Water Treatment Plants: Technology and Approaches 1774076551, 9781774076552

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Water Treatment Plants: Technology and Approaches

Water Treatment Plants: Technology and Approaches

Edited by:

Saeed Farrokhpay

ARCLER

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www.arclerpress.com

Water Treatment Plants: Technology and Approaches Saeed Farrokhpay

Arcler Press 224 Shoreacres Road Burlington, ON L7L 2H2 Canada www.arclerpress.com Email: [email protected] e-book Edition 2021 ISBN: 978-1-77407-854-9 (e-book) This book contains information obtained from highly regarded resources. Reprinted material sources are indicated and copyright remains with the original owners. Copyright for images and other graphics remains with the original owners as indicated. A Wide variety of references are listed. Reasonable efforts have been made to publish reliable data. Authors or Editors or Publishers are not responsible for the accuracy of the information in the published chapters or consequences of their use. The publisher assumes no responsibility for any damage or grievance to the persons or property arising out of the use of any materials, instructions, methods or thoughts in the book. The authors or editors and the publisher have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission has not been obtained. If any copyright holder has not been acknowledged, please write to us so we may rectify.

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ABOUT THE EDITOR

Dr Saeed Farrokhpay is a Chemical Engineer with over 15 years of experience in mineral & material processing. He obtained his PhD from Ian Wark Research Institute, University of South Australia in 2005. He is a Senior Research Scientist at University of Lorraine. Previously, he has worked at University of South Australia, and University of Queensland. He has published more than 80 papers in high ranked journals and conference proceedings and has served as an editorial board member of several journals.

TABLE OF CONTENTS

List of Figures ........................................................................................................xi List of Tables .......................................................................................................xiii List of Abbreviations ............................................................................................xv Preface........................................................................ ................................... ....xix Chapter 1

Introduction to Water Treatment Technologies ........................................ 1 1.1. Introduction ........................................................................................ 2 1.2. Bioremediation ................................................................................... 4 1.3. Membrane Bioreactors (MBRs)............................................................ 9 1.4. Advanced Oxidation Processes (AOPs) ............................................. 11 1.5. Homogeneous Advanced Oxidation Processes (AOPs) Without Energy ................................................... 13 1.6. Homogeneous Advanced Oxidation Process Using Ultrasound .............................................................. 20 1.7. Homogeneous Process Utilizing Solar Energy ................................... 22 1.8. Heterogeneous Catalytic Oxidation .................................................. 23 1.9. Heterogeneous Wet Oxidation.......................................................... 24 1.10. Heterogeneous Photocatalysis ........................................................ 27 References ............................................................................................... 29

Chapter 2

Introduction To Wastewater Treatment Processes .................................. 45 2.1. Introduction ...................................................................................... 46 2.2. Sources ............................................................................................. 46 2.3. Treatment Objectives ........................................................................ 47 2.4. Wastewater Treatment Process .......................................................... 47 2.5. Description Of Process Operations ................................................... 48 2.6. Discharge Criteria ............................................................................. 55 References ............................................................................................... 56

Chapter 3

Coagulation and Flocculation.................................................................. 63 3.1. Introduction ...................................................................................... 64 3.2. Colloidal Suspensions ....................................................................... 64 3.3. Coagulation ...................................................................................... 67 3.4. Flocculation...................................................................................... 74 3.5. Conventional Plants .......................................................................... 76 3.6. An Amalgamation Of Flocculation, Coagulation, And Sedimentation......................................................................... 77 3.7. Operation Of The Flocculation And Coagulation Process .................. 78 3.8. Cataloging Of Settling Behavior ........................................................ 79 3.9. Ideal Settling ..................................................................................... 81 References ............................................................................................... 82

Chapter 4

Activated Sludge Processes..................................................................... 89 4.1. Introduction ...................................................................................... 90 4.2. Comparison Between As (Activated Sludge Process), Wetland System and Percolating Filtration...................................... 93 4.3. Types Of Activated Sludge (AS) Process ............................................. 95 4.4. As Process Kinetics And Designs ....................................................... 98 4.5. Key Process Criteria Of Design ....................................................... 102 References ............................................................................................. 105

Chapter 5

Fundamentals of Membranes For Water Treatment .............................. 111 5.1. Introduction .................................................................................... 112 5.2. Membrane Characteristics .............................................................. 113 5.3. Membrane Materials ....................................................................... 116 5.4. Membrane Modules........................................................................ 116 5.5. Theory ............................................................................................ 117 References ............................................................................................. 122

Chapter 6

Microfiltration and Nanofiltration ........................................................ 127 6.1. Introduction .................................................................................... 128 6.2. Pressure Driven Membranes ........................................................... 130 6.3. Vacuum Driven Hollow Fiber Membrane – The Zeeweed Membrane ................................................................................... 132 6.4. Treatment With Microfiltration (Mf) Membranes .............................. 135

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6.5. Application Of Nanofiltration (Nf) Membranes For Drinking Water Treatment ............................................................ 140 References ............................................................................................. 145 Chapter 7

Water Purification With Reverse Osmosis ............................................ 151 7.1. Introduction .................................................................................... 152 7.2. Overview of Ro Application............................................................ 154 7.3. Principle of Reverse Osmosis (RO).................................................. 157 7.4. Ro Membrane Description .............................................................. 158 7.5. Ro Membrane Performance ............................................................ 159 7.6. Review of Ro Process and Its Applications ...................................... 162 References ............................................................................................. 181

Chapter 8

Membrane Bioreactor Technology For Treating Micropollutants .......... 189 8.1. Introduction .................................................................................... 190 8.2. Classification Of Micropollutants .................................................... 191 8.3. Water Treatment Technologies ......................................................... 193 8.4. Inorganic Micropollutants In Membrane Processes ......................... 197 8.5. Membrane Technologies And Their Limitations In Treating Inorganic Micropollutants ......................... 201 8.6. Potential Hybrid Process For Removal Of Inorganic Micropollutants ............................................................................ 210 8.7. Organic Micropollutants (OMPs) In Membrane Processes............... 217 8.8. Classification of Organic Micropollutants (OMPs)........................... 217 8.9. Membrane Technologies And Their Limitations In Treating Organic Micropollutants (OMPs).................................................. 222 8.10. Mechanisms For Removal Of Organic Micropollutants (OMPs)..... 224 8.11. Removal of Plant Care Products And Pesticides And Challenges Created By Them To Membrane Processes .................. 226 8.12. Removal of Chlorinated Solvents And Their Challenges To Membrane Processes ............................................................... 228 8.13. Removal of Phenol Derivatives And Their Challenges To Membrane Processes ............................................................... 229 8.14. Removal of Various Hydrocarbons And Their Challenges To Membrane Processes ............................................................... 231 8.15. Removal of Various Personal Care Products (PCPs) And Their Challenges To Membrane Processes .................................... 232 8.16. Hybrid Processes For Treatment For Organic Micropollutants (OMPs)......................................................................................... 233 ix

References ............................................................................................. 241 Index ..................................................................................................... 257

LIST OF FIGURES Figure 1.1. Mechanism of phytoremediation. Figure 1.2. Schematic description of the membrane bioreactor. Figure 1.3. Membrane bioreactor configuration. Figure 1.4. Schematic classification of advanced oxidation processes. Figure 2.1. The traditional treatment process of wastewater. Figure 2.2. Schematic design of anaerobic digestion procedure. Figure 2.3. Schematic diagram of the activated sludge process. Figure 3.1. In-line mixer on left and baffled chamber on right for coagulation. Figure 3.2. Two alternatives of the mechanical mixing chamber utilized for coagulation. Figure 3.3. Three kinds of flocculator: (a) propeller, (b) turbine, (c) walking beam. Figure 3.4. Conventional water treatment method involving solid elimination process. Figure 3.5. Alternate water treatment method using solid elimination technique. Figure 3.6. Schematic diagram of a joint procedure involving coagulation, flocculation, and sedimentation. Figure 4.1. Schematic illustration of the AS (activated sludge) process. Figure 5.1. Range of nominal membrane pore sizes. Figure 5.2. SEM images showing top surfaces of: (a) a phase inversion membrane, (b) a track-etched membrane, and (c) an expanded film membrane. Figure 5.3. SEM images of: (a) cross-section of an anisotropic microporous membrane and (b) cross-section of a thin-film composite membrane. Figure 5.4. SEM image of hollow fiber cross-section. Figure 5.5. Schematic of: (a) plate and frame, (b) tubular, (c) spiral wound, and (d) hollow fiber modules. Figure 6.1. The filtration spectrum. Figure 6.2. Modes of filtration. Figure 6.3. Filtration modes – hollow-fiber membranes. Figure 6.4. Operational concept of an outside-in, immersed, shell-less membrane. Figure 6.5. PFD of an immersed membrane microfilter. Figure 6.6. Scanning electron micrographs of cryptosporidium and giardia parasites.

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Figure 6.7. Typical ZeeWeed treatment plant for a complex groundwater. Figure 7.1. Schematic flow of RO membrane. Figure 7.2. Principle osmosis and of reverse osmosis (RO). Figure 7.3. Structure of RO membrane. Figure 7.4. Feed concentration effect on RO membrane rejection. Figure 7.5. Basic mechanisms of how (A) osmosis and (B) reverse osmosis work. Figure 7.6. Basic components of reverse osmosis. Figure 7.7. Schematic representation of RO systems. Figure 7.8. (A) Preconditioning/pre-filters, reverse osmosis membranes, and posttreatment disinfection system of reverse osmosis. (B) Filtration components and key steps involved in the reverse osmosis process. Figure 7.9. Detailed of various filtration methodologies and their cut-offs molecular size exclusions are illustrated. The figure indicates an example of different molecules and particles that excluded from each type of filtration system. Figure 8.1. The route for contamination of the environment by micropollutants. Figure 8.2. Flowchart of water contamination by micropollutants and their removal. Figure 8.3. Schematic illustration of the activated sludge process along with MBR. Figure 8.4. Nitrogen movement in water bodies. Figure 8.5. Schematic illustration of the ion transport mechanism in tan ion-exchange bioreactor. Figure 8.6. Typical small scale IEMB setup. Figure 8.7. The concentration of chlorates and nitrates in water treated by IEMB run at different treatment rates per unit membrane area. Figure 8.8. The chemical architecture of pesticides A) organochloride; B) pyrethroid; C) carbamate; D) organophosphate. Figure 8.9. OMPs removal mechanism in membrane processes: (a) size-exclusion or retention, (b) adsorption, (c) electrostatic repulsion, (d) adsorption. Figure 8.10. The removal efficiency of trace OMPs using MBR treatment. Figure 8.11. The overall efficiency of hybrid MBR treatment for removal of organic micropollutants utilizing different membranes (a) NF-270, (b) NF-90, (c) BW-30 (d) ESPA-2. Figure 8.12. Concentration of trace contaminants in different membranes (a) NF-270, (b) NF-90, (c) BW-30 (d) ESPA-2

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LIST OF TABLES

Table 4.1. Comparison of activated sludge process, wetland system, and percolating filtration parameters Table 5.1. Typical osmotic pressure values for solutions at 25°C Table 6.1. Surface water treatment data – direct mf with an immersed membrane Table 6.2. US EPA’s TOC removal requirements Table 6.3. Typical results of microfiltration enhanced coagulation Table 6.4. Mechanisms for groundwater contaminant removal Table 6.5. Results of well water treatment in new Brunswick, Canada Table 6.6. Results of well water treatment in Egypt using ZeeWeed Table 6.7. Summary of field testing results using nanofiltration for treatment of surface water Table 7.1. Operating conditions and effect on RO system performance Table 7.2. Feed pH effect on RO membrane rejection Table 7.3. Commonly used water purification methods Table 7.4. Average purification efficiency of RO membranes* Table 7.5. Common basic components used in RO systems Table 7.6. Energy recovery system used in RO system Table 8.1. Limits and characteristics of heavy metals, transition metals and metalloids in drinking water Table 8.2. Permissible amount of various heavy metals Table 8.3. Concentrations of NO3–, BrO3– and ClO4– in treated water and in the biocompartment and anion fluxes through the membrane for an IEMB run at steady state Table 8.4. Sources of various organic micropollutants in the aquatic environment Table 8.5. The maximum permissible concentration of various pesticides as recommended by the WHO

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LIST OF ABBREVIATIONS

AC

activated carbon

AOPs

advanced oxidation processes

AS

activated sludge

BAT

best available technology

BOD

biochemical oxygen demand

BPA

bisphenol A

CA

cellulose acetate

CAS

conventional activated sludge

Cl

chlorine

CMC

critical micelle concentration

CNS

central nervous system

CNT

carbon nanotubes

COD

chemical oxygen demand

CPs

chlorophenols

CT

carbon tetrachloride

CTA

cellulose tri-acetate

CVOCs

chlorinated volatile organic chemical compounds

CWWTP

conventional wastewater treatment plant

DAF

dissolved air flotation

DD

Donnan dialysis

DO

dissolved oxygen

ED

electrodialysis

EDB

ethylene dibromide

EDCs

endocrine-disrupting compounds

EDs

endocrine disrupters

EPA

Environmental Protection Agency

HAA

haloacetic acids

HAN

haloacetonitriles

HCB

hexachlorobenzene

HFF

hollow fine fiber

HT

hydrotalcite-like

IE

ion exchange

IEMB

ion-exchange membrane bioreactor

LC-MS

liquid chromatography-mass spectrometry

MBR

membrane bioreactor

MF

microfiltration

MLSS

mixed liquor suspended solids

MLVSS

mixed liquor volatile suspended solids

MW

molecular weight

MWCO

molecular weight cut-off

NaAlO2

sodium aluminate

NF

nanofiltration

NH4

ammonium

NOM

natural organic matter

NPRI

National Pollutant Release Inventory

NRC

Nuclear Regulatory Commission

O3

ozonation

OMPs

organic micropollutants

pCBA

para-Chlorobenzoate

PCE

perchloroethylene

PCPs

personal care products

PV

pervaporation

RO

reverse osmosis

SCWO

supercritical wet oxidation

SS

suspended solids

TCE

trichloroethene

TDS

total dissolved solids

TFC

thin-film composite

TFM

thin-film material

THMs

trihalomethanes

TiO2

titanium oxide

+

xvi

TMP

transmembrane pressure

TOC

total organic carbon

UF

ultrafiltration

UV

ultraviolet

UVA

ultraviolet A

VC

vinyl chloride

VOCs

volatile organic compounds

WPO

Wet Peroxide Oxidation

WTP

water treatment plant

WWTPs

wastewater treatment plants

xvii

PREFACE

The discharge and buildup of organic and inorganic contaminants in the aquatic environment by anthropogenic and natural sources have led to severe environmental issues; this consequently results in adverse effects on humans and other species on earth. Therefore, to inhibit the detrimental effects of environmental pollutants, the development of newer or combined technological approaches for the remediation of contaminated waters and wastewaters has great importance. This book covers the technologies which are applied to treat and purify water. This treatise on water treatment is also closely associated with solid-liquid separations. The subject of water treatment is wide-ranging and the technologies discussed in this book not just confined to only physical methods of water pollution control. A comprehensive illustration of technologies pertinent to the treatment of both groundwater and wastewater (i.e. industrial, pharmaceutical, and municipal) is embodied in this book. The approaches and technologies summarized are a combination of chemical, physical, and biological techniques. There are eight chapters in this book. The first chapter deals with the fundamentals of water treatment technologies. The concepts of water treatment using biological, chemical, and oxidative technologies are discussed in the chapter. Chapter 2 briefly discusses the fundamentals of wastewater treatment processes. The detailed description of the wastewater sources and wastewater treatment process operations are discussed in the chapter. Coagulation and flocculation are considered the basic filtration operations for treating polluted water. Chapter 3 focuses on the description of principles pertaining to coagulation, flocculation, and combined settling processes. The activated sludge (AS) process has gained enormous attention during the last two decades due to its effectiveness in treating wastewaters. Chapter 4 discusses the fundamentals of the AS process. Moreover, the process kinetics and design criteria of the AS process are also discussed in the chapter. The development of water treatment membranes for treating different types of wastewater has revolutionized water treatment technology. Chapter 5 focuses on the fundamental characteristics of membranes, membrane materials, and

different types of membrane modules used for the treatment of water. There is a wide range of membranes used for water treatment. However, microfiltration (MF) and nanofiltration (NF) are considered as the advanced membrane processes. Chapter 6 discusses the basic concepts and working principles of micro and nano membranes. Apart from MF and NF, reverse osmosis (RO) is also an attractive water treatment technique nowadays. Chapter 7 focuses on the fundamental principles of RO membranes and their working principle. Moreover, different applications of the RO process are also discussed in the chapter. There are different types of micropollutants that are responsible for polluting the water. Chapter 8 comprehensively discusses the types of micropollutants and their effects on the quality of waters. Membrane bioreactor (MBR) technologies coupled with conventional membrane technologies are used to treat water affected by micropollutants. The principles and mechanisms of wastewater treatment using MBR technology are discussed in Chapter 8. In this book, particular stress is given to water treatment technologies which are not only environmentally friendly but also cost-effective. This book can be treated as a ready reference for all the topics related to water pollution and water treatment. Students, researchers, and industrialists in the field of water treatment can use this book to solve the problems related to water purification technologies.

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CHAPTER 1

Introduction to Water Treatment Technologies

CONTENTS 1.1. Introduction ........................................................................................ 2 1.2. Bioremediation ................................................................................... 4 1.3. Membrane Bioreactors (MBRs)............................................................ 9 1.4. Advanced Oxidation Processes (AOPs) ............................................. 11 1.5. Homogeneous Advanced Oxidation Processes (AOPs) Without Energy ................................................... 13 1.6. Homogeneous Advanced Oxidation Process Using Ultrasound .............................................................. 20 1.7. Homogeneous Process Utilizing Solar Energy ................................... 22 1.8. Heterogeneous Catalytic Oxidation .................................................. 23 1.9. Heterogeneous Wet Oxidation.......................................................... 24 1.10. Heterogeneous Photocatalysis ........................................................ 27 References ............................................................................................... 29

2

Water Treatment Plants: Technology and Approaches

1.1. INTRODUCTION The accumulation and release of inorganic and organic contaminants in the environment by anthropogenic and natural sources have caused severe environmental problems; this consequently results in harmful effects on wildlife and human health. Thus, to avert the adverse effects of environmental pollutants, the development of combined and newer technologies for the remedy of wastewaters and contaminated water has been of importance. This chapter describes membrane bioreactor (MBR), bioremediation, and the AOPs (advanced oxidation processes) as other forms of wastewater and water treatment technologies. MBR and bioremediation are eco-friendly, efficient, and inexpensive methods of detoxification. The AOPs have also appeared as the set of adaptable water and wastewater treatment methods. AOPs have been extensively applied for the elimination of a wide range of pollutants including pharmaceuticals, surfactants, pesticides, dyes, herbicides, endocrine-disrupting chemicals, disinfection by-products. Conversely, there are still continuing the investigation on the effect of the oxidants utilized, nanoparticles, and subsequent metabolites on the human health and environment. In the past three eras, worries about environmental pollution have amplified around the world and have an outcome in the declaration of more preventive environmental laws. With the progress of advanced analytical methods for the quantification and identification of the pollutants, there is propensity to continue increasing those restrictions to look after the environment with a distinct focus on the control of toxic substances, carcinogenic, and mutagens precursors and carcinogenic substances. Especially, significant are the guidelines placed on the water from water decontamination plants, municipal, and industrial wastewater treatment plants (WWTPs) (Stackelberg et al., 2007). Currently, there has been amplified concern regarding the quality of water released from industrial and purification plants as particular recalcitrant xenobiotic compounds have been noticed in the groundwater systems, surface water, and drinking water. A large number of EDCs (Endocrine Disrupting Chemicals) and pharmaceuticals have been observed in wastes from the sewage treatment systems, surface water and less often, in drinking water (Andreozzi et al., 2003; Castiglioni et al., 2004; Brown et al., 2006). These pharmaceuticals vary from simple antibiotics to the anticonvulsants, cytostatic drugs, betablockers, and X-ray contrasting media (Focazio et al., 2008; Wang et al., 2011). The basic design of these chemicals averts their attack by microbes

Introduction to Water Treatment Technologies

3

utilized in biological treatment of the municipal sewage and so they persevere in the waste during discharge of the treated wastewater. Natural water and the treated wastewater signify the most important exposure pathways for the Endocrine Disrupting Chemicals to biota as these chemicals are only partially eliminated by wastewater treatment procedures (Daughton and Ternes, 1999). An assessment of the potential environmental impact of EDCs has been presented (Ikehata and Gamal El-Din, 2005a, b; Correia et al., 2007). Xenobiotic end products like dyes, pesticides, insecticides, and herbicides, and the other precursor chemicals like nitro and chlorobenzenes have also been noticed in water samples (Gilliom et al., 1999; Cortes et al., 2000; Gültekin and Ince, 2007). From the former, it is obvious that the current conventional techniques like coagulation, flocculation, adsorption on the activated carbon (AC), membrane processes and biological treatment for the sanitization of drinking water and treatment of the wastewater are unsuccessful for the elimination of particular recalcitrant organics in the water (Erickson, 2002). One major drawback of some of these methods is the concentration of contaminants on the secondary medium or in the solution and therefore the generation of the sludge which is more rigorous and more intensely toxic as compared to the individual pollutant. The proper removal of this secondary produced pollutant thus offers another problem. At present, the membrane waste sludge contains various stabilized xenobiotic organic compounds that are shrunk into sewers and the surface waters where they present further environmental threats (Fukuhara et al., 2006; Tambosi et al., 2009). Also, while adsorption on the AC displays promises for the elimination of the organics, reports on use in real wastewater exposed that adsorption of the certain organics was decreased by thousand times when compared to the outcomes from the laboratory scale experiments (Choi et al., 2005; Oller et al., 2011). Coupled with this point is the reuse of the AC is restricted to the particular number of cycles and produced spent carbon needs proper treatment. This poses a major drawback to this technology. Photo-degradation and biological degradation are a few of the main technologies applied for the elimination of toxic chemicals like chlorinated phenols, pesticides, and aromatics (Oller et al., 2011). However, in the context of the remediation defined by the European Commission Joint Center, these traditional technologies have been observed as unsatisfactory for the handling of industrial wastewater as numerous of the existing organics are resilient to biological treatment. Possibly, some of these methods when carried out in biosorption mode use cheap adsorbents and the decrease in running cost might become an advantage.

4

Water Treatment Plants: Technology and Approaches

This chapter describes MBR, bioremediation, and the AOPs as other forms of wastewater and water treatment technologies. The basic principles of application and restrictions of these technologies are discussed.

1.2. BIOREMEDIATION Bioremediation involves the utilization of live organisms like microorganisms, algae, enzymes, fungi, and plants to breakdown complex materials into the simple end products. It can be defined as the addition of the biological materials to polluted environments to speed up the natural biodegradation procedures, the addition of the terminal electron donor/acceptor or control of temperature and moisture conditions in order to form microbial consortia (Tabak et al., 1991; Singh, 2014). It is the skilled technique of eliminating pollutants from wastewater and water. It is the simple and cheap method of wastewater and water treatment which outcomes to the nontoxic byproduct. There are two key approaches to bioremediation that are discussed in subsections.

1.2.1. Bioaugmentation In this process, the microbes are added to increase the present microbial population. In this way, bioremediation takes place with more efficiency due to increased microbes.

1.2.2. Biostimulation It includes the addition of nutrients or the other growth-restricting cosubstrates to encourage the growth of original degraders. Some fungi and microbes which are commonly utilized in bioremediation include Bacillus sp, Klebsiella sp, Pseudomonas sp, Pandoraea sp, Mycobacterium sp,Agrocybe semi orbicularis, Phanerochaete chrysosporium, Auricularia auricula, Dichomitus squalens, Coriolus Versicolor, Flammulina velupites, Pleurotus ostreatus, Hypholoma fasciculare, Avatha discolor and Stereum hirsutum, genera Flavobacterium, Azotobacter, Arthrobacter, and Burkholderia are the commonly utilized bacterial species, while the P. putida theoretical demand of oxygen enzyme and the fungal enzymes laccase, peroxidases, and oxidoreductases have been described to have noticeable application in the elimination of pollutants in the environment (Rani and Dhania, 2014; Pirzada et al., 2015).Recent studies have displayed that bioremediation utilizing algae can escalate biomass by using effluent as

Introduction to Water Treatment Technologies

5

nutrients and thus can help in resolving issues created by effluents (Mehta and Gaur, 2005). Hence, microalgae cultures have been practically made by various authors for the treatment of contaminated wastewater, soils, and water (Zayadan et al., 2014). Chekroun et al. (2014) presented an overview of mechanisms utilized by the microalgae for bioremediation of the organic contaminants in aquatic ecosystems and the impact of the genetically altered microalgae on the xenobiotic degradation in order to reduce their effect on the environment. Chekroun et al. (2014) determined that the use of microalgae in the biomonitoring and restoration of the aquatic systems favors phytoextraction and biodegradation of the organic contaminants; however, there are still some persistent organic contaminants which are hard to break down by the microalgae. They preached that genetic engineering might be employed to enhance the bioremediation and absorption of numerous organic contaminants and increase microalgal acceptance to these pollutants. In Egypt, bioremediation of the Al-Sayyadin lagoon polluted water utilizing wild and mutant strains of the microalgae as evaluated by Zayadan et al. (2014). They reported that mutant and wild strains of the P. kessleri had quite higher effectiveness in phytoremediation as compared to the mutant and wild strains of C. reinhardtii and that mutant alga of the P kessleri PC displayed higher growth rate and elimination effectiveness than the wild type. Sheela and Beebi (2014) concluded a comprehensive review of the bioremediation of the ammonia from contaminated wastewaters. They highlighted the biological procedures for ammonia-nitrogen elimination; which comprises of nitrification/denitrification, instantaneous nitrification, and denitrification, nitrification/denitrification through nitrite pathway, autotrophic nitrification/denitrification, anammox process, and fractional nitrification/anammox. The factors disturbing the effectiveness of ammonia bioremediation are also discussed. Finally, the debate that anammox system was discovered to be achievable and extremely appropriate for treatment of the high concentrated ammonia comprising of industrial wastes, and that the main disadvantages for the anammox bacteria are their segregation in pure form and the slow growth rate. Olawale (2014) examined the bioremediation capability of Pseudomonas aeruginosa in eliminating heavy metals from the industrial effluent wastewater. Olawale described that wastewater treated with the P. aeruginosa exhibits that the concentrations of Se, As, Pb, and Cd tainted by 96.43%, 99.94%, 99.80%, and 90.38%, correspondingly, at the end of 15 experimental days.

Water Treatment Plants: Technology and Approaches

6

Furthermore, bioremediation has been described as the new technique of oil spill clean-up which is more efficient as compared to mechanical and chemical methods (2014 ).

1.2.3. Phytoremediation Phytoremediation is a promising technology that utilizes green plants to integrate or detoxify inorganic and organic contaminants from polluted water, soils, and wastewater. Plants possess the natural ability to remove, immobilize, or detoxify environmental pollutants in the growth matrix with the help of various biological processes (Pirzadah et al., 2015); therefore, phytoremediation can be made practical to a wide range of pollutants, including heavy metals, chemicals, radionuclides, organics. This technology keeps the environment in the natural state, enhances soil quality, it is comparatively inexpensive and mild, the plants can be observed, and valuable pollutants can easily be recuperated and re-used. However, it is restricted to the following: depth and surface area occupied by the roots, moderate plant growth and various growing seasons needed to accomplish the wanted level of refinement, leaching of pollutants into groundwater which might not be completely prohibited, toxicity of the polluted land and general condition of the soil which affects the existence of the plants, and bio-accumulation of the pollutants which might pass into the chain of food. Therefore, the plant species utilized in phytoremediation are designated based on the ability to degrade or extract the pollutants of worry, adaptation to the local climates, depth root structure, high biomass, compatibility with soils, ease of planting and preservation, growth rate, and the capability of plant to take up large amounts of water via roots. Furthermore, the effectiveness of phytoremediation is found out by the bioconcentration factor and biomass production. The bioconcentration factor is the measure of the ability of the plants to take up and transport pollutants to shoots, which are parts of the plant that can be harvested easily (Dmuchowski et al., 2014) Mechanism of Phytoremediation: Pirzadah et al. (2015) and Favas et al. (2014) have discussed the mechanisms and features affecting phytoremediation. These crucial mechanisms (Figure 1.1) include: • • • •

Phytoextraction; Phytostabilisation; Phytodegradation; Phytostimulation;

Introduction to Water Treatment Technologies

• •

7

Phytovolatilization; Rhizofiltration.

Figure 1.1. Mechanism of phytoremediation. Source: https://www.researchgate.net/publication/286194963_Water_Treatment_Technologies_Principles_Applications_Successes_and_Limitations_of_ Bioremediation_Membrane_Bioreactor_and_the_Advanced_Oxidation_Processes/link/5666c02308ae15e74634db64/download.

1.2.3.1. Phytoextraction Through this process, the plants actively eradicate pollutants from water or soil and transfer them to the higher tissues where contaminants are accumulated. Plants are therefore harvested as needed and managed consequently, i.e. the plants might be burnt, composted or utilized as firewood, for example. Proper disposal is crucial for plant biomass which gathers the contaminants, while, ash should be disposed of in dangerous waste landfill if plants are incinerated. For effective phytoextraction, the pollutant must dissolve into what the roots of the plant can absorb and the roots of the plant must absorb and pass the pollutants. The plant must also be able to store the contaminants safely, and finally, the plant should adjust to any harm the pollutants might have triggered during storage and transportation.

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Water Treatment Plants: Technology and Approaches

1.2.3.2. Phyto Stabilization This is the collaboration amongst the plants’ rhizosphere, substrate, and the contaminants in order to decrease the mobility of pollutants in the environment. This process is utilized to confine pollutants in the groundwater and soil via absorption, adsorption, and gathering by roots, or the precipitation within the rhizosphere. Phytostabilisation reduces the mobility of pollutants, averts movement to the air or groundwater, and also reduces bioavailability for entrance into the chain of food.

1.2.3.3. Phytodegradation This is the process by which plants transform or metabolize the pollutants to less injurious products with the help of the enzymes formed by plants. The contaminants are generally broken down into smaller simpler molecules and are then integrated into the tissues of the plant to support the plants ‘quick growth.

1.2.3.4. Phytostimulation This is the improvement of soil microbial action for the deterioration of pollutants, usually by organisms that associate with roots. Rhizosphere degradation takes place mostly in the soil; however, stimulation of the microbial activities in the root zone of the aquatic plants might possibly occur.

1.2.3.5. Phytovolatilization The utilization of plants to collect contaminants from water, soil, or wastewater and transfer them into shoots, where the pollutants or the modified form of contaminants might be volatilized.

1.2.3.6. Rhizofiltration This process involves filtration of water with the mass of roots to eliminate hazardous substances or excess nutrients. The contaminants stay absorbed in, or normally adsorbed to the roots. Examples of the plants known to eliminate contaminants through rhizofiltration are water spinach, duckweed, and calamus. Ndimele et al. (2014) described the aptitude of water hyacinth to absorb and transfer Iron and Copper. Their investigation exposed that Copper had better transfer capacity than Iron, while the gathering potential of Iron

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by water hyacinth is usually higher than Copper. They determined that Eichhornia crassipes can also be effectively used for phytoremediation and sufficient for biomonitoring programs for polluted water. Syuhaida et al. (2014) presented the comparison of the phytoremediation abilities of the water mimosa and Eichhornia crassipes. Their results specified that Neptunia oleracea normally has lower heavy metals acceptance and value of bioconcentration factor compared to the Eichhornia crassipes. However, it was stated that both plants gather high heavy metals in roots compared to leaves and stems and that both of the plants utilize the rhizofiltration process to eliminate heavy metals. Pedro et al. (2013) assessed the capability of Salicornia ramosissima on Cd phytoremediation under distinct salinities and Cd concentrations in the greenhouse experiment. They stated that the highest Cadmium accumulation was observed in the roots, and reduced with upsurge salinity and Cadmium concentration. They recommended S. ramosissima as the potential candidate for Cadmium phytoremediation at salinities near to zero and that the capability of S. ramosissima in Cadmium phytoaccumulation and phytostabilization was discovered to be remarkable. The phytoremediation of the water contaminated by thallium, zinc, cadmium, and lead with the utilization of Callitriche cophocarpa was described by Augustynowicz et al. (2014). After the incubation period of ten days, shoots of the C. cophocarpa were discovered to efficiently biofiltration the water therefore that it met suitable quality standards. They stated that C. cophocarpa considerably eliminated contaminated water toxicity according to Microtox bioassay and that the C. cophocarpa exposed to the metallic pollution didn’t display significant variations in its physiological status as compared with the control. Marrugo-Negrete et al. (2015) examined the phytoremediation of the mercury-polluted soils by Jatropha curcas (J. curcas). The authors evaluated the behavior of growth, mercury gathering, transfer, and bioconcentration aspects of the J. curcas and stated that the distinct tissues in the J. curcas were in order of reducing gathering Hg as roots > leaves > stems, and that the J. curcas species exhibited low translocation factor and high bioconcentration factor, and their utilization could be an auspicious method to remediate mercury-polluted soils.

1.3. MEMBRANE BIOREACTORS (MBRS) MBRs are systems assimilating the biological degradation of contaminants with membrane filtration (Figure 1.2). MBR is an emerging progressive wastewater treatment technology that has been applied successfully in

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Water Treatment Plants: Technology and Approaches

the industrial and municipal wastewater treatment, drinking water and groundwater abatement, solid effluent digestion and scent control (Newman et al., 1999). Cicek (2003) presented the critical review of MBRs and their possible application in the agricultural wastewater treatment. Studies have also exhibited that specific configurations of MBRs would preserve, concentrate, and subsequently break down hormones, pesticides, herbicides, industrial chemicals, pharmaceuticals. without needing sophisticated tertiary treatment procedures (Figure 1.3).

Figure 1.2. Schematic description of the membrane bioreactor. Source: https://www.esciencecentral.org/ebooks/ebookdetail/water-treatmenttechnologies-principles-applications-successes-and-limitations-of-bioremediation-membrane-bioreactor-and-the-advanced-oxidation-processes.

Figure 1.3. Membrane bioreactor configuration. Source: https://www.esciencecentral.org/ebooks/ebookdetail/water-treatmenttechnologies-principles-applications-successes-and-limitations-of-bioremediation-membrane-bioreactor-and-the-advanced-oxidation-processes.

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Membrane units can be made side stream or submerged in the bioreactor (Figure 1.3). The submerged is the most usual type of MBR because of its low energy requirement and compactness. In the submerged MBR, membrane segments are installed directly in the activated sludge (AS) reactor vessel, with waste sucked out of membrane segments by the support of the permeate pump and suspended solids (SS) retreat into the basin. In sidestream MBR, membrane segments are situated outside the reactor basin, therefore, the mixed liquor from the reactor is pumped into external membrane segments. This makes side stream MBR configuration to ingest more energy and side stream it also needs additional space. However, it must be observed that the selection of any configuration is dependent on the application requirements.

1.4. ADVANCED OXIDATION PROCESSES (AOPS) AOPs were defined by Glaze et al. (1987) at near ambient pressure and temperature. AOPs are water treatment procedures which include generation of the hydroxyl radicals in adequate quantities to affect water purification. Hydroxyl radical is conventionally considered to be active species responsible for the destruction of the pollutants (Yasar et al., 2007; Pimentel et al., 2008; Kim and Ihm, 2011). As per the recent evidence, the revision may be needed in this definition as numerous publications have stated use of the other radicals like sulfate radical and azide radical produced for degradation of the organic contaminants in wastewater (Sun et al., 2009; An et al., 2010). The use of AOPs also known as AOTs (Advanced Oxidation Technologies) has been stated to bring complete mineralization of the poisonous xenobiotic organics regularly detected in wastewater and water (Ikematsu et al., 2004; Ayoub et al., 2010). For the removal of diseases, that cause pathogens in water, AOPs have also been applied successfully (Klavarioti et al., 2009; Mendez-Arriaga et al., 2009; Olmez-Hanci et al., 2010). The use of AOPs for elimination of endocrine disruptors, pesticides, surfactants, pharmaceuticals, and broad range of the other industrial toxicants like humic acid, benzene derivatives and phenols commonly observed in-ground, surface, and wastewater have been reported by various researchers (Bandala et al., 2008; Martin et al., 2009; Abdessalem et al., 2010). These methods are superior to traditional chlorination of the water as they can enhance to also oxidize decontamination by-products like the haloacetic acids (HAA) and halomethanes related to chlorination of the water (Chin and Bérubé, 2005; Feng et al., 2010). The end products of the entire mineralization are water and carbon dioxide, the AOPs offer another

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important advantage to avoid the production of the secondary contaminant encountered while using other conventional methods. The AOPs have been made functional to various real waste situations like pulp mill wastes high in phenolics, landfill leachates dye wastes from dye industry and drinking water (Catalkaya and Kargi, 2007; Cortez et al., 2011; Jamil et al., 2011). When compared with the conventional processes, these exhibit better treatment efficiency. In shared mode with the other methods, they have also been utilized as post-treatment or pre-treatment unit processes as part of the multiple-barrier method for purification of water (Grafias et al., 2010; Rahman et al., 2010). In the hybrid water treatment procedures, the possible use of AOPs based on catalytic wet air oxidation and adsorption has recently proposed (Rosenfeldt et al., 2007; Lebigue et al., 2010; Mandal et al., 2010). AOPs utilize different reagent systems which comprise photo-catalysis, chemical oxidation processes, and photochemical degradation processes, they all generate hydroxyl radicals which are highly non-selective and reactive (Skoumal et al., 2006; Benitez et al., 2011). By using ozone, sulfate radicals (SO4)2– produced are more selective than the hydroxyl radical (Yang et al., 2009). The schematic grouping of AOPs is shown in Figure 1.4.

Figure 1.4. Schematic classification of advanced oxidation processes. Source: https://www.researchgate.net/publication/286194963_Water_Treatment_Technologies_Principles_Applications_Successes_and_Limitations_of_ Bioremediation_Membrane_Bioreactor_and_the_Advanced_Oxidation_Processes/link/5666c02308ae15e74634db64/download.

Abbreviations: H2O2 peroxide, UV – ultraviolet; AC – activated carbon; TiO2 – Titanium Oxide; WO – wet peroxide oxidation; WPO – wet peroxide oxidation; O3 – ozonation; SCWO – supercritical wet oxidation.

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These AOPs summarized in Figure 1.4 might be classified as heterogeneous or homogeneous (Hoigné, 1997; Alaton et al., 2002). Between these two wide classes, the main distinctive factor is the lack of the solid phase promoter or catalyst inhomogeneous process. Inhomogeneous procedures reagents like soluble O3, H2O2, and aqueous metals are utilized to produce OH radicals in the aqueous phase for the oxidizing dissolved organics. Energy sources like thermal, microwave, solar, and others shown in Figure 1.4 aren’t considered as the distinct phase from aqueous medium. Conversely, heterogeneous processes use the solid catalyst surface or phase upon which the radicals are produced. The production of radicals by UV produced holes on TiO2 solids spread in the aqueous medium is a usual example. Others like O3 break down on the solid metal oxides might be definitely regarded as the catalytic procedures and are widely classified under heterogeneous procedures. Homogeneous procedures can be further split into processes which don’t operate on the external energy sources and procedures which are driven by the external energy. These energy sources comprise ultrasonic (sonolysis), electrochemical, thermal, microwave, gamma radiation, UV light, and solar energy.

