Treatment of Tannery Effluents by Membrane Separation Technology [1 ed.] 9781617285967, 9781607418368

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Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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TREATMENT OF TANNERY EFFLUENTS BY MEMBRANE SEPARATION TECHNOLOGY

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No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

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Chemical Engineering Methods and Technology Series Treatment of Tannery Effluents by Membrane Separation Technology

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Sirshendu De, Chandan Das, and Sunando DasGupta 2009 ISBN: 978-1-60741-836-8

Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

TREATMENT OF TANNERY EFFLUENTS BY MEMBRANE SEPARATION TECHNOLOGY

SIRSHENDU DE, CHANDAN DAS Copyright © 2009. Nova Science Publishers, Incorporated. All rights reserved.

AND

SUNANDO DASGUPTA

Nova Science Publishers, Inc. New York

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Copyright © 2009 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA De, Sirshendu. Treatment of tannery effluents by membrane separation technology / authors, Sirshendu De, Chandan Das, Sunando DasGupta. p. cm. Includes bibliographical references and index. ISBN 978-1-61728-596-7 (E-Book) 1. Tanneries--Waste disposal. 2. Sewage--Purification. 3. Membrane separation. 4. Water-Purification--Membrane filtration. I. Das, Chandan. II. DasGupta, Sunando. III. Title. TD899.T3D385 2009 675'.230286--dc22 2009021047

Published by Nova Science Publishers, Inc. Ô New York

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CONTENTS

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Preface

vii

Chapter 1

Introduction

1

Chapter 2

Treatment Methods

7

Chapter 3

Treatment of Soaking Effluent

21

Chapter 4

Treatment of Liming Effluent

43

Chapter 5

Treatment of Deliming-Bating Effluent

61

Chapter 6

Treatment of Pickling Effluent

79

Chapter 7

Treatment of Degreasing Effluent

91

Chapter 8

Treatment of Tanning Effluent

101

Chapter 9

Treatment of Dyeing Effluent

115

Chapter 10

Treatment of Fatliquoring Effluent

137

Index

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PREFACE

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.

Membrane separation process has emerged as a powerful separation technique and has become an integral part of modern process industries. This has found wide applications in the field of biotechnology, beverage, dairy industry, clarification of fruit juice, waste water treatment etc. The advantages of these processes are their unique separation capabilities, low energy consumption, high throughput, and ease of scaling etc., compared to conventional separation processes. Tannery is one of the most polluting industries. In order to recover the process water and costly chemicals, membrane based processes can be effectively used to treat the effluent emerging from each of the tannery units. This book presents a systematic and comprehensive study to develop a greener route to treat such effluents. It is to be emphasized that no such book dealing with application of membrane filtration in tannery waste exists currently. Therefore, this book obviously has significant advancement compared to existing books on membrane technology. This book can be used as one of the texts for the course like “Novel Separation Processes” taught in postgraduate level. Of course, this book can be an extremely useful reference book for the students and professionals in Chemical and Environmental Engineering. We believe this book will have two fold impacts. Firstly, its academic value is quite high; Secondly, it will have remarkable impact of scaling up such system in actual industrial scale from pilot plant data in an emerging area. This proposed book presents detailed description of the membrane based processes to treat the effluent from various units operations of a tannery. The results are analyzed in full detail. We believe this book is a first kind of its own in this field. Since the topic is an emerging area and most of the work presented has potential of field application, the possibility of this book being out of date is rare in near future. Of course, this book can be an extremely useful reference book for the students and professionals in Chemical and Environmental Engineering. Apart from academic value, it will have remarkable impact on scaling up such system in actual industrial scale from pilot plant data in an emerging area.

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Chapter 1

INTRODUCTION TANNERY AS SOURCE OF POLLUTING EFFLUENT In a typical tannery, several unit operations are involved, namely, soaking, liming, pickling, deliming-bating, tanning, dyeing, etc. Each of these steps is extensively chemical consuming and produces huge amount of wastewater. Figure 1 describes the steps involved in a typical tannery. Soaking

Flaying

Liming Fleshing

Curing

Beam house

Deliming Bating

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Pickling Degreasing Tanning Neutralization

Dyeing

Fat liquoring

Finishing

Figure 1. Steps in a typical tannery.

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First step is flaying, i.e., flaying of skins and hides from mortal body. Small and thin leather (before tanning) is called skin and big, fat and strong leather (before tanning) is called hide. Flayed skins and hides are cured to prevent decomposition and putrefaction. There are three types of curing processes, namely, i) wet salting, ii) dry salting and iii) drying. During curing, hides and skins lose large quantity of its physiological content of water and unless the former regains this water during soaking operation, good quality leather cannot be produced. The first tannery operation is soaking, which is treatment of hides and skins with water. The objectives of soaking are, therefore, to rehydrate the skin proteins, to open up the contracted fibrous structure of the skin, to remove the curing salt and to clean the surface filth. Thus, the soaking operation produces huge amount of effluent containing high loading of biological oxygen demand (BOD) and chemical oxygen demand (COD). About 30-40 liter of water is used for the production of 1 kg leather. Soaking unit employs about 25% of total water consumed in tannery [1]. As a result, recycling of effluent is needed to reduce the water consumption. Liming process employs the treatment of soaked hides and skins with milk of lime with or without the addition of sharpening agents like sulphides, cyanides, amines, marcaptans, etc. The objectives of liming operation are to remove the hairs, nails, hooves and other keratinous matters, natural grease and fats, to swell up and to split up the fibers to the desired extent, to bring the collagen to a proper condition for satisfactory tannage. Liming unit employs about 15% of total water consumed in a tannery so recycling is required for reducing consumption of water [1]. During liming, pelts (limed skins and hides) become enlarged and sticky flesh adhered to the flesh side can easily be removed by machine in fleshing step. Excess lime and other alkalis used in liming is removed either by repeated washing in water or by chemical treatment using acids and/or acidic salts or by both in deliming step. In the bating operation, the pelts are treated with a solution of proteolytic enzymes derived from fermented warm infusions of hen and pigeon dungs [2]. The aims of bating process are: i) to remove all swelling and plumping, ii) to produce silky grain, iii) to remove most of the limes, iv) to remove dirt, short hairs, grease and lime soaps, dark colored pigments, v) to loosen the hypodermic tissue (flesh) so that it can easily be removed by scrapping, etc. Therefore, deliming-bating operation results in huge amount of wastewater. This wastewater can be further treated for safe disposal or recycled back to reduce the water and chemical consumption. To condition the pelts for tanning, the delimed and bated pelts are treated with a solution of acid and salt in pickling step. In most cases, sulphuric acid and common salt are used. Formic acid is also used in place of inorganic acid. The proportions of acid, salt, water vary from tannery to tannery. A required degree of acidity of pelts is obtained in pickling step. Two approaches, namely, recycle/reuse of pickle liquor for the subsequent batches and resorting to pickle free alum-chrome combination tanning systems have been studied. Pickling liquor from the first batch has been recycled for subsequent batches with appropriate replacements. The use of potash alum for the pH reduction process has been employed in place of the conventional process. Significant reduction of total dissolved solids (TDS) was observed using both approaches [3]. Depickling may be necessary in some cases to remove free excess acids by treating pickled pelts with sodium thiosulphate solution. Appreciable amount of grease may contain after liming process. Then the residual grease will be responsible for uneven dyeing, waxy patches in alum tanned leather, pink stains in

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Introduction

3

chrome blues, etc. There are three methods for the removal of grease, namely, aqueous emulsification, solvent extraction and pressure degreasing. Animal skins and hides are converted to commercial products (leather) by tanning process. It prevents putrefaction and develops the resistance to heat hydrolysis. Tanning is done mainly either by basic chrome sulphate or by vegetable tannage. Almost all leather (more than 90% of all tanneries) made from lighter-weight cattle hides and from the skin of sheep, lambs, goats, and pigs is chrome tanned. Chrome tanning is performed using a onebath process that is based on the reaction between the hide and a trivalent chromium salt, usually a basic chromium sulfate. In the typical one bath process, the hides are in a pickled state at a pH of 3 or lower, the chrome tanning materials are introduced, and the pH is raised. Following tanning, the chrome-tanned leather is piled down, wrung, and graded for the thickness and quality, split into flesh and grain layers, and shaved to the desired thickness. The grain leathers from the shaving machine are then separated for retanning, dyeing, and fat liquoring [4, 5]. Alum tanning (using potash alum) was the only method before the discovery of chrome tanning. Vegetable tannin is divided into two types, namely, pyrogallol type and catechol type. According to modern classification, tannin is of two types, hydrolysable type and condensed type. Effluent from chrome tanning step containing toxic chromium to the level of 2000 to 4000 mg/l is treated separately to recover and reuse chromium and the treated effluent can either be discharged or reused. Tanning effluent is the most toxic in nature [6]. The pH of the chrome tanned leather becomes 3.2 to 4.2. During ageing, after chrome tanning, acidity further increases due to the generation of sulphuric acid from chrome sulphate and water. Hence, neutralization is essential prior to dyeing operation. 1% borax solution is used for neutralization and the final pH of leather becomes around 6.9. Dyeing increases the commercial value of leather and improves certain aspects of skins and makes attractive and suitable for use. The nature of both dye and fibre of leather determine the mechanism of dyeing. Coating of leather surface with thin layer of oil is done by fatliquoring process. If the leather is not fatliquored, fibres break when bent and become hard on dyeing. Fatliquoring improves the tensile strength, stitch-tear resistance, abrasive resistance, etc., of dyed leather. Several fatty acids (palmitic, stearic, oleic, linoleic and linolenic), as well as few common oils, namely, castor oil, linseed oil, olive oil, etc., are used for fatliquoring. Finishing is the last and most important operation of a tannery where leather surface is coated with film of film forming material which is flexible, durable, stretchy, good looking. The characterizations of effluents emerging from different steps are summarized in Table 1. Wastewaters generated from these units are considered one of the most pollutant wastes due to presence of appreciable amount of organic materials (mainly dissolved fats, flesh, keratin, bones, etc.) and inorganic chemicals (various salts like sodium chloride, sodium sulfate, sodium sulfide, calcium hydroxide, etc). The presence of these substances causes high chemical oxygen demand (COD), biological oxygen demand (BOD), total dissolve solid (TDS), total solid (TS), conductivity etc. In the process, considerable amount of organic as well as inorganic chemicals are discharged, causing widespread aqueous and soil pollution. With increasing awareness of environmental conservation, government policy is now becoming stricter everyday and proper treatment of industrial wastewater (especially tannery effluents) has become an important social issue. In fact, in several states in India, especially in Tamil Nadu, the state government has issued order to tannery for having a zero discharge plant. This implies the tanneries must have their own facilities for treatment of wastewater in their site itself and the treated water should be reused in the plant.

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Table 1. Effluent characteristics pH

COD (mg/l)

BOD (mg/l)

TS (g/l)

S2-(g/l)

TSS (g/l)

Cr3+(g/l)

Cl- (g/l)

TDS (g/l)

Soaking

6-10.5

5000-11800

2000-5000

2.4-56.8

0-0.7

2.3-6.7

0

17-50

0.1-35.1

Liming

12-13.4

14500-40000

5000-20000

7-60.9

2-3.3

6.7-25

0

3.3-25

0.3-29.5

Deliming-bating

6-13

2500-10000

1000-4000

2.55-46.6

0.025-0.25

2.5-10

0

2.5-15

0.05-36.9

Pickling

1-6

800-3000

100-700

30-70

0

1-3

0

9-30

29-67

Degreasing

7.5-9.0

3000-5000

1000-1500

20-40 (29)

0

8-14

0

1.0-2.0

15-25

Tanning

3.2-4.2

400-1500

250-700

1.5-126

0

1.4-46.5

2.0-4.4

1.1-2.4

0.045-49.5

Neutralization

4-5

200-300

75-125

3.8-4.5

0

1.0-2.0

0

0.4-0.8

3.0-4.0

Dyeing

3.5-10

14000-75000

6000-15000

10.1-29.1

0

10.0-20.0

0-0.6

5-10

0.1-3.4

Fat liquoring

4-10

2500-7000

1000-2500

3-8

0

0.6-1.0

0-0.2

0.5-1

0.1-7.0

Finishing

4-9

2500-8000

1000-3000

10.2-25.0

0

2-4

0-0.2

5-10

13-21.0

Introduction

5

REFERENCES [1]

[2] [3]

[4]

[5]

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[6]

J. Raghava Rao, N.K. Chandrababu, C. Muralidharan, B.U. Nair, P.G. Rao and T. Ramasami, Recouping the wastewater: a way forward for cleaner leather processing. J. Cleaner Prod., 11 (2003) 591-599. G. Vidal, J. Nieto, K. Cooman, M. Gajardo and C. Bornhardt, Unhairing effluents treated by an activated sludge system. J. Hazard. Mater., B112 (2004) 143-149. V. Sivakumar, VJ Sundar, T Rangasami, C. Muralidharan and G. Swaminathan, Management of total dissolved solids in tanning process through improved techniques, J. Cleaner Prod., 13 (2005) 699-703. A. Cassano, R. Molinari, M. Romano and E. Drioli, Treatment of aqueous effluents of the leather industry by membrane processes A review. J. Membr. Sci., 181, (2001) 111126. A.F. Viero, A.C.R. Mazzarollo, K. Wada and I.C. Tessaro, Removal of hardness and COD from retanning treated effluent by membrane process, Desalination 149 (2002) 145-149. S.S. Datta, An introduction to the principles of leather manufacture, 4th Edition, Indian Leather Technologists’ Association, Kolkata, India, 1999.

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Chapter 2

TREATMENT METHODS

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CONVENTIONAL METHODS OF TREATMENT PROCESS Around 30 to 40 liters effluent are generated for every kilo finished leather production in India [1, 2]. It is around 75 to 80 liters in USA [2]. The primary treatment of effluent includes mixing of alkaline effluent of beam-house with acidic effluent from pickling and chrome tanning. It not only adjusts pH but also precipitates chromium hydroxide. Then the effluent is settled in settling tanks or lagoons. The supernatant is subjected to secondary treatment step. The first secondary treatment of effluent is chemical treatment with sulphuric acid (to bring down pH in the range of 5.2 to 5.5) followed by precipitation by alum. By adding ferric chloride after adjusting pH at 6.0 gives better removal of suspended materials. Sand bed filtration is also recommended in this regard. Secondary biological treatment by biodegradation is done thereafter. Both aerobic and anaerobic degradation systems are useful. Most popular aerobic biodegradations are activated sludge method and trickle filtration method. Anaerobic biodegradation is more or less identical with the aerobic activated sludge method except there is no use of oxygen. First mention of treatment of a tannery effluent in literature dates back to 1929 [3]. Combined effluent from tannery and degreasing plant of Winslow Bros. and Smith Co. in Norwood, USA, was collected in sedimentation tank. Sedimentation tank effluent was treated by aluminum sulphate and sulphuric acid [3]. The tannery wastes were mixed into a storage basin. These wastes were pumped from the storage basin to a mixing tank. Flue gas, taken from the power plant by tapping into the chimney, was blown in through a spider at the bottom of the tank until the waste solution was nearly saturated and the pH had been lowered to between 6.7 and 6.4. Lime water was added until the solution was basic. The system was well mixed and allowed to settle, or was pumped to another tank where it was allowed to settle. After about 3 hours, the supernatant liquid was shifted to another vessel and the treatment was repeated. After settling for 2 to 3 hours, the supernatant liquid was drawn and discharged to a stream or sewer. The sludge was collected from the tanks and dewatered [4]. Literature study reveals that the most universal treatment of tannery waste involves some form of coagulation and sedimentation of common effluent [5]. Treatment by activated sludge system is also used [6-8]. Thanikaivelan et al. have developed an enzyme-based dehairing using low amount of sodium sulfide, which completely avoids the use of lime [9]. An alternative bioprocess, based on α-amylase for fiber opening, has been attempted after enzymatic unhairing. This totally eliminates the use of lime in

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leather processing. This method enables subsequent processes and operations in leather making feasible without a deliming process [9]. Biological degradation of tannery effluent was carried out in a sequencing batch boifilm reactor (SBBR) and is combined with chemical oxidation by ozone [10]. The exhausted bath from chromium tannage contains about 30 - 40% of initial salt and it is normally sent to a cleaning-up plant. Here chromium salts end into the sludge creating serious problems for their disposal. The traditional method for chromium recovery from tanning effluent is based on the precipitation of chromium salt with NaOH followed by the dissolution of Cr(OH)3 in sulfuric acid [11, 12]. However, the quality of the recovered solutions is not always optimal due to the presence of metals, lipidic substances and other impurities. Extensive studies have been made to recycle the spent tanned liquor [13, 14]. Chromium precipitation from tanning spent liquor using industrial alkaline residues is also reported [15]. Kanagaraj et al have reported a recovery and reuse scheme of chromium by precipitation using neutralized wattle extract, one of the vegetable tannins left in the tanning bath [16]. Another method using actinimycets for biological treatment is reported [17]. However, recycling of chromium solution for tanning leads to accumulation of neutral salts, which reduces the uptake of chromium during tanning [18]. Chromium is recovered by employing precipitation, solvent extraction, and electrowinning steps [19]. Electrodialysis may be useful for selective separation of neutral salts from spent tanned liquor. However, the economic viability of the technique is yet to be established [20]. The IERECHROM (Ion Exchange REcovery of CHROMium), process is based on the use of a macroporous carboxylic resin, allowing removal and separation of almost pure (99%) chromium from other interfering metals (i.e., Fe, Al) and organic compounds for reuse. In the first regeneration step, chromium and aluminum are separated and the latter can be reused as coagulant in the general wastewater treatment plant. Chromium is recovered as chromate, and it could be reused (after makeup) in the plating industry or in the same tanneries after reduction to Cr(III). A final polishing step of the resin allows recovery of ferric sulfate and reuse as flocculating agent [21]. A coagulation/flocculation process is applied for the treatment of tannery effluent in which alkaline FeC13 is used as flocculating agent and Ca(OH)2 as base/precipitant [22]. In another study, Song et al. demonstrated that coagulation with aluminium sulphate and ferric chloride was an effective method to clarify tannery wastewater by reducing the COD, suspended solids (SS) and chromium content of the wastewaters [19]. Chromium tanned wastes generated as shaving dust is used as adsorbent. The adsorbent after tannin sorption was effectively used in the manufacture of chromium (III) sulfate for tanning process. The leathers made by employing the prepared and recycled chromium salts had satisfactory properties, comparable to conventionally processed leathers [23]. It may be mentioned here that generally tanneries are not using the modern technique of ultrasound assisted dyeing which could reduce the dye content in the exhausted liquor [24]. The effectiveness of the electrochemical process as a final treatment of vegetable tannery wastewater, allowing the complete removal of COD, tannin, and ammonium and decolorization was investigated [25]. An attempt was made to study the effectiveness of electroflotation for tannery wastewater remediation by Murugananthan et al. [26, 27]. Golder et al., investigated the removal of basic chrome sulphate (BCS) by electrocoagulation using mild steel and aluminum as electrode materials from tanning effluent [28-30].

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Treatment Methods

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LIMITATIONS OF CONVENTIONAL PROCESSES Chemical precipitation is the most widely used process for removal heavy metals from various industrial metal contaminated water and wastewater. About 75% of electroplating industries use precipitation treatment for heavy metals removal using either hydroxide precipitation, carbonate precipitation or sulfide precipitation or some combination of the three. Cr(VI) or As(III) cannot be removed by hydroxide precipitation. When hexavalent chromium is present, reduction of Cr(VI) to Cr(III) with sulfuric acid and with a reducing agent such as ferrous sulfate, sulfur dioxide, sodium bisulfite, sodium metabisulfite or sodium hydrosulfite is required. The reduction of Cr(VI) to Cr(III) is not 100% effective and the residual non-reduced Cr(VI) depends on reaction time, pH of the solution and concentration and type of reducing agents used. As(III) must be oxidized to arsenic As(V) before introducing into the precipitator. Metal hydroxide precipitates tend to re-solubilize if pH of the solution changes. Removal of various metals from multiple metals bearing wastewater may not be effective because the solubility of different metals is found to be in different pH ranges. Formation of metal hydroxide is ineffective in dilute metal bearing effluents. Due to relatively small particle size of the precipitates a filtration step is generally necessary after precipitation/ sedimentation. Cyanide interferes with heavy metal removal by hydroxide precipitation. Presence of complexing agents adversely affects the removal efficiency of metals. Removal efficiency is low at pH 1,000 Å; molecular weight of the solutes to be separated: >1,00,000. Following are major advantages of membrane separation processes over conventional processes; i) physical separation process; ii) no chemicals are used; iii) generally less energy requirement; iv) treatment of heat sensitive materials e.g., fruit juice; v) mild operating condition (operable under ambient temperature); vi) almost no damage to the species under processing; vii) no phase change; viii) less capital and operating cost; ix) easy to scale up (modular in nature) and x) simple process management (low manpower). Despite considerable progress, a membrane process has certain limitations that prevent its widespread applicability. There are two primary factors namely, concentration polarization and irreversible membrane fouling, which result in a decline in permeate flux. As the filtration progresses, the feed components accumulate over the membrane surface, known as concentration polarization. The polarized layer offers an extra resistance against the solvent flux leading to the decrease in permeation rate [48]. Concentration polarization directly leads to blocking of membrane pores by adsorption of solutes inside the pores. This reduces the permanent loss of membrane permeability. This phenomena is known as irreversible fouling. The combined effect of concentration polarization and irreversible fouling leads to decrease in permeate flux i.e., the productivity of the process. In recent years, membrane technologies have been developing rapidly and their cost is continuing to reduce while the application possibilities are ever extending [49, 50]. Research utilizing membrane separation for tannery effluent shows two distinct trends. First, effluent from different steps (except chrome tanning) are combined and are subjected to membrane separations preceded by an adequate pretreatment protocol. The second trend of research emphasizes that since each of tannery step is highly chemical consuming and generates sufficient wastewater, effluent from each step should be treated separately using hybrid membrane separation processes, with an appropriate pretreatment method. Nowadays, it is a common practice to treat the different waste streams separately rather than mixing them all [6]. The treated water has two streams. Chemical rich stream can be recycled back and thus lowering the operating cost. The other stream which has quite low BOD and COD content can be recycled as process water. This idea was first

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conceptualized by Cassano et al. (2001) [51] and lots of research works aiming at treatment of specific effluent have been reported. Application of UF, NF and RO for treatment of degreasing effluent is also reported [52-54]. Use of UF and NF for the treatment of effluent from liming and deliming-bating steps is available [55-57]. However, for the recovery and recycling of primary resources, e.g., chemicals, water, etc., membrane processes offer good opportunities [58]. Ahmed et al. have worked on treatment of liming effluent by nanofiltration [59]. They have studied the effect of pressure, initial feed concentration and pH in nanofiltration to reduce the conductivity, turbidity and COD of the pollution generated by sulfides in the effluent. Unhairing effluent treatment by activated sludge system is also reported [60]. More than 99% of BOD and around 80% of COD have been removed by activated sludge system. Cassano et al. (2001) proposed UF to treat the deliming - bating effluent. In the treated effluent, COD and fatty acid are substantially reduced and the clear solution can be reused for preparation of bating bath or wash water. Ultrafiltration (UF) and nanofiltration (NF) are used in conjunction to treat the liming effluent [61, 62]. More than 98% sulfates was separated by NF of the pickling wastewater and permeate quality was good enough to be reused [63]. Two approaches, namely, recycle/reuse of pickle liquor for the subsequent batches and resorting to pickle free alum-chrome combination tanning systems have been studied. Pickling liquor from the first batch has been recycled for subsequent batches with appropriate replacements. The use of potash alum for the pH reduction process has been employed in place of the conventional process. Significant total dissolved solids (TDS) reduction was observed using both approaches [64]. Pickling wastewater was reclaimed applying NF process. The retentate stream, with a high sulphates concentration was reused in pickling baths whereas the permeate stream, with a high chlorides concentration was pumped to the soaking drums [65]. NF membrane was used for both separating sulphates from a simulated pickling wastewater and reusing the chloride concentrated stream (permeates) in the pickling [66]. Tanning effluent is the most toxic in nature and large number of references using membrane based processes for its treatment is available [58, 59, 67-85]. Effluent from chrome tanning step containing toxic chromium to the level of 2000 to 4000 mg/l is treated separately to recover and reuse chromium and the treated effluent can either be discharged or reused. Tanning effluent is the most toxic in nature and large number of references using membrane based processes for its treatment is available [58, 59, 67-85]. Almost all leather made from lighter-weight cattle hides and from the skin of sheep, lambs, goats, and pigs is chrome tanned. Chrome tanning is performed using a one-bath process that is based on the reaction between the hide and a trivalent chromium salt, usually a basic chromium sulfate. In the typical one bath process, the hides are in a pickled state at a pH of 3 or lower, the chrome tanning materials are introduced, and the pH is raised. Following tanning, the chrome-tanned leather is piled down, wrung, and graded for the thickness and quality, split into flesh and grain layers, and shaved to the desired thickness. The grain leathers from the shaving machine are then separated for retanning, dyeing and fat liquoring [61]. However, application of dualmembrane systems including MF/NF, UF/RO and NF/RO for treatment of tannery waste and recovery of chrome and/or other chemicals is challenged by the presence of considerable BOD load and proteins, which may cause fouling and subsequent system failure temporarily or permanently [86].

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UF for colour removal of leather dyeing effluent is also reported [87]. But this work does not give a complete treatment procedure of the dyeing effluent as the treated water contains high BOD and COD level. In view of this, some additional work is needed. Use of membrane bioreactor (MBR) [88, 89] and a combination of MBR with RO treatment process is found to be highly effective for the removal of organic pollutants and suspended solids from mixed tannery effluent [90]. Suthanthararajan et al. [91] propose an exhaustive treatment for the pretreated wastewater from a tanning industry consisting of a sand filter, a photochemical oxidation step, carbon filter, a softener, chemicals dosage (acid, anti-oxidant and anti-scaling), cartridge filter, NF and RO. This proposed treatment showed excellent results but from an economical point of view the investment can not be afforded by most of tanneries. Integrated wastewater was treated by physico-chemical treatment, 20 micron cartridge filter, UF and RO for water reuse [92]. It is not advisable to treat the combined effluent because tanning effluent contains toxic chromium. Even if the streams except tanning are mixed, this creates two types of problems regarding treatment by membrane based processes. First, the organic and inorganic loading is so high that the membranes get blocked quickly. Second, recovery of chemicals after treatment will not be specific and their re-use in appropriate units will not be proper. To overcome these difficulties, the strategy for treatment of effluent from individual unit should be considered. Prior to membrane filtration, an appropriate pretreatment method should be used to reduce the load of organic, inorganic and suspended materials, so that subsequent filtration will be feasible. Although there are reports for treatment of effluent from some of the units of a tannery but several crucial issues need to be answered. First, the appropriate pretreatment protocol, identification of coagulant, its dose etc., should be established. Second, even after pretreatment, the eligibility of the effluent for membrane treatment should be evaluated. Third, if they are suitable for membrane treatment, selection of the membrane is crucial. Fourth, need for multi step filtration should be evaluated to confirm the quality of the final product. Finally, even if the filtration steps are identified, the selections of suitable operating conditions are essential. In the subsequent chapters of this book, all these issues have been addressed. A comprehensive coverage of membrane based treatment for various effluent from different units of a tannery and related important issues are addressed and presented in the subsequent chapters of this book. Different membrane separation operations for the treatment of individual tannery effluent are discussed in detail. Suitable membrane separation processes are selected first. To minimize fouling, effluent is pretreated prior to membrane filtration. Fertilizer value of the sludge obtained after pretreatment is identified. Effect of transmembrane pressure drop and change in hydrodynamics (laminar, turbulent and laminar with turbulent promoter zone) on steady state permeate flux and permeate concentration are also identified. The percentage enhancements of the permeate flux in laminar regime with turbulent promoters are studied. Variation of permeate quality (in terms of COD for soaking, liming, deliming-bating, pickling, degreasing and dyeing and chromium concentration for tanning effluent) with different operating conditions are discussed.

