10.5%. Chutani et al. [74], used an enzyme cocktail of cellulase and xylanase produced from Trichoderma longibrachiatum MDU-6 for the deinking of different paper wastes (ONP, examination paper and laser printed papers]. The authors reported improved results in the case of ONP deinking (brightness 52%) when compared with control [42.4%). In the case of examination papers and laser printed papers deinking, any remarkable improvement was not found. The mixture of xylanase, lipase and amylase were tested for the deinking of photocopy prints, MOW and ONP by [110]. A maximum deinking efficiency of 13.9% in

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the case of photocopy prints and 19.9% deinking efficiency in case of MOW were reported after deinking with enzyme mixture. There was no improvement after enzymatic deinking of ONP, in comparison to maximum deinking efficiency of 12.2% obtained in traditional alkaline deinking. Kumar et al. [75] tested a cellulase-xylanase system (low molecular weight-14 KDa] produced by Escherichia coli SD5 having 51.95 U/ mg protein of cellulase and 24.64 U/mg protein of xylanase for deinking of ONP. About 10% difference in pulp brightness was observed for diverse treatment time after deinking with the enzyme complex. Increased tear factor was also reported after enzymatic deinking. A study aimed to reduce deinking chemicals using coproduced xylanase and pectinase enzymes by Bacillus pumilus AJK [MTCC-10,414) during ONP recycling was explored by [76]. The application of enzyme mixture showed around 40% reduction in deinking chemicals. The paper handsheets made from enzymatic deinked pulp showed 5.82%, 8.57%, 6.45% increment in breaking length, burst factor and tear factor, respectively. In a combined project of Van Houtum Papier (VHP], Netherlands and Enzymatic Deinking Technologies (EDT), USA; enzymes mixture was used on recycled paper furnishes in a mill trial of 2 weeks. The results showed improvement in pulp brightness by 2.7 points along with furnish savings [111]. In other mill-scale results, visible and subvisible dirt was reduced by 50% after enzymatic deinking. 35% reduction in effective residual ink concentrations and 30–50% stickies reductions were observed within ONP/OMG in mills using enzymes for deinking. 2% improvement in pulp yield was reported in tissue and towel producing mills [112]. In an Indian paper mill, reduced residual ink count, increased brightness and cost-saving of almost 50% in sodium hydroxide, 37% saving in sodium silicate and complete elimination of hydrogen peroxide was observed with multi-grade furnish like coated book stock, MOW and ONP for producing writing and printing paper [113]. Magnin et al. [114] reported a reduction in specks in the final deinked pulp in pilot-scale trials and a mill. EDT commercialized the Enzynk process developed by Eriksson’s group using a mixture of enzymes, surfactants and chemicals [115]. The Stora Dalum deinking plant used the EDT process [116]. About 35% reduction in dirt specks, 50% reduction in stickies, 1.2% ISO brightness levels before bleaching and 2.2% ISO brightness after bleaching was reported. The mill also experienced a 1.8% higher yield (average 8 tonnes per day increased production) with reduced chemicals after applying enzymes [112, 117, 118]. Under EUREKA Enzyrecypaper Project, 2 ISO points pulp brightness improvement of 100% multi-prints furnishes was reported after treatment with cellulase and amylase enzymes at pH 7.0–7.5 in the laboratory investigation [119]. During the mill trial, up to 8 ISO points pulp brightness improvement was achieved with highly specific amylases.

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6.9 Factors affecting biodeinking Enzyme concentration, reaction time, disintegration time, consistency during enzyme reaction, and reaction temperature produced statistically significant effects on the deinking process. A strong interaction was noted between enzyme concentration and reaction time, clearly demonstrating that these factors must be carefully controlled to prevent excessive fiber degradation. Enzyme concentration, time of surfactant addition, and disintegration time significantly affected most variables used to quantify ink removal: brightness, residual ink count, and ink area. Disintegration time produced the largest positive effects. Brightness and ink count reduction increased by flotation, while residual ink areas after flotation decreased with increasing disintegration time. The joint impact of increasing enzyme concentration and disintegration time is noteworthy. At low concentrations and short reaction times, handsheet made after flotation contained both numerous and large areas of ink. Increasing levels of both factors gave not only lower ink counts of large ink particles but also large reductions in residual ink areas [37, 42]. Pulping consistency have role in the determination of the microfibrils size. Pulping at higher pulp consistency resulted in higher or rougher fiber sizes and texture and reduced deinking efficiency. Mechanical agitation helps to disperse and mix the enzyme preparations in the reaction system, enhances enzyme contact time with pulp and prevents accumulation of toner on the fiber surfaces. Too much agitation (higher agitation rate) has negative effect on pulp and paper properties. Therefore, lower agitation rate for continuous mixing helps to improve the pulp brightness by the removal of toner particles immediately after its detachment from the fiber surfaces due to enzyme action [107]. The optimal enzyme dose is crucial to avoid any negative effect on fibre/ paper strength and quality, if used in excessive amount [45]. High enzyme loading also reported to reduce pulp brightness due to accumulation of enzyme particles on the fiber’s surfaces [43]. The enzyme activity is reduced by the sizing agents present in the paper due to higher fiber hydrophobicity which act as a physical shield to hinder the attachment of enzyme by covalent bonds with cellulose of the fibres. The reduction in enzyme efficiency depends on the types of the sizing agents. Alkyl succinic anhydride increases hydrophobicity along with covalent bonds formation with the exposed groups of cellulose. The lowest deinking efficiency was reported for non-impact printed papers sized with rosin alum. Coatings, dyes, metals, and other additives may be responsible for the denaturation or inhibition of enzymes [77].

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6.10 Advantages Paper recycling reduces the requirement of virgin pulp and thus reduces stress on the environment. Conventional deinking requires extensive use of expensive and highly regulated wastewater treatment due to the use of chemicals. Enzymatic deinking is reported to reduce the chemical consumption and the production cost effectively to replace alkaline deinking chemicals such as sodium hydroxide and sodium silicate. Additionally, enzymatic deinking significantly reduces the pollution load of the wastewater and smoothes the progress of environmental protection [70, 89]. In comparison to enzymatic deinking, conventional chemical deinking methods are relatively ineffective to remove the ink from MOW due to toners and other non-impact polymeric inks from laser printing. Recycling of this waste paper presents technical and economic challenges to the paper recycler because of the non-dispersion of toner in the conventional repulping process creating difficulty in removal by flotation or washing steps. In the enzymatic process, the size and shape of the removed ink particles can be efficiently controlled to take full advantage of the flotation process. The enzymatic deinking process may not require dewatering and dispersion steps thus save electrical energy and capital costs involved to install these steps. Enzymatic deinking requires less bleaching chemicals than used for conventional chemical deinking, thus reduces wastewater treatment costs and impact on the environment. Enzyme treated pulp also shows improved pulp drainage, physical properties and pulp brightness along with lower residual ink in comparison to conventional chemically deinked pulp. Improvement in drainage results in higher machine runnability thus saving significant energy and cost.

6.11 Limitations and future recommendations However, enzymatic deinking has been established as an eco-friendly and vital technology for the deinking of different kinds of waste paper still faces some problems which limit the proper commercialization of this technology. Most of the commercially available enzymes are too expensive to compete with conventional chemicals for deinking. Enzymes are very sensitive to environmental fluctuations. They have a narrow range of pH and temperature for their optimal activity. Their activity continuously decreased with storage time. So, operating conditions must be accurately controlled for maintaining enzymatic activity. Enzymatic processes are generally slower and may be difficult to retrofit into existing pulp and paper mill conditions [17, 120]. The effectiveness of the enzymatic deinking process depends more critically also on the waste paper furnish characteristics. Some kinds of waste paper such as toner or non-impact ink printed waste paper still difficult to be deinked. There is a wide variety of waste paper supplies in the paper mills and this is probably the most important

References

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disadvantage of this technology for recycling [101]. Enzymes may seriously deteriorate the physical properties of pulp when used in higher doses. The huge doses of enzymes for deinking also cause the yellowness in paper and may reduce the pulp brightness [43]. Because of the above, there is a necessity of improvements in various areas of this technology including economical enzyme production, selection and application of enzymes based on waste paper furnish, type of inks used for printing and development of more efficient enzyme formulations to optimal biodeinking process development.

6.12 Conclusions Enzymes have been established as promising bioactive compounds in the deinking of waste papers, especially for mixed office prints. The use of enzymes reduces or eliminates the use of deinking chemicals and thus making it an environmentally friendly alternative to conventional chemical deinking. Enzymatically deinked pulps possess better optical and mechanical strength properties than chemically deinked recycled pulps. The application of enzymes at mill-scale requires extensive customization of the enzyme formulation and control of process variables to get maximum results. Still, several pilot-plant and mill-scale trials have been taken with promising results. Many mills around the world have already reduced the consumption of chemicals after applying the enzymes for deinking.

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Amit Kumar, Chhotu Ram and Adebabay Tazeb

7 Enzyme-assisted pulp refining: an energy saving approach Abstract: Energy conservation has become an essential step in pulp and paper industry due to diminishing fossil reserves and high cost of energy. Refining is a mechanical treatment of pulp that modifies the structure of the fibres in order to achieve desired paper-making properties. However, it consumes considerable amount of energy. The electrical power consumption has a direct impact on paper manufacturing cost. Therefore, there is a requirement to minimize the energy cost. Enzyme-assisted refining is the environment friendly option that reduces the energy consumption for papermaking. Enzyme-assisted refining is defined as mechanical refining after pretreatment of pulp with enzymes such as cellulases and hemicellulases. It not only reduces the energy consumption but also improves the quality of finished paper. Enzymes improve the beatability of pulp at same refining degree (°SR) and desired paper properties can be achieved at decreased refining time. The selection of suitable enzyme, optimization of enzyme dose and appropriate reaction time are the key factors for energy reduction and pulp quality improvement during enzyme-assisted refining. Keywords: mechanical refining, PFI mill, enzyme-assisted refining, pulp freeness, CSF, water retention value

7.1 Introduction High quality of paper is expected from modern paper production technologies in sustainable, cost effective and energy efficient way. The quality of paper depends on quality of cellulosic fiber and the papermaking operations. For the production of good quality paper, fiber must be matted into a uniform sheet and must also develop strong bonds at the point of contact. Beating or refining of pulp is a mechanical treatment that modifies the structure of the pulp fibres in order to achieve desired papermaking properties. Refining is an essential fiber-modifying process prior to paper manufacturing. Refining results such bonding between cellulosic fibers. It is one of the important unit operations that determine the drainage of pulp and physical strength properties of paper [1–4]. Pulp refining is one of the most energy intensive processes during papermaking that consumes 150–500 kWh/ton paper and accounting for 30–50% of total energy used for paper production [5, 6]. Mechanical refining This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, A., Ram, C., Tazeb, A. Enzyme-assisted pulp refining: an energy saving approach Physical Sciences Reviews [Online] 2020, 5. DOI: 10.1515/psr-2019-0046 https://doi.org/10.1515/9783110592412-007

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affects pulp and paper properties both positively and negatively. Therefore, some alternative approaches are required that can reduce the energy consumption during papermaking. Enzymes are the environment friendly option that reduces the energy consumption for papermaking.

7.2 Mechanical refining and its effect on paper properties Mechanical pulp refining (beating) is a complex process that is dependent upon the function of the friction among fibers and the mechanical shear force and it develops optimum papermaking properties among the fibers. The first effect of mechanical refining on fiber layer is internal fibrillation. This is delamination of P and S1 layers, resulted in by cyclic compression action of forces inside the refiner (Figure 7.1) [8, 9]. It also results in several other structural changes in the fibers including external fibrillation, fines generation, shortening or cutting of fibers, and straightening of fibers. Mechanical refining also causes the changes in cellulose crystallinity and surface composition of pulp fibers [9–11]. All structural changes during refining have been given in Figure 7.2. Refining of pulp improves fibers flexibility and produces denser paper, which means bulk, opacity and porosity decrease [11].

Figure 7.1: Wood fiber structure showing middle lamella, primary wall, secondary wall (S1, S2 and S3 layers) and lumen (Modified from [7]).

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Figure 7.2: Structural changes in fiber during mechanical refining [8, 9, 11–25].

Internal refining is believed one the main reason for improvement of paper strength properties, hydrogen bonds among microfibrils are broken during internal fibrillation and results in water absoption, specific volume, and flexibility of the fibers. The polar hydroxyl groups on the molecular structure of cellulose and hemicelluloses attract water molecules to penetrate amorphous region of cellulose that causes swelling. Swelling improves specific surface area of cellulosic fibers and

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promotes the fiber–fiber bonding that improves physical strength of paper significantly (Figure 7.3). During refining, compression of fiber also results in the collapse of the lumen. The improvement in swelling also causes the better conformability during sheet formation [24–27].

Figure 7.3: SEM micrographs of ethanol-soda pulp of Eulaliopsis binata at different magnifications: (A) without refining at 500 X, (B) after PFI refining at 500X, (C) without refining at 2000 X and (D) after PFI refining at 2000 X.

