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DecolourisationCoagulation

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larly in urban areas, is considerable. Wastewater generation fromthis sector has been estimated as 55 106 m3 per day, of which68.5 103 m3 are dumped directly into local rivers and streamswithout prior treatment (MOWR, 2000). The developing countriescontribute the largest amount of textile wastewater. For anexample, developing countries of South Asia contributed around35% textile wastewater out of total industrial wastewater generatedby South Asian countries in 2001 (World Bank, 2005). Out of seven

Abbreviations: BOD, biochemical oxygen demand; COD, chemical oxygendemand; TDS, total dissolved solid; AOX, absorbable organic halides; SS, suspendedsolid; TSS, total suspended solid; DS, dissolved solid; NTU, nephelometric turbidityunit; DO, dissolved oxygen; UV, ultraviolet; nm, nanometer; AOP, advancedoxidation process; PACl, polyaluminium chloride; PFCl, polyferric chloride; PFS,polyferric sulphate; PAFCl, polyaluminium ferric chloride.* Corresponding author. Tel.: 91 674 2300 714; fax: 91 674 2301 983.

E-mail addresses: akshaya@iitbbs.ac.in (A.K. Verma), rrdash@iitbbs.ac.in

Contents lists available at

Journal of Environm

journal homepage: www.els

Journal of Environmental Management 93 (2012) 154e168(R.R. Dash), pbhunia@iitbbs.ac.in (P. Bhunia).(BOD), chemical oxygen demand (COD), pH, temperature, turbidityand toxic chemicals. The direct discharge of this wastewater intothe water bodies like lakes, rivers etc. pollutes the water and affectsthe ora and fauna. Efuent from textile industries containsdifferent types of dyes, which because of high molecular weightand complex structures, shows very low biodegradability (Hsu andChiang, 1997; Pala and Tokat, 2002; Kim et al., 2004; Gao et al.,2007). Also, the direct discharge of this industrial efuent intosewage networks produces disturbances in biological treatment

metals either in free form in efuents or adsorbed in the suspendedsolids are either carcinogenic (Tamburlini et al., 2002; Bayramogluand Arica, 2007) or may cause harm to children even before birth,while others may trigger allergic reactions in some people.

Industrial emissions and the waste efuents generated from thefactories are associated with heavy disease burden (WHO, 2000,2002) and this could be one of the reasons for short life expec-tancy of 64 years in India compared to developed countries such asJapan, where life expectancy is 83 years (UNICEF, 2008). Althoughthe industrial sector only accounts for 3% of the annual waterwithdrawals in India, its contribution to water pollution, particu-FlocculationTextile wastewater

1. Introduction

Textile industries are one of thcomplex chemicals during textile prstages. The unused materials from thwastewater that is high in colour,0301-4797/$ e see front matter 2011 Elsevier Ltd.doi:10.1016/j.jenvman.2011.09.012have been reviewed. Based on the present review, some novel pre-hydrolysed coagulants such as Pol-yaluminium chloride (PACl), Polyaluminium ferric chloride (PAFCl), Polyferrous sulphate (PFS) and Pol-yferric chloride (PFCl) have been found to be more effective and suggested for decolourisation of thetextile wastewater. Moreover, use of natural coagulants for textile wastewater treatment has also beenemphasised and encouraged as the viable alternative because of their eco-friendly nature.

2011 Elsevier Ltd. All rights reserved.

st users of water andg at various processingesses are discharged asmical oxygen demand

processes. These efuents produce high concentrations of inorganicsalts, acids and bases in biological reactors leading to the increase oftreatment costs (Gholami et al., 2001; Babu et al., 2007). Moreover,traditionally produced fabric industries generate residuals ofchemicals that evaporate into the air that we breathe or areabsorbed through our skin. Some of the chemicals such as heavyKeywords:Dyetextile wastewater by means of cheaper and environmental friendly technologies is still a major chal-lenge. In this manuscript, several options of decolourisation of textile wastewater by chemical meansdeveloping technologies that can diminish the environmental damage. However, colour removal fromA review on chemical coagulation/occufrom textile wastewaters

Akshaya Kumar Verma, Rajesh Roshan Dash, PuspeDepartment of Civil Engineering, School of Infrastructure, Indian Institute of Technology

a r t i c l e i n f o

Article history:Received 13 April 2011Received in revised form26 August 2011Accepted 15 September 2011Available online 5 October 2011

a b s t r a c t

Textile industry is one of thpotable water. It generatesincluding dyes in the formof this wastewater into envAs environmental protectAll rights reserved.tion technologies for removal of colour

u Bhunia*

ubaneswar, Orissa 751 013, India

ost chemically intensive industries on the earth and the major polluter ofe quantities of complex chemical substances as a part of unused materialsastewater during various stages of textile processing. The direct dischargenment affects its ecological status by causing various undesirable changes.becomes a global concern, industries are nding novel solutions for

SciVerse ScienceDirect

ental Management

evier .com/locate/ jenvman

core countries of South Asia (Bangladesh, Bhutan, India, Maldives,Nepal, Pakistan, and Sri Lanka), India is the major manufacturer oftextiles which constitute 83 composite mills and 2241 semicomposite processing units (COINDS, 2000). Hence, it can be saidthat India may be the major contributor of textile wastewater inSouth Asia. The textile industries in India are mainly located inMumbai, Surat, Ahmedabad, Coimbatore, Ludhiyana and Kanpur.

1.1. Characteristic and composition of textile wastewater

On the basis of waste and wastewater (or efuent) generation,the textile mills can be classied into two main groups namely dryprocessing mills and wet processing mills (ISPCH, 1995). In the dryprocessing mills, mainly solid waste is generated due to the rejectsof cotton. In the other group, desizing, scouring, bleaching, mer-cerising, dyeing, printing, and nishing are the main processingstages. The wastewater generated by textile industry includescleaning wastewater, process wastewater, noncontact coolingwastewater, and storm water. The amount of water used varieswidely in this industry, depending on the specic processes oper-ated at the mill, the equipment used, and the prevailing philosophyof water use. The components of major pollutants involved atvarious stages during wet processing of cotton-based textileindustry are shown in Fig. 1. On account of the involved complexityof different processes at different stages, textile wastewater typi-cally contains a complex mixture of chemicals. Apart from this,large numbers of associated hazards have also been reported by the

various chemicals used in different stages of textile processing (Leeet al., 2006; Jadhav et al., 2007; Shi et al., 2007; Anouzla et al.,2009).