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[20] R.R. Rao, B.G.S. Prasad, V. Narasimhan, T. Ramasami, P.R. Shah and A.A. Khan, Electro dialysis in the recovery and reuse of chromium from industrial effluent. J. Membr. Sci., 46 (1989) 215-224. [21] D. Petruzzelli, R. Passino and G. Tiravanti, Ion exchange process for chromium removal and recovery from tannery wastes, Ind. Eng. Chem. Res., 34 (1995) 2612-2617. [22] J.I. Garrote, M. Bao, P. Castro and M.J. Bao, Treatment of tannery effluent by a two step coagulation/flocculation process, Water Res., 29 (1995) 2605-2608. [23] K.J. Sreeram, S. Saravanabhavan, J. Raghava Rao and B.U. Nair, Use of chromiumcollagen wastes for the removal of tannins from tannery wastewater, Ind. Eng. Chem. Res., 43 (2004) 5310-5317. [24] V. Sivakumar and P.G. Rao, Power ultrasound-assisted cleaner leather dyeing technique: influence of process parameters, Environ. Sci. Technol.; 38(5) 2004; 16161621. [25] M. Panizza and G. Cerisola, Electrochemical oxidation as a final treatment of synthetic tannery wastewater, Environ. Sci. Technol. 38, 2004, 5470-5475. [26] M. Murugananthan, G.B. Raju and S. Prabhakar, Separation of pollutants from tannery effluent by electro flotation, Sep. Purif. Technol., 40 (2004) 69–75 [27] M. Murugananthan, G.B. Raju and S. Prabhakar, Removal of sulfide, sulfate and sulfite ions by electro coagulation, J. Hazard. Mater. B109 (2004) 37–44. [28] A.K. Golder, A.N. Samanta and S. Ray, Removal of trivalent chromium by electrocoagulation, Sep. Purif. Technol., 53 (2007) 33–41. [29] A.K. Golder, A.N. Samanta and S. Ray, Removal of Cr3+ by electrocoagulation with multiple electrodes: Bipolar and monopolar configurations, J. Hazard. Mater. 141 (2007) 653–661. [30] A.K. Golder, A.N. Samanta and S. Ray, Trivalent chromium removal by electrocoagulation and characterization of the process sludge, J. Chem. Technol. Biot., 82 (2007) 496-503. [31] Metcalf and Eddy, Inc, Wastewater Engineering Treatment and Reuse, 4th Edition, Tata McGraw-Hill Publishing Company, New Delhi, India. [32] S.L. Daniels, Removal of heavy metals by iron salts and polyelectrolyte flocculants, AIChE Symp. Ser., 71(151) (1975) 265-271. [33] R.P. Peters and Y. Ku, Batch precipitation studies for heavy metal removal by sulfide precipitation, AIChE Symp. Ser., 81(243) (1985) 9-27. [34] L. Charerntanyarak, Heavy metals removal by chemical coagulation and precipitation, Water Sci. Technol., 39 (10/11) (1999) 135-138. [35] G.M. Ayoub, L. Semerjian, A. Acra, M. El Fadel and B. Koopman, Heavy metal removal by coagulation with seawater liquid bittern, J. Environ. Eng., 127 (3) (2001) 196-202. [36] Y. Li, X. Zeng, Y. Liu, S. Yan, Z. Hu and Y. Ni, Study on the treatment of copperelectroplating wastewater by chemical trapping and flocculation, Sep. Purif. Technol., 31 (2003) 91-95. [37] B.K. Dutta, Principles of Mass Transfer and Separation Processes, Prentice-Hall of India Private Limited, India, 2007. [38] P. Pelosi and J. McCarty, Preventing fouling of ion exchange resins: II, Chem. Eng., 89(18) (1982) 125-128.

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[39] A. Blanco, B. Sanz, M.J. Llama and J.L. Serra, Biosorption of heavy metals to immobilised Phormidium laminosum biomass, J. Biotechnol., 69 (1999) 227-240. [40] V. Boonamnuayvitaya, C. Chaiya, W. Tanthapanichakoon and S. Jarudilokkul, Removal of heavy metals by adsorbent prepared from pyrolyzed coffee residues and clay, Sep. Purif. Technol., 35 (2004) 11-22. [41] M. Kobya, E. Demirbas, E. Senturk and M. Ince, Adsorption of heavy metal ions from aqueous solutions by activated carbon prepared from apricot stone, Bioresour. Technol., 96 (2005) 1518-1521. [42] Y. Suzuki, T. Kametani and T. Maruyama, Removal of heavy metals from aqueous solution by nonliving Ulva seaweed as biosorbent, Water Res., 39 (2005) 1803-1808. [43] S.S. Ahluwalia and D. Goyal, Microbial and plant derived biomass for removal of heavy metals from wastewater, Bioresour. Technol., 98(12) (2007) 2243-2257. [44] K. Chojnacka, Biosorption and bioaccumulation of microelements by Riccia fluitans in single and multi-metal system, Bioresour. Technol., 98(15) (2007) 2919-2925. [45] M.A. Hanif, R. Nadeem, M.N. Zafar, K. Akhtar and H.N. Bhatti, Kinetic studies for Ni(II) biosorption from industrial wastewater by Cassia fistula (Golden Shower) biomass, J. Hazard. Mater., 145(3) (2007) 501-505. [46] M. Ziagova, G. Dimitriadis, D. Aslanidou, X. Papaioannou, E. Litopoulou Tzannetaki and M. Liakopoulou-Kyriakides, Comparative study of Cd(II) and Cr(VI) biosorption on Staphylococcus xylosus and Pseudomonas sp. in single and binary mixtures, Bioresour. Technol., 98(15) (2007) 2859-2865. [47] M.Y.A. Mollah, R. Schennach, J.R. Parga and D.L. Cocke, Electrocoagulation (EC) science and applications, J. Hazard. Mater., B84 (2001) 29-41. [48] R. Rautenbach and R. Albrecht, Membrane Processes, John Wiley and Sons, 1994, pp 84. [49] R.W. Baker, Membrane Separation Systems-Recent development, future direction, Noyes Data Corporation (1991) pp. 329. [50] P. Ball, Scale-up and scale down of membrane based separation processes. Membr. Technol., 117 (1999) 10–13. [51] A. Cassano, R. Molinari, M. Romano and E. Drioli, Treatment of aqueous effluents of the leather industry by membrane processes A review, J. Membr. Sci., 181, (2001) 111126. [52] A. Cassano, E. Drioli and R. Molinari, Recovery and reuse of chemicals in unhairing, degreasing and chromium tanning processes by membranes, Desalination 113 (1997) 251-261. [53] A. Cassano, E. Drioli and R. Molinari, Introduction to ultrafiltration into unhairing and degreasing operation, J. Soc. Leather Technologists Chemists 82 (1998) 130-135. [54] A. Cassano, A. Criscuoli, E. Drioli and R. Molinari, Clean operations in the tanning industry: aqueous degreasing coupled to ultrafiltration: experimental and theoretical analysis, Clean Prod. Processes 1 (4) (1999) 257-263. [55] P. Thanikaivelan, J. Raghava Rao, B.U. Nair and T. Ramasami, Approach towards zero discharge tanning: role of concentration on the development of eco-friendly limingrelinig processes, J. Cleaner Prod., 11 (2003) 79–90. [56] M.T. Ahmed, S. Taha, T. Chaabane, D. Akretche, R. Maachi and G. Dorange, Treatment of the tannery effluents from a plant near Algiers by nanofiltration (NF): experimental results and modeling, Desalination 165 (2004) 155-160.

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[57] M.T. Ahmed, S. Taha, T. Chaabane, D. Akretche, R. Maachi and G. Dorange, Nanofiltration process applied to the tannery solutions, Desalination 200 (2006) 419420. [58] A. Cassano, L. Della Pietra and E. Drioli, Integrated membrane process for the recovery of chromium salts from tannery effluents, Ind. Eng. Chem. Res., 46 (2007) 6825-6830. [59] M.T. Ahmed, S. Taha, T. Chaabane, N. BenFarès, A. Brahimi, R. Maachi and G. Dorange, Treatment of sulfides in tannery baths by nanofiltration, Desalination 185, (2005), 269-274. [60] G. Vidal, J. Nieto, K. Cooman, M. Gajardo and C. Bornhardt, Unhairing effluents treated by an activated sludge system, J. Hazard. Mater., B112 (2004) 143-149. [61] E. Drioli, A. Caggiano and C. Cammisa, Uno studio sull’introdu-. zione dell’ultrafiltrazione nel processo di concia delle pelli, Acqua and Aria 4 (1982) 391– 398 [62] M.T. Ahmed, S. Taha, T. Chaabane, N. BenFarès, A. Brahimi, R. Maachi and G. Dorange, Treatment of sulfides in tannery baths by nanofiltration, Desalination 185 (2005) 269-274. [63] M.V.G. Aleixandre, A.I. Clar, A.B. Pia, J.A.M. Roca, B.C. Uribe and M.I.I. Clar, Nanofiltration for sulfate removal and water reuse of the pickling and tanning processes in a tannery, Desalination 179 (2005) 307-313. [64] V. Sivakumar, V.J. Sundar, T. Rangasami, C. Muralidharan and G. Swaminathan, Management of total dissolved solids in tanning process through improved techniques, J. Cleaner Prod., 13 (2005) 699-703. [65] M.H. Davis and J.G. Scroggie, Theory and practice of direct chrome liquor recycling, Das Leder 31 (1980) 1-8. [66] B.C. Uribe, A.I. Clar, A.B. Piá, J.A.M. Roca, M.V.G. Aleixandre and M.I.I. Clar, Nanofiltration of a simulated tannery wastewater: influence of chlorides concentration, Desalination 191 (2006) 132-136. [67] W. Scholz and W. Bowden, Application of membrane technology in the tanning industry, Leather 201 (4694) (1999) 17–18. [68] A.F. Viero, A.C.R. Mazzarollo, K. Wada and I.C. Tessaro, Removal of hardness and COD from retanning treated effluent by membrane process, Desalination 149(1-3), (2002) 145-149. [69] N.P. Slabbert, Recycling in the tanning industry, J. Soc. Leather Traders Chem., 64 (1980) 89-92. [70] O.O. Hart, A.E. Simpson, CA Buckley, GR Groves, FGND Wild, The treatment of industrial effluents with high salinity and organic contents, Desalination 67( 1987) 395407. [71] A. Cassano, E. Drioli, R. Molinari and C. Bertolutti, Quality improvement of recycled chromium in the tanning operation by membrane processes, Desalination 108 (1996) 193-203. [72] C. Fabiani, F. Ruscio, M. Spadoni and M. Pizzichini, Chromium (III) salts recovery process from tannery wastewaters, Desalination 108 (1997) 183-191. [73] M.A. Chaudry, S. Ahmad, M.T. Malik, Supported liquid membrane technique applicability for removal of chromium from tannery wastes, Waste manage., 17 (1997) 211-218.

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[74] M. Aloy and B. Vullermet, Membrane technologies for the treatment of tannery residual floats, Industrie du Cuir 2 (1998) 43-48. [75] A. Cassano, R. Molinari and E. Drioli, Saving of water and chemicals in tanning industry by membrane processes, Water Sci. Technol., 40 (1999) 443-450. [76] P. Padilla, E.L. Tavani, Treatment of an industrial effluent by reverse osmosis, Desalination 126 (1999) 219-226. [77] W.S.W. Ho, T.K. Poddar, New membrane technology for removal and recovery of chromium from wastewater, Environ. Prog., 20(2001)44-52. [78] H.F. Shaalan, M.H. Sorour and S.R. Tewfik, Simulation and optimization of a membrane system for chromium recovery from tanning wastes, Desalination 141 (2001) 315-324. [79] V.J. Sundar, J. Raghava Rao and C. Muralidharan, Cleaner chrome tanning-emerging options, J. Cleaner Prod., 10 (2002) 69-74. [80] A.I. Hafez, M.S.E. Manharawy and M.A. Khedr, RO membrane removal of unreacted chromium from spent tanning effluent. A pilot-scale study, Part 2, Desalination 144 (2002) 237-242. [81] A. Cassano, J. Adzet, R. Molinari, M.G. Buonomenna, J. Roig and E. Drioli, Membrane treatment by nanofiltration of exhausted vegetable tannin liquors from the leather industry, Water Res., 37 (2003) 2426-2434. [82] W. Scholz and M. Lucas, Techno-economic evaluation of membrane filtration for the recovery and re-use of tanning chemicals, Water Res., 37 (2003) 1859-1867. [83] A. Hafez and S.E. Manharawy, Design and performance of the two-stage/two-pass RO membrane system for chromium removal from tannery wastewater. Part 3, Desalination 165 (2004) 141-151. [84] L.M. Ortega, R. Lebrun, I.M. Noël and R. Hausler, Application of nanofiltration in the recovery of chromium (III) from tannery effluents, Sep. Purif. Technol., 44 (2005) 4552. [85] C. Covarrubias, R. García, R. Arriagada, J. Yánez, H. Ramanan, Z. Lai and M. Tsapatsis, Removal of trivalent chromium contaminant from aqueous medium using FAU-type zeolite membranes, J. Membr. Sci., 312 (2008) 163-173. [86] H.F. Shaalan, M.H. Sorour and S.R. Tewfik, Simulation and optimization of a membrane system for chromium recovery from tanning wastes, Desalination 141 (2001) 315-324. [87] A.M.B. Alves and M.N. de Pinho, Ultrafiltration for colour removal of tannery dyeing wastewater, Desalination 130 (2000) 147-154. [88] T. Reemtsma, B. Zywicki, M. Stueber, A. Kloepfer and M. Jekel, Removal of sulfurorganic polar micropollutants in a membrane bioreactor treating industrial wastewater, Environ. Sci. Technol.; 2002; 36(5); 1102-1106. [89] T. Reemtsma, O. Fiehn, G. Kalnowski and M. Jekel, Microbial transformations and biological effects of fungicide-derived benzothiazoles determined in industrial wastewater, Environ. Sci. Technol. 1995, 29(2) 478-485. [90] W.G. Scholz, P. Rouge, A. Bodalo and U. Leitz, Desalination of mixed tannery effluent with membrane bioreactor and reverse osmosis treatment, Environ. Sci. Technol.; 2005; 39(21); 8505-8511.

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[91] R. Suthanthararajan, E. Ravidranath, K. Chitra, B. Umamaheswari, T. Ramesh and S. Rajamani, Membrane application for recovery and reuse of water from treated tannery wastewater, Desalination 164 (2004) 151-156. [92] M.F. Roger, J.A.M. Roca, M.V. Aleixandre, A.B. Pia´, B.C. Uribe and A.I. Clar Reuse of tannery wastewaters by combination of ultrafiltration and reverse osmosis after a conventional physical-chemical treatment, Desalination 204 (2007) 219-226.

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Chapter 3

TREATMENT OF SOAKING EFFLUENT ABSTRACT As mentioned earlier, soaking is first tannery operation. In this operation, skins and hides are soaked with water and small quantities of imbibing substances to rehydrate the skin proteins, to remove salts used in curing step (for preventing putrefaction and decomposition) and also to solubilize the denatured proteins. Small and thin leather (before tanning) is called skin and big, fat and strong leather (before tanning) is called hide. During curing, hides and skins lose large quantity of its physiological content of water. They regain water during soaking operation and therefore, good quality leather is produced. Soaking operation produces huge amount of effluent containing high BOD and COD.

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3.1. SOAKING EFFLUENT The characterization of a typical soaking effluent collected from M/s, Alison Tannery, Kolkata, India is shown in Table 3.1. Effluent is collected after one to two hours of the completion of the soaking operation. As discussed in earlier chapter, the soaking effluent is subjected to a suitable pretreatment process prior to membrane filtration. Table 3.1. Characterization of soaking effluent Effluent

Soaking

10.5

Conductivity (S/m) 5.38

TS (g/l) 56.8

TDS (g/l) 35.1

COD (ppm) 9280

BOD (ppm) 3569

Cl(g/l) 20.59

Ca++ (g/l) 1.0

7.25

4.85

43.1

32.3

4120

1585

22.4

1.18

pH Feed After alum dose

3.2. PRETREATMENT The soaking effluent is subjected to coagulation by alum. Coagulation study using commercial potassium alum has been carried out in eight graduated cylinders of 50 ml

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capacity with different dosages of alum, namely, 0.02, 0.1, 0.3, 0.5, 1, 2, 3 and 4% (weight by volume) for twenty four hours. It may be pointed out that the rate of coagulation remains almost unchanged beyond half an hour. The optimum alum dose is established by examining various properties (e.g., pH, TDS, conductivity, TS, COD, turbidity) of supernatant solutions. Various properties of the clear liquid after coagulation at different alum concentrations are presented in Figure 3.1.

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Figure 3.1. Determination of optimum alum dose for soaking effluent.

It may be observed from the Figures 3.1 (a) and (b) that beyond an alum concentration of 2%, TDS, conductivity and TS concentration increase significantly. COD of the clarified liquor decreases with alum concentration and beyond 2%, the change is gradual. It may also be noted that with increasing concentration of alum, the turbidity of the solution decreases (with more settling of solids) and beyond 2% the turbidity increases rapidly. It is also observed from Figure 3.1 (b) that the pH of the clear solution is close to the normal pH (~7.25) at 2% alum concentration and it decreases further with increase in alum dose. From these observations, 2% is selected as the optimum concentration of alum for coagulation. The clarified liquor after optimum alum dosing is subjected to membrane filtration after a fine cloth filtration [1].

3.3. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of membrane with an appropriate molecular weight cut-off (MWCO) is the most crucial step. The permeate flux and qualities are important aspects in this regard. A high permeate flux is necessary for filtration to be practical and economic, and product quality should at least meet as those obtained by other standard treatment techniques [2]. The permeate flux during filtration depends on operating conditions (transmembrane pressure, temperature and turbulence), nature of membrane, molecular weight cut off and nature of feed solution. Keeping the other factors constant, water flux increases with MWCO because the membrane permeability is proportional to the square of the pore radius [3].

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3.3.1. Membranes Membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. These membranes are supplied by M/s, Permionics Membranes Pvt. Ltd., Gorwa, Vadodara, India. NF membrane of MWCO 400, consisting of a polyamide skin over a polysulphone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. Polyamide thin film composite membrane is used for reverse osmosis. The membrane is also supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. Constituting polymeric material and the pure water permeability of the membranes are reported in Table 3.2 for UF and NF membranes and those for membranes employed in RO are provided in Table 3.3. Table 3.2. Specifications of UF and NF membranes used during filtration MWCO

Lp×1011(m/Pas)

Membrane material

20 kDa

5.85

Poly ether sulfone (PES)

15 kDa

5.10

Poly amide (PA)

10 kDa

4.20

Poly ether sulfone (PES)

5 kDa

3.91

Thin film composite (TFC)

1 kDa

3.35

Poly amide (PA)

400 Da

2.60

Thin film composite (TFC)

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Table 3.3. Specifications of RO membrane used during filtration Lp×1012(m/Pas)

Membrane material

8.19

Thin film composite (TFC)

3.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred experiments are conducted in a 650 ml capacity filtration cell in a continuous mode. The feed tank of 5 liter capacity is connected to a single cylinder reciprocating pump. The pump discharge is fed to the cell. The stirrer speed is set using a variac (variable A.C. transformer for smooth control of voltage and thereby stirring speed) and it is measured by a hand held digital tachometer (Agronic, India). Inside the cell, a circular membrane is placed over a base support. The membrane diameter is 6.7 cm and the effective membrane area is 35.26 cm2. The permeate is collected from the bottom outlet of the cell. The schematic of the experimental set up is shown in Figure 3.2. The operating pressure used during experiment is fixed at 414 kPa and 828 kPa for UF and NF, respectively. The stirring speed is 1000 rpm. The duration of each experiment is 45 minutes. All the experiments are conducted at a room temperature of 30±20C.

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Figure 3.2. Schematic of the experimental setup: (1) Feed tank; (2) Pump; (3) Filtration cell; (4) Mechanical stirrer; (5) Pressure gauge; (6) Retentate valve; (7) Bypass valve; (8) Permeate sampling.

The steps used in the experiments are as follows: The tannery effluent is cloth-filtered to remove suspended impurities. A fresh membrane is compacted at a pressure higher than maximum operating pressure for 3 h using distilled water. Membrane permeability is determined using distilled water. Flux values at various operating pressures are measured and the slope of flux versus pressure plot gives the permeability. The stirrer speed is set using a variac (variable A.C. transformer for smooth control of voltage and thereby stirring speed) and it is measured by a hand held digital tachometer (Agronic, India). Controlling the valve before the rotameter, the flow rate is controlled independently. Cumulative volumes of permeate are collected during the experiment. Values of permeate flux are determined from the slopes of cumulative volume versus time plot. Permeate samples are collected at different times for analysis. The duration of the cross-flow experiment is 1 h. Once an experimental run is over, the membrane is thoroughly washed, in situ, with distilled water for 15 minutes applying a maximum pressure of 200 kPa. After dismantling the set up, the membrane is rinsed with water and dipped in 0.12 (N) HCl solution for three hours. Next, the membrane is washed carefully with distilled water to remove traces of acid or surfactant. The cell is reassembled and the membrane permeability is again measured using distilled water. It is observed that the membrane permeability remains almost constant between successive runs.

3.3.3. Performance Testing of Various Membranes Pretreated effluent is subjected to the stirred cell in continuous mode using various MWCO of membranes, starting from NF (MWCO 400 Da) to UF (MWCO ranging from 1 kDa to 20 kDa). Transient flux decline behavior using various membranes is shown in Figure 3.3. In Figure 3.3, operating pressure for all the UF runs is 414 kPa and that for NF is 828 kPa. General trend for permeate flux profiles in Figure 3.3 is that the flux decreases with the operating time. This is due to concentration polarization. As filtration progresses, solute

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particles deposit over the membrane surface, forming a polarized layer which grows in thickness. Some of the pores in the membrane are also clogged by the solutes. This is confirmed by observing the fact that pure water flux reduces when the same membrane is used without any chemical cleaning (cleaning by water only) just after the experiment. Combined effects of these phenomena lead to a decline in flux. It is observed that about 37% decline in flux occurs for 20 kDa MWCO membrane over the 20 minutes of operating time. This is about 37%, 40% and 37% for 10 kDa, 5 kDa and 1 kDa cut-off membranes. Interestingly, flux decline over the duration of the experiment for NF membrane is only about 9%. It may be pointed out that although the value of flux increases from 1 kDa to 20 kDa cutoff membranes, the flux decreases steadily during the filtration time. On the other hand, for NF membrane, flux becomes almost constant beyond 10 minutes of operation. Since, the retentate and permeate streams are recycled to the feed chamber, both the feed volume and feed concentration remain unchanged and therefore, the value of steady state permeate flux remains same beyond 10 minutes as evident from Figure 3.3. This indicates that the UF membranes having larger pore size (in the increasing order of 1 kDa to 20 kDa), they are more susceptible to pore clogging by the solute particles, resulting in steady decline in flux, although flux decline becomes gradual later on. For NF membrane, larger solute particles cannot enter the pores at all, leading to formation of a polarized layer over the membrane surface whose thickness remains constant by external stirring and hence almost a steady state flux is resulted beyond 10 minutes and flux decline is also minimum in this case. Suitability of a membrane separation process depends not only on its permeate flux, i.e., productivity but also on the permeate quality. It is observed from Table 3.4 that reduction in COD is only 1.2% in 20 kDa, 16% for 10 kDa, 24% for 5 kDa, 33% for 1 kDa and 78% for NF membrane. Variation of total solid concentration in all UF membranes is almost insignificant (about 40 g/l TS concentration with respect to 43 g/l in the alum treated feed). But NF membrane shows about 26% retention of total solids.

Figure 3.3. Flux decline of pretreated soaking effluent using various membranes.

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Table 3.4. Permeate properties of soaking effluent MWCO 20 kDa 10 kDa 5 kDa 1 kDa 400 Da

COD (mg/l) 4072 [4120]* 3451 3120 2744 897

TS (g/l) 42.9 [43.1]* 42.5 40.0 38.6 31.7

TDS (g/l) 32.3 [32.3]* 31.2 30.2 30.0 25.9

*

Values indicate the properties corresponding to feed.

As expected, the TDS retention (i.e. retention of inorganic solutes) by UF membrane is marginal, whereas 400 Da NF membrane shows about 20% retention of inorganic solutes in terms of TDS. Therefore, as far as the quality of the permeate is concerned, NF membrane shows the most promising performance. It may be noted here that the permeate quality after NF is still not adequate to discharge in the sewage (discharge limit for COD is 250 mg/l and for BOD is 30 mg/l). For this, the permeate from nanofiltration may be subjected to RO. Therefore, 400 Da MWCO NF membrane followed by RO should be the selected membrane process for the treatment of pretreated soaking effluent.

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3.4. DETAILED STUDY OF TREATMENT OF SOAKING EFFLUENT A scheme is proposed to treat the soaking effluent using a hybrid process, including alum coagulation, nanofiltration and reverse osmosis. The supernatant of pretreated liquor is subjected to continuous cross flow nanofiltration followed by reverse osmosis. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 3.4.

Figure 3.4. Proposed scheme for the treatment of soaking effluent.

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3.4.1. Cross Flow Cell and Operating Conditions

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A rectangular cross-flow cell, made of stainless steel, is designed and fabricated. The cell consists of two matching flanges as shown in Figure 3.5a. The inner surface of the top flange is mirror polished. The bottom flange is grooved, forming the channels for the permeate flow. The channels in the bottom flange with the internal grid structure are shown in Figure 3.5b. A porous stainless steel plate is placed on the lower plate that provides mechanical support to the membrane. Two neoprene rubber gaskets are placed over the membrane; the top view of which is shown in Figure 3.5c. Sixteen equispaced thin wires of diameter 0.19 mm are placed laterally (along the width of the channel) in between the two gaskets (shown in Figure 3.5d), as turbulent promoters. The spacing between the turbulent promoters is 14.0 mm. The two flanges are tightened to create a leak proof channel. The effective length and width of the membrane available for filtration are 26 cm and 4.9 cm, respectively. The height of the flow channel is determined by the thickness of the gaskets after tightening the two flanges and is found to be 3.4 mm. The obstruction in the flow path due to the wires promotes localized turbulence.

The schematic of the experimental setup is shown in Figure 3.6. The pretreated effluent is placed in a stainless steel feed tank of 10 l capacity. A high pressure reciprocating pump is used to feed the effluent into the cross-flow membrane cell. The retentate stream is recycled to the feed tank routed through a rotameter. The permeate stream is also recycled to maintain a constant concentration in the feed tank. A bypass line from the pump delivery to the feed tank is provided. The two valves in the bypass and the retentate lines are used to vary the pressure and the flow rate through the cell, independently.

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Figure 3.5. Membrane module assembly.

Figure 3.6. Experimental setup for cross flow membrane module.

Experiments are carried out in the cross flow cell to observe the effect of cross flow rate and transmembrane pressure drop. The operating conditions for all membrane experiments are presented in Table 3.5.

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Sirshendu De, Chandan Das and Sunando DasGupta Table 3.5. Operating conditions for cross flow experiments Reynolds number

Step

Transmembrane pressure (kPa)

Laminar and with promoter

Turbulent

NF RO

828, 966 and 1104 1515, 1725 and 1932

680, 1020 and 1360 1020

4762, 5442 and 6122 4762, 5442 and 6122

3.4.2. Analysis of Transient Flux Decline

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Nanofiltration The experimental flux decline for transient state in the three flow regimes at a constant pressure of 828 kPa and varying Reynolds number is shown in Figure 3.7. Flux decline at constant Reynolds number (680 for laminar and 4762 for turbulent) and varying pressure is shown in Figure 3.8. It can be clearly seen from the Figure 3.7 that the time required to reach steady state decreases with increase in Reynolds number. For example, it can be observed from Figure 3.7 that the steady state is attained in about 9.3 min, for Re=4762 and 828 kPa pressure, whereas at the same pressure but at Re=5442 and Re=6122, the steady states are attained within 7.3 min and 5.5 min, respectively.

Figure 3.7. Permeate flux decline profile with time for NF membrane at 828 kPa.

The flux decline is about 25% of the initial value for Re=4762, about 20% with increase in Re= 5442, and 15 % at Re=6122. Similar trends can be observed for flux decline in laminar regime with and without promoters. As the cross flow velocity increases, the growth of the polarized layer over the membrane surface is slower because of enhanced forced convection. This leads to the onset of steady state at an earlier time. For the above reason, the resistance to

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the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Figure 3.7, at Re=680 and 828 kPa, the steady state is attained in about 17.1 min without promoter and about 12.7 min with promoter at the same operating condition. The flux decline is about 31% without promoter at Re=680 and 828 kPa pressure; but only 21% using promoter at the same operating condition. Use of the turbulent promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the polarized layer is controlled quickly, establishing steady state earlier than without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less compared to the no promoter case.

Figure 3.8. Permeate flux decline profile with time for NF membrane at various pressures.

From Figure 3.8 it can be observed that at a fixed Reynolds number, the steady state is attained faster with an increase in operating pressure. For example in Figure 3.8, steady state is attained in about 4.9 min for an operating condition of Re=4762 and at 1104 kPa pressure. Whereas at the same Reynolds number, the time required to attain steady state is about 9.3 min for an operating pressure 828 kPa.

Reverse Osmosis The permeate from NF is collected and treated using RO in the same cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating pressure difference. The flux versus time data of experimental values are plotted in Figure 3.9 for all three working regimes of operation namely: laminar, laminar with promoter and turbulent.

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Figure 3.9 clearly shows that as in the case of NF, the time required to reach steady state decreases with increase in cross flow velocity and transmembrane pressure and also in presence of turbulent promoters. Extent of flux decline also follows similar trends for reasons already discussed earlier.

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Figure 3.9. Permeate flux decline profile with time for RO membrane.

3.4.3. Analysis of Steady State Flux Nanofiltration The variations of steady state permeate flux with pressure at different Reynolds number under turbulent flow, laminar flow without and with turbulent promoters are shown in Figure 3.10. The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier. Reverse Osmosis The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to higher turbulence induced by higher Reynolds number. The effects of trans-membrane pressure and Reynolds number on steady state flux in RO are shown in Figure 3.11. The figure shows that the permeate flux increases with operating pressure and Reynolds number as in NF. At 1518 kPa, an increase in Reynolds number from 4762 to 6122 results in about 26% increase in permeate flux. From Figure 3.11 it can be seen that at 1932 kPa, the flux increment is about 40% for laminar flow with promoter at Re=1020.

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Figure 3.10. Variation of permeate flux with transmembrane pressure in NF.

Figure 3.11. Variation of permeate flux with transmembrane pressure in RO.