1.5. HOMOGENEOUS ADVANCED OXIDATION PROCESSES (AOPS) WITHOUT ENERGY 1.5.1. Ozonation in an Alkaline Medium In aqueous medium, ozone is unstable decomposing instinctively by the complex mechanism which includes the production of the hydroxyl radicals. Degradation of the organic compounds takes place by the action of O3 itself along with through radicals produced in alkaline medium. For a generation of the radical, the scheme has been broadly argued (Chelme-Ayala et al., 2010). Hoigne (1997) exhibited that in the aqueous solution ozone decomposition proceeds through the formation of the hydroxyl radicals as displayed in Equations (1)–(8). Hydroxyl ion has the part of the initiator, in the reaction mechanism.

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The mechanism also clarifies the role used by H2O2 as its formation during decomposition of the ozone as exhibited in Equation (2). It is thus clear that the addition of the H2O2 to the O3 aqueous solution would increase the O3 decomposition with the formation of the OH radicals. This makes the basis for the utilization of another process centered on H2O2 and O3: the peroxone process. Alaton et al. (2002) proposed the less complicated scheme for the decomposition of the O3 to radicals. The use of ozonation has been widely applied for the decomposition of the organics in wastewater and water. The wastes from paper and pulp industry include a number of poisonous compounds especially chlorinated derivatives and phenols. The paper making procedure uses a large quantity of water and their wastes laden with the xenobiotic organics are discharged directly into the surface water streams causing harmful effects to aquatic life. Catalkaya and Kargi (2007) used the ozonation process as the pretreatment operation for wastes from the pulp mill in Turkey. To remove the color from wastes this method was considered very effective. For the treatment of pesticides existent in the membrane concentrate from the membrane plant in Alberta, Canada, Chelme-Ayala et al. (2010) used the ozonation process. The workers noted the removal efficiency of 63% for trifluralin and 87% for bromoxynil from the concentrates. For the treatment of several organics in the wastewater through the application of ozonation, a large number of articles published, are available in the literature (Dombi et al., 2002; Consejo et al., 2005; Schröder and Meesters, 2005). The major constraint to the utilization of O3 for the water treatment lies with the operational expense of the procedure itself. When in water the lifetime of O3 is just a few minutes, as O3 is quite unstable gas. It thus has to be yielded in-situ when needed. Its application needs well-designed gas-liquid contacting strategies to bring the efficient mass transfer of the O3 from gas to liquid phase (Wu et al., 2004, 2007a, b).

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1.5.2. Oxidation Using H2O2 Oxidation by H2O2 (peroxide) alone is not considered as an advanced oxidation process because H2O2 is not a radical. H2O2 is generally stable in water and the oxidation is centered on the inherent oxidative power of peroxide molecule being the natural oxidizing agent. In practical uses, H2O2 goes through solar photolysis giving radicals in the same way as UV photolysis of H2O2 and the participation of OH• during the H2O2 oxidation can’t be taken out. The procedure of H2O2 breakdown is slow without the catalyst and has been considered as inefficient for the treatment of the organics in wastewater. In the research by Coca et al. (2005), brown-colored molasses obtained from the fermentation factory in Spain and the peroxide added to it, even after 18 hours of contact didn’t bring any variation in color of wastewater. Until 23 hours of continuous addition of peroxide stepwise, no substantial change in brown color of syrupy wastewater occured. The use of UV combined separately with H2O2 and chlorine as a novel, Wang et al. (2012) evaluated the treatment of trichloroethylene in the drinking water through AOP. In their study, the rate of the trichloroethylene decay by UV/chlorine, UV/H2O2, and UV at discrete pH values were examined and compared. They stated that at pH, the UV/chlorine procedure was more effective than the UV/H2O2 procedure, but in alkaline and neutral pH range, UV/H2O2 procedure became more effective (Wu et al., 2004). Prado and Esplugas (1999) ihave exhibited that at high pH in lack of UV light formation of the hydroxyl radical by the hydrogen peroxide was only favorable. They stated that at pH 6.8 and 4.8, the effects of the variable pH on oxidation of the atrazine, over the 24 hour period atrazine elimination of 9% and 4% was observed. However, within 180 min 100% elimination of pesticide was attained at higher basic pH. The process at high pH for the generation of the radicals assumed to be intricated in the oxidation process has not been investigated. In another study in Pakistan, Yasar et al. (2007) studied the utilization of H2O2for controlling of the pathogens in industrial wastes. Results attained under improved oxidant dose of around 170 mg/L exhibited that the oxidant was proficient in eliminating 99% of pathogens present in wastewater. The disadvantage of its utilization was highlighted by the authors that after the incubation period pathogens halted by the H2O2 were reactivated, which changed depending on reaction condition. When H2O2 was utilized in combination with other procedures like O3 and UV, this problem was overcome.

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Based on the literature review, we mayt assume that the utilization of H2O2 for water treatment is only effective at alkaline pH, and it can be considerably affected by the pH. When applied as the disinfectant for pathogens, the colony regrowth is repressed when H2O2 is utilized in combination with other procedures. The existence of residual peroxide is another disadvantage of this procedure, which might need further consideration after the water treatment. The utilization of peroxide as the oxidant for organics has been also reported by other researchers (Ledakowicz et al., 2006).

1.5.3. Fenton and Fenton-Like Procedures In the presence of Fe(II) as the catalyst, oxidation power of the H2O2 on specific organic molecules in which the OH radicals are formed from H2O2 was described by H.J. Fenton, a century ago. Walling (1975) proposed the reaction mechanism, later on, and is given by Equations (9)–(11). Fe2 + + H2O2→Fe3 + + OH− + OH•

(9)

Fe + OH →Fe + OH

(11)

Fe3 + + H2O2→Fe2 + + H + + HOO• 2+

•

3+

–

(10)

For this old reactive system, the renewed concern of researchers is today highlighted by the considerable number of investigations dedicated to its application for the treatment of the wastewater. The Fenton procedure is one of the most researched and widely applied processes for the treatment of the recalcitrant compounds due to its simplicity. The utilization of Fenton reagent for degradation of the wastewater gathered from industrial waste plants in India was examined by Mandal et al. (2010). The effect of operational variables like temperature, peroxide, pH, and Fe(II) dose was examined for the reduction of the COD (chemical oxygen demand) in the wastewater sample. The Fenton procedure needs strict pH control and numerous published data puts optimal pH for the procedure in the range: 2.5–3.5. The COD elimination was increased by an additional 5% only, after extending the time to 24 hours. For the total elimination of the organics in wastewater, the use of the procedure in combination with the biological treatments was recommended; it was unproductive for the low concentrations of organic. For degradation of the model contaminant in aqueous solution, the Fenton process was also utilized by Chen et al. (2010). After 150 min, salicylic acid elimination effectiveness under optimized conditions was 96% and COD reduction was 80%. For degradation of the salicylic acid, the Fenton procedure was efficient, as exhibited by results. The entire mineralization wasn’t as effective as the degradation of contaminants.

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For amelioration of the organics in the wastewater with the use of the Fentonprocedure was discussed by numerous reports in the literature (Benitez et al., 2002; Al-Momani et al., 2007; Sun et al., 2008). All procedures which include the utilization of Fe3+ ions or oxidizing metals like permanganate are mentioned as the Fenton like processes. Specifically important between them is ferrioxalate complex, it has the strong absorption at wavelengths above 400 nanometers and thus undergoes reactions which can be photo catalyzed by the visible light. The photodegradation of oxalate and hydroxyl complexes of Fe(III) create Fe2+ ions which catalyze the decomposition of the H2O2 to OH radicals. Equations (12)–(16) display the involvement of Fe as oxalate in the generation of the hydroxyl radical.

Furthermore, other active species and free radicals are produced as exhibited by Equation (16). An important advantage of the Fenton like the process is an evasion of Fe3+ sludge related to the classical Fenton process.

1.5.4. Ozone/H2O2 Process H2O2 is to some extent disassociated to the hydroperoxide ion (HO2−) in an aqueous solution, which reacts with the ozone thus giving rise to the series of chain reactions containing OH radicals (Sánchez-Polo et al., 2007). Equations (17) and (18) demonstrate direct attack of the O3 by hydroperoxide and H2O2 ion to yield HO2 and OH radicals respectively. The effectiveness of the peroxone method is centered on the production of these radicals. H2O2 + 2O3→ 2OH• + 3O2

(17)

HO2– + O3→HO2• + O3•

(18)

HO2– + H+ ↔ H2O2

(19)

By decomposition of the H2O2 hydroperoxide, an ion is produced which recombines reversibly with the hydrogen ions to yield H2O2 in the solution (Equation 19). At high pH, decomposition of the ozone also yields the hydroperoxide ion which reversibly yields peroxide as exhibited in Equation 2. According

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to Le Chatelier’s principle, decomposition of the ozone would be increased by external addition of the H2O2 to the aqueous solution of ozone in order to yield OH radicals The impact of pH is also apparent that the active species is conjugate base OH2– in ozone decomposition mechanism whose concentration is firmly dependent on the pH. The addition of H2O2 and an increase in pH to the aqueous solution of the ozone would outcome in higher production of the hydroxyl radical and in radical chain decomposition; procedure the higher firm state concentration of the OH is attainted. It is important to note that no substantial change in the industrial reactor or in apparatus is observed by the adoption of AOP as it is only essential to add the H2O2 dosing system. For the treatment of the landfill leachates in Portugal, peroxone system was applied by Cortez et al. (2011) with the intention of raising the biodegradability of treated leachate in the combined biological/chemical process. While both peroxone and ozonation processes considerably enhanced the BOD/COD Ratios (BOD and COD ratios) implying enhanced biodegradability, higher ratios are presented by the peroxone process and higher biodegradability of treated leachates. Application of one procedure over the other would be dependent on the running cost of the chemicals weighed against the anticipated quality of waste needed for successive biological treatment. To increase the production of non-selective OH, utilization of ozone in amalgamation with H2O2 has been broadly accepted during ozonation leading to enhanced removal of the refractory organic compounds (Sánchez-Polo et al., 2007; Catorceno et al., 2010).

1.5.5. Homogeneous Advanced Oxidation Processes (AOPs) Using UV Homogeneous AOPs using UV radiation is usually applied for degradation of the compounds which adsorb UV radiation. These are generally aromatic hydrocarbons and dyes proficient in absorbing UV energy. The UV isn’t considered the distinct phase from the aqueous medium where it is made functional, as mentioned previously (José et al., 2010).

1.5.6. UV/Ozone AOPs using UV and ozone proceeds through photolysis of the aqueous ozone to generate hydroxyl radicals, as exhibited by r Equations (20)–(21). For ozone absorption, an aqueous solution is exposed with UV light at the

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wavelength of around 254 nm. At this wavelength, extinction coefficient of the ozone is normally 177 times higher than that of the peroxide. HO2 + O3 + UV→ 2OH + O2 2OH → H2O2

(20) (21)

For the degradation of phenol, this technique has been utilized. Outcomes were compared with procedures run without the use of UV radiation (José et al., 2010). The comparison revealed the effect of UV radiation on the oxidation procedure. The UV/O3 procedure was capable of 99.2% elimination of the dissolved organic carbon than the O3 alone (58.2%). Some researchers have applied this process equally for degradation of the diazepam, clofibric acid, diclofenac, and carbamazepine and observed the degradation products through LC-MS (liquid chromatography-mass spectrometry) (Wang et al., 2009). Under optimized conditions of the ozonation by UV photolysis, complete elimination of these recalcitrant pharmaceuticals was reported. Trihalomethanes (THM) and Haloacetic acid are the main group of disinfection by-products and normally have been recognized as carcinogens. For mineralization of the dichloroacetic acids and trichloro, UV/O3 was described to have been appropriate, during the disinfection of the water by chlorination these traditional HAA are formed (Vogelpohl, 2007).

1.5.7. UV/H2O2 For the treatment of the wastewater polluted with a wide variety of organics, the utilization of UV to bring photolysis of the hydrogen peroxide has normally been adopted. This procedure involves the photolytic symmetrical splitting of the peroxide molecule to yield two hydroxyl radicals as exhibited by Equation (22). Typical UV absorption of the H2O2 is around 257 nm.

H 2O 2

+ UV → 2OH

(22)

2UV/H2O2 system can mineralize totally any organic compound and breaking it to water and CO2. In real-life scenarios, the drastic process is not compulsory. Since the toxicity of the oxidation products is easily biodegraded, therefore it is not a problem anymore (Ledakowicz et al., 2001). The major disadvantage of this procedure is smaller extinction coefficient of H2O2 at 257 nm ( i.e 18.6 M–1cm–1),therefore, only the comparatively small portion of incident UV light is exploited for AOPs. For water purification, numerous studies have been initiated to optimize this procedure. This is accepted by comparative ease of handling the cost of the chemicals and H2O2. Prior to the biological treatment with dehydrogenase of the raw textile wastewater utilization of UV/H2O2 for detoxification of

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the same has been stated (Kestioğlu et al., 2005). When UV was utilized alone to 26% for UV/ H2O2 the inhibitory effect was decreased from 30% was attributed to the synergistic effect of combined treatment. The enzyme activity was repressed by raw wastewater by around 47%. For detoxification of the textile wastewater, the AOPs were an efficient technique as per the conclusion. During subsequent biodegradation on the AS, considerable decrease of the inhibitory action of the microbial growth demonstrates the positive impact. For the treatment of the raw waste from the olive treatment plant in Turkey, Kestioglu et al. (2005) applied the UV/ H2O2 procedure. The waste was characterized by high COD, total phenol and very low pH. Treatment periods of 1440 and 400 min decrease the COD to 3650 and 3060 mg/L. The total phenol for the optimized peroxide dose was decreased to 22 and 10 mg/L, accounting for 99% of COD and phenol elimination. For the treatment of the pollutant in wastewater, utilization of this technique as an AOP has been stated in other studies (Çatalkaya and Şengül, 2006; Sauer et al., 2006; Thiruvenkatachari et al., 2007).

1.5.8. UV/Fenton Process The degradation rate of the organic contaminant with the Fenton-Fenton like substances is strongly enhanced by irradiation with the UV-visible light (Huang et al., 2008; Prato-Garcia et al., 2009). This is an addition of the Fenton process which uses the benefit of the energy obtained from photons in the UV-visible irradiation at the values of wavelength higher than 300 nanometers. In these conditions, photolysis of the Fe(III) hydroxide complex, which is, the product of the Fenton process (Equation 11) permits for the regeneration of the Fe2+ as demonstrated by the Equation (23).

1.6. HOMOGENEOUS ADVANCED OXIDATION PROCESS USING ULTRASOUND The formation of OH radicals when ultrasounds lead by high pressure and temperature conditions are attained inside the bubbles produced by the ultrasounds. The process is founded on the ‘hot-spot’ theory. According to this theory, sonolysis triggers liquid cavitation given rise to the energy bubble which breakdown generating radicals. In the circumstance of reactions where the governing mechanism is a radical attack, the utilization of ozone

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and hydrogen peroxide improves the degradation because of the generation of the additional free radicals. Normally, this type of advanced oxidation process reduces cost as no radiation is required, and can be combined readily with other methods (Liou et al., 2003; Kusvuran and Erbatur, 2004; Muñoz et al., 2006). Sonolysis has been stated to be sludge free, safe, and without the creation of secondary pollutants. It has the capability to enter cloudy waters and offers better energy conservation schemes. Despite its benefits, there are economic and technical challenges restricting the adoption and scaleup of this technology for industrial wastewater treatment. The technology mandatory is still in the budding stage (He et al., 2007; Saritha et al., 2007). The reactions which take place in the existence of ultrasound and ozone are given in Equations (24)–(31) (Shemer and Narkis, 2005; He et al., 2007).

He et al. (2007) utilized this ultrasound technique to accomplish the efficient degradation of the p-aminophenol, an intermediate in the production of the paracetamol. For the preliminary p-aminophenol concentration of around 10 mmol/L, the degradation efficiency of 99% was accomplished within 30 min. The mineralization of p-aminophenol was 77% at this time. The dose of ozone was at around 5.3 g/h at 25oC with a pH of about 11 and the ultrasonic energy density of around 0.3 W/mL. It was noticed that the degradation was affected by the pH, ozone dose, and temperature, but unaffected by the rise in the energy density per unit volume of ultrasound energy. By combining H2O2and ultrasounds, it was probable to accomplish the creation of free radicals in the gaseous phase of cavitation bubbles produced during the sonication. The proposed mechanism for the production of the free radicals was presented by Shemer and Narkis (2005) and is exhibited in Equations (32)–(34). H2O2→OH• + OH•

H2O2 + O2→2HO2

•

(32) (33)

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Water Treatment Plants: Technology and Approaches

H2O2 + OH•→ HO2• + H2O

(34)

1.7. HOMOGENEOUS PROCESS UTILIZING SOLAR ENERGY The current drift in energy management is to advance technologies that run on renewable energies. Especially significant in this respect are those which run on universally ample solar energy. Radiant energy from the sun comprises the broadband of energized photons proficient of producing hydroxyl radicals through photolysis of the peroxide in a manner similar to the utilization of UV-visible light source. Non-selective radical produced then destroys the organics. The utilization of solar energy as the source of energy in order to motivate photolytic cleavage of the H2O2 and Fenton process was accepted by Muruganandham and Swaminathan (2007) for decolorization of the reactive yellow-14 in heterogeneous and the homogeneous media. The experiments were performed under the sunny circumstances in the borosilicate glass tubes with the solar irradiation out in the open-air condition. The solar intensity was observed every 30 min using the LT Lutron LX-10/A and the average intensity of light over the duration of the experiment was calculated. They stated that utilization of the solar irradiation in the existence of H2O2 improved the de-colorization of dye solution occasioning in escalation in the percentage elimination from 12.7% to 71.2%. The reaction at lower pH has been more auspicious. The percentage elimination of dye utilizing the process of Fenton in the existence of solar energy also increased from 73.7% to 80.3%. In another experiment utilizing TiO2 and solar energy, the dye elimination after 80 min was around 82.1%. In the solar/TiO2 process, TiO2 uses the UV part of the solar spectrum to form e– and h+. The highly oxidative h+ reacts directly with the surface adsorbed molecule of dye or directly oxidizes organic compounds through the creation of OH radical. Studies on the outcome of solar light intensity on de-colorization displayed that the increase of solar intensity of light from 700 to 1250 Lux escalates the decolorization from 73.2% to 91.25, 85.3 to 93.4% and 28.7 to 42.1% for TiO2/solar, solar/H2O2/ Fe2+ and solar/H2O2 processes. The improvement of the rate of removal is because of an increase in hydroxyl radical production (Markan et al., 2000; García-Molina et al., 2005). Light intensity regulates the number of photons absorbed by the catalyst. With an increase in solar power, catalyst absorbs a greater number of photons and creates more hydroxyl radicals (Kusvuran et al., 2004; Debellefontaine and Foussard, 2000).

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1.8. HETEROGENEOUS CATALYTIC OXIDATION In the current years, a huge amount of investigation and development has been made in the field of photocatalytic water and heterogeneous catalytic purification processes because of their efficiency in mineralizing and degrading recalcitrant organic compounds along with the possibility of using UV, solar, and visible spectrum. Applied catalysts comprise tempered metals, AC, and metal oxide nanoparticles (Lesko et al., 2006). Various projects have been focused on progressing the synthesis and working of various shapes and sizes of the semiconductor and the metal nanoparticles. The aims of these research projects are chiefly to enhance the utilization and performance of nanoparticles in numerous applications that comprise the AOPs (Horikoshi et al., 2007; Wang et al., 2009). Whereas mechanism of the mineralization utilizing AC in advance oxidation process, is carried out by the adsorption on carbon surface trailed by quick oxidation of the concentrated contaminant on carbon interface. Mechanism for the metals and the nanoparticles in advance oxidation process is by thermal- initiated or complex photoredox reactions which causes either direct mineralization of organics or generation of the free radicals with following mineralization. The heterogeneous catalyst used in AOP might be made practical in one of the three modes: (1) As hetero-catalyst at ambient pressure and temperatures to catalyze an improved AOP like ozonation, peroxide or Fenton processes; (2) as the hetero-catalyst at raised temperatures to catalyze the wet oxidation procedures and (3) as the catalyst functional in photo-processes in order to bring photocatalytic degradation of the organic substrate or with other advanced oxidation process like UV/Fenton or UV/ozonation (Guwy et al., 1998; Ayanda and Petrik, 2014).

1.8.1. Heterogeneous Catalysis at the Ambient Conditions Materials functional as the catalyst at the ambient conditions comprise of AC, metals supported on the metal oxide surfaces (Al2O3, ZrO3, TiO2, CeO2, MnO2, Cu-Al2O3, FeOOH, and Cu-TiO2) or supported on the AC. The catalyst support assists to increase the surface area of the catalyst, reduce sintering, and enhance hydrophobicity along with the chemical and thermal stability of material (Ma et al., 2004; Rivera-Utrilla et al., 2008). Heterogeneous catalytic ozonation is the advanced oxidation process where an oxidative property of the ozone is enhanced by adding the solid catalytic materials. The process is possibly an inexpensive advanced oxidation process that has been worked fruitfully at ambient environments by numerous workers and

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might be comparatively easier to apply in the water treatment plant (WTP) (Bourbigot et al., 1996). It has been revealed that AC can improve ozone transformation into the OH radicals (Sánchez-Polo et al., 2006). Electrons in graphene layers and the basic surface groups on AC are the chief factors accountable for the decomposition of the O3 into OH• (Akyurtlu et al., 1998). These produced hydroxyl radicals are accountable for the oxidation of the organics utilizing the O3/ GAC process. In addition to the catalytic role, AC also aids as a vital adsorbent for organics been handled and would be efficient for the elimination of hydrophobic micropollutants which can’t be oxidized by the ozone. Sanchez-Polo et al. (2005) applied the O3/ GAC advanced oxidation process for the treatment of the pCBA (ParaChlorobenzoate) and compared the effectiveness of this procedure with the traditional homogeneous ozonation in an alkaline medium and peroxone processes. pCBA was chosen for the research due to its low reactivity with the O3 and the slow adsorption kinetics on AC. Their report exhibited that the O3/GAC system was poorer than the homogeneous processes. Radical generation in heterogeneous systems included the adsorption of the ozone onto AC and its consequent breakdown to radicals. As this process needs more phases, it is thus slower as compared to a system where the O3 and species starting its breakdown are normally in the same phase. Despite this drawback, the utilization of heterogeneous catalysts for the ozone breakdown is meeting interest. This is due to the possibility of the catalyst regeneration and reprocesses out ways the ineffectiveness of heterogeneous processes. More so, utilization of metals in the homogeneous process creates another difficulty as these metals whether poisonous or not would need elimination after the treatment process (Doocey et al., 2004; Yu et al., 2011; Pathan and Patel, 2013).

1.9. HETEROGENEOUS WET OXIDATION Application of the heterogeneous catalyst to the wet air oxidation or wet peroxide AOPs have been reported (Arena et al., 2010; Benitez et al., 2011). The use of proper catalysts for the CWAO not only decreases the severity of reaction conditions but also decomposes more easily even refractory contaminants, thereby decreasing operational and capital cost (Gomes et al., 2002; Larachi, 2005). Heterogeneous catalysts made practical in these procedures possess high physical, chemical, temperature, and mechanical stability. They would also retain high resistance to erosion over the wide temperature and pressure range (Bhargava et al., 2006; Zhan et al., 2010).

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1.9.1. Catalytic Wet Air Oxidation Numerous heterogeneous catalysts comprising of noble metals, mixed metal oxides, and metal oxides have been widely studied to improve the efficiency of CWAO. In the 3 phase CWAO process, the organic contaminants are oxidized by the activated O2 species in existence of the solid catalyst, creating biodegradable intermediates, or are normally mineralized to CO2, associated inorganic salts and water. Various recently advanced heterogeneous catalysts, comprising of noble metals and transition metal oxides deposited on diverse supports, have exhibited good catalytic activity in the CWAO of organic contaminants present in waste and drinking water (Levec and Pintar, 2007; Liu et al., 2010). The utilization of mixed metal oxides for the CWAO of the phenol in water was stated (Benitez et al., 2011). Batch experiments were performed using changing combinations of zinc, copper, and aluminum oxides in the autoclave and per reactor. The effect of functioning variables like catalyst loading, temperature, catalyst composition, oxygen partial pressure, stirring speed and initial phenol concentration was reported. The time needed for the total phenol conversion varied from 0.5 hr to 2 hr and was reliant on the experimental conditions. When the reaction amongst phenol and oxygen species was initiated from the room temperature, degradation was noticed to proceed through two regimes. Firstly, the period of induction, after which there was a transition to the much higher activity regime. On the other hand, when phenol was presented after the preheating period of the solution saturated with O2, no period of induction was observed.

An overview of the catalytic pattern of the homogeneous and ceria supported transition metals in CWAO of the phenol has been described (Doocey and Sharratt, 2004). Catalyst tests were performed at 150°C and 1.4 MPa using the PTF-lined autoclave comprising 0.25 mL, 1000 mg/L phenol and worked in the semi-batch mode. Catalytic homogeneous wet air oxidations were noticed to proceed through an indiscriminating autocatalytic free-radical pathway leading to the refractory C1-C2 acids, whereas CWAO on the heterogeneous ceria gave better efficiency and was considered for by Langmuir-Hinshelwood mechanism. In the recent study, Yu, and the co-workers made functional Ru supported on the Ce/γ-Al2O3 and γ-Al2O3 for CWAO of the isopropyl alcohol, acetic acid, phenol, and N,Ndimethylformamide (Bhargava et al., 2006). The effectiveness of catalysts was noticed to escalate with increasing temperature. The Ru-Ce/γ-Al2O3 catalyst formed superior elimination effectiveness compared to Ru-γ-Al2O3 for all of the substrates examined and attributed to better dispersion of Ru particles on the Ce/γ-Al2O3 surface along with an increase in the number of

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efficiently active sites on cluster derived catalyst surface. The efficiency of Pt supported on the AC and on the multi-walled CNT (carbon nanotubes), as a heterogeneous catalyst in the CWAO of chosen pharmaceuticals was examined by Benitez et al. (2011). The supported platinum (Pt) catalyst has been described to be highly efficient in the oxidation of the organic compounds (Mikulová et al., 2007). Operating variables like catalyst type, temperature, dose, and oxygen pressure were considered. There was a noteworthy enhancement in the elimination of pharmaceuticals utilizing CWAO comparative to WAO. Pt reinforced on AC normally gave greater elimination efficiency compared to Platinum supported on CNT under similar conditions, reflecting the contribution of the adsorption effects to oxidation paths for the elimination of pharmaceuticals under study.

1.9.2. Catalytic Wet Peroxide Oxidation (WPO) The mechanistic path for the production of hydroxyl radicals from the decomposition of the hydrogen peroxide anticipated by Li et al. (1991) includes the initiation of the chain reaction by reaction of the hydrogen peroxide with heterogeneous or homogeneous species existing in the reactor system. According to this proposed path, one way of enhancing the efficiency and production of hydroxyl radicals, and therefore lowering the working price would be to announce the heterogeneous catalyst into the reactor system. In this viewpoint, various workers have applied numerous catalysts to the WPO processes. Doocey and Sharratt (2004) made functional iron-loaded zeolite for the elimination of the chlorinated phenolic effluent from the aqueous waste. The efficient utilization of produced hydroxyl radicals is maintained by choosy adsorption of phenolic effluent onto ironloaded zeolite trailed, by the in-situ Fenton oxidation by hydroxyl radicals on the catalyst surface. This Fenton-type oxidation utilizing iron supported on AC and alumina has also been described for CWPO of the halogenated organic compounds in the groundwater. Over the former decade, HT (hydrotalcite-like) Compounds, otherwise known as LDHs(Layered Double Hydroxide) have gained increasing attention due to their different applications particularly in catalysis (Centi and Perathoner, 2008; Debecker et al., 2009). These compounds centered on mineral hydrotalcite have been produced and employed as the catalyst in WPO procedures (Wang et al., 2009). CWPO of the phenol by Ni-Cu-Al hydrotalcite was reported recently (Madhavan et al., 2010a, b; Zhou et al., 2011). There was the synergistic effect when the catalyst was made functional in the existence of hydrogen peroxide, getting about a complete elimination of phenol within two hours

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when the system was functioned at 30oC. When applied alone, metal HT and the hydrogen peroxide brought only about 15.7% and 39.7% elimination of phenol, respectively. The effect of working variables like catalyst dose, oxidant/phenol ratio, and the temperature was also reported (Dubey et al., 2003; Kanna et al., 2005).

1.10. HETEROGENEOUS PHOTOCATALYSIS The most usual nanoparticle made functional as the heterogeneous photocatalyst in the advanced oxidation process is TiO2. To this end, huge numbers of articles have been dedicated to study the use of this oxide to numerous contaminants, and amalgamation with other procedures like ozonation and the ultrasound-assisted-photolytic procedures (Ahmed et al., 2010, 2011). Other heterogeneous catalysts made functional in AOP comprise ZnO, Al2O3, In2O3, SnO2, ZnS, CeO2, ZrO2, Fe2O3, CuO, MnO2, SiO2, and CdS. While the chemical, physical, and catalytic properties of the catalysts change markedly, the principle of the operation as photo-catalysts stays the same. Adsorption of the electromagnetic energy by the electrons on the surface of the photo-catalyst brings an excitation of the surface electrons. When the absorbed energy is greater than band-gap energy, excited electrons transfer from valence-band to conduction-band. This mechanism produces active species which encourage redox reactions.

1.10.1. Photo-Catalytic Oxidation Early applications of the photo-catalyst comprise of coating of the selfcleaning windshields and windows, and floor-tiles utilized in hospitals to decrease the density of groups of the microorganisms in hospital floors and walls. They have been made functional in architectural constructions, creating deodorizing, self-cleaning surfaces, and mold-preventing (Lachheb et al., 2002). In current times, the use has stretched to water purification for the elimination of the organics in water. The application of UV/TiO2 for the treatment of the dye solutions and the other refractory organic contaminants in the water has been described (Heller, 1995). Mascolo et al. (2008) made functional the UV/TiO2 procedure for the elimination of MTBE (methyltert-butyl ether) from laboratory-made solutions comprising MTBE and groundwater samples comprising MTBE collected from the petrochemical site in Southern Italy.

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Saritha et al. (2007) made functional the UV/TiO2 procedure for elimination of the 4-chloro-2-nitrophenol, the USEPA listed contaminant, extensively available in pesticide wastes and bulk drug. Under similar working conditions, outcomes for the UV/TiO2 method were better than UV/ H2O2, peroxide, Fenton, and UV processes. The UV/TiO2procedure brought around 85% mineralization of contaminants in 120 min. In the deficiency of TiO2, the UV mineralization was quite slow because the 4-chloro-2nitrophenol suffers light absorptions which don’t contribute to the elimination of the compound. Because of surface area restrictions and poor adsorption capability of TiO2 current research is focused on including photo-catalysts on the porous supports which will absorb target environmental contaminants more efficiently former to consequent oxidation by photo-catalyst.

1.10.2. Photo-Catalytic Ozonation The fundamental mechanism of the photocatalytic ozonation might be given by the Equations (35)–(38).

Even though the genuine mechanism might be more complex, the principle has been extensively applied for the treatment of the environmental contaminants in water (Černigoj et al., 2007; Jing et al., 2011). The UV/ TiO2/O3 procedure was described to be better than O3, UV/TiO2/O2 and UV/O3, and the process for degradation of the neonicotinoid insecticides in water. The synergistic effect of the O3 on TiO2 was apparent at acidic and neutral pH. At basic pH, the decomposition of O3 by the reaction with OH– was the dominant mechanism for the OH radical production (Rajeswari and Kanmani, 2009). A similar synergistic influence was also stated for photocatalytic ozonation of the dimethyl phthalate utilizing laboratory prepared TiO2. The rate constant for the UV/TiO2/O3 process was 2.5 which are 5.2 times more than in UV/TiO2/O2 (UV/O3) process. The TOC elimination of photo-catalytic ozonation procedure increased with rising ozone dosage and was defined by the Langmuir-Hinshelwood model (Beltrán et al., 2009, 2010).

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Journal of Hazardous Materials, 185(1), 353–358. Jing, Y., Li, L., Zhang, Q., Lu, P., Liu, P., & Lü, X., (2011). Photocatalytic ozonation of dimethyl phthalate with TiO2 prepared by a hydrothermal method. Journal of Hazardous Materials, 189(1/2), 40–47. José, H. J., Gebhardt, W., Moreira, R. F. P. M., Pinnekamp, J., & Schröder, H. F., (2010). Advanced oxidation processes for the elimination of drugs resisting biological membrane treatment. Ozone: Science and Engineering, 32(5), 305–312. Kannan, S., Dubey, A., & Knozinger, H., (2005). Synthesis and characterization of CuMgAl ternary hydrotalcites as catalysts for the hydroxylation of phenol. Journal of Catalysis, 231(2), 381–392. Kestioğlu, K., Yonar, T., & Azbar, N., (2005). Feasibility of physicochemical treatment and advanced oxidation processes (AOPs) as a means of pretreatment of olive mill effluent (OME). Process Biochemistry, 40(7), 2409–2416. Kim, K. H., & Ihm, S. K., (2011). Heterogeneous catalytic wet air oxidation of refractory organic pollutants in industrial wastewaters: A review. Journal of Hazardous Materials, 186(1), 16–34. Klavarioti, M., Mantzavinos, D., & Kassinos, D., (2009). Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes. Environment International, 35(2), 402–417. Kusvuran, E., & Erbatur, O., (2004). Degradation of aldrin in adsorbed system using advanced oxidation processes: Comparison of the treatment methods. Journal of Hazardous Materials, 106(2/3), 115– 125. Kusvuran, E., Gulnaz, O., Irmak, S., Atanur, O. M., Yavuz, H. I., & Erbatur, O., (2004). Comparison of several advanced oxidation processes for the decolorization of reactive red 120 azo dye in aqueous solution. Journal of Hazardous Materials, 109(1–3), 85–93. Lachheb, H., Puzenat, E., Houas, A., Ksibi, M., Elaloui, E., Guillard, C., & Herrmann, J. M., (2002). Photocatalytic degradation of various types of dyes (Alizarin S, Crocein orange G, Methyl red, Congo red, Methylene blue) in water by UV-irradiated titania. Applied Catalysis B: Environmental, 39(1), 75–90. Larachi, F., (2005). Catalytic wet oxidation: Micro-meso-macro methodology from catalyst synthesis to reactor design. Topics in Catalysis, 33(1–4), 109–134.

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82. Lebigue, C. J., Andriantsiferana, C., Ayral, C., Mohamed, E., Wilhelm, A. M., Delmas, H., & Fowler, G. D., (2010). Application of sludgebased carbonaceous materials in a hybrid water treatment process based on adsorption and catalytic wet air oxidation. Journal of Environmental Management, 91(12), 2432–2439. 83. Ledakowicz, S., Michniewicz, M., Jagiella, A., Stufka-Olczyk, J., & Martynelis, M., (2006). Elimination of resin acids by advanced oxidation processes and their impact on subsequent biodegradation. Water Research, 40(18), 3439–3446. 84. Ledakowicz, S., Solecka, M., & Zylla, R., (2001). Biodegradation, decolourisation and detoxification of textile wastewater enhanced by advanced oxidation processes. Journal of Biotechnology, 89(2/3), 175–184. 85. Lesko, T., Colussi, A. J., & Hoffmann, M. R., (2006). Sonochemical decomposition of phenol: Evidence for a synergistic effect of ozone and ultrasound for the elimination of total organic carbon from water. Environmental Science and Technology, 40(21), 6818–6823. 86. Levec, J., & Pintar, A., (2007). Catalytic wet-air oxidation processes: A review. Catalysis Today, 124(3/4), 172–184. 87. Li, L., Chen, P., & Gloyna, E. F., (1991). Generalized kinetic model for wet oxidation of organic compounds. AIChE Journal, 37(11), 1687– 1697. 88. Liou, M. J., Lu, M. C., & Chen, J. N., (2003). Oxidation of explosives by Fenton and photo-Fenton processes. Water Research, 37(13), 3172– 3179. 89. Liu, W. M., Hu, Y. Q., & Tu, S. T., (2010). Active carbon–ceramic sphere as support of ruthenium catalysts for catalytic wet air oxidation (CWAO) of resin effluent. Journal of Hazardous Materials, 179(1–3), 545–551. 90. Ma, J., Sui, M. H., Chen, Z. L., & Wang, L. N., (2004). Degradation of refractory organic pollutants by catalytic ozonation—activated carbon and Mn-loaded activated carbon as catalysts. Ozone: Science and Engineering, 26(1), 3–10. 91. Madhavan, J., Grieser, F., & Ashokkumar, M., (2010a). Degradation of orange-G by advanced oxidation processes. Ultrasonics Sonochemistry, 17(2), 338–343. 92. Madhavan, J., Kumar, P. S. S., Anandan, S., Zhou, M., Grieser, F.,

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112. Pirzadah, T. B., Malik, B., Tahir, I., Kumar, M., Varma, A., & Rehman, R. U., (2015). Phytoremediation: an eco-friendly green technology for pollution prevention, control, and remediation. Soil Rem Plants Prospects Chall, 5, 107–126. 113. Prado, J., & Esplugas, S., (1999). Comparison of different advanced oxidation processes involving ozone to eliminate atrazine. Ozone: Science and Engineering, 21(1), 39–52. 114. Prato-Garcia, D., Vasquez-Medrano, R., & Hernandez-Esparza, M., (2009). Solar photoassisted advanced oxidation of synthetic phenolic wastewaters using ferrioxalate complexes. Solar Energy, 83(3), 306– 315. 115. Rahman, M. F., Jasim, S. Y., Yanful, E. K., Ndiongue, S., & Borikar, D., (2010). Advanced oxidation treatment of drinking water: Part II. Turbidity, particles and organics removal from Lake Huron water. Ozone: Science and Engineering, 32(5), 295–304. 116. Rahman, M. F., Yanful, E. K., Jasim, S. Y., Bragg, L. M., Servos, M. R., Ndiongue, S., & Borikar, D., (2010). Advanced oxidation treatment of drinking water: Part I. Occurrence and removal of pharmaceuticals and endocrine-disrupting compounds from Lake Huron water. Ozone: Science and Engineering, 32(4), 217–229. 117. Rajeswari, R., & Kanmani, S., (2009). A study on synergistic effect of photocatalytic ozonation for carbaryl degradation. Desalination, 242(1–3), 277–285. 118. Rani, K., & Dhania, G., (2014). Bioremediation and biodegradation of pesticide from contaminated soil and water-a noval approach. Int. J. Curr. Microbiol. App. Sci, 3(10), 23–33. 119. Rivera-Utrilla, J., Sánchez-Polo, M., Méndez-Díaz, J. D., Ferro-García, M. A., & Bautista-Toledo, M. I., (2008). Behavior of two different constituents of natural organic matter in the removal of sodium dodecylbenzene sulfonate by O3 and O3-based advanced oxidation processes. Journal of Colloid and Interface Science, 325(2), 432–439. 120. Rosenfeldt, E. J., Chen, P. J., Kullman, S., & Linden, K. G., (2007). Destruction of estrogenic activity in water using UV advanced oxidation. Science of the Total Environment, 377(1), 105–113. 121. Sánchez-Polo, M., Rivera-Utrilla, J., Méndez-Díaz, J. D., Canonica, S., & Von Gunten, U., (2007). Photooxidation of naphthalene sulfonic acids: Comparison between processes based on O3, O3/activated carbon

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CHAPTER 2

Introduction to Wastewater Treatment Processes

CONTENTS 2.1. Introduction ...................................................................................... 46 2.2. Sources ............................................................................................. 46 2.3. Treatment Objectives ........................................................................ 47 2.4. Wastewater Treatment Process .......................................................... 47 2.5. Description Of Process Operations ................................................... 48 2.6. Discharge Criteria ............................................................................. 55 References ............................................................................................... 56

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2.1. INTRODUCTION IThe wastewater treatment is a comparatively modern practice. Even though building sewers for removing smell of the foul water was very common in early Roman times, it was not till the 19th century that big cities started to understand the need of decreasing the number of pollutants in the used water being discharged to the environment by them (Chen, 2004; Göbel et al., 2007). In spite of huge supplies of freshwater as well as the atural capability of surface waters to get cleaned with the passage of time, still by 1850, the world populations had become so concentrated that epidemics of several life-threatening diseases became routine. These epidemics were traced to pathogenic bacteria in the contaminated water (Tchobanoglous et al., 1992; Bigda, 1995) (Figure 2.1).