Wet processing operations (including preparation, dyeing, andnishing) generate the majority of textile wastewater having veryhigh COD, BOD, TDS and very deep colour as shown in the Table 1.Large numbers of chemical constituents such as alkali, acids,bleaching chemicals, enzymes, starch, dyes, resins, solvents, waxes,oils etc. are used in the various steps during textile processing andnally comes out in the efuent after its consumption. Desizing, orthe process of removing size chemicals from textiles, is one of theindustrys largest sources of wastewater pollutants (Bisschops andSpanjers, 2003; Dos Santos et al., 2007). In this process, largeamount of size chemicals used in weaving processes are discarded.Dyeing operation generates a large portion of the industrys totalwastewater. The primary source of wastewater in dyeing operationsis spent dye bath and wash water. Such wastewater typicallycontains by-products, residual dyes, and auxiliary chemicals.Additional pollutants include cleaning solvents, such as oxalic acid(USPEA, 1997). Of the 700,000 tons of dyes produced annuallyworldwide (Papic et al., 2004; Lee et al., 2006; Riera-Torres et al.,2010), about 10e15 percent of the dyes are disposed off in efuentfrom dyeing operations (Snowden-Swan, 1995; Husain, 2006; Haiet al., 2007; Gupta and Suhas, 2009).

Dyes in wastewater may be chemically bound to fabric bers(ATMI, 1997). Dyeing and rinsing processes for disperse dyeinggenerate about 91e129 m3 of wastewater per ton of product

Constituents Process Wastewater characteristics

BOD (34-50% of total), high

Sizing

Desizing

n

Yarn waste, unused starch-based sizes

Enzymes, starch,

High BOD, medium COD

A.K. Verma et al. / Journal of Environmental Management 93 (2012) 154e168 155Scouring

Bleaching

Mercerisatio

Dyeing

Finishing

Printing

waxes, ammonia

Disinfectants and insecticides residues, NaOH, surfactants , soa p s ,

H 2 O 2 , AOX, NaOCl, organics

NaOH

Colour, metals, sulphide, salts, acidity/alkalinity, formaldeh y de

Urea, solvents , colour, metals

Chlorinated compounds, resins, spent solvents, softeners, waxes, a ce tat e Fig. 1. The component of major pollutants involved at various stages of a textile manufacturCharoenlarp and Choyphan, 2009).High pH, TDS

Oily fats, BOD (30% of total), high pH, temp. (70- 80 C), dark colour

COD, temp. (70-80 C)

Low alkalinity, low BOD, high toxicit y

High BOD, high pH suspended solids

High toxicity, BOD (6% of total), high dissolved solids, high pH,

High toxicity, high COD, high BOD, high dissolved solids, hi g h p H, stron g colou r ing industry (SEAM Project, 1999; Yusuff and Sonibare, 2004; Joseph, 2007; Paul, 2008;

/L)

060

0

I : A

men(Snowden-Swan, 1995). Similar processes for reactive and directdyeing generate even more wastewater, about 113e151 m3 per tonof product (Snowden-Swan, 1995; Karcher et al., 2002; Riera-Torreset al., 2010). This can be attributed by the fact that disperse dyesshow higher percentage of xation to the ber as compared to acidand reactive dyes. Finishing processes typically generate waste-water containing natural and synthetic polymers and a range ofother potentially toxic substances (Snowden-Swan, 1995). Textileindustries typically generate 200e350 m3 of wastewater per ton ofnished product (Ranganathan et al., 2007; Gozlvez-Zafrilla et al.,2008) resulting in an average pollution of 100 kg COD per ton offabric (Jekel, 1997).

Raw textile wastewater can be characterised bymeasurement ofBOD, COD, colour, suspended solids (SS), dissolved solids (DS) andheavy metals etc. Typical characteristics of textile industry waste-water generally include a wide range of pH, COD, dissolved solidsand strong colour (El-Gohary and Tawk, 2009; Lau and Ismail,2009; Ciabatti et al., 2010; Debik et al., 2010; Phalakornkule et al.,2010), which may be comparable to moderate municipal waste-water (Rott and Minke, 1999). However, the main challenge is to

Table 1Major characteristics of real textile wastewater studied by various researchers.

pH COD (mg/L) BOD5 (mg/L) TSS (mg/L) TDS (mg

8.8e9.4 595 131 379 110 276 76 e11.2 2276 660a e 47.95e10 1100e4600 110e180 e 506.5e8.5 550e1000 e 100e400 e2.7 7000 e 440 93013.56 2968 e e e12e14 1500e2000 e e e10 1150 170 150 e9 750 160 e e2e10 50e5000 200e300 50e500 e8.32e9.50 278e736 137 85e354 1715e618.7 0.2 17900 100 5500 100 23900 50 1200 59.30 3900 e e e7.8 810 50.4 188 15.2 64 8.5 e13 1 2300 400 e 300 100 e6.95 3422 1112 e7.86 340 210 300 e7.5 0.3 131 18 e 75 13 1885 8a BOD7 and efuent is from reactive dye bath.b Integral of the absorbance curve in the whole visible range (400e800 nm), ADM

A.K. Verma et al. / Journal of Environ156eliminate the colour of wastewater, which is due to the remainingdyes. The major characteristics of real textile wastewater have beendescribed in the Table 1. It has been observed from the Table 1, thatthe textile wastewaters exhibit wide range of pH from 2 to 14, CODfrom 50 mg/L to approximately 18,000 mg/L, TDS from 50 mg/L toover 6000 mg/L and very strong colour. This wide variation in thecharacteristics of textile wastewater is due to complexity of mate-rials used in the textile industry during the processing of textiles.