Dewatering or drainage of pulp is an important parameter for paper manufacturing. It depends on the fibrillation of pulp and influences the runnability of the paper machine. There are some common methods that are used for determination of pulp’s drainage. Schopper-Riegler (°SR), Canadian Standard Freeness (CSF) and water retention value (WRV) are used to measure drainage and are useful methods for determining the level of refining. Refining is easily monitored by drainage rate of water through pulp [11, 28]. The freeness value should not be more than 30 °SR for

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paper production at industrial scale [5]. According to TAAPI test method T 227om-99, if pulp is drained at normalized conditions the volume of excess of water from overflow is measured as CSF [29]. It is the indirect measurement of pulp drainage and is affected by external fibrillation and amount of fibers. A high drainage means a high CSF. The lower value of CSF indicates a higher amount of refining. However, the higher WRV of pulp is indicates to degree of refining or beating. If CSF value is lower, paper machine will operate relatively slowly that will affect the paper production cost [11, 25]. Drainage of pulp strongly affects the energy efficiency of paper machine and thus cost efficiency of papermaking [30].

7.3 Enzyme-assisted refining Mechanical pulp refining is energy intensive process. The energy consumption directly affects the paper production cost and profitability of paper mill. Therefore, several efforts have been made to minimize the energy consumption during pulp refining. Different approaches such as plate pattern adaptation, variation in pulp consistency, variation in refining temperature, use of chemicals, increased intensity through refiner, or redesigning refining strategy. Some of these adaptations require high initial investments but simpler methods that require lower implementation cost are preferable. Enzyme-assisted refining is defined as mechanical refining after pretreatment of pulp with enzymes such as cellulases and hemicellulases. Therefore, enzyme-assisted refining represents an effective and environment friendly alternate that improves the pulp properties [31–33]. Enzyme-assisted refining decreases the energy consumption during mechanical operation stage that is performed by PFI mill or valley beater in research laboratories and conical or disc refiners in paper mill. Enzyme-assisted refining not only reduces the energy consumption but also improves the quality of finished paper. Cellulases, xylanases and pectinases are the enzymes that are applied during beating or refining of pulp. The effect of enzyme treatment on fiber properties depends on the type of substrate and enzyme such as cellulase and xylanase [32, 34].

7.3.1 Enzymatic modification of fiber and its effect on pulp and paper properties Cellulases and xylanases are the enzymes that are used dominantly for enzyme-assisted refining. Cellulases and xylanases act on cellulose and xylan available in pulp fiber. Cellulases are multi-component enzyme system that consists of three enzyme including endo-β-1, 4 glucanase, exo-β-1, 4 glucanase and β-glucosidase. Endoglucanases randomly hydrolyze the β-1, 4 glycosidic linkages on the amorphous part of cellulose away from chain ends. Exoglucanases attack on reducing and non-reducing ends of cellulosic chains to release cellobiose. β-glucosidase

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converts cellobiose to glucose. FPase represents the activity of all three components of cellulase complex system. Xylanases solubilises the xylan present in hemicellulose [35, 36]. Cellulases and xylanases are used singly or in combinations for enzyme-assisted refining and found effective for the modification of pulp fibers surfaces. Mechanical refining removes outer layers from the fiber that results in the swelling of fiber and decreases the fiber length. It improves fiber conformability, fiber flexibility and fiber–fiber bonding, and strength properties as described in Figure 7.1 in details. Similarly, cellulase treatment appears to produce the same effect on pulp. Cellulase action removes primary wall and S1 layer (rich in cellulose). It also fibrillate S2 layer, and also improves surface area and specific volume. The pretreatment of pulp prior to mechanical refining results in the higher fibrillation of fibers, that improves inter-fiber bonding and enhances the tensile strength of pulp [24, 31, 37, 38] (Figure 7.4). It has been observed that cellulase treatment results in the loss of fiber intrinsic strength and usually affects the tear strength negatively [31, 39]. Several researchers have hypothesized that xylanase action also modifies the fiber, causes the separation of secondary wall, increases the surface area by fibrillation and reduces the pore volume [24, 31, 40]. Xylanase treatment also improves the flexibility of fibers [41]. Hemicellulases treatment of cellulosic fibers results in the degradation of associated hemicelluloses or xylans and the removal of these polysaccharides facilitates the penetration of water molecules into spaces within cellulosic fibers that may break a fraction of hydrogen bonds connecting cellulose chains. This loosens the 3D structure of cellulosic fibers that makes fiber more flexible. Thus, enzyme treatment improves fiber flexibility that increases the compactness of paper, i.e. apparent sheet density along with strength properties.

Figure 7.4: SEM micrographs of eucalyptus kraft pulp refined for 1500 PFI revolutions: (a) without enzyme treatment and (b) after cellulase treatment (Adapted with permission [24]).

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The amount of fines also affects the apparent sheet density due to better packing of fibers and fine in sheet [5, 28, 32]. Furthermore, fibrillation is another factor that influences the conformability and hence apparent sheet density. Internal fibrillation influences conformability, flexibility, and collapsibility positively. External fibrillation increases hydrogen bond-assisted compaction. Thus, all of these effects may improve the apparent sheet density [32, 42, 43]. Liu et at. [44] studied effect of enzyme-assisted refining on crystallinity index of bleached kraft pulp and observed that cellulose crystallinity index was declined up to 55.11% from its initial value of 60.54% using cellulase enzyme.

7.3.2 Requirement of controlled enzymatic treatment Several factors such as temperature, pH, enzyme dose, reaction time, ratio of different enzymes, type of enzyme and type of substrate determines the effect of enzyme treatment on refining. Optimum effect of enzyme treatment is attained by controlling these parameters [24, 32]. Excessive enzyme treatment either in the form of higher enzyme dose or longer reaction time deteriorate the pulp quality. Therefore, selection of suitable enzyme, optimization of enzyme dose and appropriate reaction time are the key factors for energy reduction and pulp quality improvement during enzymeassisted refining [28]. It has been reported that pulp hydrolysis less than 3% is acceptable for enzyme-assisted refining [39, 45]. A study on cellulase-assisted refining of bleached Eucalyptus globulus kraft pulp showed pulp hydrolysis up to 1.1% and pulp viscosity was decreased by 22.5% without affecting the fiber length significantly [39]. Singh et al. [32] performed a study on cellulase and xylanase-assisted refining of mixed hardwood pulp, and observed a balance between energy saving and strength properties at reaction time of 2 h with enzyme dose of 0.06 IU/g of OD pulp. When enzyme dosages were increased to 0.08 and 0.10 IU/g tensile index was lowered significantly [32]. A study on three types of kraft pulp from Pinus radiata showed that different types of pulp respond in a different way to same enzyme from different sources. Mansfield et al. [46] assessed the refining ability of two xylanase preparations namely Xylanase E (Genecor International, USA) and Pulpzyme HC (Novo Nordisk, Demmark) at enzyme dosages of 1.8 µkat/g OD pulp. The response of three different pulps namely ITC, K70 and LCB from Pinus radiata was measured in terms of solubilised arabinoxylan by enzyme treatment. Pulpzyme HC solubilised 7.5, 10.5 and 9.4% of arabinoxylan from LCB, ITC and K70 pulp samples, respectively, while another enzyme Xylanase E hydrolysed 15.9 and 18.3% arabinoxylan from pulp samples ITC and K70, respectively, significantly higher compared to Pulpzyme HC [46].

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7.3.3 Fiber length and other pulp & paper properties Fiber length is usually as indication of good quality of pulp. But at industrial scale pulp fibers are kinked and curl; therefore, fiber length cannot be utilized as efficiently as laboratory studies indicate. Furthermore, a high fiber length also has adverse effect on sheet formation. Longer fiber shows more flocks forming tendency [47, 48]. Several studies proved that the pulp quality becomes superior by xylanase treatment without affecting the fiber length [5, 28, 49]. Wong et al. [49] carried out xylanase-assisted PFI refining of unbleached softwood kraft pulp and observed no loss in fiber length while sheet densification was improved. Plazl et al. [28] studied the enzyme-assisted refining of five different bleached kraft pulps with different enzyme preparations including Novozym 342 (mainly endoglucanase), Novozym 476 (endoglucanase), Novozym 51,024 (endoxylanase) and Pulpzyme HC (endoxylanase). When different pulps were treated with these enzyme preparations individually, there was no significant effect on fiber length while fine contents were increased slightly. Different combinations of cellulase and xylanase (342 + 51,024, 342 + HC, 476 + 51,024 and 476 + HC) significantly reduced the fiber length and increased the fine content. These results proved that cellulases and xylanases hydrolyze the complex fiber structure in synergistic manner [28]. Therefore, the impact of enzymatic treatment depends upon a balance between activities of cellulases and hemicellulases [5, 28]. A study on enzyme-assisted PFI-mill refining of kraft pulp describes the effect of Thermomyces lanuginosus xylanase, Aspergillus sp. cellulase and ‘NS-22,086ʹ having both xylanase and cellulase activities. At target freeness value of 30 °SR, the treatments with Aspergillus sp. cellulase and ‘NS-22,086ʹresulted in higher pulp susceptibility for refining in shorter treatment time. The average fiber length was also decreased significantly. Thus, these treatments deteriorated the strength properties of paper. The treatment with T. lanuginosus xylanase resulted in the hydrolysis of xylan, a fraction of this xylan is associated with cellulosic fibers. Thus, slightly loosened the fiber structures and affected the pulp susceptibility for refining positively with improvement in strength properties. Finally, T. lanuginosus xylanase caused significant saving in refining energy without any adverse effect on strength properties [5]. As described in Section 2.3.1 enzymes show same effect as mechanical refining. Enzymatic treatment improves fibrillation, surface area, inter fiber boding, fiber flexibility, and fiber conformability. Therefore, enzymatic treatment significantly improves sheet density, smoothness, burst index, tensile index and stiffness while decrease the opacity, bulk, tear strength, porosity and roughness [24]. The effect of different enzymes on strength properties have been discussed in Table 7.1. A study on analysis of xylanase-treated alkaline peroxide mechanical Aspen pulp properties was performed and xylanase-treated pulp showed 14, 23, and 18% improvement in tensile, tear and burst indexes, respectively, compared to untreated pulp. Fine content and kink index were decreased to some extent with xylanase treatment. The

7.3 Enzyme-assisted refining

163

enzyme treatment also increased the total carboxyl content, crystallinity index, and WRV significantly [38]. The improvement in strength properties may be due increased carboxyl content and crystallinity index. The decrease in fine content and kink index as well as the removal of a fraction of xylan and lignin from the fiber surfaces also favoured the improvement in physical strength properties [38]. As explained in Figure 7.1 refining results external fibrillation and fines have high surface area therefore fines improve the bonding but affects drainage negatively. Although most of the enzymes-assisted refining studies have been performed on fixed value of CSF or °SR, both of these parameters represents drainage of pulp indirectly (Table 7.2.1). If enzyme-assisted-refining is performed on same level of refining energy than improvement or decrement in drainage has been reported by different researchers. Cui et al. [3] carried out the PFI refining of kraft pulp using cellulase and xylanase and reported improvement in drainage after enzymatic treatment of post refining. If enzymatic treatment was performed before refining, drainage was declined [3]. Controlled enzymatic treatment removes only elements that have a higher great attraction for water molecules but contribute less to the fiber– fiber bonding. Enzyme treatment removes these surface components selectively that results in reduced pulp-water interactions. Therefore, mechanical strength properties are not affected significantly while a significant improvement occurs in drainage. If the enzyme treatment becomes longer then fibrillation becomes pronounced that leads a poor drainage [25]. A study performed on enzyme-assisted refining of bleached E. globulus pulp reported 80% improvement in °SR at 1500 PFI revolutions with 3.33 N/mm of refining intensity. At same level of refining and energy input WRV was improved by 17.5%, this small increase in WRV compared to ° SR may be due the fact that WRV is associated with internal fibrillation [39]. The penetration of enzymes into fiber wall is reported as being limited [4, 54]. Another study on enzyme-assisted PFI refining of bleached mixed hardwood pulp showed 14% enhancement in WRV compared to control. An enzyme dose of 0.06 IU/g of OD pulp was used for 2 h of treatment [32]. Liu et al. [44] carried out cellulase-assisted refining of bleached kraft pulp at 40,000 PFI revolutions and found a improvement in °SR by 12.98% compared to control (no enzyme treatment). Several commercial enzyme preparations of cellulases and xylanases have been found effective for refining of various kinds of pulp. Different commercial enzyme preparations that have been used for refining are listed in Table 7.2.

7.3.4 Energy saving during enzyme-assisted refining Energy conservation has become an essential step in pulp and paper industry due to diminishing of fossil reserves and high cost of energy. Refining is the step in paper making that requires substantial energy to develop the desired paper properties [44]. The main objective of utilization of enzymes before refining is to minimize

Source of enzyme

Commercial enzyme

Novozymes “Cel B”

Novozym (NS)

Megazyme, Creative Enzymes, Sigma-Aldrich

Enzymes

Cellulase, xylanase

Cellulase, xylanase

Cellulase

Xylanase Cellulase

Kraft pulp

Mixed hardwood pulp

. g enzymes/kg OD pulp

. IU/g OD pulp

Varied with different enzymes

Bleached pine kraft pulp

– g/T Mixed softwood pulp bleached kraft pulp

Type of pulp

Enzyme dose

Effect of enzyme-assisted refining on pulp and paper properties

To achieve the freeness of  °SR, PFI revolutions were decrease by .% compared to control

Refining energy reduced by % at enzyme dose of  g/T of pulp

To achieve  ml of CSF, refining energy reduced by %

[]

[]

– Pure xylanase improved the tensile strength – Xylanase with cellulase decreased tear resistance – WRV were lower for enzyme treated pulp

[]

[]

References

– Tear index was decreased by % – Braking length improved

– Tensile and burst index were improved by  and %, respectively – Tear index was reduced up to % – Apparent density of handsheets was improved – WRV of enzyme treated pulps increased

PFI revolutions decreased by – Tensile index, burst index, and % to achieve same internal bonds strength were freeness level improved by ., . and .%, respectively, compared to control

Effect of enzyme treatment on energy requirement

Table 7.1: Effect of enzyme-assisted refining on refining energy requirement and pulp and paper properties.