3.4.4. Analysis of Various Resistances The permeate flux at any point of time is expressed as,

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Sirshendu De, Chandan Das and Sunando DasGupta

J=

ΔP μ[ Rm + R p (t )]

(3.1)

where, Rm is the membrane resistance and Rp is the polarized layer resistance, which is a function of time. From the steady state flux values obtained from the experimental results, the polarized layer resistance at the steady state is calculated as,

R ps =

ΔP − Rm μ J ss

(3.2)

The variation of dimensionless steady state polarized layer resistance with Reynolds number are presented in Figure 3.12, for turbulent flow regime and Figure 3.13, for laminar and with promoter conditions. It is observed from these figures that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure of 828 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 7.2 to 6.6 with an increase in Reynolds number from 680 to 1020. Rp values increase with the transmembrane pressure. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number (680) and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the resistance to 5.0 compared to 7.2 in laminar

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flow. This reduction in

Rps

is more than 34% in some of the experiments leading to a

significant enhancement of the permeate flux. These figures also show further reductions for the case of purely turbulent flows for reasons already discussed. It may be observed from Figures 3.12 and 3.13 that Reynolds number has a significant effect on the polarized layer resistance. For laminar flow with and without promoter, polarized layer resistance is the major contributing resistance. For example, in case of pure laminar flow, at Reynolds number=680 and transmembrane pressure at 1104 kPa Rm and Rp constitute about 12% and 88% of the total resistance, respectively. In case of laminar flow with promoter, at the same operating condition, contribution of Rp decreases to 84% of the total resistance. For turbulent flow regime, effects of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the range of Reynolds number studied herein,

Rps varies between 1.2 to 1.9 times of Rm. At Reynolds number =4762,

Rp contributes about 65% of total resistance, whereas, at Reynolds number =6122, it is about 55%.

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Figure 3.12. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during NF in turbulent flow regime.

Figure 3.13. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during NF in laminar and with turbulent promoter flow regime.

Figure 3.14 represents the variation of dimensionless steady state polarized layer resistance with transmembrane pressure, for all the hydrodynamic conditions in RO. The

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steady state values of Rp increase marginally with the transmembrane pressure and decrease significantly with increase in Reynolds number as discussed earlier.

Figure 3.14. Variation of the ratio of polarized layer and hydraulic resistances at steady state with transmembrane pressure in RO.

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3.4.5. Enhancement of Steady State Permeate Flux Nanofiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 3.15. All the increases are calculated taking the laminar flow results under same operating conditions as the basis. The formation of polarized layer over the membrane surface is significantly reduced in presence of the turbulent promoters. This causes a corresponding increase in permeate flux. It may be observed from Figure 3.15 that the flux increment is in the range of 30 to 43% for laminar flow with promoter. However, it can be clearly seen from Figure 3.15 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases. Reverse Osmosis At a Reynolds number of 1020, the percentage flux enhancement for three operating pressures (1518, 1725 and 1932 kPa) are 33%, 35% and 40%, respectively.

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Figure 3.15. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.

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3.4.6. Permeate Quality Analysis Nanofiltration The permeate quality after NF, for various operating conditions, is presented in Table 3.6. The conductivity of the permeate is same as the feed which signifies that almost all the salt present in the feed solution has permeated through the NF membrane. Table 3.6. Permeate analysis after nanofiltration Sr. No

Pressure kPa

Reynolds number

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

Turbulent regime 1

828

4762

22700

29400

8.0

3.60

15000

428

2

828

5442

22650

29250

7.99

3.57

15300

487

3

828

6122

22600

29200

8.05

3.55

15900

552

4

966

4762

22600

29100

8.03

3.64

15900

542

5

966

5442

22650

29000

7.77

3.59

16500

592

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Sirshendu De, Chandan Das and Sunando DasGupta Table 3.6. (Continued) Sr. No

Pressure kPa

Reynolds number

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

6

966

6122

22500

28950

7.93

3.56

17250

696

7

1104

4762

22500

28900

7.95

3.66

18000

636

8

1104

5442

22400

28750

7.86

3.63

19800

660

9

1104

6122

22400

28700

7.88

3.57

20250

708

Laminar regime 10

828

680

23600

30600

7.83

3.72

17600

386

11

828

1020

23600

30500

7.87

3.69

19900

394

12

828

1360

23600

30400

7.93

3.63

18180

350

13

966

680

23300

30300

7.82

3.70

16700

386

14

966

1020

23200

30100

7.89

3.65

18480

440

15

966

1360

23150

30050

7.91

3.62

18160

454

16

1104

680

23100

30100

7.84

3.66

18280

480

17

1104

1020

23000

29900

7.84

3.63

18180

496

18

1104

1360

22950

29750

7.83

3.60

18380

402

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With turbulent promoter 19

828

680

23000

29800

7.7

3.62

19400

392

20

828

1020

22900

29600

7.4

3.59

19800

324

21

828

1360

22850

29450

7.6

3.56

20000

286

22

966

680

22900

29650

7.38

3.66

15800

410

23

966

1020

22800

29500

7.55

3.61

16380

406

24

966

1360

22800

29400

7.69

3.58

16840

386

25

1104

680

22700

29300

7.59

3.68

16520

400

26

1104

1020

22650

29150

7.64

3.65

17000

442

27

1104

1360

22600

29050

7.72

3.59

16180

406

Variations of permeate COD with transmembrane pressure at the operating Reynolds number in turbulent, laminar and with turbulent promoter are shown in Table 3.7. It is observed that with increase in transmembrane pressure and Reynolds number, the permeate quality improves. With increase in pressure, the solvent flux increases linearly, while the solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [4]. This indicates that with increasing pressure, more solvent passes through the membrane along with a fixed amount of the solute; the permeate becomes purer and hence the permeate quality (expressed as COD) increases. The similar trends are observed for laminar flow with promoter and turbulent flow. It can be seen from Table 3.7 that at 828 kPa pressure and Re=1360, COD decreases by about 13% in presence of promoter compared to the base case (laminar at same operating conditions). Percentage decrease in COD is found to be about 22% at 966 kPa pressure and Re=1360 and about 30% at 1104 kPa

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pressure and Re=1360. At Re=4762, as the transmembrane pressure increases from 828 kPa to 1104 kPa, COD decreases by 38%. Table 3.7. Variation of COD under different operating conditions for NF 400 (TFC) and RO (TFC)

Operating condition Flow Reynolds regime number 4762 Turbulent 5422 6122 680 Laminar 1020 1360 680 Laminar 1020 (with promoter) 1360

Nanofiltration COD (ppm) 966 828 kPa kPa 921.6 645.0 829.4 599.0 783.4 537.6 822.6 765.0 786.0 754.0 776.0 744.0 713.9 620.8 667.4 605.3 651.8 574.2

Reverse osmosis 1104 kPa 574.2 558.7 496.6 711.6 696.0 667.4 496.6 478.6 465.6

1518 kPa 128.0 112.0 88.0

1725 kPa 112.0 96.0 88.0

1932 kPa 102.0 88.0 80.0

96.0

88.0

80.0

88.0

74.0

64.0

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Reverse Osmosis The effects of transmembrane pressure and cross flow velocity on permeate quality in terms of COD for turbulent regime, in RO, are also shown in Table 3.7. The table illustrates that the decrease of permeate quality with decrease in Reynolds number and pressure is significant. The permeate qualities in terms of other properties for various operating conditions are presented in Table 3.8. It may be observed from Table 3.8 that COD in the permeate varies from about 128 to 64 ppm in the pressure range of 1518 to 1932 kPa which is substantially lower than the permissible limit (250 ppm). Table 3.8. Permeate Analysis after reverse osmosis

Sr.No

Reynolds number

Pressure kPa

TDS ppm

TS ppm

pH

Conductivity (S/m)

Ca++ ppm

Clppm

Turbulent regime 1

4762

1518

7470

7770

7.05

1.25

40

2200

2

5442

1518

8300

8630

7.1

1.37

45

2210

3

6122

1518

7650

7950

7.15

1.31

36

2140

4

4762

1725

5280

5490

7.12

0.93

49

2180

5

5442

1725

5170

5380

7.05

0.92

45

2220

6

6122

1725

5430

5650

7.1

0.93

43

2160

7

4762

1932

5610

5830

6.98

0.98

33

2140

8

5442

1932

5930

6170

7.09

1.01

39

2120

9

6122

1932

5800

6030

7.12

0.99

38

2180

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40

Sirshendu De, Chandan Das and Sunando DasGupta Table 3.8. (Continued)

Sr.No

Reynolds number

Pressure kPa

TDS ppm

TS ppm

pH

Conductivity (S/m)

Ca++ ppm

Clppm

Laminar regime 1

1020

1518

5540

5760

7

0.95

41

2240

2

1020

1725

5620

5840

7.04

0.97

41

2200

3

1020

1932

5480

5900

7.11

0.93

45

2120

With turbulent promoter 1

1020

1518

5500

5720

7.05

0.94

36

2160

2

1020

1725

5820

6050

7.15

0.99

32

2180

3

1020

1932

5920

6260

7.1

1.01

33

2140

From Table 3.8, it may also be observed that the conductivity of the permeate is very small signifying that significant amount of the salt present in the feed has been retained by the RO membrane. This salt rich retentate stream can be recycled to the pickling process. These informations are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality.

3.4.7. Sludge Characterization

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The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 3.9. It is observed from Table 3.9 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.2 kg from 40 liters of effluent) can be used as a good fertilizer. Table 3.9. Fertilizer quality of sludge obtained from soaking effluent Sample

pH

Sludge from Soaking Vermi-compost

6.8 7.1-7.8

Organic Carbon (wt %) 11.25 9.97-10.62

Nitrogen (wt %) 1.28 1.80

Phosphorous (wt %) 0.11 0.90

Potassium (wt %) 0.44 0.40

CONCLUSION Effluent from a soaking unit has been successfully treated using a combined process of coagulation by alum and membrane separation. The retentate of the RO, rich in sodium chloride, can be recycled to pickling process. The time required to reach steady state decreases with increase in Reynolds number and applied pressure. The use of turbulent promoters in laminar regime results in substantial increase in flux (30-43% for NF and 3340% for RO) compared to the laminar case. The treatment of the permeate of the NF process by RO successfully retains most of the dissolved salts. Both in NF and RO, polarization resistance is the major contributor to overall resistance to the solvent flow. The results are in

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corroboration to the theory that with increasing pressure, polarized layer resistance increases and with increasing flow rate it decreases. For NF, the contributions of polarization resistance is 1.2 to 1.9 times to that of membrane resistance in turbulent flow regime, 4.0 to 5.3 times for laminar with turbulent promoter regime and maximum for laminar regime with 6.2 to 7.3 times. Hence, maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime. The values of COD (~92.3 ppm) and BOD (~28.5ppm) in the permeate of RO are well below the discharge limit of the same.

REFERENCES [1] [2] [3]

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[4]

C. Das, S. DasGupta and S. De, Treatment of soaking effluent from a tannery using membrane separation processes, Desalination 216 (2007) 160–173. C. Das, S. DasGupta and S. De, Selection of membrane separation processes for treatment of tannery effluent” J. Environ. Prot. Sci., 1 (2007) 75-82. M. Charyan, Ultrafiltration and Microfiltration Handbook (Technomic Publishing Company, Lancaster PN, USA) (1998) pp. 83-85. P. M. Bungay, H. K. Lonsdale, and M. N. de Pinho, Synthetic Membranes: Science, Engineering and Application, published by D. Reidel Publishing Company, (1983) pp. 312.

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Chapter 4

TREATMENT OF LIMING EFFLUENT ABSTRACT Liming (unhairing, fleshing and trimming) removes the extraneous tissue of epidermic materials including interfibrillar proteins (albumins, globulins, etc.), hair and eliminates mucoids and swelling of derms. It is the second tannery operation, as mentioned earlier. Lime, sodium sulphide are used to eliminate flesh from skins and hides.

4.1. EFFLUENT CONTENT

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Table 4.1 shows the characterization of a typical liming effluent collected from M/s, Alison Tannery, Kolkata, India. Like soaking, liming effluent is also collected after one to two hours of the completion of the liming operation. Table 4.1. Characterization of liming effluent* Effluent Liming

13.14

Conductivity (S/m) 4.40

TS (g/l) 60.9

TDS (g/l) 29.5

COD (ppm) 15040

BOD (ppm) 5784.6

Cl(g/l) 22.98

Ca++ (g/l) 1.4

6.8

2.87

47.2

19.0

3500

1346

21.0

1.3

pH Feed After alum dose

*

All properties are reported average of three measurements.

4.2. PRETREATMENT The liming effluent is brought from the plant and is subjected to coagulation by alum. Nine graduated cylinders of 50 ml capacity with different dosages of alum are used for coagulation study using commercial alum. To get the optimum alum dose, concentrations of 0.05, 0.1, 0.2, 0.3, 0.5, 1, 2, 3 and 5% (weight by volume) are used. The liming effluent is used after a week of gravity settling. Before alum coagulation, the supernatant is siphoned out. Coagulation experiments are conducted with different dosages of alum for twenty four hours. Rate of coagulation does not vary appreciably beyond 30 minutes. The optimum alum

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dose is established by examining various properties (e.g., pH, TDS, conductivity, TS, COD, turbidity) of supernatant solutions. Figure 4.1 (a) represents the variation of total dissolved solids (TDS) and total solids (TS) with alum concentration of the supernatant of gravity settled liquor.

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Figure 4.1. Determination of optimum alum dose for liming effluent.

It can be observed that beyond 2% alum concentration, TDS, and TS concentration increase significantly. Variation of conductivity and pH of the clear liquid after coagulation at different alum concentrations are presented in Figure 4.1 (b). The pH of the supernatant is close to normal pH (~6.8) at 2% alum concentration and it decreases further with increase in alum dose. Figure 4.1 (c) shows that COD of the clarified liquor decreases with alum concentration and beyond 2%, the change is gradual. It may also be observed from Figure 4.1 (c) that with increase of alum concentration, the turbidity of the solution decreases (with more settling of solids) and beyond 2% the turbidity increases rapidly. From these observations, 2% is selected as the optimum concentration of alum for coagulation. The supernatant liquor is then prefiltered through a fine cloth and is subjected to membrane filtration [1]

4.3. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of the appropriate membrane for the filtration is of utmost importance. Selection should primarily be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

4.3.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide

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skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

4.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

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4.3.3. Performance Testing of Various Membranes Pretreated liming effluent is subjected to various UF (from 20 kDa to 1 kDa MWCO) membranes and 400 Da MWCO NF membrane in a stirred cell. The profiles of permeate flux are shown in Figure 4.2. It is observed from the figure that the flux decline is due to concentration polarization and beyond about 5 minutes of operation, flux becomes steady in all the cases. Attainment of steady state flux indicates that the pore blocking is completed quickly unlike the soaking effluent. Flux decline is 15% for 20 kDa, 13% for 10 kDa, 11% for 5 kDa, 9% for both 1 kDa and 400 Da MWCO NF membranes. Therefore, as far as flux decline is concerned, performance of 5 kDa, 1 kDa and 400 Da MWCO NF membranes is almost same. Regarding absolute flux value, 5 kDa UF membrane exhibits 60% more flux compared to NF membrane and 14% more flux compared to 1 kDa UF membrane. Quality of the permeate from various types of membrane is presented in Table 4.2. Reduction in COD is 1.1 % for 20 kDa, 32% for 10 kDa, 74% for 5 kDa, 75% for 1 kDa and 77% for 400 Da MWCO NF membrane. Therefore, as far as COD reduction is concerned, 5 kDa UF membrane shows almost same performance compared to 1 kDa UF and 400 Da MWCO NF membrane. 20 and 10 kDa UF membranes show almost no reduction in concentration of total solids. Whereas, 23% reduction in TS is observed in 5 kDa membrane. This value is 26% for 1 kDa UF and 30% for 400 Da MWCO NF membrane. Reduction in concentration of TDS is marginal for 20 kDa and 10 kDa UF membrane. This value is 25% for 5 kDa, 29% for 1 kDa and 36% for 400 Da MWCO NF membrane. Therefore, from the point of permeate quality, performance of 5 kDa UF membrane is almost equivalent to 1 kDa and NF membrane. Moreover, permeate flux is significantly more in case of 5 kDa UF membrane compared to 1 kDa UF membrane and 400 Da MWCO NF membrane. Therefore, 5 kDa UF membrane is selected as the possible membrane among the studied ones for treatment of liming effluent. Still the COD (842 mg/l) and BOD (326 mg/l) values of the UF permeate are substantially higher than the permissible limit as mentioned earlier. Therefore, permeate from ultrafiltration is collected and is subjected to NF using 400 Da MWCO NF membrane. Thus, the pretreated liming effluent should be treated with 5 kDa UF membrane followed by 400 Da MWCO NF membrane.

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Figure 4.2. Flux decline of pretreated liming effluent using various membranes.

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Table 4.2. Permeate properties of liming effluent MWCO 20 kDa 10 kDa 5 kDa 1 kDa 400 Da

COD (mg/l) 3460 [3500]* 2390 921 860 782

TS (g/l) 46.4 [47.2]* 46.3 36.2 35.1 33.0

TDS (g/l) 18.3 [19.0]* 18.1 14.2 13.4 12.1

*

Values indicate the properties corresponding to feed.

4.4. DETAILED STUDY OF TREATMENT OF LIMING EFFLUENT In this chapter, a scheme is proposed to treat the liming effluent using a hybrid process, including alum coagulation, ultrafiltration and nanofiltration. The optimum alum dose is established. The fertilizer value of the sludge produced is tested. The supernatant liquor is subjected to continuous cross flow ultrafiltration followed by nanofiltration. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 4.3.

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Figure 4.3. Proposed scheme for the treatment of liming effluent.

4.4.1. Cross Flow Cell and Operating Conditions The same cross flow cell described in section 3.4.1 is used for liming effluent also. The operating conditions for all membrane experiments are presented in Table 4.3. In turbulent flow regime, ultrafiltration experiments are conducted at three different operating pressures of 759, 828 and 897 kPa. At 759 kPa, experiments are carried out at Reynolds numbers of 4762, 5442 and 6122, whereas at 828 and 897 kPa experiments are conducted at a Reynolds number of 4762 only. Table 4.3. Operating conditions for cross flow experiments

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Step

UF NF

Transmembrane pressure (kPa) 759 828 897 276, 414 and 552 828, 966 and 1104

Reynolds number Laminar and with promoter ------680, 1020 and 1360 680, 1020 and 1360

Turbulent 4762, 5442 and 6122 5442 6122 4762, 5442 and 6122 4762, 5442 and 6122

4.4.2. Analysis of Transient Flux Decline Ultrafiltration Figure 4.4 represents the experimental flux decline for transient state in the turbulent flow regimes. The experimental permeate flux decline profile with time of laminar and with promoter is shown in Figure 4.5. Figures 4.4 and 4.5 illustrate that the time required to reach steady state decreases with increase in Reynolds number and transmembrane pressure. For example, it can be observed from Figure 4.4 that the steady state is attained in about 3.38 min, for Re=4762 and 759 kPa pressure, whereas at Re=5442 and Re=6122, the steady states are attained within 2.87 min

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and 2.65 min, respectively at the same transmembrane pressure difference. As shown in Figure 4.4, the flux decline is about 19% of the initial value for Re=4762, about 16% with for Re= 5442, and 15 % for Re=6122. As Reynolds number increases, the growth of the polarized layer over the membrane surface decreases due to enhanced forced convection and steady state reaches at an earlier time. The resistance to the solvent flux also decreases with increase of cross flow velocity and permeate flux increases. Hence, the flux decline is lower at higher cross flow velocities. On the other hand, steady state is attained faster with an increase in operating pressure at a fixed cross flow velocity. For example, in Figure 4.4, for Re=4762, steady states are attained in about 2.77 and 2.3 min respectively for 828 and 897 kPa pressures. Whereas at the same Re=4762, time required to attain steady state is about 3.38 min for an operating pressure of 759 kPa. Steady state is achieved faster using turbulent promoter compared to laminar flow. It can be observed from Figure 4.5 that at Re=680 and 276 kPa, the steady state is attained in about 15.75 min without promoter and about 13.1 min with promoter at the same operating condition. The flux decline is about 38 % without promoter at Re=680 and 276 kPa pressure; but only 32 % using promoter at the same operating condition. Turbulent promoters generate local turbulence and hence reduce the concentration polarization at the membrane surface. Steady state is established faster than without promoter as the growth of the polarized layer is controlled quickly. Therefore, the flux decline is also lower compared to the purely laminar condition.

Figure 4.4. Permeate flux decline profile with time of turbulent flow regime in UF.

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Figure 4.5. Permeate flux decline profile with time of laminar and with promoter flow regime for UF membrane.

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Nanofiltration The permeate from the UF is collected and treated using NF in the same cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating conditions. Figure 4.6 represents the transient permeate flux decline profile at a constant transmembrane pressure of 828 kPa and varying Reynolds number. The flux versus time data of experimental values are plotted in Figure 4.7 for a constant Reynolds number (680 for laminar and 4762 for turbulent) and varying pressure.

Figure 4.6. Permeate flux decline profile with time for NF membrane at 828 kPa.

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Figure 4.7. Permeate flux decline profile with time for NF membrane at various pressures.

As in the case of UF, Figures 4.6 and 4.7 clearly show that the time required to reach steady state decreases with increase in cross flow velocity and transmembrane pressure and also in presence of turbulent promoters. Extent of flux decline also follows similar trends for reasons already discussed earlier.

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4.4.3. Analysis of Steady State Flux Ultrafiltration Figure 4.8 represents the variations of steady state permeate flux with pressure at different Reynolds number under turbulent flow, laminar flow without and with turbulent promoters. The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier. Nanofiltration The effects of transmembrane pressure and Reynolds number on steady state permeate flux in NF are shown in Figure 4.9. The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to higher turbulence induced by higher Reynolds number. The figure shows that the permeate flux increases with operating pressure and Reynolds number as in UF. At 1104 kPa, an increase in Reynolds number from 4762 to 6122 results in about 24.5% increase in permeate flux. From Figure 4.9 it can be seen that at 828 kPa, the flux increment is about 47.2% for laminar flow with promoter at Re=1020.

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Figure 4.8. Variation of permeate flux with transmembrane pressure in UF for liming effluent.

Figure 4.9. Variation of permeate flux with transmembrane pressure in NF for liming effluent.

4.4.4. Analysis of Various Resistances The permeate flux at any point of time is expressed as presented by Eq. (3.1). The steady state polarized layer resistance is given by Eq. (3.2). The variation of dimensionless steady state polarized layer resistance with Reynolds number for laminar regime and laminar with

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promoter regime is presented in Figure 4.10. It is observed from the figure that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure of 276 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 2.4 to 1.7 with an increase in Reynolds number from 680 to 1360. Rp values increase with the transmembrane pressure. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number (680) and transmembrane pressure (276 kPa), the presence of turbulent promoters reduces the resistance to 1.5 compared to 2.4 in laminar flow. This reduction in

Rps

is more than 42.6% in some of the

experiments leading to a significant enhancement of the permeate flux. Table 4.4 shows further reductions for the case of purely turbulent flows for reasons already discussed. It may be observed from Figure 4.10 and Table 4.4 that Reynolds number has a significant effect on the polarized layer resistance. For laminar flow with and without promoter, polarized layer resistance is the major contributing resistance. For example, in case of pure laminar flow, at Reynolds number=680 and transmembrane pressure at 414 kPa, Rm and Rp constitute about 26% and 74% of the total resistance, respectively. In case of laminar flow with promoter, at the same operating condition, contribution of Rp decreases to 63.5% of the total resistance. For turbulent flow regime, effects of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the range of Reynolds number studied herein,

Rps varies

between 0.61 to 0.9 times of Rm. At a

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transmembrane pressure of 759 kPa and Reynolds number =4762, Rp contributes about 45.8% of total resistance, whereas, at Reynolds number =6122, it is about 37.8%.

Figure 4.10. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during UF.

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Sirshendu De, Chandan Das and Sunando DasGupta Table 4.4. Variation of ratio of polarized layer resistance to hydraulic resistance in turbulent flow regime

Transmembrane pressure (kPa) 759 828 897

R ps /Rm Re=4762 0.845 0.864 0.9

Re=5442 0.762 ---

Re=6122 0.607 ---

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Figures 4.11 and 4.12 represent the variation of dimensionless steady state polarized layer resistance with transmembrane pressure, for turbulent flow conditions and laminar and with promoter conditions, respectively, in RO. The steady state values of Rp increase marginally with the transmembrane pressure and decrease significantly with increase in Reynolds number as discussed earlier.

Figure 4.11. Variation of the ratio of polarized layer and hydraulic resistances at steady state with transmembrane pressure difference in turbulent flow regime.

4.4.5. Enhancement of Steady State Permeate Flux Ultrafiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 4.13. All the increases are calculated taking the laminar flow results under same operating conditions as the basis. The formation of polarized layer over the membrane surface is significantly reduced in presence of the turbulent promoters. This causes a corresponding increase in permeate flux. It may be observed from Figure 4.13 that the flux increment is in the range of 34.9 to 44.2% for laminar flow with promoter. However, it can be clearly seen from Figure 4.13 that the permeate flux

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in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

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Figure 4.12. Variation of the ratio of polarized layer and hydraulic resistances at steady state with transmembrane pressure difference in laminar and with promoter flow regime.

Figure 4.13. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in UF.

Nanofiltration The flux enhancement is about 47.2% for laminar flow with promoter at Re=1020 at 828 kPa pressure taking the laminar flow results under same operating conditions as the basis. The permeate flux enhancement varies from 26.7% to 47.2% for various operating conditions.

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4.4.6. Permeate Quality Analysis Ultrafiltration For different operating conditions, the permeate quality after UF, is shown in Table 4.5 Almost all the salt present in the feed solution has permeated through the UF membrane. Hence, the permeate conductivity remains almost same as that of feed. Table 4.5 also represents the variation of COD with transmembrane pressure difference at the operating Reynolds number (turbulent, laminar and with turbulent promoter). With increase in transmembrane pressure difference the permeate quality in terms of COD decreases and with Reynolds number, permeate quality improves. With increase in pressure, the solvent flux as well as solute flux increase linearly and thus COD of the permeate increases. As Reynolds number increases, the growth of the polarized layer over the membrane surface is reduced due to enhanced forced convection as discussed earlier. So the solute concentration of the permeate decreases and as a result of which COD increases.

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Table 4.5. Permeate quality analysis after UF Pressure Reynolds Sr. number No kPa Turbulent flow regime 1 759 4762 2 759 5442 3 759 6122 4 828 4762 5 897 4762 Laminar flow regime 1 276 680 2 276 1020 3 276 1360 4 414 680 5 414 1020 6 414 1360 7 552 680 8 552 1020 9 552 1360 With turbulent flow regime 1 276 680 2 276 1020 3 276 1360 4 414 680 5 414 1020 6 414 1360 7 552 680 8 552 1020 9 552 1360

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

COD ppm

15500 15300 15200 15700 15900

41500 41200 41000 41800 42000

7.6 7.68 7.7 7.63 7.72

2.42 2.46 2.34 2.30 2.42

20200 20040 20100 20200 20200

1275 1275 1280 1245 1260

902 880 848 928 960

16100 15900 15800 16300 16100 16000 16400 16200 16100

42200 42000 41800 42900 42000 40800 43800 43000 42600

7.47 7.44 7.48 7.48 7.49 7.52 7.58 7.61 7.6

2.47 2.54 2.50 2.48 2.42 2.40 2.40 2.42 2.42

20200 20400 20000 20040 20040 19920 20200 20100 20400

1280 1260 1245 1225 1150 1130 1275 1275 1225

776 766.2 757 788.2 780 773 800 792 783.6

15500 15200 15000 15900 15600 15400 16500 16200 16000

42000 41800 41500 42500 42200 42000 43100 42700 42300

7.5 7.59 7.51 7.51 7.47 7.58 7.49 7.51 7.53

2.47 2.51 2.50 2.51 2.50 2.51 2.40 2.35 2.36

20400 20100 20200 20040 20100 20100 20200 20400 19920

1250 1275 1245 1160 1225 1275 1245 1275 1225

745 738 730 754 748 740 763 756 745

It can also be observed from Table 4.5 that at 276 kPa pressure and Re=680, COD is 776 where as at 552 kPa pressure and at same Re, the COD is 800. It is also observed that at 552

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kPa pressure and Re=680, COD decreases by about 5% in presence of promoter compared to laminar at same operating conditions. At Re=4762, as the transmembrane pressure difference increases from 759 kPa to 897 kPa, COD increases by 6%. For a transmembrane pressure difference of 759 kPa, COD reduces from 902 to 848 with an increase in Reynolds number from 4762 to 6122. Hence, the COD removal is around 75.9%. In another study, Cassano et al. has removed 61.8% COD [2]. Since all the salt present in the feed solution has permeated through the UF membrane, the permeate conductivity remains almost same as that of feed.

Nanofiltration Table 4.6 shows other properties for various operating conditions in NF. Permeate conductivity is same as the feed which indicates that almost all the salt present in the feed solution has permeated through NF membrane. Variations of permeate COD with transmembrane pressure at the operating Reynolds number in turbulent, laminar and with turbulent promoter are also shown in Table 4.6.