1.2. Effects of textile wastewater

Textile wastewaters generated from different stages of textileprocessing contains huge amount of pollutants that are veryharmful to the environment if released without proper treatment.Harmful direct and indirect effects of textile wastewater have beensummarized in Fig. 2. The release of textile wastewater to theenvironment causes aesthetic problems as the changed colour ofthe water bodies such as lakes and rivers, after releasing ofwastewater from the industry, cannot be tolerated by the localpeople. Also, the accumulation of colour hinders sunlight pene-tration, disturbs the ecosystem of receiving water (Georgiou et al.,2003; Merzouk et al., 2010). Ground water systems are also getaffected by these pollutants due to leaching through the soil(Namasivayam and Sumithra, 2005; Khaled et al., 2009). Apart fromthis, several dyes and their decomposition derivatives have provedtoxic to aquatic life (aquatic plants, microorganisms, sh andmammals) (Georgiou et al., 2002; Kim et al., 2004; Ustun et al.,2007). Additionally, fairly intensive studies has inferred that suchcoloured allergens may undergo chemical and biological assimila-tions, cause eutrophication, consume dissolved oxygen, prevent re-oxygenation in receiving streams and have a tendency to sequestermetal ions accelerating genotoxicity andmicrotoxicity (Walsh et al.,1980; Foo and Hameed, 2010). In a wider sense, sporadic andexcessive exposure to coloured efuents is susceptible to a broadspectrum of immune suppression, respiratory, circulatory, centralnervous and neurobehavioral disorders presage as allergy, auto-immune diseases, multiple myeloma, leukemia, vomiting, hyper-ventilation, insomnia, profuse diarrhea, salivation, cyanosis,jaundice, quadriplegia, tissue necrosis, eye (or skin) infections,irritation to even lung edema (Anliker, 1986; Foo and Hameed,2010).

1.3. Present practices for treatment of textile wastewater

Colour Turbidity (NTU) References

e e El-Gohary and Tawk, 2009e e Golob et al., 20051450e1475(ADMI) e Dos Santos et al., 20077.50e25.50b 15e200 Ciabatti et al., 2010

2140 Al-Malack et al., 19993586 (C.U) 120 Joo et al., 2007Dark blue e Gozalvez-Zafrilla et al., 20081.24436nm e Selcuk, 2005e e Schrank et al., 2007>300 (C. U) e Lau and Ismail, 2009e e Phalakornkule et al., 2010e e Rodrguez et al., 2008e 240 Paschoal et al., 20090.15669nm e Haroun and Idris, 2009e e Debik et al., 2010e 5700 Bayramoglu et al., 2004>200 (Pt-Co) 130 Merzouk, 2010e e Ustun et al., 2007

merican dye manufacturer institute, C.U: Colour Unit.

tal Management 93 (2012) 154e168The available literature shows a large number of well estab-lished conventional decolourisation methods involving physico-chemical, chemical and biological processes, as well as some ofnew emerging techniques like sonochemical or advanced oxidationprocesses. However, there is no single economically and technicallyviable method to solve this problem and usually two or threemethods have to be combined in order to achieve adequate level ofcolour removal (Kang and Chen, 1997; Robinson et al., 2001).Researches on chemical coagulation/occulation, is observed asone of the most practised technology. Regardless of the generationof considerable amount of sludge, it is still used in developed and indeveloping countries. Because the mechanism of coagulant appliedto decolourise wastewater is still not absolutely clear, colourremoval by coagulation is found in some cases very effective, insome cases however, has been failed completely.

Hence, the objectives of this review are the analysis of variouschemical technologies developed for decolourisation of textilewastewater, giving more stress upon chemical coagulation/occu-lation technology, short description and critical appraisal ofdecolourisation methods, comparison of their relative advantagesand disadvantages and propose for the effective and cheaperalternatives.

menA.K. Verma et al. / Journal of Environ2. Chemical composition and structure of colour causing dyesin textile processing

A dye is used to impart colour to a material, of which it becomesan integral part. An aromatic ring associatedwith a side chain usuallyrequired for resonance and thus to impart colour. Characterisation ofdyes is based on their chemical structure and application. They arecomposed of the atoms responsible for the dye colour called chro-mophores as well as an electron withdrawing or donating substit-uent that causes or intensies the colour of chromophores, calledauxochrome (Christie, 2001). Usual chromophores are eC]Ce(ethenyl), eC]O (carbonyl), eC]Ne (imino), eCH]S (thio-carbonyl), eN]Ne (azo), eN]O (nitroso), eNO2 (nitro) and usualauxochromes are eNH2 (amino), eCOOH (carboxylic), eSO3H (sul-phonyl) and eOH (hydroxyl) (Van der Zee, 2002). The intensity ofcolour depends upon the number of such groups. Compounds ofbenzene containing chromophore radicals are called chromogens.Such compounds, though coloured, are not dyes, since they do nothave the afnity or the ability to unite with tissue. To be a dye,a compoundmust containnotonly the chromophore groups, but alsotheadditional group(s) calledAuxochrome(s). These auxiliarygroupsare responsible for imparting theproperty of electrolytic dissociation

Fig. 2. Schematic representation of the effect otal Management 93 (2012) 154e168 157i.e., the separation of the dyemolecule into its components or atomsand to form saltswith either acid or alkali. They can also belong to theclasses of reactive, acid, basic, direct,mordant, disperse, pigment, vat,anionic, sulphur and disperse dye (Welham, 2000). Anthraquinonedyes possesswide range of colours in thewhole visible spectrumandconstitute the second most important class of textile dyes after azodyes, which are used to give blue, green and violet colours (Christie,2001; Fontenot et al., 2003). The characteristics of different dyes thatare used widely in the textile industry have been summarised inTable 2. It has been observed from the Table 2 that reactive dyes arewidely used to colour the cotton which contribute as half of theworldwide textile-ber market. The reactive dyes that are used forcotton, show poorest rate of xation due to which textile efuentpossesses strong colour. The chemical structure of different azo dye,vatdye andanthraquinonedyemolecules comprisingof auxochromeand chromophore has been illustrated in Fig. 3.