Figure 2.1. The traditional treatment process of wastewater. Source: https://www.researchgate.net/figure/1-Conventional-sewage-treatment-process_fig1_301626104.

The procedure of treating wastewater in a treatment plant is somewhat the same as what happens naturally in a lake, ocean, stream, or river. The wastewater treatment plant (WWTP) speeds up this natural process of cleaning. By employing different technically sound chemical, biological, physical, and mechanical available methods, wastewater is collected and methods for its treatment are continually perfecting. Consequently, the quality of water and public health are protected in a better manner today (Muga and Mihelcic, 2008).

2.2. SOURCES We can define wastewater as the flow of used water being discharged from industries, homes, businesses, and commercial activities. With the help of a

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carefully designed network of pipes, the used water is directed to the WWTPs (Murakami et al., 2009). Later, this wastewater is additionally categorized and defined as per the source of its origin. We use the term “domestic wastewater” for referring to water which is discharged mainly from the residential sources and it is produced by various household activities such as preparation of food, cleaning, washing clothes, and personal hygiene (Kümmerer, 2001; Sörme and Lagerkvist, 2002). Commercial/ Industrial wastewater is used for referring to water being generated and discharged from theindustrial and commercial activities including food and drinks processing, printing, and production. The term institutional wastewater refers to the wastewater generated by big institutions for example schools, colleges, and hospitals (Eriksson et al., 2003; Kolodziej et al., 2004; Jackson and Sutton, 2008). Normally, around 200 to 500 liters of wastewater are generated for each person each day. The quantity of flow handled by a treatment plant depends on the season of the year as well as the time of the day (Klein et al., 1974; Focazio et al., 2008; Fernández-Nava et al., 2010).

2.3. TREATMENT OBJECTIVES The general purposes of wastewater treatment are removal of the contaminants as well as the protection and conservation of the natural resources of water. The major concern is safeguarding the public health by destroying the pathogenic organisms existing in wastewater before entering the clean water.

2.4. WASTEWATER TREATMENT PROCESS Process, by definition, implies a chain of actions or changes. Water treatment facilities include several processes that jointly achieve the desired the quality of water. A few of these processes include separation, removal, and discarding of the contaminants present in the wastewater (Ratola et al., 2012). Traditionally, four basic techniques are used for the treatment of wastewater, namely, physical technique, mechanical technique, biological technique, and chemical technique. In the physical technique of water treatment, tanks, and various other structures designed for containing and controlling the flow of wastewater are used in order to promote the removal of the pollutants (Ruppert et al., 1993; Bernard et al., 2001; Bolzonella et al., 2005).

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The mechanical water treatment method involves the use of both simple and complex machines. The water treatment using bacteria and other microbes is categorized as a biological technique of treatment. Biological technique plays a key role in removing the contaminants which cannot be efficiently achieved through other methods. Chemical treatment techniques increase the effectiveness of water treatment process operations and offer specialized treatment as a result of their addition at different stages of treatment (Tong et al., 1980; Karvelas et al., 2003; Holenda et al., 2008).

2.5. DESCRIPTION OF PROCESS OPERATIONS In order to understand the process operations associated with the treatment of wastewater, it is indispensable to become familiar with the expressions and terms commonly used in this field of work. The subsequent sections briefly discuss the specifics of wastewater treatment (Speece, 1983; Babuponnusami and Muthukumar, 2014).

2.5.1. Collection System The collection (can be called a sewer system) is a series of pipes designed especially for transporting the millions of liters of wastewater which is generated each day. We categorize sewer piping by the type of flow which is transported by it, such as storm, sanitary, and combined sewers. For example, in some cities, the wastewater collection system is constructed as a separated storm sewer and separate sanitary sewer collection system. The sanitary system is directed to the treatment plant for the wastewater (Easter et al., 2005; Servos et al., 2005). Whereas, in modern cities, the storm system is directed to the storm water management systems which is located strategically within the city. Contrarily, in older developed city areas, storm water is discharged directly to the river. Treatment plants associated with a storm or combined sewers obtain much higher flow than normal flow in the course of snow melts and heavy rainfalls. Wherever it is possible, we design collection systems as gravity flow systems. Gravity flow sewer piping is positioned with such a steep slope that can maintain a wastewater flow velocity of around 0.75 ms–1. This velocity is high enough to possess all of the suspended materials in its flow. In places where it is not possible to allow for gravity flow due to geographically restrictions, lift stations are provided which receive wastewater from low lying regions. In such cases, pumps are employed for further transportation

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of the wastewater through gravity sewers. In order to prevent occurring of inflow, infiltration, or exfiltration inside the system, it is imperative to properly maintain the complex sewer collection system. The functionality and integrity of the collection system are accomplished by routine inspection, repair, and cleaning as well as planned replacement of system components (Sonune and Ghate, 2004).

2.5.2. Preliminary Treatment Affluent reaching the treatment plants include rags, wood pieces, all sorts of plastics and other debris like this. Various inorganic materials such as eggshells and sand is also present in the flow besides the presence of various organic matter from industrial commercial, household, as well as institutional water usage. During preliminary treatment, the large debris and heavy inorganic material present in the wastewater flow is removed (Andreozzi et al., 2002; Zhang et al., 2006). One of the foremost operations of the treatment involves screening the flow of influent wastewater. For removal of such debris, mechanical screens composed of stepped plates or parallel bars are positioned at an angle in the pathway of the wastewater flow. Mechanical rakes clear out wreckage from the bars. After that, the screening products are washed and then compressed for removing surplus water and finally disposed of by burying in a landfill. By removing the debris and such waste material, in this manner, the piping and downstream equipment of the treatment plant is protected from blockage and/or damage (Jiang and Graham, 1998; Cooper et al., 2002). After the screening operations, the wastewater flow is then passed into aerated channels which are designed in a manner to slow down the velocity of the flow to 0.3 ms–1. At this point, heavy inorganic materials get separated from the wastewater and settle down. The inorganic material which is settled down is known as grit. Every so often, the settled grit is taken away from the channels, washed, and finally disposed of by burying in a landfill. Grit is very coarse and if removed earlier in the treatment process results in reducing wear on pumps and such other equipment. This inorganic material would ultimately settle in other process areas and take up a considerate amount of capacity as well as of effective treatment.

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2.5.3. Primary Treatment After removal of the large debris and grit from wastewater, it is then directed to the primary treatment operations. Taking volume into account, wastewater is greater than 99.9% water and less than 0.1% suspended solid material, dissolved, and settable solids. Even though this might appear as a very minute quantity of material, still, if it is not treated and discharged, it can cause grave negative effects in the receiving waters. The separation, as well as removal of a substantial share of this material, is achieved during primary treatment (Xu et al., 2009; Gupta et al., 2012). In the course of primary treatment, wastewater flows into and through clarifiers or big settling tanks where the velocity of the flow is reduced to an extent that it can afford hydraulic retention times of 2 to 4 hours. In this case, initial separation takes place, and 40 to 50% of the heavier settleable solids produces a primary sludge on the foot of the settling tanks; whereas, the lighter materials float to the surface of tanks (Kruithof et al., 2007). This sludge, contains a characteristic volatile solids content of around 75%, and is gathered and then discharged to other process operations for advance treatment. The floated materials, mainly consisting of oils, fats, and grease are scanned from the surface of tanks and are also directed for advance treatment operations (Sorg and Logsdon, 1978; Carballa et al., 2005) The non-settleable suspended solid materials outstanding in the wastewater flow which exit the settling tanks is known as primary effluent, and are directed to the other process operations for advance treatment. In primary effluent, approximately 60 to 70% of the total solids of the plant influent are present.

2.5.4. Anaerobic Digestion In the course of primary treatment, raw sludge and primary effluent are produced as a result of the initial separation of the materials that are present in the wastewater. The physical characteristics as well as the organic strength of each product are measured through the biochemical oxygen demand (BOD) and are unique for each product. Secondary treatment processes are used for further treatment of these products and are normally biological types of treatment (Batstone et al., 2002; Chen et al., 2008) (Figure 2.2).

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Figure 2.2. Schematic design of anaerobic digestion procedure. Source: https://www.researchgate.net/figure/Schematic-diagram-of-the-anaerobic-digestion-process_fig1_315838727.

The sludge formed after primary settling is directed to huge enclosed tanks known as digesters and does not contain any molecular oxygen. Anaerobic bacteria make use of the organic material existing in the sludge as a source of food and produce CO2 and methane gas. As a result of these reactions by anaerobic bacteria, primary sludge is stabilized and its characteristics are altered improving its dewaterability for advance processing. During the digestion phase, normally ranging from 15 to 28 days, conditions favorable to increase the biological activity of the anaerobic bacteria are sustained. The contents of the digester tank are heated for maintaining a temperature of 35–37°C, mixed to provide a contact of bacteria with organic material and to avoid the formation of a scum blanket (Gujer and Zehnder, 1983; HolmNielsen et al., 2009). Anaerobic digestion normally occurs in two stages. Contents of the 2nd stage digestion tanks are intermittently allowed to rest in order to boost settling down of the stabilized digested biosolids. After that, solids are withdrawn and later directed to solids handling operations for removal of surplus water and additional processing. The final products of digestion are stabilized biosolids and a comparatively clear liquid known as supernatant which overflows the secondary digestion tanks. Additionally, the supernatant is returned back to the plant influent for undergoing treatment once more for removing any material if it contains (Appels et al., 2008). The gas which is formed during the digestion process comprises of 65% methane and 35% CO2 by volume and is employed for fueling the facilities such as cogeneration engines and hot water boilers. Heat energy formed is

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used for heating of domestic building and digester; whereas, the generated electrical energy is utilized on-site and offsets the total amount of electricity needed to be purchased.

2.5.5. Activated Sludge (AS) The organic material existing in the primary effluent overflowing the primary settling tanks displays certain characteristics which need further treatments. Such organic material comprises dissolved as well as finely divided suspended or, else colloidal solids which are responsible for the turbid look of the primary effluent. Naturally, the dissolved organic material existing in the influent will persist in solution in the liquid flow during the course of primary treatment (Henze, 1992; Ahn et al., 2010). The mass and size of colloidal solids present are very small and do not settle throughout primary treatment. It is impossible as well as not practical to rise the detention time of the wastewater in the primary tanks in order to remove such colloidal solids. Raising detention times will result in promoting the development of septic conditions inside the settling tanks which will result in decreasing the efficiency of solid removal (Sakai et al., 1997; Lie, 2003) (Figure 2.3).

Figure 2.3. Schematic diagram of the activated sludge process. Source: https://www.researchgate.net/figure/Schematic-diagram-of-a-conventional-activated-sludge-wastewater-treatment-system_fig1_295186388.

For the treatment of a waste stream of primary effluent, a secondary biological treatment method is used and is referred to as the activated sludge

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(AS) process. This procedure successfully eliminates the dissolved organic material as well as a portion of the colloidal matter. Residual colloidal material is then converted to a biological sludge that settles quickly. AS contains particles of sludge formed by the growth of organisms in the company of free dissolved oxygen (DO). We use the term “activated” since the particles are alive and teeming with fungi, bacteria, and protozoa. These microbes clean the wastewater by making use of the organic material as a source of food or their growth and reproduction. They further stabilize soluble or colloidal solids through partial oxidation producing CO2, water, nitrate, and sulfate compounds. There are numerous modifications of the AS procedure; nevertheless, basic principles of operation are applicable to all. Wastewater which needs to be treated is comprehensively mixed with the AS to produce mixed liquor which then flows through big aeration basins allowing detention times amongst 4 to 6 hours. At this time, oxygen is dissolved into the mixed liquor through blowing of air over the flow or through mechanical surface mixers that wallow the mixed liquor into the air permitting oxygen from the air to be dissolved. After this aeration phase, the aerobic organisms existing in the mixed liquor are further directed to a tributary clarifier where they flocculate and then settle to create sludge. A ration of this settled sludge is directed back to the start of the process as return AS in order to maintain and continue the whole process. Surplus sludge is either wasted or discharged from the treatment structure back to the primary settling tanks or else is sent for separate sludge thickening operation.

2.5.6. Chemical Treatment Even though both primary and secondary treatment operations are effective in eliminating the majority of wastewater contaminants, there are few pollutants that need special forms of treatment in order to remove them. One such pollutant is Phosphorous. If it is left untreated and remains in the final effluent of a WWTP, it might cause a gravely negative impact on receiving waters (Legrini et al., 1993; Prairie et al., 1993). Phosphorous is one of the significant nutrients connected to the growth of aquatic plants. Few sources of phosphorous comprise human waste materials, corrosion control chemicals employed in industrial discharges and water supplies, and detergents composed of phosphate additives. The high amount of phosphorous in receiving waters is responsible for promoting extreme growth of algae and such aquatic plants which might disturb the natural

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ecological balance of the receiving water. A prompt weakening in the quality of water can accelerate the eutrophication course of the receiving body of the water. We can characterize methods of Phosphorus removal as either biological or chemical precipitation methods. Currently, a metal salt is used which reacts with soluble phosphorous to produce an insoluble precipitate. During the settling operations, this precipitate settles with the sludge and is therefore removed from the wastewater flow. The most commonly used metal salt is ferrous chloride which is also referred to as “pickled liquor.” The salt solution of ferrous chloride is a commercially available waste derivative of steelmaking operations. It is imperative that chemicals employed for phosphorous precipitation should be closely mixed with the wastewater in order to guarantee uniform dispersion for achieving maximum removal effectiveness (Westerhoff et al., 2005; Kurniawan et al., 2006). One more essential chemical treatment being used at WWTPs is the disinfection of the ultimate effluent. Pathogenic microbes are possibly present in all wastewaters owing to the human discharges. It is necessary to remove or kill these microbes prior to the discharge of treated wastewater into receiving waters. Chlorination for purpose of disinfection leads to the destruction of nearly all of the pathogenic microbes and hence inhibits the spread of waterborne diseases. For adding a layer of protection to the receiving waters, sodium bisulfate is added subsequent to the disinfection of the wastewater effluent before its discharge.

2.5.7. Dewatering Dewatering operation is a solid handling method. Stabilized bio-solids which result from the secondary anaerobic digestion is headed for dewatering in order to remove extra water. Dewatering increases the dryness and decreases the volume of the solids for further processing (Zhai et al., 2012). The feed solids are acclimatized with a polymer-coagulating agent after whom they are squeezed amongst woven mesh filter belts. The surplus water is removed, and wash water is employed for cleaning the woven filter belts after which it is headed back into the wastewater flow for treatment (Bien et al., 1997; He et al., 2005).

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2.6. DISCHARGE CRITERIA The principal components for which discharge criteria are set are (Kilgour et al., 2005; Kim et al., 2008): • • • • • • • • •

Hydraulic capacity rating; TP total phosphorous; EC E. coliform; TKN total Kjeldahl nitrogen; TSS total suspended solids (SS); TOD total oxygen demand; CR chlorine residual; BOD – biochemical oxygen demand; NH3 ammonia nitrogen.

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22. Gujer, W., & Zehnder, A. J., (1983). Conversion processes in anaerobic digestion. Water Science and Technology, 15(8/9), 127–167. 23. Gupta, V. K., Ali, I., Saleh, T. A., Nayak, A., & Agarwal, S., (2012). Chemical treatment technologies for waste-water recycling—an overview. Rsc. Advances, 2(16), 6380–6388. 24. He, D. Q., Wang, L. F., Jiang, H., & Yu, H. Q., (2015). A Fentonlike process for the enhanced activated sludge dewatering. Chemical Engineering Journal, 272, 128–134. 25. Henze, M., (1992). Characterization of wastewater for modeling of activated sludge processes. Water Science and Technology, 25(6), 1–15. 26. Holenda, B., Domokos, E., Redey, A., & Fazakas, J., (2008). Dissolved oxygen control of the activated sludge wastewater treatment process using model predictive control. Computers and Chemical Engineering, 32(6), 1270–1278. 27. Holm-Nielsen, J. B., Al Seadi, T., & Oleskowicz-Popiel, P., (2009). The future of anaerobic digestion and biogas utilization. Bioresource Technology, 100(22), 5478–5484. 28. Jackson, J., & Sutton, R., (2008). Sources of endocrine-disrupting chemicals in urban wastewater, Oakland, CA. Science of the total Environment, 405(1–3), 153–160. 29. Jiang, J. Q., & Graham, N. J., (1998). Preliminary evaluation of the performance of new pre-polymerized inorganic coagulants for lowland surface water treatment. Water Science and Technology, 37(2), 121– 128. 30. Karvelas, M., Katsoyiannis, A., & Samara, C., (2003). Occurrence and fate of heavy metals in the wastewater treatment process. Chemosphere, 53(10), 1201–1210. 31. Kilgour, B. W., Munkittrick, K. R., Portt, C. B., Hedley, K., Culp, J., Dixit, S., & Pastershank, G., (2005). Biological criteria for municipal wastewater effluent monitoring programs. Water Quality Research Journal, 40(3), 374–387. 32. Kim, E., Jun, Y. R., Jo, H. J., Shim, S. B., & Jung, J., (2008). Toxicity identification in metal plating effluent: Implications in establishing effluent discharge limits using bioassays in Korea. Marine Pollution Bulletin, 57(6–12), 637–644.

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33. Klein, L. A., Lang, M., Nash, N., & Kirschner, S. L., (1974). Sources of metals in New York City wastewater. Journal (Water Pollution Control Federation), 2653–2662. 34. Kolodziej, E. P., Harter, T., & Sedlak, D. L., (2004). Dairy wastewater, aquaculture, and spawning fish as sources of steroid hormones in the aquatic environment. Environmental Science and Technology, 38(23), 6377–6384. 35. Kruithof, J. C., Kamp, P. C., & Martijn, B. J., (2007). UV/H2O2 treatment: A practical solution for organic contaminant control and primary disinfection. Ozone: Science and Engineering, 29(4), 273– 280. 36. Kümmerer, K., (2001). Drugs in the environment: Emission of drugs, diagnostic aids and disinfectants into wastewater by hospitals in relation to other sources–a review. Chemosphere, 45(6/7), 957–969. 37. Kurniawan, T. A., Chan, G. Y., Lo, W. H., & Babel, S., (2006). Physico– chemical treatment techniques for wastewater laden with heavy metals. Chemical Engineering Journal, 118(1/2), 83–98. 38. Legrini, O., Oliveros, E., & Braun, A. M., (1993). Photochemical processes for water treatment. Chemical Reviews, 93(2), 671–698. 39. Leita, L., & De Nobili, M., (1991). Water-soluble fractions of heavy metals during composting of municipal solid waste. Journal of Environmental Quality, 20(1), 73–78. 40. Liu, Y., (2003). Chemically reduced excess sludge production in the activated sludge process. Chemosphere, 50(1), 1–7. 41. Muga, H. E., & Mihelcic, J. R., (2008). Sustainability of wastewater treatment technologies. Journal of Environmental Management, 88(3), 437–447. 42. Murakami, M., Shinohara, H., & Takada, H., (2009). Evaluation of wastewater and street runoff as sources of perfluorinated surfactants (PFSs). Chemosphere, 74(4), 487–493. 43. Pharand, D., (1971). Oil pollution control in the Canadian arctic. Tex. Int’l LJ, 7, 45. 44. Prairie, M. R., Evans, L. R., Stange, B. M., & Martinez, S. L., (1993). An investigation of titanium dioxide photocatalysis for the treatment of water contaminated with metals and organic chemicals. Environmental Science and Technology, 27(9), 1776–1782.

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45. Ratola, N., Cincinelli, A., Alves, A., & Katsoyiannis, A., (2012). Occurrence of organic microcontaminants in the wastewater treatment process. A mini review. Journal of Hazardous Materials, 239, 1–18. 46. Ruppert, G., Bauer, R., & Heisler, G., (1993). The photo-Fenton reaction—an effective photochemical wastewater treatment process. Journal of Photochemistry and Photobiology A: Chemistry, 73(1), 75–78. 47. Said-Pullicino, D., Erriquens, F. G., & Gigliotti, G., (2007). Changes in the chemical characteristics of water-extractable organic matter during composting and their influence on compost stability and maturity. Bioresource Technology, 98(9), 1822–1831. 48. Sakai, Y., Fukase, T., Yasui, H., & Shibata, M., (1997). An activated sludge process without excess sludge production. Water Science and Technology, 36(11), 163–170. 49. Servos, M. R., Bennie, D. T., Burnison, B. K., Jurkovic, A., McInnis, R., Neheli, T., & Ternes, T. A., (2005). Distribution of estrogens, 17β-estradiol and estrone, in Canadian municipal wastewater treatment plants. Science of the Total Environment, 336(1–3), 155–170. 50. Snucins, E., Gunn, J., Keller, B., Dixit, S., Hindar, A., & Henriksen, A., (2001). Effects of regional reductions in sulphur deposition on the chemical and biological recovery of lakes within Killarney Park, Ontario, Canada. Environmental Monitoring and Assessment, 67(1/2), 179–194. 51. Sonune, A., & Ghate, R., (2004). Developments in wastewater treatment methods. Desalination, 167, 55–63. 52. Sorg, T. J., & Logsdon, G. S., (1978). Treatment technology to meet the interim primary drinking water regulations for inorganics: Part 2. Journal-American Water Works Association, 70(7), 379–393. 53. Sörme, L., & Lagerkvist, R., (2002). Sources of heavy metals in urban wastewater in Stockholm. Science of the Total Environment, 298(1–3), 131–145. 54. Speece, R. E., (1983). Anaerobic biotechnology for industrial wastewater treatment. Environmental Science and Technology, 17(9), 416A–427A. 55. Tchobanoglous, G., Burton, F. L., & Stensel, H. D., (1991). Wastewater engineering. Management, 7, 1–4.

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56. Tong, R. M., Beck, M. B., & Latten, A., (1980). Fuzzy control of the activated sludge wastewater treatment process. Automatica, 16(6), 695–701. 57. Westerhoff, P., Yoon, Y., Snyder, S., & Wert, E., (2005). Fate of endocrine-disruptor, pharmaceutical, and personal care product chemicals during simulated drinking water treatment processes. Environmental Science and Technology, 39(17), 6649–6663. 58. Xu, G. R., Yan, Z. C., Wang, Y. C., & Wang, N., (2009). Recycle of Alum recovered from water treatment sludge in chemically enhanced primary treatment. Journal of Hazardous Materials, 161(2/3), 663– 669. 59. Zhai, L. F., Sun, M., Song, W., & Wang, G., (2012). An integrated approach to optimize the conditioning chemicals for enhanced sludge conditioning in a pilot-scale sludge dewatering process. Bioresource Technology, 121, 161–168. 60. Zhang, J., Zhang, F., Luo, Y., & Yang, H., (2006). A preliminary study on cactus as coagulant in water treatment. Process Biochemistry, 41(3), 730–733.

CHAPTER 3

Coagulation and Flocculation

CONTENTS 3.1. Introduction ...................................................................................... 64 3.2. Colloidal Suspensions ....................................................................... 64 3.3. Coagulation ...................................................................................... 67 3.4. Flocculation...................................................................................... 74 3.5. Conventional Plants .......................................................................... 76 3.6. An Amalgamation Of Flocculation, Coagulation, And Sedimentation......................................................................... 77 3.7. Operation Of The Flocculation And Coagulation Process .................. 78 3.8. Cataloging Of Settling Behavior ........................................................ 79 3.9. Ideal Settling ..................................................................................... 81 References ............................................................................................... 82

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3.1. INTRODUCTION Surface and groundwater comprise of both suspended and dissolved particles. Coagulation and flocculation are utilized to isolate the suspended solid percentage from the water. Suspended particles differ in source, particle size, shape, charge, and density. The correct application of coagulation and flocculation is dependent upon these factors. SS portion in water has a negative charge and as they have a similar kind of surface charge, they generally repel one another when they are brought closer (Harif et al., 2012). Thus, SS will normally remain in suspension and won’t clump together and settle down out of the water, until proper coagulation and flocculation are utilized (Aziz et al., 2007; Guibal and Roussy, 2007). Coagulation and flocculation take place in successive steps, permitting particle strike and growth of the flocs. This is then trailed by sedimentation. If coagulation is partial, flocculation will be ineffective, and if the flocculation step is partial, sedimentation will be ineffective (Hassan et al., 2009; Verma et al., 2010). In most of the wastewater treatment workings and wetland methods, sedimentation is generally the chief process utilized in the primary treatment. Primary sedimentation eliminates between 50% to 70% of the SS, which comprise of between 25% and 40% of the BOD from wastewater, including the urban runoff. Sedimentation is usually defined as the elimination of solid particles from the suspension by settling under the gravity. Clarification is a similar term, which specifically refers to the function of a sedimentation tank in eliminating suspended matters from the water in order to give the clarified effluent (Shammas, 2005). Thickening takes place in sedimentation tanks and is the procedure whereby settled contaminations are concentrated and condensed on the floor of tanks and in sludge hoppers. The concentrated contaminations eliminated from the tank bottom are known as sludge and the contaminations floating to the tank surface are called scum.

3.2. COLLOIDAL SUSPENSIONS In wastewater and water treatment, there are basically two kinds of colloidal systems, which generally possess water as the disperse point: colloidal suspensions and emulsions. Features of the colloidal suspension are given below:

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1.

Colloids cannot be eliminated from the suspension by normal filtration. Though, they can be eliminated by ultrafiltration (UF) or dialysis through the specialized membranes. 2. Colloidal particles are not visible under the normal microscope. Though, they can be perceived as the fragments of light with the microscope when the beam of light is moved through the suspension. This is triggered by the Tyndall effect, which is defined as a scattering of the light by colloidal particles. 3. Brownian motion averts the accommodation of particles under gravity. Some of the colloids can be eliminated by centrifugation. 4. There exists a natural trend for the colloids to coagulate and then precipitate. Sometimes, this trend is contradicted either by the common repulsion of particles or by strong attraction amongst the medium and particles. If these outcomes are strong and coagulation doesn’t take place, the suspension is stable. When the chief factor triggering stability of the colloidal suspensions is the attraction amongst water and particles, the colloids are considered as hydrophilic. When there is not great attraction amongst water and particles, and stability is dependent on mutual repulsion, then the colloids are considered to be hydrophobic (Gast et al., 1983; Löwen, 1994). Hydrates of Fe (iron) and Al (aluminum) produce hydrophobic colloids in the water; stability takes place through common electrostatic repulsion. Proteins, fats, or starches produce hydrophobic colloids in the water. Stability can be accomplished through the attraction amongst water and particles. Stable hydrophilic colloids are quite difficult to coagulate (Gast and Zukoski, 1989; Park et al., 2009). Colloids possess large surface areas; for example, if the 10 mmcube was fragmented into cubical particles having dimensions of around 102 mm, the surface area would be escalated by the factor of 106 to 6 hundred m2. Surface effects are thus of importance; of these, the two given below are important: 1.

The propensity for the substances to concentrate on the surfaces; and 2. The propensity for surfaces of the substances in touch with water to obtain electrical charges, giving the electro-kinetic properties. Electrical charge at the surface outcome from the colloidal material’s attraction, from few ions in water, or from ionization of some atoms which vacate the colloid. The surface charge attracts the ions carrying the charge

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with opposite sign and therefore creates the cloud of counter-ions, in which the concentration reduces with the increase in distance from the particle (Vermant and Solomon, 2005; Piazza and Parola, 2008). As an instance, clay particles in the water are charged due to the isomorphous substitution, where specific cations of crystal lattice creating the mineral might have been substituted by cations of a similar size but having a lower charge. For instance, Si4+ might be substituted by Al3+ and Al3+ by the Mg2+. In both of the cases, the lattice is left with the residual negative charge, which needs to be balanced by the suitable number of recompensing cations in dry clay. These might be large ions like Ca2+ or Na2+, which can’t be adjusted in the lattice structure; therefore these ions are movable and might diffuse into the solution when the dry clay is dipped in water, occasioning in negatively charged particles (Moeller et al., 2007; Placci et al., 2010). For these and some other reasons, the colloidal particles in water are normally charged. The bulk of particles that came across in the natural waters are generally negatively charged. The suspension of the colloidal particles has no net charge, as the surface charge of particles is precisely balanced by an equal number of the oppositely charged counterions in the solution. Moreover, the circulation of these counter-ions is not random, as by electrostatic attraction these counter-ions tend to gather around charged particles. The mutual system of surface charge of the particle and the linked counter-ions in the solution is called the electrical double layer (Bergenholtz and Fuchs, 1999). In addition to forces associated with electrical charges, the colloidal particles, when brought close together, are subjected to the van der Waals forces. These initiate in the behavior of the electrons, which are a chunk of the molecular or atomic system. The forces of the attraction become important only at short distances (Monovoukas and Gast, 1989). Source of the energy might either be from the Brownian motion or from the relative movement in the water. If the energy of the collision is insufficient, the residual repulsion will compel the particles apart, creating a stable colloidal suspension. When the material is actually dissolved in water, the material is dispersed as molecules or ions. The particle sizes of the dissolved material are normally in the range between 2 × 10–4 mm and 10–3 mm. The particles can’t settle and can’t be eliminated by ordinary filtration. True solutions and true colloidal suspensions are readily differentiated, but there exists no piercing line of demarcation. Colloidal particles are described as the

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particles in the range between 103 mm and 1 mm. In, for example, urban water runoff, the colloidal solids normally comprise of fine silts, bacteria, clay, viruses, and color-causing particles.

3.3. COAGULATION Coagulants with opposite charges than those of the SS are added to water in order to neutralize the negative charge on non-settable solids. Once the negative charge is neutralized, the suspended particles are proficient in sticking together. These marginally larger particles are known as micro flocs and are not visible to the bare eye. Water adjacent to the newly created micro flocs must be clear. If not, the coagulation and some particle charge have not been neutralized. More coagulants might essential to be added (Hulka et al., 1996; Chrysohoou et al., 2004). The high-energy, rapidly mix to appropriately scatter coagulant and promote particle impact is required to accomplish good coagulation (Vassalli and McCluskey, 1964, 1965). Over-mixing does not affect thecoagulation, but inadequate mixing will leave behind this step incomplete. The contact time in the rapid-mix chamber is generally one to three minutes (Figures 3.1 and 3.2).

Figure 3.1. In-line mixer on left and baffled chamber on right for coagulation. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

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Figure 3.2. Two alternatives of the mechanical mixing chamber utilized for coagulation. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

3.3.1. Coagulant Selection The selection of coagulant chemicals is dependent upon kind of the SS to be eliminated, raw water conditions, cost of chemical, and facility design. The final choice of coagulants needs to be done with jar testing and plant scale evaluation. Consideration should be given to the required effluent quality, cost, the effect upon the downstream treatment procedure performance, cost, and method of sludge disposal and handling (Amuda and Amoo, 2007; Ahmad et al., 2011).

3.3.1.1. Inorganic Coagulants Inorganic coagulants like iron salts and aluminum are commonly used in water treatment industry. When these chemicals are added to the water, the highly charged ions act to neutralize the suspended particles. The inorganic hydroxides which are formed yield shorter polymer chains which improve micro flocs formation. Inorganic coagulants normally offer the lowest cost per pound, are broadly available, and, when appropriately applied, are efficient in eliminating most SS. They are also proficient in eliminating the portion of organic precursors which might combine with chlorine (Cl) to create disinfection by-products. Inorganic coagulants generally yield large volumes of the floc which can also deceive bacteria as they settle (Xu and Zhu, 2004; Aboulhassan et al., 2006).

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Inorganic coagulants might vary the pH of water as they consume alkalinity. When applied in the process of lime soda ash softening, iron salts and alum produce demand for soda ash and lime. They also need corrosionresistant storing and feed equipment. Ferric sulfate, ferric chloride, and alum lower the pH and alkalinity as described in the following reactions: 1. Ferric Sulfate: Fe2(SO4)3

+ 3Ca(HCO3)2 ------------> 2Fe(OH)3 + 3CaSO4 + 6CO2

Ferric + Calcium Ferric + Calcium + Carbon

Sulfate Bicarbonate Hydroxide Sulfate Dioxide 2. Ferric Chloride: 2FeCl3 + 3Ca(HCO3)2 ------------> 2Fe(OH)3 + 3CaCl2 + 6CO2 Ferric + Calcium Ferric + Calcium + Carbon

Chloride Bicarbonate Hydroxide Chloride Dioxide 3. Alum: A12(SO4)3

+ 3Ca(HCO3)2 ------------> 2Al(OH)3 + 3CaSO4 + 6CO2

Aluminum + Calcium Aluminum + Calcium + Carbon Sulfate Bicarbonate Hydroxide Sulfate Dioxide

3.3.1.2. Polymers Polymers are becoming more extensively used in water and wastewater treatment. These materials can be utilized as the coagulant aids along with regular inorganic coagulants. Anionic polymers are frequently utilized with metal coagulants. Low to medium weight cationic polymers might be utilized alone, or in amalgamation with alum ferric coagulants in order to attract the suspended solid and neutralize the surface charge. Manufacturers can yield a wide variety of polymers that meet the variety of source water conditions by monitoring the type and amount of charge and molecular weight (MW) of polymers. Polymers are efficient over the wider pH range as compared to the inorganic coagulants. They can be made practical at lower doses, and do not consume alkalinity. They yield smaller volumes of concentrated, quickly settling floc. Floc produced from utilization of the properly chosen polymer will usually be more resilient to shear, occasioning in less carryover and the cleaner effluent.

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Polymers are usually several times more costly in price than the inorganic coagulants. The choice of proper polymer needs substantial jar testing under replicated plant conditions, tracked by the pilot or the plant-scale trials. All polymers should also be sanctioned for potable water utilization by regulatory agencies.

3.3.2. Coagulation Processes It is essential to decrease or remove the energy necessary to destabilize the hydrophobic colloidal suspension in order to allow coagulation to take place. The van der Waals forces cannot be controlled, but the electrical forces can. The principle procedures are as follows: Neutralization or reduction of charges on the colloids: 1.

Increase of density of the counter-ions field, and therefore a decrease of the range of repulsive effect; 2. Permanent contact between particles through the molecular bridges. Neutralizing charges on the colloids might be achieved by the addition of multivalent ions or the colloids that have the opposite charge. These are often added as the chemical coagulant. The coagulating power of the chemical rises quickly with its valence. For instance, Al3+ and SO42 ions are generally several hundred times more efficient than Na+ and Clions. In water, the cloud of the counter-ions is extensively dispersed. With an upsurge in the ionic strength of the solution, the cloud of counter-ions becomes more concentrated close to the colloidal particles, countering the charges, and the field of impact of the colloidal charges becomes more restricted. Thus, an upsurge in the ionic strength of the colloidal suspension will incline to trigger destabilization and subsequently coagulation (Baskan and Pala, 2010; Guo et al., 2010).

3.3.3. Coagulation Chemicals Chemicals utilized for the chemical coagulation of contaminations in water treatment must be inexpensive and not leave behind any poisonous or other unwanted residues in the water. Coagulation in the water treatment takes place mainly by two mechanisms: 1.

Adsorption of soluble hydrolysis types on the colloid following and destabilization; and

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2.

Sweep coagulation everywhere the colloid is entangled within the precipitation product. The particular mechanism taking place depends on both the alkalinity and turbidity of water being treated. The reactions in destabilization and adsorption are very fast and take place within 1 and 0.1 s. Sweep coagulation is significantly slower and takes place in the range between 3 s and 17 s. The differences between the two kinds of coagulation in terms of quick mixing aren’t normally outlined in the literature. It is vital that the coagulant is dispersed in the water as quickly as possible so that the products of hydrolysis which develop within 1 and 0.01 s will be the reason for the destabilization of the colloids (Gawryl and Hoyer, 1982; Choo et al., 2007). The aluminum salt used commonly is aluminum sulfate. It is available in powdered, solid, granular, or liquid form. The chemical formula of aluminum sulfate is Al2 (SO4)3.nH2O, where n is dependent on the technique used by the manufacturers but is normally in the range amid 12 and 16. When hydrolyzed, aluminum sulfate yields sulfuric acid along with the hydrate. For instance, when creating the basic hydrate Al(OH)3, the equation below describes the hydrolysis reaction. Al2(SO4)3 + 6H2O→Al(OH)3 + 3H2SO4

(1)

Consequently, it is considered as an acidic salt and the water should contain enough alkalinity to react with acid as it produces to sustain the pH within the anticipated range for better coagulation and flocculation. It can be proved that 1 mg/L alum is utilized for 0.5 mg/L alkalinity, which outcomes in the formation of 0.44 mg/L CO2. After coagulation, the requirements of taste dictate the remaining alkalinity of 30 mg/L. Sodium aluminate (NaAlO2) is an alkaline salt formed by treating the aluminum oxide Al(OH)3 with caustic soda (NaOH). The equation below gives the hydrolysis reaction. NaAlO2 + 2H2O→ Al(OH)3 + NaOH

(2)

It can be utilized either in combination with aluminum sulfate or on its own for the waters, which do not have adequate natural alkalinity. However, consequent coagulation might not be as worthy as when aluminum sulfate is utilized in conjunction with alkali as the divalent sulfate ions presented with the aluminum sulfate have an auspicious influence on coagulation (Yun et al., 1993; Lee et al., 2009).

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Ferric ions are hydrolyzed and then precipitated as the ferric hydroxide at pH greater than 4.5. Poor coagulation takes place in the pH range amid 7.0 and 8.5. The ferric ion hydrolyzes to produce hydrates and acid. Enough alkalinity should be present to merge with acid and sustain an appropriate pH for better coagulation. Unlike Al(OH)3, the ferric hydroxide does not redissolve in the alkaline solutions and there is no specific upper pH limit for the ferric coagulation. Additionally, ferric hydroxide floc is normally heavier and therefore, faster settling as compared to the aluminum hydroxide flocs (Yasar and Guzeler, 2011; Chen et al., 2012). Keeping in mind the coagulation of the colored acid waters, the existence of divalent sulfate ion supports coagulation to a greater degree than the monovalent chloride ion. The ferric salt used commonly in water treatment is FeCl3 (ferric chloride), which is extremely corrosive in the existence of water. Therefore, it needs transportation in glass containers or rubber-lined tanks. Ferric sulfate is usually accessible as an anhydrous material, thus making the transportation easier. In some circumstances, ferrous salts are utilized. To be efficient, the ferrous ion must be oxidized to ferric form when in the solution (Srichaikul et al., 1975). At pH values greater than 8.5, DO (dissolved oxygen) in the water can cause oxidation. Conversely, at the lower pH values, chlorine (Cl) can be utilized as an oxidizing agent. Ferrous sulfate, also called copperas, is normally available as the granular material. Because it needs a pH of around 8.5 in order to be oxidized by DO, it can’t be utilized on its own in the natural waters. When utilized in combination with lime, it is valuable in coagulating the precipitate acquired in lime softening of the water, and the elimination of excessive manganese and iron from waters (Valencia et al., 2010). Chlorinated copperas can be utilized for water with a relatively low pH. It is made by mixing the solutions of chlorine and copperas (Eq. (3.3)): 6FeSO4 + 3Cl2 →2Fe2(SO4)3 + 2FeCl3

(3)

Furthermore, an excess of Cl can be utilized to disinfect water. Copperas can be readily utilized than the ferric salts since it is not so corrosive and can also be handled by simple chemical feeding equipment. Lime is not normally utilized as a coagulant in the water treatment, even though it is frequently applied in the wastewater treatment. Lime is utilized in water treatment as the pH control step for adding alkalinity and in the softening waters (Ognibene et al., 1971; Bektas et al., 2004).