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Table 4.6. Permeate quality analysis after NF Sr. Pressure Reynolds number No kPa Turbulent flow regime 1 828 4762 2 828 5442 3 828 6122 4 966 4762 5 966 5442 6 966 6122 7 1104 4762 8 1104 5442 9 1104 6122 Laminar flow regime 1 828 680 2 828 1020 3 828 1360 4 966 680 5 966 1020 6 966 1360 7 1104 680 8 1104 1020 9 1104 1360 With turbulent flow regime 1 828 680 2 828 1020 3 828 1360 4 966 680 5 966 1020 6 966 1360 7 1104 680 8 1104 1020 9 1104 1360

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

COD ppm

11800 11700 11600 11800 11400 11300 11600 11600 11500

20500 20400 20300 21300 21300 21000 20800 20600 20200

7.4 7.39 7.4 7.44 7.4 7.37 7.41 7.4 7.4

1.79 1.73 1.74 1.79 1.74 1.71 1.73 1.76 1.74

1090 1080 1090 1100 1080 1090 1090 1080 1070

18020 18000 18100 18060 18100 18040 18000 18020 18040

168 162 158 155 152 145 142 136 132

12000 11900 11800 11800 11800 11700 11800 11800 11800

25600 25300 25000 24800 24700 24500 24000 23600 23000

7.35 7.34 7.35 7.4 7.41 7.39 7.35 7.42 7.42

1.80 1.81 1.79 1.79 1.78 1.78 1.78 1.78 1.78

1080 1060 1070 1090 1080 1040 1070 1090 1070

18100 18020 18080 18060 18100 17980 18040 18000 18020

206 200 196 200 192 188 186 180 172

11800 11700 11600 11700 11600 11500 11500 11700 11800

23500 23200 23000 22600 22500 22500 22100 22000 21800

7.41 7.4 7.41 7.39 7.42 7.41 7.41 7.42 7.44

1.74 1.79 1.74 1.78 1.72 1.79 1.74 1.78 1.78

1060 1040 1050 1040 1040 1050 1070 1040 1060

17960 17960 18020 17980 18020 18010 18100 18000 17940

196 192 186 184 178 170 162 156 150

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It is observed that with increase in transmembrane pressure difference and Reynolds number, the permeate quality improves. With increase in pressure, the solvent flux increases linearly, while the solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [3]. This indicates that with increasing pressure, more solvent passes through the membrane along with a fixed amount of the solute; the permeate becomes purer and hence the permeate quality (expressed as COD) increases. The similar trends are observed for laminar flow with promoter and turbulent flow. It can be seen from Table 4.6 that at 828 kPa pressure and Re=1360, COD decreases by about 11% in presence of promoter compared to the base case (laminar at same operating conditions). Percentage decrease in COD is found to be about 13% at 966 kPa pressure and Re=1360 and also about 13% at 1104 kPa pressure and Re=1360. At Re=4762, as the transmembrane pressure difference increases from 828 kPa to 1104 kPa, COD decreases by 15%. From Table 4.6, it may also be observed that COD in the permeate varies from about 206 to 132 ppm in the pressure range of 828 to 1104 kPa which is much lower than the permissible limit (250 ppm). These informations are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality.

4.4.7. Sludge Characterization The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 4.7. It is observed from Table 4.7 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.3 kg from 40 liters of effluent) can be used as a good fertilizer.

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Table 4.7. Fertilizer quality of sludge obtained from liming effluent Sample

pH 7.3

Organic Carbon (wt %) 10.35

Nitrogen (wt %) 1.24

Phosphorous (wt %) 0.098

Potassium (wt %) 0.35

Sludge from Liming Vermi-compost

7.1-7.8

9.97-10.62

1.80

0.90

0.40

CONCLUSION The viability of liming unit effluent treatment using a combined process of coagulation by alum and membrane separation is established in this study. The values of COD (~164 ppm) of NF are well below the discharge limit. With increase in Reynolds number and applied pressure the time required to reach steady state decreases. Permeate flux enhancements using turbulent promoters in laminar regime (35-44% for UF and 27-47% for NF) are observed. Polarization resistance is the major contributor to overall resistance, both in UF and NF, to the solvent flow for laminar and laminar with promoter case. With increasing pressure, polarization resistance increases and with increasing flow rate it decreases and it is 1.0 to 1.8 times for laminar with turbulent promoter regime and maximum for laminar regime

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with 1.7 to 3.0 times for UF. However, for turbulent flow conditions, polarized layer resistances are 0.6 to 0.9 times the membrane hydraulic resistance for UF and 0.7 to 1.2 times for NF. Also maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime. Pulverized sludge obtained after sun drying can be used as organic fertilizer.

REFERENCES [1] [2]

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[3]

C. Das, S. De and S. DasGupta, Treatment of liming effluent from a tannery using membrane separation processes, Sep. Sci. Technol., 42 (2007) 517-539. A. Cassano, A. Cassano, E. Drioli and R. Molinari, Recovery and reuse of chemicals in unhairing, degreasing and chromium tanning processes by membranes. Desalination 113 (1997) 251-261. P.M. Bungay, H.K. Lonsdale, and M.N.de Pinho, Synthetic Membranes: Science, Engineering and Application, published by D. Reidel Publishing Company, (1983) pp. 312.

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Chapter 5

TREATMENT OF DELIMING-BATING EFFLUENT ABSTRACT Excess lime, hair residue and degraded proteins are removed in deliming-bating step. Pigments and hair roots are also removed during this process. During liming, hides and skins become alkaline. In deliming-bating step, a salt of strong acid and week base is mixed with proteolytic enzymes together with alkaline skins and hides. The derived effluent contains appreciable amount of calcium salt, ammonium salt, sulfide residues, degraded collagens and hairs and residual proteolytic enzymatic agents.

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5.1. EFFLUENT CONTENT Table 5.1 shows the characterization of a typical deliming-bating effluent collected from M/s, Alison Tannery, Kolkata, India. Like soaking and liming, deliming-bating effluent is also collected after one to two hours of the completion of the deliming-bating operation. Table 5.1. Characterization of deliming-bating effluent* Effluent Delimingbating

10.4

Conductivity (S/m) 5.61

TS (g/l) 46.6

TDS (g/l) 36.9

COD (ppm) 8120

BOD (ppm) 3123

Cl(g/l) 15.2

Ca++ (g/l) 0.4

7.25

2.40

44.3

15.9

2732

1050.8

15.2

0.38

pH Feed After alum dose

*

All properties are reported average of three measurements.

5.2. PRETREATMENT The supernatant of the deliming-bating effluent after settling is tested for optimum alum dosing for coagulation. Eight graduated cylinders of 50 ml capacity with different dosages of alum are used for coagulation study using commercial alum. To get the optimum alum dose, concentrations of 0.1, 0.2, 0.3, 0.5, 1, 2, 3 and 5% (weight by volume) are used. The optimum alum dose is established by examining various properties (e.g., pH, TDS, conductivity, TS,

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COD, turbidity) of supernatant solutions. The effluent is subjected to coagulation with optimum alum dosing in a 40 litre bucket. The sludge is kept under sun light for drying. Figure 5.1 (a) represents the variation of total dissolved solids (TDS) and total solids (TS) with alum concentration of the supernatant of the liquor. It is observed from Figure 5.1 (a) that beyond an alum concentration of 2%, TDS, conductivity and TS concentration increase significantly.

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Figure 5.1. Determination of optimum alum dose for deliming-bating effluent.

It may also be noted from Figure 5.1 (b) that with increasing concentration of alum, the turbidity of the solution decreases (with more settling of solids) sharply and beyond 2%, the decrease is gradual. It is also observed that the pH of the clear solution is close to the normal pH (~7.25) at 2% alum concentration and it decreases further with increase in alum dose. Figure 5.1 (c) shows that the COD of the clarified liquor decreases with alum concentration and beyond 2%, the change is gradual. From these observations, 2% is selected as the optimum concentration of alum for coagulation. The clarified liquor after optimum alum dosing is subjected to membrane filtration after filtering using a fine cloth.

5.3. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of membrane with an appropriate molecular weight cut-off is crucial for a membrane separation unit. Selection should be primarily based on maximum productivity of the system with desired quality of the permeate.

5.3.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

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5.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

5.3.3. Performance Testing of Various Membranes

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Pretreated deliming-bating effluent is subjected to UF of various cut-off membranes as well as NF with 400 Da MWCO membrane in a stirred cell. Profiles of permeate flux decline are presented in Figure 5.2. It is observed from Figure 5.2 that permeate flux declines over the duration of filtration and beyond five minutes of operation, flux attains a steady state. Development of deposited (polarized) layer thickness is controlled by the external stirring. Decline in flux turns out to be 16% for 20 kDa, 14% for 10 kDa, 12% for 5 kDa, 10% for 1 kDa and 9% for NF membrane. Therefore, as far as flux decline is concerned, 5 kDa UF membrane shows slightly higher flux decline compared to 1 kDa UF and NF membrane. But with respect to absolute flux value, it delivers 22% more flux compared to1 kDa membrane and 61% more flux compared to NF membrane. Therefore, permeate quality of these three membranes have to be compared before final selection. Properties of the permeate of various membranes are listed in Table 5.2. It is observed from this table that reduction of COD is 6%, 17%, 77%, 78% and 80% for 20, 10, 5, 1 kDa UF and 400 Da MWCO NF membrane. Therefore, 5 kDa UF membrane shows almost equivalent performance in terms of percentage reduction of COD compared to 1 kDa and NF membrane.

Figure 5.2. Flux decline of pretreated deliming-bating effluent using various membranes.

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Table 5.2. Permeate properties of deliming-bating effluent MWCO 20 kDa 10 kDa 5 kDa 1 kDa 400 Da

COD (mg/l) 2564 [2732]* 2282 620 608 540

TS (g/l) 42.1 [44.3]* 39.5 24.7 22.4 21.2

TDS (g/l) 15.7 [15.9]* 15.2 13.7 12.7 10.8

*

Values indicate the properties corresponding to feed.

Reduction of total solids is insignificant for 20 and 10 kDa membranes. It is 44%, 49% and 52% for 5 kDa, 1 kDa and NF membrane. Reduction of total dissolved solids is 1.3%, 4.4%, 13.8%, 20.1% and 32.1% for 20, 10, 5, 1 kDa UF and NF membranes. Based on permeate quality and permeate flux, 5 kDa membrane is selected for treatment of delimingbating effluent. But, COD (620 mg/l) and BOD (238 mg/l) values are still above the permissible level. To make the treated effluent disposable, ultrafiltered effluent may be subjected to NF using 400 Da MWCO membrane.

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5.4. DETAILED STUDY OF TREATMENT OF DELIMING-BATING EFFLUENT In this chapter, a scheme is proposed to treat the deliming-bating effluent using a hybrid process, including alum coagulation, ultrafiltration and nanofiltration. The optimum alum dose is established. The fertilizer value of the sludge produced is tested. The supernatant liquor is subjected to continuous cross flow ultrafiltration followed by nanofiltration. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 5.3.

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5.4.1. Cross Flow Cell and Operating Conditions The same cross flow cell which was used for soaking effluent treatment is used for deliming-bating effluent also. The operating conditions for all membrane experiments are presented in Table 5.3. In turbulent flow regime, ultrafiltration experiments are conducted at three different operating pressures of 759, 828 and 897 kPa. At 759 kPa, experiments are carried out at Reynolds numbers of 4762, 5442 and 6122, whereas at 828 and 897 kPa experiments are conducted at a Reynolds number of 4762 only. As indicated in the Table, the experiments are conducted at 276, 414 and 552 kPa pressure under laminar flow conditions with the Reynolds numbers 680, 1020 and 1360. Under the same conditions, promoters are also used to observe the effects of the turbulent promoters on the system performance. Table 5.3. Operating conditions for cross flow experiments

Step

UF NF

Transmembrane pressure (kPa) 759 828 897 276, 414 and 552 828, 966 and 1104

Reynolds number Laminar and with Turbulent promoter --4762, 5442 and 6122 --5442 --6122 680, 1020 and 1360 4762, 5442 and 6122 680, 1020 and 1360 4762, 5442 and 6122

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5.4.2. Analysis of Transient Flux Decline Ultrafiltration The transient flux decline in the turbulent flow regime is shown in Figure 5.4. Flux decline for laminar with and without promoter is shown in Figure 5.5. It can be seen from the figures that the time required to reach steady state decreases with increase in Reynolds number. Figure 5.4 shows that the steady state is attained in about 4.3 min, for Re=4762 and 759 kPa pressure, whereas at Re=5442 and Re=6122, the steady states are attained within 3.5 min and 3 min, respectively at the same transmembrane pressure. As shown in Figure 5.4, the flux decline is about 21% of the initial value for Re=4762, about 19% for Re= 5442, and 16% for Re=6122. As Reynolds number increases, the growth of the polarized layer over the membrane surface decreases due to enhanced forced convection and steady state is attained at an earlier time. The resistance to the solvent flux also decreases with increase in cross flow velocity and permeate flux increases. Hence, the flux decline is lower at higher cross flow velocities. On the other hand, steady state is attained faster with an increase in operating pressure at a fixed cross flow velocity. For example, in Figure 5.4, for Re=4762, steady states are attained in about 3.6 min and 3 min respectively for 828 and 897 kPa pressures. Whereas at the same Re=4762, time required to attain steady state is about 3.5 min for an operating pressure of 759 kPa. Steady state is achieved faster using turbulent promoter compared to laminar flow. It can be observed from Figure 5.5 that at Re=680 and 276 kPa, the steady state is attained in about 17.1 min without promoter and about 13.6 min with promoter at the same operating condition.

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Figure 5.4. Permeate flux decline profile with time of turbulent flow regime in UF.

Figure 5.5. Permeate flux decline profile with time for UF membrane at various pressures in laminar flow regime.

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The flux decline is about 34 % without promoter at Re=680 and 276 kPa pressure; but only 28 % using promoter at the same operating condition. Turbulent promoters generate local turbulence and hence reduce the concentration polarization at the membrane surface. Steady state is established faster than without promoter as the growth of the polarized layer is controlled quickly. Therefore, the flux decline is also lower compared to the purely laminar flow condition.

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Nanofiltration The permeate from the UF is collected and treated using NF in the same cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating conditions. The flux decline for transient state in the three flow regimes in NF at a constant pressure of 828 kPa and varying Reynolds number is shown in Figure 5.6. Flux decline at constant Reynolds number (680 for laminar and 4762 for turbulent) and varying pressure is shown in Figure 5.7. It can be clearly seen from the figure that the time required to reach steady state decreases with increase in Reynolds number. For example, it can be observed from Figure 5.6 that the steady state is attained in about 7.2 min, for Re=4762 and 828 kPa pressure, whereas at the same pressure but at Re=5442 and Re=6122, the steady states are attained within 6.4 min and 5.5 min, respectively. The flux decline is about 22% of the initial value for Re=4762, about 20% with increase in Re=5442, and 18% at Re=6122. Similar trends can be observed for flux decline in laminar regime with and without promoters. As the cross flow velocity increases, the growth of the polarized layer over the membrane surface is arrested faster because of enhanced forced convection. This leads to the onset of steady state at an earlier time.

Figure 5.6. Permeate flux decline profile with time for NF membrane at 828 kPa.

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Figure 5.7. Permeate flux decline profile with time for NF membrane at various pressures.

For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities. It is also observed that the steady state is achieved faster using turbulent promoter compared to laminar flow. For example, in Figure 5.6, at Re=680 and 828 kPa, the steady state is attained in about 15.7 min without promoter and about 9.7 min with promoter at the same operating condition. The flux decline is about 31% without promoter at Re=680 and 828 kPa pressure; but only 20.5% using promoter at the same operating condition. Use of the turbulent promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the polarized layer is controlled quickly, establishing steady state earlier than without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less compared to the no promoter case.

5.4.3. Analysis of Steady State Flux Ultrafiltration The variations of steady state permeate flux with pressure at different Reynolds number under turbulent flow, laminar flow without and with turbulent promoters are shown in Figure 5.8. The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier.

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Figure 5.8. Variation of permeate flux with transmembrane pressure in UF for deliming-bating effluent.

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Nanofiltration The variations of steady state permeate flux with pressure at different Reynolds number under turbulent flow, laminar flow without and with turbulent promoters are shown in Figure 5.9.

Figure 5.9. Variation of permeate flux with pressure drop in NF. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier. It may be noted here that the variation of permeate flux with pressure at various Reynolds number in NF experiments is different from the UF runs (Figures 5.8 and 5.9). The increase in flux values is more with the transmembrane pressure in UF (Figure 5.8) compared to Figures 5.9. For example, at Re=1360 (without promoter), the flux increase from 17.64 l/m2h to about 31.68 l/m2h when the transmembrane pressure increases from 276 to 552 kPa, indicating about 80% increase. Under the same Reynolds number, in case of NF, flux enhancement is only 27% when pressure increases from 828 to 1104 kPa. Similar trend is observed for other operating conditions, as well. These results indicate that the deposition over the membrane surface is more compact in case of NF. The organic materials permeated through UF form a compact cake type layer over the NF membrane. Hence, the flux enhancement by pressure and cross flow velocity is less in case of NF.

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5.4.4. Analysis of Various Resistances The permeate flux at any point of time is expressed as presented by Eq. (3.1). The steady state polarized layer resistance is given by Eq. (3.2). The variation of dimensionless steady state polarized layer resistance with Reynolds number for laminar regime and laminar with promoter regime is presented in Figure 5.10. It is observed from the figure that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure drop of 552 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 2.3 to 1.5 with an increase in Reynolds number from 680 to 1020. Rp values increase with the transmembrane pressure drop. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced turbulence near the membrane surface induced by the promoters. At the same Reynolds number (680) and transmembrane pressure (552 kPa), the presence of turbulent promoters reduces the resistance to 1.5 compared to 2.3 in laminar flow. s

This reduction in Rp is more than 41% in some of the experiments leading to a significant enhancement of the permeate flux. Table 5.4 shows further reductions for the case of purely turbulent flows for reasons already discussed. It may be observed from Figure 5.10 and Table 5.4 that Reynolds number has a significant effect on the polarized layer resistance. For laminar flow with and without promoter, polarized layer resistance is the major contributing resistance. It may be observed from Figure 5.10 that Reynolds number has a significant effect on the polarized layer resistance. For laminar flow with and without promoter, polarized layer resistance is the major contributing resistance. For example, in case of pure laminar flow, at Reynolds number=1360 and transmembrane pressure drop at 552 kPa, Rm and Rp constitute about 30% and 70% of the total resistance, respectively. In case of laminar flow with promoter, at the same operating condition, contribution of Rp decreases to 61% of the total resistance. For turbulent flow regime, effects

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of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the range of Reynolds number studied herein,

Rps

varies between 0.6 to 0.8 times of Rm. At Reynolds number =4762, Rp contributes about

42% of total resistance, whereas, at Reynolds number =6122, it is about 38%.

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Figure 5.10. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during UF.

Table 5.4. Variation of ratio of polarized layer resistance to hydraulic resistance in turbulent flow regime

Transmembrane pressure (kPa) 759 828 897

R ps /Rm Re=4762 0.721 0.759 0.797

Re=5442 0.674 ---

Re=6122 0.607 ---

Figures 5.11 and 5.12 represent the variation of dimensionless steady state polarized layer resistance with transmembrane pressure, for turbulent flow conditions and laminar and with promoter conditions, respectively, in RO. The steady state values of Rp increase marginally with the transmembrane pressure and decrease significantly with increase in Reynolds number as discussed earlier.

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Figure 5.11. Variation of the ratio of polarized layer and hydraulic resistances at steady state with transmembrane pressure difference in turbulent flow regime.

Figure 5.12. Variation of the ratio of polarized layer and hydraulic resistances at steady state with transmembrane pressure difference in laminar and with promoter flow regime in NF.

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5.4.5. Enhancement of Steady State Permeate Flux

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Ultrafiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 5.13. All the increases are calculated taking the laminar flow results under same operating conditions as the basis. The formation of polarized layer over the membrane surface is significantly reduced in presence of the turbulent promoters. This causes a corresponding increase in permeate flux. It may be observed from Figure 5.13 that the flux increment is in the range of 27 to 38% for laminar flow with promoter. However, it can be clearly seen from Figure 5.13 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

Figure 5.13. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in UF.

Nanofiltration The flux enhancement is about 53% for laminar flow with promoter at Re=1360 at 828 kPa pressure taking the laminar flow results under same operating conditions as the basis. The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are 45 to 66%. All the increases are calculated taking the laminar flow results under same operating conditions as the basis. The formation of polarized layer over the membrane surface is significantly reduced in presence of the turbulent promoters as discussed earlier. This causes a corresponding increase in permeate flux. It may be noted here that flux enhancement in NF with turbulent promoter is more (45 to 66%) compared to that in

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UF (27 to 38%). This indicates that the deposition over the membrane surface is much less sticky in case of NF, constituting mainly the lower molecular weight organic materials and inorganic substances. The results for turbulent flow regime cannot be directly compared to the laminar flow results for flux enhancement calculations, as the operating conditions are different. However, it can be clearly seen from Figure 5.9 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

5.4.6. Permeate Quality Analysis Ultrafiltration For different operating conditions, the permeate quality after UF, is shown in Table 5.5 Almost all the salt present in the feed solution has permeated through the UF membrane. Hence, the permeate conductivity remains almost same as that of feed.

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Table 5.5. Permeate quality analysis after UF Sr. Pressure Reynolds No (kPa) number Turbulent flow regime 1 759 4762 2 759 5442 3 759 6122 4 828 4762 5 897 4762 Laminar flow regime 1 276 680 2 276 1020 3 276 1360 4 414 680 5 414 1020 6 414 1360 7 552 680 8 552 1020 9 552 1360 With turbulent flow regime 1 276 680 2 276 1020 3 276 1360 4 414 680 5 414 1020 6 414 1360 7 552 680 8 552 1020 9 552 1360

TDS (ppm)

TS (ppm)

pH

Conductivity (S/m)

Cl(ppm)

Ca++ (ppm)

COD (ppm)

13200 13200 13000 13300 13500

31100 30300 29500 32700 33200

7.68 8.01 7.98 7.63 7.90

2.02 2.00 1.97 2.04 2.07

13600 13420 13200 13800 14100

320 310 300 340 342

512 462 400 530 552

13700 13500 13300 14000 13800 13700 14400 14300 13900

27100 26600 26000 28300 27500 27000 29400 28700 28000

7.76 7.81 7.78 7.81 7.81 7.50 7.64 7.72 7.79

2.11 2.08 2.05 2.16 2.13 2.11 2.22 2.20 2.14

12430 12540 12630 12620 12720 12830 12790 12850 12920

280 280 290 290 300 310 300 320 320

440 432 426 456 452 442 472 464 458

13300 13400 13100 13700 13500 13400 14100 13900 13700

26000 25800 25800 27600 27200 27000 28800 28100 27600

7.50 7.59 7.51 7.51 7.47 7.58 7.49 7.51 7.53

2.05 2.06 2.01 2.10 2.08 2.06 2.17 2.13 2.10

12390 12490 12600 12580 12670 12780 12730 12800 12870

270 280 300 280 290 300 290 300 300

428 422 418 446 440 432 460 452 446

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Table 5.5 also represents the variation of COD with transmembrane pressure difference at the operating Reynolds number (turbulent, laminar and with turbulent promoter). With increase in transmembrane pressure difference the permeate quality in terms of COD decreases and with Reynolds number, permeate quality improves. With increase in pressure, the solvent flux as well as solute flux increase linearly and thus COD of the permeate increases. As Reynolds number increases, the growth of the polarized layer over the membrane surface is reduced due to enhanced forced convection as discussed earlier. So the solute concentration of the permeate decreases and as a result of which COD increases. It can be observed from Table 5.5 that at 276 kPa pressure and Re=680, COD is 440 ppm where as at 552 kPa pressure and at same Re, the COD is 472 ppm. It is also observed that at 276 kPa pressure and Re=680, COD decreases by about 3% in presence of promoter. At Re=4762, as the transmembrane pressure increases from 759 kPa to 897 kPa, COD increases by 8%. For a transmembrane pressure drop of 759 kPa, COD reduces from 512 to 400 ppm with an increase in Reynolds number from 4762 to 6122. Since all the salt present in the feed solution has permeated through the UF membrane, the permeate conductivity remains almost same as that of feed.

Nanofiltration Table 5.6 shows other properties for various operating conditions in NF. Permeate conductivity is same as the feed which indicates that almost all the salt present in the feed solution has permeated through NF membrane. Variations of permeate COD with transmembrane pressure at the operating Reynolds number in turbulent, laminar and with turbulent promoter are also shown in Table 5.6.

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Table 5.6. Permeate quality analysis after NF Sr. Pressure Reynolds number No kPa Turbulent flow regime 1 828 4762 2 828 5442 3 828 6122 4 966 4762 5 966 5442 6 966 6122 7 1104 4762 8 1104 5442 9 1104 6122 Laminar flow regime 1 828 680 2 828 1020 3 828 1360 4 966 680 5 966 1020 6 966 1360 7 1104 680 8 1104 1020 9 1104 1360

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

COD ppm

8690 8870 8610 9860 6870 8500 8640 8490 8910

17900 18100 17500 17000 17300 16800 16700 16700 16400

7.94 8.00 8.04 7.98 7.99 8.00 7.98 7.90 7.97

1.32 1.34 1.30 1.49 1.04 1.29 1.31 1.29 1.34

11080 11040 11010 11000 11010 10980 10960 11000 10980

220 210 205 210 200 195 200 210 190

145 138 126 120 112 96 102 86 80

9400 9100 9150 9200 8170 8310 8600 8830 9180

19500 19100 18900 18800 18300 18000 17700 17200 16500

8.11 7.99 7.93 7.95 7.84 7.78 7.90 7.71 7.74

1.48 1.45 1.43 1.44 1.40 1.35 1.34 1.31 1.28

10960 10960 10920 10900 10860 10860 10880 10840 10800

200 205 190 195 190 195 185 180 180

124 112 96 90 82 76 80 72 66

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Table 5.6. (Continued)

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Sr. Pressure Reynolds number No kPa With turbulent flow regime 1 828 680 2 828 1020 3 828 1360 4 966 680 5 966 1020 6 966 1360 7 1104 680 8 1104 1020 9 1104 1360

TDS ppm

TS ppm

pH

Conductivity (S/m)

Clppm

Ca++ ppm

COD ppm

9080 9010 8870 8800 8720 8670 8680 8600 8540

19000 18700 18200 18300 18000 17600 17100 16900 16900

7.92 8.01 8.00 7.74 7.62 7.51 7.52 7.78 7.70

1.42 1.38 1.38 1.39 1.24 1.26 1.30 1.34 1.39

11010 10920 10900 10860 10800 10840 10800 10780 10800

195 190 180 190 195 180 180 185 180

96 92 80 80 72 64 70 62 58

It is observed that with increase in transmembrane pressure difference and Reynolds number, the permeate quality improves. With increase in pressure, the solvent flux increases linearly, while the solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [2]. This indicates that with increasing pressure, more solvent passes through the membrane along with a fixed amount of the solute; the permeate becomes purer and hence the permeate quality (expressed as COD) increases. The similar trends are observed for laminar flow with promoter and turbulent flow. It can be seen from Table 5.6 that at 828 kPa pressure and Re=680, COD decreases by about 23% in presence of promoter compared to the base case (laminar at same operating conditions). Percentage decrease in COD is found to be about 16% at 966 kPa pressure and Re=1360 and about 12% at 1104 kPa pressure and Re=1360. At Re=4762, as the transmembrane pressure increases from 828 kPa to 1104 kPa, COD decreases by 30%. It may be observed from Table 5.6 that for most of the experiments, COD values are well within the discharge limit valid in India (i.e., 250 ppm COD and 30 ppm for BOD). These informations are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality.

5.4.7. Sludge Characterization The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 5.7. It is observed from Table 5.7 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.1 kg from 40 liters of effluent) can be used as a good fertilizer. Table 5.7. Fertilizer quality of sludge obtained from deliming-bating effluent Sample

pH 6.6

Organic Carbon (wt %) 10.5

Nitrogen (wt %) 1.55

Phosphorous (wt %) 0.074

Potassium (wt %) 0.5

Sludge from Deliming-bating Vermi-compost

7.1-7.8

9.97-10.62

1.80

0.90

0.40

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CONCLUSION Effluent from a deliming-bating unit has been successfully treated using a combined process of coagulation by alum and membrane separation processes consisting of UF followed by NF. The optimum alum dose is found to be 2 % (wt/vol). The time required to reach steady state decreases with increase in Reynolds number and applied pressure. The use of turbulent promoters in laminar regime results in substantial increase in flux (27-38% for UF and 45-66% for NF) compared to the laminar case. Both in UF and NF, polarization resistance is the major contributor to overall resistance to the solvent flow (70% for UF and 77% for NF). With increasing pressure, polarization resistance increases and with increasing flow rate it decreases and it is 1.2 to 1.7 times for laminar with turbulent promoter regime and maximum for laminar regime with 2.4 to 3.3 times for NF. However, for turbulent flow conditions, polarized layer resistances are 0.6 to 0.8 times the membrane hydraulic resistance for UF and 0.7 to 1.1 times for NF. Also maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime. Increase of permeate flux with transmembrane pressure is more pronounced in UF (77-85%) compared to NF (20-34%). The retentate of the NF can be recycled to bating process and permeate can be used as wash water. The sun dried sludges obtained from process can be used as organic fertilizer. The COD (~92 ppm) and BOD (~25 ppm) values in the permeate of NF are well below the discharge limit of the same.