3. Physico-chemical methods for removal of colour fromtextile wastewaters

Plenty of Physico-chemical methods in the form of pretreat-ment, post treatment or main treatment have been investigated by

f textile wastewater into the environment.

thod

menvarious researchers throughout the World. A brief discussion onthese methods along with a comprehensive discussion particularly,on the chemical coagulation and occulation technology for colourremoval has been presented in this section.

One of the most commonly known methods is the ltrationtechnology. Filtration methods such as ultraltration, nano-ltration and reverse osmosis have been used for water reuse andthe chemical recovery (Marcucci et al., 2001; Fersi and Dhahbi,2008). In the textile industry, these ltration methods can beused for both ltering and recycling of not only pigment richwastewaters, but also mercerising and bleaching wastewaters. Thespecic temperature and chemical composition of the wastewatersdetermines the type and porosity of the lter to be applied. Further,the utilisation of membrane technology for dye removal fromtextile wastewater is very effective as reported by variousresearchers (Ledakowicz et al., 2001; Ahmad et al., 2002). However,the main drawbacks of membrane technology are the high cost,

Table 2Characterisation of different class of dyes mainly used in textile industry and its me2009).

Class Characteristics Substrate (bre)

Acid Anionic, water soluble Nylon, wool, silkBasic Cationic, water soluble Modied nylon, polyesterDirect Anionic, water soluble Cotton, rayon, leather, nylon

Disperse Very low water solubility Polyester, poly-amide, acetate,plastic, acrylic

Reactive Anionic, water soluble Cotton, nylon, silk, wool

Sulfur Colloidal, insoluble Cotton, rayon

Vat Colloidal, insoluble Cotton, rayon

A.K. Verma et al. / Journal of Environ158frequent membrane fouling, requirement of different pretreat-ments depending upon the type of inuent wastewaters, andproduction of concentrated dyebath which further needs propertreatment before its safe disposal to the environment (Robinsonet al., 2001; Akbari et al., 2006). For membrane ltration, properpretreatment units for removing SS of the wastewaters are almostmandatory to increase the life time of the membranes. These makethe process more expensive and thereby limit the application ofthis expensive technology for wastewater treatment.

Another most popular method is adsorption technology. Adsorp-tionmethod for colour removal is based on the afnity of various dyesforadsorbents. It is inuencedbyphysical andchemical factors suchasdyeeadsorbent interactions, surface area of adsorbent, particle size,temperature, pH and contact time (Anjaneyulu et al., 2005; Patel andVashi, 2010). Themain criteria for selectionof adsorbents are basedonthe characteristics like high afnity, capacity of target compound andpossibility of adsorbent regeneration (Karcher et al., 2002). Activatedcarbon ismost commonlyused adsorbent and canbe veryeffective formany dyes (Walker and Weatherly, 1997; Pala and Tokat, 2002).However, efciency is directly dependent upon the type of carbonmaterial used and wastewater characteristics (Robinson et al., 2001).The limitations of this technology are the eco-friendly disposal ofspent adsorbents, excessive maintenance costs, and pretreatment ofwastewater to reduce the SS under acceptable range before it is fedinto the adsorption column. Because of these reasons, eld scaleapplication of adsorption technology is limited not only for colourremoval of textile wastewaters but also for other water and waste-water treatment.

Chemical methods mainly involve use of oxidising agents suchas ozone (O3), hydrogen peroxide (H2O2) and permanganate(MnO4) to change the chemical composition of compound or groupof compounds, e.g. dyes (Metcalf and Eddy, 2003). Among theseoxidants, ozone is widely used because of its high reactivity withdyes and good removal efciencies (Alaton et al., 2002). However, itis also been reported that ozone is not efcient in decolourisingnonsoluble disperse and vat dyes which react slowly and takelonger reaction time (Marmagne and Coste, 1996; Rajeswari, 2000).The decoulorisation efciency also depends upon the pH. As the pHdecreases, ozonation of hydrolysed dyes (Reactive Yellow 84)decreases (Rein, 2001; Konsowa, 2003). It has also been reportedthat the O3/UV as the more effective method for decolourising ofdyes compared to oxidation by UV or ozonation alone. However,

of application (Easton, 1995; Akbari et al., 2002; Hees et al., 2002; Lau and Ismail,

Dye-bre interaction Method of application

Electrostatic, Hydrogen bonding Applied from neutral to acidic dyebathsElectrostatic attraction Applied from acidic dyebathsIntermolecular forces Applied from neutral or slightly

alkaline baths containingadditional electrolytes

Hydrophobic- Solid statemechanism

Fine aqueous dispersions oftenapplied by high temperaturepressure or lower temperaturecarrier methods

Covalent bonding Reactive site on dye reacts withfunctional group on bre to binddye covalently under inuence ofheat and pH(alkaline)

Covalent bonding Aromatic substrate vatted with sodium sulde and re-oxidised toinsoluble sulfur-containingproducts on bre

Impregnation and oxidation Water insoluble dyes solubilisedby reducing with sodium hydrosulte,then exhausted on bre and re-oxidised

tal Management 93 (2012) 154e168Perkowski and Kos, (2003) have reported no signicant differencesbetween ozonation and O3/UV in terms of colour removal. This maybe due to the fact that production of hydroxyl radical (HO) duringphotodecomposition of ozone may improve the degradation oforganics. However, most of the UV light gets absorbed by the dyesand hence very small amount of hydroxyl free radical can beproduced to decompose the dyes. Therefore, approximately samecolour removal efciencies using O3 and O3/UV could be expected.