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In some types of waters, the coagulation is very poor, even though with an optimal dose of coagulant. In these circumstances, the coagulant aid is utilized. The two most usually coagulant aids are polyelectrolytes and clay. Clay is beneficial in some of the waters which are lacking in the negatively charged flocs. The clay colloids offer nuclei for the creation and growth of the flocs, along with the medium to add weight to the particles. In some circumstances, adsorptive qualities of clay might be helpful in removing odor, color, and taste (Maynard et al., 1977; Tejada et al., 2007). Polyelectrolytes are long-chain macromolecules which have ionizable groups or electrical charges. Cationic polyelectrolytes are the polymers which, when dissolved, yielding positively charged ions. They are extensively utilized because the colloidal and SS generally found in water are usually negatively charged (Barron and O’Hern, 1966; Rodder et al., 2006). Cationic polymers can also be applied as the main coagulant or as a support to conventional coagulants. There are various advantages of utilizing this coagulant aid: the amount of the coagulant can be decreased, less sensitivity to the pH, flocs settle better, and flocculation of the living organisms is improved (Ballard and MacKay, 2005; Choi et al., 2006). Anionic polyelectrolytes are the polymers which dissolve to create negatively charged ions and are utilized to eliminate positively charged solids. Anionic polyelectrolytes are utilized mainly as the coagulant aids with iron or aluminum coagulants. The anionic chemicals upsurge flow size, progress settling, and normally produce stronger flocs. They aren’t considerably affected by alkalinity, hardness, pH, or turbidity (Adgar et al., 2005; Isobe et al., 2011). Non-ionic polyelectrolytes are the polymers that have a neutral or balanced charge, but upon dissolving, they release both negatively and positively charged ions. Non-ionic polyelectrolytes might be utilized as coagulant aids or as coagulants. Although they should be added in quite larger amounts than other kinds, they are cheap (Butt et al., 1941; Golob et al., 2005).

3.3.4. Rapid Mixing The first junction of coagulant with the water is the crucial period in the whole process of coagulation. The coagulation reaction takes place rapidly, so it is important that the colloidal particles and coagulant come instantaneously into contact with each other. After adding the coagulant, the water must be agitated aggressively for a few seconds to boost the maximum number of

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collisions feasible with the suspended particles (Bubley and Dyer, 1997; Wong et al., 2004). There are three main types of mixers utilized in the rapid mixing stage: 1. 2. 3.

Mechanical devices; Pumps and conduits; and Baffled chambers. Impeller, propeller, and turbine-kind mechanical mixers are extensively applied due to their positive control characteristics (Dolgopya, 1998; Jerrum and Sinclair, 1988). Detention times in the chambers are quite short, normally less than 1 minute. Mechanical mixers can be mounted straight in the pipeline. Unlike pump and conduit type mixers, in-line mixers can also be adjusted to deliver the precise degree of mixing. As an in-line mixer needs no special tank, so it is a cheap device that is increasing in popularity (Gibson, 1969; Groves et al., 1997).

3.4. FLOCCULATION Flocculation, the gentle mixing stage, upsurges the size of the particle from submicroscopic micro-floc to the visible suspended particles. Micro floc particles strike to yield larger, visible flocs known as pin flocs. Floc size stays to build with the additional interaction and collisions with the added inorganic or organic polymers. Macroflocs are made and polymers with high molecular weight, known as coagulant aids, might be added to support bind, bridge, and strengthen the floc, increase settling rate and add weight. Once floc reaches its optimal strength and size, water is then ready for sedimentation (Aguilar et al., 2002, 2005). Design junction times for the flocculation range from about 15 or 20 minutes to one hour or more, and flocculation needs careful observation of the amount of mixing energy and mixing velocity. To avoid floc from shearing or tearing apart, the mixing energy and velocity are generally tapered off as the floc size increases. Once the flocs are torn apart, it is difficult to get them to restructure to their optimum strength and size (Wang et al., 2007; Moghaddam et al., 2010) (Figure 3.3).

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Figure 3.3. Three kinds of flocculator: (a) propeller, (b) turbine, (c) walking beam. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

When the colloid becomes destabilized, the progress of flocs is encouraged and improved by gentle mixing. This flocculation process takes place primarily in two stages called peri-kinetic and ortho-kinetic flocculation (Tambo and Watanabe, 1979; Jekel, 1986). In the peri-kinetic stage, the particles strike to each other as they move casually under the impact of Brownian motion. The time reserved for the particles to become so large that these particles are no longer considerably disturbed by the Brownian motion is dependent on the frequency of the collisions. The chance for collisions is proportional to the concentration of the particles, thus peri-kinetic flocculation is quick in concentrated suspensions. In the ortho-kinetic stage, the particles are stirred together by the mild motion of water. The flocculation rate is dependent on the particle size, concentration, and nature, and velocity shear gradient of water. Numerous mathematical models of this method have been made, even though they are all hard to apply in practice. The flocculation rate is proportional to: 1. Velocity shear gradient; 2. The volume of particle zone of impact; 3. The square of the numerical concentration of the particles. Although the preliminary rate of floc creation is proportional to the velocity shear gradient, the high-velocity gradients can also cause very large flocs to be torn apart as an outcome of surface shear stress erosion, internal tension, or both. The agitation essential to encourage flocculation is produced in several ways:

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1. 2. 3. 4.

Hydraulic agitation: baffled channels and jets; Air injection: rising bubbles; Mechanical agitation: turbine, and propeller and paddle and reel; Solids contact: sludge blanket.

3.5. CONVENTIONAL PLANTS Conventional plants isolate the coagulation stage from the flocculation stage. These stages are tailed by sedimentation and then filtration. Plants designed for the direct filtration course water directly from flocculation to the filtration. These systems usually have a higher raw-water quality. Conventional plants might have changeable mixing speeds in the slow-mix and rapid-mix equipment. Several feed points for polymers, flocculants, coagulants, and the other chemicals can also be offered and there is usually sufficient space to distinct the feed points for mismatched chemicals. Conventional plants generally have conservative rise-rates and retention times. This normally results in necessities for the large process basins and the large piece of land for the plant site. On-site pilot plant assessment, by the capable engineer accustomed to the quality of water, is suggested prior to design (Figures 3.4 and 3.5).

Figure 3.4. Conventional water treatment method involving solid elimination process. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

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Figure 3.5. Alternate water treatment method using solid elimination technique. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

Retention time is the time which water spends in the process. It is estimated by dividing the volume of liquid of the basin by plant flow rate. Actual detention time in the basin will be quite less than the one calculated due to the short-circuiting and dead areas, which could be because of the inadequate baffling. Retention time = Rise rate is determined by dividing the flow (gallons per minute) by net water surface area of basin (square feet). Rise Rate =

3.6. AN AMALGAMATION OF FLOCCULATION, COAGULATION, AND SEDIMENTATION Some designs include flocculation, coagulation, and sedimentation as a single unit. Most up-flow solids interaction units utilize recirculation of formerly formed floes to improve floc creation and maximize utilization of treatment chemicals. Sludge blanket units normally force newly creating flocs to move upward through the suspended bed of the floc (Pieterse and Cloot, 1997; Winterwerp, 1998). In both styles of the units, the cross-sectional surface of basin upsurges from the bottom to top, triggering water flow to slow down as it upsurges, and permitting floc to settle out. Combination units usually use shorter detention time and higher rise rates than conventional treatment. Many manufacturers

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market registered units established on these design ideas. These units are compact and need less land for the plant site location (Roussy et al., 2005; Vandamme et al., 2011) (Figure 3.6).

Figure 3.6. Schematic diagram of a joint procedure involving coagulation, flocculation, and sedimentation. Source: https://www.mrwa.com/WaterWorksMnl/Chapter%2012%20Coagulation.pdf.

3.7. OPERATION OF THE FLOCCULATION AND COAGULATION PROCESS There are three vital steps in operating the flocculation and coagulation process: 1. Choosing the chemicals; 2. Applying the chemicals; and 3. Observing the process efficiency. The choice of chemical coagulants and the coagulant aids is the continuing effort of test and evaluation, generally utilizing the jar test. When choosing chemicals, the characteristics of raw water given below to be treated must be measured: 1. 2. 3.

Temperature; Alkalinity; pH;

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4. Color; 5. Turbidity. The jar test, even though the most extensively utilized coagulation control check, is subjective and is dependent mainly on the human eye for assessment and interpretation. The operator must measure pH, filterability, zeta potential, and turbidity to obtain further information regarding the flocculation and coagulation process (Drikas et al., 2001; Rivas et al., 2004). However, jar test conclusions assist to find out the kinds of chemicals or chemical and their corresponding optimal amount to be used. Most jar test outcomes are given in mg/L. This unit should be transformed to the corresponding full-scale quantity in kg/day or m3/day. The operator of the treatment plant consequently applies the chemical to water by setting the automatic or manual metering of the chemical feed system to the anticipated dosage rate (Haydar and Aziz, 2009). Even though jar test gives a good hint of the anticipated results, the full-scale plant operation might not always resemble these results. The actual performance of the plant should be observed for sufficient flash mixing, adequate flocculation time, gentle flocculation, and settled and purified water quality. The zeta potential experiment is increasingly being utilized in water treatment studies in order to find out the best dosage and pH for the cationic coagulants like Al3+ and Fe3+ as well as cationic polymers. The control procedure needs observing of the zeta potential of the coagulating water and varying the chemical amount when zeta potential differs outside the range known to yield the lowest turbidity. The range, although adjustable from one plant to another, is between 6 mV and 10 mV.

3.8. CATALOGING OF SETTLING BEHAVIOR The hydrodynamic issue of settling has been examined by numerous researchers, and most commonly recognized classification pattern is as follows: • Class I: Unobstructed settling of the discrete particles. • Class II: Settling of a dilute suspension of the flocculent particles. • Class III: Obstructed and zone settling. • Class IV: Compression settling. Regarding Class I settling, a single particle in the liquid of the lower density must be considered for the cause of this explanation. Particles

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will speed up until the settling velocity has reached the point where the gravitational force (G) is balanced by frictional drag force. Settling velocity can be found using either Newtons’ law (Eq 4), or Stoke’s law (Eq. 3) dependent on whether the flow is turbulent or laminar, respectively.

where: Us → Settling velocity;

Ss → Specific gravity of solid matter;

g → Gravitational constant;

d → Diameter of the solid matter; V → Kinematic viscosity. In several practical circumstances, even in the relatively dilute suspension, the particles merge to create particle aggregates with improved settling velocities. The degree of such flocculation is the function of numerous variables, including concentration and suspension type, the dominant velocity gradient, and time. In wastewater engineering, maximum sewage, including the urban runoff, displays Class II settling in the primary tanks (Krumbein, 1942; Chien, 1994; Tang et al., 2002). Regarding Class III settling, with an increase in the concentration of the particles in the suspension, a point is obtained where the particles are very near to one another that they do not settle distinctly, but the velocity fields of fluid displaced by the nearby particles overlap. This gives growth to the net upward flow of the liquid displaced by the settling particles. This outcome in the reduction of settling velocity and is known as “hindered settling.” For the traditional wastewater plants, Class III settling is extensively displayed in sludge blanket clarifier, where the concentration of particles is so high that the entire suspension inclines to settle as the “blanket.” Moreover, most of the urban runoff shows Class II or Class IV in the densely planted built treatment wetland (Buscall et al., 1982; Camenen, 2007). At the bottom of the settling column, as the settling continues, the compressed layer of particles starts to form. The particles in the particular region seemingly form the configuration having close physical contact amongst the particles. Therefore, the obstructed settling region comprises of a gradient in the solids concentration upsurging from that discovered at the boundary of the

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settling area to that recognized in the compression zone. This area of Class IV settling is chiefly significant when seeing the strategy of thickeners for the activated sludge (AS) (Davies, 1968; Burns and Rosa, 1980).

3.9. IDEAL SETTLING The behavior of perfect sedimentation tank, working on a continuous basis with the distinct suspension of particles, can be examined as follows: 1. 2. 3.

Suppose quiescent circumstances in the settling region. Suppose uniform flow through the settling region. Suppose uniform solid concentration as the flow arrives the settling region. 4. Suppose that solids arriving at the sludge region aren’t suspended again. Equation (5) gives the way to calculate the overflow rate, v, it must be considered as the design settling velocity. v= Q/A

(5)

where: V → Overflow rate (m/s); Q → Discharge (m3/s); A → Tank surface area (m2).

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46. Moeller, K., Kobler, J., & Bein, T., (2007). Colloidal suspensions of nanometer-sized mesoporous silica. Advanced Functional Materials, 17(4), 605–612. 47. Moghaddam, S. S., Moghaddam, M. A., & Arami, M., (2010). Coagulation/flocculation process for dye removal using sludge from water treatment plant: Optimization through response surface methodology. Journal of Hazardous Materials, 175(1–3), 651–657. 48. Monovoukas, Y., & Gast, A. P., (1989). The experimental phase diagram of charged colloidal suspensions. Journal of Colloid and Interface Science, 128(2), 533–548. 49. Ognibene, A. J., O’leary, D. S., Czarnecki, S. W., Flannery, E. P., & Grove, R. B., (1971). Myocarditis and disseminated intravascular coagulation in scrub typhus. American Journal of Medical Sciences, 262(4), 233–239. 50. Palacci, J., Cottin-Bizonne, C., Ybert, C., & Bocquet, L., (2010). Sedimentation and effective temperature of active colloidal suspensions. Physical Review Letters, 105(8), 088304. 51. Park, S., An, J., Jung, I., Piner, R. D., An, S. J., Li, X., & Ruoff, R. S., (2009). Colloidal suspensions of highly reduced graphene oxide in a wide variety of organic solvents. Nano Letters, 9(4), 1593–1597. 52. Piazza, R., & Parola, A., (2008). Thermophoresis in colloidal suspensions. Journal of Physics: Condensed Matter, 20(15), 153102. 53. Pieterse, A. J. H., & Cloot, A., (1997). Algal cells and coagulation, flocculation, and sedimentation processes. Water Science and Technology, 36(4), 111–118. 54. Rivas, F. J., Beltrán, F., Carvalho, F., Acedo, B., & Gimeno, O., (2004). Stabilized leachates: Sequential coagulation-flocculation + chemical oxidation process. Journal of Hazardous Materials, 116(1/2), 95–102. 55. Roder, H., Maki, K., & Cheng, H., (2006). Early events in protein folding explored by rapid mixing methods. Chemical Reviews, 106(5), 1836–1861. 56. Roussy, J., Van Vooren, M., Dempsey, B. A., & Guibal, E., (2005). Influence of chitosan characteristics on the coagulation and the flocculation of bentonite suspensions. Water Research, 39(14), 3247– 3258.

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57. Shammas, N. K., (2005). Coagulation and flocculation. In: Physicochemical Treatment Processes (Vol. 1, pp. 103–139). Humana Press. 58. Srichaikul, T., Punyagupta, S., Nitiyanant, P., & Alkarawong, K., (1975). Disseminated intravascular coagulation in adult Dengue haemorrhagic fever: Report of three cases. Southeast Asian Journal of Tropical Medicine and Public Health, 6(1), 106–114. 59. Tambo, N., & Watanabe, Y., (1979). Physical aspect of flocculation process—I: Fundamental treatise. Water Research, 13(5), 429–439. 60. Tang, P., Greenwood, J., & Raper, J. A., (2002). A model to describe the settling behavior of fractal aggregates. Journal of Colloid and Interface Science, 247(1), 210–219. 61. Tejada, L., Gomez, R., & Fernández-Salguero, J. O. S. É., (2007). Sensory characteristics of ewe milk cheese made with three types of coagulant: Calf rennet, powdered vegetable coagulant and crude aqueous extract from Cynara cardunculus. Journal of Food Quality, 30(1), 91–103. 62. Valencia, P. M., Basto, P. A., Zhang, L., Rhee, M., Langer, R., Farokhzad, O. C., & Karnik, R., (2010). Single-step assembly of homogenous lipid-polymeric and lipid-quantum dot nanoparticles enabled by microfluidic rapid mixing. ACS Nano, 4(3), 1671–1679. 63. Vandamme, D., Pontes, S. C. V., Goiris, K., Foubert, I., Pinoy, L. J. J., & Muylaert, K., (2011). Evaluation of electro-coagulation-flocculation for harvesting marine and freshwater microalgae. Biotechnology and Bioengineering, 108(10), 2320–2329. 64. Vassalli, P., & McCluskey, R. T., (1964). The pathogenic role of the coagulation process in rabbit Masugi nephritis. The American Journal of Pathology, 45(4), 653. 65. Vassalli, P., & McCluskey, R. T., (1965). The coagulation process and glomerular disease. The American Journal of Medicine, 39(2), 179– 183. 66. Verma, S., Prasad, B., & Mishra, I. M., (2010). Pretreatment of petrochemical wastewater by coagulation and flocculation and the sludge characteristics. Journal of Hazardous Materials, 178(1–3), 1055–1064.

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67. Vermant, J., & Solomon, M. J., (2005). Flow-induced structure in colloidal suspensions. Journal of Physics: Condensed Matter, 17(4), R187. 68. Wang, J. P., Chen, Y. Z., Ge, X. W., & Yu, H. Q., (2007). Optimization of coagulation-flocculation process for a paper-recycling wastewater treatment using response surface methodology. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 302(1–3), 204–210. 69. Winterwerp, J. C., (1998). A simple model for turbulence induced flocculation of cohesive sediment. Journal of Hydraulic Research, 36(3), 309–326. 70. Wong, S. H., Ward, M. C., & Wharton, C. W., (2004). Micro T-mixer as a rapid mixing micromixer. Sensors and Actuators B: Chemical, 100(3), 359–379. 71. Xu, X., & Zhu, X., (2004). Treatment of refectory oily wastewater by electro-coagulation process. Chemosphere, 56(10), 889–894. 72. Yasar, K., & Guzeler, N., (2011). Effects of coagulant type on the physicochemical and organoleptic properties of Kashar cheese. International Journal of Dairy Technology, 64(3), 372–379. 73. Yun, J. J., Kiely, L. J., Kindstedt, P. S., & Barbano, D. M., (1993). Mozzarella cheese: Impact of coagulant type on functional properties. Journal of Dairy Science, 76(12), 3657–3663.

CHAPTER 4

Activated Sludge Processes

CONTENTS 4.1. Introduction ...................................................................................... 90 4.2. Comparison Between As (Activated Sludge Process), Wetland System and Percolating Filtration...................................... 93 4.3. Types Of Activated Sludge (AS) Process ............................................. 95 4.4. As Process Kinetics And Designs ....................................................... 98 4.5. Key Process Criteria Of Design ....................................................... 102 References ............................................................................................. 105

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4.1. INTRODUCTION The activated sludge (AS) process is presently the most extensively used biological wastewater treatment procedure in the modern world. Since its beginning in the late 19th century and consequent development into the fullscale process by Lockett and Arden in 1913 at Davyhulme sewage treatment operates in Manchester, UK, the basic procedure has been broadly accepted and further established, giving it the exclusive flexibility of operation. Treatment wetlands constructed have the possibility to replace expensive AS processes, mainly in developing countries. Furthermore, wetland systems are frequently utilized to polish the waste from AS systems throughout the entire world. The AS process comprises of two distinct phases: 1. Aeration settlement; and 2. Sludge settlement. This unit is normally functioned with no settlement permitted in the aeration tanks, and the entirely distinct settlement tank with nonstop sludge elimination and arrival to aeration tank is thus functioned after the aeration tank (Ekama and Marais, 1979; Van Haandel et al., 1981; Ekma et al., 1986). In the first stage, wastewater is added to the aeration tank containing the mixed microbial population. Air or at times pure oxygen (O2) is added either by the surface agitation or through diffusers utilizing compressed air. The aeration has the dual function: 1.

To provide oxygen (O2), for respiration, to aerobic microorganisms in the reactor. 2. To preserve the microbial flocs in the endless state of disturbed suspension, confirming maximum contact amongst the surface of flocs and the wastewater. The uninterrupted mixing action is significant not only to guarantee that the sufficient supply of food gets to microorganisms but supports the maximum oxygen(O2) concentration gradient to improve mass transfer and disperse the metabolic pollutants from within the flocs (Dold and Marais, 1986; Sollfrank and Gujer, 1991). As the established wastewater enters into the aeration tank, it shifts the mixed liquor into the sedimentation tank. This is the second phase, where flocculated biomass settles quickly out of the suspension to create sludge, and where the elucidated waste, which is nearly free from the solids, is settled as the final waste. (Henze, 1992; Ahn et al., 2010)

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In the conventional AS process, between 0.5 kg and 0.8 kg sludge is formed for every kg of eliminated BOD5. The sludge is somewhat like a weak slurry comprising between 0.5% and 2.0% dry solids, and thus it can be pumped easily. As the content of solids increases, the viscosity quickly becomes greater, even though under normal working conditions AS is hard to concentrate to greater than 4% dry solids by gravity alone (Sakai et al., 1997; Liu, 2003). Most of the AS is returned to aeration tanks to perform as inoculum of the microorganisms, confirming that there is sufficient microbial population in order to oxidise the wastewater fully during its retention time within the aeration tanks. The excess sludge needs treatment before disposal (Dold and Ekama, 1981; Majone et al., 1999) (Figure 4.1).

Figure 4.1. Schematic illustration of the AS (activated sludge) process.

The most vital function in the AS procedure is the flocculent behavior of microbial biomass. Not only the flocs should be effective in adsorption and consequent absorption of organic matters in the wastewater, but they should also be effectively and rapidly isolated from the treated waste within the sedimentation tanks. Any variation in the process of reactor will bring variations in the behavior of flocs, which can harmfully affect the entire process in many ways; most particularly, poor settlement can outcome in turbid wastes, and the damage of microbial biomass (Fuhs and Chen, 1975; Liu and Tay, 2001). Although some alternatives of the AS process are utilized to treat manure which has only been de-gritted and screened, the majority of AS processes utilize settled effluent as the feedstock. The sludge yielded from the process must not be mixed up with primary sludge, as it is entirely made up of microbial biomass and the adsorbed particulate matter.

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The main parts of all AS systems are described below: 1.

Reactor: This generally can be a lagoon, tank, or ditch. The key design criteria for the reactor are that the content can be sufficiently mixed and aerated. The reactor is usually called the basin or the aeration tank. 2. Activated Sludge (AS): AS is the microscopic biomass within the reactor, which contains bacteria and other microflora and microfauna. The sludge is the flocculant suspension of the organisms and is generally known as the mixed liquor. The normal concentration is assessed as the MLSS (mixed liquor suspended solids (SS)) normally between 2000 mg/L and 5000 mg/L. 3. Aeration and Mixing System: Aeration and mixing of AS and the arriving wastewater are crucial. While these procedures can be commenced independently in distinct tanks, they are usually combined with one another using the single system. Either diffused air or surface aeration is used. 4. Sedimentation Tank: Final settlement of the AS evacuated from the aeration tanks by arriving wastewater is needed. This isolates the microscopic biomass from the treated waste. 5. Returned Sludge: Some of the AS in the sedimentation tanks is returned back to the reactor in order to sustain the microbial population at the essential concentration to guarantee the continuation of treatment. Ideally, the AS process must be operated near to the food-limited situation as probable to inspire endogenous respiration. This is where every microorganism is using its own cellular content, therefore decreasing the amount of biomass yielded. During the phase of endogenous respiration, the rate of respiration will fall to the minimum value, which is adequate for the maintenance of cells only. Though, under normal working conditions, the development of microbial population and the gathering of non-biodegradable solids result in an increased amount of AS produced (Koottatep et al., 2005; Matamoros et al., 2007). The two main elimination mechanisms in the AS process are described below: 1.

Assimilation: Usage of effluent to produce new biomass. Colloidal and the soluble BOD is converted into biomass, which is settled out later; and

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2.

Mineralization: Oxidation of effluent to inactive end products, which are released into the atmosphere or are left behind in solution in the waste. AS plants can be functioned to favor either of the two processes. In plants subjugated by assimilation, there is quick elimination of BOD with the consistently high production of the sludge, which means that the sludge treatment expenses are high. Plants that function in the way that supports mineralization need long aeration times. Therefore, the operating expenses increase because of the increased oxygen or air requirements. This is balanced by the decreased production of the sludge and low sludge treatment expenses respectively (Rosenberger et al., 2005; Sinha et al., 2008). Whichever technique is preferred, the wastewater itself should contain sufficient nutrients for biological growth. This nutrient necessity is normally given in terms of carbon (C), nitrogen (N), and phosphorus (P) ratio (Eq. 4.1). Observe that the five-day BOD is utilized as a measure for carbon content. Five-day BOD: N: P = 100:5:1

(1)

4.2. COMPARISON BETWEEN AS (ACTIVATED SLUDGE PROCESS), WETLAND SYSTEM AND PERCOLATING FILTRATION The comparative advantages and disadvantages of the AS process, wetland system, and percolating filtration parameters are discussed in Table 4.1 (Hiley, 1995; Maehlum, 1995; Cui et al., 2008). Table 4.1. Comparison of Activated Sludge Process, Wetland System and Percolating Filtration Parameters Activated Sludge Process

Percolating Filtration

Wetland System

Capital cost

Low

High

Very low

Operating cost

High

Low

Very low

Area of land

Low; beneficial where land obLarge; ten times tainability is limited or expensive more area needed

Large; ten times more area needed

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Technical control

High; microbial activity can be controlled closely; needs a continuous and skilled operation

Little feasible; doesn’t need skilled or continuous operation

Little feasible; doesn’t need skilled or continuous operation

Influence of weather

Works quite well in the wet weather; marginally worse in the dry weather; less affected by the low winter temperatures

Works better in summer but likely ponding in winter

Independent of the weather, if the wetland is adequately large

Hydrostatic head

Small; low pumping condition; appropriate for the site where available hydraulic head is restricted

Large; site should offer natural hydraulic head, else pumping is needed

Small; appropriate for the site where the accessible hydraulic head is restricted

Nature of the wastewater

Sensitive to the toxic shocks and variations in loading; trade wastewaters can cause bulking problems

Strong wastewaters tolerable; able to bear variations in loading and poisonous discharges

Strong wastewaters tolerable; able to bear variations in loading and poisonous discharges

Nuisance

Low scent and no-fly issues; noise might be the problem in both rural and urban areas

Moderate scent; severe fly issue in summer

No undesirable nuisance because of an ecological equilibrium

Secondary sludge

Large volume; high content of water; hard to dewater; less stabilized

Small volume; less water; highly stabilized

Virtually no sludge production

Final effluent quality

Poor nitrification but lower in the suspended solids, excluding when the separation problems happen

Highly nitrified; a comparatively high load of the suspended solids

High-quality waste unless the system is overloaded

Synthetic detergents

Likely foaming; particularly with Little or no foam diffusers

No problem

Energy requirement

High; needed for aeration, mixing and preserving sludge flocs in the suspension, and recycling sludge

Low; natural ventilation; gravitational flow

Virtually no energy needed

Robustness

Not very strong; the high extent of maintenance on motors; not probable to function without power supply

Very sturdy; low maintenance; probable to function without power

Very sturdy; low maintenance; probable to function without power

Source: Decamp and Warren, 2000; Uggetti et al., 2010; Vincent et al., 2011.

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4.3. TYPES OF ACTIVATED SLUDGE (AS) PROCESS 4.3.1. Conventional Whole Mix AS Process From the time of its introduction, the AS process has suffered many changes and adaptations. In some situations, these adaptations have an outcome from fundamental research into the main principles of the procedure, but more frequently, they have ascended as the empirical solutions to specific issues in plant operation (De Gisi et al., 2009; Valderrama et al., 2012). Regarding the conventional AS process, also known as the whole mix, the waste from the main sedimentation tank is aerated into the aeration tank. This aeration normally lasts 6 to 12 hours at three DWF. The MLSS (mixed liquor suspension solids) are kept just high in order to upsurge the effectiveness of the biological reactions (Tellez et al., 2002; Liew et al., 2015). Typically MLSS are between 3.0 mg/L and 3.5103 mg/L. The rate of organic loading for the units is between 3.0 and 3.5 kg BOD5/m3/day. The air delivered to the aeration units is generally in the order of six m3/m3 effluents. This kind of plant does not yield a nitrified effluent (Washington and Symons, 1962; Stricker et al., 2009). fThe waste quality of treated effluent after 2 hours of sedimentation is given as follows: BOD5 in the middle of 10 mg/L and 15 mg/L, and SS of around 20 mg/L. The aeration in the aeration tank is accomplished at a rationally uniform rate over the entire length of the tank. This causes probable oxygen scarcity at the inlet. Reactions might thus be oxygen-limited (Heidler and Halden, 2007; Gander et al., 2009). Therefore, process efficiency might be reduced. The conventional system offers good buffering against the toxic conditions and shock loads and is therefore mainly appropriate for the treatment of industrial wastes (Radjenović et al., 2009; Jelic et al., 2011).

4.3.2. Series or the Plug Flow System In this type of arrangement, the effluent to be processed and the return AS are presented at one end of the channel, in which the number of aerators is positioned, and the treated mixed liquor is released at the other end. With the mechanical aerators, there might be as many as around ten aerators in the line, but it is normal to have greater number lines with 3 or 4 aerators per line. Compared to the single line, this gives a higher preliminary degree of the aeration to untreated waste. Domestic sewage is generally treated by this approach. The advantage of this method is that the progressively decreasing substrate-level yields sludge with enhanced settling characteristics.

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4.3.3. Tapered Aeration This is the modification of the plug flow system. The procedure seeks to regulate the supply rate of oxygen throughout the entire length of the tank to the demand rate of oxygen. Therefore, the higher proportion of total air supply is familiarized at the inlet end and the supply rate is tapered towards the tank outlet. The rate at the outlet is usually set to the value impending that of the endogenous respiration. This system is mainly suitable for the treatment of the strong, eagerly biodegradable effluents.

4.3.4. Step Feed AS Process This is another variant of the plug flow process, which tries to balance the oxygen demand and supply. In this situation, instead of changing the supply rate of oxygen along with the aeration tank, limited equalization of demand is accomplished by adding the effluent to the aeration tanks at several points along the entire length of the tank. There is no obvious benefit for this system regarding the treatment of domestic sewage.

4.3.5. High-Rate AS Process This is sometimes known as a modified aeration process. This kind of plant functions at much higher food to microorganism ratios as compared to the conventional process: 1.

Conventional: 0.2e0.4 kg BOD5/kg MLVSS (mixed liquor volatile SS)/day. 2. High rate: 1.5e5.0 kg BOD5 kg/ MLVSS /day. This high rate of loading is accomplished by the reduced hydraulic retention time and the lower MLSS. The air supply is roughly 3 m3/m3, and the BOD5 reductions between 60% and 70% are possible. The process outcome in excess of the arriving organic matters being produced to sludge organisms. Thus, the total oxygen necessities are to some extent less than in the conventional AS processes, but the demand rate of oxygen is much higher per unit of the MLVSS.

4.3.6. Extended Aeration This alternate is described by the very low food to microorganism ratio, higher MLSS, low net sludge produce, and higher hydraulic retention times. This makes the procedure appropriate for small separated communities.

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The drawbacks of the aeration systems for very small communities are described below: 1.

Effect of the high hydraulic loading, leading to the loss of AS solids; 2. Widely changing BOD loads; 3. Uneconomic demands of the conventional aeration systems for regular attention. In the comprehensive aeration plant, major sedimentation can be lost so that only the sludge from the wastage line is disposed of. It follows that the process is suitable for rural locations. The protracted aeration system offers an aeration tank capacity of about >24 h DWF, which is the large buffer for high immediate loadings. However, the final settlement tanks must be made for the treatment of the peak loads. The rate of loading applied to the systems is in the middle of 0.24 and 0.32 kg BOD5/m3/day. The hydraulic retention period is between 24 h and 36 h. This should yield the waste of normally 40 mg/L BOD5 and 50 mg/L SS. One benefit of these aeration systems is that the process of aeration is taken quite fine into the endogenous stage of respiration. As a result, the sludge production is minimized. The age of sludge is ten days.

4.3.7. Contact Stabilization Contact stabilization attempts to exploit the quick reduction in the BOD of untreated waste by bio flocculation and biosorption with the return AS. In this system, untreated waste and the return AS are aerated in the contact stage with the retention between 0.5 h and 3 h at DWF. The sludge, isolated by settlement, is then passed to the stabilization tank, where the sludge is re-aerated for the period based on the AS return flow to finish the oxidation of adsorbed BOD. The benefit of the contact stabilization is that sludge re-aeration is performed in the small tank. Therefore, the tank volume and capital expenses are low. Large volumes of the sludge are yielded in this procedure, and aerobic digestion is thus adopted sometimes to decrease the yield. The usual design criteria are given below: 1.

The BOD loading must be 480 mg/L for combined contact and the re-aeration stages.

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2.

The capacity of the contact stage based on the DWF must be between 0.5 h and 2 h.

4.4. AS PROCESS KINETICS AND DESIGNS 4.4.1. Diffused Air Aeration The objectives of the aerating wastewater are: 1.

To introduce pure oxygen or air into the wastewater with sunken diffusers or the other aeration devices. 2. To stir the wastewater mechanically, in order to promote the solution of the air from the atmosphere. The diffused air system comprises of diffusers sunken in the wastewater, blowers from which the air passes and the header pipe air mains. In the past, several diffusion devices have been categorized as either coarse-bubble or fine-bubble aeration systems, with the implication that the fine bubbles were effective in transferring oxygen. The first choice is to classify the diffused aeration systems by physical features of the equipment. The following three groups have been defined: 1. 2. 3.

Porous or the fine-pore diffusers; Nonporous diffusers; and Other diffusion devices, like jet aerators, U-tube aerators and aspirating aerators. Porous diffusers are built in numerous shapes; most commonly used are plates, discs, domes, and tubes. Plate type diffusers are expensive to install and hard to maintain, while the discs and domes are easy to eliminate and can be built from porous plastic or ceramics. It is necessary that the air delivered to these devices free from dust and is clean, which may clog the diffusers (Pei et al., 2009; Alvarez and Myerson, 2010). Nonporous diffusers are accessible in many types. The fixed and the valve orifice diffusers yield larger bubbles than the porous diffusers. As a result, they have lower aeration effectiveness, but the benefits of less maintenance, lower cost, and the nonexistence of severe air purity necessities offset the marginally lower efficiency (Aziz and Klinzing, 1990; Lansing et al., 2008). In static tube aerators, the air is provided at the bottom of the circular tube, which can change in height between 0.5 m and 1.25 m. internally the tubes are fixed with the interchangeably positioned deflection plates in order

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to raise the contact of air with the wastewater. Mixing is achieved since the tube aerator performs as the airlift pump (Liotta et al., 2014; Ridder et al., 2014). Other kinds of diffuser are accessible in the market. Jet aeration, for instance, unites liquid pumping with air diffusion. The pumping systems recirculate liquid in the aeration basin and then discharge it with compressed air through the nozzle assembly. This system is beneficial for deep tanks. Aspirating aeration comprises of the motor-driven aspirator pump. U-tube aeration is utilized in the deep shaft procedure. Typical transfer effectiveness for these devices varies between 9% and 40%.

4.4.2. Mechanical Aerators This group of aerators is generally divided into two groups: 1. Aerators with the vertical axis; and 2. Aerators with the horizontal axis. Both of the groups can be classified further as surface or submerged aerators. Surface aerators with the vertical axis are made to persuade either downdraft or updraft flows through the pumping action. High-speed aerators are utilized in ponds and lagoons. The water level changes so that the fixed support is impossible. Thus, nearly floating devices are utilized in practice. Low-speed aerators are utilized in the AS process, normally on the fixed platform. They are made to release wastewater into the atmosphere in the shape of small droplets. Submerged devices with the vertical axis are dependent on the violent agitation of surface and consequent air entrainment to accomplish improved oxygen transfer. These aren’t popular devices, even though they have been utilized in pure oxygen systems. Mechanical aerators with the horizontal axis can be sub-classified into the surface and the submerged devices. The surface aerator kinds are based on Kestner brush aerators, the device usually utilized in the oxidation ditches. The brush kind aerator has the horizontal axis with the bristles fixed above the water surface. The bristles are submerged and the cylinder is rapidly rotated, scattering wastewater across the tank, supporting circulation, and entraining the air. Submerged aerators with the horizontal axis are just alike the surface type aerators, apart from that they utilize disks or paddles fixed to the rotating

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shaft in order to stir the fluid. Disk aerators have been utilized previously in the oxidation ditches. The disks are then submerged to roughly between 0.125 and 0.375 of the diameters. Usual efficiency figures for the mechanical aerators are between 8% and 15%.

4.4.3. Process Design An empirically made relationship amongst biological growth and the substrate utilization is expressed in Eq. (4.1) which is commonly utilized in the biological systems stabilizing the organic and inorganic waste. The net growth rate of the microorganisms = Birthrate – Death rate

(1)

The ratio of food to microorganisms called the process loading factor, the specific utilization, and the substrate removal rate. The fractional growth, microorganism growth rate, and food to microorganism growth rate are linked directly with each other. However, the following suppositions are to be made: 1. 2.

All the essential nutrients for the growth are present; Temperature and the pH are controlled to accomplish the optimal growth rate; 3. The equations are applicable only to that area of the waste which is soluble or recyclable. The four main types of reactors for the biological wastewater treatment are categorized according to the hydraulic features as a batch, complete mix, plug, and arbitrary flow. In plug flow, the fluid particles move through the tanks and are released in a similar order in which they enter. The particles preserve their individuality, and they stay in the tank for the time equal to theoretical detention time. Complete mixing happens when particles arriving at the tank are instantaneously distributed throughout the tank. The particles vacate the tanks in proportion to the statistical population. The batch reactor is generally characterized by neither the fact that the flow neither leaves nor enters nor the reactor on a continuous basis (Henze et al., 1987; Hamoda and Al-Attar, 1995). Arbitrary flow signifies any degree amongst plug flow and partial mixing and is tough to describe mathematically. Hence, complete mix flow or ideal plug flow models are normally assumed. The reactor is mixed entirely, and no organisms are left in the influent. The mean cell residence time and the hydraulic retention time are the same. The

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mass balance for microorganisms is given as follows: Rate of change of the organism concentration in reactor is equal to the net rate of the organism growth in reactor subtracted by the rate of the organism discharge from the reactor (Marais and Ekama, 1976; Kappeler and Gujer, 1992). The complete mix model can be utilized to simulate the traditional anaerobic treatment systems and some amended AS processes, lagoons, and oxidation ponds, given that the allocation is made for adjusting where suitable. The treatment effectiveness of the process with high cell residence time might be restricted by the reduction of nutrients, oxygen (O2) transfer problems, and issues with the mixing of large microorganism mass consequential from the prolonged cell residence times (Nejjari et al., 1999; Li et al., 2009). It is supposed that the content of the reactor is mixed completely and that there remain no microorganisms in waste effluent. As such, kind of system involves the settling unit; further streamlining assumptions should be made: 1. 2.

Waste stabilization happens only in the reactor; The volume utilized to develop the mean cell residence time includes only the reactor volume. For this type of system, mass balance is described as follows: Rate of change of the organism concentration in reactor is equal to the net rate of the organism growth in reactor subtracted by the rate of the organism discharge from the reactor. There isn’t any need to regulate the quantity of the biological solids in the system or the quantity of food utilized. The quantified percentage of cell mass in this type system should be wasted every day to regulate the rate of growth of microorganisms and therefore the extent of waste stabilization (Spanjers et al., 1996; Pol et al., 2004). Not like the complete mix scheme, in most of the biological treatment procedures, cell wastage takes place from the sludge recycle line. Both the sludge microorganism and mixed liquor concentration must be known if the cells are unused from the sludge recycle line. In the plug flow model, all the particles entering the reactor will stay there for a similar amount of time. Therefore, simplifying suppositions can be made (Poduska and Andrews, 1975; Dytczak et al., 2008). For example, influent microorganism concentration is roughly equal to the waste microorganism concentration. Moreover, the additional microorganisms are missed from the reactor discharge and not from the recycle line. Theoretically, the system of plug flow recycle is more effective at waste stabilization as compared to the system of complete mix recycle. In

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practice, though, longitudinal dispersion stops the regime of true plug flow. The system of plug flow is also more vulnerable to shock loads. Thus, the aeration tank is usually divided into the sequence of complete mix reactors to give enhanced treatment together with better resistance to the shock loads (Schmidt and Ahring, 1994; Sipma et al., 2010).