REFERENCES [1]

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[2]

C. Das, S. De and S. DasGupta, Treatment of deliming-bating effluent from tannery using membrane separation processes, J. Environ. Prot. Sci., 2 (2008) 11- 24. P.M. Bungay, H.K. Lonsdale, and M.N.de Pinho, Synthetic Membranes: Science, Engineering and Application, published by D. Reidel Publishing Company, (1983) pp. 312.

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Chapter 6

TREATMENT OF PICKLING EFFLUENT ABSTRACT Leather industries generate enormous wastewater including high concentrations of organic materials, salts and other pollutants. The limed or bated skins must be adjusted to a suitable acidity for tannage by pickling. To pickle the pelt means its acidification. This is usually done in presence of salt. Pickling slows down the chrome tanning process and does not allow the tanning agent for deeper penetration. Pickled effluent has a pH of about 1.5 [1].

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6.1. EFFLUENT CONTENT Table 6.1 shows the characterization of a typical pickling effluent collected from M/s, N.A. Trading, Bantala Leather Complex, Kolkata, India. The effluent is collected after one to two hours of the completion of the pickling operation. Table 6.1. Characterization of pickling effluent* Effluent Pickling

pH Feed After CaO dose

1.4 7.26

Conductivity (S/m) 6.80 5.73

TS (g/l) 145.0 127.6

TDS (g/l) 44.9 37.7

COD (ppm) 1120 432

BOD (ppm) 249.0 96

*

All properties are reported average of three measurements.

6.2. PRETREATMENT The pH of the pickling solution is 1.4. The total solid is 145.0 g/l. Hence, the effluent is brought from the plant and is subjected to coagulation by calcium oxide (CaO) prior to membrane operation. Six graduated cylinders of 50 ml capacity with different dosages of alum are used for coagulation study using calcium oxide. To get the optimum dose, concentrations of 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0% (weight by volume) are used. The pickling effluent is used after a week of gravity settling. Before CaO coagulation, the supernatant is

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siphoned out. Coagulation experiments are conducted with different dosages of CaO for twenty four hours. The optimum CaO dose is established by examining various properties (e.g., pH, TDS, conductivity, TS, COD) of supernatant solutions. Figure 6.1 (a) represents the variation of total dissolved solids (TDS) and total solids (TS) with CaO concentration of the supernatant of gravity settled liquor.

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Figure 6.1. Determination of optimum alum dose for pickling effluent.

It can be observed that TS value decreases with CaO concentration and beyond 0.4% concentration, it increase significantly. Variation of conductivity and pH of the clear liquid after coagulation at different CaO concentrations are presented in Figure 6.1 (b). The pH of the supernatant is close to normal pH (~7.26) at 0.4% CaO concentration and it increases significantly further with increase in CaO dose. Figure 6.1 (c) shows that COD of the clarified liquor decreases with CaO concentration and beyond 0.4%, the change is gradual. From these observations, 0.4% is selected as the optimum concentration of CaO for coagulation. The supernatant liquor is then prefiltered through a fine cloth and is subjected to membrane filtration [2].

6.3. SELECTION OF MEMBRANE SEPARATION PROCESSES As mentioned in earlier chapters, selection of the membrane separation process should be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

6.3.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. The permeabilities of these membranes are listed in Table 3.2.

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6.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

6.3.3. Performance Testing of Various Membranes

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Pretreated pickling effluent is subjected to various UF (from 20 kDa to 1 kDa MWCO) membranes and 400 Da MWCO NF membrane in a stirred cell. The profiles of permeate flux are shown in Figure 6.2. It is observed from the figure that the flux decline is due to concentration polarization and beyond about 12 minutes of operation, flux becomes steady in all the cases. Flux decline is 7.0% for 20 kDa, 11.0% for 10 kDa, 6.6% for 5 kDa, 7.2% for 1 kDa and 4.7% for 400 Da MWCO NF membranes. Therefore, as far as flux decline is concerned, performance of 5 kDa, 1 kDa and 400 Da MWCO NF membranes is almost same. Quality of the permeate from various types of membrane is presented in Table 6.2. Reduction in COD is 64 % for 20 kDa, 69% for 10 kDa, 80% for 5 kDa, 82% for 1 kDa and 88% for 400 Da MWCO NF membrane.

Figure 6.2. Flux decline of pretreated pickling effluent using various membranes.

Therefore, as far as COD reduction is concerned, 5 kDa UF membrane shows almost same performance compared to 1 kDa UF and 400 Da MWCO NF membrane. 20 and 10 kDa UF membranes show almost no reduction in concentration of total solids. Whereas, 6% reduction in TS is observed in 5 kDa membrane. This value is 8% for 1 kDa UF and 13% for 400 Da MWCO NF membrane. From Table 6.2, it is also observed that COD value of the

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permeate is well below the discharge limit (250 ppm in India) for 5kDa membrane. For lower cut off membranes, COD values of permeate are still less but at the cost of permeate flux, i.e., productivity of the system. Therefore, from the point of permeate quality, performance of 5 kDa UF membrane is almost equivalent to 1 kDa and NF membrane. Moreover, permeate flux is significantly more in case of 5 kDa UF membrane compared to 1 kDa UF membrane and 400 Da MWCO NF membrane. Therefore, 5 kDa UF membrane is selected as the possible membrane among the studied ones for treatment of pickling effluent. Table 6.2. Permeate properties of pickling effluent MWCO 20 kDa 10 kDa 5 kDa 1 kDa 400 Da

COD (mg/l) 406 [1120]* 344 216 203 136

TS (g/l) 142.6 [145]* 143.4 136.4 133.6 126.2

TDS (g/l) 39.8 [44.9]* 40.3 39.0 38.8 38.1

*

Values indicate the properties corresponding to feed.

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6.4. DETAILED STUDY OF TREATMENT OF PICKLING EFFLUENT In this chapter, a scheme is proposed to treat the pickling effluent using a hybrid process, CaO coagulation and ultrafiltration. The optimum CaO dose is established. The fertilizer value of the sludge produced is tested. The supernatant liquor is subjected to continuous cross flow ultrafiltration. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 6.3.

Figure 6.3. Proposed scheme for the treatment of pickling effluent.

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6.4.1. Cross Flow Cell and Operating Conditions The same cross flow cell described in section 3.4.1 is used for pickling effluent also. The operating conditions for all membrane experiments are presented in Table 6.3. Both in laminar and turbulent flow regime, ultrafiltration experiments are conducted at four different operating pressures of 276, 414, 552 and 690 kPa. Table 6.3. Operating conditions for cross flow experiments Reynolds number

Step

Transmembrane pressure (kPa)

Laminar and with promoter

Turbulent

UF

276, 414, 552 and 690

606, 909 and 1212

4242, 4848 and 5454

6.4.2. Analysis of Transient Flux Decline

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The pretreated pickling effluent is subjected to membrane separation process with a 5 kDa ultrafiltration membrane. The experiments are conducted in three different flow regimes: laminar, laminar with promoter and purely turbulent. Figure 6.4 represents the variation of permeate flux behavior with time at laminar regime with and without promoters at 276 kPa and 690 kPa pressures.

Figure 6.4. Variation of permeate flux with time in both laminar and laminar with promoter flow regime.

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It indicates that the time required to reach steady state is decreased with increase in Reynolds number. For example, it can be observed from Figure 6.4 that the steady state is attained in about 18.7 min for Re=606 and 276 kPa pressure, whereas at the same pressure but Re=1212, the steady state is attained within 18.1 min. Similarly, the steady state is attained in 3.7 min for Re=606 and 690 kPa pressure, whereas at the same pressure but Re=1212, the steady state is attained within 3.2 min for laminar with promoter condition. It is also found that the steady state is achieved faster using turbulent promoters compared to laminar flow. The turbulent promoters reduce the concentration polarization at the membrane surface due to local turbulence and growth of polarized layer is controlled which establishes steady state faster compared to purely laminar condition. Figure 6.5 represents similar trends in turbulent regime at 276 kPa and Re=4242, 5454. Steady state is attained at 18.45 and 12.8 mins, respectively.

Figure 6.5. Variation of permeate flux with time in turbulent flow regime.

6.4.3 Analysis of Steady State Flux Figure 6.6 represents the variations of steady state permeate flux with pressure at different Reynolds number under laminar and laminar with turbulent promoter flow conditions. The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier. Variation permeate flux with transmembrane pressure in the turbulent flow regime is presented in Figure 6.7. With increasing transmembrane pressure and

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Reynolds number, permeate flux increases. The reason behind this is already discussed earlier.

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Figure 6.6. Variation of permeate flux with transmembrane pressure in UF for pickling effluent in laminar and laminar with turbulent promoter flow regime.

Figure 6.7. Variation of permeate flux with transmembrane pressure in UF for pickling effluent in turbulent flow regime. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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6.4.4. Analysis of Various Resistances The permeate flux at any point of time is expressed as presented by Eq. (3.1). The steady state polarized layer resistance is given by Eq. (3.2). The variation of dimensionless steady state polarized layer resistance with Reynolds number for laminar regime is presented in Figure 6.8. It is observed from the figure that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure of 276 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 2.2 to 1.5 with an increase in Reynolds number from 606 to 1212. Rp values increase with the transmembrane pressure. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number (1212) and transmembrane pressure (690 kPa), the presence of turbulent promoters reduces the resistance to 1.1 compared to 1.6 in laminar flow. This reduction in

Rps

is more than 30.3% leading to a significant enhancement of the permeate

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flux.

Figure 6.8. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during UF.

The value of

Rps

reduces further in case of purely turbulent flows for reasons already

discussed. For turbulent flow regime, effects of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the

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range of Reynolds number studied herein,

Rps varies between 0.6 to 1.2 (Table 6.4) times of

Rm. At a transmembrane pressure of 414 kPa and Reynolds number =4848, Rp contributes about 50.6% of total resistance. Table 6.4. Variation of ratio of polarized layer resistance to hydraulic resistance in turbulent flow regime

R ps /Rm

Transmembrane pressure (kPa) Re=4242 0.80 0.91 0.96 1.16

276 414 552 690

Re=4848 0.71 0.80 0.90 1.08

Re=5454 0.61 0.71 0.85 0.97

6.4.5. Permeate Quality Analysis For different operating conditions, the permeate quality after UF, is shown in Table 6.5. Almost all the salt present in the feed solution has permeated through the UF membrane. Hence, the permeate conductivity remains almost same as that of feed. Table 6.5. Permeate quality analysis after UF

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Properties Permeate

pH 7-7.5

Conductivity ( S/m) 5.6-5.9

TDS (g/l) 36.5-38.5

TS (g/l) 52-61

The variation of permeate COD of UF with various operating conditions is presented in Table 6.6. It can be observed from Table 6.6 that COD values of the permeate are in the range of 87 to 198 ppm, which is considerably lower than the discharge limit (250 ppm) in India. Table 6.6. Variation of COD with operating conditions in UF Operating Condition

Laminar

Laminar with promoter

Turbulent

Re (kPa) Pressure 276 414 552 690

606

909

1212

606

909

1212

4242

4848

5454

198 170 149 130

182 162 138 123

178 157 127 101

183 163 147 128

178 158 136 120

167 153 125 98

179 161 145 116

171 156 131 106

163 150 123 87

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6.4.6. Sludge Characterization The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 6.7. It is observed from Table 6.7 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.6 kg from 100 liters of effluent) can be used as a good fertilizer. Table 6.7. Fertilizer quality of sludge obtained from pickling effluent Sample

pH

Sludge from Pickling Vermi-compost

6.8 7.1-7.8

Organic Carbon (wt %) 1.23 9.97-10.62

Nitrogen (wt %) 0.23 1.80

Phosphorous (wt %) 0.14 0.90

Potassium (wt %) 0.56 0.40

CONCLUSION

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The viability of pickling unit effluent treatment using a combined process of coagulation by CaO and membrane separation is established in this study. The values of COD (87 to 198 ppm) of UF are well below the discharge limit. With increase in Reynolds number and applied pressure the time required to reach steady state decreases. In UF, polarization resistance is the major contributor to overall resistance to the solvent flow for laminar case. With increasing pressure, polarization resistance increases and with increasing flow rate it decreases and it is 1.5 to 2.3 times for laminar flow regime. However, for turbulent flow conditions, polarized layer resistances are 0.6 to 1.2 times the membrane hydraulic resistance. Pulverized sludge obtained after sun drying can be used as organic fertilizer.

REFERENCES [1] [2]

S.S. Dutta, An Introduction to the Principles of Leather Manufacture; Indian Leather Technologists’ Association (4th Edition), Kolkata, India(1999) C. Prabhavathy, C. Das, S. DasGupta and S. De, Treatment of pickling effluent from tannery using membrane separation processes, International Conference on Catalysis in Membrane Reactors, CGCRI, Kolkata, India, (2007)., 136-139.

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Chapter 7

TREATMENT OF DEGREASING EFFLUENT ABSTRACT Degreasing of skins and hides are done by leaching these with organic solvents or surfactants. For sheepskin containing about 30 to 40% fat substances, this operation is very essential. The effluent generated from this step has a very high COD due to the presence of these fatty substances and gives troubles in biological treatment plants [1-3].

7.1. EFFLUENT CONTENT

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Table 7.1 shows the characterization of a typical degreasing effluent collected from M/s, N.A. Trading, Bantala Leather Complex, Kolkata, India. The effluent is collected after one to two hours of the completion of the degreasing operation. Table 7.1. Characterization of degreasing effluent Effluent Degreasing

8.6

Conductivity (S/m) 2.64

TS (g/l) 31.0

TDS (g/l) 17.5

COD (ppm) 3700

BOD (ppm) 1000

7.0

2.9

27.4

19.1

730

200

pH Feed After alum dose

7.2. PRETREATMENT Degreasing effluent has a COD of 3700 ppm and TDS of 31.0 g/l. As a result of which, pretreatment is necessary prior to membrane separation operation. After bringing the effluent from the plant, it is subjected to coagulation by alum. A series of control experiments are conducted as discussed in earlier chapters and various properties of the effluent are noted. Following the similar procedure as described in previous chapters, the optimum alum dose is found to be 0.7% (wt/vol). Various properties at this alum dose is presented in Table 7.1. The

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supernatant liquor is then prefiltered through a fine cloth and is subjected to membrane filtration.

7.3. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of the appropriate membrane for the filtration is of utmost importance. Selection should primarily be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

7.3.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

7.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

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7.3.3. Performance Testing of Various Membranes Pretreated degreasing effluent is subjected to various UF (from 20 kDa to 1 kDa MWCO) membranes and 400 Da MWCO NF membrane in a stirred cell. Transient flux decline behavior using various membranes is shown in Figure 7.1. In Figure 7.1, operating pressure for all the UF runs is 414 kPa and that for NF is 828 kPa. General trend for permeate flux profiles in Figure 7.1 is that the flux decreases with the operating time. This is due to concentration polarization. As filtration progresses, solute particles deposit over the membrane surface, forming a polarized layer which grows in thickness. Some of the pores in the membrane are also clogged by the solutes. This is confirmed by observing the fact that pure water flux reduces when the same membrane is used without any chemical cleaning (cleaning by water only) just after the experiment. Combined effects of these phenomena lead to a decline in flux. It is observed that about 8% decline in flux occurs for 20 kDa MWCO membrane over the 14 minutes of operating time. This is about 11%, 15% and 18% for 10 kDa, 5 kDa and 1 kDa cut-off membranes. Interestingly, flux decline over the duration of the experiment for NF membrane is only about 12%. It may be pointed out that although the absolute flux increases from 1 kDa to 20 kDa cut-off membranes, the flux decreases steadily during the filtration time.

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Figure 7.1. Flux decline profile of pretreated degreasing effluent using various membranes.

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Table 7.2. Permeate properties of degreasing effluent MWCO

COD (mg/l)

TS (g/l)

TDS (g/l)

20 kDa 10 kDa 5 kDa 1 kDa 400 Da

720 [730]* 685 622 560 110

27.4 [27.4]* 26.08 24.87 23.41 12.75

19.0 [19.1]* 18.55 17.41 16.04 10.15

*

Values indicate the properties corresponding to feed.

On the other hand, for NF membrane, flux becomes almost constant beyond 8 minutes of operation. Since, the retentate and permeate streams are recycled to the feed chamber, both the feed volume and feed concentration remain unchanged and therefore, the value of steady state permeate flux remains same beyond 8 minutes as evident from Figure 7.1. This indicates that the UF membranes having larger pore size (in the increasing order of 1 kDa to 20 kDa), they are more susceptible to pore clogging by the solute particles, resulting in steady decline in flux, although flux decline becomes gradual later on. For NF membrane, larger solute particles cannot enter the pores at all, leading to formation of a polarized layer over the membrane surface whose thickness remains constant by external stirring and hence almost a steady state flux is resulted beyond 8 minutes and flux decline is also minimum in this case.

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Suitability of a membrane separation process depends not only on its permeate flux, i.e., productivity but also on the permeate quality. It is observed from Table 7.2 that reduction in COD is only 1.4% in 20 kDa, 6.2% for 10 kDa, 15% for 5 kDa, 23.3% for 1 kDa and 85% for NF membrane. Total solid concentration in all UF membranes is almost insignificant (about 25.8 g/l TS concentration with respect to 27.4 g/l in the alum treated feed). But NF membrane shows about 53.5% retention of total solids. As expected, the TDS retention (i.e. retention of inorganic solutes) by UF membrane is marginal, whereas 400 Da NF membrane shows about 47% retention of inorganic solutes in terms of TDS. Therefore, as far as the quality of the permeate is concerned, NF membrane shows the most promising performance. Therefore NF 400 membrane alone is sufficient to treat degreasing effluent.

7.4. DETAILED STUDY OF TREATMENT OF DEGREASING EFFLUENT

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In this chapter, a scheme is proposed to treat the degreasing effluent using a hybrid process, alum coagulation and nanofiltration. The optimum alum dose is identified as 0.7% (wt/vol). The fertilizer value of the sludge produced is tested. The supernatant liquor is subjected to continuous cross flow ultrafiltration. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 7.2.

Figure 7.2. Proposed scheme for the treatment of degreasing effluent.

7.4.1. Cross Flow Cell and Operating Conditions The same cross flow cell described in section 3.4.1 is used for degreasing effluent also. The operating conditions for all membrane experiments are presented in Table 7.3. Both in laminar and turbulent flow regime, nanofiltration experiments are conducted at four different operating pressures of 828, 966, 1104 and 1242 kPa.

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Table 7.3. Operating conditions for cross flow experiments in NF Reynolds number Transmembrane pressure (kPa) 828, 966, 1104 and 1242

Laminar and with promoter

Turbulent

606, 909 and 1212

4242, 4848 and 5454

7.4.2. Analysis of Transient Flux Decline

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The experimental flux decline for transient state in turbulent flow regime is shown in Figure 7.3. Figure 7.4 represents the transient flux decline profile in laminar and with turbulent promoter flow regime. It can be clearly seen from the Figure 7.3 that the time required to reach steady state decreases with increase in Reynolds number. For example, it can be observed from Figure 7.3 that the steady state is attained in about 20.5 min, for Re=4242 and 966 kPa pressure, whereas at the same pressure but at Re=5454, the steady state is attained within 18.9 min. The flux decline is about 10% of the initial value for Re=4242, about 8.5% with increase in Re= 5454. Similar trends can be observed for flux decline in laminar regime with and without promoters (Figure 7.4). As the cross flow velocity increases, the growth of the polarized layer over the membrane surface is slower because of enhanced forced convection. This leads to the onset of steady state at an earlier time.

Figure 7.3. Permeate flux decline profile with time of turbulent flow regime in NF.

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Figure 7.4. Permeate flux decline profile with time for NF membrane at various pressures in laminar and with promoter flow regime.

For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Figure 7.4, at Re=606 and 966 kPa, the steady state is attained in about 27.3 min without promoter and about 21.8 min with promoter at the same operating condition. The flux decline is about 17.2% without promoter at Re=606 and 966 kPa pressure; but only 8.9% using promoter at the same operating condition. Use of the turbulent promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the polarized layer is controlled quickly, establishing steady state earlier than without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less compared to the no promoter case.

7.4.3. Analysis of Steady State Flux Figure 7.5 represents the variations of steady state permeate flux with pressure at different Reynolds number under purely turbulent, laminar flow and laminar with promoter conditions. The figure shows the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier.

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Figure 7.5. Variation of permeate flux with transmembrane pressure in NF for degreasing effluent.

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7.4.4. Analysis of Various Resistances The permeate flux at any point of time is expressed as presented by Eq. (3.1). The steady state polarized layer resistance is given by Eq. (3.2). Figure 7.6 represents the variation of dimensionless steady state polarized layer resistance with Reynolds number for laminar and with promoter flow regime. It is observed from the figure that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure of 966 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 7.2 to 6.3 with an increase in Reynolds number from 606 to 1212. Rp values increase with the transmembrane pressure. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number (1212) and transmembrane pressure (1242 kPa), the presence of turbulent promoters reduces the resistance to 5.1 compared to 6.5 in laminar flow. This reduction in

Rps

The value of

is around 22% leading to a significant enhancement of the permeate flux.

Rps

reduces further in case of purely turbulent flows for reasons already

discussed. For turbulent flow regime, effects of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the range of Reynolds number studied herein,

Rps varies between 2.8 to 3.6 (Table 7.4) times of

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Rm. At a transmembrane pressure of 1242 kPa and Reynolds number =5454, Rp contributes about 76.4% of total resistance.

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Figure 7.6. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during NF in laminar and with promoter flow regime.

Table 7.4. Variation of ratio of polarized layer resistance to hydraulic resistance in turbulent flow regime

Transmembrane pressure (kPa) 828 966 1104 1242

R ps /Rm Re=4242 3.1 3.2 3.4 3.6

Re=4848 3.0 3.3 3.4 3.4

Re=5454 2.8 3.2 3.2 3.2

7.4.5. Enhancement of Steady State Permeate Flux The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 7.7. All the increases are calculated taking the laminar flow results under same operating conditions as the basis. The formation of polarized layer over the membrane surface is significantly reduced in presence

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of the turbulent promoters. This causes a corresponding increase in permeate flux. It may be observed from Figure 7.7 that the flux increment is in the range of 23 to 37% for laminar flow with promoter. However, it can be clearly seen from Figure 7.7 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

Figure 7.7. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.

7.4.6. Permeate Quality Analysis For different operating conditions, the permeate quality after NF, is shown in Table 7.5. It can also be observed from Table 7.5 that COD values of the permeate are in the range of 70 to 170 ppm, which is considerably lower than the discharge limit (250 ppm) in India. pH of the permeate varies from 6.4 to 7.5, which is also within the discharge limit (5.5 to 9.5). Table 7.5. Permeate quality analysis after NF Properties

pH

Conductivity ( S/m)

TDS (g/l)

COD (ppm)

TS (g/l)

Permeate

6.4-7.5

1.35-1.95

8.5-13.0

70-170

10.5-15.5

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7.4.5. Sludge Characterization The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 7.6. It is observed from Table 7.6 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.1 kg from 40 liters of effluent) can be used as a good fertilizer. Table 7.6. Fertilizer quality of sludge obtained from degreasing effluent Sample

pH 6.8

Organic Carbon (wt %) 12.1

Nitrogen (wt %) 5.5

Phosphorous (wt %) 0.35

Potassium (wt %) 0.13

Sludge from Degreasing Vermi-compost

7.1-7.8

9.97-10.62

1.80

0.90

0.40

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CONCLUSION The viability of degreasing unit effluent treatment using a combined process of coagulation by alum and membrane separation is established in this study. The values of COD (70 to 170 ppm) of NF are well below the discharge limit. Permeate flux enhancements using turbulent promoters in laminar regime (23 to 37% for NF) is observed. With increase in Reynolds number and applied pressure the time required to reach steady state decreases. In NF, polarization resistance is the major contributor to overall resistance to the solvent flow for laminar case. With increasing pressure, polarization resistance increases and with increasing flow rate it decreases and it is 6.2 to 7.4 times for laminar flow regime. However, for turbulent flow conditions, polarized layer resistances are 2.8 to 3.6 times the membrane hydraulic resistance. Pulverized sludge obtained after sun drying can be used as organic fertilizer.

REFERENCES [1]

[2]

[3]

B. Cortese and E. Drioli, Esperinze di ultrafiltrazion su liquami provenienti dallo sgrassaggio delle pelli in un’industria conciaria, Cuiro Pelli, Mat. Concianti, 5 (2) (1978) 167. Cassano, A. Criscuoli, E. Drioli and R. Molinari, Clean operations in the tanning industry: aqueous degreasing coupled to ultrafiltration: experimental and theoretical analysis, Clean Product Process, 1 (4) (1999) 257-263. S.S. Dutta, An Introduction to the Principles of Leather Manufacture; Indian Leather Technologists’ Association (4th Edition), Kolkata, India(1999)

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Chapter 8

TREATMENT OF TANNING EFFLUENT ABSTRACT

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Degresed skins and hides are tanned by two types of tanning operations, namely, chrome tanning and vegetable tanning. Chrome tanning process protects the hides and skins from its decay and gives the resistance against high temperature and bacterial damage. During chrome tanning, cross linking of chromium ions take place with free carboxylic groups into the collagen. Basic chrome sulfate is widely used as tanning substrate [1]. Around 30% chromium of initial BCS (basic chrome sulfate) bath is found in the exhausted bath after chrome tanning process. This huge chromium creates serious problems during disposal [2]. Hence, chromium recovery from tanning effluent plays a vital economical assistance in a tannery in terms of minimization of ETP (effluent treatment plant) load and its reuse in the tanning process itself.

8.1. TANNING EFFLUENT The characterization of a typical tanning effluent collected from M/s, Alison Tannery, Kolkata, India is shown in Table 8.1. Effluent is collected after one to two hours of the completion of the tanning operation. As discussed in earlier chapter, the tanning effluent is subjected to a suitable pretreatment process prior to membrane filtration. Table 8.1. Characterization of tanning effluent pH Feed

pH 4.1

Conductivity (S/m) 7.5

TS (g/l) 126

TDS (g/l) 49.5

COD (ppm) 1150

BOD (ppm) 442

8.2. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of membrane with an appropriate molecular weight cut-off (MWCO) is the most crucial step. Selection should primarily be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

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8.2.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

8.2.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

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8.2.3. Performance Testing of Various Membranes Pretreated (cloth filtered) effluent is subjected to the stirred cell in continuous mode using various MWCO of membranes, starting from NF (MWCO 400) to UF (MWCO ranging from 1 kDa to 20 kDa). Transient flux decline behavior using various membranes is shown in Figure 8.1. In Figure 8.1, operating pressure for all the UF membranes is 414 kPa and that for NF is 828 kPa. Stirring speed is 1000 rpm for all the cases. General trend for permeate flux profiles in Figure 8.1 is that the flux decreases with the operating time. This is due to concentration polarization. As filtration progresses, solute particles deposit over the membrane surface, forming a polarized layer which grows in thickness. Some of the pores in the membrane are also clogged by the solutes. Combined effects of these phenomena lead to a decline in flux. It is observed that about 12% decline in flux occurs for 20 kDa MWCO membrane for the 14 minutes of operating time. This is about 14%, 18% and 21% for 10 kDa, 5 kDa and 1 kDa cut-off membranes. It may be pointed out that although the absolute flux increases from 1 kDa to 20 kDa cutoff membranes, the flux decreases steadily during the filtration time. On the other hand, for NF membrane flux decline is almost 22%. Suitability of a membrane separation process depends not only on its permeate flux, i.e., productivity but also on the permeate quality. It is observed from Table 8.2 that reduction in Cr3+ concentration is only 8.5%, 21%, 23% and 25% for 20, 10, 5 and 1 kDa UF membranes whereas it is 95.7% for NF membrane and reduction in COD is only 2.8% in 20 kDa, 6.4% for 10 kDa, 11% for 5 kDa, 15.5% for 1 kDa and 32.2% for NF membrane. Total solid concentration in all UF membranes is almost insignificant (about 107 to 122 g/l TS concentration with respect to 126 g/l in the feed). But NF membrane shows about 41.5% retention of total solids. As expected, the TDS retention (i.e. retention of inorganic solutes) by UF membrane is marginal, whereas 400 NF membrane shows about 14% retention of inorganic solutes in terms of TDS. Therefore, as far as the quality of the permeate is concerned, NF membrane shows the most promising performance.

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Table 8.2. Permeate properties of tanning effluent MWCO

Cr3+ concentration (mg/l)

COD (mg/l)

TS (g/l)

TDS (g/l)

20 kDa

2104 [2300]*

1118 [1150]*

121.5 [126.1]*

49.4 [49.5]*

10 kDa

1815

1076

118.2

48.6

5 kDa

1751

1024

112.1

46.7

1 kDa

1712

972

106.6

45.0

400 Da

98.5

780

73.8

42.4

*

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Values indicate the properties corresponding to feed.

Figure 8.1. Flux decline of pretreated tanning effluent using various membranes.

It may be noted here that the permeate quality after NF is still not adequate to discharge in the sewage (discharge limit for COD is 250 mg/l and for BOD is 30 mg/l). For this, the permeate from NF 400 may be subjected to RO. Therefore, 400 MWCO NF membrane followed by RO should be the selected membrane process for the treatment of pretreated tanning effluent [3].