164 7 Enzyme-assisted pulp refining: an energy saving approach

Commercial enzyme “Cel F”

Novozymes NS

Sukehan Co., Novozymes Biologicals Inc

Commercial enzyme

Commercial enzyme (AUPE & ,)

Cellulase

Cellulase, xylanase

Cellulase, xylanase

Cellulase, xylanase

Xylanase

Italian black Poplar branches pulp

 IU/g pulp

Refining energy reduced by %

To achieve  ml of CSF, refining energy reduced by –%

Mixed hardwood pulp

 g/T of OD pulp

To achieve the freeness of  °SR, PFI revolutions were decrease by % compared to control

To achieve  ml of CSF, refining energy was reduced by %

Refining energy minimized up to %

Bleached pine kraft pulp

Mixed hardwood pulp

Both cellulase Poplar APMP pulp & xylanase  IU/g OD pulp

Varied with different enzymes

. IU/g OD pulp

[]

– CSF was decreased by – ml – Cellulase pretreatment improved breaking length, tear index, burst index and folding number by ., ., . and %, respectively – Xylanase pretreatment improved breaking length, tear index, burst index and folding number by ., ., . and %, respectively

– Physical and optical properties improved – Fiber length increased slightly – Fine content decreased

(continued )

[]

[]

[]

– Breaking length improved slightly – Tear resistance was decreased by % – WRV decreased by % – Fiber length decreased by % – Amount of fines increased by .%

– Tensile index, burst index, tear index and double fold improved by , ., ., and .%, respectively – Drainage improved by %

[]

– Pulp viscosity decreased by % – Tensile and burst index improved by  and %, respectively – Tear index decreased by %

7.3 Enzyme-assisted refining

165

Bleached kraft pulp

Pinus radiata ubleached kraft pulp

.– g/Kg of OD pulp

 µkat/ g of pulp

– IU/g of OD pulp

Novozym  Novozym  Novozym , Pulpzyme HC

Pulpzyme HC, Novo Nordisk

Celluclast . L®, Viscozyme L®

Cellulase, xylanase

Xylanase

Cellulase, Xylanase, βglucanase, arabinase Bleached Eucalyptus globulus kraft pulp

Mixed hardwood pulp

 g/T of OD pulp

Commercial enzyme

Cellulase, xylanase

Type of pulp

Enzyme dose

Source of enzyme

Enzymes

Table 7.1 (continued )

–

Escher-Wyss (conical refiner) resulted % saving in energy to achieve a given apparent density

Refining energy reduced by %

To achieve  ml of CSF, refining energy was reduced by . and % at laboratory and plant scale trail, respectively

Effect of enzyme treatment on energy requirement

– °SR enhanced by % at fixed refining energy level ( PFI revolutions) – At IU/g of enzyme dose, tensile strength was improved by .% – Pulp viscosity decreased by .% but fiber length was not affected significantly

– Xylanase treatment resulted in a slight decrease in fiber length (approximately %) – Xylanase treatment also resulted in significantly higher flexibility – Apparent density was improved

– Pulp quality deteriorated – Average fiber length decreased significantly – Fine fraction increased

– Drainage improved by .% – Breaking length improved by .% – Burst and tensile strength improved marginally while tear index reduced marginally

Effect of enzyme-assisted refining on pulp and paper properties

[]

[]

[]

[]

References

166 7 Enzyme-assisted pulp refining: an energy saving approach

7.3 Enzyme-assisted refining

167

Table 7.2: Commercial enzyme preparations used for pulp refining. Commercial enzyme name

Producer company

Source of enzyme

Type of enzyme activity

Cel B

Novozymes, Banglore, India Novozymes, Denmark

–

Cellulase & xylanase

Bacillus sp.

Xylanase

Hemicellulase “Amano”  Cartazyme HS Irgazyme S Bleachzyme F Novozym 

Amano Pharmaceuticals Co. Ltd., Japan Sandoz Chemicals, UK Genencor, Finland Biocon, India Novozymes Deutschland GmbH, Germany

–

Xylanase

Novozym 

Novozymes Deutschland GmbH, Germany Novozymes Deutschland GmbH, Germany Novozymes

Pulpzyme HC

Novozym , Novozym (NS) NS  Celluclast . L® Viscozyme .L®

Novozymes, Denmark Novo Nordisk Novo Nordisk

Novozyme SP

Novo Nordisk, Denmark

FiberZymeTM

Dyalic International Inc., USA

Xylanase E

Genencor International, USA

– – – Humicola sp

Xylanase Xylanase Xylanase Endo-,-βglucanase, Cellobiohydrolase, Xylanase Aspergillus Endo-,-βsp. glucanase Thermomyces Endo-,-β- xylanase lanuginosus – Endo-,-βglucanase – Cellulase & xylanase T. reesei Cellulase Aspergillus Cellulases, aculeatus hemicellulases, arabinase, βglucanases – Endo-,-βglucanase, xylanase, mannanase – Endo-,-βglucanase, xylanase, mannanase T. reesei Xylanase

Reference [] [, , ] [] [] [] [] []

[] [] [] [] [] []

[]

[]

[]

the energy requirement as well as to make the process environment friendly [5]. Several researchers have investigated and reported the energy saving during enzyme-assisted refining (Table 7.2.1). Cellulases and xylanases fibrillate the pulp to reduce the energy demand in paper making process. Hemicellulases are capable to assist the refining of pulps by increasing the rate of fiber hydration. Enzymes improve the beatability of pulp at same refining degree (°SR) and desired paper properties can be achieved at decreased refining time [53, 55, 58, 59]. A study performed on cellulase/hemicellulases-assisted refining of bleached and unbleached mixed

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7 Enzyme-assisted pulp refining: an energy saving approach

(hardwood and bamboo) pulp showed 15–20% reduction in refining energy and concluded that enzymes are more effective for bleached pulp compared to unbleached pulp. Most probably it might be due to removal of lignin and exposure of hydroxyl groups of cellulose and hemicelluloses [60]. Liu et al. [44] studied the effect of cellulose binding domain (CBD) on enzymatic reaction with northern bleached softwood kraft pulp and 13–24% of refining energy was saved by cellulase pretretament of pulp before refining. The cellulases without CBD were found less effective compared to cellulase with CBD for adsorption and hydrolysis of fiber [44]. Singh et al. [34] performed enzymatic pre-treatment of bleached mixed hardwood pulp with cellulase as biocatalyst and after that cellulase treated pulp was refined in PFI mill. This treatment achieved reduction in refining energy by approximately 29% (specific energy consumption reduced from 1.33 kWh/kg of pulp to 0.94 kWh/kg pulp) at dosage of 0.07 IU/g of OD pulp after reaction time of 90 min with improved pulp quality.

7.4 Enzyme-assisted refining of recycled fiber Use of recycled fiber for paper, paperboard and newsprint production increased rapidly but recycled fiber have limited recyclability and pulp properties deteriorates on each recycling [61]. Waste paper recycling and utilization has been recognized because of concerns about environmental issues and economic aspects. In spite of several advantages of waste paper recycling there are several disadvantages are also associated with waste paper recycling. Waste paper recycling shows the problems such as ink removal during deinking, drainability of recycled pulp, stickies contamination and lower physical strength properties [30]. Successive drying and wetting of recycled fiber result in hornification of fiber that causes lower strength properties and drainability of recycled fiber. Hornified fibers become more rigid and produces less dense paper sheet with decreased inter-fiber bonding. Refining is used to recover the strength properties of recycled fiber. Although refining improves the strength properties of recycled fiber, it also increases the fine content and decreases the drainability of recycled pulp. Lower drainage rate decreases the speed of paper machine [62–64]. Pulp dewatering properties determines the energy efficiency of paper machine and thus cost efficiency of papermaking. Conventionally, dewatering is improved by using drainage aids wet-end section and/or higher wet pressing in press section. Drainage aids adversely affect sheet formation and higher wet press level decreases the bulk of the end product. Therefore, alternatives for dewatering are required so that paper machine can operate at high speed [65, 66]. Enzymatic treatment of pulp can improve drainage without adversely affecting the sheet formation and bulk of end product. Beating can effectively improve reclamation quality of recycled cellulose fibers. A study on refining of recycled unbleached eucalyptus cellulosic fiber explained the beating effect on fiber length, fine content and physical strength properties such as tensile strength, breaking length and stretch.

7.5 Enzyme-assisted refining studies at mill scale

169

Tensile index was improved by 39.9, 27.8, and 18.9% after first, second and third beaten cycle, respectively, compared to unbeaten pulp. Breaking length and stretch were also increased with increasing beating time. Furthermore, fine content, fiber swelling and WRV were increased significantly. WRV improved by 32.1% during first beaten cycle compared to unbeaten pulp [12]. Pommier et al. [67] performed enzymatic treatment of recycled fiber to study its effect on drainage. Cellulases and combination of celluloses and hemicellulases improved pulp drainability without affecting strength properties of pulp. At same drainability value, improved strength properties were observed when mechanical refining was used before enzymatic treatment compared to untreated pulp [67]. Maximino et al. [64] carried out a study on drainability and strength properties of recycled pulp containing old corrugated container fibers, kraft liners and white office paper. Enzymatic treatment of recycled fiber without refining increased freeness of pulp without any loss in tensile strength. Combined treatment (enzymatic and mechanical refining) resulted higher tensile index with significant improvement in drainability [64]. Ghosh et al. [68] studied physical strength properties of recycled fibers (old corrugated container pulp) during refining using ProLab refiner. At similar refining energy, enzymatic treatment increased short span compression (SCT) by 1.5 points compared to control. At the same SCT, freeness was significantly improved for enzymatic treatment. The enzyme treatment exhibited a significant improvement in burst strength and pulp with enzymatic treatment showed higher sheet density compared with control at similar refining energy. Hossein et al. [69] investigated the effect of changing the sequence of refining and enzymatic treatment on the properties of deinked pulp from mixed office waste paper (MOW). After enzymatic treatment and refining, deinking of waste paper was performed in each sequence. Drainage, optical and strength properties were improved for refining-enzymatic treatment-deinking (RED) sequence compared to enzymatic treatment- refining-deinking (ERD). During ERD sequence, some undesirable effects on fibers and ink particles were observed. Mechanical properties except tensile strength were adversely affected. Brightness and opacity were also decreased while ERIC value was increased significantly [69].

7.5 Enzyme-assisted refining studies at mill scale Several studies reported enzyme-assisted refining of various kinds of pulp at plant scale. Tripathi et al. [53] conducted a plant scale study on enzyme-assisted refining of mixed hardwood bleached unrefined pulp at one of the units of Ballarpur Industries Limited a wood based paper mill in western part of India. Five enzyme preparations named (Enzyme 1, Enzyme 2, Enzyme 3, Enzyme-4 and Enzyme 5) was taken for the study. Of these 5 enzyme preparations two were selected for plant trial based on laboratory scale experiments. Plant scale trial was conducted with Enzyme-3 for a period of 12 days at enzyme dose of 50–100 g/t. This treatment resulted in an improvement

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7 Enzyme-assisted pulp refining: an energy saving approach

in °SR by 2 units. Breaking length of paper was improved from 2961 to 3720 m in machine direction and 1567 to 1813 m in cross direction while tear index was reduced marginally. In another trial with Enzyme-4, °SR was improved by 5 units with a treatment period of 5 days. During this trial, refining energy requirement was reduced by approximately 20%. Breaking length was improved from 2961 to 3475and 1567 to 1798 in machine direction and cross direction, respectively. Tear index was decreased marginally [53]. During high-strength kraft pulp manufacturing, refining energy reduced by 25 kWh/TP with enzyme-assisted refining in process-scale trials at the mill producing packaging paper. During enzyme-assisted refining steam consumption was also reduced by 0.6 ton/ton of paper. Enzymatic treatment did not affect the physical strength properties. Even though, high-strength paper having low Gurley porosity was produced [70]. The same enzyme was used for another mill scale experiment producing coated paper and the enzymatic treatment resulted in a reduction in refining energy by 70 kWh/TP in softwood pulp and 30 kWh/TP in hardwood pulps. The steam consumption was reduced by approximately 0.5T/T of paper during enzymeassisted refining. The physical strength properties were also not affected [71, 72].