4.5. KEY PROCESS CRITERIA OF DESIGN 4.5.1. Loading Criteria The aspects to be well-thought-out in the design of AS systems are given below: 1. Loading criteria l; 2. Sludge production l; 3. Selection of the type of reactor l; 4. Nutrient requirements; 5. Solid-liquid separation; 6. Oxygen necessities and transfer l; 7. Environmental necessities; 8. Effluent characteristics; The parameters given below are normally used to regulate and design the AS process: 1. Food to microorganism ratio; 2. Mean cell residence time. Normally, the ratio of food to microorganisms is between 0.2 and 0.5. Values for mean cell residence time are between 7 days and 15 days and outcome in high quality and stable effluent. The conforming sludge must be simple to be dewatered. This empirical method of design is based on the concern of the maximal organic loading, together with the requirements of detention time. The method overlooks the concentration of mixed liquor, mean cell residence time, and food to microorganism ratio. Still, it has the benefit of requiring the minimum aeration tank volume which must be sufficient for satisfactory treatment, supposing the proper choice of the other design criteria.

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4.5.2. Reactor Types Initial construction, function, and maintenance expenses will affect the selection process of the reactor in practice. Operational factors that should be well-thought-out include the following: 1. Reaction kinetics controlling the treatment process; 2. Oxygen transfer requirements; 3. The nature of wastewater to be treated; 4. Local environmental conditions. For the 1st-order substrate elimination kinetics, the total volume needed for the series of the complete mix reactors is significantly less than that needed for the single complete mix reactor. Certainly, the volume differential turns out to be more noticeable as the removal effectiveness increases and also with an increase in the order of the substrate elimination kinetics. In practice, though, neither the complete mix nor the plug flow reactor functions as supposed in theory (Glasser et al., 1987; Xu et al., 2005). With conventional systems of plug flow aeration, inadequate oxygen is delivered to fulfill the oxygen necessities of the reactor and the input. Modifications to enhance the route of oxygen transfer can be briefed as follows: 1.

Tapered aeration, where the supply of air is matched to the demand for oxygen; 2. Step aeration, where incoming effluent and the return solid are dispersed along the length of the reactor; 3. The complete mix AS process, where the aeration equals the oxygen demand. In the complete mix reactor, the arriving waste is dispersed uniformly. As an outcome, it can bear shock loads better than the plug flow reactor. Therefore, a complete mix reactor is generally preferred for industrial wastewater treatment (Liu and Tay, 2002; Guo et al., 2008). The amount of sludge yielded is important to the design of sludge treatment and disposal facilities. The volumetric sludge making is dependent on the volume of the reactor and the effectiveness of the final settling tank.

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4.5.3. Oxygen Demand The requirement of oxygen is calculated from the BOD of the untreated water and from the number of cells missed from the system every day. Assuming that the complete BOD is transferred to end products, total demand for oxygen can be calculated by transforming BOD5 to BODult and then subtracting the BODult of cells missed from the system. Utilizing molecular weight (MW) relationships for the typical waste and the linked oxygen requirement, it goes on that the demand for oxygen can be projected by the food used every day decreased by around 1.42 times of the organisms wasted every day. If the oxygen transferal effectiveness of aeration system is identified, the requirement of air can be found. The least concentration of dissolved oxygen (DO) throughout the reactor must be between 1 mg/L and 2 mg/L. usually, an aeration system must be capable of providing 150% of normal air requirements.

4.5.4. Nutrient Requirements Sufficient nutrients should be accessible for the proper functionality of the treatment plant. The main nutrients for all the biological treatment systems, containing built wetlands, are phosphorus and nitrogen. Assuming the composition of the cell of C5H7NO2, then 12.4% nitrogen (N) by weight of a mass of the organisms yielded each day is needed. The corresponding phosphorus (P) demand is around 0.2 of nitrogen demand. Generally, wastewater comprises of all the nutrients needed for cell growth. Conversely, industrial wastewaters might need additional nutrients for optimal biodegradation.

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12. Fuhs, G. W., & Chen, M., (1975). Microbiological basis of phosphate removal in the activated sludge process for the treatment of wastewater. Microbial Ecology, 2(2), 119–138. 13. Gander, M., Jefferson, B., & Judd, S., (2000). Aerobic MBRs for domestic wastewater treatment: A review with cost considerations. Separation and Purification Technology, 18(2), 119–130. 14. Glasser, D., Crowe, C., & Hildebrandt, D., (1987). A geometric approach to steady flow reactors: The attainable region and optimization in concentration space. Industrial and Engineering Chemistry Research, 26(9), 1803–1810. 15. Guo, W. Q., Ren, N. Q., Wang, X. J., Xiang, W. S., Meng, Z. H., Ding, J., & Zhang, L. S., (2008). Biohydrogen production from ethanol-type fermentation of molasses in an expanded granular sludge bed (EGSB) reactor. International Journal of Hydrogen Energy, 33(19), 4981–4988. 16. Hamoda, M. F., & Al-Attar, I. M. S., (1995). Effects of high sodium chloride concentrations on activated sludge treatment. Water Science and Technology, 31(9), 61–72. 17. Heidler, J., & Halden, R. U., (2007). Mass balance assessment of triclosan removal during conventional sewage treatment. Chemosphere, 66(2), 362–369. 18. Henze, M., (1992). Characterization of wastewater for modeling of activated sludge processes. Water Science and Technology, 25(6), 1–15. 19. Henze, M., Grady, Jr. C. L., Gujer, W., Marais, G. V. R., & Matsuo, T., (1987). A general model for single-sludge wastewater treatment systems. Water Research, 21(5), 505–515. 20. Hiley, P. D., (1995). The reality of sewage treatment using wetlands. Water Science and Technology, 32(3), 329. 21. Jelic, A., Gros, M., Ginebreda, A., Cespedes-Sánchez, R., Ventura, F., Petrovic, M., & Barcelo, D., (2011). Occurrence, partition, and removal of pharmaceuticals in sewage water and sludge during wastewater treatment. Water Research, 45(3), 1165–1176. 22. Kappeler, J., & Gujer, W., (1992). Estimation of kinetic parameters of heterotrophic biomass under aerobic conditions and characterization of wastewater for activated sludge modeling. Water Science and Technology, 25(6), 125–139.

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23. Koottatep, T., Surinkul, N., Polprasert, C., Kamal, A. S. M., Koné, D., Montangero, A., & Strauss, M., (2005). Treatment of septage in constructed wetlands in tropical climate: Lessons learnt from seven years of operation. Water Science and Technology, 51(9), 119–126. 24. Lansing, S., Víquez, J., Martínez, H., Botero, R., & Martin, J., (2008). Quantifying electricity generation and waste transformations in a lowcost, plug-flow anaerobic digestion system. Ecological Engineering, 34(4), 332–348. 25. Li, J., Zheng, G., He, J., Chang, S., & Qin, Z., (2009). Hydrogenproducing capability of anaerobic activated sludge in three types of fermentations in a continuous stirred-tank reactor. Biotechnology Advances, 27(5), 573–577. 26. Liew, W. L., Kassim, M. A., Muda, K., Loh, S. K., & Affam, A. C., (2015). Conventional methods and emerging wastewater polishing technologies for palm oil mill effluent treatment: A review. Journal of Environmental Management, 149, 222–235. 27. Liotta, F., Chatellier, P., Esposito, G., Fabbricino, M., Van Hullebusch, E. D., & Lens, P. N., (2014). Hydrodynamic mathematical modeling of aerobic plug flow and nonideal flow reactors: A critical and historical review. Critical Reviews in Environmental Science and Technology, 44(23), 2642–2673. 28. Liu, Y., & Tay, J. H., (2001). Strategy for minimization of excess sludge production from the activated sludge process. Biotechnology Advances, 19(2), 97–107. 29. Liu, Y., & Tay, J. H., (2002). The essential role of hydrodynamic shear force in the formation of biofilm and granular sludge. Water Research, 36(7), 1653–1665. 30. Liu, Y., (2003). Chemically reduced excess sludge production in the activated sludge process. Chemosphere, 50(1), 1–7. 31. Maehlum, T., (1995). Treatment of land fill leachate in on-site lagoons and constructed wetlands. Water Science and Technology, 32(3), 129– 135. 32. Majone, M., Dircks, K., & Beun, J. J., (1999). Aerobic storage under dynamic conditions in activated sludge processes: The state of the art. Water Science and Technology, 39(1), 61. 33. Marais, G., & Ekama, G. A., (1976). The activated sludge process part I-steady state behavior. Water sA, 2(4), 163–200.

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34. Matamoros, V., Arias, C., Brix, H., & Bayona, J. M., (2007). Removal of pharmaceuticals and personal care products (PPCPs) from urban wastewater in a pilot vertical flow constructed wetland and a sand filter. Environmental Science and Technology, 41(23), 8171–8177. 35. Nejjari, F., Dahhou, B., Benhammou, A., & Roux, G., (1999). Nonlinear multivariable adaptive control of an activated sludge wastewater treatment process. International Journal of Adaptive Control and Signal Processing, 13(5), 347–365. 36. Pei, J., Li, Q., Lee, M. S., Valaskovic, G. A., & Kennedy, R. T., (2009). Analysis of samples stored as individual plugs in a capillary by electrospray ionization mass spectrometry. Analytical Chemistry, 81(15), 6558–6561. 37. Pholchan, M. K., Baptista, J. D. C., Davenport, R. J., & Curtis, T. P., (2010). Systematic study of the effect of operating variables on reactor performance and microbial diversity in laboratory-scale activated sludge reactors. Water Research, 44(5), 1341–1352. 38. Poduska, R. A., & Andrews, J. F., (1975). Dynamics of nitrification in the activated sludge process. Journal (Water Pollution Control Federation), 1, 2599–2619. 39. Pol, L. H., de Castro, L. S. I., Lettinga, G., & Lens, P. N. L., (2004). Anaerobic sludge granulation. Water Research, 38(6), 1376–1389. 40. Radjenović, J., Petrović, M., & Barceló, D., (2009). Fate and distribution of pharmaceuticals in wastewater and sewage sludge of the conventional activated sludge (CAS) and advanced membrane bioreactor (MBR) treatment. Water Research, 43(3), 831–841. 41. Ridder, B. J., Majumder, A., & Nagy, Z. K., (2014). Population balance model-based multiobjective optimization of a multisegment multiaddition (MSMA) continuous plug-flow antisolvent crystallizer. Industrial and Engineering Chemistry Research, 53(11), 4387–4397. 42. Rosenberger, S., Evenblij, H., Te Poele, S., Wintgens, T., & Laabs, C., (2005). The importance of liquid phase analyses to understand fouling in membrane assisted activated sludge processes—six case studies of different European research groups. Journal of Membrane Science, 263(1/2), 113–126. 43. Sakai, Y., Fukase, T., Yasui, H., & Shibata, M., (1997). An activated sludge process without excess sludge production. Water Science and Technology, 36(11), 163–170.

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44. Schmidt, J. E. E., & Ahring, B. K., (1994). Extracellular polymers in granular sludge from different up flow anaerobic sludge blanket (UASB) reactors. Applied Microbiology and Biotechnology, 42(2/3), 457–462. 45. Sinha, R. K., Bharambe, G., & Chaudhari, U., (2008). Sewage treatment by vermifiltration with synchronous treatment of sludge by earthworms: A low-cost sustainable technology over conventional systems with potential for decentralization. The Environmentalist, 28(4), 409–420. 46. Sipma, J., Osuna, B., Collado, N., Monclús, H., Ferrero, G., Comas, J., & Rodriguez-Roda, I., (2010). Comparison of removal of pharmaceuticals in MBR and activated sludge systems. Desalination, 250(2), 653–659. 47. Sollfrank, U., & Gujer, W., (1991). Characterization of domestic wastewater for mathematical modeling of the activated sludge process. Water Science and Technology, 23(4–6), 1057–1066. 48. Spanjers, H., Vanrolleghem, P., Olsson, G., & Doldt, P., (1996). Respirometry in control of the activated sludge process. Water Science and Technology, 34(3/4), 117–126. 49. Stricker, A. E., Barrie, A., Maas, C. L., Fernandes, W., & Lishman, L., (2009). Comparison of performance and operation of side-by-side integrated fixed-film and conventional activated sludge processes at demonstration scale. Water Environment Research, 81(3), 219–232. 50. Tellez, G. T., Nirmalakhandan, N., & Gardea-Torresdey, J. L., (2002). Performance evaluation of an activated sludge system for removing petroleum hydrocarbons from oilfield produced water. Advances in Environmental Research, 6(4), 455–470. 51. Uggetti, E., Ferrer, I., Llorens, E., & García, J., (2010). Sludge treatment wetlands: A review on the state of the art. Bioresource Technology, 101(9), 2905–2912. 52. Valderrama, C., Ribera, G., Bahí, N., Rovira, M., Giménez, T., Nomen, R., & Martinez-Lladó, X., (2012). Winery wastewater treatment for water reuse purpose: Conventional activated sludge versus membrane bioreactor (MBR): A comparative case study. Desalination, 306, 1–7. 53. Van Haandel, A. C., Ekama, G. A., & Marais, G., (1981). The activated sludge process—3 single sludge denitrification. Water Research, 15(10), 1135–1152.

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54. Vincent, J., Molle, P., Wisniewski, C., & Liénard, A., (2011). Sludge drying reed beds for septage treatment: Towards design and operation recommendations. Bioresource Technology, 102(17), 8327–8330. 55. Washington, D. R., & Symons, J. M., (1962). Volatile sludge accumulation in activated sludge systems. Journal (Water Pollution Control Federation), 1, 767–790. 56. Xu, J. L., Li, Y. X., & Wong, T. N., (2005). High speed flow visualization of a closed loop pulsating heat pipe. International Journal of Heat and Mass Transfer, 48(16), 3338–3351.

CHAPTER 5

Fundamentals of Membranes for Water Treatment

CONTENTS 5.1. Introduction .................................................................................... 112 5.2. Membrane Characteristics .............................................................. 113 5.3. Membrane Materials ....................................................................... 116 5.4. Membrane Modules........................................................................ 116 5.5. Theory ............................................................................................ 117 References ............................................................................................. 122

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5.1. INTRODUCTION In the 1960s, feasible earnings of water purification with the progress of high presentation synthetic membranes were Membranes. By consuming more membranes made from new materials, application of membranes for water treatment has proceeded and worked in various configurations. An increasing scarcity in freshwater sources fueled a push towards alternative resources such as ocean water. For water desalination by utilizing membranes examination initiated in the 1970s from saltwater (Nataraj et al., 2006; Matin et al., 2011). In the water management marketplace, membranes became a feasible substitute for evaporation-based technologies by proving effective at making purified water. Over the years, an excess of new submissions has seemed, and purified water standards have become stricter (Amjad, 1993; Murray-Gulde et al., 2003). Water treatment procedures employ many types of membranes (Baker, 2002). They contain microfiltration (MF), reverse osmosis (RO, ultrafiltration (UF)), and nanofiltration (NF) membranes (Figure 5.1). MF membranes classically discard big particles and many microorganisms and have the major pore size. UF membranes have slighter pores than MF membranes and, therefore, in addition to great particles and microorganisms, bacteria, and soluble macromolecules such as proteins can also be rejected by them (Green and Maloney, 1997; Hamid et al., 2002). RO membranes are effectively non-porous and, therefore, exclude particles and even many low molar mass species such as salt ions, organics. NF membranes are often called “loose” RO membranes and they are comparatively fresh (Sami et al., 2000). They are absorbent membranes, but subsequently, the size of pores is almost ten angstroms or less, they show a presentation between that of RO and UF membranes (Baker et al., 1998; 2010, 2012).

Figure 5.1. Range of nominal membrane pore sizes (Matin et al., 2011). Source: https://www.researchgate.net/publication/251435747_Fundamentals_ of_Membranes_for_Water_Treatment.

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5.2. MEMBRANE CHARACTERISTICS Generally, membranes are categorized as isotropic or anisotropic. Isotropic membranes are unvarying in physical nature and composition across the cross-section of the membrane. Over the membrane cross-section, anisotropic membranes are non-uniform, and they classically contain layers that differ in structure and/or chemical composition. In many subcategories, isotropic membranes can be separated. For example, isotropic membranes may be microporous. Microporous membranes are often prepared from rigid polymeric materials with large voids that create interconnected pores (Wang et al., 1996). The most common microporous membranes are phase inversion membranes (Figure 5.2a). A cast film creates them from the solution of polymer and solvent and for the polymer the cast film in a nonsolvent submerging. Many polymers utilize in such applications are hydrophobic, so the most common nonsolvent is water (Sagle and Freeman, 2004). Upon interaction with water, the membrane is made by the precipitations of the polymer. The track-etched membrane is an additional type of microporous membrane (Figure 5.2b). Preservation of a polymer film with charged particles that bout the polymer chains, leaving damaged molecules after, this type of membrane is organized. The film is then passed through an etching solution, and for the creation of cylindrical pores, the damaged molecules melt, several of which are vertical to the membrane surface (Wang et al., 2000a, b, 2002). A fewer shared microporous membrane is a prolonged-film membrane (Figure 5.2c). Spaces formed by an extrusion and stretching process the crystalline polymers are arranged by expanded film membranes. First, using a quick draw-down rate close to its melting temperature the material is extruded. Then, the extruded material is annealed, cooled, and stretched up to 300% of its original length. 200 to 2500 Å is the size of slit-like pores forming by this stretching process. To reduce the membrane efficiently non-porous (Di Luccio et al., 2000a), the so minor isotropic membranes which either deficient pores or have pores can be dense. Solution casting monitored by solvent evaporation or melt extrusion form these films (Wang et al., 1996, 1999).

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Figure 5.2. SEM images showing top surfaces of a) a phase inversion membrane, b) a track-etched membrane, and c) an expanded film membrane. Source: https://www.researchgate.net/publication/251435747_Fundamentals_ of_Membranes_for_Water_Treatment.

There are two types of anisotropic membranes: thin-film composite (TFC) membranes and phase separation membranes. Loeb Sourirajan membranes are often the second name of the anisotropic phase separation membranes. These phase-separated membranes are similar in chemical composition but not in their structure. Loeb-Sourirajan membranes are produced via phase inversion techniques such as those described above, except that the pore sizes and porosity varies across the membrane thickness (Figure 5.3a). A solid sheet of polymer on the surface of a gradually porous layer is included in Loeb-Sourirajan membranes (Trushinski et al., 1993, 1994, 1998; Di Luccio et al., 2000a). TFC membranes are together structurally and chemically heterogeneous (Figure 5.3b). A highly porous substrate coated with a thin dense film of a different polymer is included in TFCs. They can be formed through many methods comprising of solution coating, interfacial polymerization, plasma polymerization or surface treatment (Sagle and Freeman, 2004; El-Ghaffar and Tieama, 2017).

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Figure 5.3. SEM images of a) cross-section of an anisotropic microporous membrane and b) cross-section of a thin-film composite membrane (Wang et al., 2006). Source: http://www.sciencepublishinggroup.com/journal/paperinfo?journalid =243&doi=10.11648/j.cbe.20170202.11.

Mention to flat sheet configurations the explanations of isotropic and anisotropic membranes is given above. Though, membranes can also be shaped as hollow fibers. These fibers can either be isotropic or anisotropic just similar to flat sheets. Their nature may be dense or porous. With a dense outer layer around a porous tube, common fibers used in industry today are anisotropic (Figure 5.4). One advantage of hollow fiber membranes is that as compared to flat sheet membranes hollow fiber membrane is larger in size (Rezac et al., 1994; Pinnau and Freeman, 1999)

Figure 5.4. SEM image of hollow fiber cross-section (Nune et al., 2011). Source: https://pubs.acs.org/doi/abs/10.1021/bk-2000-0744.ch007.

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5.3. MEMBRANE MATERIALS Many MF, UF, RO, and NF membranes are artificial organic polymers. The same material is utilized to form MF and UF membranes, but under dissimilar membrane formation conditions, they are ready so that different pore sizes are formed. Typical MF and UF polymers comprise poly(vinylidene fluoride), poly(acrylonitrile), polysulfone, and poly(acrylonitrile)-poly(vinyl chloride) copolymers. For UF membranes poly(ether sulfone) is often used. MF membranes also have poly(tetrafluoroethylene) and cellulose acetate (CA) -cellulose nitrate blends, nylons. RO membranes are characteristically either poly(tetrafluoroethylene) or CA with aromatic polyamides (Taniguchi and Belfort, 2004; Yune et al., 2011). Similar the RO membranes NF membranes are formed from polyamide composites or CA blends or polyamide composites, or they could be the modification of UF membranes such as sulfonated polysulfone (Nunes and Peinemann, 2006, 2010). Inorganic materials such as ceramics or metals can also form membranes. Ceramic membranes are chemically resistant, microporous, thermally stable, and often used for MF (Peinemann et al., 2007). Though, disadvantages like their vast usage have been stalled by high cost and mechanical fragility. Stainless steel forms metallic membranes and can be very superbly porous. Gas separations are their main application, but at high temperatures, they can also be recycled for water filtration or as membrane support (Iarikov et al., 2011).

5.4. MEMBRANE MODULES There are four key forms of modules: tubular, plate-and-frame, hollow fiber, and spiral wound (Figure 5.5). The plate-and-frame module is the meekest configuration, containing the flat sheet membrane, two end plates, and spacers. In tubular modules, the membrane is frequently on the inside of a tube, and the feedstuff solution is propelled through the tube. The greatest widespread module in manufacturing for NF or converse osmosis membranes is the spirally twisted module. This module has an even sheet membrane enfolded around a perforated permeate gathering tube (Coˆte et al., 2005; Pellegrino and Sikdar, 2017). The feed streams on one side of the membrane. Permeate is composed on the other side of the membrane and spirals near the center gathering tube.

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Figure 5.5. Schematic of: (a) plate and frame, (b) tubular, (c) spiral wound, and (d) hollow fiber modules (Coˆte et al., 2005). Source: https://www.nist.gov/publications/membrane-technology-fundamentals-bioremediation.

Hollow fiber modules applied for seawater purification contain bundles of hollow fibers in a pressure vessel. They can have a shell-side feed conformation where the feed permits along with the outdoor of the threads and exoduses the fiber end (Baker et al., 1991; Mallevialle et al., 1996). Muffled fiber modules can also be applied in a bore side feed conformation where the feed is spread through the fibers. Hollow fibers used for wastewater action and in membrane bioreactors (MBRs) are not continuously applied in pressure vessels. Packages of fibers can be deferred in the feed solution, and pervade is composed of one end of the fibers (Bird t al., 2002).

5.5. THEORY The theory leading fluid transportation through membranes is frequently uttered as follows (Bird et al., 2002):

where ρA is the mass density of constituent A, DAB is the effective diffusion coefficient of component A in the membrane, NA is the mass flux of constituent A through the membrane, is the mass average velocity of the fluid through

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the membrane, and ΔρA is the mass density gradient. In membranes where pore movement donates meaningfully to flux, Darcy’s law is frequently used to describe the mass average velocity (Van der Bruggen and Vandecasteele, 2002).

where µ is the fluid viscosity, κ is the Darcy Law permeability of the medium Δp is the pressure gradient, µ is the gravity vector, ρ is the solution density. Presenting Equation (2) into Equation (1), limiting carriage to only the x-direction, which would characteristically be the direction perpendicular to the membrane surface, and abandoning gravity, produces:

The first term in Equation (3) signifies mass flux due to pressuredriven convection through pores, and the second term signifies flux due to dispersion. Diffusion through absorbent membranes is characteristically insignificant comparative to convection. In this circumstance, the flux is directly proportional to the pressure incline across the membrane (Wade, 1993; Ryoo et al., 2003). The used pressure difference across the membrane, often known as the transmembrane pressure (TMP) difference, is the driving force leading conveyance of liquid through a porous membrane (Frost et al., 1998; Konagaya et al., 2000; Wade, 2001). In smearing the convective term of Equation (3) to conveyance through MF and UF membranes, the penetrability, κ, is contingent, frequently in a complicated way, on factors such as the tortuosity and the porosity of the membrane. Tortuosity, τ, is the ratio of the regular length of the “tortuous” track that the liquid must travel to permit through the membrane to the membrane width. For example, a tubular pore perpendicular to the surface has a tortuosity of one. Most phase transposal membranes have tortuosities from 1.5 to 2.5. Porosity, ε, is the void portion of the membrane. MF and UF membrane porosity characteristically vary from 0.3 to 0.7 (Mallevialle et al., 1996; Water, 2004). Since RO membranes are efficiently non-porous, the conveyance of a molecule across the membrane is dispersion measured. This means that the second term of Eq. 3 regulates the flux across the membrane. Water molecules sorb into the upstream expression of the membrane, desorb from

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the downstream face of the membrane, and diffuse down the chemical potential gradient across the membrane (Baker et al., 1991; Ettouney et al., 2002). The second stage, dispersion through the membrane, is the ratedefining step in water conveyance across the membrane. This instrument of mass conveyance across membranes is generally mentioned as the “solutiondiffusion” model. Opening with the more general model of mass conveyance being motivated by chemical potential gradients rather than concentration gradients, the solution-diffusion conveyance equation for converse osmosis can be resultant (Wijmans and Baker, 1995; Strathmann, 2001): NAw = L(∆p −∆π)

(4)

L= DSV/RTl

(5)

Where ∆p is the TMP difference, NAw is the water flux through the membrane, L is a constant relating the physical features of the membrane itself and ∆π is the difference in osmotic pressure between the permeate and the feed. Within the context of the solution-diffusion model used to define conveyance in nonporous films, L is given by: where R is the ideal gas constant, D is the water diffusivity in the membrane, V is the molar volume of water, S is the water solubility in the membrane, l is the membrane thickness, and T is the ambient temperature. A complete beginning can be found in the Wijmans and Baker review of the solutiondiffusion model and in Paul’s current re-examination of the solutiondiffusion prototypical for RO (Paul, 2004). As understood from Equation (4), the osmotic pressure of the permeate solutions and feed plays a role in the parting. Osmotic pressure is the pressure required to cause a solvent to leave a solution and infuse through the membrane. For an ideal solution, with complete detachment of salt ions, osmotic pressure is well-defined as (Cadotte and Lloyd, 1985; Freeman, 2003): π= CRT

(6)

where C is the salt ion concentration, π is the osmotic pressure, T is the solution temperature, and R is the ideal gas constant. The salt ion concentration, C is given by the number of ions in solution per gram of water divided by the specific volume of water. Table 5.1 offers the osmotic pressure for numerous solutions relevant to water treatment applications (Sourirajan, 1977).

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Table 5.1. Typical Osmotic Pressure Values for Solutions at 25°C (Avlonitis et al., 1992) Solute

Concentration (mg/L) Osmotic Pressure (psi)

NaCl Seawater NaCl Brackish water

2,000 32,000 35,000 2,000–5,000

23 339 397 15–39

In opposite osmosis, salt conveyance across a membrane is as significant as water conveyance. Though dissimilar to water flux, which is determined by both osmotic pressure and applied TMP, the salt flux is only a function of salt concentration: Ns = B(C feed −C permeate)

(7)

Where B is the salt permeability constant relating the physical features of the membrane, Ns is the salt flux through the membrane, Cpermeate is the salt concentration in the permeate solution and Cfeed is the salt concentration in the feed solution. Equivalent to L in the solution diffusion equation, B is given by: B=DsKs/l

(8)

Where Ds is the salt diffusivity in the membrane, Ks is the salt partition coefficient, and l is the membrane thickness. However, instead of reporting salt flux values, most membrane performance specifications provide salt rejection values. Salt rejection, R, is defined as follows (Tella et al., 2011):

Additionally, salt flux and water flux be contingent on each other. Equation (10) narrates the salt flux, Ns18 and water flux, NAw: (10) where Cpermeate is the salt concentration in the permeate and Cw is the water concentration in the permeate. By replacing Equations (4) and (7) into (10) and reorganizing terms, the following expression for rejection may be resultant (Riley et al., 1967; Stevens, 1999):

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Equation (11) narrates salt refusal to the physical features of the membrane (which effect B and L), the osmotic pressure difference between permeate and the feed and the used TMP difference. Equation (11) permits one to forecast the salt refusal of the membrane based on the investigational circumstances and the membrane features (Loeb, 1962; Mason, 1991).

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Amjad, Z., (1993). Reverse Osmosis: Membrane Technology, Water Chemistry and Industrial Applications (Vol. 1, pp. 1–10). Chapman & Hall. Ang, D., Li, K., & Teo, W. K., (2000a). Porous PVDF asymmetric hollow fiber membranes prepared with the use of small molecular additives. Journal of Membrane Science, 178(1/2), 13–23. Avlonitis, S., Hanbury, W. T., & Hodgkiess, T., (1992). Chlorine degradation of aromatic polyamides. Desalination, 85(3), 321–334. Baker, R. W., & Lokhandwala, K., (2008). Natural gas processing with membranes: An overview. Industrial and Engineering Chemistry Research, 47(7), 2109–2121. Baker, R. W., (2002). Future directions of membrane gas separation technology. Industrial and Engineering Chemistry Research, 41(6), 1393–1411. Baker, R. W., (2010). Research needs in the membrane separation industry: Looking back, looking forward. Journal of Membrane Science, 362(1/2), 134–136. Baker, R. W., (2012). Membrane Technology and Applications (Vol. 1, pp. 1–15). John Wiley & Sons. Baker, R. W., Cussler, E. L., Eykamp, W., Koros, W. J., Riley, R. L., & Strathman, R. H., (1991). Membrane Separation Systems, 1, 1–15. Baker, R. W., Wijmans, J. G., & Kaschemekat, J. H., (1998). The design of membrane vapor-gas separation systems. Journal of Membrane Science, 151(1), 55–62. Bird, R. B., Stewart, W. E., & Lightfoot, E. N., (2002). Transport Phenomena (Vol. 2, No. 1, pp. 10–25). John Wiley & Sons. Inc., Hoboken, NJ. Cadotte, J. E., & Lloyd, D. R., (1985). Materials science of synthetic membranes. In: ACS Symposium Series (Vol. 269, pp. 273–294). American Chemical Society Washington, DC. Coˆte, P., Siverns, S., & Monti, S., (2005). Comparison of membranebased solutions for water reclamation and desalination. Desalination, 182(1–3), 251–257. Di Luccio, M., Nobrega, R., & Borges, C. P., (2000a). Microporous anisotropic phase inversion membranes from bisphenol-A

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26. Mason, E. A., (1991). From pig bladders and cracked jars to polysulfones: An historical perspective on membrane transport. Journal of Membrane Science, 60(2/3), 125–145. 27. Matin, A., Khan, Z., Zaidi, S. M. J., & Boyce, M. C., (2011). Biofouling in reverse osmosis membranes for seawater desalination: Phenomena and prevention. Desalination, 281, 1–16. 28. Murray-Gulde, C., Heatley, J. E., Karanfil, T., Rodgers, Jr. J. H., & Myers, J. E., (2003). Performance of a hybrid reverse osmosisconstructed wetland treatment system for brackish oil field produced water. Water Research, 37(3), 705–713. 29. Nataraj, S. K., Hosamani, K. M., & Aminabhavi, T. M., (2006). Distillery wastewater treatment by the membrane-based nano filtration and reverse osmosis processes. Water Research, 40(12), 2349–2356. 30. Nunes, S. P., & Peinemann, K. V., (2006). Membrane Technology: In the Chemical Industry (Vol. 1, pp. 10–20). John Wiley & Sons. 31. Nunes, S. P., & Peinemann, K. V., (2010). 1.06-advanced polymeric and organic–inorganic membranes for pressure-driven processes. Comprehensive Membrane Science and Engineering, 2017, 113–129. 32. Nunes, S. P., Behzad, A. R., Hooghan, B., Sougrat, R., Karunakaran, M., Pradeep, N., & Peinemann, K. V., (2011). Switchable pH-responsive polymeric membranes prepared via block copolymer micelle assembly. ACS Nano, 5(5), 3516–3522. 33. Paul, D. R., (2004). Reformulation of the solution-diffusion theory of reverse osmosis. Journal of Membrane Science, 241(2), 371–386. 34. Peinemann, K. V., Abetz, V., & Simon, P. F., (2007). Asymmetric superstructure formed in a block copolymer via phase separation. Nature Materials, 6(12), 992. 35. Pellegrino, J., & Sikdar, S. K., (2017). Membrane technology fundamentals for bioremediation. In: Fundamentals and Applications of Bioremediation (Vol. 1, pp. 457–509). Routledge. 36. Pinnau, I., & Freeman, B. D., (1999). Formation and modification of polymeric membranes: Overview. In: ACS Symposium Series (Vol. 744, pp. 1–22). American Chemical Society. 37. Rezac, M. E., Le Roux, J. D., Chen, H., Paul, D. R., & Koros, W. J., (1994). Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes. Journal of Membrane Science, 90(3), 213–229.

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38. Riley, R. L., Lonsdale, H. K., Lyons, C. R., & Merten, U., (1967). Preparation of ultrathin reverse osmosis membranes and the attainment of theoretical salt rejection. Journal of Applied Polymer Science, 11(11), 2143–2158. 39. Ryoo, M. W., Kim, J. H., & Seo, G., (2003). Role of Titania incorporated on activated carbon cloth for capacitive deionization of NaCl solution. Journal of Colloid and Interface Science, 264(2), 414–419. 40. Sagle, A., & Freeman, B., (2004). Fundamentals of membranes for water treatment. The Future of Desalination in Texas, 2(363), 137. 41. Sami, S., Hamid, A., Lafri, D., Semmar, D., & Kharchi, R., (2000). Optimization of capture surfaces of solar installations for collective water heating. Rev. Energ. Ren .: Chemss, 1, 25–31. 42. Sourirajan, S., (1977). Reverse osmosis and synthetic membrane. National Council Canada, 1, 1–12. 43. Stevens, M. P., (1999). Polymer Chemistry, an Introduction (Vol. 3, pp. 5–25). Oxford University press. 44. Strathmann, H., (2001). Membrane separation processes: Current relevance and future opportunities. AIChE Journal, 47(5), 1077–1087. 45. Taniguchi, M., & Belfort, G., (2004). Low protein fouling synthetic membranes by UV-assisted surface grafting modification: Varying monomer type. Journal of Membrane Science, 231(1/2), 147–157. 46. Teella, A., Huber, G. W., & Ford, D. M., (2011). Separation of acetic acid from the aqueous fraction of fast pyrolysis bio-oils using nano filtration and reverse osmosis membranes. Journal of Membrane Science, 378(1/2), 495–502. 47. Trushinski, B. J., Dickson, J. M., Childs, R. F., & McCarry, B. E., (1993). Photochemically modified thin-film composite membranes. I. Acid and ester membranes. Journal of Applied Polymer Science, 48(2), 187–198. 48. Trushinski, B. J., Dickson, J. M., Childs, R. F., McCarry, B. E., & Gagnon, D. R., (1994). Photochemically modified thin-film composite membranes. II. Bromoethyl ester, dioxolan, and hydroxyethyl ester membranes. Journal of Applied Polymer Science, 54(9), 1233–1242. 49. Trushinski, B. J., Dickson, J. M., Smyth, T., Childs, R. F., & McCarry, B. E., (1998). Polysulfonamide thin-film composite reverse osmosis membranes. Journal of Membrane Science, 143(1/2), 181–188.

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50. Van der Bruggen, B., & Vandecasteele, C., (2002). Distillation vs. membrane filtration: Overview of process evolutions in seawater desalination. Desalination, 143(3), 207–218. 51. Wade, N. M., (1993). Technical and economic evaluation of distillation and reverse osmosis desalination processes. Desalination, 93(1–3), 343–363. 52. Wade, N. M., (2001). Distillation plant development and cost update. Desalination, 136(1–3), 3–12. 53. Wang, D., Li, K., & Teo, W. K., (1996). Polyethersulfone hollow fiber gas separation membranes prepared from NMP/alcohol solvent systems. Journal of Membrane Science, 115(1), 85–108. 54. Wang, D., Li, K., & Teo, W. K., (1999). Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes. Journal of Membrane Science, 163(2), 211–220. 55. Wang, D., Li, K., & Teo, W. K., (2000b). Highly permeable polyethersulfone hollow fiber gas separation membranes prepared using water as non-solvent additive. Journal of Membrane Science, 176(2), 147–158. 56. Wang, D., Teo, W. K., & Li, K., (2002). Removal of H2S to ultra-low concentrations using an asymmetric hollow fiber membrane module. Separation and Purification Technology, 27(1), 33–40. 57. Wang, Y. Q., Wang, T., Su, Y. L., Peng, F. B., Wu, H., & Jiang, Z. Y., (2006). Protein-adsorption-resistance and permeation property of polyethersulfone and soybean phosphatidylcholine blend ultrafiltration membranes. Journal of Membrane Science, 270(1/2), 108–114. 58. Water, P., (2004). Coalbed Natural Gas Resources: Beneficial Use Alternatives (Vol. 9, pp. 1–10). GasTIPS®. 59. Wijmans, J. G., & Baker, R. W., (1995). The solution-diffusion model: A review. Journal of Membrane Science, 107(1/2), 1–21. 60. Yune, P. S., Kilduff, J. E., & Belfort, G., (2011). Fouling-resistant properties of a surface-modified poly (ether sulfone) ultrafiltration membrane grafted with poly (ethylene glycol)-amide binary monomers. Journal of Membrane Science, 377(1/2), 159–166.

CHAPTER 6

Microfiltration and Nanofiltration

CONTENTS 6.1. Introduction .................................................................................... 128 6.2. Pressure Driven Membranes ........................................................... 130 6.3. Vacuum Driven Hollow Fiber Membrane – The Zeeweed Membrane ................................................................................... 132 6.4. Treatment With Microfiltration (Mf) Membranes .............................. 135 6.5. Application Of Nanofiltration (Nf) Membranes For Drinking Water Treatment ............................................................ 140 References ............................................................................................. 145

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6.1. INTRODUCTION The main substances in the drinking water sources can be presented as colloidal, suspended, and dissolved materials. The suspended substance which is usually characterized by turbidity can be removed by most common treatment methods (Jacangelo et al., 1995, 1997; Mourato, 1998), the most known of which are chemically aided coagulation after that filtration or filtration and clarification. The coagulant amount, in this case, is usually similar to the turbidity level in the source. The existence of Giardia cysts and Cryptosporidium and oocysts and further parasites in drinking water sources has revealed a fresh field of application for the membranes in the drinking water sector. The incompetence of common filtration plants to purify and filter these pathogens from the drinking water has forced engineers to look into new techniques. Membranes are the natural reply to resolve their problem because these are complete barriers to parasites which size increased the membrane’s pore size (Tan and Sudak, 1992; Taylor et al., 1992). Common treatment methods are also usually not active when total organic carbon (TOC) and color are present in advanced levels in the feed water. As the suspended and colloidal portions of these constituents are comparatively high, they are not readily removed by gravity and settling filtration. Lastly, great levels of manganese and iron in well waters have been tough to treat with the common green sand method and again, these have underway to be decent candidate plants for membrane technologies (Cooper, 1993; Wiesner et al., 1994; Cath et al., 2013). Nanofiltration (NF) and microfiltration (MF) membranes are becoming progressively more used in the drinking water field. For some uses, MF membranes are currently seen as a recognized technology. This comprises Giardia cysts and Cryptosporidium and oocysts parasites removal and turbidity elimination with color and MF and salty water treatment with NF. Benefits associated with the use of membranes in potable water treatment are complete barrier effect to microorganisms, low energy requirements, less chlorine requirement for disinfection, low chemical (if any) usage, and smaller footprint. The kind of membrane used also impacts some particular advantages (Lee et al., 1999; Ince et al., 2010; Madaeni et al., 2013). This chapter will present the usual applications of both kinds of membranes in the drinking water field. Subjects discussed here are: 1.