8.3. DETAILED STUDY OF TREATMENT OF TANNING EFFLUENT In this chapter, nanofiltration (NF) followed by reverse osmosis (RO) technique has been used to treat the chrome tanning effluent in a cross flow cell. Retentate stream of NF may be

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recycled to the tanning chamber after make up of the required chromium concentration. The permeate stream of NF (which contains most of the natural salts) is passed through a reverse osmosis unit to get clean water and concentrated salt solution for reuse. The effects of different operating conditions, e.g., transmembrane pressure drop and cross flow on permeate flux and observed retention are studied. The experiments are conducted in both laminar and turbulent regimes as well as using thin wires as turbulent promoters. The proposed treatment scheme for tanning effluent is presented in Figure 8.2.

Figure 8.2. Proposed scheme for tanning effluent treatment.

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8.3.1. Cross Flow Cell and Operating Conditions Nanofiltration experiments are conducted at three pressures (828, 966, 1104 kPa) with Reynolds numbers of 636, 954 and 1272 in laminar regime both with and without promoter. Reynolds numbers of 4453, 5089 and 5725 are used in turbulent regime. Reverse osmosis experiments are conducted at four different pressures of 1380, 1518, 1725, and 1932 kPa with Reynolds numbers of 4453, 5089 and 5725 in turbulent regime. In laminar regime, with and without promoter, the RO experiments are conducted at 1725 kPa pressure and at Reynolds numbers of 636 and 954 only. The operating conditions for all membrane experiments are presented in Table 8.3. Table 8.3. Operating conditions for cross flow experiments

Step NF RO

Transmembrane pressure (kPa) 828, 966 and 1104 1380, 1518, 1725 and 1932 1725

Reynolds number Laminar and with promoter

Turbulent

636, 954 and 1272 -636, 954

4453, 5089 and 5725 4453, 5089 and 5725 --

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8.3.2. Analysis of Transient Flux Decline

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Nanofiltration Figures 8.3 and 8.4 represent the flux decline behavior of the effluent at 828 kPa and 1104 kPa pressures respectively. It can be clearly seen from these two figures that the time required to reach steady state decreases with increase in cross flow velocity. For example, it can be observed from Figure 8.3 that the steady state is attained in about 25.3 min, at Re=4453 and 828 kPa pressure, whereas at the same pressure but at Re=5725, the steady state is attained within 21.7 min. In addition, the flux decline is about 36% of the initial value for Re=4453, about 33% with increase in velocity to Re=5089, and 24% at Re=5725. As the cross flow velocity increases, the growth of the concentration boundary layer over the membrane surface is arrested faster. This leads to the onset of the steady state at an earlier time. For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities as observed. It can also be seen from Figures 8.3 and 8.4 that at a fixed cross flow velocity the steady state is attained faster with an increase in operating pressure. For example in Figure 8.4, steady state is attained in about 20.2 min for an operating condition of Re=4453 and at 1104 kPa pressure. Whereas at the same cross flow velocity, the time required to attain steady state is about 25.3 min for an operating pressure 828 kPa. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Figure 8.4, at Re=636 and 828 kPa, the steady state is attained in about 44.6 min without promoter and about 34.1 min with promoter at the same operating condition. The flux decline is about 26 % without promoter at Re=954 and 828 kPa pressure; but only 18% using promoter at the same operating condition.

Figure 8.3. Transient flux data at 828 kPa pressure in NF.

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Figure 8.4. Transient flux data at 1104 kPa pressure in NF.

Similar trends are observed in Figure 8.4 as well. Use of the turbulence promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the concentration boundary layer is checked quickly, establishing steady state earlier than the case of without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less than the no promoter case.

Reverse Osmosis The flux versus time data of experimental values are plotted in Figure 8.5 for all three working regimes of operation namely: laminar, laminar with promoter and turbulent. Figure 8.5 presents the flux decline behavior with transmembrane pressure and cross flow velocity in RO. The results clearly show that as in the case of NF, the time required to reach steady state decreases with increase in cross flow velocity and applied pressure or in presence of turbulence promoters. Extent of flux decline also follows similar trends for reasons already discussed in the section describing NF operations. It can be observed that the flux decline is about 26% of the initial value at Re=954 and 1725 kPa pressure and is 20% at the same operating conditions but with promoters. The system reaches steady state in about 13.3 min with promoter compared to about 28.3 min without promoter in laminar regime. It may also be observed from Figure 8.5 that at lower operating pressure in turbulent regime, flux decline is marginal; whereas at higher operating pressure, it is significant due to concentration polarization effects. For example, flux decline is about 26% at 1932 kPa and Re=5725.

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Figure 8.5. Transient flux data in RO.

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8.3.3. Analysis of Steady State Flux Nanofiltration The variations of steady state permeate flux with pressure at different cross flow rate under laminar flow and with turbulent promoters are shown in Figure 8.6. The variations of permeate flux under turbulent regimes at different operating conditions are shown in Figure 8.7. The figures show the usual trend that the permeate flux increases with the operating pressure and cross flow rate. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with cross flow rate is because of decreasing concentration polarization as discussed earlier. Reverse Osmosis The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to the higher operating pressure (driving force) and turbulence present near the membrane surface. The effects of transmembrane pressure Reynolds number on steady state flux in RO are shown in Figure 8.8. The figure shows that the permeate flux increases with operating pressure and Reynolds number as in NF. At 1932 kPa, an increase in Reynolds number from 4453 to 5725 results in about 64% increase in permeate flux.

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Figure 8.6. Variation of permeate flux with pressure drop in laminar regime in NF.

Figure 8.7. Variation of permeate flux with Pressure drop in turbulent regime in NF.

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Figure 8.8. Variation of permeate flux with pressure drop in RO.

Figure 8.9. Variation of the ratio of polarized layer and hydraulic resistances at steady state with the cross flow velocity during NF.

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8.3.4. Analysis of Various Resistances The variation of steady state polarized layer resistances with Reynolds number for some typical experiments is presented in Figure 8.9. As can be observed from Figure 8.9, steady state polarized layer resistance R ps decreases with increase in Reynolds number in all the cases. For example, for a transmembrane pressure drop of 828 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 20 to 17.6 with an increase in Reynolds number from Re=636 to 954. Significant reductions in polarized layer resistance, compared to laminar flow, are achieved using turbulent promoters at the same Reynolds number. At the same operating Re=636 and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the resistance to 12.38 compared to 20 in laminar flow. For the case with the promoters, the gel type layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters.This reduction in R ps is more than 61% in some of the experiments leading to a significant enhancement of the permeate flux. The figure also shows further reductions for the case of purely turbulent flows for reasons already discussed.

8.3.5. Enhancement of Steady State Permeate Flux

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Nanofiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 8.10.

Figure 8.10. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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All the increases are calculated taking the laminar flow results under same operating conditions as the base. The concentration boundary layer over the membrane surface is significantly disturbed in presence of the turbulent promoters. This causes reduction in the membrane surface concentration and thereby increase in the effective driving force (ΔP-Δπ, Δπ is the osmotic pressure) and hence an increase in permeate flux. It may be observed from Figure 8.10 that the flux increment is in the range of 31 to 57% for laminar flow with promoter. The results for turbulent flow regime cannot be directly compared with laminar flow results for flux enhancement calculations, as the operating conditions are different. However, it can be clearly seen from Figures 8.6 and 8.7 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

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8.3.6. Permeate Quality Analysis Nanofiltration The permeate quality is expressed in terms of retention of chromium. Variations in permeate quality with trans-membrane pressure at three different cross flow velocities in laminar and turbulent regimes are shown in Table 8.4, respectively. It is observed that with increase in pressure drop and cross flow velocity, the permeate quality improves. With increase in pressure, the water flux increases in a linear fashion, while solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [4]. The result is that, with increasing pressure, more water passes through the membrane along with a fixed amount of the solute; the water is thus purer and hence the permeate quality (expressed as observed rejection) improves. The effects of turbulence promoter are also investigated in the laminar flow regime. The results are summarized in Table 8.4. It can be seen from Table 8.4 that at 828 kPa pressure and Re=1272, chromium retention characteristic improves slightly (by about 3%) in presence of promoter compared to the base case (laminar at same operating conditions). Percentage improvement in quality is found to be in about the same range as in the turbulent regime as shown in Table 8.4. Table 8.4 illustrates that the chromium retention increases with cross flow velocity and pressure for reasons already described earlier. For example, at 1104 kPa, an increase in Reynolds number from 4453 to 5725 results in an increase in chromium retention from 95.65 % to 97.8 %. At Re=4453, as the transmembrane pressure increases from 828 kPa to 1104 kPa, the observed retention of chromium increases by 5.1%. The permeate quality after NF, for various operating conditions, is also presented in Table 8.4. It may be observed from the table that for the laminar regime, the retention of chromium varies from about 92% to 97.3%. Similar result was obtained by Cassano et al. [5]. The COD of the permeate remains quite high, higher than the permissible limit (250 mg/lit) in India. The conductivity of the permeate is same as the feed which signifies that almost all the salt present in the feed solution has permeated through the NF membrane. From Table 8.4, it can be observed that the presence of turbulent promoters or high cross flow velocity (turbulent regime) does not affect the chromium retention characteristics of the membrane to a great extent, rather it contributes to the considerable increase in the permeate flux.

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Sirshendu De, Chandan Das and Sunando DasGupta Table 8.4. Permeate analysis after nanofiltration

Sr. No

Pressure (kPa)

Re

TDS (g/l)

TS (g/l)

pH

Conductivity (S/m)

COD mg/l

BOD mg/l

R0 of chromium

4452

42.8

73.0

4.4

6.48

859.1

330.4

91.0

5088

42.5

71.4

4.4

6.44

822.4

316.3

91.5

5724

42.3

71.0

4.4

5.18

797.9

306.9

91.9

4452

42.3

72.0

4.4

4.86

675.5

259.8

93.6

5088

42.3

72.1

4.4

4.62

614.4

236.3

94.5

5724

43.0

69.0

4.4

4.31

569.5

219.0

95.1

4452

43.0

68.3

4.4

5.00

532.8

204.9

95.7

5088

43.0

67.1

4.4

4.58

496.1

190.8

96.2

5724

43.5

67.6

4.4

4.36

381.9

146.9

97.8

636

42.9

71.4

4.3

6.50

797.9

306.9

91.9

42.0

71.8

4.3

5.86

740.8

284.9

92.7

4.4

5.66

695.9

267.7

93.3

Turbulent Regime 1 2

828

3 4 5

966

6 7 8

1104

9 Laminar regime 10 11

954

12

1272

42.0

72.2

13

636

43.0

69.4

4.4

5.16

712.3

273.9

93.1

954

42.0

70.8

4.4

4.94

622.5

239.4

94.4

15

1272

42.0

68.2

4.4

4.63

512.4

197.1

95.9

16

636

40.9

67.2

4.4

6.18

577.7

222.2

95.0

954

40.9

68.5

4.4

5.94

512.4

197.1

95.9

1272

37.3

68.2

4.4

5.66

418.6

161

97.3

636

37.2

68.1

4.4

5.64

691.9

266.1

93.4

954

35.6

67.7

4.4

5.11

565.4

217.5

95.2

1272

42.3

67.5

4.3

5.09

512.4

197.1

95.9

636

42.6

67.1

4.3

6.45

659.2

253.5

93.9

14

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828

17

966

1104

18

With turbulent promoter 19 20

828

21 22 23

966

954

36.9

65.3

4.4

6.12

504.2

193.9

96.1

24

1272

38.9

65.1

4.3

5.89

426.7

164.1

97.2

25

636

39.2

65.1

4.3

5.94

540.9

208.1

95.5

954

37.3

64.5

4.4

5.66

475.7

183

96.5

1272

35.6

64.2

4.4

5.39

402.3

154.7

97.5

26 27

1104

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Reverse Osmosis The effects of transmembrane pressure and cross flow velocity on permeate quality in terms of chromium retention for turbulent regime, in RO, are shown in Table 8.5. The figure illustrates that the improvement of permeate quality with Reynolds number and pressure is marginal. For example, at 1932 kPa, an increase in Reynolds number from 4453 to 5725 results in an increase in retention of chromium from 99.62 % to 99.74%. The permeate qualities in terms of other properties for various operating conditions are presented in Table 8.5. It may be observed from Table 2.53 that the concentration of chromium in the permeate varies from about 1.0 to 0.2 ppm in the pressure range of 1380 to 1932 kPa which is within the permissible limit (1.0 ppm [6]). Furthermore, the COD of permeate is substantially lower than the permissible limit (250 mg/lit). From Table 8.5, it may also be observed that the conductivity of the permeate is very small signifying that almost all the salt present in the feed has been retained by the RO membrane. This salt rich retentate stream can be recycled to the tanning process. The informations are essential for choosing the operating conditions and thereby improving the economics of the process without loss of product quality. Table 8.5. Permeate analysis after reverse osmosis

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Sr. No

Pressure (kPa)

Re

Turbulent regime 1 4452 1380 2 5088 3 5724 4 4452 1518 5 5088 6 5724 7 4452 1725 8 5088 9 5724 10 4452 11 1932 5088 12 5724 Laminar regime 13 1725 636 14 1725 954 Laminar with promoter 15 1725 636 16 1725 954

TDS (g/l)

TS (g/l)

pH

Conductivity (S/m)

COD (mg/l)

BOD (mg/l)

R0 of chromium

Cr Conc. (mg/l)

5.35 4.80 5.02 5.80 5.74 5.61 4.36 5.36 6.36 7.36 8.36 9.36

5.56 4.99 5.22 6.03 5.97 5.83 4.53 5.57 6.61 7.65 8.69 9.73

6.2 6.1 5.9 5.7 5.9 6.1 6.1 6.4 5.8 5.7 5.7 5.6

1.01 0.93 0.80 1.09 1.05 1.02 1.39 1.09 0.90 1.49 1.18 0.96

160.3 148.7 127.0 94.1 94.1 78.4 94.1 78.4 62.7 77.1 71.1 63.8

61.7 57.2 48.8 36.2 36.2 30.2 36.2 30.2 24.1 29.7 27.4 24.6

98.9 99.0 99.1 99.4 99.6 99.6 99.5 99.6 99.7 99.6 99.7 99.7

1 0.91 0.73 0.49 0.36 0.32 0.47 0.35 0.29 0.33 0.31 0.23

1.49 1.43

1.55 1.49

5.8 5.6

0.23 0.22

73.8 67.2

28.4 25.8

99.6 99.7

0.31 0.25

1.44 1.36

1.50 1.41

5.8 5.6

0.22 0.21

70.5 60.5

27.1 23.3

99.7 99.8

0.28 0.20

CONCLUSION Effluent from a chrome tanning unit has been successfully treated by a combination of NF followed by RO process. The retentate of the NF, rich in chromium, can be recycled.

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However, most of the salt is not retained by the process. The time required to reach steady state decreases with increase in cross flow velocity and applied pressure. The use of turbulent promoters in laminar regime results in substantial increase in flux compared to the laminar case (31-57% for NF and 42.6-44.4% for RO). Effluent quality also increases with increase in pressure and cross flow velocity. Nevertheless, the permeate contains higher than permissible amounts of chromium and higher COD. The treatment of the permeate of the NF process by RO successfully addresses these problems including retaining most of the dissolved salts. The permeate RO has less than 1 ppm of chromium. The values of COD (~92.3 ppm) and BOD (~28.5ppm) in the permeate of RO are well below the discharge limit of the same. It is found that the resistance decreases with increase in cross flow velocity and significantly decreases by the presence of turbulent promote. For NF, the contributions of polarization resistance is 6.1 to 11.6 times to that of membrane resistance in turbulent flow regime, 7.0 to 12.4 times for laminar with turbulent promoter regime and maximum for laminar regime with 10.0 to 20.0 times. Hence, maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime.

REFERENCES [1] [2] [3]

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[4]

[5] [6]

S.S. Datta, An introduction to the principles of leather manufacture, 4th Edition, Indian Leather Technologists’ Association. 1999. J. Gauglhofer, Environmental aspects of tanning with chromium, J. Soc. Leather Technol. Chem., 70 (1) (1986) 11-13. C. Das, P. Patel, S. De and S. DasGupta, Treatment of tanning effluent using nanofiltration followed by reverse osmosis, Sep. Purif. Technol., 50 (2006) 291-299. P.M. Bungay, H.K. Lonsdale, and M.N.de Pinho, Synthetic Membranes: Science, Engineering and Application, published by D. Reidel Publishing Company, (1983) pp. 312. A. Cassano, L.D. Pietra and E. Drioli, Integrated membrane process for the recovery of chromium salts from tannery effluents. Ind. Eng. Chem. Res., 46 (2007) 6825-6830. A. Cassano, R. Molinari, M. Romano and E. Drioli, Treatment of aqueous effluents of the leather industry by membrane processes A review. J. Membr. Sci., 181, (2001) 111126.

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Chapter 9

TREATMENT OF DYEING EFFLUENT ABSTRACT Dyeing process, another posttanning process, improves the aspects of leathers and increases their commercial values. Leathers are died with dyes in aqueous solutions [1]. Effluent generated from dyeing process mainly contains dyestuff residue, organic substances, etc.

9.1. DYEING EFFLUENT

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The characterization of a typical dyeing effluent collected from N.A. Trading, Bantala Leather Complex, Kolkata, India is shown in Table 9.1. Effluent is collected after one to two hours of the completion of the dyeing operation. Table 9.1. Characterization of dyeing effluent pH 5.83

Conductivity (S/m) 0.52

TS (g/l) 29.2

TDS (g/l) 3.43

COD (ppm) 14592

BOD (ppm) 4169

Cl(ppm) 4250

Cr+3 (ppm) 6.0

9.2. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of the appropriate membrane for the filtration is of utmost importance. Selection should primarily be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

9.2.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide

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skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

9.2.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

9.2.3. Performance Testing of Various Membranes

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Effluent contains single dye with concentration 358 mg/l, with very high COD (14592 mg/l) and BOD (4169 mg/l) values. pH of the effluent is 5.83 and conductivity, TS, TDS, Clconcentration are 52.1 S/m, 29.2 g/l, 3.43 g/l, 4250 mg/l, respectively. Since, this effluent is generated after chrome tanning, it contains about 6 mg/l of toxic chromium. As dyeing is almost last step in a tannery, it does not contain huge amount of organic materials. High COD values are due to presence of chemicals and dye. Therefore, this effluent is not subjected to any pretreatment process and directly subjected to membrane filtration. Figure 9.1 presents the flux decline behavior of all the membranes. Flux becomes steady beyond five minutes of operation.

Figure 9.1. Flux decline of pretreated dyeing effluent using various membranes.

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Table 9.2. Permeate properties of dyeing effluent MWCO

COD (mg/l)

TS (g/l)

TDS (g/l)

Dye (mg/l)

Cr3+ (mg/l)

20 kDa

14216 [14592]*

24.6 [29.2]*

3.2 [3.4]*

320 [358]*

5.5 [6.0]*

15 kDa

13520

20.1

3.0

295

5.2

5 kDa

12856

18.6

2.9

281

5

1 kDa

12160

18.0

2.8

256

4.7

400 Da

1657

3.0

2.3

0

0

*

Values indicate the properties corresponding to feed.

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As far as flux decline is concerned, all the membranes show a decline in permeate flux in the range of 13 to 16%. Therefore, a suitable membrane is to be selected based on the quality of the permeate. Permeate quality after various membrane systems is presented in Table 9.2. Percentage COD reduction is 3%, 7%, 12%, 17% and 89% for 20, 15, 5, 1 kDa UF membranes and NF membrane, respectively. Reductions in TS are 16%, 31%, 36%, 38% and 90% for these membranes. Similar results are obtained by Alves at al. [2]. Retention of total dissolved solids is also maximum for NF membrane. Interestingly, NF membrane completely retains dye, whereas UF membranes show upto only 28% of dye retention (for 1 kDa membrane) as evident from Table 9.2. Chromium concentration is also zero in the permeate of NF membrane. Therefore, NF 400 Da MWCO membrane is selected for treatment of dyeing effluent. Although chromium and dye are completely removed by NF membrane, COD and BOD of the treated effluent are beyond the disposable limits. Hence, the NF treated effluent may be subjected to RO. Therefore, 400 Da MWCO NF followed by RO should be the selected combination of membrane separation process for treatment of dyeing effluent.

9.3. DETAILED STUDY OF TREATMENT OF DYEING EFFLUENT The present work aims at development of a viable membrane based separation scheme to treat the leather dyeing effluent. Use of direct RO may result extremely large removal of dye, COD and BOD but at the cost of very small throughput due to high loading of suspended solids and organic materials. Therefore, another membrane process is required prior to RO. UF can not be used as it does not retain dye and COD. Hence, the selected treatment scheme consists of NF followed by RO. Performance of separation is evaluated by characterizing each stream in terms of BOD, COD, TDS, TS, etc. Changes in hydrodynamic conditions in the flow channel are also investigated to enhance the permeate flux. The proposed scheme of the treatment process is presented in Figure 9.2 [3].

9.3.1. Cross Flow Cell and Operating Conditions A rectangular cross-flow cell, made of stainless steel, was designed and fabricated. The cell consisted of two matching flanges as shown in Figure 9.3a. The inner surface of the top flange was mirror polished. The bottom flange was grooved, forming the channels for the

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permeate flow. The channels in the bottom flange with the internal grid structure were shown in Figure 9.3b. A porous stainless steel plate was placed on the lower plate that provided mechanical support to the membrane. Two neoprene rubber gaskets were placed over the membrane; the top view of which was shown in Figure 9.3c.

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Figure 9.2. Proposed scheme for the treatment of dyeing effluent.

Figure 9.3. Membrane module assembly for treatment of dyeing effluent.

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Two flanges were tightened to create a leak proof channel. Equi-spaced, thin U shaped stainless steel wires of diameter 1.68 mm were placed laterally (along the width of the channel) in between the two gaskets, to act as turbulent promoters. The spacing between the turbulent promoters was 15.0 mm. These were shown in Figure 9.3d. The number of such promoters was fifteen for NF and nine for RO. The effective length and width of the membrane surface available were 26.2 cm and 4.6 cm, respectively. The height of the flow channel was determined by the thickness of the gaskets after tightening the two flanges and was found to be 3.4 mm. For the experiments with turbulent promoters, the obstruction in the flow path due to the wires promoted localized turbulence. The operating conditions for all cross flow membrane operations are presented in Table 9.3. Table 9.3. Operating conditions Operating condition Laminar (with and Nanofiltration without promoter) Turbulent Laminar (with and Reverse without promoter) Osmosis Turbulent

Reynolds number 682, 1022 and 1363 4431, 5794 and 7157 682, 1022 and 1363

Pressure (kPa) 828, 966, 1104 and 1242 1518, 1656, 1794 and 1932

4431, 5794 and 7157

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9.3.2. Optical Studies and Image Analysis The fouling (deposition of dye and other organic particles) of the membrane during operation are measured using an optical microscopy based image acquisition and analysis method. The optical measurements are done in reflection mode of incident light. The membrane sample pieces are viewed under the microscope using a high resolution digital camera and a digital display interfaced with the microscope. The captured images are digitized and then analyzed to evaluate the thickness of different layers of the membrane, deposition over the membrane surface. For each such mounted membrane, images are captured at different points and averaged to take into account local fluctuations. To obtain the cross-sectional view of the deposited membrane, a number of pieces are cut from various locations as marked in Figure 9.4a for no-promoter case. The sampling of the membrane after each experiment is done carefully to encompass the whole area of the membrane. For experiments with promoters, the variation in solute deposition in the proximity of the promoter is investigated by taking snaps of points at regular intervals (Figure 9.4b) using a micrometer bench attached to the microscope. The quantifications on the digitized images of the cross-section of the fouled membrane are done using image analyzing software. Guiding lines are drawn to demarcate different layers of membrane and deposition over the membrane. The distances between the lines demarcating the top surface of the deposition and the top of the membrane region are measured to obtain thickness of the deposition. Measurements are taken at several positions and averaged to eliminate local fluctuations [4].

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For experiments with promoter, a specific axial location is selected and the deposition thicknesses are measured at five different locations along the membrane width. Each data point reported in this paper is obtained by measuring the thicknesses at the same specific axial location for four such promoters (i.e. in total 20 measurements for each point reported herein). The consecutive points where such measurements are performed are 1 mm apart from each other in the flow direction around a promoter. Again, as deposition profile varies in the transverse direction the line average of depositions is calculated separately on seven different equi-spaced parallel lines covering the channel width (Figure 9.4b). Finally, the deposition thickness is calculated from these seven values by averaging over the entire membrane width. The standard deviations of the measurements are calculated for each reported data in the paper and are approximately equal to ± 0.7 μm with standard errors less than ± 0.2 μm. A standard grating element having 2000 lines per inch is used to calibrate the magnification of the microscope.

Figure 9.4. Schemes for optical measurements of deposition thickness, (a) for experiments with nopromoter conditions, (b) for experiments using curved shaped promoters.

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The software based image analysis for the fouled membranes is described elsewhere in detail [4]. In case with promoters, the retentate solution encounters the promoters and flows around them through the gap between the promoters and the top and bottom (membrane) surfaces. The extents of the disturbed zones of deposition prior and beyond the promoters are termed as ‘front length’ and ‘tail length’ respectively. It is anticipated that the ‘tail length’ will increase with the cross flow velocity. Increasing the number (decrease in lateral spacing) of promoters should further enhance the flux but at the cost of axial pressure drop across the channel. A representative figure showing the image of a fouled membrane used in RO filtration with promoter condition is shown in Figure 9.5. The variation of deposition around any promoter position in case of laminar flow (Average Re = 742) and 966 kPa of transmembrane pressure over NF membrane is shown in Figure 9.5. The contour of deposition is quite similar in all cases. As expected, the effect of the presence of the promoter is felt more at the downstream side as compared to the upstream side. The deposition progressively decreases as the promoter is approached, being the least at the bottom. It can also be seen from the figure that the deposition thickness after the promoter is quite small compared to the deposition thickness at an equivalent position at the upstream.

Figure 9.5. Dye deposition around a promoter in nanofiltration (laminar flow with average Re equal to 742 at 966 kPa of transmembrane pressure).

As the feed passes along the length of the channel it encounters the promoters placed across its movement. Consequently, there will be a variation of Reynolds number in the promoter region and Re will reach its maximum value just below the promoter. About 8.8% increase is calculated on the length averaged value of Re over the membrane area in all cross flow velocity conditions. The average values are listed in Table 9.5.

9.3.3. Analysis of Transient Flux Decline Nanofiltration The flux decline for transient state in the three flow regimes at a constant pressure of 828 kPa and varying Reynolds number is shown in Figure 9.6. Flux decline at constant Reynolds number (682 for laminar and 4431 for turbulent) and varying pressure is shown in Figure 9.7.

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Figure 9.6. Permeate flux decline profile with time for NF membrane at 828 kPa.

Figure 9.7. Permeate flux decline profile with time for NF membrane at various pressures.

It can be clearly seen from the Figure 9.6 and Table 9.4 that the time required to reach steady state decreases with increase in Reynolds number. It is also observed that the steady state is achieved faster using turbulent promoter compared to laminar flow. For example, it can be observed from Figure 9.6 that the steady state is attained in about 2.17 min, for Re

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equal to 4431 and 828 kPa pressure, whereas at the same pressure but at Re equal to 5794 and Re equal to 7159, the steady states are attained within 2 min and 1.83 min, respectively. The flux decline is about 23% of the initial value for Re equal to 4431, about 21% with increase in Re equal to 5794, and 19 % at Re equal to 7157. Similar trends can be observed for flux decline in laminar regime with and without promoters. As the cross flow velocity increases, the growth of the polarized layer over the membrane surface decreases due to enhanced sweeping motion of the liquid. This leads to the onset of steady state at an earlier time. For the above reason, the resistance to the solvent flux also decreases with the cross flow velocity, resulting in higher permeate flux. Therefore, the flux decline is lower at higher cross flow velocities. Table 9.4. Variation of time required to reach the steady state with different operating conditions Turbulent

Laminar

Time to Re

Pressure

reach

(kPa)

steady

Time to Re

Pressure

reach

(kPa)

steady

state (min) 4431

682

2.0

1022

7157

1.8

1363

4431

2.0

682

1.9

1022

1.8

1363

1.8

682

1.7

1022

7157

1.7

4431

5794

828

966

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7157 4431 5794

5794 7157

1104

1242

Time to Re

Pressure

reach

(kPa)

steady

state (min)

2.2

5794

Laminar with promoter

state (min)

5.2

682

4.9

1022

4.8

1363

4.8

682

4.5

1022

4.5

1363

4.5

682

4.3

1022

1363

4.1

1363

2.7

1.7

682

3.9

682

2.8

1.6

1022

3.9

1022

1.6

1363

3.7

1363

828

966

1104

1242

3.7 828

3.6 3.4

966

3.5 3.2 3.2

1104

1242

3.1 2.9

2.6 2.4

For example, in Figure 9.6, at Re equal to 682 and 828 kPa, the steady state is attained in about 5.17 min without promoter and about 3.67 min with promoter for the same operating conditions. The flux decline is about 36% without promoter at Re equal to 682 and 828 kPa pressure; but only 30% using promoter at the same operating condition. Use of the turbulent promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the polarized layer is controlled quickly, establishing steady state earlier than without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less compared to the no promoter

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case. Flux decline at constant Reynolds number (682 for laminar and 4431 for turbulent) and varying pressure is shown in Figure 9.7.