In H2O2/UV process, HO radicals are formed when water con-taining H2O2 is exposed to UV light, normally in the range of200e280 nm (Metcalf and Eddy, 2003). The H2O2 photolysis occursas per the reaction shown in Equation (1).

H2O2 UV l 200 280 nm/HO$ HO$ (1)This process is most widely used in Advanced Oxidation Process

(AOP) technology for the decomposition of chromophores presentin the dyes (Ferroro, 2000; Kurbus et al., 2002) and consequentlyrelies complete decolourisation. Fenton reaction is also an exampleof AOP inwhich hydrogen peroxide is added in an acid solution (pH2e3) containing Fe2 ions (Equation (2)).

Fe2 H2O2/Fe3 HO$HO (2)As compare to ozonation, this method is relatively cheap and

also presents high COD reduction and decolourisation efciencies

menA.K. Verma et al. / Journal of Environ(Van der Zee, 2002). The main drawback is high sludge generationdue to the occulation of reagents and dye molecules (Robinsonet al., 2001; Azbar et al., 2004). Most of the AOP for textile waste-waters are highly expensive and its effectiveness varies widely withthe type of constituents present in the textile wastewaters. Also,from the several reports it is observed that the in some cases, atcertain conditions, these technologies give very attractive results,however, in some other cases, their application has been reportednot worthy considering the cost and complexity involved in thesetechnologies.

Chemical coagulation and occulation in wastewater treatmentinvolves the addition of chemicals to alter the physical state ofdissolved and suspended solids and facilitate their removal bysedimentation. In some cases the alteration is slight, and removal isaffected by entrapment within a voluminous coagulate consistingmostly the coagulant itself. Another result of chemical addition isa net increase in the dissolved constituents in the wastewater.Coagulation is used for removal of the waste materials in sus-pended or colloidal form that do not settle out on standing or maysettle by taking a very long time. Inwater treatment, coagulation aspretreatment is regarded as the most successful pretreatment(Huang et al., 2009; Leiknes, 2009).

Coagulation of dye-containing wastewater has been used formany years as main treatment or pretreatment due to its lowcapital cost (Anjaneyulu et al., 2005; Golob et al., 2005). However,the major limitation of this process is the generation of sludge andineffective decolourisation of some soluble dyes (Anjaneyulu

Fig. 3. Structure of various azo dyes showing chromophore and auxochrome (Van der Zee,Anthraquinone dye (Kim et al., 2004) and vat dye (Silvia et al., 2007).tal Management 93 (2012) 154e168 159et al., 2005; Hai et al., 2007). Further, the sludge production canbe minimised if only a small volume of highly coloured efuenttreated directly after the dyeing bath (Golob et al., 2005). Thereasons could be the non-availability of other chemical additivesexcept hydrolysing and xing agents in the efuent coming fromdyeing bath. The chemical additives that are normally present inthe textile wastewaters provide hindrance to the colour removal.If interfering chemical additives are absent in the textile waste-water except colour causing dyes, then less coagulant dosagemight be required which in turns will reduce the sludgeproduction.

On account of this, coagulation of water soluble dyes ischallenging due to their high solubility. Further, due to devel-opment of synthesis technology, large number of innovativedyes with complex structures have been synthesised and still inprocess of synthesis, which provides difculties for the selectionof appropriate coagulant (Yu et al., 2002). In general colourremoval decreases with increase in dye concentration and dyesolubility (Bouyakoub et al., 2009; Zahrim et al., 2010). There-fore, re-evaluation of optimum conditions for coagulation ofdifferent types of dyes is necessary. Moreover, the effectivenessof the coagulation can be improved by appropriate selection ofcoagulant, occulant aids, optimization of process parameterssuch as pH, dosage of coagulant/occulant aids, mixing time,settling time, etc. The relative advantages and disadvantages ofdifferent physico-chemical methods have been summarisedin Table 3.

2002; Yang and McGarrahan, 2005; Dos Santos et al., 2007; Riera-Torres et al., 2010),

.te,

n

e

cale

menOperating cost and time required for the desired degree oftreatment may be the major criteria for the selection of suitablemethod. It can be observed from the Table 3 that each and everymethod is associated with some type of limitations such as oza-nation gives good colour removal but not signicant COD reduction,also the expensive method. Good colour removal and COD reduc-tion can be achieved by using Fentons reagent but comparativelylonger treatment time and handling of iron contaminated sludge isthe major problem. Oxidation using H2O2-UV is not very effective

Table 3Advantages and limitations of various methods of dye removal from textile efuents

Physical/chemicalmethods

Method description Advantages

Ozonation Oxidation using ozone gas Application in gaseous stano alteration of volume

Fenton reagents Oxidation using H2O2-Fe(II) Effective decolourisationof both soluble andinsoluble dyes

Photochemical Oxidation using mainly H2O2-UV No sludge productionSonolysis Destruction of chemical bond by

producing free radicalusing ultrasound

No Extra sludge productio

Adsorption Dye removal basedon solid support

Excellent removal ofwide variety of dyes

Membrane ltration Physical separation Removal of all types of dy

Ion exchange Ion exchange resin Easy regeneration

Electro-coagulation Treatment based onanode and cathode

Good removal of dye

Irradiation Treatment based on ionizing radiation

Effective oxidation at lab s

Biological Process Treatment based onmicrobiological degradation

Environmental friendly

Chemical coagulationand occulation

Addition of coagulantsand occulants

Economically feasible,excellent colour removal

A.K. Verma et al. / Journal of Environ160since colour and COD reduction is not very signicant, moreover itis not applicable to all types of dyes, produces large number of by-products and also suffers from UV light penetration limitation.Sonolysis gives good colour removal by the destruction of chemicalbond present in the dye structure with the help of free radicalproduction, however it requires enormous amount of dissolveoxygen and involves high electricity cost. Good removal of widevariety of dyes can be achieved by adsorption but regeneration isexpensive and it also necessitates costly disposal. Removal of alltypes of dyes may be achieved by selecting appropriate membranebut production of concentrated sludge and high cost of themembrane are again major limitations. Easy regeneration andefcient recovery of dyes may be possible in the ion exchangemethod but cost of regeneration is high and the method is notapplicable to all types of dyes since, ion exchange resins are dyespecic.