Removal of iron and Mn by combining oxidation with MF;

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Removal of color and TOC by NF; Removal of parasites and turbidity by direct MF – disinfection MF; 4. Removal of TOC and color by combining superior coagulation with MF. Membrane filtration works on the principle of specific separation based on a pore size distribution and pore size. MF membranes have pore sizes that differ from 0.075 microns to 3 microns. Depending on the membrane selected, it will permit to detach suspended solids (SS) above 0.45 microns, cysts, bacteria, and many other parasites which diameter are larger than the greater pore size of the membrane (Carroll et al., 2002; Tahri et al., 2012). NF membranes have pore sizes range from 0.005 microns to 0.001 microns and with such an insignificant pore size are capable to remove large molecular weight (MW) molecules, for example, certain humic acids and salts. This allows for the production of a parasite and solids-free water without the need for chemicals (Saboyainsta and Maubois, 2000; Wu et al., 2016) (Figure 6.1).

Figure 6.1. The filtration spectrum. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

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Membranes are composed of several materials, with ceramic, polymers, and sintered metals being the most common types of membranes. However, ceramic and sintered metals generally have industrial applications, polymeric membranes are becoming a common tool for municipal uses and drinking water treatment. Membranes need transmembrane pressure (TMP) to force the clean water over the membrane, leaving the concentrate comprising the solids and separated particles. The TMP compulsory to drive membrane plants can be induced by vacuum or by pressure (Shirazi et al., 2010; Masmoudi et al., 2014). Likewise, there are a large number of filtration paths which are generally found in membranes: Dead-end filtration, where the filtrate practices a cake as the sieve becomes plugged, cross-flow filtration in which the filtrate is moved away from the membrane, this evading fast filter plugging and osmosis where the water is clean over a semi-permeable membrane. This chapter emphasis on Cross-flow filtration membranes (Winzeler and Belfort, 1993; Van der Bruggen et al., 2003a, b) (Figure 6.2).

Figure 6.2. Modes of filtration. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

Different types of membranes are discussed in the following sections of this chapter.

6.2. PRESSURE DRIVEN MEMBRANES The primarily commercially accessible membranes were formed using flat sheets rolled to make spiral wound membranes. These membranes

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may perhaps not tolerate solids and necessary great pressures to function. The great working cost of these membranes caused by occasional use and slight municipal uses in the MF model (Chen et al., 2013; Pearce, 2007). Spiral wound membranes are usually met in reverse osmosis (RO) and NF applications and are normally used for seawater and desalting brackish water for the production of clean water (Gupta et al., 2012; Jhaveri and Murthy, 2016). Hollow fiber membranes were advanced in the last ten years to approach MF requirements while by fewer energy costs to work. These membranes shortly became an industry customary and a large number of companies started producing these high surface area membranes and applying them to the potable water field (Jain and Pradeep, 2005; Macedoni and Drioli, 2008). Two kinds of pressure-driven hollow fiber membranes are found: 1.

2.

Inside-out membranes, in which the influent is forced inside the membrane’s lumen (inside) and the clean water moves from the interior of the membrane to the outside; and Outside-in membranes in which the influent is forced from the outside of the membrane and the clean water moves from the outside to the interior (lumen) of the membrane (Figure 6.3).

Figure 6.3. Filtration modes – hollow-fiber membranes. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

Entire pressure-driven, hollow-fiber membranes are fixed inside pressure vessels, essential to apply the pressure for appropriate fluid transfer (Wiesne and Chellam, 1992; Drioli et al., 2006). The usual functioning pressure for these membranes is 15 to 30 psi.

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6.3. VACUUM DRIVEN HOLLOW FIBER MEMBRANE – THE ZEEWEED MEMBRANE The ZeeWeed™ centered potable water practice is a revolutionary less energy membrane method that contains outside-in hollow-fiber MF components absorbed in raw feed-water. This micro-filter has a 0.085 micron minimal and a 0.2-micron entire pore size, confirming that no particulate matter above 0.2 micron will seepage to the treated water stream (Wang et al., 2009; Abu-Zeid et al., 2015). The membranes work under a small suction created inside the hollow fibers by an infiltrate pump. The preserved water passes over the membrane, enters the hollow fibers and is pumped out to circulation by the invade pumps (Bhaumik et al., 2004; Lee and Kim, 2014). Airflow is presented at the bottom of the membrane component to form a disorder that scrubs and wipes the outside of the membrane fibers letting them operate at a great flux rate. This air will also oxidize Fe (iron) and further organic compounds, producing improved quality water than provided by MF only (Mavrov et al., 2003; Sun and Chung, 2013; Wang and Chung, 2013) (Figure 6.4).

Figure 6.4. Operational concept of an outside-in, immersed, shell-less membrane. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

Being an outside-in hollow fiber membrane, the plant does not require pretreatment, though the feed water has fine particles and clays. So, in a particular step, it swaps the flocculation, coagulation, clarification, and sand

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purification steps of common plants, but also removes the pretreatment essential by inside-out membranes and spiral (Abdallah et al., 2013). A plant of this kind works with a process tank containing a set of immersed membranes. The water moves over the membranes and the permeate is driven out. The air necessary to retain the membrane clean is produced by an air blower. The plant is easy to function but also easy to gather into slight containerized plants which can be fixed in small to large groups. The plant’s process flow diagram is given in Figure 6.5.

Figure 6.5. PFD of an immersed membrane microfilter. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

Moreover, the ZeeWeed outside-in immersed membrane gives extra advantages that are discussed in subsections.

6.3.1. High Solids Outside-in membranes in which water flow is from the external of the membrane to the internal of the hollow fiber, a sense that the inside just sees clean, micro-filtered water. So the algae cysts, solids, and clays to be removed stay outside the membrane and not ever move in the membrane initiating membrane plugging and fouling. This feature evades the usage of interior recirculation of the permeate to clean the membranes. Moreover, submerged membranes are not fixed inside pressure vessels; they are as a substitute, immersed inside the process tanks, so resistant to the existence of high solids in the tank. It means that in surface water plants, the performance

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of the membrane is self-determining of the feed water’s high solids peaks and regular turbidity.

6.3.2. Oxidation and Volatilization of Contaminants In the meantime, the membrane’s scrubbing air is injected in the feedwater; it becomes accessible for destroying eagerly oxidizable organics, micro precipitating certain metals, for example, Fe, and H2S and scrubbing volatile organics, therefore generating potable water of improved quality than when treated by MF only.

6.3.3. Energy Efficiency Cure with immersed outside-in membranes is carried out in an energy effective manner because the membrane works underneath a small suction (–2 to –5 psi) and with a too-small blower pressure (5.2 psi). Moreover, in plants constructed at the water level, the membranes can be fallen openly into the raw feedstuff well, evading feed pumping costs. Lastly, there is no requirement for spending energy in interior recirculation pumping costs because there is no particles trap within the membrane body.

6.3.4. Chlorine Resistance The ZeeWeed membrane is resilient to chlorine and any other oxidant in concentrations is as high as 200 mg/L. It means that a plant can prechlorinate its water for a zebra mussel regulator deprived of having to increase a dechlorination step. Resistance to oxidants permits for the addition of oxidation pretreatment stages along with for easy decontamination of the plant and the membranes.

6.3.5. Low Particle Counts At no time, the ZeeWeed membranes are back-washed or stressed below pressure. The product is that submerged membrane plants have the lowermost particle amounts in the MF field, normally with under 3 counts/mL. This permits for on-line 24 hours observing of membrane integrity.

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6.4. TREATMENT WITH MICROFILTRATION (MF) MEMBRANES 6.4.1. Surface Water Treatment – Disinfection by Direct Microfiltration (MF) The usage of a 0.2 micron MF membrane in a potable water filtration plant permits to address, in a single step, a few of the most explained current issues with present technologies (Jacangelo, 1995, 1997). The elimination of Giardia cysts, coliforms, Cryptosporidium oocysts, and other parasites and SS; 1. The decrease of settling chemicals; 2. The decrease in the use of disinfection chemicals; 3. The decrease of sludge for disposal; 4. The decrease in viruses. This type of treatment is accomplished with any of the MF membranes explained above. Usual results achieved in drinking surface water treatment by using MF are existing below (Mourato, 1998) (Table 6.1). Table 6.1. Surface Water Treatment Data – Direct MF with an Immersed Membrane Feed Water Element

Treated Water Quality

Giardia and Cryptosporidium

Non-Detectable > 6 log removal

Coliforms

< 10 cfu/100 mL

Suspended Solids

Non-Detectable

Particle Counts

< 3 particles/mL

Turbidity

< 0.1 NTU

*Results from work performed in Alberta river water treatment and Egypt on canal water treatment. Giardia cysts and Cryptosporidium are now a huge problem in insecure surface water reservoirs. These oocysts and cysts are found because of

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contamination by human manure but also by normal living organisms that evacuate inside the water. These parasites are just two between several make clean waters harmful to drink. It seems that daily the WHO is finding additional water parasites that are intimidating human life (Jolis et al., 1999; Sethi and Juby, 2002). The elimination of cysts with membranes is an easy task because the diameter of these is greater than the diameter of many MF membranes. Figure 6.5 displays the size of two parasites usually present in North American waters: Giardia and Cryptosporidium when got under a scanning electron microscope. It is easy to understand from these pictures why these parasites would be eagerly removed by a 0.2 micron pore MF membrane deprived of the requirement of chemicals or other treatment procedures (Langlais et al., 1992; Guo et al., 2015) (Figure 6.6).

Figure 6.6. Scanning electron micrographs of cryptosporidium and giardia parasites. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

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6.4.2. Surface Water Treatment – Enhanced Coagulation With Microfiltration (MF) Several surface potable water supplies are greatly colored. The majority of soluble organics existing in natural water supplies contain humic materials. These compounds are comparatively high MW polar organic compounds, which feature the yellow to brown color observable in some surface supplies. Whereas these substances themselves do not create any health alarms, chlorination of these waters can end in the creation of trihalomethanes (THM) which are supposed to be dangerous to health, and which are coming under progressively stringent government strategies (Zhu et al., 2005; Arnaldos and Pagilla, 2010). When joined with coagulation, MF has the capability to remove organic carbon and color from water sources. This is achieved by precipitating dissolved organics into micro-flocs which can then be disconnected by the membrane (Carroll et al., 2000; Lee et al., 2000). Color and TOC are in large amounts in the certain river and lake water supplies, the most conventional drinking water sources in North America. The United States, EPA ruling for TOC elimination differs with water alkalinity (Leiknes et al., 2004; Kimura et al., 2008). Table 6.2 presents these requirements. Table 6.2. US EPA’s TOC Removal Requirements TOC in Water Alkalinity Levels in the Feed Water (mg/L CaCO3) mg/L 0–60 mg/L 60–120 mg/L > 120 mg/L 2.0–4.0 35% 25% 15% 4.0–8.0 45% 35% 25% 8.0 50% 40% 30%

MF only does not remove TOC or color from the water. Nevertheless, when joined with coagulation, these can be efficiently removed, therefore combining the absolute blockade advantage of MF with coagulation procedures. This exceptional process for TOC, color, and THM precursor elimination has been developed by using ZENON’s submerged MF membrane technology ZeeWeed. The capability to build high solids levels in the process tank permits, by a mutual mechanism of co-precipitation, coagulation, and adsorption on solids, to reach high levels of TOC elimination with minor dosages of coagulants. Two coagulants can be used: iron chloride or alum.

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Realizing the water’s chemistry, higher levels of elimination can be achieved with greater dosages of coagulants and with modifying the water’s pH. Removals as great as 95% color elimination and 85% TOC removal are achievable with an improved process. Process optimization often needs pH modification which translates in the usage of more chemicals and can be tougher to work in small plants. Lebeau et al. (1998) have joined the used of the immersed MF through coagulant and powder activated carbon (AC) as a means to efficiently eliminate natural organic matter (NOM) from surface water. Though this procedure is more difficult to function, it considerably improves the quality of the finished water with slight chemical consumption. Classic results of MF improved coagulation with ZeeWeed are given in Table 6.3. Table 6.3. Typical Results of Microfiltration Enhanced Coagulation Feedwater Color: 35 units Feedwater TOC: 10 mg/L Alum coagulation FeCl3 coagulation Permeate color (% Removal): Permeate TOC (% Removal): Permeate THM (% Removal):

(60 mg/L) 74% 49% 48%

(60 mg/L) 66% 66% 66%

Note: The maximum TOC removal using non-membrane coagulation was 40%.

6.4.3. Groundwater Treatment by Microfiltration (MF) Wells water frequently has manganese and iron which needs to be removed in advance human consumption. Several small communities depend on communal groundwater supplies and need systems that guarantee removal of turbidity, metals, microorganisms, and hydrogen sulfide while reducing chemical usage and sludge production (Ellis et al., 2000). Wells with great levels of manganese and iron are conventional in certain parts of the world, dependent on the geological development. Common technologies like green sand and oxidation/settling are operational at low to medium concentrations. When well waters have Iron in surplus of 5 mg/L and Manganese in surplus of 1 mg/L, common technologies are no longer effective because of filter blinding produced by the iron bacteria films and precipitated iron. Moreover, many wells under the effect of surface

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waters also contain microorganisms, oocysts, and cysts that essential to be effectively removed for safe potable water intake. Deep wells also usually have H2S and organics which also need to be removed, frequently resulting in a more difficult treatment plant than essential by these clear waters (Thompson et al., 1995; Drewes et al., 2003). The ZeeWeed membrane, because of its design features resolves many of these problems deprived of the adding of needless steps (Table 6.4). Table 6.4. Mechanisms for Groundwater Contaminant Removal Contaminant Removal

Removal Mechanism

Fe

Air oxidation

Turbidity, microorganisms

Direct microfiltration

Giardia, Cryptosporidium

Direct microfiltration

H 2S

Air scouring

Mn

In-line Oxidant admixing

The process flow diagram for the outside-in immersed membrane process for the treatment of complex groundwater is given in Figure 6.7.

Figure 6.7. Typical ZeeWeed treatment plant for a complex groundwater. Source: https://pdfs.semanticscholar.org/7c03/d6949ad668cfcaf50d05df7daf29d22fe880.pdf.

Usual results attained with an immersed outside-in membrane in groundwater usage are presented in Tables 6.5 and 6.6. These results have been gathered from full-scale plant operation in New Brunswick in an iron and manganese polluted well under the impact of surface water and in Egypt, in deep wells comprising a mixture of manganese, iron, H2S, organics, and bacteria (Lee et al., 2000; Soares et al., 2000).

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Table 6.5. Results of Well Water Treatment in New Brunswick, Canada Contaminant

ZeeWeed Treated Water

Well Water Feed

Manganese

< 0.1 mg/L

2–10 mg/L

Iron

< 0.1 mg/L

2–10 mg/L

Note: The ZeeWeed plant is enhanced with permanganate injection for Mn removal. In more polluted waters, the other process mechanisms come into play. This is mainly true when we preserved well water in Egypt. These waters contained great levels of biodegradable BOD and ammonia, causing sewage pollution of the well. The air in the ZeeWeed procedure tank oxidized the H2S but also curved the process tank into a bio-oxidizing, bioreactor the biodegradable BOD and nitrifying the ammonia (Bellona and Drewes, 2007). Typical results of this work are presented in Table 6.6: Table 6.6. Results of Well Water Treatment in Egypt using ZeeWeed Contaminant

ZeeWeed Treated Water (mg/L)

Well Water Feed (mg/L)

H 2S

Non-detectable

10

Manganese

< 0.1

0.5–10

Ammonia

2 TCU in the permeate upheld for more than 1200 hours under improved operating conditions. Provincial objects for TOC (5 mg/L), Turbidity (1 NTU) and THMFP (350 ug/L) were also attained (Table 6.7).

TOC

Turbidity

THMFP

1

2

5

ND

ND

100

25

65

Rawdon, Quebec (1991) (Lac Vail)

ND Contra Costa Water District, CA (Sacramento-San Joaquin)

45

Lac Deux Montagnes

Sept-Isles, Quebec 90 (1992) (Lac desRapides)

ND

Caramat Lake, Ontario – 1992

East Bay CA (1991) (Molkelumme River)

Ottawa, Ontario (1991) (Ottawa River)

98

ND

ND

94

97

96

99

95

7.4

1.4

5.2

10.4

7

6

14

10

1.1

0.4

2.1

1.6

1.1

1.8

2.6

2.8

85

74

60

85

84

70

81

72

1.4

0.64

12

ND

1.3

10

0.3

0.65

0.05

0.05

0.05

ND

92

>96

72

160

50

330

154

260

200

1350

1200

27

6.5

205

71

60

64

150

175

83

87

38

54

77

68

89

85

(TCU) Removal Raw Permeate Removal Raw Permeate Removal Raw (mg/L) Removal (%) (%) (mg/L) (mg/L) (%) (NTU) (NTU) (%) (mg/L)

110

Raw (TCU)

Color

Fauquier, Ontario (1993)

Location

Table 6.7. Summary of Field Testing Results Using Nanofiltration for Treatment of Surface Water

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An initial economic assessment was undertaken to associate the expenses of the transverse flow module to a common package management process (coagulation, sedimentation, filtration, with powdered initiated carbon addition) capable of providing similar water feature for the water supply at Fauquier. The cost study displayed that NF was less costly than common treatment plus powdered initiated carbon for flows up to 100 gpm. These assumptions are in congruence with various researchers (Wiesner et al., 1993; Liang et al., 2014), which have shown that NF in both hollow fiber (cross flow) and crosswise flow formations are cost-competitive. The treatment process of these membranes is improved by using powdered initiated carbon while GAC/ozone is required to attain THM limitations (Watson and Hornburg, 1989; Tuhkanen et al., 1994; Patterson et al., 2011).

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34. Masmoudi, G., Trabelsi, R., Ellouze, E., & Amar, R. B., (2014). New treatment at source approach using combination of microfiltration and nanofiltration for dyeing effluents reuse. International Journal of Environmental Science and Technology, 11(4), 1007–1016. 35. Mavrov, V., Erwe, T., Blöcher, C., & Chmiel, H., (2003). Study of new integrated processes combining adsorption, membrane separation, and flotation for heavy metal removal from wastewater. Desalination, 157(1–3), 97–104. 36. Mourato, D., (1998). Microfiltration and nanofiltration. In: Regional Symposium on Water Quality: Effective Disinfection (Vol. 1, pp. 1–17). CEPIS. 37. Patterson, C., Anderson, A., Sinha, R., Muhammad, N., & Pearson, D., (2011). Nanofiltration membranes for removal of color and pathogens in small public drinking water sources. Journal of Environmental Engineering, 138(1), 48–57. 38. Pearce, G., (2007). Introduction to membranes: Filtration for water and wastewater treatment. Filtration and Separation, 44(2), 24–27. 39. Riera-Torres, M., Gutiérrez-Bouzán, C., & Crespi, M., (2010). Combination of coagulation-flocculation and nanofiltration techniques for dye removal and water reuse in textile effluents. Desalination, 252(1–3), 53–59. 40. Saboyainsta, L. V., & Maubois, J. L., (2000). Current developments of microfiltration technology in the dairy industry. Le Lait., 80(6), 541– 553. 41. Sethi, S., & Juby, G., (2002). Microfiltration of primary effluent for clarification and microbial removal. Environmental Engineering Science, 19(6), 467–475. 42. Shirazi, S., Lin, C. J., & Chen, D., (2010). Inorganic fouling of pressuredriven membrane processes—A critical review. Desalination, 250(1), 236–248. 43. Soares, M. I., Brenner, A., Yevzori, A., Messalem, R., Leroux, Y., & Abeliovich, A., (2000). Denitrification of groundwater: pilot-plant testing of cotton-packed bioreactor and post-microfiltration. Water Science and Technology, 42(1/2), 353–359. 44. Sun, S. P., & Chung, T. S., (2013). Outer-selective pressure-retarded osmosis hollow fiber membranes from vacuum-assisted interfacial

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CHAPTER 7

Water Purification With Reverse Osmosis

CONTENTS 7.1. Introduction .................................................................................... 152 7.2. Overview of Ro Application............................................................ 154 7.3. Principle of Reverse Osmosis (RO).................................................. 157 7.4. Ro Membrane Description .............................................................. 158 7.5. Ro Membrane Performance ............................................................ 159 7.6. Review of Ro Process and Its Applications ...................................... 162 References ............................................................................................. 181

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7.1. INTRODUCTION About 25% of the global population has no access to safe and clean potable water. Though freshwater is accessible in many parts of the world, most of these water sources polluted by natural means or through human activity. Along with human consumption, industries require clean water for machinery operation and product development. With the industrial growth and population boom, the demand for clean water is constantly increasing, and freshwater supplies are being polluted and sparse. Along with human migrations, the water pollution in current farming societies is mainly ascribable to anthropogenic causes, for example, the overutilization of endowed agrochemicals―synthetic chemical fertilizers, fungicides, pesticides, and herbicides. The use of these synthetic chemicals continues to pollute many of the valuable water resources globally. Furthermore, other parts where the groundwater polluted with arsenic, fluorides, and radioactive material present naturally in the soil. Even though the human body is capable to detoxify and discharge toxic chemicals, when the inherent natural capacity overdone, the kidneys or liver, or both body parts may fail (Wu et al., 2018; Zia et al., 2018). Succeeding constant consumption of contaminated water, when the situations are not in favor and the body’s thresholds are go beyond, depending on the kind of toxin and pollutants, cardiac, liver, brain, or renal catastrophe may occur. Therefore, safe, and clean water provided at a reasonable price is not just increasingly renowned, but similarly a human right and exceptionally important. Most of the domestic filters and techniques used for the water refinement remove only the particulate substance. The traditional approaches, comprising domestic water sieves and even some of the newer approaches like ultra-filtration, do not eradicate most of the toxic chemicals or heavy metals from water that can hurt humans. The latter is accomplished by the practice of ion-exchange methods and reverse osmosis (RO) technology. Well-designed RO methods get rid of > 95% of all potential toxic impurities in a one-step method. This review describes the RO method in simple terms and sums up the effectiveness of this technology in particular situations in unindustrialized countries (Wahlqvist et al., 2009; Scholz and Grabowiecki, 2009). Water is a conventional chemical substance critical for the existence of nearly all known living organisms. Water holds 71% of the earth’s surface, but 97% of this water is present as saltwater in the oceans. Of all surface water, icecaps, and glaciers hold about 2%, and lakes and freshwater rivers have just 1%. Nevertheless, several societies around the world do not give

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attention and devotion to maintaining this vital product that is in limited supply (Wahlqvist, 2009a, b; Rajapakse et al., 2016). Nearly 2 billion people in the world, (around 25% of the world’s population) do not have an approach to safe potable water (Wahlqvis and Kuo, 2009; Wimalawansa, 2013). So, water consumption associated deaths (extending from 5 to 7 million deaths per year) are perhaps the biggest single source of deaths in the world. It is expected that in 2020, at the present rate, 75 million people will pass away each year of avoidable water associated deaths (Gleick, 2002; Boonyabancha, 2004; Rathnamalala et al., 2011). Most of these deaths are affected by secondary diarrhea and infectious diseases (Hunter et al., 2001). Nevertheless, a great number of deaths happen secondary to consuming non-pathogen water impurities (Fleisher, 1990; Wimalawansa, 2014). Managements in many countries carry on to abandonment the most susceptible people who do not have an easy approach to clean water. This produced, however in part, by the lack of suitable resources, lack of importance, and/or disrespect for the difficulty of people who do not have a voice, and the deficiency of potable water and sanitary services (Casola, 1975; Gabriel, 1975; Abu-Halaweh et al., 2005). To tie this need, several helpful organizations have stepped in to offer this crucial life-saving service. In the past twenty years, some techniques were established to convert polluted water and salty water to clean drinking water (Baumgarten et al., 2009; Sarkar et al., 2011). This chapter discusses one such main technology, which established in the early 1970s at the University of California, Berkley, and is important for many countries: specifically, the RO process (Klumb, 1975; Van De Poll and Harmsen, 1981). Since its development, this technique has been applied in a variety of applications, as well as in hospitals and the food and pharmaceutical productions (Nielsen et al., 1974; Alečković et al., 2010). By cleaning a finer particle size, RO systems eliminate much reduced dissolved particles than do any carbon filters or ultra-filtration. Not like the latter two, the RO systems eliminate heavy metals, like cadmium, lead, arsenic, and copper, and unstable organic compounds, nitrates, sodium, phosphate, cysts, fluoride, total dissolved solids (TDS), petrochemical, and agrochemical pollutants, and pharmaceutical impurities in a one-step procedure (Lozier and Ortega, 2010; Leo et al., 2013). So, RO technology is a vital solution for producing safe drinking water. Furthermore, the RO

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process also takes out salinity (i.e. brackishness; ionicity) and several biological and microbial pollutants (Weeraratne and Wimalawansa, 2015; Wimalawansa and Wimalawansa, 2016). The exclusion of components that are not dangerous to health, for example, color, hardness, taste, odor, and the smell is elective but generally combined as a part of the RO process. In the past few years, different water management technologies have developed that supply to particular purposes, for example, the bio-filters and activated carbon (AC), which are often fitted to water taps. Nevertheless, such filters take away just components that adsorbed by carbon and are incapable to remove fluoride and heavy metals efficiently (Rahaman et al., 1979; Victora et al., 2000). However, eliminating chemical pollutants remains a tough problem. Particular defluoridation filters have planned based on either resins or activated alumina. These can be used in applications where fluoride is the single water contaminant that origins health problems, like skeletal and dental fluorosis. Due to the very slight pore sizes in the membranes used in RO, the technique also removes biological impurities without needing any extra time or costs. Though the RO method disables all these problems, possibly high start-up costs, handling of wastewater, the need for regular back-flushing and requirement of electricity and/or replacement of membranes and filters remain hurdles to this technology (Joyce et al., 2001; Kaufman et al., 2010). RO can clean chemically polluted water, salty water, or seawater, eliminating minerals, toxins, chemicals, and undissolved and dissolved substances. In places where there is no centrally cleaned pipe-borne water source or natural disasters and after the flood with water pollution, RO units can offer safe, drinking water to societies and can be used for industrial needs (Jeong et al., 2007; Bereschenko et al., 2008). Skid-mounted moveable RO systems are perfect for emergencies, like following floods, tsunamis, and earthquakes to offer potable water to affected societies. Furthermore, many industries promoted by recycling wastewater by using RO plants in the production method (Spahn and Davin, 1983; Chakraborti et al., 2015).

7.2. OVERVIEW OF RO APPLICATION RO is a separation method that is appropriate for a comprehensive range of applications, particularly when salt and/or liquefied solids need to be removed from a solution. Therefore, RO can be used for seawater and

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saltwater desalination, give both glasses of water for industrial use, and drinking water. It can also be useful for the generation of ultrapure water (e.g. pharmaceutical, semiconductor industries) and boiler feed water. Furthermore, RO membrane systems are used for water reuse and wastewater treatments. RO is now considered to be the most effective and economic process for water purification. Therefore, it is often a suitable method to treat solutions having salt concentrations from 100 to over 50,000 milligrams per liter. Solutions with salinity make surface water to seawater, and even seawaters can be preserved by the RO membrane (Bhattacharya, 2001). Crossflow is the configuration useful for membrane separation by using an RO membrane. As shown in Figure 7.1, the feed water stream moves indirectly to the membrane surface. A portion of the water in this feed stream passes over the membrane, while the majority of the feed flow travels alongside the surface. Therefore, two streams are composed: 1. 2.

Pervade, nearly pure water having a low concentration of ions; and Concentrate, containing a high concentration of small elements and dissolved ions.

Figure 7.1. Schematic flow of RO membrane. Source: https://puretecwater.com/reverse-osmosis/what-is-reverse-osmosis.

In operation, the RO membrane system is constantly provided with feed water which gives a continuous water drive from feed to concentrate. When in the cross-flow process, there is little growth of the prohibited solutes and scaling or fouling can be decreased. Users may already know RO from common under-sink home water cure systems, but RO is applied in various applications and at several scales. RO can filter use less water to the point where it’s almost free of esthetic issues and health risks (McCoy et al., 2008). To know how RO works, a reference of osmosis is in order. Osmosis is the method through which

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liquids move over a semi-permeable membrane through molecular kinetic energy. Particularly, it is the technique through which cells transport water in biological structures. In RO, though, the pressure is enforced to liquid to move it over a semipermeable membrane. Due to their small size, water molecules tend to move easily over an RO membrane, whereas contaminants and minerals with greater molecules are stopped. Membranes are made of a range of materials, particularly polymers and ceramic. RO is applied for a host of applications like hemodialysis, ice-making, CIP water recovery, seawater desalination, in manufacturing, and in the recycling of sewage water (a current survey of water industry specialists regarded RO maximum among technologies pliable to water recycling. RO is normally used as one level of treatment in the company of other treatment approaches, including DAF (dissolved air flotation), ultrafiltration (UF), disc filtration, advanced oxidation, and ultraviolet (UV) treatment processes. Treatment systems with RO (R component may require important customization to modify them to particular supplies and to meet necessary standards (Dziuban et al., 2006; Liang et al., 2006). The four kinds of membranes applied in RO are low-pressure, highrejection, fouling-resistant, and heat-sanitized. Polluting is a persistent problem in RO. Scale can form on membranes, so it’s conventional practice to condition flush membranes and feed water to raise membrane life and evade degradation, clogging, or covering.

7.2.1. Addressing Water Scarcity Two of RO’s most vital uses in the era of growing universal water stress are for water recycle and the purification of salty water or seawater. New developments in artificial membranes have improved the efficiency of RO purification, and decentralized treatment approaches have extended access. Now RO purification stands poised to take freshwater to dry areas wide-reaching, stabilizing economies and nations, and building deserts bloom.

7.2.2. Industrial Applications Ultrapure and high quality water is essential in a number of uses in the beverage and food, microelectronics, and pharmaceutical industries, along with in power generation. For example, the Ataqa power plant present

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in Egypt is presently using seawater demineralization for process water production, and in Israel, Fluence has produced a multistage water cure system to give ultrapure water and cooling system water for the cutting-edge Ashalim solar plant’s condensation boiler.

7.2.3. Containerized Treatment The containerization of flexible RO purification and water recycle systems has been taking treatment to places where it wasn’t before presented. One example is Fluence’s NIROBOX™ SW, a seawater purification unit contained in a normal shipping container that can yield 1,500 m3/d of freshwater from the seawater. Progressive technology is engineered into a small footprint for plug-and-play installation and easy transport—even in isolated areas—with a slight environmental disorder. Another decent example is the completely automated and remotely supervised Nirobox system at the Reserva Conchal route in Costa Rica. The drought-stricken route desired a way to guarantee a freshwater supply deprived of disrupting the area’s natural beauty.

7.3. PRINCIPLE OF REVERSE OSMOSIS (RO) Osmosis is a usual phenomenon that can be explained as the movement of clean water over a semi-permeable membrane from a low to a high concentration solution (see Figure 7.2). The membrane is porous to water and some the ions but discards nearly all ions and dissolved solids. This process (movement of water) happens until the osmotic equilibrium is accomplished, or until the chemical potential is equivalent on both sides of the membrane. A difference of height is detected between both sections when the chemical potential is stable. The change in height expresses the osmotic pressure alteration between the two solutions (Park et al., 2011). RO is a process that arises when pressure, the osmotic pressure, is useful to the concentrated solution. Water is enforced to flow from the concentrated to the dilute side, and solutes are retained by the membrane (Figure 7.2).

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Figure 7.2. Principle osmosis and of reverse osmosis (RO). Source: http://lpt.lanxess.com/uploads/tx_lxsmatrix/01_lewabrane_manual_ ro_theory.pdf.

7.4. RO MEMBRANE DESCRIPTION RO membranes can be delivered in both flat sheet and hollow fine fiber (HFF) structural layouts. The flat sheet RO membrane is composed of three layers. As shown in Figure 7.3, there is a polysulfone layer, a nonwoven polyester support layer, and on top of the polyamide obstacle layer. The obstacle layer is made by the polyamide of which the molecular structure is shown in Figure 7.3.

Figure 7.3. Structure of RO membrane. Source: http://lpt.lanxess.com/uploads/tx_lxsmatrix/01_lewabrane_manual_ ro_theory.pdf.

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7.5. RO MEMBRANE PERFORMANCE The performance of a RO membrane is explained by numerous parameters. The significant parameters are defined below (Bonetta et al., 2011).

7.5.1. Flow Rate In RO device, there are three main streams. The feed stream is parted by RO membrane into permeate and concentrate streams. Movement of these streams is generally expressed in m³/h (cubic meters per hour) or gpm (gallons per minute). The feed flow rate is explained as the amount of water entering the RO system. Permeate flow rate is explained as the amount of water passing over the RO membrane, and concentrate flow rate is explained as the amount of flow which has not passed over the RO membrane and comes out from the RO system with rejected ions.

7.5.2. Permeate Flux Permeate flux explains the amount of permeate formed during membrane separation per unit of time and RO membrane area. The flux is measured in Lmh (liters per square meters per hour) or gfd (gallons per square feet per day).The flux is explained by:

where: S = area of the membrane, JV = permeate flux, Qp = permeate flow rate.

7.5.3. Salt Rejection Salt rejection is a percentage that defines the extent of solute retained by the RO membrane. The retaining is given by: where: Cp, permeate concentration, R, rejection Cc, concentrate concentration, Cf, feed concentration. The cave is average feed concentration, which is calculated as follows: Cave = (Cf + Cc)

(3)

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7.5.4. Recovery Rate The recovery rate is explained as the portion of the feed flow which passes over the membrane. It is generally expressed in percentage. (4) where: Y= recovery rate, Qf = feed flow rate Qp= permeate flow rate,

7.5.5. Differential Pressure (Pressure Drop) The pressure drop is the difference between the concentrate and feeds pressure during water flow through one or more RO membrane elements. The pressure drop (dp) is explained as: dp=Pf −Pc

(5)

where: Pc = concentrate pressure, Pf = feed pressure.

7.5.6. Transmembrane Pressure (TMP) Transmembrane pressure (ΔP or TMP) is described as the change in pressure between the permeate side and the feed side of the membrane. This pressure is generally measured in psi or bar and is the driving force for permeate production and membrane separation. Generally, an increase in the TMP surges the flux through the membrane. The TMP is defined by:

where: Pp, permeate pressure, Pc, concentrate pressure, Pf, feed pressure.

7.5.7. The Tendency of RO Performance The central parameters of RO membrane unit are usual permeate flux which describes the salt rejection and the permeate production which defines the worth of permeate. These two parameters can be subjective by the operating parameters for example feed concentration, feed pressure, temperature. These aspects influence the RO membrane system performance and are sum up in the following Table 7.1. When considering variations of feed concentration, the salt rejection rises with the rise of the concentration first. Getting the maximum value of rejection in the normal feed salinity range of 300–500 mg/l. After that, the salt rejection drops as the feed

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concentration rises. The above discussion inconsistency of salt rejection is shown schematically in Figure 7.4.

Figure 7.4. Feed concentration effect on RO membrane rejection. Source: http://lpt.lanxess.com/uploads/tx_lxsmatrix/01_lewabrane_manual_ ro_theory.pdf.

The effect of the pH of the feed solution on the rejection of solutes is very difficult since the pH fluctuation disturbs the charge of the membrane surface along with the state of the solutes and dissociation rate. Nevertheless, the following trends of rejection can be shown in Table 7.2. Table 7.1. Operating Conditions and Effect on RO System Performance Increase of Conditions

Tendency Flux

Rejection

Reason for Membrane Performances Change

Temperature

Permeate flux rises with temperature (3%/°C) mostly due to a decrease in water viscosity. Solute permeation rate rises with temperature more than permeating flux.

Feed Concentration

Net driving pressure declines by osmotic pressure. At lower salinity (ex. < 400 mg/l), the salt rejection reduces because of the charged effect of the RO membrane.

Concentrate Flow Rate

At a low flow rate, concentration polarization happens, therefore, the concentration at the membrane surface becomes greater, and osmotic pressure increases

Feed Pressure

Permeate flux is proportional to net driving pressure. Solute permeation rate does not rise with pressure. Therefore, salt rejection and the flux increase.

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Table 7.2. Feed pH Effect on RO Membrane Rejection Chemicals

pH Range pH (Acidity) Range (Alkalinity)

Reason for Membrane Rejection Change

Basic Com- High pounds

Low

The dissociation of alkaline compounds at acidic pH improves the rejection due to the charge repulsion occurring between membrane surface and compounds.

Acidic Low Compounds

High

The dissociation of acids at alkaline pH improves the rejection due to the charge repulsion happening between the membrane surface and compounds.

Boron

Low

High

A rise of pH adjusts the ionization of boron from boric to borate, thus increasing the rejection.

SiO2

Low

High

An increase of pH adjusts the ionization of silica from silica acid to silicate, thus increasing the rejection.

7.6. REVIEW OF RO PROCESS AND ITS APPLICATIONS Freshwater is not just a right of people but also the main requirement to have healthier lives. Most countries have enacted environmental safety laws that comprise maintaining water resources. Nevertheless, execution levels of these laws are greatly inconstant, and obedience often is poor (Spahn and Davin, 1983; Tu et al., 2013). Mainly essential is the prevention of biological and industrial waste-disposal, pollution, and pollution of water sources and air pollution (Glater, 1998; Vrouwenvelder et al., 2010). Nevertheless, not all impurities are anthropogenic or only man-made. Global warming has also harmed environmental pollution. Environmental pollution is an unintentional product of anthropogenic causes and enhanced by human activities (Ekanem and Achinewhu, 2006). However, there are also natural phenomena. Collected these increase the climate-change brought cyclones, typhoons, hurricanes, floods, and droughts, all of which lead to important groundwater pollution; these actions are becoming more common and are key, but frequently forgotten, sources of water pollution (Fujioka et al., 2013; Ducos and Tabugo, 2015).

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7.6.1. The Gravity of Consuming Contaminated Water Each year, many million people die as they consumed polluted water. The massive majority of these deaths happen in inferior and agricultural societies in economically deprived countries (Elimelech and Phillip, 2011; Gao et al., 2011). Though large amounts of these deaths are attributable to microbial infection, leading to situations like dysentery, a growing number of people die after consuming pollutants and chemicals-contaminated water (Greenlee et al., 2009; Schroeder, 2015). In most cases, the reasons for these deaths are not well clear, so they are not ascribed to water “poisoning”; so are underestimated. Mainly, this is because there is neither the knowledge nor the equipment presented to make the right diagnosis of the cause of death in many parts of the world (Kelkar et al., 2003). Nearly 60% of the population in economically poor and emerging economies countries continues to rely on wells, rivers, reservoirs, and natural streams for regular water requirements. Alternatively, nearly all city dwellers get centrally cleaned pipe-borne water supplies; which they have occupied for granted. Furthermore, the quality of drinking water in municipal areas secure through programs to ensure potable water is safe and free from harmful toxins, chemicals, and pathogenic microorganisms (Humphries and Wood, 2004). Though, no such programs occur in distant villages, where almost 65% to 70% of the population lives in economically, and developing disadvantaged countries. The majority of them do not have the liberty to a pipe-borne water supply.