7.6 Limitations and future prospective Enzymes have been proved effective for development of desired papermaking properties. Enzymes improve the drainage and physical strength properties along with reduction in energy consumption [24]. The use of enzymes also reduces the steam consumption in dryer section. Although enzyme-assisted refining modifies the cellulosic fibers positively, higher enzyme dose or longer treatment time can reduce the pulp viscosity. Excessive enzyme treatment may also be detrimental for physical strength properties of pulp. Therefore, controlled enzyme action is required during enzyme-assisted refining. Most of research on enzyme-assisted refining has been reported at laboratory scale. The utilization of enzyme of at industrial scale has also been reported but more research is required at industrial scale to develop effective enzyme-assisted refining. Temperature, pH, enzyme dose, reaction time, ratio of different enzymes, type of enzyme and type of substrate determines the effect of enzyme treatment on refining. Therefore, further optimization studies are required to explain the effect enzyme-assisted refining on different substrates by the enzymes from various sources at different treatment conditions.

7.7 Conclusion Mechanical pulp refining develops optimum papermaking properties among the fibers. It results in several structural changes in the fibers including external

References

171

fibrillation, internal fibrillation, fines generation, shortening or cutting of fibers, and straightening of fibers. Mechanical pulp refining requires a large amount of electrical energy. To minimize the energy consumption several strategies including plate pattern adaptation, variation in pulp consistency, variation in refining temperature, use of chemicals, or redesigning refining strategy have been used. But enzyme-assisted refining is more popular due to its cost effectiveness and simple method of application. Enzyme-assisted refining reduces the energy consumption up to 50% and improves drainage and physical strength properties of pulp.

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[4]

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[17] Kerekes RJ, Soszynski RM, Doo PA. Characterizing refining action in PFI mills. TAPPI J. 2005;4:9–13. [18] Zeng X, Retulainen E, Heinemann S, Fu S. Fibre deformations induced by different mechanical treatments and their effect on zero-span strength. Nord Pulp Pap Res J. 2012;27:335. [19] Wistara N, Young RA. Properties and treatments of pulps from recycled paper. Part I. Physical and chemical properties of pulps. Cellulose. 1999;6:291–324. [20] Pattara PS Low consistency refining of chemical pulp, investigating the effect of intensity on fibre cutting. Thailand: Thesis, Asian Institute of Technology, Pulp and Paper Technology, 2012. [21] Clark JD. Fibrillation, free water, and fiber bonding. Tappi J. 1969;52:335–40. [22] Mou H, Iamazaki E, Zhan H, Orblin E, Fardim P. Advanced studies on the topochemistry of softwood fibres in low-consistency refining as analyzed by FE-SEM, XPS, and ToF-SIMS. BioResour. 2013;8:2325–36. [23] Maloney T, Paulapuro H. The formation of pores in the cell wall. J Pulp Pap Sci. 1999;25:430–6. [24] Singh R, Bhardwaj NK. Enzymatic refining of pulps: an overview. IPPTA. 2010;22:109–15. [25] Torres CE, Negro C, Fuente E, Blanco A. Enzymatic approaches in paper industry for pulp refining and biofilm control. Appl Microbiol Biotechnol. 2012;96:327–44. [26] Motamedian HR, Halilovic AE, Kulachenko A. Mechanisms of strength and stiffness improvement of paper after PFI refining with a focus on the effect of fines. Cellulose. 2019;26:4099–124. [27] Lin X, Wu Z, Zhang C, Liu S, Nie S. Enzymatic pulping of lignocellulosic biomass. Ind Crop Prod. 2018;120:16–24. [28] Znidaršiè-Plazl P, Rutar V, Ravnjak D. The effect of enzymatic treatments of pulps on fiber and paper properties. Chem Biochem Eng Q. 2009;23:497–506. [29] TAPPI. TAPPI standard test methods. 2007. [30] Verma PK, Bhardwaj NK, Singh SP. Upgradation of recycled pulp using endoglucanase enzyme produced by Pycnoporus sanguineus NFCCI-3628. Iran J Chem Chem Eng. 2017;36:191–201. [31] Cui L, Meddeb-Mouelhi F, Laframboise F, Beauregard M. Effect of commercial cellulases and refining on kraft pulp properties: Correlations between treatment impacts and enzymatic activity components. Carbohydr Polym. 2015;115:193–9. [32] Singh R, Bhardwaj NK, Choudhury B. An experimental study of the effect of enzyme- assisted refining on energy consumption and paper properties for mixed hardwood pulp. Appita J. 2014;67:226–31. [33] Lecourt M, Sigoillot J, Petit-conil M. Cellulase-assisted refining of chemical pulps : impact of enzymatic charge and refining intensity on energy consumption and pulp quality. Process Biochem. 2010;45:1274–8. [34] Singh R, Bhardwaj NK, Choudhury B. Cellulase-assisted refining optimization for saving electrical energy demand and pulp quality evaluation. J Sci Ind Res. 2015;74:471–5. [35] Kumar A, Dutt D, Gautam A. Production of crude enzyme from Aspergillus nidulans AKB-25 using black gram residue as the substrate and its industrial applications. J Genet Eng Biotechnol. 2016;14:107–18. [36] Pathak P, Bhardwaj N, Singh A. Production of crude cellulase and xylanase from Trichoderma harzianum PPDDN10 NFCCI-2925 and its application in photocopier waste paper recycling. Appl Biochem Biotechnol. 2014;172:3776–97. [37] Kumar A, Gautam A, Dutt D, Yadav M, Sehrawat N, Kumar P. Applications of microbial technology in the pulp and paper industry. In: Kumar V, Singh G, Aggarwa N, editors. Microbiology and biotechnology for a sustainable environment. Nova Science Publishers, Inc., New York, USA, 2017:185–206.

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[60] Ahmad S, Jain R, Mediratta R, Prasad KD, Arora SS. Enzymatic treatment on chemical pulp in beating/refining process-an attempt towards energy conservation. Ippta J. 2006;18:127–32. [61] Gautam A, Kumar A, Dutt D. Effects of ethanol addition and biological pretreatment on soda pulping of Eulaliopsis binata. J Biomater Nanobiotechnol. 2016;7:78–90. [62] Maximino MG, Taleb MC, Adell AM. Influence of the enzyme addition point on recycled industrial pulp properties. BioResour. 2013;8:1089–99. [63] Bawden AD, Kibblewhite RP. Effects of multiple drying treatments on kraft fibre walls. J Pulp Pap Sci. 1997;23:J34–346. [64] Maximino MG, Taleb MC, Adell AM, Formento JC. Application of hydrolitic enzymes and refining on recycled fibers. Cellul Chem Technol. 2011;45:397–403. [65] Verma PK, Bhardwaj NK, Singh SP. Improving the material efficiency of recycled furnish for papermaking through enzyme modifications. Can J Chem Eng. 2016;94:430–8. [66] Antunes E, Garcia FA, Ferreira P, Blanco A, Negro C, Rasteiro MG. Use of new branched cationic polyacrylamides to improve retention and drainage in papermaking. Ind Eng Chem Res. 2008;47:9370–5. [67] Pommier JC, Fuentes JL, Goma G. Using enzymes to improve the process and quality in the recycled paper industry. Tappi J. 1989;71:187–91. [68] Ghosh A, Thornton B, Hart PW. Effect of pH and enzymes on strength of recycled fibers during refining. Tappi J. 2018;17:407–15. [69] Hossein MA, Talaeipour M, Hemmasi A, Bazyar B, Mahdavi S. Effects of sequencing enzyme application and refining on DIP properties produced from mixed office waste paper. BioResour. 2015;10:4768–83. [70] Bajpai P, Mishra SP, Mishra OP, Kumar S, Bajpai PK. Use of enzymes for reduction in refining energy- laboratory studies. Tappi J. 2006;5:25–32. [71] Bajpai P, Bajpai PK, Mishra SP, Mishra OP, Kumar S Enzymatic refining of pulp-case studies. In: Paperex, 7th International conference on pulp paper and conversion industry. New Delhi, 2005:143–59. [72] Bajpai P. Fiber modification. In: Bajpai P, editor. Biotechnology for pulp and paper processing. Singapore: Springer Singapore, 2018:241–71.

Amit Kumar

8 Dissolving pulp production: Cellulases and xylanases for the enhancement of cellulose accessibility and reactivity Abstract: Dissolving pulps are high-grade cellulose pulps that have minimum amount of non-cellulosic impurities. Dissolving pulps are the basic source for the manufacturing of several cellulosic products such as viscose, lyocell, cellulose acetates, cellulose nitrates, carboxymethyl-cellulose, etc. Dissolving pulps are mainly manufactured by pre-hydrolysis kraft and acid sulphite pulping. A high reactivity of dissolving pulps is desirable for its eco-friendly utilization for several purposes. Several approaches including mechanical, chemical, ultrasonic, and enzymatic treatments have been employed for the improvement of pulp reactivity. This review mainly focussed on pulp reactivity improvement through enzymatic approaches. Cellulases and xylanase have been proved effective for the improvement of pulp reactivity of dissolving pulp from different sources. The different combinations of cellulase, xylanase, and mechanical refining have been tested and found more effective rather than the single one. Keywords: Viscose, pre-hydrolysis kraft pulping, cellulose reactivity, Fock reactivity, cellulases, pulp viscosity

8.1 Introduction Dissolving pulps contributes approximately 3% pulp manufactured around the world. Recently, the production of dissolving pulps has grown rapidly all over the world and showed important role in the pulp market [1]. Dissolving pulp is a special chemical pulp consisting of 90–99% cellulose, low content of hemicelluloses (< 4%), and traces of lignin and resin. It shows uniform molecular weight distribution and high brightness [2–4]. Dissolving pulp acts as raw material for the manufacturing of a range of cellulose derivatives such as regenerated cellulose (e.g. viscose rayon), cellulose esters (e.g. acetates and nitrates), cellulose ethers (e.g. carboxymethyl- and ethyl-cellulose), and other cellulose-based products (nano- and micro-crystalline celluloses) [5–7]. The desired alpha cellulose content of dissolving pulps is dependent upon the end use of the dissolving pulps. The desired alpha cellulose contents

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Kumar, A., Dissolving pulp production: Cellulases and xylanases for the enhancement of cellulose accessibility and reactivity Physical Sciences Reviews [Online] 2021, 6. DOI: 10.1515/psr-2019-0047 https://doi.org/10.1515/9783110592412-008

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for rayon/cellophane, cellulose acetate and nitrocellulose are 90–92, 95–97, and 98%, respectively [8–10]. Most of dissolving pulps are produced by pre-hydrolysis kraft and acid sulphite processes. The quality of dissolving pulp is essential for cellulosic products such as carboxymethyl-cellulose, viscose, cellulose film, and sausage skin. The properties of raw materials and pulping process determine the quality of dissolving pulp. Dissolving pulps should have some special properties such as a high purity, uniform molecular weight distribution, and good reactivity and accessibility of the cellulose to chemicals. The reactivity of cellulose pulp is one of the essential properties of dissolving pulp that is defined as the capacity of dissolving pulp to participate in diverse chemical reactions [11, 12]. The processability, reactivity, and viscosity are very important parameters for dissolving pulp. High reactivity and low viscosity can enhance the homogeneity and quality of cellulose-end products and minimizes the demands of reactants. But, it is difficult to control of viscosity and increase reactivity due to the compact fibrillar structure of cellulose. Several treatments including chemical, mechanical, and biological have been evaluated for the improvement of dissolving pulp properties. Among these, cellulase treatment has proved to be promising method for this purpose that is eco-friendly approach and performed under mild processing conditions [13–16]. Cellulases act on cellulose structure that increases its accessibility towards reactants and facilitates the xanthation reactions during rayon production process [11, 15].

8.2 Methods of dissolving pulp production Presently, the commercial production of dissolving pulp is carried out by acid sulphite pulping and PHK pulping. Recently, dissolving pulp production has also been carried out by upgradation of paper grade pulp [18]. There are some issues in dissolving pulp production such as lower yield (generally less than 30%), higher chemical costs, and higher inventories requirements as technical conditions for pulping and bleaching that makes production investment more than other chemical-grade pulps [19, 20].

8.2.1 Acid sulphite pulping method Acid sulphite pulping is performed under acidic conditions. In a typical ammoniabased sulphate process, hemicelluloses, lignin, and other minor components are removed and then dissolved in spent sulphite liquor that is not easily recovered for further utilization. After further purification by hot alkali extraction, or multi-bleaching steps process, brown stock is converted into dissolving pulp [21, 22].

8.2 Methods of dissolving pulp production

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8.2.2 Pre-hydrolysis kraft pulping The PHK process is performed by two steps including acidic pre-hydrolysis and alkaline kraft pulping. PHK is generally employed for raw material with high hemicelluloses content and produces dissolving pulp with low hemicelluloses content [21]. PHK is popular for dissolving pulp production due to intrinsic advantages such as capital investment, operation, and environmental compatibility. Moreover, it is compatible with variety of raw materials and alkali is recovered more effectively [6, 22, 23]. During prehydrolysis stage, acetic acid is generated that initiates auto-hydrolysis. The auto-hydrolysis process results in the removal of majority of hemicelluloses and a fraction of lignin. In the next step of kraft pulping, a majority of lignin is dissolved from the raw material, while in the same process a significant amount of hemicelluloses and a fraction of cellulose are also removed. Furthermore, the residual lignin is removed using bleaching process after kraft pulping [5]. The low reactivity of dissolving is the main challenge for pulp produced by PHK method as compared to acid sulphite pulping. The low reactivity of PHK pulp is due to the peeling reactions that cannot generate the accessible hydroxyl groups as much as acid hydrolysis of acid sulphite pulping. Therefore, reactivity improvement is necessary for PHK pulp after dissolving pulp production [23].