Apart from these, various researchers have also proposed theenzymatic degradation of synthetic dyes. In this direction Bhuniaet al. (2001) investigated the application of novel enzymes horse-radish peroxidase for decomposition and precipitation of azo dyes.The degradation rate was dependent on the pH of the wastewater.In another study, Bumpus et al. (1991) revealed that the enzymefrom white rot fungus degraded Crystal Violet via N-demethylationin a considerable amount. Hence, after standardisation and facili-tation of accurate dosage, effective dye degradation performancecan be achieved. Simplicity in application and rapid modicationaccording to the character of dye to be removed makes it attractivechoice for decolourisation of textile wastewater. However, themajor limitation may be the denaturation of enzyme due the effectof temperature since wastewater coming out from the textileindustry is generally having high temperature.

Both soluble and insoluble dyes can be effectively removed byelectro-coagulation process but high cost of electricity and gener-ation of secondary pollutants from chlorinated organics, heavymetals are themajor limitations. Complete decolourisationmay notbe achieved by the treatment with the help of irradiation for alltypes of dyes. Though, there is no sludge production in this tech-nology, but high cost of electricity may be the limitation. Cost

Disadvantages References

Short half-life (20 min),High cost

Hao et al., 2000; Ince and Tezcanli, 2001;Robinson et al., 2001; Gogate and Pandi., 2004

Sludge generation andits handling

Hao et al., 2000; Arslan and Balcioglu, 2001;Meric et al., 2004

Formation of by-products Konstantinou and Albanis, 2004; Hai et al., 2007Requires a lot of dissolvedoxygen, High cost

Adewuyi, 2001; Arslan-Alaton, 2003

Regeneration difculties,costly disposal of adsorbent

Hao et al., 2000; Fu and Viraraghavan, 2001;Hai et al., 2007

Production of concentratedsludge, High cost

Marcucci et al., 2001;Barredo-Damas et al., 2006

Not effective for all dyes Slokar and Marechal, 1998;Robinson et al., 2001; Hai et al., 2007

High cost, less electrodereliability

Chen et al., 2005; Merzouk et al., 2010;Phalakornkule et al., 2010

Not effective for all dyes,High cost

Robinson et al., 2001; Hai et al., 2007

Slow Process, need of adequatenutrients, narrow operatingtemperature range

Lin and Peng, 1996; Sandhya andSwaminathan, 2006; Togo et al., 2008

Sludge production Hao et al., 2000; Fu and Viraraghavan,2001; Aboulhassan et al., 2006;Gao et al., 2007; Ciabatti et al., 2010

tal Management 93 (2012) 154e168effectiveness of biological treatment process makes it attractivewhich can efciently remove most of the dyes used in the textileindustry because dyes generally possess high level of adsorption onto the activated sludge. However, longer duration of treatment,toxicity of dyes and its low biodegradability are the major limita-tions. Excellent colour removal may be achieved by coagulation-occulation which can remove most of the dyes used in thetextile industry. Though the sludge production is the major limi-tation in this process, cost effectiveness of the treatment ascompared to other methods makes it one of the attractive optionsfor treatment of textile wastewaters.

Addition of some chemicals (polyelectrolyte) enhances coagu-lation by promoting the growth of large, rapid settling of ocs.Polyelectrolytes are high-molecular-weight polymers, whichcontain absorbable groups and when small dosages of poly-electrolyte (1 mg/L to 5 mg/L) are added in conjunction withcoagulant, these are also referred as coagulant aids. The poly-electrolyte is substantially unaffected by pH variations and serve asa coagulant itself by reducing the effective charge on colloids. Itproduces a large amount of ions in water and shows properties ofboth polymers and electrolytes. The most practical benet ofpolyelectrolyte is the formation of massive ocs. These massiveocs speed up the oc settling velocity, reduce the expense ofdecolourisation and also decrease the settled sludge volume(Bidhendi et al., 2007).

Since Coagulation occulation is cost effective technology andgives excellent colour removal for wide variety of dyes, it becomespromising technology for decolourisation of textile wastewater.Sludge production can also be minimised by optimising process

parameters and suitable selection of coagulant and occulant. Dueto scarcity of landll sites, the disposal of sludge becomes moreproblematic and expensive. Therefore, recycle of sludge becomesthe only viable option. In this regard, the use of sludge as a buildingmaterial (Balasubramanian et al., 2006), a soil conditioner (Pearsonet al., 2004; Rosa et al., 2007; Islam et al., 2009) or as a fuel (Van derBruggen et al., 2005) has been studied by several researchers.

Coagulation by means of biopolymers is very effective methodfor treatment of industrial wastewater. Chitosan as a bioocculantcan be successfully applied for the removal of both particulate anddissolved substances (Renault et al., 2009). The main reasons forthe success of this biopolymer in wastewater treatment usingcoagulation/occulation are, chitosan has the advantage of beinga non-toxic material, non corrosive and safe to handle (nonhazardous product, not irritating for skin and eyes) (Bolto andGregory, 2007; Bratby, 2007). Moreover, chitosan is also efcientin cold water and at much lower concentration than metal salts.The lower concentration of polymers reduces the volume of sludge

more rapid occulation and strong ocs than that of alum atequivalent dosage. This can be attributed by the fact that thesecoagulants are pre-neutralised, have smaller effect on the pH ofwater and so reduce the need of pH correction. Most of the dyesused in textile industries are of negatively charged and hencecationic polymer is preferred over anionic and nonionic polymersdue to the better dye removal performance. However, the mecha-nisms of these products are not well established yet. Further, toconduct and evaluate the work, it is necessary to consider only themost critical controlling parameters. Various authors have sug-gested the most important parameters to be consider in coagula-tion are pH and concentration of applied metal ions (coagulant)such as alum (El-Gohary and Tawk, 2009), FeCl3 (Kim et al., 2003;Bidhendi et al., 2007), MgCl2 (Tan et al., 2000; Semerjian andAyoub, 2003; Gao et al., 2007), polyaluminium chloride (PACl)(Sanghi and Bhattacharya, 2005; Choo et al., 2007), lime (Mishraet al., 2002; Georgiou et al., 2003) and ferrous sulphate andorganic polymeric coagulants (Mishra et al., 2002; Bidhendi et al.,