7.6.2. Options for Generating Clean Water Whereas the economically well-to-do people and those who reside in and around cities providing with fresh water through the pipe-borne water systems, the majority of villagers, mainly people in the low- to middleincome constituencies, depending on their sources for water supplies. So, their health can extremely affect, dependent on the cleanliness of the water they consume. This is mainly important in agricultural societies. Table 7.3 illustrates the most frequently used methods for water purification (PérezGonzález et al., 2012; Wang and Karnik, 2012).

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Table 7.3. Commonly Used Water Purification Methods Process

Method Use

Economical and most usually used methods

Elimination of particles, suspended solids, grit Odor control and slush sedimentation Chlorination and Filtration

Chemical and mechanical methods

Coagulation and Aeration Filtration and Flocculation Disinfection Carbon adsorption

Expensive but effective methods

Ion-exchange, Distillation methods Electro-dialysis, reverse osmosis (RO)

Most of the filtration systems applied in developing countries centered on simple mechanical filtration methods (Table 7.3). These remove particulate matter by a mechanical method based on the physical extent. These methods might remove some greater metals and inorganic matter that are in the particulate forms, but not liquefied in the water. Some filters have an extra AC component, which adsorbs some elements to the surface of the carbon. Nevertheless, not like with absorption methods, adsorption relies on the existing surface area of the material; and therefore the capacity is inadequate (Chung et al., 2012). The three most conventional heavy metal impurities that causing ill health, lead, cadmium, and arsenic in water are in the dissolved form and therefore normally cannot be removed by these filtration techniques. Since the mechanism of impurity removal in AC filters is through adsorption, instead of absorption, the ability is insignificant and these filters saturate quickly; therefore, the capacity lasts just a few days, in spite of claims by manufacturers. Furthermore, these filters will not eliminate considerable amounts of fluoride or heavy metals from water (Drewes et al., 2003). Costly options are the usage of bottled water, everyday transportation of water to villagers through water transporters/bowsers, delivery of water filters to distinct households, and the fixing of wells, containing deep tube wells. In the case of water pollution following environmental disasters and floods, it is likely to use purification tablets, chemical flocculation methods, and emergency moveable, skid-mounted RO systems; all these can conventional

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quickly. Though, field experience in emerging countries. With our own experiences proposes that not only are the generally used filters incompetent in removing impurities, but the use of these filters also is inadequate. If a freshwater supply is accessible upstream, it is more inexpensive to tap that source. Because the present and normally used systems are not working, new out-of-the-box, methods are warranted (Quanrud et al., 2003).

7.6.3. Understanding Osmotic Pressure Numerous methods exist for measuring osmotic pressure. It is measured from the lowering of the vapor pressure of a solution, by the equivalent of the ideal gas law equation, or by depression of the freezing point. Furthermore, numerous commercially existing devices can measure osmotic pressure straight. An additional way to determine the osmotic pressure of a solution is to measure the water flux over a module under working conditions at many pressures. If a plot of water flux versus pressure generalized to zero water flux, the intercept would be the osmotic pressure. This provides the operative osmotic pressure, containing concentration polarization. This ultimately measures the pressure that is essential to stop the flow of water through a membrane (Crittenden and Harza, 2005; Leo et al., 2013). Direct osmotic pressure dimension in a solution by working at a pressure just enough to get zero flow is impractical since the membranes are not flawless semipermeable membranes. This method would measure the alteration in osmotic pressure among the output water and the feed-water. At low pressures, not just is the salt rejection reduced, but the measured osmotic pressure too could be inferior to the real value. The osmotic pressure of a solution rises with the concentration of a solution. Generally, this is defined as centered on sodium chloride. The osmotic pressure rises by nearly 0.01 psi for each milligram of solvent/ liter. Even though this is a good calculation for most water impurities, pollutants with great molecular weight and organic impurities may produce a comparatively lower osmotic pressure. For example, in association with NaCl, sucrose yields a significance of nearly 0.001 psi, a tenfold less for each milligram/liter. Generally, the osmotic pressure of a water supply that needs demineralization is 10 psi per 1,000 mg/L (ppm) of TDS.

7.6.4. Definitions of Reverse Osmosis (RO) Purification 1.

Osmosis: It is described as the spontaneous passage or impassive diffusion of water or a solvent over a semipermeable membrane

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because of osmotic pressure. Liquid passages from a dilute to a more concentrated solution through a semi-permeable membrane (Figure 7.5).

Figure 7.5. Basic mechanisms of how (A) osmosis and (B) reverse osmosis work. Source: https://pdfs.semanticscholar.org/1477/7e52b5efa8ff94e529051b24fad 15808df6a.pdf.

Through osmosis, without applying pressure through a membrane, a lower-concentration solution or water molecules will “filter” or gravitate to the greater concentration solution, therefore diluting the latter till equilibrium is reached. The movement of the solvent decreases the free energy of the system by aligning solute concentrations on both sides of the membrane and producing the same osmotic pressure. The transference of liquid from one side of the membrane to the other remains up until the head or pressure is great enough to avoid any net transfer of the solvent (e.g. water) to the extra concentrated solution (Figure 7.5). Reliant on the size of the pores in the membrane, it stops the route of dissolved solutes and particulate matter to the contrasting side of the membrane. At this balance, the quantity of water or the solvent passing in any direction is alike, and the osmotic pressure of the solution on any side of the membrane is equal (Blais and Cooper, 1980).

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The osmosis flow is upturned in the RO process. By put on hydraulic pressure to the great concentration side of the solution, it moves solvents to filter over the membrane, against a pressure gradient to the lower-concentrate solution. In RO, by using a mechanical pump, the pressure is applied to a solution through one side of the semi-permeable membrane to come over inherent osmotic pressure: a thermodynamic parameter. The process also eliminates particulate and matter soluble, containing salt from seawater in purification from the solution of interest (Bereschenko et al., 2008). While pressure applied on the concerted side of the semi-permeable membrane outside the osmotic pressure of the solution, the solvent initiates to flow to the less concentrated side (Figure 7.5). Solvent from the concerted solution (water) passes over the membrane to the solution of lower concentration; therefore, the concentration of solute is greater inside where the pressure is applied. Most frequently, RO well-known for its use in potable water purification from seawater and producing freshwater from salty water, and practice in the milk processing and pharmaceutical industry. RO can eliminate many kinds of ions and molecules from solutions; therefore, it is used equally in industrial processes and the production of drinking water. The product is that the solute reserved on the pressurized side of the pure solvent and membrane, which in many cases is water, forced over the membranes to the new site, where it is collected. RO is applied in various applications, containing wastewater treatment, recycling, food, and beverage processing, and the production of energy. Numerous technologies and processes combine the usage of RO treatment plants. RO is one of the uncommon operative ways to remove volatile organic compounds (VOCs), minerals, fluoride, and other chemical impurities from potable water supplies (Sauvet-Goichon, 2007).

7.6.5. Mechanism of Purification by Reverse Osmosis (RO) The RO is slightly similar to other membrane technology applications, like ultra-filtration, but there are alterations among RO and other filtration. The elimination mechanism of filtration is damaging or size exclusion, and pore sizes are greater than with RO membranes. The ultra-filtration method, however, in theory, give good exclusion of particles, irrespective of the operative variability, containing solute concentrations and pressure. Nevertheless, since the pore sizes are greater, all heavy metals, inorganic components, and microbial agents pass over the ultra-filtration method (Zhang and He, 2015) (Figure 7.6).

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Figure 7.6. Basic components of reverse osmosis. Source: https://pdfs.semanticscholar.org/1477/7e52b5efa8ff94e529051b24fad 15808df6a.pdf.

Since RO relies on a diffusive mechanism, separation efficiency differs based on solute concentration (TDS), water temperature, and pressure applied (Kang and Cao, 2012). High-pressure pumps in RO systems force water over the pores of the membranes (permeate), and the residual water with greater concentrations of solutes is forced out as wastewater (brine) (Figure 7.7). Basic components of a RO system are explained in Figure 7.6.

Figure 7.7. Schematic representation of RO systems.

Source: https://pdfs.semanticscholar.org/1477/7e52b5efa8ff94e529051b24fad 15808df6a.pdf.

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Moreover, to toxins and agrochemicals, one of the crucial benefits of RO is its capability to eliminate salinity, fluoride, and heavy metals from water, while most other methods, containing activated-charcoal sieves and even ultra-filtration-based technologies, flop to eliminate these ions. In greater RO units, when the high-pressure water opening coupled with a motor or a turbine, it can reprocess some of this else wasted energy to run the permeate pumps, pressure pumps, or other electrical applications. Mechanistic constituents and flow cycle of a common RO system explained in Figure 7.6. The curved membranes are made from one or more membrane envelopes coiled around a pierced central tube. The permeate passes from the membrane into the envelope and helixes inward to the central tube for gathering. Table 7.4 specifies the average best refinement efficiencies of several inorganic water impurities by optimum RO units. Entire RO units’ work in the same method. Several have the same basic components, but the main difference is the value of the membranes and filters inside the unit (Table 7.5). These define the quality of the production water, operational cost, capital costs, and durability (Soderland et al., 2010; Nanayakkara et al., 2012). Table 7.4. Average Purification Efficiency of RO Membranes* Component

Efficiency %

Component

Efficiency %

Sodium

94%

Lead

93%

Sulfate

94%

Arsenic

95%

Calcium

97%

Magnesium

96%

Potassium

93%

Nickel

95%

Nitrate

90%

Fluoride

95%

Iron

95%

Manganese

96%

Zinc

95%

Cadmium

95%

Mercury

94%

Barium

95%

Selenium

94%

Cyanide

92%

Phosphate

95%

Chloride

92%

Agrochemicals

98%

Petrochemicals

95%

Organic compound

98%

Particulate matt

99%

*Percentages may differ based on the pore size, membrane type, and water quality, temperature, pressure, and TDS. *Data are averaged from numerous sources.

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The constancy and quality of the membranes is the main factor that affects the performance and durability excellence of any RO unit. Other features that affect performance are the pressure of the water cove, the concentration of the solutes, water temperature, and density of the particulate matter, and the TDS in the water. Table 7.5. Common Basic Components Used in RO Systems Components

Mechanics and Detail

Pre-filters

Generally, the inlet water source enters the RO system through the pre-filter. Relying on the TSD and the of inlet quality water, some units use a sequence of pre-filters to eliminate particles along with oxidative components, for example, chlorine, that potentially harm RO membranes. The most frequently used pre-filters are remainder filters (multi-media filters) used to remove sand, dirt, silt, particulate, and other residue material. Charcoal filters are used to eliminate oxidizing compounds, for example, chlorine, to protect the membranes, mainly thin-film material (TFM) and thin-film composite (TFC) membranes. Carbon pre-filters are not normally used when the system uses cellulose tri-acetate (CTA) membranes, but most companies use the TFC/TFM filters.

Inlet water line valve

The valve that fixed on the inlet water streamline to control the water source inflowing the RO system or the pre-filtration device.

Pressure pumps

High-pressure pumps and switch valves that control the flowthrough system and produce filtration pressure for RO.

RO membranes

The RO membrane is crucial to the system. The most frequently used membranes are spiral wound. The CTA is comparatively chlorine tolerant, while the TFM and TFC membranes are not.

Post-filters

Among the RO unit, the storing tank, and the clean water outlet, water flows over one or more post-filters to seizure any undesirable matter. These post-filters comprise of activated carbon in either carbon block or granular form. These allow any extra impurities to get adsorbed, containing organic components and any other material that may have avoided the RO membranes. They also remove irregular taste or odor in the effluent water.

Check valve

A check valve is positioned at the outlet end of the membrane housing. It avoids the regressive flow of freshwater from the storing tank to the unit and prevents harmful membranes.

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Automatic shutoff/floater valve

To preserve water, the RO systems have built-in automatic shut-off valves (a floater). When the storing tank is filled, the valve shuts off the water from arriving at the membrane section. This avoids freshwater production, discharges the conserves water and pump pressure, Once water out from the tank, the pressure in the tank drops, and the shut-off/floater valves open, re-establishing the water stream to the membrane.

Flow restrictor

Water flow over the membrane is controlled by flow control, which is placed in the RO drain line. These flow control devices preserve the flow rate essential to gain high-quality drinking water, in part based on the quality and the capability of the membrane. They also aid retain pressure on the inlet side of the membrane. Flow restrictors are essential to retain the pressure within the membrane chamber permitting RO to take place. They also avoid incoming water taking the path of minimum resistance, flowing down the drain line.

Permeate pump

Pumps inserted among the flow restrictor and the RO module to retain the membrane pressure and produce power that else goes to waste from the permeate water stream.

Storage tank

The cleaned water from the RO membranes is focused on an overhead storing tank. The capability of storing tanks differs depending on the capability of the membranes, the water volume, and the pressure unit.

Faucet

The valve that controls the RO unit or the above tank outlet flow.

Drain line

The drain line goes through the outlet end of the RO membrane covering to the drain, comprising a higher concentration of impurities.

7.6.6. The Importance of the Quality of Membranes and Filters in an RO Plant High-pressure RO systems have used broadly since the mid-1970s for cleansing of seawater and salty to drinking water and to produce clean water for industrial, medical, and domestic applications. High-quality components inside the unit are significant for the quantity and the quality of clean water output (Senevirathna et al., 2012). While considering designing or obtaining an RO system, the questions to reflect include the quality of the materials and the kinds of influences used, containing the plastic products and influences, probability of leaks, interior pressure capacity and built-in detection systems for example for TDS and pressure, the quality, the quality of the material used, the membrane pore size, and durability, quality, and the ability of the multi-media sieves and the capability and the frequency required to back-plashing filters, the quality of the micron-filters and AC, tolerance, and accuracy of the specifications

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providing by the manufacturer for individually component, and potential for impurity or water avoiding the filtration system.

7.6.7. Mechanisms Involved in Reverse Osmosis (RO) The membranes used for RO have thick layers in the polymer matrix where the chemical isolation occurs (Kong et al., 2012). In many cases, the membrane is designed to permit merely water to pass from this dense layer with cut-off limit is nearly 200 Daltons, while avoiding the passage of solutes, like salt ions, organic molecules, and heavy metals. Applied pressure differs on the surface of the membrane, commonly among 2 and 17 bars (30–250 psi) for fresh and saltwater, and 40 and 82 bars (600–1200 psi) for seawater. The later has an osmotic pressure of 27 bars (390 psi). Several systems integrate UV lamps for disinfecting the water and killing the microbes that may leak filtering over the RO membrane. A flow chart of systematic constituents of a RO system is presented in Figure 7.8.

Figure 7.8. (A) Preconditioning/pre-filters, reverse osmosis membranes, and post-treatment disinfection system of reverse osmosis. (B) Filtration components and key steps involved in the reverse osmosis process. Source: https://pdfs.semanticscholar.org/1477/7e52b5efa8ff94e529051b24fad 15808df6a.pdf.

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7.6.8. Membrane Cleaning Process The percentage recovery of cleaned water relies on numerous factors; containing temperature, membrane pore size, membrane surface area, and operating pressure. One of the main problems with membranes is residue deposition, which affects the membranes. Thus, when the intake water has greater hard water or TDS, it is a condition to remove sediment either through anti-scale injection systems or by using water softeners. Recovery of freshwater based on numerous factors, like the TDS, water temperature, and the ability to produce consistent pressure on the RO membranes. With time, RO membrane elements practice a decline in performance because of the buildup of deposits on the membrane surfaces. Mineral scale, insoluble organic compounds, colloidal particles, and biological matter lead to membrane polluting. When the generation of RO system descents, by more than 10% or the differential pressure rise by around 15% over the typical functioning conditions, membrane cleaning should be performed (Madala et al., 2012). Water flows downward over the media while some solids expected to accumulate in the media bed. The refined water, permeate passes over to downstream processes. When the filter initiates to clog or when the pressure decrease through the bed increases, flow rates are drop. When the recovery of RO system reduced (i.e. wastewater percentage rises), effective impurity removal rates also tend to reduce; therefore, water TDS will carry on to increase, and membrane failure may happen (Kang and Cao, 2012). To avoid degradation of water features, at this point, the movement needs to be reversed. This can be done either by hand or semi-automatically directing over the controller valve to drain, carrying with it, the particulate matter that has made up during service. The essential flow is particular to the media and is crucial to proper scrubbing of the media bed. For media filters, the necessary backwash flow is constantly greater than the service flow rate. Filters need periodic backwashing to arrange of the accumulated remains. This is achieved by backwashing clean water over the unit and then disposing of the waste. During this procedure, the various sizes of media separate into layers, arranging the filter bed for service. Nevertheless, when operating smaller, double or triple unit systems, the optimal backwash flow rate is less; thus, these systems can be operated at higher service flow rates. Both caustic cleaning chemicals and acid used for the membrane cleaning procedure. Acid cleaners usually used at pH of around, which removes

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iron and inorganic deposits. Alkaline cleaners are used nearly about pH 12, which will eliminate organic foulants, biological matter, and silica deposits.

7.6.9. Membrane Pore Size and RO Unit Capacities RO membranes are made in two conventional configurations: hollow-fiber and spiral-wound. RO is considered as a “hyperfiltration” since it gets rid of particles greater than 0.1 nm. Membrane pore sizes can differ from 0.1 nm (3.9 × 10–9 inches) to 5,000 nm (0.00020 inches), dependent on the filter type. Generally, particle filtrations take away particles of 1 micrometer (3.9 × 10–5 inches) or larger than that. Microfiltration (MF) eliminates particles of 50 nm or greater. Ultrafiltration removes particles of approximately 3 nm or larger. Nanofiltration (NF) eliminates particles of 1 nm or greater. Specifics of different filtration methods and their molecular extents exclusions are indicated in Figure 7.9.

Figure 7.9. Detailed of various filtration methodologies and their cut-offs molecular size exclusions are illustrated. The figure indicates an example of different molecules and particles that excluded from each type of filtration system. Source: https://pdfs.semanticscholar.org/1477/7e52b5efa8ff94e529051b24fad 15808df6a.pdf.

7.6.10. Other Uses of Reverse Osmosis (RO) Systems In developed countries, military organizations and emergency services normally use RO water refinement units on the battleground and in training. The extents of these units range from 1,500 to 150,000 imperial gallons

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(6,800 to 680,000 L) per day, dependent on the need. The most conventional of these are the units with the capability of 1,000 and 3,000 gallons per hour, which are proficient in purifying salty and saltwater, and water polluted with chemical, radiological, biological, and nuclear agents. At usual operating variables, one of these units can yield 12,000 to 60,000 imperial gallons (55,000 to 270,000 L) of water per 24-hour time, with a necessary 4 hour maintenance window to check systems, elements, pressure pumps, and the generators. Therefore, a single unit can aid nearly 3,000 to 7,000 people. RO is also used in industry to take away minerals to avoid scaling from boiler water at power plants and clean wastes in salty groundwater. The procedure of RO is also used for the production of hospitals, deionized water, pharmaceutical production, and concentration of milk in dairy production (Stevens et al., 2011). RO systems also useful in the food industry. Furthermore, to desalination, RO is a more inexpensive technique for concentrating food liquids (such as fruit juices) than are common heat-treatment or lyophilization procedures. RO methodology widely used in the dairy industry for the making of concentrating milk and to whey protein powders reduces shipping costs. In whey applications, the whey, the liquid leftover after cheese manufacture, is concerned with RO from 6% total solids to 10% to 20% total solids formerly ultra-filtration treating. The ultra-filtration material used to generate several whey powders. Furthermore, the ultra-filtration of milk simplifies the concentration of lactose as of 5% total solids to 18% to 22% total solids; this distinctly decreases the drying and crystallization costs of the lactose and milk powder. Several aquariums also use RO systems to regulate salinity in the synthetic mixture of seawater that suits sea mammals and fish. Normal tap water often comprises excessive chlorine, chloramines, nitrites, copper, nitrates, silicates, phosphates, and other chemicals that are harmful to the sensitive organisms in a ridge environment. Meanwhile, pollution with phosphates and nitrogen-containing compounds can lead to extreme algae growth and increase the cost of conservation. An operative combination of both RO and deionization (RO/ DI) is the most conventional treatment process used in ridge aquariums. This technique is favored over the other refining processes due to its relatively low operating and capital costs. Though, when chlorine and chloramines are

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existing in the tap water, AC filtration is desired before the water is passed to the membrane tool.

7.6.10.1. Seawater Reverse Osmosis (RO) This is a high-pressure RO procedure used for purification that has commercially existed for the past four decades. This method does not need heating, and the energy necessity is around 3 kWh/m3, which is great in comparison to other sophisticated purification methods. Nonetheless, due to the high osmotic pressure because of NaCl, this process needs the generation of higher pressures, so comparatively higher amounts of electricity, like 0.1 to 1 kWh/m3, are necessary than are needed for the cleansing of salty and stream water. Consequently, based on this technique, instead of the 65% to 80% recovery achieved with salty water, only about 50% of the seawater input can recover as fresh drinkable water. Nevertheless, larger plants allow the production of the beneficial by-products salt and electricity.

7.6.10.2. Brackish Water Reverse Osmosis (RO) Salty water or briny water is water that has greater salinity than freshwater but much less than seawater. It may end from the involvement of seawater with fresh water, as in estuaries and lagoons, or it may happen in brackish fossil aquifers. This water may hold among 0.5 and 30 grams of salt per liter— frequently expressed as 0.5 to 30 parts per thousand (ppt). The percentage regaining of water from these structures differ with the system designs and salinity of the feed-water: normally 30% for small seawater systems, 50% for greater seawater systems, and as much as 80% for brackish water. The concentrate flow normally is only 3 bars (50 psi) lower than the feed pressure, so it still transfers much of the high-pressure pump input energy. The process of refinement for salty water is similar to that for purification of water, but the inlet water has much lower salt content than does seawater and therefore requires less pressure to enforce water through the membrane. Sources of such water contain river saline and estuaries – or other chemical-contaminated wells and watercourses. The procedure is alike to that of seawater RO but needs lower pressures and less energy than does purification. In these systems, as much as 80% of the water input can improve as pure water.

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7.6.11. Pre-Treatment Pre-treatment is vital when working with NF or RO membranes due to the nature of their spiral-wound design. The spiral-wound designs do not let back pulsating with water or air agitations to fresh the membrane surface and elimination of solids and adsorbed ions. Since accumulated material cannot be detached from the membrane surface systems, they are very vulnerable to fouling―loss of production capability (a reduction in the efficiency of the system). So, pre-treatment is an essential part of these two systems of water cleansing. Generally, the pre-treatment systems have numerous main components, as described here.

7.6.11.1. Size-Exclusion Screening of Solids Before water directed through the membranes, the solids in the inlet water essential to remove to avoid contaminating the membranes by microbial growth or fine particles. This also avoids a potential loss to high-pressure pump components. 1. 7.6.11.2. Cartridge Filtration String-wound polypropylene filters applied to remove particles of 1 to 5 µm diameter. 1.

Dosing: In some RO systems, oxidizing constituents, like chlorine, added to destroy bacteria, after that bisulfite dosing to eliminate chlorine, and by triggered carbon filters to remove oxidizing components, for example, chlorine, to avoid thin-film composite (TFC) membrane loss.

7.6.11.3. Pre-Filtration pH Adjustment Feed-water pH, rigidity (mainly, calcium carbonate), and alkalinity cause scaling of membranes and pipes, which distinctly decrease the productivity of an RO unit. Thus, RO systems use water treatment to reduce the hardness of water to avoid scaling, and by altering carbonate and phosphate to soluble chemical forms, to avoid interacting with calcium. Calculated quantities of antiscalants, acid, or softeners inserted into the intake water source to preserve carbonates in soluble carbonic acid form, therefore preventing its scale formation and precipitation within the system. The basic chemistry of these reactions: CO32– + H3O+ = HCO3– + H2O; HCO3– + H3O+ = H2CO3 + H2O

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Conversion of carbonate to carbonic acid avoids it from relating with calcium to form calcium carbonate, thus preventing scaling. Calcium carbonate scaling tendency is valued by using the Langelier saturation index. Adding more sulphuric acid to switch carbonate scales may end in scaling creation with calcium sulfate, strontium sulfate, or barium sulfate on the osmosis membranes.

7.6.11.4. Prefiltration Antiscalants The addition of scale inhibitors (also known as antiscalants) avoids the creation of all types of scales compared with acid, which can avoid only the formation of calcium phosphate and calcium carbonate scales. Antiscalant prevents not only phosphate and carbonate scales but also fluoride and sulfate scales, furthermore to dissolving colloids and metal oxides. The main advantage is that antiscalants can switch acid-soluble scales at a portion of the dosage essential to control the same scale by using sulphuric acid. Some of the small-scale purification RO units are located on seashores or in close vicinity to the seashore. These intake amenities are comparatively simple to build, and seawater essentials to pre-treat through filtration through the subsurface sand in the area of source water withdrawal; this is done in place of using comparatively expensive multi-media filters. By association with direct seawater, inlets using beach wells offer comparatively better quality in terms of solids (TDS), oil, salt, and grease, natural organic pollution, and aquatic microorganisms. Beach intakes may also produce source water of somewhat lower salinity, which needs less energy to purify.

7.6.12. Pressure Pump A high-pressure pump is essential to pressurize water to force over the membrane to stimulate the RO phenomenon. Distinctive pressures for salty water range from 225 to 375 psi (15.5 to 26 bars, or 1.6 to 2.6 MPa). Seawater/desalination pumps need three to four times’ greater pressures, ranging from 800 to 1,180 psi (55 to 81.5 bars or 6 to 8 MPa), thus needing a greater amount of energy. When an energy recovery method used (through energy recovery devices), as with the larger-scale RPO units, incomplete amounts of energy improved to work the high-pressure pump, therefore reducing the system’s overall extra energy necessity.

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7.6.13. Pressure Recovery Pump Effective energy recovery systems can decrease energy consumption by roughly 50%. High-pressure pump input energy improved over the effluent flow and directed into an energy retrieval device. Energy recovery devices can decrease the energy requirements and therefore the costs of RO. A reciprocating piston pump (or a turbine) via the pressurized concentrate flow is functional to one side of each piston to drive the membrane feedflow from the opposing side. Some systems also use an infuse pump, using the energy from the permeate water fluid from the membrane component. This simple energy recovery method associations the energy recovery and high-pressure pump in a single self-regulating unit. These approaches are used less regularly on smaller low-energy systems that use 3 kWh/m3 or less energy but are valuable components in decreasing the energy desires of larger systems. Devices that been used for energy recovery are described in Table 7.6. Table 7.6. Energy Recovery System Used in RO System Recovery Method

Description

Permeate pumps

These used among the RO membrane and the flow restrictors, seizing the energy from the outflow permeate water.

Turbocharger

A water turbine focused by the concentrate flow, openly connected to a centrifugal pump, which increases the high-pressure pump output pressure, decreasing the pressure required from the high-pressure pump and therefore its energy input.

Turbine or Pelton wheel

A water turbine focused by the pressurized concentrate flow, linked to the high-pressure pump drive shaft to offer input power. Positive transposition axial piston motors can use in place of turbines on minor systems.

Pressure exchanger

The pressurized concentrate flow fixed to a piston or a turbine openly to convert mechanical energy to electrical energy. A boost pump used to raise the pressure, usually in the range of 3 bars (50 psi), to feed the inlet water to the membrane. In common, this can decrease the load on the high-pressure pump by an expanse equal to the concentrate flow/ the waste, typically by about 60%. These are broadly used on greater low-energy RO systems that have 3 kWh/m3 or less energy depletion.

7.6.14. Re-Mineralization and pH Adjustment In some systems, the cleansed water is stabilized to defend downstream pipelines and storing tanks by adding caustic soda or lime to avoid the decay

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of pipes and concrete-lined surfaces. Lime is used to modify the pH of 6.8 and 8.0 to meet the drinkable water specifications in a given country but also for operative disinfection and corrosion control. Furthermore, demineralization with calcium may be essential to add the natural flavor and replace some of the minerals detached from the water by the RO process.

7.6.15. Disinfection Methods Though it is not crucial, most RO plants have post-treatment filtration or disinfection systems. Post-treatment comprises suitably preparing the water for circulation after filtration. Though RO is an effective obstacle to many odor, pathogens, and chemicals, post-treatment methods offer secondary protection against extra and potential concessions in membranes, instrument, and pipe pollution, or equipment letdowns. System failure can happen with the pollution of membranes, downstream system, or distribution failures, and throughout backwashing procedures. The two most conventional methods used are disinfection by using chlorination, or UV lamps, or chloramination (adding chlorine and ammonia) to defend alongside pathogens. Due to woven construction and pore size of the membrane, RO avoids harmful impurities and pathogens from entering into the clean waterfront of the system. Though, it also strips the good components, for example, taste, and healthful minerals, from the water. Therefore, it may be essential to re-mineralize the dematerialized clean water for human intake. Thus, bottled water companies add sodium chloride or calcium and/or potassium chloride to water to restore the original water taste. The Swiss Federal Institute of Aquatic Sciences and Technology has described a useful and cost-effective, solar water cleansing method for treating water to make it safe to drink in emerging countries. It includes using clear PET (chemically inert, food-grade packaging plastic) bottles occupied with water and placed in the sun for six hours. The ultraviolet A (UVA) rays in sunlight used to destroy pathogens like viruses, parasites, and bacteria. This process described working even at lower temperatures and in most liberties.

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CHAPTER 8

Membrane Bioreactor Technology for Treating Micropollutants

CONTENTS 8.1. Introduction .................................................................................... 190 8.2. Classification Of Micropollutants .................................................... 191 8.3. Water Treatment Technologies ......................................................... 193 8.4. Inorganic Micropollutants In Membrane Processes ......................... 197 8.5. Membrane Technologies And Their Limitations In Treating Inorganic Micropollutants ......................... 201 8.6. Potential Hybrid Process For Removal Of Inorganic Micropollutants ............................................................................ 210 8.7. Organic Micropollutants (OMPs) In Membrane Processes............... 217 8.8. Classification of Organic Micropollutants (OMPs)........................... 217 8.9. Membrane Technologies And Their Limitations In Treating Organic Micropollutants (OMPs).................................................. 222 8.10. Mechanisms For Removal Of Organic Micropollutants (OMPs)..... 224 8.11. Removal of Plant Care Products And Pesticides And Challenges Created By Them To Membrane Processes .................. 226 8.12. Removal of Chlorinated Solvents And Their Challenges To Membrane Processes ............................................................... 228 8.13. Removal of Phenol Derivatives And Their Challenges To Membrane Processes ............................................................... 229 8.14. Removal of Various Hydrocarbons And Their Challenges To Membrane Processes ............................................................... 231 8.15. Removal of Various Personal Care Products (PCPs) And Their Challenges To Membrane Processes .................................... 232 8.16. Hybrid Processes For Treatment For Organic Micropollutants (OMPs)......................................................................................... 233 References ............................................................................................. 241

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8.1. INTRODUCTION Water is the ultimate source of life which is considered as one of the most essential resources for the existence and development of humans and animals. Although, earth surface contains 71% water, freshwater resources which could be directly utilized by humans and animals, such as natural water lakes, groundwater, and river water, constitute approximately 0.03% of the entire water present on earth. Moreover, the quick growth of industries and escalating human activities has resulted in the production and release of many organic and inorganic micropollutants which are adversely affecting the freshwater resources and ecological environment. Various industrial processes and products such as fertilizers, tanneries, metal plating, mining, paper production, pesticides, and batteries are responsible for creating a significant amount of water pollution (Camargo and Alonso, 2006; Schaider et al., 2014). In recent times, the infiltration of different harmful substances in water bodies has resulted in a significant impact on the health of humans and animals. Among these harmful substances are rapidly emerging micropollutants (typically organic and pharmaceuticals) which are present in both industrial and household wastewater in immense concentrations. The concentrations of these micropollutants usually range from µg per liter to nanograms per liter. Some of these micropollutants called endocrine disrupters (EDs) are regarded as exogenous stimulators which interfere in the production, transportation, secretion, binding, and removal of natural hormones present in the body which are accountable for maintenance, development, reproduction, and behavior systems (Schug et al., 2016). The sources of these micropollutants include pharmaceuticals, steroid hormones, personal care products (PCPs), pesticides, industrial chemicals, and much other affluence. Various researchers have exploited the effect of these species present in water bodies. Harmful impacts of micropollutants include anomalies in endocrine systems, reduction in sperm count, cancer, hepatitis, endometriosis. (Gomes and Lester, 2003; Lopes et al., 2016). Moreover, micropollutants can react actively with various other substances and aggravate the adverse effects (Hamid and Eskicioglu, 2012; Luo et al., 2014). The existence of micropollutants in aqueous media has also been linked with the ability of microorganisms to develop resistance against antibiotics. However, low concentrations and the broad diversity of micropollutants have made the precise detection and analysis of micropollutants a challenge

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for wastewater treatment industries (Bila and Dezotti, 2003, 2007). Figure 8.1 illustrates the route of environmental pollution by micropollutants (Matamoros et al., 2008; Silva et al., 2017).

Figure 8.1. The route for contamination of the environment by micropollutants. Source: https://pdfs.semanticscholar.org/ee83/0122aaee6b1eabb46b564f7247 dda42e3cef.pdf.

8.2. CLASSIFICATION OF MICROPOLLUTANTS There are three major categories of micropollutants which include organic, inorganic, and pharmaceutical micropollutants. However, the third category of micropollutants called pharmaceutical micropollutants may contain compounds from organic, inorganic or both of these sources. Inorganic micropollutants consist of metallic species and nutrients. Heavy metal ions present in water are quite extremely difficult to biodegrade, and they possess the ability to penetrate the human body through water intake, causing a chain of irreparable physiological ailments. For example, mercury (Hg) ions can affect the central nervous system (CNS), resulting in headaches, gastroenteritis, and stomatitis (Tchounwou et al., 2003; Bolong et al., 2009). Lead (Pb) ions can result in an insufficient supply of oxygen and nutrients, causing brain tissue damage (Daniel et al., 2004). Excessive intake of lead ions by children in their development and growth stages may result in developmental delay, hearing impairment and loss of appetite. Cadmium

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ions have the ability to substitute calcium ions present in the bones and hinder the normal calcium accumulation in the bone which may lead to cartilage disease (Miyahara et al., 1984). The presence of excess arsenic amounts in the human body can impede the normal cellular metabolism which causes the cell lesions and resulting in organ damage. Organic micropollutants (OMPs) in aquatic environments such as hydrocarbons, pesticides, phenols, fertilizes, biphenyls, plasticizers, oils, greases, pharmaceuticals, fatty acids, and detergents are predominant derivatives of agriculture, paper industry, domestic sewage, and food items. These OMPs require a substantial amount of oxygen during the oxidative decomposition process, which results in the decline of dissolved oxygen (DO) amount in water, thereby threatening aquatic ecosystems (Lonnen et al., 2005; Göktaş and MacLeod, 2016). Therefore, the development of green wastewater treatment processes and efficient water treatment materials are essential for solving the problem of micropollutants in the water. Flowchart of micropollutants’ distribution is explained in Figure 8.2 (Matlok et al., 2002; Barbosa et al., 2016).

Figure 8.2. Flowchart of water contamination by micropollutants and their removal. Source: https://www.sciencedirect.com/science/article/abs/pii/ S0043135416301063.

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8.3. WATER TREATMENT TECHNOLOGIES The treatment technologies presently employed by water treatment plants (WTPs) and wastewater treatment plants (WWTPs) are typically not effective in eliminating these micropollutants. The persistent nature of these micropollutants can be attributed to the fact that these are extremely resistant to degradation and hard to separate. The problem of micropollutants is usually aggravated due to lack of monitoring for most of these substances, which are typically only existent in trace amounts (Rengaraj et al., 2001) Micropollutants present in the municipal wastewater are difficult to remove effectively by conventional wastewater treatments methods including activated sludge (AS), coagulation, and flotation (Carballa et al., 2005; Joss et al., 2006). Therefore, secondary effluents need to be further treated using an advanced treatment process, i.e. single membrane processes or hybrid membrane processes.

8.3.1. Removal of Micropollutants by Membrane Processes Although many studies have investigated the micropollutants’ removal RO or NF their removal, mechanisms still need to be understood well. The fundamental factors influencing the elimination of micropollutants using various membrane processes are illustrated below: 1.

Characteristics of water with respect to ionic strength, pH, and the concentration of organic compounds and hardness. 2. Physicochemical features of the solute with respect to acid disassociation constant (i.e. pKa), diffusion coefficient, molecular weight (MW), octanol-partition coefficient of water (Kow) and molecular diameter. 3. Membrane properties such as molecular weight cut-off (MWCO) pore size, roughness, zeta potential and contact angle (Bellona et al., 2004). Membrane processes are better than conventional treatment processes in terms of water treatment (Chong et al., 2010). However, these processes also have some limitations which, ultimately, affect the performance of the membranes. For example, membranes undergo fouling and scaling during the operation which restricts the membrane efficiency. Fouling is basically the accumulation of solid pollutants on the surface of the membrane resulting in blockage of pores (Achilli et al., 2009). Fouling results in lower permeation of the filtrate which ultimately results in high process cost due to the requirement of high pressure (to force the filtrate through the membrane).

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However, there is some membrane whose performance is not impacted by the fouling phenomenon and their efficiency remains constant or sometimes improved. For example, in the case of nanoporous membranes, the fouling doesn’t block the pores and the spacing between particles of the fouling layer/substance is higher than the pore size. In such cases, the performance of the membrane remains unaffected. On the other hand, there are some membrane systems that exhibit higher efficiency due to fouling. For example, the pore diameter of the membrane is reduced due to the presence of fouling substance which, in turn, blocks the entry of micropollutants (which were easily passing through the original membrane) (Zularisam et al., 2010). From the above discussion, it can be inferred that fouling may be beneficial for some membranes and detrimental for others which depends on the variety of micropollutants and the membrane. Nghiem and Espendiller (2014) showed that the effect of membrane fouling on the retention and rejection of solute by using nanofiltration (NF) membrane (NF-90 with small pore size) was negligible. However, a significant effect of membrane fouling on solute rejection (for both pharmaceutical residues and inorganic salts) was observed with the loose NF membrane (NF-270) having a larger pore size (Gutman and Herzberg, 2013). These effects can be ascribed to a phenomenon called cake-enhanced concentration polarization. This phenomenon mainly occurs due to the low back‐diffusion of salt and various other species through the fouling layer which is mainly because of the presence of colloids or bacteria in the feed water (Aboagye, 2014). It is important to note that such effects are highly foulant-dependent. Before the discharge of domesticated municipal wastewater to lakes, and rivers, proper removal of nutrients (i.e. phosphorus, and nitrogen) is also required as phosphorus and nitrogen can strongly instigate the eutrophication (Renou et al., 2008). However, solo-membrane processes are not suitable for efficient removal of inorganic micropollutants, i.e. phosphorus, and nitrogen. Therefore, MBR and CBWT processes can be coupled with some suitable membrane processes (e.g. aerobic, anaerobic, NF, reverse osmosis (RO)) to augment the removal of nutrients (Al-Rifai et al., 2011). It is indispensable to cultivate new technologies for the elimination of micropollutants to inhibit their bio-accumulation and the consequential exacerbation of the harmful effects on animal and human health. Recently, MBR based processes coupled with some advanced membrane processes are being exploited as efficient technologies for the exclusion of micropollutants. These combined processes are known as hybrid membrane processes that exhibit a strong potential for treating micropollutants with

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minimized drawbacks. A suitable combination of techniques is selected for the treatment of specific micropollutants which ultimately results in higher solute rejection efficiencies (Boleda et al., 2011; Castillo et al., 2013).