8.2.3 Upgradation of paper grade pulp In last decade, research has been focussed on upgradation of paper-grade pulp to dissolving pulp as an alternative to sulphite and PHK processes. A paper-grade pulp contains a higher hemicelluloses and lignin content. So, selective hydrolysis and extraction of hemicelluloses to maintain the high reactivity of pulp is the main challenge for paper-grade pulp conversion to dissolving pulp. Alkali-soluble hemicelluloses show dissolution in steeping liquor that affects reactivity and quality of resultant viscose. Therefore, the alkali-soluble hemicelluloses are essentially removed for viscose production. The upgradation of paper-grade pulp to dissolving pulp requires an additional step to meet the necessary standards of dissolving pulp [24, 25]. Several treatments including hot caustic extraction, cold caustic extraction, organosolv extraction, ionic liquor extraction, and the combination of enzymatic treatment and caustic extraction have been studied for selective removal of hemicelluloses from paper-grade pulp [5, 18, 24].

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8.3 Dissolving pulp reactivity improvement The chemical composition, fiber structure, and fiber morphology are the main factors that determine the reactivity of dissolving pulp. Intra- and intermolecular bonds along with hydrophobic interactions affects the reactivity of cellulose. Intra- and intermolecular bonds results in compact fibrillar structure of cellulose. These factors generally cause limited accessibility and reactivity of cellulose towards reagents [26, 27]. There is high demand for the dissolving pulp having high pulp reactivity and suitable pulp viscosity. The Fock reactivity is another term for pulp recativity and it is defined as the capacity of cellulose to react with CS2 under defined conditions. The reactivity of dissolving pulp shows it’s’ processability for viscose and gives the measurement of reacted cellulose in percentage during the xanthation reaction [10, 28, 29]. The reactivity of pulp sometimes referred as accessibility that is an intrinsic property of the pulp that is determined by chemical and structural properties. Pulp reactivity is expressed as the availability of functional groups (reaction sites) in the pulp for reaction with chemical agents and/or solvents [18]. To improve dissolving pulp reactivity several methods including mechanical [23, 26], chemical [30, 31], ultrasonic [32], thermal degradation [18], and enzymatic methods [28, 33] have been employed (Figure 8.1). A

Figure 8.1: Methods of reactivity improvements of dissolving pulp.

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179

number methods including Fock test, viscose filterability, iodine sorption, water retention value, phosphitylation, and quantitative 31 P NMR spectroscopy are available for determination of dissolving pulps reactivity. Among them Fock test and viscose filterability test are most popular in viscose rayon industry [16, 20].

8.3.1 Mechanical treatment Several mechanical treatments including ball milling [34, 35], PFI refining [16], Hollander beating [26], and grinding [16] have been evaluated for the reactivity improvements of dissolving pulps. The ball milling treatment results in size reduction and decreases the crystallinity and degree of polymerization of cellulose. The use of ball milling is not feasible due to higher energy consumption and poor energy efficiency [16, 34, 35]. PFI refining has been employed for the reactivity improvement of PHK pulps. PFI refining results in the external and internal fibrillation of pulp rather than the fiber cutting and this fibrillation assists inter- and intra fibril/fiber hydrogen bond formation. This would be partially responsible for the lower increase in by PFI refining [16, 26, 36]. Hollander beating or Valley beater works with a lower pulp consistency (1.57%) and it can perform a greater cutting action compared with the main fibrillation of PFI refining [36]. The excessive mechanical treatment may decrease the pulp viscosity and strength of resultant rayon products. A better strategy for accessibility improvement of inner regions of pulp fibers might be the use of different combinations of mechanical, enzymatic and chemical treatments [18, 33, 37].

8.3.2 Chemical treatments A substantial amount of hemicelluloses are available in bleached pulps such as 5– 20% in kraft wood pulp. This amount of hemicelluloses can be removed either by enzymatic methods or chemical methods such as hot water extraction, cold caustic extraction and ionic liquid extraction, ozone treatment, and acid treatment [38, 39]. The cold caustic extraction method is considered as an effective method for the removal of hemicelluloses due to high yield and purity of dissolving pulp. After extraction, the remaining solution (i.e. used lye) retains a substantial amount of alkali (approximately 100 g/L). Therefore, it can be reused as cooking liquor for alkaline pulping or mixed to black liquor in the chemical recovery system [38]. He et al. [38] performed cold caustic extraction process and reutilized the caustic lye to manufacture dissolving pulp and the findings demonstrated that lye can be reutilized about 12 times. Acidic treatments are also employed for the reactivity improvement of dissolving pulp. Acidic treatment causes the hydrolysis of hemicelluloses and their removal from the fiber. It also causes the degradation of cellulose to reduce its degree

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of polymerization [18, 40]. Wang et al. [30] used phosphotungstic acid for reactivity improvement and control of viscosity of dissolving pulp. Phosphotungstic acid is a heteropoly acid that got much attention in recent years due to its excellent physiochemical properties such as strong Bronsted acidity, good structural stability, and low corrosion to metal equipment [30, 41]. The phosphotungstic acid treatment improved the Fock reactivity from 49.1% to 74.1% due to increased fiber accessibility [30]. Ionic liquid treatment has emerged as a potential method to produce dissolving pulp production. There are two types of ionic liquid treatments: first one ionic liquid that dissolve all components of biomass including cellulose while second ionic liquid treatment selectively extract lignin and hemicelluloses. The latter approach generates cellulose rich pulp [42]. Laine et al. [43] carried out ionic pretreatment of softwood kraft pulp to produce dissolving pulp while hemicelluloses were also recovered. The pulp treatment with 1-ethyl-3-methylimidazolium acetate and water resulted in a total of 95.5 wt.% of dissolving pulp and hemicellulose fraction. The obtained pulp showed high purity with an alkali resistance (R18) value of 97.8%.

8.3.3 Ultrasonic treatment Ultrasonic treatment is green and energy-saving pretreatment method for lignocellulosic biomass. Ultrasound treatment of biomass causes the alterations in surface structures and generation of oxidizing radicals which act on lignocellulosic biomass [44]. The diffusion of ultrasonic waves in the liquid medium generates a large amount of energy. During ultrasonic treatment, gas/vapour bubbles grow to a certain critical size and becomes unstable to collapse abruptly. This process creates a high temperature (2000–5000 K) and pressure (1800 atm) [32, 45]. Zhou et al. [32] performed the ultrasonic treatment of softwood PHK-based dissolving pulp to improve its reactivity and accessibility through cavitation of ultrasound. The ultrasonic treatment decreased the cellulose crystallinity and increased specific surface area, average pore diameter in fibers, and water retention value. The findings showed that Fock reactivity of resultant pulp increased from 53.3% to 73.2% and the viscose filtering value decreased from 3743 to 15.4s due to cavitation of ultrasound after 5 min of treatment at 540 W. Moreover, the intrinsic viscosity and fiber length were also decreased slightly.

8.3.4 Enzymatic treatment for reactivity improvement Enzymes are biocatalysts that are green alternatives for pulp and paper processing. Cellulases, xylanases, mannanases, and laccases have been employed for the environment-friendly and efficient manufacturing of dissolving pulp. The enzymatic treatments are found to purify cellulose, increase brightness, and improve the reactivity

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of resultant pulp. The manufacturing of dissolving pulp commonly utilizes cellulases, xylanases or their combinations (Table 8.1) [2, 46]. Table 8.1: Effect of enzymatic treatment on reactivity and viscosity of dissolving pulp. Type of treatment

Type of pulp

Treatment conditions

Effect on pulp reactivity and viscosity

Xylanase

Bamboo dissolving pulp

Xylanase dosage: U/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

After xylanase treatment, pentosan content was decreased from . to % and reactivity improved drastically

[]

Xylanase + refining

Kraft bamboo pulp

Xylanase treatment + refining improved hemicelluloses removal by . to % Fock reactivity was improved by % compared to control

[]

Cellulase

Semibleached dissolving pulp

Xylanase dosage: – mL/tonne OD pulp, temperature: –° C, pH: .–. PFI refining: –, revolutions, pulp consistency: % Cellulase dosage: . U/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

Hypochlorite treatment resulted in reactivity of .% Cellulase treatment showed the reactivity of %

[]

Cellulase

Harwood kraft dissolving pulp

Cellulase dosage:  U/g OD pulp

After enzymatic treatment, reactivity of pulp increased from .% to .%

[]

Cellulase

Harwood kraft dissolving pulp

Cellulase dosage: .– U/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

Fock reactivity of pulp increased from .% to .% Pulp viscosity–of resultant pulp was decreased from . mL/ g to . mL/g

[]

Endoglucanase dosage:  ECU/g OD pulp, pH: ., temperature: °C, treatment time . h, pulp concentration: %

After enzymatic treatment, reactivity of pulp increased from .% to .%

[]

Endoglucanase Softwood sulphite dissolving pulp

References

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Table 8.1 (continued ) Type of treatment

Type of pulp

Treatment conditions

Effect on pulp reactivity and viscosity

Endoglucanase Softwood sulphite dissolving pulp

Endoglucanase dosage:  With enzymatic treatment % reactivity was ECU/g OD pulp, pH: ., achieved temperature: °C, treatment time  min, pulp concentration: %

Cellulase + refining

Harwood kraft dissolving pulp

Refining pretreatment:  PFI revolutions, consistency: % Cellulase dosage: . U/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

Cellulase + xylanase

Xylanase + cellulase

Xylanase + cellulase

References

[]

Combined treatment increased Fock reactivity from .% to .% Pulp viscosity of resultant pulp was decreased from . mL/g to  mL/g

[]

Enzyme dosage: – U/ g OD pulp (based on endoglucanase), pH: ., temperature: °C, treatment time  min, consistency: % Eucalyptus Xylanase treatment: pH: globulus ., temperature: °C, kraft pulp treatment time  min, consistency: % Endoglucanase dosage: – U/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

Fungal enzyme improved the pulp reactivity from .% to .% at enzyme dosage of  U/g OD pulp

[]

Xylanase pretreatment resulted in increased pulp reactivity and brightness Endoglucanase posttreatment resulted in adjustment of degree of polymerization and improvement in pulp viscosity

[]

Xylanase dosage: – EXU/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: % Endoglucanase dosage: – ECU/g OD pulp, pH: ., temperature: °C, treatment time  min, consistency: %

Endoglucanase treatment improved cellulose reactivity and reduced pulp viscosity Alkali extraction after xylanase treatment followed by endoglucanase treatment significantly reduced xylan content and the reactivity reached up to –%

[]

Bleached sulphite dissolving pulp

Bleached hardwood kraft pulp

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8.3.4.1 Xylanases treatment for reactivity improvement Hemicellulases are the enzymes that perform the breakdown of hemicelluloses. Hemicellulases are categorized into several groups including endoxylanase, β-xylosidase, α-L-arabinofuranosidase, α-D-glucuronidase, acetyl xylan esterase, and feruloyl & coumaroyl esterase based on their action on specific sites of hemicelluloses. Endoxylanases are generally called as xylanases [47]. Xylanases are utilized for several purposes such as pre-bleaching of pulp, deinking of waste paper, enzyme-assisted refining, and debarking of wood logs in pulp and paper industry [48]. Xylanases treatment followed by alkali extraction is effective method for hemicellulose removal for dissolving pulp production. The action of xylanases lowers the content of pentosans in dissolving pulp and thus improves the its reactivity [49]. Xylanases hydrolyze 1, 4-β-Dxylosidic linkages in xylan that is a major component of hemicelluloses. Xylan is located at interface between lignin and cellulose and protects cellulose microfibrils against biodegradation [50]. Xylanase treatment results in loosening of fiber due to partial depolymerization of hemicelluloses. It has been showed reprecipitated xylan on the surface of fibers after kraft pulping acts as barrier for extraction of residual lignin. Xylanase treatment partially hydrolyzes this precipitated xylan and improves the permeability of pulp for subsequent alkali extraction process [51–53]. Xylanases are substrate specific and do not cause any harm to native cellulose in dissolving pulp. But, very high dosages of xylanases results in structural modifications in the cellulose matrix. A too high dosage of xylanase may cause more dense fiber due to internal collapse of fiber, such fiber have low accessibility for endoglucanase for subsequent stages [50, 54]. Therefore, optimum xylanase dosage is necessary for desired properties of resultant pulp. Viscose manufacturing was carried out by xylanase pretreatment of mixed hardwood kraft pulp. Xylanase treatment prior to alkali extraction had significant effect on pentosans removal. An enzyme dosage of 50 AXU/g was found optimum for Fock reactivity and viscosity improvement. Alkali extraction without xylanase pretreatemnt showed decreased Fock reactivity, alpha cellulose, ISO brightness and viscosity [50]. Dissolving pulp production was performed by three stage process consisting of sequential xylanase pretreatment, alkaline extraction and cellulase treatment of bleached kraft softwood (Southern pine) pulp. The three stage process resulted in the improvement of pulp solubility that reached up to 81% from the initial value of 29%. The cellulose crystallinity and specific surface area were not affected during enzymatic treatment [54]. Mechanical refining and xylanase treatment were found more effective for hemicelluloses removal. The xylan content was affected slightly by mechanical refining alone. When xylanase treatment was performed after mechanical refining, there was improvement of 17.4 to 25% depending upon the PFI revolutions during mechanical refining [33, 55]. The removal of hemicelluloses is a critical step for upgradation of paper pulp to dissolving pulp. The upgradation of paper-grade pulp into dissolving pulp showed challenges for retaining high reactivity. This requires a gentle treatment that is able