ulph

lysinalts

gula

m

fe

ch

oride

m

su

A.K. Verma et al. / Journal of Environmental Management 93 (2012) 154e168 161production compared to sludge obtained with alum. In addition, asbiopolymers are biodegradable, hence, the sludge can be efcientlydegraded by microorganisms. Various studies have been reportedthat the sludge produced from the treatment of milk processingplant wastewater (Chi and Cheng, 2006) and kaolinite suspensions(Divakaran and Pillai, 2001) was non-toxic and could be used tostimulate growth in plants.

4. Chemical coagulation technologies

Chemical coagulation is a complex phenomenon involvingvarious inter-related parameters, hence it is very critical to denethat how well coagulant will function under given conditions. Onthe basis of effectiveness to decolourise the textile wastewater,chemical coagulants can be categorised in the three parts asdescribed in the following Fig. 4.

It has been reported that pre-hydrolysed metallic salts are oftenfound to be more effective than the hydrolysing metallic salts suchas aluminium sulphate (alum), ferric chloride and ferric sulphatethose are readily soluble in water (Jiang and Graham, 1998). Pre-hydrolysed coagulants such as Polyaluminium chloride (PACl),Polyaluminium ferric chloride (PAFCl), Polyferrous sulphate (PFS)and Polyferric chloride (PFCl) seem to give better colour removaleven at low temperature and may also produce lower volume ofsludge. In this connection, Gregory and Rossi (2001) have studiedthe effectiveness of various pre-hydrolysing coagulants for thetreatment of wastewater, and reported that PACl products give

Polyaluminium

Polyferric

Polyferrous s

(PAFCl)

Ferric chloride

Ferric sulphate

Magnesium chloride

Alum

Hydrolysingmetallic salts

Pre- hydrometallic s

Chemical coa

Polyaluminiu

chl

PolyaluminiuFig. 4. Categorisation of chemical coagula2007).Apart from these, mixing speed and time (Gurses et al., 2003),

temperature and retention time (Ong et al., 2005; Naimabadi et al.,2009) also inuence the colour removal efciency. Hence, theoptimisation of these factors may signicantly increase the processefciency. Different coagulants affect different degrees of destabi-lisation. The higher the valence of the counter ion, the more is itsdestabilising effect and the less is the dose needed for coagulation.If pH is below the isoelectric point of metal hydroxide whileprecipitation of colloids by different coagulants supported bysuitable polymer, then the positively charged polymers will prevailand adsorption of these positively polymers can destabilise nega-tively charged colloids by charge neutralisation. Above theisoelectric point, anionic polymers will predominate where particledestabilization may take place through adsorption and bridgeformation. At high dose of metal ions (coagulant), a sufcientdegree of oversaturation occurs to produce a rapid precipitation ofa large quantity of metal hydroxide, enmeshing the colloidalparticles which are termed as sweep oc (Peavy et al., 1985). Forexample, when Fe(III) salts are used as coagulants, monomeric andpolymeric ferric species are formed, the formation of which ishighly pH dependent (Abo-Farha, 2010). Some of the reportedchemical coagulation technology and their performance have beensummarised in Table 4.

The studies made by various researchers as described in theTable 4 show that the natural pH of ferric chloride solution is acidic.However, the effective colour removal can be achieved when the

Aminomethyl polyacrylamide

Polyalkylene

Polyamine

Polyethylenimine

Polydiallyldimethyl ammonium

chloride (poly-DADMAC)

ate (PFS)

g Synthetic cationic polymers

nts

rric chloride

loride (PACl) (PFCl)

lphate (PAS)nts according to their effectiveness.

pH is maintained near to neutral, but it again depends upon thetype of dyes to be removed (Kim et al., 2004; Guendy, 2010;Moghaddam et al., 2010). Hence, the addition of base to maintainthe pH becomes prime requirement. Lime or NaOH can be used forthis purpose. However, addition of lime may produce extra sludge.Whereas, addition of polyelectrolyte as a coagulant aid generallyimproves the performance of coagulant. It can be seen from theTable 4 that optimum pH for alum is near to neutral and hencehigher colour removal efciency can be obtained at this pH.Moreover, addition of polyelectrolyte generally improves the colourremoval performance. However, generation of large amount ofsludge associated with this process makes it unattractive.

PACl products are aluminium-based coagulants. They are similarto alum, with several important differences:

- Partially pre-neutralised (Higher basicity than alum)- Contains Cl instead of SO42

- Contains up to three times the aluminium content- Rapid aggregation velocity, bigger and heavier ocs

Moreover, PACl shows better colour removal efciency ina wider pH range of 7e10. The optimum pH for FeSO4 is alkaline inthe range 7e9 and gives higher colour removal at this pH range.Various researchers have also revealed that the addition of poly-electrolyte generally increases turbidity and volume of settledsludge. This undesired effect may be eliminated if the usedconcentration of polyelectrolyte is less than 2 mg/L (Bidhendi et al.,2007).

The optimum pH for magnesium chloride varies between 9 and

of basic efuent after treatment, may not be considered as the goodcoagulants. Though, both the ferric chloride and alum give roughlyhigh efciency, at low concentration, colour removal efciency isreported less for ferric chloride (Kim et al., 2004; Golob et al., 2005;Bidhendi et al., 2007). However, signicant improvement in thecolour removal has been reported if ferric chloride used with smallamount of cationic polymer (Suksaroj et al., 2005). Very limitedinformation is available based on the coagulant studies using PFS.PFS makes it attractive coagulant because it is practically soluble inwater, and forms large amount of polynucleic complex ions like(Fe2(OH)3)3, (Fe2(OH)2)2, (Fe8(OH)20)4, which are prone torender occulation. It is advantageous as regards the followingpoints:

- Fast settling of ocs- Broad pH compatibility- Low iron contamination- High heavy metal removal rate- Easy dehydration of sludge etc.