Reverse Osmosis The permeate from the NF is collected and treated using RO in another cross flow cell in purely laminar, laminar with turbulent promoters and in turbulent conditions at different operating conditions. The flux decline for transient state in the three flow regimes at a constant pressure of 1518 kPa and varying Reynolds number is shown in Figure 9.8. Flux decline at constant Reynolds number (682 for laminar and 4431 for turbulent) and varying pressure is shown in Figure 9.9.

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Figure 9.8. Permeate flux decline profile with time for RO membrane at 1518 kPa.

Figure 9.9. Permeate flux decline profile with time for RO membrane at various pressures.

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9.3.4. Analysis of Steady State Permeate Flux

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Nanofiltration The variations of steady state permeate flux with pressure at different cross flow rates under laminar flow, with turbulent promoters and turbulent regimes are shown in Figure 9.10. The figure shows the usual trend that the permeate flux increases with the operating pressure and cross flow rate. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with cross flow rate is because of decreasing concentration polarization as discussed earlier.

Figure 9.10. Variation of permeate flux with pressure drop in NF.

Reverse Osmosis The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to higher turbulence induced by higher Reynolds number. The effects of transmembrane pressure and Reynolds number on steady state flux in RO are shown in Figure 9.11. It is interesting to note that the values of permeate flux is almost independent of pressure for laminar flow, with and without promoters as well as for the turbulent flow. This indicates that the filtration is gel-layer controlling in this case. Left over organics present in the permeate of NF forms a gel-type layer over the membrane surface that grows in time and finally gets stabilized by forced convection imparted by the cross-flow of the feed. The figure shows that the permeate flux increases with Reynolds number as in NF.

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Figure 9.11. Variation of permeate flux with pressure drop in RO.

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9.3.5. Analysis of Various Resistance Nanofiltration The variation of the polarized layer resistance with Reynolds number for laminar flow regime experiments is presented in Figure 9.12. As observed from Figure 9.12, the polarized s

layer resistance R p decreases with increase in Reynolds number and increases with transmembrane pressure in all the cases. With increase in Reynolds number, the polarized layer resistance decreases due to high shear imposed by the cross flow. With increase in pressure, more solutes are convected towards the membrane surface resulting into an increase in the polarized layer resistance. For transmembrane pressure drop of 828 kPa in laminar flow, the ratio of the polarized and hydraulic resistance reduces from 3.15 to 2.62 with an increase in Reynolds number from 682 to 1363. Significant reductions in polarized layer resistance, compared to laminar flow, are achieved using turbulent promoters. At the same Reynolds number (682) and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the polarized layer resistance to 1.65 compared to 3.15 in laminar flow.

R ps /Rm increases from 2.88 to 3.04 when transmembrane pressure drop increases from 828 kPa to 1242 kPa at a Reynolds number of 1022 in pure laminar regime. For the case with the promoters, the polarized layer resistance decreases significantly (by 48 %) at the pressure of 828 kPa and Reynolds number of 1022. Further reduction in polarized layer resistance is s

observed in case of purely turbulent flow. It is found that R p accounts about 76% of the total resistance in the case of laminar flow with lowest Reynolds number (682) and at a Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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transmembrane pressure of 1242 kPa, whereas at the same pressure in turbulent flow regime,

R ps accounts about 57% of the total resistance.

Figure 9.12. Variation of the ratio of polarized layer resistance and membrane hydraulic resistances at steady state with Reynolds number in laminar regime in NF.

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Variation of the ratio of polarized layer resistance and membrane hydraulic resistances at steady state with Reynolds number in turbulent flow regime is presented in Figure 9.13.

Figure 9.13. Variation of the ratio of polarized layer resistance and membrane hydraulic resistances at steady state with Reynolds number in turbulent flow regime in NF. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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Reverse Osmosis Compared to NF, similar trends of variation of R ps /Rm with the operating conditions are observed. It can be observed from Figures 9.14 and 9.15 that this ratio of resistance varies in the range of 1.33 to 2.04 in the laminar flow regime, whereas, it is in the range of 0.6 to 1.08 in case of laminar flow with turbulent promoters for various operating conditions. For pure turbulent flow, the polarized layer resistance decreases further and the above ratio is in the range of 0.3 to 0.64 for different operating conditions.

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Figure 9.14. Variation of the ratio of polarized layer resistance and membrane hydraulic resistances at steady state with Reynolds number in laminar regime in RO.

Figure 9.15. Variation of the ratio of polarized layer resistance and membrane hydraulic resistances at steady state with Reynolds number in turbulent flow regime in RO.

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9.3.6. Microscopy Based Quantification of Fouling Nanofiltration The microscopic observation confirms that deposition thickness decreases with cross flow velocity and increases with transmembrane pressure (Table 9.5). For example, at 828 kPa and at a Re = 682, the deposition thickness is 13.3 µm and it decreases to 11.6 µm when Re equal to 1363 at the same transmembrane pressure. Decrease in deposition is caused due to the shearing action of the faster moving liquid. It can clearly be observed from Table 1 that deposition thickness increases with transmembrane pressure. When the transmembrane pressure is increased from 828 kPa to 1242 kPa at Re = 682, the deposition thickness increases from 13.3 to 14.6 µm. As pressure increases, the convection of the dye molecules towards the membrane surface increases. For the case with the promoters, the deposited layer thickness decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number 1363 and transmembrane pressure difference 828 kPa, the presence of turbulent promoters reduces the average value of deposition thickness to 8.8 µm compared to 11.6 µm in laminar flow. Table 9.5 also shows further reductions in dye deposition thickness for the case of purely turbulent flows for reasons already discussed. Table 9.5. Average deposition over the membrane surface under different operating conditions for NF 400 (TFC) and RO (TFC) Nanofiltration

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Operating condition Flow

Reynolds

regime

number

Reverse osmosis

Deposition over the membrane

Deposition over the membrane

surface (μm)

surface (μm)

828 kPa

966

1104

1242

1518

1656

1797

1932

kPa

kPa

kPa

kPa

kPa

kPa

kPa

13.6

14.0

14.6

3.6

3.6

3.8

3.9

3.3

3.4

3.6

2.3

2.3

2. 4

682

13.3

1022

12.4

3.1

1363

11.6

2.8

Laminar

742*

10.2

with promoter)

1112*

9.7

2.7

1483*

8.8

2.5

4431

5.9

5794

5.4

1.9

7157

4.6

1.8

Laminar

Turbulent

10.6

6.1

10.8

6.8

11.4

7.3

3.2

2.2

*

Averaged over membrane area.

Figure 9.16 shows depositions at the promoter-inactive zones over TFC NF membrane surface for the case of laminar flow with promoters (Average Re = 742, 1112 and 1483, respectively) at a transmembrane pressure of 828 kPa. As expected, deposition thickness

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decreases with enhanced cross flow velocity. This is due to shearing effect of tangential flow of feed over the membrane surface and is consistent with the basic understanding of the process.

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Figure 9.16. Deposition over TFC NF membrane surface at the promoter-inactive zones. Parametric conditions: Laminar (with promoter), Pressure-828 kPa; Average Reynolds number over membrane area-[A] 742, [B] 1112 and [C] 1483.

Reverse Osmosis Variation of deposition thickness for various operating conditions in case of reverse osmosis is presented in Table 9.5. During NF, a large amount of organic as well as inorganic particles are retained. Therefore, the deposition thicknesses for all the experimental conditions in RO are small compared to NF experiments. The trends are similar to those of NF. However, the variation in deposition with pressure is rather small. For example, at Re = 682 the deposition thickness remains unchanged at 3.6 µm when transmembrane pressure increases from 1518 kPa to 1656 kPa (Table 9.5). Further increase in pressure to 1797 kPa and 1932 kPa, the deposition thickness increase is only 5.5% and 8.3%, respectively. With increase in pressure, the permeate flux will increase, resulting in enhanced convection of the solute molecules towards the membrane surface. However increase in pressure will make the deposited layer more compact [5], thereby reducing its thickness. These two opposite effects probably have caused the less than expected increase in the deposition thickness. However, the deposition is more at higher pressures both for the promoter and the no-promoter cases. The microscopic observation confirms the steady decrease in deposition thickness with increase in cross flow velocity at any constant pressure. At 1518 kPa and at a Re = 682, the dye deposition thickness is 3.6 µm and it decreases to 2.8 µm when Re = 1363 at the same transmembrane pressure. The reasons are already discussed earlier.

9.3.7. Enhancement of Steady State Permeate Flux Nanofiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 9.17. All the increases are calculated taking the laminar flow results under the same operating conditions as the base. The concentration boundary layer over the membrane surface is significantly disturbed in presence of the turbulent promoters. This causes reduction in the membrane surface concentration and thereby increase in the effective driving force and hence an increase in

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permeate flux. It may be observed from Figure 9.17 that the flux increment is in the range of 54 to 57% for laminar flow with promoter.

Figure 9.17. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.

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Reverse Osmosis The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions vary from 45% to 46.5% for all operating conditions (Figure 9.18).

Figure 9.18. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in RO.

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9.3.8. Permeate Quality Analysis Nanofiltration Table 9.6 represents the variations of permeate COD with transmembrane pressure in turbulent, laminar and with turbulent promoter cases. It is observed that with increase in transmembrane pressure difference and Reynolds number, the permeate quality improves. With increase in pressure, the solvent flux increases linearly, while the solute flux is nearly independent of pressure for less open membranes (RO and in some cases for NF membranes) [6]. This indicates that with increasing pressure, more solvent passes through the membrane along with a fixed amount of the solute; the permeate becomes purer and hence the permeate quality (expressed as COD) increases. Similar trends are observed for laminar flow with promoter and turbulent flow. It can be seen from Table 9.6 that at 828 kPa pressure and Re equal to 1363, COD decreases by about 11% in presence of promoter compared to the base case (laminar at same operating conditions). Percentage decrease in COD is found to be about 14% at 966 kPa pressure and Re equal to 1363 and also about 26% at 1104 kPa pressure and Re equal to 1363. At Re equal to 4431, as the transmembrane pressure difference increases from 828 kPa to 1242 kPa, COD decreases by 33%. From the figure it may also be observed that COD in the permeate varies from about 1676 to 528 ppm in the pressure range of 828 to 1242 kPa. Table 9.6. Variation of COD in ppm with transmembrane pressure drop in NF Operating condition

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Flow regime

Laminar

Laminar with promoter)

Turbulent

COD (ppm) 966

1104

1242

kPa

kPa

kPa

1676

1490

1366

1180

1022

1490

1335

1226

1071

1363

1273

1133

1055

962

742*

1366

1211

1040

854

1112*

1273

1071

885

683

1483*

1133

978

776

621

4431

1288

1211

1055

869

5794

1148

1040

916

698

7157

962

900

760

528

Reynolds number

828 kPa

682

*

Averaged over membrane area.

Table 9.7 shows other properties for various operating conditions in NF. Permeate suspended solid (TS-TDS) is 1.76 g/l which is substantially lower compared to feed suspended solid of 25.77 g/l. It indicates that almost all the suspended solids are retained by the NF membrane. On the other hand, the concentration of total dissolved solids is not reduced.

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Table 9.7. Permeate quality after nanofiltration Sr. Reynolds Pressure No. number (kPa) Turbulent regime 1 4431 828 2 5794 828 3 7157 828 4 4431 966 5 5794 966 6 7157 966 7 4431 1104 8 5794 1104 9 7157 1104 10 4431 1242 11 5794 1242 12 7157 1242 Laminar regime 1 682 828 2 1022 828 3 1363 828 4 682 966 5 1022 966 6 1363 966 7 682 1104 8 1022 1104 9 1363 1104 10 682 1242 11 1022 1242 12 1363 1242 With turbulent promoter 1 742* 828 2 1112* 828 3 1483* 828 4 742* 966 5 1112* 966 6 1483* 966 7 742* 1104 8 1112* 1104 9 1483* 1104 10 742* 1242 11 1112* 1242 12 1483* 1242

TDS (ppm)

TS (ppm)

pH

Conductivity (S/m)

Cl(ppm)

BOD (ppm)

2870 2820 2720 2620 2640 2530 2590 2530 2460 2490 2470 2360

3600 3800 4100 4000 3900 3700 4200 3800 4200 4000 3800 3900

6.83 6.85 6.80 6.88 6.82 6.80 6.80 6.84 6.85 6.82 6.88 6.85

0.44 0.43 0.42 0.40 0.40 0.38 0.39 0.39 0.37 0.38 0.38 0.36

3730 3742 3734 3810 3751 3745 3787 3720 3758 3760 3751 3746

372 328 276 350 298 256 300 264 221 251 202 153

2170 2150 2210 2320 2040 1720 2100 1980 1840 2050 1980 1740

4000 3800 3600 4200 4000 3800 4200 3600 3400 4000 3400 3200

6.89 6.85 6.82 7.00 6.84 6.85 6.87 6.82 6.84 7.00 6.85 6.84

0.33 0.33 0.34 0.35 0.31 0.26 0.32 0.30 0.28 0.31 0.30 0.27

3720 3756 3755 3749 3764 3781 3760 3801 3742 3781 3789 3724

479 426 364 421 381 324 390 350 302 337 306 275

2260 2140 2100 2190 2240 2180 2250 2200 2190 1960 1950 1860

4000 3500 4100 4100 4200 4800 4000 3700 4100 4200 3900 3800

6.76 6.72 6.84 6.80 6.84 6.85 6.90 6.91 6.86 6.89 7.00 6.82

0.33 0.33 0.34 0.35 0.31 0.26 0.32 0.30 0.28 0.31 0.30 0.27

3720 3756 3755 3749 3764 3781 3760 3801 3742 3781 3789 3724

401 361 320 340 306 282 298 253 220 242 201 178

*

Averaged over membrane area.

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Reverse Osmosis The effects of transmembrane pressure and cross flow velocity on permeate quality in terms of COD in RO, are shown in Figure 9.19. The figure illustrates that the variation of COD with Reynolds number and pressure is significant. COD decreases with cross flow velocity and pressure. It is shown that COD of the permeate varies from 80 to 0 ppm for all the hydrodynamic conditions studied herein. The permeate qualities in terms of other properties for various operating conditions are presented in Table 9.8. It may be observed from Table 9.8 that BOD in the permeate varies from about 23 to 0 ppm in the pressure range of 1518 to 1932 kPa which is substantially lower than the permissible limit (30 ppm). From Table 9.8, it may also be observed that pH of the permeate is around 7.6 which is also around neutral pH.

Figure 9.19. Variation of COD with pressure drop in RO.

Table 9.8. Permeate quality after reverse osmosis Sr. Reynolds No. number Turbulent regime 1 4431 2 5794 3 7157 4 4431 5 5794 6 7157 7 4431

Pressure (kPa)

TDS (ppm)

TS (ppm)

pH

Conductivity (S/m)

Cl(ppm)

BOD (ppm)

1518 1518 1518 1656 1656 1656 1794

188 190 185 186 180 182 210

300 280 290 280 240 240 380

7.8 7.6 7.5 7.6 8.0 7.9 7.8

0.03 0.03 0.03 0.03 0.03 0.03 0.03

176 173 162 170 167 168 167

18 14 9 14 14 9 9

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Table 9.8. (Continued) Sr. Reynolds Pressure No. number (kPa) 8 5794 1794 9 7157 1794 10 4431 1932 11 5794 1932 12 7157 1932 Laminar regime 1 682 1518 2 1022 1518 3 1363 1518 4 682 1656 5 1022 1656 6 1363 1656 7 682 1794 8 1022 1794 9 1363 1794 10 682 1932 11 1022 1932 12 1363 1932 With turbulent promoter 1 742* 1518 2 1112* 1518 3 1483* 1518 4 742* 1656 5 1112* 1656 6 1483* 1656 7 742* 1794 8 1112* 1794 9 1483* 1794 10 742* 1932 11 1112* 1932 12 1483* 1932

7.7 7.2 7.5 7.7 7.4

Conductivity (S/m) 0.03 0.03 0.03 0.03 0.03

Cl(ppm) 160 156 168 163 152

BOD (ppm) 9 5 5 0 0

290 300 250 280 280 300 280 260 280 260 310 280

7.6 7.5 7.2 7.8 7.7 7.5 7.4 7.6 7.6 7.4 7.5 7.5

0.03 0.03 0.03 0.04 0.04 0.04 0.05 0.05 0.05 0.04 0.04 0.05

168 171 159 166 155 167 171 168 162 170 156 155

23 23 18 18 14 14 14 9 5 5 0 0

280 350 290 300 240 270 260 270 280 300 310 260

7.6 7.4 7.4 7.7 7.8 7.8 7.6 7.9 7.5 7.5 7.6 7.5

0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.05 0.05 0.05

164 167 168 170 163 155 165 150 164 173 155 157

19 14 9 14 9 5 9 5 0 0 0 0

TDS (ppm) 200 180 191 194 185

TS (ppm) 310 290 320 290 270

190 180 180 190 170 180 160 180 170 160 170 180 160 190 180 170 160 170 180 170 190 170 170 180

pH

*

Averaged over membrane area.

CONCLUSION Effluent from dyeing unit has been successfully treated using an integrated membrane separation processes consisting of NF followed by RO. The time required to reach steady state decreases with increase in Reynolds number and applied pressure. Polarization resistance is the major contributor to overall resistance, both in NF and RO, to the solvent flow for laminar and laminar with promoter case. With increasing pressure, polarization resistance increases and with increasing flow rate it decreases. For NF, the contributions of

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polarization resistance is 0.8 to 1.3 times to that of membrane resistance in turbulent flow regime, 1.4 to 1.7 times for laminar with turbulent promoter regime and maximum for laminar regime with 2.6 to 3.2 times. Also maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime. The use of turbulent promoters in laminar regime results in substantial increase in flux (54-57% for NF and 45-47% for RO) compared to the laminar case. NF process retains 100% dye and almost 93% suspended solids. COD (~32 ppm) and BOD (~9 ppm) values in the permeate of RO are well below the discharge limit of the same in India. The permeate of RO can be used as wash water and retentate can be used for new dyeing bath. Image analyzing video microscopy is successfully used to accurately quantify the deposition of dye and organic molecules on the membrane surface. Both for NF and RO, the deposition thickness decreases with cross flow velocity and increases with transmembrane pressure. The increases in deposition thickness are less pronounced with increase in pressure. The effects of introduction of promoters are studied in detail. The shape of the deposition profiles (including the front and the tail regions around a promoter) and the experimentally measured values of the permeate flux are found to be consistent with the basic understanding of the process. Significant decreases in the deposition are measured as a result of the incorporation of the promoters and these are further corroborated by substantial increases in permeate flux. At 966 kPa pressure and at a Reynolds number of 1022, 57.2% flux enhancement is observed with curved promoters compared to the no promoter condition in NF.

REFERENCES [1]

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[2] [3] [4] [5] [6]

S.S. Datta, An introduction to the principles of leather manufacture, 4th Edition, Indian Leather Technologists’ Association. 1999. A.M.B. Alves, M.N.de Pinho, Ultrafiltration for colour removal of tannery dyeing wastewater, Desalination 130 (2000) 147-154. C. Das, S. DasGupta and S. De, Treatment of dyeing effluent from tannery using membrane separation processes, Int. J. Environ. Waste Manage., (2009). S. Pal, A. Ghosh, T.B. Ghosh, S. De and S. DasGupta, Optical quantification of fouling during nanofiltration of dyes, Sep. Purif. Technol. 52 (2006) 372-379. S. Sahin and L Bayindirli, The effect of depectinization and clarification on the filtration of sour cherry juice, J. Food Eng., 19 (1993) 237-245. P. M. Bungay, H. K. Lonsdale, and M. N. de Pinho, Synthetic Membranes: Science, Engineering and Application, published by D. Reidel Publishing Company, (1983) pp. 312.

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Chapter 10

TREATMENT OF FATLIQUORING EFFLUENT

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ABSTRACT During beam house operations, natural fats are cleaned from skins and hides to improve tanning by preventing undesired reactions and increasing reaction sites for tanning. This removal of natural oils makes the leather hard and horny material having no value as leather as fibers cohered together. The objective of fatliquoring is to make the leather soft by adding oil layer into the fibrils by influencing the degree of fiber cohesion [1-3]. It is utmost significant step for producing leather for garments and upholstery (furniture and automotives). Fatliquoring process controls the differential shrinkage of grain of the leather during drying, thus playing an important role in controlling the degree of tightness of the leather. Fatliquoring operation also persuades water proofness, tensile strength, wetting properties, extensibility and water and air permeabilities [4]. In addition, it also controls the handle, drape, flexibility, durability, stretch, and water resistance of leather, and also increases its strength. Fatliquors are useful in aqueous emulsions. The oil content in soft leather is very small, being about 10 to 15% on dry weight basis [5]. Fatliquoring is usually carried out in a drum at about 60 to 650C for full chrome tanned leather and 450C for vegetable tanned leather.

10.1. EFFLUENT CONTENT Table 10.1 shows the characterization of a typical fatliquoring effluent collected from M/s, N.A. Trading, Bantala Leather Complex, Kolkata, India. The effluent is collected after one to two hours of the completion of the fatliquoring operation.

10.2. PRETREATMENT Fatliquoring effluent has a COD of 13,700 ppm and TS of 21.5 g/l. As a result of which, pretreatment is necessary prior to membrane separation operation. After bringing the effluent from the plant, it is subjected to a two-step pretreatment process, namely, ferrous sulfate, followed by calcium oxide. Using the same procedure, as discussed in earlier chapters, 0.3% (wt/vol) ferrous sulfate and 1.5 g/l calcium oxide are found to be the optimum concentration

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for coagulation. The supernatant liquor is then prefiltered through a fine cloth and is subjected to membrane filtration. Various properties of the pretreated effluent are tabulated in Table 10.1. Table 10.1. Characterization of fatliquoring effluent Effluent

Fatliquoring

4.15

Conductivity (S/m) 1.18

TS (g/l) 21.5

TDS (g/l) 7.8

COD (ppm) 13,700

BOD (ppm) 3045

3.8

1.06

18.4

7.1

4900

1300

8.0

0.9

17.4

4.1

4255

951

pH Feed After FeSO4 dose After CaO dose

10.3. SELECTION OF MEMBRANE SEPARATION PROCESSES Selection of the appropriate membrane for the filtration is of utmost importance. Selection should primarily be based on maximum permeate flux (productivity of the system) with desired quality of the permeate.

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10.3.1. Membranes As discussed in section 3.3.1, same membranes of five molecular cut-off, namely, 1, 5, 10, 15 and 20 kDa are used, for UF. NF membrane of MWCO 400, consisting of a polyamide skin over a polysulfone support is supplied by M/s, Genesis Membrane Sepratech Pvt. Ltd., Mumbai, India. The permeabilities of these membranes are listed in Table 3.2.

10.3.2. Membrane Experiments Stirred Cell and Operating Conditions The stirred cell used for the experiments and the corresponding operating conditions for UF and NF are same as presented in section 3.3.2.

10.3.3. Performance Testing of Various Membranes Pretreated fatliquoring effluent is subjected to various UF (from 20 kDa to 1 kDa MWCO) membranes and 400 Da MWCO NF membrane in a stirred cell. Transient flux decline behavior using various membranes is shown in Figure 10.1. In Figure 10.1, operating pressure for all the UF runs is 414 kPa and that for NF is 828 kPa. General trend for permeate flux profiles in Figure 10.1 is that the flux decreases with the operating time. This is due to concentration polarization.

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Figure 10.1. Flux decline of pretreated fatliquoring effluent using various membranes.

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Table 10.2. Permeate properties of fatliquoring effluent MWCO

COD (mg/l)

TS (g/l)

TDS (g/l)

20 kDa 10 kDa 5 kDa 1 kDa 400 Da

4,100 [4,255]* 4,052 3,884 3,838 2,716

21.1 [21.5]* 20.08 19.15 18.44 10.5

7.8 [7.8]* 7.1 6.4 5.8 4.5

*

Values indicate the properties corresponding to feed.

Combined effects of polarized layer formation and clogging of some membrane pores lead to a decline in flux. It is observed that about 18.5% decline in flux occurs for 20 kDa MWCO membrane over the 14 minutes of operating time. This is about 19.5%, 18.9% and 18.9% for 10 kDa, 5 kDa and 1 kDa cut-off membranes. Interestingly, flux decline over the duration of the experiment for NF membrane is only about 14.2%. It may be pointed out that although the absolute flux increases from 1 kDa to 20 kDa cut-off membranes, the flux decreases steadily during the filtration time. On the other hand, for NF membrane, flux becomes almost constant beyond 12 minutes of operation. Since, the retentate and permeate streams are recycled to the feed chamber, both the feed volume and feed concentration remain almost unchanged and therefore, the value of steady state permeate flux remains same beyond 12 minutes as evident from Figure 10.1. This indicates that the UF membranes having larger pore size (in the increasing order of 1 kDa to 20 kDa), they are more susceptible to pore

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clogging by the solute particles, resulting in steady decline in flux, although flux decline becomes gradual later on. For NF membrane, larger solute particles cannot enter the pores at all, leading to formation of a polarized layer over the membrane surface whose thickness remains constant by external stirring and hence almost a steady state flux is resulted beyond 8 minutes and flux decline is also minimum in this case. Suitability of a membrane separation process depends not only on its permeate flux, i.e., productivity but also on the permeate quality. It is observed from Table 10.2 that reduction in COD is only 3.6% in 20 kDa, 4.8% for 10 kDa, 8.7% for 5 kDa, 9.8% for 1 kDa and 36.2% for NF membrane. Total solid concentration in all UF membranes is almost insignificant (about 19.7 g/l TS concentration with respect to 21.5 g/l in the alum treated feed). But NF membrane shows about 51.2% retention of total solids. As expected, the TDS retention (i.e. retention of inorganic solutes) by UF membrane is marginal, whereas 400 Da NF membrane shows about 42.3% retention of inorganic solutes in terms of TDS. Therefore, as far as the quality of the permeate is concerned, NF membrane shows the most promising performance. It may be noted here that the permeate quality after NF is still not adequate to discharge in the sewage (discharge limit for COD is 250 mg/l and for BOD is 30 mg/l). For this, the permeate from nanofiltration may be subjected to RO. Therefore, 400 Da MWCO NF membrane followed by RO should be the selected membrane process for the treatment of pretreated fatliquoring effluent.

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10.4. DETAILED STUDY OF TREATMENT OF FATLIQUORING EFFLUENT In this chapter, a scheme is proposed to treat the fatliquoring effluent using a hybrid process, alum coagulation and nanofiltration. The optimum alum dose is identified. The fertilizer value of the sludge produced is tested. The supernatant liquor is subjected to continuous cross flow ultrafiltration. Effects of operating pressure and change in hydrodynamics (laminar, laminar with turbulent promoter and turbulent flow regime) on the permeate flux are observed. The treatment performance is finally evaluated in terms of various properties like BOD, COD, TS, conductivity, etc. The proposed scheme of the treatment process is presented in Figure 10.2.

Figure 10.2. Proposed scheme for the treatment of fatliquoring effluent.

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10.4.1. Cross Flow Cell and Operating Conditions The same cross flow cell described in section 3.4.1 is used for fatliquoring effluent also. The operating conditions for all membrane experiments are presented in Table 10.3. Both in laminar and turbulent flow regime, nanofiltration experiments are conducted at four different operating pressures of 828, 966, 1104 and 1242 kPa. Reverse osmosis experiments are conducted at four different pressures of 1311, 1449, 1587 and 1725 kPa. Table 10.3. Operating conditions for cross flow experiments Reynolds number Step

Transmembrane pressure (kPa)

Laminar and with promoter

Turbulent

NF

828, 966, 1104 and 1242

606, 909, 1212 and 1515

4242, 4848, 5454 and 6060

RO

1311, 1449, 1587 and 1725

606, 909, 1212 and 1515

4242, 4848, 5454 and 6060

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10.4.2. Analysis of Transient Flux Decline Nanofiltration Figure 10.3 represents the flux decline behavior of the effluent at Reynolds number equal to 6060 in nanofiltration. It can be clearly seen from the figure that the time required to reach steady state decreases with increase in transmembrane pressure. For example, it can be observed from Figure 10.3 that the steady state is attained in about 8.8 min, at Re=6060 and 828 kPa pressure, whereas at the same Re but at 1242 kPa pressure, the steady state is attained within 5.1 min. It is also observed that the steady state is achieved faster using turbulence promoter compared to laminar flow. For example, in Figure 10.4, at Re=1515 and 828 kPa, the steady state is attained in about 15.2 min without promoter and about 11.2 min with promoter at the same operating condition. The flux decline is about 15.2 % without promoter at Re=1515 and 828 kPa pressure; but only 8.7 % using promoter at the same operating condition. Use of the turbulence promoters creates local turbulence, thus reducing the concentration polarization at the membrane surface and the growth of the concentration boundary layer is checked quickly, establishing steady state earlier than the case of without promoter. Since the concentration polarization is reduced due to the presence of the promoters, the flux decline is also less than the no promoter case. Reverse Osmosis The flux versus time data of experimental values are plotted in Figure 10.5 for turbulent flow regime in RO. It may also be observed from Figure 10.5 that at lower operating pressure in turbulent regime, flux decline is marginal; whereas at higher operating pressure, it is significant due to concentration polarization effects. For example, flux decline is about 11.1% at 1725 kPa and Re=5454.

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Figure 10.3. Permeate flux decline profile with time of turbulent flow regime in NF.