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to reduce hemicelluloses content to desired level. Xylanase treatment has proved effective for hemicellulose removal and improves the pulp reactivity to achieve a high quality of dissolving pulp [50]. Gehmayr et al. [39] studied the effect of enzymatic treatment on oxygen-delignified pulp of Eucalyptus globulus paper-grade kraft pulp during total chlorine free bleaching. The two step treatment process consisting xylanase pretreatment and cold caustic extraction (with reduced alkalinity) resulted in efficient removal of hemicelluloses and improved pulp brightness significantly due removal of hexenuronic acid content. Improved pulp reactivity and viscose dope quality were also obtained after xylanase pretreatment. The degree of polymerization of pulp was precisely adjusted and viscose pulp reactivity was also improved with endoglucanase post-treatment. 8.3.4.2 Cellulases treatment for reactivity improvement Cellulases are multi-component enzyme system that composed of endoglucanase, exoglucanase and β-glucosidase. Endoglucanase acts on amorphous part of cellulose, it randomly cleave the β-1, 4-glycosidic bond. Exoglucanase attacks reducing and non-reducing chain ends of crystalline cellulose that release the cellobiose unit. The cellobiose is hydrolyzed into glucose by the action of β-glucosidase [61, 62]. Cellulase treatment is a promising approach for the activation of dissolving pulp in terms of viscosity and reactivity [63]. The cellulase treatment of pulp not only adjusts the pulp viscosity but also improves the pulp reactivity. Endoglucanase randomly acts on β-glycosidic linkage in cellulose and preferentially degrade the amorphous part of cellulose located on the fiber surface and in between microfibrils. The degradation of amorphous region of cellulose exposes the crystalline region and improves swelling ability and reactivity of pulp [1, 39]. The cellulase treatment breaks hydrogen bonds to expose the hydroxyl groups available in cellulosic chains. The increase in the freely available hydroxyl groups improves the pulp reactivity due to the involvement of hydroxyl groups in derivatization reactions. Moreover, the action of cellulase on cellulose improves the pore volume of the dissolving pulp due to creation of additional openings in cellulosic fibers structure [28]. The physical close contact of enzyme molecule and cellulose is prerequisite for the effectiveness of cellulase treatment. Generally, cellulase molecules preferably adsorb on the substrates having a high cellulose accessibility that is the combined result of smaller size, high specific surface area, large porosity and low crystallinity [13, 64, 65]. Duan et al. [64] performed the cellulase treatment of three pulp fractions namely long fiber (LF), mid-fiber (MF), and short fiber (SF) from hardwood based dissolving pulp. The cellulase treatment with enzyme dosage of 0.23 U/g OD pulp was carried out at 10% consistency of each fraction at pH of 4.8 and temperature 55°C. SF showed highest accessibility, lowest viscosity, and maximum cellulase adsorption capacity while LF showed opposite trend for these parameters. At a fixed value of viscosity (approximately 460 mL/g), cellulase treatment based combined process (20, 40, 120 min

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for SF, MF and LF, respectively) caused a higher Fock reactivity (85.6%) as compared to conventional cellulase treatment (76.3%). The production of high-value cellulose materials required efficient removal of cellulose and hemicelluloses from pulp fiber. The cellulase pretreatment, followed by cold caustic extraction of softwood sulphite pulp showed suitable morphological changes in the dissolving pulp fiber. The specific surface area, pore volume & diameter, and water retention value were improved. These changes resulted in improved fiber swelling in subsequent CCE treatment that caused increased hemicelluloses removal. Combined cellulase and CCE treatment resulted in higher cellulose purity (97.9%) and hemicelluloses removal selectivity reached up to 76.49% at cellulase dosage of 1 mg/g [66]. 8.3.4.2.1 Approaches for cellulase efficiency improvement The combinations of mechanical refining and enzymatic treatment have also been reported for the improvement of cellulose accessibility and pulp reactivity. The mechanical refining of pulp results in internal fibrillation, external fibrillation, shortening and cutting of fibers, and fines generation. The pre-refining of pulp assists to open structure of cellulosic fiber that improves the specific surface area for adsorption of enzyme molecules [59, 62, 67]. Yang et al. [59] studied the effect of pre-refining and cellulase treatment on the reactivity and viscosity of hardwood dissolving pulp. The combined process consisting of mechanical refining and polydiallyldimethylammonium chloride (PDADMAC) assisted cellulase treatment improved the Fock reactivity from 41.5% to 88.7% and the pulp viscosity was decreased from 628.8 mL/g to 407.8 mL/g. The combined treatment decreased the mean fiber length from 0.72 mm to 0.59 mm while cellulase treatment alone showed the mean fiber length of 0.71 mm. The combined treatment also increased the fine content from 24.1% to 36.4%. The lower mean fiber length and higher fine content is responsible for higher surface area. The reactivity or accessibility of pulp towards reactants depends upon the specific surface area of pulp [59, 68]. Miao et al. [69] studied the effect of enzymatic and mechanical treatment of hardwood kraft pulp and found the improvement in Fock reactivity from 49.6 to 75.8% with cellulase treatment (dosage of 0.3 U/g, reaction time of 2 h). The Fock reactivity further improved up to 81.7% by mechanical treatment prior to enzymatic treatment. Moreover, the specific surface area and fine content ware also increased substantially by the combined treatment as compared to control. The results indicated that mechanical refining assists the enzymatic treatment and improves efficiency of enzymes [69]. A recent study reported integrated process of depth refining and cellulase treatment for upgradation of PHK pulp. The integrated treatment resulted in the improvement in Fock reactivity from 54.8% (19 °SR) to 78.0% (50 °SR) that was mainly due to intermolecular hydrogen bond changes, verified by FTIR analysis. Furthermore, the cellulase adsorption ratio of refined pulp (30–50 °SR) was improved in a range of

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39.7–71.2%, which was supported by increased enzymatic accessibility. The integrated process showed improved reactivity at enzyme dose of 0.5 mg/g pulp as compared to control (1 mg/g pulp) [23]. Cellulase treatment has been proved an efficient method to increase pulp reactivity. But, the cost of enzyme is high that limits the scale up applications of enzymes. However, different approaches have been tested to improve the efficiency of enzyme that may lower of cost of the process. The use of surfactant during cellulase treatment can improve the efficiency of enzyme. Tween-80 was introduced during enzymatic treatment and results showed improvement in the cellulase efficiency. The addition of Tween-80 (0.1 g/L) during enzymatic hydrolysis reduced surface tension by 28% and showed cellulose chain scission rate more than 2 times higher compared to control (enzymatic treatment without surfactant). Tween-80 assisted cellulase treatment resulted in increased Fock reactivity and decreased viscosity as compared to control [63]. The adsorption of cellulase molecules on the cellulosic fibers is critical for catalytic efficiency of cellulases. Improved adsorption of cellulase molecules increases the efficiency of enzyme that makes the process cost-effective by minimizing the required cellulase dosage. Qin et al. [70] performed two step process for reactivity improvement of kraft-based dissolving pulp due to improved cellulase efficiency. The phosphotungstic acid (PTA)-assisted pre-refining was followed by the cellulase treatment. PTA-assisted pre-refining at 8000 revolutions and 90°C resulted in improvement of cellulase adsorption from 29.1% to 49.7%. The Fock reactivity of resultant pulp was improved from 31.5% to 74.4% at low cellulase loading (0.5 mg cellulase/g ODP) while the viscosity of pulp was decreased from 665 to 430 mL/g due to higher cellulase adsorption and reaction efficiency. Furthermore, the use of PTA may also be beneficial due to high usability (more than 86%) during PTA-assisted pre-refining. Therefore this two step process showed potential for large-scale manufacturing of high-grade dissolving pulp. The recovery or recycling of cellulase is a promising approach to minimize the enzyme cost. After enzymatic treatment of pulp, cellulase molecules remain in the aqueous phase and the recovery and reuse of this liquor reduces the cost of enzyme significantly [71, 72]. Several researchers studied the recovery cellulase during enzymatic hydrolysis of lignocellulosic biomass [73, 74]. Cellulase recycling with fresh enzyme addition strategy was adopted to improve the Fock reactivity of hardwood kraft-based dissolving pulp. The results showed 35.1–48.8% recovery of cellulase from filtered liquor after five rounds of recycling that was reused for enzymatic treatment. The recycled cellulase supplemented with 1 mg/g of fresh cellulase resulted in viscosity and Fock reactivity of 470 mL/g and 80%, respectively, that was comparable with cellulase dosage of 2 mg/g [72]. The enzymatic treatment is performed at low, medium, and high consistency. The enzymatic treatment that is performed at higher pulp consistency improves the interaction of cellulase molecules and cellulosic fibers. This improves the adsorption of cellulase on cellulose thus increases the enzymatic efficiency [13, 75]. Wang

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et al. [13] studied the effect of consistency on cellulase treatment of prehydrolysis kraft based dissolving hardwood pulp. The cellulase adsorption ratio was found to increase at high consistency (20–24%) as compared to low consistency (3%). At 20% of consistency, pulp viscosity decreased from 510 mL/g to 471 mL/g as compared to low consistency of 3% at 24 h while the Fock reactivity improved from 70.3% to 77.3% at higher consistency. The analysis of alpha cellulose, alkali solubility and molecular weight distribution also supported that cellulase treatment at high consistency was more effective as compared to low consistency. The fiber hornification is a substrate related factor that affects the efficiency of enzymatic treatment. The process of drying after any aqueous treatment of lignocellulosic material causes the irreversible pore collapse in the microstructure of cellulosic fiber and water binding ability of fiber is also lost irreversibly. This process is called as fiber hornification. It decreases the enzymatic hydrolysis of cellulosic pulp [71, 76]. The impact of fiber hornification on cellulose accessibility and enzymatic viscosity control was determined for kraft dissolving pulp. The cellulase enzyme was used for the treatment of oven dried (OD), air dried (AD), and never dried (ND) pulp samples and results demonstrated that the enzymatic efficency in terms of pulp viscosity decrease was governed by drying process. The strongest viscosity drop was observed in the ND samples, followed by the AD samples. The minimum drop in pulp viscosity was found from the OD samples. The results indicated that enzymatic treatment must be performed in its bleaching plant before drying of pulp [76].

8.4 Applications of dissolving pulp Dissolving pulp is a uniform product that contains more than 90% of alpha-cellulose. It is a raw material for the production of cellulose-based derivatives and regenerates. Cellulose-based chemical derivatives are utilized for cellulose plastic materials and lacquers. The cellulose regenerates are mainly employed for the production of manmade fiber [77, 78]. Presently, a large share of dissolving pulps is used in textile industries for the production of viscose rayon and other fibers. The use of dissolving pulp is extensive, over 70% of dissolving pulp is consumed for the production of viscose alone. Viscose is manufactured mainly by dissolving pulp and cotton. In recent years, the production of cotton has decreased by 7% and consumption has increased. Therefore, dissolving has become the chief raw material for viscose production [2, 10]. The cellulose-based derivatives include cellulose acetates, cellulose nitrates, carboxymethyl cellulose, cellulose ethers and other derivatives The cellulose derivatives are used for production of paints, binders, glues, artificial leather, cigarette filters and various commodities manufactured by food, pharmaceutical, and cosmetic industries [77, 79].

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8.5 Conclusion The global demand of dissolving pulp has been increased in recent years and it is expected to increase more in future. The supply of high purity dissolving pulp through environmentally sustainable approaches is essential. Enzymes are environmentally friendly and efficient alternatives to achieve high pulp reactivity and accessibility that control viscosity and adjust the molecular weight also. Cellulases and xylanases have been mainly employed for dissolving pulp manufacturing. The cost of enzymes is a critical issue for their utilization at industrial scale. Several approaches such enzyme recycling, surfactant supplementation, mechanical refining pretreatment, and high pulp consistency treatment have been utilized to improve the efficiency of enzymes that may decrease the process cost.