8.3.2. Membrane Bioreactor (MBR) Technologies In recent years, the treatment process based on membrane bioreactor (MBR) has gained significant attention for treating wastewater due to high-quality effluents and low sludge production. The MBR process has substantially replaced the conventional biological wastewater treatments (CBWT), which are composed of an AS coupled with secondary sedimentation. As compared to CBWT, the MBR treatment is effective for the elimination of the mainstream of water pollutants, including organic substances, nutrients, and pharmaceutical residues. It has been reported by Clara et al. (2005) that the MBR based process efficiently removed some pharmaceutical and endocrine-disrupting micropollutants such as bezafibrate, tonalite, ibuprofen, bisphenol-A, and galaxolide (can be completely removed via biological processes, i.e. >95%). Though, non-biodegradable pharmaceutical products (e.g. carbamazepine, and diclofenac) and endocrine disruptors (e.g. nonylphenoxyethoxyacetic acid and nonyl phenoxy acetic acid) pose a challenge for MBR processes. Therefore, coupled membrane and biological processes seem to be a more viable solution for the removal of these micropollutants (Karim and Mark, 2017). Schematic of a typical MBR is illustrated in Figure 8.3.

Figure 8.3. Schematic illustration of the activated sludge process along with MBR. Source: https://en.wikipedia.org/wiki/Membrane_bioreactor.

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8.3.3. Membrane Bioreactor (MBR) Hybrid System In the recent few years, the MBR based process has gained significant consideration as a pretreatment process for NF and RO membrane processes because of its high-quality effluents and lower sludge productions. The combined processes have the potential to substitute the conventional biological and wastewater treatment which consists of the AS systems for bio-treatment trailed by the sedimentation schemes for the separation process of the treated effluent. MBR based processes exhibit substantial efficiency for removal of the mainstream of water impurities, including organic chemical compounds, nutrients, PPCPs, and endocrine disruptors (EDs). However, it also exhibits some limitations in removing all of the micropollutants from municipal wastewater. As described earlier, Clara et al. (2005) reported that the MBR based process effectively eliminated micropollutants including bisphenol-A, ibuprofen, tonalite, bezafibrate, surfactants, and galaxolide. These micropollutants are typically excluded during the biological processes with efficiency greater than 95%. Nevertheless, nonbiodegradable micropollutants such as Diclofenac, nonylphenoxyacetic acid, carbamazepine, and nonylphenoxyethoxyacetic acid were not eliminated because of their resistance to bio-transformation. Therefore, dense membranes (i.e. NF, and RO) should be coupled for substantial removal of remaining micropollutants to enhance efficiency (i.e. greater than 95%). A lot of theoretical and practical studies have been carried out for the removal of micropollutants using different membrane processes for the efficient recovery of municipal wastewater. These research studies have mostly focused on the utilization of RO coupled with MBR. High quality of effluents can also be obtained with the application of Nano-filtration membranes in place of RO but NF membranes offer the advantage of being able to moderate at relatively low pressures. However, a detailed investigation of the NF assembly installed downstream of MBR (i.e. NF receiving MBR effluents) is rarely performed. Therefore, there is an urgent need to study this coupling process. Generally, the MBR process is operated with AS and it has built-in membranes (usually Microfiltration membranes-MF) for separation of effluent after biological treatment. An air blower or diffuser is applied to provide oxygen for aerobic reaction (microbial respiration) and is employed to augment the nitrification and it also helps to removes membrane fouling deposits in submerged MBR. The permeate from MBR is intermittently

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collected out after the fixed interval and backwashed frequently at fixed intervals with the filtrated water to ensure that the filtration membrane fouling deposits are removed through backflow and the membrane filtration remains effective in next cycle.

8.4. INORGANIC MICROPOLLUTANTS IN MEMBRANE PROCESSES Industrial wastewaters possess site and industry-specific characteristics. The water quality depends on a broad range of factors. The water quality may have seasonal fluctuations because of changed evaporation, temperature, rainfall. Moreover, the site or industry-specific characteristics of the water are characterized by the analysis of compounds present at the site. For example, the water quality obtained from mines is strongly influenced by the physiognomies of bedrock and ore treating techniques (Karim and Mark, 2017). Mine waters typically have low concentrations of organic substances but inorganic compounds such as heavy metallic elements and nutrients are sparingly present. The mine-water can exhibit a low pH contingent to the bedrock quality and its propensity to form acid mine drainage. Lime is broadly used to treat the acidic waters by increasing its pH increase while making the metallic species to precipitate. Lime typically enhances the pH level when acidic water is not exposed to the environment. During the functioning stage mine, wastewater may emerge from enrichment practices and drainage coming from waste rocks (Johnson, 2003). The fertilizer and many other chemical industries also produce wastewaters, which result in the increment of the concentrations of inorganic species in natural water bodies. Apatite ore is typically utilized to produce phosphorous fertilizers. Sometimes, phosphorus-rich wastewater contains heavy metals that are taken from the earth’s crust or landfill. A brief description of inorganic micropollutants based on their classification is given in the following sections. These micropollutants include heavy metals, nitrogen compounds, sulfates, chlorides, and phosphorous compounds (Wu et al., 2011).

8.4.1. Metallic Species in Water Metallic species in water are usually categorized into transition metals, heavy metals, and metalloids. Heavy metals are typically characterized by their density; however, density does not solely define the features of the elements

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perfectly. For example, arsenic possesses some similar characteristics to heavy metals. However, it is a metalloid ( it has the visual appearance of metal, but is a poor conductor of electricity) and this is the manner arsenic has been referred to in this report. Heavy metals (e.g. Pb, Sn, Cd) are found in soil and the earth’s crust. Though heavy metals naturally occur in the environment, significant apprehensions towards these species have been raised because of the anthropogenic actions. Heavy usually metals float into the aquatic environments from various sources which include power plants, petroleum processing industries, petrochemical combustion, agrochemical feed production, pulp, and paper industry, metal industry, and mining industry (Duffus, 2002; Kumar et al., 2015). Some other sources of heavy metals include geogenic activities, natural soil erosion, and urban runoff which instigate the drifting of heavy metals into waters (Periasamy and Namasivayam, 1996; Salati and Moore, 2010). Heavy metals do not degrade and cannot be destroyed. Therefore, possess the tendency to accumulate naturally. Heavy metals also have the ability to move into the food chain and subsequently into human and animal tissues (Baby et al., 2010). Heavy metals are extremely injurious even at low concentrations. The health impacts of heavy metals mainly depend on the nature of heavy metal and its oxidation state. Heavy metals are usually classified as carcinogenic compounds. Their concentrations can become momentous in sediments of water bodies which results in higher levels of harmfulness (Nabulo et al., 2011). Many environmental laws restrict the release of these harmful elements into water bodies because of their severe health hazards (Saltelli et al., 2006). Apart from different heavy metals, some supplementary micropollutants such as metalloids have analogous characteristics to heavy metals. Arsenic is one of the most famous metalloids, which is a toxic element at very low concentrations. Arsenic can be present in the environment naturally but may also be disseminated in the environment by various anthropogenic activities. Arsenic has the ability to dissolve in waters in the form of arsenite or arsenate depending on the pH, biological processes, and redox potential. Arsenic can also be found in the form of dimethyl arsenate and methylarsonate. Anthropogenic sources include domestic wastewaters, metal refining, metal/ alloy manufacturing, synthesis of chemicals and non-ferrous metal smelting. Arsenic can exist in different concentrations depending on the environment, i.e. 1.5–1.7 μg per liter in seawater, 0.1–2 μg per liter in groundwater, up to

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3400 μg per liter in volcanic rocks or areas of sulfide mineral deposits and up to 48000 μg per liter in mining areas (Panel, 2012). Transition metals are also present in the water bodies containing inorganic micropollutants. Manganese is a well-known transition metal element that is considered as one of the most abundant elements present in the Earth’s crust. The oxidation states of manganese may fluctuate between + 2 to + 7. The most common oxidation state is + 2 exhibited in the water at a pH range of around 4 to 7. On the other hand, higher oxidation states of manganese are exhibited in basic conditions. Manganese is a typical trace element that is vital for humans. However, the manganese levels in water above 0.1 mg/L cause unpleasant taste and stain while it forms a layer on pipes at levels above 0.2 mg/L. The WHO recommendation for the permissible amount of Mn in drinking water is approximately 0.05 mg/L. However, the health effects usually occur at concentrations above 0.4 mg/L. The severe health impacts of Mn include anomalies in the nervous system (Frisbie et al., 2012, 2015). Typical characteristics of metalloids, heavy metals, and transition metals and their concentration limits illustrated in Table 8.1 (Villanueva et al., 2014). Table 8.1. Limits and Characteristics of Heavy Metals, Transition Metals and Metalloids in Drinking Water Metallic Species

Toxic Symptoms

Drinking Water Quality Guideline

Cadmium

Carcinogenic (Group 1), lung cancer and prostate cancer. Accumulation in kidneys causes dysfunction

3 μg/L

Arsenic (Metalloid)

Long-term Effects: Skin problems, Hypopigmentation, bladder cancer, dermal lesions, and lung cancer.

10 μg/L

Chromium

Carcinogenic. Cr6+ fits in Group 1 and Cr + 3 are in Group 3. Carcinogenic, causes e.g. stomach, and lung cancer.

3 μg/L

Nickel

It can cause allergic reactions when in contact with skin. Carcinogenic when inhaled (i.e. Group 1).

70 μg/L

Manganese (transition metal)

Above 0.1 mg/L concentration results in deterioration of water taste and hence lower concentrations are preferred

0.05 mg/L

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Copper

High concentrations can cause nausea, gastric irritation, anemia, liver issues, and kidney damage.

2 mg/L (and over 1 mg/L cause staining)

Zinc

No effect on health at concentrations found in drinking water. However, above 4 mg/L deteriorates drinking water taste

No health-based guidelines

8.4.2. Nutrients in Water Phosphorous is considered a critical nutrient that cannot be substituted by alternative constituents in animal and plant growth. P is a non-toxic nutrient; however, when assimilated at high concentrations some digestive problems may occur. Phosphorous has alleviated the development of modern agriculture and farming industry. Apart from agriculture, phosphorous is also utilized in chemicals and detergents in the form of phosphoric acid. Phosphorous typically exists in the form of phosphates in water. Higher levels of phosphate in surface water can cause problems because of eutrophication (Van Vuuren et al., 2010). For example, a lake undergoing eutrophication contains total phosphorous concentrations above 35 µg/L. Phosphorous can exist in a broad range of industrial wastewaters. Phosphorus is present in high concentrations in agriculture waters and municipal wastewaters. Drifting of phosphorous into waters can take place from agriculture, fertilizer industry, and municipal wastewaters. The discharge limit of phosphorus from a WTP to the Sea for over-all phosphorous content is roughly 1 mg per liter. Generally, the limit for the discharge of phosphorus from a chemical industry must not exceed 2 mg per cubic decimeter (Łysiak-Pastuszak et al., 2004). Nitrogen compounds typically possess nine different oxidation states (ranging from –3 to + 5). Nitrogen compounds normally tend to oxidize into nitrates (NO3–) in waters, but ammonium (NH4+) ions and nitrite (NO2–) can also be present. These nitrogen compounds have high solubility in water and they conveniently drift into groundwaters. Compounds of nitrogen usually exist in the form of nutrients and they can instigate water eutrophication in surface waters if present in high concentrations (Glass and Silverstein, 1999; Jermakka et al., 2015). Fertilizers are usually rich in nitrogen compounds. The concentration limit of nitrates in drinking water is below 50 mg/L. Studies have exhibited that nitrogen ingestion can be carcinogenic in conditions where endogenous nitration results in the formation of N-nitroso compounds. Ingestion of nitrates through edibles and drinking water has been proposed to increase the possibility of bladder cancer (Morales-SuarezVarela et al., 1995).

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Sulfates (SO42–) are common types of species present in water coming from anthropogenic and natural sources. Sulfate concentrations in water can increase because of industrial actions which include mining or chemical activities when sulfur-containing chemical compounds and minerals are used. Municipal waters also contain enhanced concentrations of sulfates i.e., 20–500 mg per cubic decimeter (Lens et al., 1998). Elemental sulfur typically oxidizes to sulfites and sulfates in waters in the presence of oxygen. Therefore, sulfates are the major concern among sulfur compounds present in many wastewaters (Akcil and Koldas, 2006). Sulfate is a non-toxic chemical species. However, it can instigate gastrointestinal effects if the drinking water contains very high concentrations of sulfates (i.e. above 500 mg/L). Sulfates content above 250 mg/L can also cause impairment of taste in drinking water. Sulfate species can cause grave environmental issues if a substantial amount is integrated into the water. It has the ability to increase the salinity of the water bodies consequently creating dead zones when heavy salty water tends to amass at the bottom of basins. Enhanced sulfate concentrations can exist in phosphorous recirculation which is responsible for the release of phosphorus from sediments, ultimately resulting in eutrophication (Simate and Ndlovu, 2014). Sulfates conveniently form salts with magnesium, calcium, and sodium, which are responsible for fouling the industrial equipment and piping. Cleaning and removal of the sulfates can be extremely challenging. Moreover, sulfate also has the ability to corrode the stainless steel equipment installed in WTPs (Pistorius and Burstein, 1992).

8.5. MEMBRANE TECHNOLOGIES AND THEIR LIMITATIONS IN TREATING INORGANIC MICROPOLLUTANTS As it has been discussed there are numbers of metallic and non-metallic inorganic micropollutants including anions chlorate(III), (V) and (VII), arsenates (III), nitrate(V) and (V), bromate(V) fluoride and borate) and various heavy metals (Hg, Sn, Pb) have been found at significantly harmful concentrations in almost all water sources and wastewaters (Velizarov et al., 2004; Hoshino et al., 2009; Alzahrani et al., 2013; Bodzek, 2013; Swangjang, 2015). Many of these inorganic micropollutants are greatly soluble in water and form ions after dissociating completely which are chemically stable in normal conditions. WHO recommendation and many other countries have set the limit for the maximum allowable concentrations of these species in drinking and wastewater which is discharged to the environment. These

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permissible limits are very low (typically between few µg per liter to a few mg per liter). Since most of the micropollutants are in ionic states, they can be considered as charged micropollutants. The contamination of the aquatic water systems with inorganic micropollutants may be either natural or anthropogenic. A number of common water treatment technologies, including sedimentation, coagulation, chemical precipitation, adsorption ionexchange (IE), evaporation, biological methods, classical solvent extraction, which are frequently employed for elimination of inorganic micropollutants from natural or wastewater bodies, had faced serious challenges (Oehmen et al., 2011). Membrane processes are being increasingly employed to remove inorganic micropollutants from the aquatic environments. Primarily, membrane processes based on RO, NF, MF and ultrafiltration (UF) have been employed, electrodialysis (ED), as well as Donnan dialysis (DD), have also been applied in this report we will be focusing on the challenges created by these inorganic micropollutants on the membrane process and to overcome those challenges how MBR with combination with various another membrane process can be used to remove these hazardous inorganic micropollutants from natural water sources and wastewater bodies.

8.5.1. Removal of Anionic and Non-Metallic Micropollutants by Membrane Processes In this section of this report removal of various anionic micropollutants including nitrates (V), bromates (V) (BrO3–), chlorates (Vll), fluorides using various membrane processes have been discussed and the challenges created by these inorganic anionic micropollutants have been highlighted

8.5.1.1. Challenges Created by Nitrates (V) Nitrites Over Membrane Processes Nitrates and nitrites are a nitrogen-containing chemical compound formed by the biological breakdown of organic nitrogen. Nitrogen is crucial for all living species as it is a component of protein. In the environment, they exist in many forms and change its forms as it moves through the nitrogen cycle. At the same time, extreme concentrations of various nitrogen-containing compounds in drinking water can be hazardous to health, especially for pregnant women and infants. Figure 8.4 shows the movement of nitrogencontaining the compound in water bodies (Swangjang, 2015).

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Figure 8.4. Nitrogen movement in water bodies. Source: http://www.sacriver.org/aboutwatershed/reportcard/section3/ section3_3/334-nitrogen-loadcycling.

As, we have seen it has a serious effect on humans, animals, and the environment, therefore if we talk about its removal using membrane processes it creates serious challenges on conventional wastewater and drinking WTP. The contamination of various waters bodies with nitrates (V) the other nitrogen-containing compound is a result of extensive use of fertilizers containing nitrogen and discarding of industrial and municipal solid and liquid residues to the environment. RO, IE, biological denitrification, ED, and are frequently used methods that have been tested and used for the removal of the excessive amount of this nitrogen-containing a chemical compound (Koltuniewicz and Drioli, 2008). Nitrates have the potential to cause several effects on living beings especially human and animal health among which the most prominent are gastric cancer, methemoglobinemia, and non-Hodgkin’s lymphoma (Raczuk, 2010). The RO membrane process decreases the amount of NO3– in drinking water up to the level mentioned in the regulations (10 mg N/L). Numerous RO membranes have been characterized by their high values of the retention coefficient for inorganic species. Therefore, the required amount of decrement can be achieved using the RO membrane. So, the required concentration of NO3– can be accomplished by adding the impure water to raw water. Monovalent nitrates ions such as NO3– are not completely stopped while using NF, for e.g. the retention coefficient of Nitrate for NF-70 membrane (obtained from Dow/FilmTec) found to be 76%, which is lower than that

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of RO membranes (Epsztein et al., 2015). So, for better result NF has the potential to be used as an initial step in the NO3– elimination process when combined with RO or IE. Nevertheless, the retention coefficient of nitrate ions during NF in the presence of sulfates decreases. In those situations, NF membranes basically do not eliminate nitrates, however, they were able to stop multivalent ions (Ca2+ and Mg2+) and it has a productive effect on IE and RO performance. Both the processes have relative comparable purification expenses with the expenses of IE as well as ED, in which costs of clearance of obtained concentrate are also included. NF and RO membranes which are used for the removal of nitrates from different types of water are twice expensive than the membranes which are used in the case of low-pressure membrane processes. So, the first and the most important challenge is the cost of these membranes is highly expensive. Additionally, they require much higher pressure so these processes are much more energy-consuming. Therefore, alternate methods comprised of UF membranes and polymers or surfactants complexing these nitrate ions are applied. Micelles or complexes which contain nitrate ions can be further retained in an UF membrane process. Other important challenges created by these nitrogen-containing inorganic micropollutants over membrane processes like RO or IE are the requirement of frequent regeneration and it also leads to the production of secondary solution which is of very high concentration which is because of accumulation of undestroyed nitrates so are of higher salinity and required to be frequently disposed of.

8.5.1.2. Challenges Created by Bromates (V) (BrO3–) Over Membrane Processes Bromates (V) (BrO3–) in drinking water are usually formed by the disinfection by-products formed during the ozonation of water which contains bromides ions (Br–). The concentration of BrO3– varies between 15–200 µg/L in natural waters, while in the groundwater its concentration is higher. Removal of BrO3– using the NF process reaches up to 78–99% with the initial concentration of 285 µg/L, while in the RO process the average retention coefficient obtained is 97% (Butler et al., 2005). Prados-Ramirez et al. (1995) achieved 78% removal of BrO3– and 64% of Br from river water having the initial concentration of BrO3– 300 µg/L using NF membrane. It was concluded that the NF membrane process was more efficient in terms of cost, mainly because of lower pressure applied in that case. The challenges observed in membrane techniques include severe deionization of

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permeate, and it needs remineralization and the production of concentrated waste outcome which is retentate, proper treatment is required before their disposal to the environment.

8.5.1.3. Challenges Created by Chlorates (VII) Over Membrane Processes Due to its widespread use, low tendency to degradation and high mobility in the natural water chlorates (VII) is found now to be a potential environmental problem. It is highly toxic and has a severe adverse effect on the development and activity of the various living organism. Application of RO and NF for the removal of ClO4– from water solutions has been reported (Logan, 2001). In the case of NF, ClO4– retention is about 76–90%, while for RO it up to 96.0% at the original content of ClO4– about 100 mg/L. Even 99.9% of ClO4– can be achieved using high RO membranes and it is lower (95%) for lowpressure RO membrane. Therefore, for a better result, further treatment of the permeate could be done before its encounter to the real filtration process; for example, adsorption on the surface of activated carbon (AC), through IE, or in a bioreactor (Logan, 2001). Normally, RO has the potential to efficient even as a stand-alone process to eliminate chlorates (VII) while producing drinking water only for low ClO4– concentrations. Challenges are the same as other cases RO and NF are not destructive processes; therefore, concentrate contains ClO4–(VII) and many other pollutants that must be removed from it before its expulsion into the environment. Generally, evaporation and biological-based treatment have been taken into consideration.

8.5.1.4. Challenges Created by Fluorides (F–) Over Membrane Processes The presence of fluorides (F–) in natural water is the result of its presence in the lithosphere and several anthropogenic industrial activities. According to WHO recommendation, the maximum limit for fluoride concentration 1.5 mg/L in drinking water. Coagulation with sedimentation, adsorption, IE and various membrane processes including RO, NF, and ED have been the frequently proposed methods for the of removal fluorides from water. RO application for fluoride elimination is associated with partial demineralization of drinking water, which is the main drawback of this process. RO membrane application for water desalination removes 97–99% of salts from it, which results in almost complete retention of fluorides, for example, lower than 0.03 mg/L for the initial concentration ranging from

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1.30 to 1.80 mg/L (Lu et al., 2014). Throughout the treatment of water with a higher concentration of fluoride, NF application is advantageous as in that case generally the remineralization of resulting water is not required. Using commercially obtainable NF membranes such as NF270 and NF90 (obtained from FilmTec) TR60 the final concentration of F– ions in permeate obtained for (Toray) was ranging from 0.05 to 4 mg/L, but it always depends on the membrane type and initial concentration (Tahaikt et al., 2008). There are many other commercial NF membranes are available for which results show that it can be used for potable water production from water with very high concentration fluorides those membranes are F-70, NTR-7250 and NTR7450, (FilmTec), Desal 51-HL and Desal-5-DL (Osmonics), SR-1 (Koch) and MT-08 (PCI) (Takdastan et al., 2015).

8.5.2. Removal of Cationic and Metallic Micropollutants by Membrane Processes In this section of this report, we have discussed the removal of hazardous cationic and metallic micropollutants including Arsenic, Chromium(VI), and challenges created by them to the membrane process during the treatment but only selected metals have been considered in this section heavy metals will be discussed in another section.

8.5.2.1. Challenges Created by Arsenic Over Membrane Processes Arsenic occurs in water as inorganic micropollutants in the forms as – As(III) and As (V), in groundwater, lower oxidation stage governs and in surface water the higher. As(III) occurs as inert molecules H3AsO3 and As (V) as AsO43–, H2AsO4– and HAsO42– at pH close to 7 which is for neutral chemicals. The form in which As (V) ions are has been found to have a direct influence on the effectiveness and choice of the method used for the treatment. In order to reduce the arsenic content in potable water, NF membranes and RO as well as the hybrid process in which coagulation with MF/UF has been applied and reported (Dilek et al., 2002; Bick and Oron, 2005; Zhang et al., 2005). It has been observed that RO membranes such as TFC-ULP (Koch) are able to remove about 99% of the arsenic from the groundwater (which is a reduction from 60 to 0.9 µg/L), while DK2540F membranes (from Desal) were able to retain 88–96% of this micropollutant (Ćurko et al., 2016; Kou et al., 2017). Reduction in the content of As(III) has always been lower than As (V), and for better results, oxidizing environments during the procedure have been recommended (Nguyen et

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al., 2009). The amount and type of dissolved organic compound and pH have a significant impact on arsenic removal.As(V) removal rate at pH = 3 reaches up to 80%, while at pH range from 5 to 10 it can reach up to 95% (NTR-729HF membrane). At lower organic matter content higher reduction in arsenic (V) (up to 90%) has been observed, as compared to higher organic matter concentration for which it is equal to about 80%. A number of many other pilot and laboratory research and on arsenic content reduction using RO membranes have also been performed and reported. NF membranes processes can also be used for removal. 97.0% elimination of As(V) has been obtained using NF-70 FilmTec membrane while using the NF-45 membrane, it fluctuates between 45.0 to 90.0%, depending on the original content of the As in water (Nguyen et al., 2009). As(III), as for RO, much lower retention coefficients have been observed and it decreases from about 20% to nearly 10% with the micropollutant concentration increment in the water. With the increase in pH the rate of reduction of As(V) content significantly increases with the application of the NF-45 membrane, it is according to the difference in Arsenic ion hydration. No influence of pH on the retention coefficient of As(III) is observed when it lies between 4 and 8. It indicates that the mechanism of arsenic elimination from water using NF membranes is mainly dependent on both the sieving separation and the electrostatic repulsion among similar charged ions and membrane surfaces. UF membrane processes and MF membrane processes can also be employed for the arsenic removal from water, but usually by adopting integrated systems (Hybrid system) with coagulation mechanism (Dabwan et al., 2015). Hence, the challenges observed for the case of arsenic are it is highly dependent on the form of Arsenic compound, pH, and initial concentration of the organic matter and arsenic present in the water bodies which makes it really difficult to decide which membrane process to use.

8.5.2.2. Challenges Created by Chromium (VI) Over Membrane Processes The solubility of Chromium compounds is highly dependent on the pH of the water in which it is present at pH 1–6 they appear as Cr2O7–2 and HCrO4– ions which are highly soluble, whereas at pH higher than 6 – CrO4–2 ions which are soluble in water are formed. For living organisms, these compounds are highly toxic; therefore, their allowable concentration in potable water should be around 0.05 mg per liter, including 3 µg per liter of Cr (VI). Various investigations have been conducted on the reduction of Cr(VI) water usually involves RO with the utilization of osmotic membranes which are Sepa-S

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type membranes manufactured by utilizing cellulose acetate (CA) (Owlad et al., 2009). It has been observed that the CA membranes were able to retain approximately 96% of Cr(VI) ions, whereas osmotic membranes could do between 80–96%, which depends on the compactness of the membrane. NF has been being a better solution for the deduction of Cr(VI) from the water. The retention coefficient exhibits an increment with respect to pH increase this effect has been observed more prominent for membranes with inferior separation capacities ranging from 47 to 94.5% for the osmotic type of membranes, compared to more dense membranes which are between 84 to 99% for osmotic membranes (Hafiane et al., 2000). For NF membranes retention coefficient also depends on the concentration of Cr in the feed, at the same time the range of the effect on reduction also depended on the pH of water. Higher retention of Cr is observed at lower pH (in the acidic solution), whereas the pH variation mainly depends on the nature of basic solution, i.e. lower retention is observed for the higher concentrations of Cr. This phenomenon can be explained in the way that it is due to the fact that the ionic form of Cr(VI) depends on the pH and it changes its nature with the change in pH. At low pH for the highly acidic environment, Cr(VI) is in the form of non-dissociated chromic acid (H2CrO4) while at pH higher than 6.5, HCrO4– ions are produced and its concentration increases with the increase in the parameter. The additional increment in pH above 7 causes the formation of CrO4–2 ions and its concentration also depend on pH. Cr2O7–2 ions are also found in the solution and their concentration depends on the initial content of the contaminant in the feed and also on pH. At high concentrations of Cr, this ion is usually dominant and in strongly acidic condition at pH between 1–7) as pH increases its concentration decreases (Hafiane et al., 2000). Therefore, challenges are to first determine the concentration and pH of the feed which becomes difficult especially to decide the initial concentration and its solubility and separation are highly dependent on the pH condition and separation also varies with pH and initial condition.

8.5.3. Removal of Heavy Metals Using Membrane Processes Heavy metals including Pb, mercury, selenium (Se), iron, nickel, manganese, cobalt, cadmium, zinc (Zn) and chromium are overall the most hazardous impurities present in wastewaters and natural water present in the form of various inorganic micropollutants. The main source of drinking water is natural water and it is also found that they present in that also which makes the situation really dangerous. The danger caused by the presence of these

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heavy metals in water is quite significant if we consider the daily, monthly, or annual water consumption. The permissible concentration is established only for part of them. Permissible amount of various heavy metals according to different organizations is mentioned in Table 8.2. Table 8.2. Permissible Amount of Various Heavy Metals Heavy Metal

Permissible Limit (mg/L) WHO

USEPA

ISI

CPCB

ICMR

Iron

0.1

-

0.3

1.0

1.0

Cupper

1.0

1.3

0.05

1.5

1.5

Mercury

0.001

0.002

0.001

No relaxation

0.001

Cadmium

0.005

0.005

0.01

No relaxation

0.01

Arsenic

0.05

0.05

0.05

No relaxation

0.05

Lead

0.05

-

0.10

No relaxation

0.05

Zinc

5.0

-

5.0

15.0

0.10

Chromium (mg/l)

0.1

-

0.05

No relaxation

-

WHO: World Health Organization, USEPA: United States Environmental Protection Agency, ISI: Indian Standard Institution, ICMR: Indian Council of Medical Research, CPCB: Central Pollution Control Board.

Conventional methods including precipitation, IE, or extraction have many limitations, especially when it comes to the processing of a large amount of water containing extremely low concentration yet very dangerous metal ions. Even after the final filtration process, it is frequent to find the concentration of these metal ions in the filtrate above the level of some mg per liter. Membrane processes like RO, NF, UF, and ED are more frequently applied for the removal of heavy metals from water at the industrial level. It has been observed by various research studies that metal ions can be conveniently eliminated from water using RO and NF, as in these cases membranes could retain dissolved salts of particle sizes not even larger than a few nanometers (Sablani et al., 2003; Agarwal et al., 2015). Various researchers have found that the removal efficiency using RO for individual heavy metals was high and it was about 98% for copper while 99% for cadmium, while in the case of NF it was above 90%. When both metals were simultaneously present, RO membranes were able to reduce the concentration of these ions from 500 mg/L to about 3 mg per liter with an efficiency of 99.4%. While NF has an efficiency of 97% and it is an emerging technology for the removal of heavy metal ions including copper-nickel and chromium(III) from wastewater (Qdais and Moussa, 2004). NF process has many benefits which mainly

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include the convenience of operation, low energy use, reliability, and higher pollutant removal efficiency. The application of NF has also been reported for the reduction of nickel ions from wastewater (Mehiguene et al., 1999; Fu and Wang, 2011). The maximum solute rejection rate observed for the nickel and cadmium ions is approximately 98.9% and 82.7%, respectively, for the feed containing an initial concentration of 5 mg per liter. These investigation studies have concluded that the NF technique can be a suitable solution for the elimination of heavy metals from wastewater up to a particular level of acceptable environmental regulations. Cation retention in the process highly depends on the type and valence of co-ions, the energy of hydration, passing through this membrane as well as on applied pressure and pH value. For example, the retention of Cu2+ and Cd2+ ions has been observed greater for the higher co-ions valence and for the higher cation hydration energy (Mehiguene et al., 1999). The retention coefficient for sulfates of cadmium (Cd) and copper (Cu) are almost 100% independent of the prevalent pressure. While in the case of nitrates and chlorides, the solute retention rates show an increment with respect to pressure up to certain specific values which mainly rely on the type of the co-ions. If we consider UF membrane process their pore sizes of are bigger than dissolved metal ions which are in the form of hydrated ions or as complexes low MW, these ions would not be retained and easily pass through UF membranes. Therefore, the challenges in case of heavy metals for membrane process has been found to be for UF the size of the ions are smaller so various surface modification, for example, uses of surfactant and other surface modifier has been reported. For RO and NF it is highly dependent on hydration energy, form in which they are present, pH, and initial concentration of the pollutants. As these heavy metals are present in extremely low concentration still highly toxic and its low, concentration really makes it a bigger challenge for membrane processes.

8.6. POTENTIAL HYBRID PROCESS FOR REMOVAL OF INORGANIC MICROPOLLUTANTS There are a lot of inorganic micropollutants which are classified in the category of ionic micro-pollutants, while the most important of them having a significant environmental impact are listed as follows: 1. 2. 3.

Perchlorate; Bromate; Arsenate;

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4. Nitrates; 5. Arsenite; 6. Heavy metals Cations (Hg+2). All of the above-listed micropollutants are usually present in hazardous quantities in many aquatic systems. World Health Organization limits for these hazardous micropollutants are very low (usually in the range of ppb), so the mainstream of them can be regarded as the ionic micropollutants. The primary challenges, for the time being, are there timely detection followed by their removal from the aquatic environment. With the advancements in technology and improvements using literature, effective detection of micropollutants can be carried out using ICP-AAS or using Dionex HPLC systems but the limitation still lies in how to treat the micropollutants. Even if some processes can remove the concentrations of these small substances, they are not sufficient to reduce them up to the WHO standards. The utilization of membranes is increasing nowadays and presents an attractive solution for eliminating micropollutant ions among two distinct liquid phases (i.e. concentrated, and purified streams of water) while avoiding the potential problems associated with their adsorption, precipitation, and coagulation. Therefore, membrane technologies are being successfully utilized nowadays for the purpose of drinking water treatment.

8.6.1. Ion-Exchange Membrane Bioreactor (IEMB) The handling of concentrated/eject streams or waste generated, however, can be problematic in some cases as it becomes difficult to handle. One of the recent solutions to overcome this problem is to utilize integrated processes, including the emerging technique of MBRs (Chaudhari and Murthy, 2010, 2013). Subsequent to the above-mentioned approach, the coupling of the DD method with the biological detoxification process for a charged micropollutant is a suitable approach typically known as the ion-exchange membrane bioreactor (IEMB) (Velizarov et al., 2000, 2002, 2003). This process has been experimentally verified for the mechanism of aqueous denitrification (Figure 8.5).

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Figure 8.5. Schematic illustration of the ion transport mechanism in tan ionexchange bioreactor. Source: https://www.researchgate.net/figure/Schematic-diagram-of-aniontransport-and-bioreduction-in-the-ion-exchange-membrane_fig1_235752004.

IEMB is a method for the treatment of contaminated water containing inorganically charged micropollutants utilizing DD principles. The ionexchange membrane is normally employed as a barrier layer between the bio-compartment and the contaminated water stream. The biocompartment is a region where an appropriate mixed microbial culture exhibits degradation (i.e. the transformation of the micropollutants to less harmful products. A schematic diagram of anionic micropollutants (ClO–, NO3–) removal is explained below. Therefore, the IEMB process is predominantly attractive if the high flux of water with decent quality is desired. Another noteworthy benefit of IEMB processes is their ordinary (convenient) configurations and low energy requirement (Fonseca et al., 2000). Figure 8.5 shows a schematic illustration of the ion transport mechanism in an ion-exchange bioreactor (Velizarov et al., 2004). An appropriate driving counter-ion is usually incorporated (i.e. chloride) in the biocompartment at considerably high levels which facilitates the coupling back of the target anionic micropollutants. Transport of co-ions (i.e. cations) through the membrane is usually negligible because of their

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213

electrostatic repulsion characteristics (i.e. Donnan exclusion) with respect to the positively charged surface of the membranes. The case for the treatment of cationic micropollutants is exactly the same; however, it involves reversing the charge of the membrane. In the recent years, extensive research has been carried out on the potential applications of the IEMB processes for playing with the case of emerging water micropollutants which mainly contain heavy metals and oxyanions. High amounts of Perchlorates are observed to hamper the consumption of iodide in the thyroid and as a consequence interfere with the production of thyroid-stimulating hormones (i.e. TSH). Present US Environmental Protection Agency (EPA) regulations have espoused the Nuclear Regulatory Commission (NRC) reference dose of perchlorate equal to 0.7 μg per kilogram body weight per day. While California has proposed a limit of 6 ppb extreme, pollutant load in drinking water and while Massachusetts has fixed its own allowed limit of 2 ppb (Velizarov et al., 2004). Moreover, Bromate ion at concentration ranging between 0.45 to 60 ppb is observed after ozonation of water containing contextual bromide (Mehiguene et al., 1999; Crespo et al., 2004; Butler et al., 2005). This anion has extremely high solubility and stability in water which makes it really tough to eliminate using conventional treatment methods. Diagnostic advances are recently successful in the detection of bromate pollutant, in both natural surface and groundwater reserves, which has provided additional stress on the bromate elimination processes. Bromate is categorized as a potential human carcinogen, with the maximum permissible pollutant concentration between 10 to 27 ppb, and this has now been implemented by many national drinking water regulations (Velizarov et al., 2004). Oxyanions such as bromates, nitrates, perchlorates, and various different ionic micropollutants have also been observed in freshwater systems at extremely hazardous concentrations. Mercury is also a toxic heavy metallic element that has the potential to attack the endocrine and CNS in humans (Nies, 1999). An excessive amount of mercury in water supplies can result in brain damage and consequently the death of the living species. The occurrence of mercury in the environment is becoming a grave issue of the ecosystem (Matos et al., 2009). The contamination of mercury in drinking water typically originates from industrial or natural sources that contain mercury in the cationic state (Hg2+).

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The subsequent section illustrates the potentials of the IEMB process for eliminating the ionic micropollutants such as perchlorates, bromates, and nitrates from polluted water.

8.6.2. Configuration Ion-Exchange Membrane Bioreactor (IEMB) The experimental architecture of the IEMB setup consists of a parallel-plate module constructed from stainless steel embedding the anion-exchange membrane which separates between the two similar quadrilateral channels. One of the IEBM module channels is usually linked to the external vessel by means of a re-circulation loop and uninterrupted bio feed is provided to the external vessel. The chloride ions act as key counter-ions for the movement of chlorates and nitrates to the bio-compartment. Ethanol is provided to the bio compartment as a source of carbon and electrons to enrich the mixed culture of nitrates and perchlorates. The other module channel is generally associated with a second tank to which polluted water is unceasingly fed and treated water content is withdrawn. The two liquid segments (biomedium and water) are recirculated in the corresponding module channels simultaneously using pumps for maintaining suitable flow rates. In a recent investigation carried out on IEBM processes, a similar experimental setup was used (Matos et al., 2009). The results acquired from the experimental work were pretty attractive with respect to the use of IEBM. A typical setup of IEMB is shown in Figure 8.6:

Figure 8.6. Typical small scale IEMB setup.

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Source: https://www.researchgate.net/figure/Schematic-diagram-of-the-membrane-bioreactor_fig1_225742580.

8.6.3. Removal of Bromates, Nitrates, and Perchlorates Using Ion-Exchange Membrane Bioreactor (IEMB) The experimental findings attained from recent research studies have exhibited that IEBM is an effective process for the removal of micropollutants. The results for IEBM obtained from numerous studies have been listed in the table below and are quite analogous to the recent experimental studies conducted by Velizarov et al. (2000) exploiting the IEMB process to treat polluted water with the below-mentioned compositions at the water treatment flux of Neosepta-ACS membrane of almost 3.1 L/(m2h). 1. 100 ppb of chlorates; 2. 200 ppb of bromates; 3. 60 ppm of nitrates. All of the above-mentioned micropollutants are instantaneously transported through the membrane within a meticulous way through the contaminated water. All of the transported ionic species are monovalent possessing the same ionic size. However, the results were remarkably great and displayed the novel capability of the mixed culture to biodegrade all the anionic micropollutants proficiently to chloride, nitrogen, bromide. Irrespective of the higher levels of nitrate ion, as an electron acceptor for carbon source (i.e. ethanol) oxidation. A research study by Velizarov et al. (2004) was also focused on higher quantities of contaminated water per membrane area to assess the IEMB treatment efficiency of the process. These purification cycles have been conducted with water containing sodium salts of NO3– and ClO4– and having amounts of 100 ppb of chlorates and 60 ppm of nitrates respectively, but operated at altered water flow rates per membrane area known as (F/A) ratios L/(m2h) as follows: 1. 2. 3. 4.

18.5; 15.4; 7.7; 3.1.

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Table 8.3 shows the concentrations of different anions in the flux before and after IEMB treatment (Velizarov et al., 2008) Table 8.3. Concentrations of NO3–, BrO3– and ClO4– in Treated Water and in the Biocompartment and Anion Fluxes Through the Membrane for an IEMB Run at Steady State Treated Water Anion Flux Target conc. (ppm) (g/m2 h) Anion

Biocompartment Polluted Conc. (ppm) Water Conc. (ppm)

Anion Flux (g/m2 h)

0.3 ± 0.2

0.2

NO3–