Figure 10.4. Permeate flux decline profile with time of laminar flow regime in NF.

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Figure 10.5. Permeate flux decline profile with time for turbulent flow regime in RO membrane.

Figure 10.6. Permeate flux decline profile with time for laminar and with promoter flow regime in RO membrane.

Figure 10.6 represents the transient flux decline profile for laminar, laminar with promoter regimes. The results clearly show that as in the case of NF, the time required to reach steady state decreases with increase in cross flow velocity and applied pressure or in presence of turbulence promoters. Extent of flux decline also follows similar trends for

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reasons already discussed in the section describing NF operations. It can be observed that the flux decline is about 13% of the initial value at Re=1212 and 1449 kPa pressure and is 10.7% at the same operating conditions but with promoters. The system reaches steady state in about 5.3 min with promoter compared to about 6.7 min without promoter in laminar regime.

10.4.3. Analysis of Steady State Flux Nanofiltration Figures 10.7 and 10.8 represent the variations of steady state permeate flux with pressure at different Reynolds number under turbulent and laminar flow condition (with and without promoter), respectively, in NF. The figures show the usual trend that the permeate flux increases with operating pressure and Reynolds number. Higher flux at higher pressure is due to enhanced driving force. The increase in flux with Reynolds number is because of decreasing concentration polarization as discussed earlier.

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Reverse Osmosis The variation of steady state permeate flux with transmembrane pressure for all the operating conditions are presented in Figure 10.9. The values of flux obtained in the turbulent regime are significantly higher than that of laminar and laminar with turbulent promoters due to the higher operating pressure (driving force) and turbulence present near the membrane surface. The figure shows that the permeate flux increases with operating pressure and Reynolds number as in NF. At Reynolds number is equal to 4242, an increase in transmembrane pressure from 1311 to 1725 kPa results in about 26.5% increase in permeate flux.

Figure 10.7. Variation of permeate flux with transmembrane pressure in NF for fatliquoring effluent for turbulent flow regime. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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Figure 10.8. Variation of permeate flux with transmembrane pressure in NF for laminar and with promoter regime.

Figure 10.9. Variation of permeate flux with transmembrane pressure in RO.

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10.4.3. Analysis of Various Resistances The permeate flux at any point of time is expressed as presented by Eq. (3.1). The steady state polarized layer resistance is given by Eq. (3.2). Figure 10.10 represents the variation of dimensionless steady state polarized layer resistance with Reynolds number for laminar regime in NF. It is observed from the figure that the steady state values of Rp decreases with Reynolds number as expected. For example, for a transmembrane pressure of 828 kPa in laminar flow, the ratio of the polarized layer and hydraulic resistance reduces from 1.5 to 1.0 with an increase in Reynolds number from 606 to 1515. Rp values increase with the transmembrane pressure. With increase in pressure, more solutes are convected towards the membrane and this enhances the concentration polarization, resulting in increase in Rp values. For the case with the promoters, the polarized layer resistance decreases significantly due to the enhanced forced convection near the membrane surface induced by the promoters. At the same Reynolds number (606) and transmembrane pressure (828 kPa), the presence of turbulent promoters reduces the resistance to 1.0 compared to 1.5 in laminar flow. This reduction in Rps is more than 33.1% leading to a significant enhancement of the permeate flux. The value of Rps reduces further in case of purely turbulent flows for reasons already discussed. For turbulent flow regime, effects of Reynolds number are really profound and polarized layer resistance becomes comparable to the membrane hydraulic resistance. For the range of Reynolds number studied herein,

Rps varies between 0.52 to 0.97 (Table 10.4) times

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of Rm. At a transmembrane pressure of 1242 kPa and Reynolds number =4242, Rp contributes about 49.3% of total resistance.

Figure 10.10. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number during NF. Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,

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Table 10.4. Variation of ratio of polarized layer resistance to hydraulic resistance in turbulent flow regime

R ps /Rm Transmembrane pressure (kPa)

Re=4242

Re=4848

Re=5454

Re=6060

828

0.67

0.63

0.59

0.52

966

0.81

0.75

0.69

0.58

1104

0.93

0.86

0.81

0.74

1242

0.97

0.93

0.87

0.81

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Figure 10.11 represents the variation of dimensionless steady state polarized layer resistance with transmembrane pressure, for all the hydrodynamic conditions in RO. The steady state values of Rp increase marginally with the transmembrane pressure and decrease significantly with increase in Reynolds number as discussed earlier.

Figure 10.11. Variation of the ratio of polarized layer and hydraulic resistances at steady state with Reynolds number in UF.

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10.4.4. Enhancement of Steady State Permeate Flux

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Nanofiltration The percentage enhancements of the permeate flux in laminar regime with turbulent promoters for all the operating conditions are presented in Figure 10.12. All the increases are calculated taking the laminar flow results under same operating conditions as the basis.

Figure 10.12. Flux enhancement with cross flow velocity and pressure in laminar regime with promoter in NF.

The formation of polarized layer over the membrane surface is significantly reduced in presence of the turbulent promoters. This causes a corresponding increase in permeate flux. It may be observed from Figure 10.12 that the flux increment is in the range of 16 to 25% for laminar flow with promoter. However, it can be clearly seen from Figure 10.12 that the permeate flux in turbulent flow at a specific pressure is considerably higher than either the laminar or the turbulent promoter enhanced cases.

Reverse Osmosis The percentage flux enhancements for all hydrodynamic conditions are 32 to 48.6%.

10.4.5. Permeate Quality Analysis For different operating conditions, the permeate quality after NF, is shown in Table 10.5. It can be observed from Table 10.5 that COD values of the permeate are in the range of 2400 to 2750 ppm, which is considerably higher than the discharge limit (250 ppm) in India. After

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RO, the COD of the permeate varies from about 125 to 210 which is lower than permissible limit. pH of the permeate varies from 7.5 to 7.9, which is also within the discharge limit (5.5 to 9.5). Table 10.5. Permeate quality analysis after NF Properties

pH

Permeate from NF Permeate from RO

7.7-7.9 7.5-7.9

Conductivity (S/m) 0.53-0.66 0.17-0.20

TDS (g/l) 3.5-4.3 1.2-1.8

COD (ppm) 2400-2750 125-210

TS (g/l) 9.0-11.0 1.0-1.4

10.4.6. Sludge Characterization The dried and pulverized sludge is analyzed for its fertilizer value and compared with vermi compost. The results are presented in Table 10.6. It is observed from Table 10.6 that the properties of the sludge are close to those of vermi compost. Therefore, the sludge produced (1.43 kg from 40 liters of effluent) can be used as a good fertilizer.

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Table 10.6. Fertilizer quality of sludge obtained from fatliquoring effluent Sample

pH 6.8

Organic Carbon (wt %) 12.1

Nitrogen (wt %) 5.5

Phosphorous (wt %) 0.35

Potassium (wt %) 0.13

Sludge from Fatliquoring Vermi-compost

7.1-7.8

9.97-10.62

1.80

0.90

0.40

CONCLUSION The viability of fatliquoring unit effluent treatment using a combined process of coagulation by alum and membrane separation is established in this study. The time required to reach steady state decreases with increase in Reynolds number and applied pressure. The use of turbulent promoters in laminar regime results in substantial increase in flux (16-25% for NF and 32-48.6% for RO) compared to the laminar case. The treatment of the permeate of the NF process by RO successfully retains most of the dissolved salts. Both in NF and RO, polarization resistance is the major contributor to overall resistance to the solvent flow. The results are in corroboration to the theory that with increasing pressure, polarized layer resistance increases and with increasing flow rate it decreases. For NF, the contributions of polarization resistance is 0.5 to 1.0 times to that of membrane resistance in turbulent flow regime, 0.8 to 1.4 times for laminar with turbulent promoter regime and maximum for laminar regime with 1.0 to 1.8 times. Hence, maximum polarization resistance is observed in laminar flow regime followed by laminar with promoter and the least in case of turbulent regime. The values of COD (125-210 ppm) in the permeate of RO are well below the

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Sirshendu De, Chandan Das and Sunando DasGupta

discharge limit of the same. Pulverized sludge obtained after sun drying can be used as organic fertilizer.

REFERENCES [1]

[2]

[3] [4]

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[5]

A. Cassano, A. Criscuoli, E. Drioli and R. Molinari, Clean operations in the tanning industry: aqueous fatliquoring coupled to ultrafiltration: experimental and theoretical analysis, Clean Product Process, 1 (4) (1999) 257-263. B. Cortese and E. Drioli, Esperinze di ultrafiltrazion su liquami provenienti dallo sgrassaggio delle pelli in un’industria conciaria, Cuiro Pelli, Mat. Concianti, 5 (2) (1978) 167. S.S. Dutta, An Introduction to the Principles of Leather Manufacture; Indian Leather Technologists’ Association (4th Edition), Kolkata, India (1999). L. M. Santos, M. Gutterres, Reusing of a hide waste for leather fatliquoring, Journal of Cleaner Production 15 (2007) 12-16. P.L. Kronick, Leather processing, Bailey’s industrial oil and fat products, In: Industrial and consumer non-edible products from oils and fats. 5th ed., vol. 5. New York: John Wiley and Sons; 1996. p. 309-316 [Chapter 8].

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INDEX

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A acid, 2, 3, 7, 8, 9, 10, 12, 13, 25, 61 acidic, 2, 7 acidification, 79 acidity, 2, 3, 79 activated carbon, 16 adsorption, 11 aerobic, 7, 9 ageing, 3 agent, 8, 9, 79 agents, 2, 9, 61 aggregates, 9 air, 137 alkaline, 7, 8, 14, 61 alternative, 7 aluminium, 8 aluminum, 7, 8, 9, 10 amide, 24 amines, 2 ammonium, 8, 61 amylase, 7 anaerobic, 7, 9 application, vii, 10, 11, 12, 19 aqueous solution, 16, 115 aqueous solutions, 16, 115 arsenic, 9 atmosphere, 11 averaging, 120 awareness, 3

B bacteria, 10 bacterial, 101 base case, 38, 58, 77, 111, 132 baths, 12, 17 behavior, 25, 84, 92, 102, 105, 106, 116, 138, 141

bioaccumulation, 16 biodegradable, 9 biodegradation, 7 biomass, 10, 16 bioreactor, 13, 18 biosorption, 16 biotechnology, vii bypass, 28

C calcium, 3, 61, 79, 137 capital cost, 10 carbon, 9, 10, 13, 16, 40, 58, 77, 89, 100, 149 carbon dioxide, 9 carboxylic, 8, 101 carboxylic groups, 101 castor oil, 3 catechol, 3 cathode, 10 cattle, 3, 12 cell, 24, 25, 28, 29, 31, 46, 48, 50, 64, 66, 68, 82, 84, 92, 94, 102, 103, 116, 117, 124, 138, 141 channels, 28, 117 chelating agents, 9 chemical oxidation, 8 chemicals, vii, 3, 11, 12, 13, 16, 18, 59, 116 chloride, 3, 7, 8, 9, 11, 12, 40 chromium, 3, 7, 8, 9, 12, 13, 14, 15, 16, 17, 18, 59, 101, 104, 111, 112, 113, 114, 116, 117 Chromium, 8, 14, 17, 117 classification, 3 clay, 10, 16 cleaning, 8, 10, 26, 92 coagulation, 7, 8, 9, 10, 15, 21, 22, 23, 27, 40, 43, 45, 47, 58, 61, 63, 65, 78, 79, 81, 83, 89, 91, 94, 100, 138, 140, 149 coffee, 16 cohesion, 137

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Index

collagen, 2, 15, 101 combined effect, 11 components, 11 compost, 40, 58, 77, 89, 100, 149 concentration, 9, 10, 11, 12, 13, 16, 17, 23, 25, 28, 31, 32, 34, 44, 45, 46, 49, 51, 53, 56, 62, 63, 68, 69, 71, 76, 80, 81, 82, 85, 87, 92, 93, 96, 97, 102, 103, 104, 105, 106, 107, 111, 113, 116, 117, 123, 125, 130, 132, 137, 138, 139, 141, 144, 146 conductivity, 3, 12, 22, 23, 27, 37, 40, 44, 45, 47, 56, 57, 61, 65, 75, 76, 80, 81, 83, 88, 94, 111, 113, 116, 140 conservation, 3 consumption, vii, 2 contaminant, 18 control, 24, 25, 91 convection, 30, 34, 49, 53, 56, 66, 68, 76, 87, 95, 97, 110, 125, 129, 130, 146 copper, 15 costs, 10 covering, 120 cross-sectional, 119 curing, 2, 21 curing process, 2 cyanide, 10

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D dairy, vii dairy industry, vii decay, 101 decomposition, 2, 21 degradation, 7, 8 delivery, 28 deposition, 71, 75, 119, 120, 121, 129, 130, 136 derivatives, 9 dialysis, 15 distillation, 10 distilled water, 25 dosage, 13 dosing, 23, 61, 63 drying, 2, 59, 62, 89, 100, 137, 150 durability, 137 duration, 24, 25, 26, 64, 92, 139 dust, 8 dyeing, 1, 2, 3, 8, 12, 13, 15, 18, 115, 116, 117, 118, 135, 136 dyes, 14, 115, 136

E economics, 40, 58, 77, 113

effluent, vii, 2, 3, 5, 7, 8, 10, 11, 12, 13, 14, 15, 17, 18, 21, 23, 25, 26, 27, 28, 40, 41, 43, 45, 46, 47, 48, 52, 58, 59, 61, 63, 64, 65, 66, 70, 77, 78, 79, 81, 82, 83, 84, 86, 89, 91, 92, 93, 94, 97, 100, 101, 102, 103, 104, 105, 114, 115, 116, 117, 118, 136, 137, 138, 139, 140, 141, 144, 149 effluents, vii, 3, 5, 9, 10, 16, 17, 18, 114 electricity, 10 electrodes, 10, 15 electroflotation, 8 electromotive force, 10 electroplating, 9, 15 emulsification, 3 emulsions, 137 energy, vii, 11 energy consumption, vii enzymatic, 7, 61 enzymes, 2, 61 equilibrium, 10, 11 experimental condition, 130 extraction, 3, 8, 10 extraction process, 10

F failure, 12 fat, 2, 3, 12, 21, 91, 150 fats, 2, 3, 137, 150 fatty acid, 3, 12 fatty acids, 3 fertilizer, 40, 47, 58, 59, 65, 77, 78, 83, 89, 94, 100, 140, 149, 150 fiber, 7, 137 fibers, 2, 137 fibrils, 137 film, 3, 10, 24 filtration, vii, 7, 9, 11, 13, 18, 21, 23, 24, 25, 45, 63, 64, 81, 92, 101, 102, 115, 116, 121, 125, 136, 138, 139 flexibility, 137 floating, 10 flocculation, 8, 9, 15 flotation, 15 flow, 27, 29, 30, 31, 32, 34, 35, 36, 37, 38, 39, 40, 47, 48, 49, 50, 51, 53, 54, 55, 56, 57, 58, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 83, 84, 85, 86, 87, 88, 89, 94, 95, 96, 97, 98, 99, 100, 103, 104, 105, 106, 107, 109, 110, 111, 113, 114, 117, 119, 120, 121, 122, 124, 125, 126, 127, 128, 129, 130, 131, 132, 134, 135, 140, 141, 142, 143, 144, 146, 147, 148, 149 flow rate, 29, 41, 58, 78, 89, 100, 107, 125, 135, 149 fluctuations, 119

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Index flue gas, 14 fouling, 10, 11, 12, 13, 15, 119, 136 fruit juice, vii, 11 fungicide, 18 furniture, 137

G gas, 7, 9, 14 gauge, 25 gel, 110, 125 generation, 3 government, 3 government policy, 3 grain, 2, 3, 12, 137 gravity, 43, 79 groups, 101 growth, 30, 49, 56, 66, 68, 69, 76, 85, 95, 96, 105, 106, 123, 141

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H hardness, 5, 17 heat, 3, 11 heavy metal, 9, 10, 15, 16 heavy metals, 9, 10, 15, 16 height, 28, 119 high pressure, 28 high resolution, 119 high temperature, 101 hybrid, 11, 27, 47, 65, 83, 94, 140 hydrodynamic, 35, 117, 134, 147, 148 hydrodynamics, 13, 27, 47, 65, 83, 94, 140 hydrogen, 9 hydrogen sulfide, 9 hydrolysis, 3 hydroxide, 3, 7, 9 hydroxides, 10

I identification, 13 image analysis, 121 images, 119 impurities, 8, 25 in situ, 10, 25 India, 3, 5, 7, 14, 15, 21, 24, 25, 43, 46, 61, 63, 77, 79, 83, 88, 89, 91, 92, 99, 100, 101, 102, 111, 115, 116, 136, 137, 138, 148, 150 Indian, 5, 14, 89, 100, 114, 136, 150 industrial, vii, 3, 8, 9, 11, 14, 15, 16, 17, 18, 150 industrial wastes, 14

153

industry, vii, 5, 8, 13, 14, 16, 17, 18, 100, 114, 150 infusions, 2 injury, iv inorganic, 2, 3, 9, 13, 27, 75, 94, 102, 130, 140 investment, 13 ionic, 10 ions, 9, 10, 15, 16, 101 iron, 9, 15 Italy, 14

K keratin, 3

L lamina, 13, 27, 30, 31, 32, 34, 35, 36, 37, 38, 40, 47, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 76, 77, 78, 83, 84, 85, 86, 87, 89, 94, 95, 96, 97, 98, 99, 100, 104, 105, 106, 107, 108, 110, 111, 114, 121, 122, 124, 125, 126, 127, 128, 129, 130, 131, 132, 135, 140, 141, 142, 143, 144, 145, 146, 148, 149 laminar, 13, 27, 30, 31, 32, 34, 35, 36, 37, 38, 40, 47, 48, 49, 50, 51, 52, 54, 55, 56, 57, 58, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 76, 77, 78, 83, 84, 85, 86, 87, 89, 94, 95, 96, 97, 98, 99, 100, 104, 105, 106, 107, 108, 110, 111, 114, 121, 122, 124, 125, 126, 127, 128, 129, 130, 131, 132, 135, 140, 141, 142, 143, 144, 145, 146, 148, 149 leaching, 91 leather, 2, 3, 5, 7, 8, 12, 13, 14, 15, 16, 18, 21, 114, 117, 136, 137, 150 limitations, 11 linear, 111 lipid, 8 liquor, 2, 8, 12, 14, 17, 23, 27, 44, 45, 47, 62, 63, 65, 80, 81, 83, 92, 94, 138, 140

M management, 11 manpower, 11 media, 10 membrane permeability, 11, 23, 25 membrane separation processes, 11, 13, 41, 59, 78, 89, 135, 136 membranes, 10, 13, 16, 18, 24, 25, 26, 38, 45, 46, 47, 58, 59, 63, 64, 65, 77, 81, 82, 92, 93, 102, 103, 111, 115, 116, 117, 121, 132, 138, 139 metabisulfite, 9 metal ions, 9, 10

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Index

metals, 8, 9, 10, 15 microbes, 10 microbial, 16, 18 micrometer, 119 microscope, 119, 120 microscopy, 119, 136 milk, 2 mirror, 28, 117 mixing, 7, 11 modeling, 16 molecular weight, 9, 11, 23, 63, 75, 101 molecules, 129, 130, 136 motion, 123 movement, 121

N natural, 2, 104, 137 neutralization, 3 New York, 150 nickel, 14 nontoxic, 10 normal, 23, 45, 63, 81 nutrients, 9

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O observations, 23, 45, 63, 81 obstruction, 28, 119 oil, 3, 10, 137, 150 oils, 3, 137, 150 olive, 3 olive oil, 3 optical, 119, 120 optical microscopy, 119 optimization, 18 organic, 3, 8, 9, 10, 13, 17, 59, 71, 75, 78, 79, 89, 91, 100, 115, 116, 117, 119, 130, 136, 150 organic compounds, 8 organic solvent, 91 organic solvents, 91 osmosis, 11, 18, 19, 24, 27, 39, 103, 104, 113, 114, 129, 130, 134, 141 osmotic, 111 osmotic pressure, 111 oxidation, 8, 9, 10, 13, 15 oxide, 10, 79, 137 oxygen, 2, 3, 7 ozone, 8

P particles, 9, 26, 92, 93, 102, 119, 130, 140 peat, 10 permeability, 11, 23, 24, 25 permeation, 11 pH, 2, 3, 4, 7, 9, 12, 21, 22, 23, 37, 38, 39, 40, 43, 44, 45, 56, 57, 58, 61, 63, 75, 76, 77, 79, 81, 88, 89, 91, 99, 100, 101, 112, 113, 115, 116, 133, 134, 135, 138, 149 photochemical, 13 physiological, 2, 21 pigments, 2 pigs, 3, 12 plants, 91 polarization, 11, 25, 31, 32, 34, 40, 46, 49, 51, 53, 58, 68, 69, 71, 78, 82, 85, 87, 89, 92, 96, 97, 100, 102, 106, 107, 114, 123, 125, 135, 138, 141, 144, 146, 149 pollutant, 3 pollutants, 13, 15, 79 pollution, 3, 12, 14 polyamide, 24, 45, 63, 92, 102, 115, 138 poor, 10 pore, 11, 23, 26, 46, 93, 139 pores, 11, 26, 92, 93, 102, 139 porous, 28, 118 potassium, 21 power, 7, 10 power plant, 7 precipitation, 7, 8, 9, 10, 14, 15 pressure, 3, 11, 12, 13, 23, 24, 25, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 47, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 65, 66, 68, 69, 70, 71, 72, 73, 74, 76, 77, 78, 83, 84, 85, 86, 87, 88, 89, 92, 94, 95, 96, 97, 98, 99, 100, 102, 104, 105, 106, 107, 108, 109, 110, 111, 113, 114, 121, 123, 124, 125, 126, 129, 130, 131, 132, 134, 135, 138, 140, 141, 143, 144, 145, 146, 147, 148, 149 process control, 137 production, 2, 7 productivity, 11, 26, 45, 63, 81, 83, 92, 94, 101, 102, 115, 138, 140 program, 10 promoter, 13, 27, 30, 31, 32, 34, 35, 36, 37, 38, 39, 40, 41, 47, 48, 49, 50, 51, 53, 54, 55, 56, 57, 58, 65, 66, 68, 69, 71, 72, 73, 74, 76, 77, 78, 83, 84, 85, 86, 88, 94, 95, 96, 97, 98, 99, 104, 105, 106, 110, 111, 112, 113, 114, 119, 120, 121, 122, 123, 129, 130, 131, 132, 133, 135, 140, 141, 143, 144, 145, 148, 149 promoter region, 121 proteins, 2, 12, 21, 43, 61

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Index proteolytic enzyme, 2, 61 protocol, 11, 13 Pseudomonas, 16 pure water, 24, 26, 92 purification, 10

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R radius, 23 range, 7, 9, 11, 34, 36, 39, 53, 54, 58, 72, 74, 88, 97, 99, 111, 113, 117, 128, 131, 132, 134, 146, 148 reaction time, 9 recovery, 8, 10, 12, 13, 15, 17, 18, 19, 101, 114 recycling, 2, 8, 12, 14, 17 reflection, 119 regeneration, 8, 10 regular, 119 rejection, 111 remediation, 8 residues, 8, 14, 16, 61 resin, 8, 10 resins, 15 resistance, 3, 11, 30, 34, 35, 40, 49, 52, 54, 58, 66, 69, 71, 72, 78, 87, 88, 89, 96, 97, 98, 100, 101, 105, 110, 114, 123, 126, 127, 128, 135, 137, 146, 147, 149 resolution, 119 resources, 12 retention, 11, 26, 27, 94, 102, 104, 111, 113, 117, 140 Reynolds number, 30, 31, 32, 34, 35, 36, 38, 39, 40, 48, 50, 51, 52, 53, 54, 56, 57, 58, 66, 68, 69, 70, 71, 72, 76, 77, 78, 84, 85, 87, 89, 95, 96, 97, 98, 100, 104, 107, 110, 111, 113, 119, 121, 122, 124, 125, 126, 127, 128, 129, 130, 132, 134, 135, 141, 144, 146, 147, 149 room temperature, 24 rubber, 28, 118

S salinity, 17 salt, 2, 3, 8, 11, 12, 37, 40, 56, 57, 61, 75, 76, 79, 88, 104, 111, 113, 114 salts, 2, 3, 8, 14, 15, 17, 21, 40, 79, 104, 114, 149 sample, 119 sampling, 25, 119 sand, 13 scaling, vii, 13 seawater, 15 seaweed, 16 sedimentation, 7, 9

155

separation, vii, 8, 10, 11, 13, 16, 26, 40, 58, 63, 81, 84, 89, 91, 94, 100, 102, 117, 137, 140, 149 sequencing, 8, 14 services, iv sewage, 27, 103, 140 shape, 136 shear, 126 sheep, 3, 12 silica, 10 sites, 137 skin, 2, 3, 12, 21, 24, 46, 63, 92, 102, 116, 138 sludge, 5, 7, 8, 9, 10, 12, 13, 14, 15, 17, 40, 47, 58, 59, 62, 65, 77, 83, 89, 94, 100, 140, 149, 150 snaps, 119 sodium, 2, 3, 7, 9, 11, 40, 43 softener, 13 software, 119, 121 soil, 3 soil pollution, 3 solubility, 9 solvent, 3, 8, 10, 11, 31, 38, 40, 49, 56, 58, 66, 69, 76, 77, 78, 89, 96, 100, 105, 123, 132, 135, 149 sorption, 8 species, 10, 11 speed, 24, 25, 102 stainless steel, 28, 117, 119 standard deviation, 120 standard error, 120 Staphylococcus, 16 steady state, 13, 26, 30, 31, 32, 34, 35, 36, 40, 46, 48, 51, 52, 53, 54, 55, 58, 64, 66, 68, 69, 70, 71, 72, 73, 78, 85, 87, 89, 93, 95, 96, 97, 98, 100, 105, 106, 107, 109, 110, 114, 122, 123, 125, 127, 128, 135, 139, 141, 143, 144, 146, 147, 149 steel, 8, 28, 117, 119 steel plate, 28, 118 storage, 7 streams, 10, 11, 13, 26, 93, 139 strength, 3, 137 students, vii substances, 3, 8, 21, 75, 91, 115 sulfate, 3, 8, 9, 12, 15, 17, 101, 137 sulfur, 9, 18 sulfur dioxide, 9 sulfuric acid, 8, 9 sulphate, 3, 7, 8 supernatant, 7, 22, 27, 43, 45, 47, 61, 65, 79, 81, 83, 92, 94, 138, 140 supply, 10 surfactant, 25 surfactants, 14, 91 swelling, 2, 43

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Index

T ultrasound, 8, 15 upholstery, 137

V values, 25, 31, 32, 34, 36, 41, 46, 50, 51, 53, 54, 58, 65, 71, 72, 77, 78, 83, 87, 88, 89, 97, 99, 100, 106, 107, 114, 115, 116, 120, 121, 125, 136, 141, 144, 146, 147, 148, 149 variation, 34, 35, 44, 52, 54, 56, 62, 71, 72, 76, 80, 84, 87, 88, 97, 110, 119, 121, 126, 128, 130, 134, 144, 146, 147 velocity, 30, 32, 37, 39, 49, 51, 55, 66, 68, 69, 71, 74, 95, 96, 99, 105, 106, 109, 110, 111, 113, 114, 121, 123, 129, 130, 131, 134, 136, 143, 148 video microscopy, 136

W wastes, 3, 7, 8, 14, 15 wastewater, 1, 2, 3, 5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 79, 136 wastewater treatment, 8, 9, 14 wastewaters, 8, 10, 17, 19 water, vii, 2, 3, 7, 9, 10, 11, 12, 13, 14, 17, 18, 19, 21, 23, 24, 25, 26, 78, 92, 104, 111, 136, 137 wetting, 137 wires, 28, 104, 119

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tanks, 7 tannin, 3, 8, 18 tanning, 3, 4, 12, 14, 101, 103 tannins, 8, 15 temperature, 11, 23, 101 tensile, 3, 137 tensile strength, 3, 137 thin film, 24 thiosulphate, 2 tissue, 2, 43 toxic, 3, 9, 12, 13, 116 transformations, 9, 18 transmembrane, 13, 23, 29, 32, 33, 34, 35, 36, 38, 39, 48, 50, 51, 52, 53, 54, 55, 56, 57, 58, 66, 70, 71, 72, 73, 76, 77, 78, 85, 86, 87, 88, 97, 104, 106, 107, 110, 111, 113, 121, 125, 126, 129, 130, 132, 134, 136, 141, 144, 145, 146, 147 transport, 11 turbulence, 23, 28, 31, 32, 49, 51, 68, 69, 71, 85, 96, 105, 106, 107, 111, 119, 123, 125, 141, 143, 144 turbulent, 13, 27, 28, 30, 31, 32, 34, 35, 36, 38, 39, 40, 47, 48, 49, 50, 51, 53, 54, 56, 57, 58, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 83, 84, 85, 86, 87, 88, 89, 94, 95, 96, 97, 98, 100, 104, 106, 107, 108, 110, 111, 112, 113, 114, 119, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 135, 136, 140, 141, 142, 143, 144, 146, 147, 148, 149 turbulent flows, 34, 53, 71, 87, 97, 110, 129, 146

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Treatment of Tannery Effluents by Membrane Separation Technology, Nova Science Publishers, Incorporated, 2009. ProQuest Ebook Central,