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Puneet Pathak, Varun Kumar, Nishi Kant Bhardwaj and Chhavi Sharma

9 Slime control in paper mill using biological agents as biocides Abstract: The environmental conditions of paper mills are suitable for the growth of slime-forming microorganisms due to the supply of nutrients, favorable temperature, and moisture. The slime formation causes the spoilage of raw materials & additives, breaks in the paper during papermaking, loss of production, reduces the hygienic quality of the end products, produces off-spec and rejected products, creates microbiological corrosion, and produces harmful gases. The main microorganisms are Bacteria (mainly Bacillus spp., Achromobacter spp., Enterobacter spp., Pseudomonas spp., Clostridium, etc.), Fungi (Aspergillus, Penicillium, Saccharomyces, etc.), and Algae. Besides the use of conventional toxic chemical biocides or slimicides, slime formation can also be controlled in an eco-friendly way using enzymes, bacteriophages, biodispersants, and biocontrol agents alone or along with biocides to remove the slime. Enzymes have shown their effectiveness over conventional chemicals due to nontoxic and biodegradable nature to provide clean and sustainable technology. Globally enzymes are being used at some of the paper mills and many enzymatic products are presently being prepared and under the trail at laboratory scale. The specificity of enzymes to degrade a specific substrate is the main drawback of controlling the mixed population of microorganisms present in slime. The enzyme has the potential to provide the chemical biocide-free solution as a useful alternative in the future with the development of new technologies. Microorganisms control in the paper mill may appear as a costly offer but the cost of uncontrolled microbial growth can be much higher leading to slime production and large economic drain. Keywords: slime control, bIocides, enzymes, biodispersants, bacteriophages, paper mill

9.1 Introduction Globally, the paper industry is under pressure to reduce their specific water consumption per ton of product (i.e. pulp, paper, or board) due to environmental restrictions [1]. As a result, other problems also arise related to higher process temperatures

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Pathak, P., Kumar, V., Bhardwaj, N. K., Sharma, C. Slime control in paper mill using biological agents as biocides Physical Sciences Reviews [Online] 2021, 6. DOI: 10.1515/psr-2019-0049 https://doi.org/10.1515/9783110592412-009

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(above 50 °C), increased concentrations of suspended solids, colloidal material, and dissolved material in the process circulation [2]. These dissolved materials act as nutrients to favor microbial growth or biofilm formation [3]. Generally, temperature range of 40–60 °C is optimum for the microbial growth to form biofilm but for some thermophilic bacteria higher temperature is favorable. Microorganisms have a natural tendency to attach in contact with wet surfaces, to multiply and to implant themselves in a slimy matrix made up of extracellular polymeric substances (EPS), forming a biofilm [4]. In the paper industry, biofilm formation is often considered to create industry, environment, and health-related problems causing failure of operation and a loss of product quality. The process conditions of paper mills are favorable for the reproduction of slime-forming microorganisms because of the supply of nutrients, temperature, and moisture. This microbial growth causes spoilage or deterioration of raw materials and additives used during papermaking resulting in the economic loss [5–7]. As per the estimation of The Institute of Paper Chemistry, about $100 million per year upwards cost to slime-related problems in the United States paper industry. This cost could be as high as ten times that without biological control systems or chemicals. In today’s market, the existing measures with the conventionally used biocides do not offer a satisfactory results to the paper industry in reference to cost benefits, production efficiency and product quality. Therefore, a novel approach could be the use of biological agents (like enzymes, bacteriophages, biodispersants, etc.) for controlling the slime or facilitate to act on cells directly due to exposure using lower dosages of biocides. This will help to minimize environmental problems indirectly.

9.2 Slime Several microorganisms grow in various processes of the paper mill in the aggregated form such as biofilm and flocs. In a biofilm, bacteria stick to the surface and start the development of adherent bacterial biofilm. The quality of the surface, on which bacteria attach to form a biofilm, is important and depends on hydrophobicity and hydrophilicity of the surface. Glycocalyx polymers adhesion involves firstly the attachment of glycoproteins with the surface followed by aggregation of bacteria with the formation of micro-colonies and lastly, the formation of biofilm by secreting EPS. Different processes during biofilm formation are briefed in Figure 9.1. EPS are generally composed of polysaccharides but may also contain proteins, nucleic acid, and polymeric lipophilic compounds [9]. EPS are considered as the major structural component of biofilm helping in the individual and interfacial interactions of microbes with each other [10, 11]. Biofilm acts as a protective layer against harmful agents as well as carbon and energy sources at times of nutrient deprivation. Biofilm also provides resistance to UV light and increased rates of genetic exchange. The organic deposits of biofilm act as a habitat for microorganisms and help in holding biofilms together for their long-term survival. Biofilm organic deposits also protect the microorganisms

9.2 Slime

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Figure 9.1: Processes governing biofilm formation [adopted from 8].

from the physiochemical changes in the system. The slime formation is a very complex process due to various factors involved during its development to provide suitable conditions. The slime formation occurs naturally due to the growth of microorganisms like bacteria, fungi, and yeast in presence of nutrients, moisture, and warmness. Secondary fiber-based paper mills using closed processes have optimal conditions for the growth of microorganisms due to high nutrient level, optimum temperature and pH [12]. The white water in the paper mill is also enriched with nutrients providing an appropriate environment for microbial growth especially having 30 °C water. The type and nature of slime changes with the different areas of mills and paper machines [13]. The basic characteristics of slimy growth are as follows: 1. Gelatinous and adherent; 2. Stringy or ropy; 3. Thick and viscous; 4. Pasty or rubbery; 5. Leathery, horny, hard or matted The biological components of the paper mill slime are all unicellular microorganisms. Bacteria, yeast, and fungi are most commonly found in slime (Figure 9.2). Bacteria, generally found in natural water, are mostly found in the paper mill. The most commonly found bacterium in slime is nonsporing rod-shaped Aerobacter aerogenes having the adaptation ability as per the oxygen availability in the surroundings. It requires the support of the network of paper fibers for forming soft and gelatinous slimes. They are mobile and are capable of moving to a certain extent for choosing the most favorable places in the paper machine for colonizing. Escherichia coli, Pseudomonas, Arthrobacter, Proteus, etc are other species responsible for slime formation. These species are responsible for growth in thickness and finally detached from the surface leaving patches behind. Pseudomonas, the group of badly defined species, is resistant to anti-slime agents in low concentrations and like water having low BOD for colonizing, therefore, they are mainly found in mills using nonrecycled process water. Slime may provide an anaerobic zone for the growth of sulfate-reducing bacteria. For example, Desulphovibrio desulphuricans reduces sulfate

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Figure 9.2: Microorganisms responsible for slime formation in mill environment.

to hydrogen sulfide and also reduces sulfites and thiosulphates and hyposulphites. This is one of several types of bacteria that promote corrosion and discoloration of painted surfaces, pulp, and various equipment. Molds or thread-type fungi are many: celled organisms composed of branched filaments with numerous systems of spore formation. Deposits formed are variable, from heavy gelatinous growth to leathery or matted slimes. Molds use a wide variety of foods from cellulose, starchy or sugary nutrients. Several mold species forming colonies with bacterial species for example, Cladosporium, Geotrichum, Mucor and fungal species, for example, Aspergillus, Penicillium, and Cephalosporium are responsible for slime formation [14, 15]. The thermo-tolerant fungi are generally present in closed process water systems in the paper mill having a surrounding temperature of above 30 °C [16]. These fungi produced slime-forming EPS, reserve materials, and excretory products. The fibrous mat-like structure produced by Basidiomycetes fungi and Penicillium species helps in the accumulation of slime deposits resulting in colored patches in the pulp thus, deteriorating wet pulp stacked sheets and finished paper products.

9.3 Sources of slime The main sources of microbiological contaminations are as follows [13]: 1. Freshwater, especially untreated surface water 2. The recycled process water

9.3 Sources of slime

3. 4. 5. 6.

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The environment of the paper machine The solutions/ suspensions having additives, pigments, starches, coating, etc. Cellulosic raw materials The brokes, having sizing & coating additives

When dissolved oxygen concentration increases, it promotes the growth of aerobic bacteria main causing agents of slime formation, and when dissolved oxygen concentration decreases anaerobic bacterial growth increases. An increase in temperature helps in the growth and development of mesophilic to thermophilic bacteria. Reuse of white water increase filamentous microorganisms in the water system. The water circuits in the paper machine systems containing three main circulations for the water is shown in Figure 9.3. Bacterial and fungal contamination generally occurs

Figure 9.3: Water circuits in the paper machine systems containing three main circulations for the water [adopted from 2].

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in the case of using recycled pulp. The microbial population in recycled pulp is multiple times higher than that of virgin pulp [13]. Occasionally, pulpwood is a major source of mill contamination. Logs near the bottom of the pile are generally covered with bacterial slime and mold growth. The lower the pile is, the higher the bacteriological counts, where these conditions occur, growth often resist the temperature of grinding (195 °F). Spore-forming bacteria, notable variants of Bacillus subtilis regularly withstand this temperature but nonspore formers are usually killed. Aerobacter aerogenes contamination was introduced by heavily slimed pulpwood. Grinding temperature is failed to eliminate these slime formers which persist and are carried into the groundwood and paper mill systems. Fungus contaminated pulpwood presents another problem in the tracking of mold spores into the mill and their dissemination by air currents to stored pulp and the process [15]. Most of the chemical pulp used is free from microbiological growth. However, groundwood lapstock is encountered with bacterial and fungal contamination. It develops slime-formers of the Coliforms and Bacillus subtilis and a putrefactive flora. 1.5 to 70 million colonies are reported per gram in groundwood laps. Fungi are active on the exposed surface of the pulp. Extreme care should be taken to prevent spoilage of lap pulp. Broke system: This system accounts for as much as 50% of total microbiological contamination. Since broke represents such a small portion of the total mill production, hence low expenditure on biocidal chemicals helps in eliminating a major part of this contamination. Starch solutions: Many paper and paperboard mills use starch in various processes. Precautions should be taken not only to protect solutions from contamination but also to prevent the development of growth and subsequent spoilage. Within the exception of extraneous source of mill contamination, existing deposits within the system also promote growth inoculation and spread. Natural breeding places in mills offer the most appropriable conditions for bacterial multiplication and development [15].

9.4 The problems in paper mills Different types of losses in paper mill processes or products due to slime deposition are summarized in the Figure 9.4. Biofilm can occur at different interfaces such as solid–liquid, solid–air, liquid–liquid, and liquid–air. The biofilms formation in the machine circuits frequently lead to paper defects or cause web breaks. Metabolites released by bacterial metabolism in the biofilm are organic and inorganic acids [13]. Due to which biofilm generate different oxygen concentration at different regions which cause patches in the pape due to its corrosiveness. The smell is produced by sulfate-reducing bacteria wherever reductive bleach is used and especially in a paper mill having closed water loops resulting in a rise in the concentration

9.4 The problems in paper mills

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Figure 9.4: Different types losses in paper mill processes or products due to slime deposition.

of volatile organic acids or compounds having a pungent smell. The problem becomes more acute for those machines which use recycled fibers and with a closed water cycle. Sulfate-reducing bacteria produce gas which is fatal particularly in an area that is poorly ventilated [13, 17]. An economical slime control program of a mill must consider the total loss caused by the slime such as loss of finished product, heat, chemicals, additives, water, filter, and fibers, production time, decreased life of equipment [17]. The most familiar problem of pink and red slime in mills mis composed of various microorganisms i.e; Micrococcus agilus, Serratia marcescens, S. piscotora, Monilia, Rhodotorula, Fusarium sp., Penicillium pinophilum [18]. Different biocides, generally nonselective in nature, are used by the papermaking system to avoid slime formation which develop resistance in microorganisms towards these biocides. Due to which most of the papermakers avoid the use of a single biocide and use the biocide in an alternating manner [17]. To understand the nature of slime formation in the paper mill, it requires an understanding of the functioning of the mill. By knowing the functioning of the paper mill, it helps in identifying the areas where these biological activities can be seen. The papermaking process is a multistep process. In the paper mill, the pulp is diluted in water to make a pulp consistancy of 3–4% representing the beginning of the wet end. A complex system of flow spreads and pressurized headboxes eject this dilute pulp slurry onto a speedily moving wire screen of paper machine. Fibers are laid onto the screen surface draining its water to form a paper sheet. Drained water is called white water responsible to provide suitable conditions for the most of the

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biological activities. Microbial contamination can occur if stored pulp is stored in open for a long time. Microbial contamination is also occurring in mills having closed-loop recycling white water [19, 20]. Factors such as temperature, pH, and nutrients also play a significant role in microbial contamination. Paper mill conditions having pH 5–8, temperature 20–78 °C, and abundant nutrients, provide an excellent environment for the growth of bacteria and fungi. The starch, glues, additives, etc. added in papermaking processes are excellent food for microorganisms. Surface water supply from ponds, rivers, lakes, and wells can also be a serious source of inorganic nutrients and bacterial contamination. The deposits formed can be classified as biological (slime) and nonbiological (scale and pitch) [13].

9.5 Methods for slime detection of in the paper industry There are many methods available for detecting slime problems in the paper mill. Few of these methods are as following [21].

9.5.1 Slime collection boards A slime collection board unit is placed in paper machine water for the formation and deposition of slime. At regular intervals, these boards are inspected for slime formation which reveals the early indication about slime formation on the machine before becoming a major problem. It is the most effective and cheaper method because of the plastic material such as Plexiglas. It is sufficient to suspend small rectangular pieces of Plexiglas with the holes. These slightly curved Plexiglas can be placed in a flowing water to detect the slime formation on these collecting boards. These boards are placed in any location at the paper machine where detection for slime has to be done and inspected at regular intervals.

9.5.2 Identification of the contaminated points There are two tests developed for the identification of stains and black areas. These spots are not of biological origin, therefore chemical tests are done (1) Ninhydrin test and (2) Tetrazolium chloride test.

9.5.3 Standard plate count method The detection of biological activity in the paper mill is generally done by the standard plate count method. The mill sample is serially diluted and poured in a sterile

9.5 Methods for slime detection of in the paper industry

201

solid nutrient media. With the help of this method, the normal population of slimeforming microbes can be calculated which is normally 10,000 microorganisms/ml of the sample to 100 million microorganisms /ml of the sample. The plates on which the sample is placed are incubated to allow organisms in the sample to reproduce and to form colonies in the solidified nutrient media. Bacteria take 48–72 h and fungi take 5–7 days for their incubation. After incubation, colonies formed by organisms are counted in the sample.

9.5.4 Dip Sticks Method Dip sticks contain a solidified nutrient having color-changing indicator. In this method, dip sticks are dipped into the water sample of the paper mill and are incubated for 24– 48 h. After incubation, colonies are counted or comparison is done on a chart to gauze the color change, which is interrelated to the standard plate count method giving a rough indication of the biological activity in the sample. This method is generally used for controlling the slime formation in the cooling tower system, but due to less precision and higher incubation time, paper mills are searching alternate more rapid sample test method to avoid the problems associated.

9.5.5 Luminescence The luminescence method quantifies the amount of adenosine triphosphate (ATP) molecule, linked with biological activity, in the liquid samples in less than 30 minutes. Different species of bacteria and fungi functions have a different level of ATP. This rapid and precise method is mostly useful for pure culture study but it is complicated and expensive.

9.5.6 Bio-Lert Method The Bio-Lert method is based on the indicator system in sterile nutrient broth placed in capped vial. The contents are mixed thoroughly by shaking the vial so that the sample gets mixed with nutrients and indicators. After incubation at 37 to 42 °C, the time taken for change from blue color to pink color depends on the biological population in the original sample and is linked to standard plate counts. This method give rapid results in one hour even with the excessive biological population. Therefore, the mill can take corrective action to minimize the the economic losses due to slime problems. This method is effective and stable in varible environment such as incubation temperature and sample pH. Due to different response of microbial species in the test,

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standard plate count is used for comparison to improve accuracy. This test provides an alert to high levels of biological activity, the time taken for the color change is several hours but if the biological activity is high, then color changes rapidly and provide sufficient time to take corrective measures before any economic loss. This simple, reliable and rapid Bio-Lert method provides accurate, valuable information about the biological activity in the pulp and paper regularly (Table 9.1). Table 9.1: Comparison of biological activity test methods [13]. Method

Time

Accuracy Comments

Ninhydrin spray  min.

Fair

Standard plates – h

Excellent Time-consuming

Rapid amino-nitrogen test (careful; do not tough spot) no quantitative

Dip-stick

 h

Yes

Time-consuming

ATPluminescence

19 h. Dosing with phage at the intervals of 3-h was resulted in no more restriction of K. pneumonia growth in comparison to culture injected only at the beginning of the test. According to them, the anticipated development of resistance in bacteria against phage attack and wide range of bacterial strains among different paper mills is also a crucial factor during the application of bacteriophage towards bacterial control in process waters [93]. An engineered bacteriophage showing expression of biofilm degrading enzyme was found to be effective to attack the cells of bacteria in the biofilm and its matrix. The biofilm cell count was reduced by more than 99.9% [94]. Under ideal environments, up to 80% biofilm removal having P. fluorescens cells were removed using bacteriophages in the early stage of development and 5-day-old biofilms [95]. Biofilms of L. monocytogenes and E. coli were inactivated by bacteriophage L. monocytogenes phage ATCC 23074-B1 and bacteriophage T4, respectively [96, 97]. 98,reported the role of mineral hydroxyapatite (HA) and eicosanoic acid (C20:0) to improve the lytic activity of bacteriophages and to destroy the biofilm structure, respectively, against Xanthomonas campestris pv campestris (Xcc) biofilm by the dysregulation of metabolic pathways by phage treatment. They also showed involvement of specific markers (amino acids, lactate and galactomannan) to control of biofilm stability [98].

9.6.5 Inhibitors for biofilm formation The main mechanism of biofilm inhibitors is to hinder the formation of the EPS layer around the cell of bacteria preventing attachment of bacterial cell and biofilm formation. The attachment of a bacterial cell to the surfaces becomes difficult in the absence of EPS having gluing property around the cell that protects them from the harmful microbicides. According to 99, N-acetyl-l-cysteine (NAC) may be used as a biofilm inhibitor on stainless steel surfaces of paper mills to hold wet-end deposits together. They reported that NAC acts as both as chemical and biological during the adhesion process of the bacterial cell (10 strains isolated from the paper mill) to stainless steel surfaces by influencing the wettability of the substratum. NAC increases the wettability of the stainless steel surface after its binding to surfaces, thus causing decreased bacterial cell adhesion and also detaching already adhered bacterial population. NAC also

9.7 Conclusion and future prospects

211

restricts the growth of single species as well as the multispecies population at various concentrations and reduces the production of EPS at concentrations not affecting the bacterial growth. NAC was not found to be responsible for the degradation of EPS. Microbially produced biosurfactants like probiotic bacterium Lactococcus lactis 53 were also found to impair the biofilm-forming abilities of four bacterial and two yeast strains on silicone rubber [100]. They showed that biosurfactants absorbed siliconerubber surface were more hydrophilic (contact-angle 48°) than bare silicone rubber (contact-angle 109°). Biosurfactant was also found to be effective to decrease the initial deposition rates of Staphylococcus epidermidis GB 9/6, Streptococcus salivarius GB 24/ 9, and S. aureus GB 2/1 from 2100, 1560 and 1255 microorganisms cm−2s−1 to 220, 137 and 135 microorganisms cm−2s−1, respectively, i.e. around 90% reduction. This deposition rates were lesser in the case of Rothia dentocariosa GBJ 52/2B, C. albicans GBJ 13/ 4A, and Candida tropicalis GB 9/9 [100]. Mireles et al. [101] reported the prevention of biofilm formation of Salmonella enteric and E. coli due to the disruption of biofilm without affecting cell growth using surfactin produced by B. subtilis. The other microbially produced (from L. monocytogenes) molecules like nisin, reuterin, and pediocin also have abilities for the control of biofilm formation [102]. According to [103], efficiencies of biological and chemical disinfectants should be tested on microorganisms growth in the biofilm mode in place of microorganisms growth in the free cell because of 100–1000 times more resistance to disinfectants action on biofilm. Although biological control is not recommended as a cost-effective alternative to the chemical disinfectants but the selection of disinfectants should also be based on effectiveness, easy rinsing off from the surfaces, no toxic residues in the final products, safety and easy applicability.

9.7 Conclusion and future prospects The use of biological approaches (i.e. enzyme, bacteriophage, biodispersants, etc.) as biocides alone and in combination with chemical biocides are promising for slime control. Even after several benefits over conventional biocides, the enzymes‘ application for controlling biofilm formation is not common in the paper industry due to the low prices of conventional chemicals used in the industry. Based on the efficacy of the different natural and eco-friendly biocides, still, there is a need for detailed and deep scientific knowledge to develop more effective biocontrol strategies for the paper mill conditions for evaluating large scale economic feasibility of the process that will help to improve the efficiency of the process and the quality of the product. The central focus of current research to develop methods for biofilm removal is towards the characterization of the various types of EPS produced by different microorganisms. The lack of reliable methods for measuring the enzymatic effects quantitatively also make restricted use of these enzymes towards biofilm control. Enzymes having a wide range of activities should be preferred over specific monocomponent enzymes to

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attain an acceptable degree of removal in complex biofilms due to the diversity of the biofilm matrix. By keeping the balance between industrial demands of efficiency and environmental sustainability, future research should also be focused to find the mechanism behind the deposit formation at paper machines in combination with developing deposit control agents.

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Chhotu Ram, Pushpa Rani, Kibrom Alebel Gebru and Mebrhit G Mariam Abrha

10 Pulp and paper industry wastewater treatment: use of microbes and their enzymes Abstract: Pulp and paper industry is coming under one of the most water polluting industries, and generated wastewater is highly toxic in nature. The paper mill requires huge quantity (~50–60 m3 of water to produce one ton of paper) of water, and accordingly huge quantity of chemical contaminated wastewater is discharged. The paper mill effluents have identified 240–250 chemicals in different stages of paper making. Various chemical constituents such as high chemical oxygen demand, biochemical oxygen demand, AOX, chlorinated compounds, color, suspended materials, lignin and their derivatives are released in the wastewater. The present review study is focused on the paper mill processes, wastewater generation and its effective treatment by microorganisms. The biological treatment has been identified as cost-effective and eco-friendly methods for the degradation of xenobiotic compounds for paper mill wastewater. Various studies have been performed so far to investigate the complex nature of wastewater by the application of bacteria, fungi and their enzymes at industrial scale. Therefore, the article discussed the importance of biological method as an effective technique for the degradation of paper mill wastewater. Keywords: paper mill effluent, treatment, bacteria, fungi, enzymes

10.1 Introduction Pulp and paper mills have important role in the economy of several countries such as USA, Canada, India and Portugal. Pulp and paper industry is considered as one of the most polluting industry [1, 2]. The major processes of paper mills are raw material preparation, pulping, bleaching and paper making which are considered as highly water and energy intensive. Pulping is considered as one of the most polluted stream in the paper mills due to removal of lignin and fibers for paper making. Bleaching process in paper mills are used to whiten and brighten the pulp. Large amount of water is required in the pulp and paper mill processes and reappear in the form of wastewater contaminated by organic compounds. As per the previous report, water consumption in paper mills mainly depends upon the production processes and require 60 m3 per

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Ram, C., Rani, P., Gebru, K. A., Abrha, M., G. M. Pulp and paper industry wastewater treatment: use of microbes and their enzymes Physical Sciences Reviews [Online] 2020, 5. DOI: 10.1515/psr-2019-0050 https://doi.org/10.1515/9783110592412-010

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ton of paper produced in spite of the best and modern available technologies [1]. The paper mill effluent contains different nature of organic and inorganic compounds and measured as higher chemical oxygen demand (COD), biochemical oxygen demand (BOD), chlorinated organic compounds (as AOX), suspended particles having fibers, tannins, fatty acids, resin acids, sulfur and sulfur compounds, lignin and its derivatives [3, 4]. Many research works showed that pulp and paper mill effluents have toxic and lethal effects on the aquatic life, e. g. planktons, daphnia, fish and other biota in the receiving water bodies [5–8]. Chandra et al. (2007, 2009) also reported that wastewater containing chlorinated organics, dioxins and furans have negative impacts of genotoxicants on public health as well as aquatic biota through the environmental contamination of drinking water supplies and food [4, 9]. A recent investigation [10] has been carried out by Bacillus aryabhattai isolate to reduce 67 % and 54 % colour and lignin, respectively, from pulp and paper mill wastewater. Chandra et al. [11] reported the residual organic pollutants of pulp and paper mill effluent after biological treatment and assessed their degradability by biostimulation. The various isolated bacteria were identified as Klebsiellapneumoniae, Enterobacter cloacae strain, Enterobacter cloacae and Acinetobacter pittii strain. One study indicated the ability of fungal consortium consisting of Nigrospora sp. and Curvularialunata to degrade BOD, COD and color, and lignin concentration was removed under catalytic enzyme activity of the pulp and paper mill effluents [12]. Limaye et al. [13]. have studied a strain of actinomycete having the capacity to produce laccase enzyme which actively degrades cellulose, hemi-cellulose and lignin of pulp and paper mill effluents under alkaline conditions. Therefore, proper treatment of pulp and paper mill wastewater is important before disposal into the surrounding environment. Literature suggests several methods of wastewater treatment where biological treatment received much attention. Therefore, biological treatment technologies involve utilization of microorganisms including bacteria, fungi, enzymes and algae in real treatment plant conditions. Biological treatment at industrial scale could be used as single-step treatment or in combination with other treatment methods such as physical and/or chemical methods [14]. Microbial degradation technique has advantages such as it is cost-effective and eco-friendly and is more suitable for complete degradation of organic and inorganic compounds present in paper mill wastewater. Therefore, bioremediation is an effective environmental pollution control method which utilizes biological systems to catalyze the degradation of various complex toxic chemicals to simpler compounds. Hence, bioremediation is widely used for the degradation of several industrial wastewaters including pulp and paper mill effluent [15–17].

10.2 Pulp and paper mill process A typical paper mill consists of four major sections: pulping, bleaching, paper making and recovery [1]. Figure 10.1 shows the particular process of paper mill generating

10.2 Pulp and paper mill process

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Figure 10.1: Pollutants released from various steps of paper manufacturing process [modified from Ref. 18].

effluent. The pulping results in lignin network degradation and removal of its soluble fractions from the plant tissue producing unbleached pulp (cellulose 80–90 %, hemicelluloses 10–15 % and residual lignin 2.5–4 %). The residual lignin is accountable for the unwanted dark colour of pulp [19]. Bleaching is a multistage process which involves treatment of pulp with a variety of chemicals in series. The first stage is chlorination by chlorine (Cl2) and/ or chlorine dioxide (ClO2) to delignify the pulp followed by an alkaline extraction (E) stage where alkali (NaOH) and oxygen (O2) and/ or hydrogen peroxide (H2O2) are used for the removal of alkali soluble lignin [20, 21]. Indian paper mills are broadly grouped into three categories, i. e. large (>100 tonnes per day), medium (30–100 tonnes per day) and small (