The principle mechanism for coagulation is similar to the PACli.e. adsorption and charge neutralisation. At high turbidity, thecoagulation may follow Sweep coagulation (Peavy et al., 1985).

Gao et al. (2001) have investigated the application of PAFCl forthe decolourisation of petrochemical wastewater and reported thatPAFCl gives better turbidity removal in 7.0e8.4 pH range and goodcolour removal for suspension dyes over the other selected coag-ulant such as PFS and PACl. In addition to the performance of bettercoagulation efciency for colour removal, it also reects the ability

lour

30)

)

A.K. Verma et al. / Journal of Environmental Management 93 (2012) 154e16816212 (Gao et al., 2007; El-Gohary and Tawk, 2009). It gives very goodcolour removal performance if used with lime. However, it gener-ates large amount of sludge which may cause sludge disposalproblem and also involves extra cost. Alum and magnesium chlo-ride, because of large amount of sludge generation and production

Table 4Effectiveness of different chemical coagulants studied by different researchers for co

Name of coagulant Optimiseddose (mg/L)

Coagulant aids (if any)

Steel industry wastewaterPotassium ferrate 100 Polyamine based polymerPolyaluminium Chloride (PACl) 10Poly-epichlorohydrin-diamine 20Alum 200 Polyacrylamide

based polymer (Cytec)Alum 5000 Copper sulphate as catalystAlum 20 Commercial cationic

occulant (Coloc-RDeCiba)Alum 7 104Ferrous Sulphate 200 PolyelectrolyteFerric chloride 400Ferric chloride 293

Ferric chloride 56 Cationic polymerMagnesium chloride 400 Polyelectrolyte (Koaret PA 32Magnesium chloride 120 LimeMagnesium chloride 800 Hydrated lime

Polyaluminium chloride (PACl) 0.1 Poly acrylamide-seed gum

Polyaluminium chloride (PACl) 800 Anionic polyacrlamide,Exeroc 204

Ferrous sulphate 400 Lime and Cationic polymerFerrous sulphate 1000 Anionic

polyelectrolyte (Henkel23500Ferrous sulphate 7 104Ferric sulphate 7 104

a Experiments were carried out at original pH of raw wastewater.to quick formation of the ocs. The ability of quick formation of theocs and superior colour removal efciency may be due to thereason that PAFCl combines the coagulatory advantages of bothaluminium and iron salts and hence able to form ocs rapidly withmore bulky and rapid sedimentation. This novel coagulant is not

removal of textile wastewater.

Type of dyespresent

OptimumpH

% Colourremoval

Reference

Disperse 4.25 99 Anouzla et al., 20096.5e8.5 95 Ciabatti et al., 20107.2 99.9 Choo et al., 20077 95 Kang et al., 20075.3 78.9 El-Gohary and Tawk, 2009

4 74 Kumar et al., 2008Reactiveand acid

Near toneutral

98 Golob et al., 2005

5.7e6.5a 74 Patel and Vashi, 2010Sulfur 9.4 90 Bidhendi et al., 2007Sulfur 8.3 100 Bidhendi et al., 2007Reactive anddisperse

6 71 Kim et al., 2004

4 92 Suksaroj et al., 2005Reactive 11 85 Tan et al., 2000

11 100 El-Gohary and Tawk, 2009Reactive anddisperse

12 98 Gao et al., 2007

Reactive,acid and direct

8.5 80 Sanghi et al., 2006

7.5 75 Tun et al., 2007

Reactive 12.5 90 Georgiou et al., 20039.5 60 Selcuk, 2005

5.7e6.5a 85 Patel and Vashi, 20105.7e6.5a 58 Patel and Vashi, 2010

al ba

coa

al co

menwidely studied for textile wastewater treatment hence very limitedinformation is available. The principle mechanism of PAFCl ischarge neutralisation and bridging (Chen et al., 2010). Recently,Ciabatti et al. (2010) have studied the use of potassium ferrate incombination of polyamine based polymer for treatment of dyeingefuents and found excellent colour removal as well as CODreduction. Since ferrate (VI) ion is a strong oxidant in entire the pHrange, hence after reduction to Fe(III) ion or ferric hydroxide duringoxidation process, it possesses the ability to act as coagulant. Hencepotassium ferrate represents a unique dual function (Oxidant andcoagulant) chemical reagent that can be an effective alternative tocurrent approaches for water and wastewater treatment.

5. Coagulation with the help of natural coagulants

Chemical coagulation with the help of above discussed coag-

AnimPlant based

Natural

Guar gum

Gum Arabic

Seed extract from

Strychnos potatorum, Moringa Oilifeira etc.

Cactus latifaria extract Potato starch

Chitosan

Fig. 5. Categorisation of natur

A.K. Verma et al. / Journal of Environulants may be the method of choice for decolourisation of textilewastewater before being fed to the biological treatment, ifnecessary. However, it has also some drawbacks as the efciencyof the treatment strongly depends on pH. Moreover, the coagu-lation process is not always efcient enough because at differentenvironmental conditions such as at extreme pH and at very lowor very high temperature, it may produce very sensitive, fragileocs, which result in poor sedimentation. These ocs may ruptureunder any type of physical forces. To improve the efciency ofcoagulation process, number of high molecular weightcompounds such as polymers from synthetic or natural originmay be recommended. These polymers can function as coagulantitself or in the form of coagulant aids/bioocculants, dependingupon the wastewater and polymer characteristics. These polymersare normally macro-molecular structure with variety of functionalgroups which can either work as coagulants by destabilising thecharged stable particles mainly through the process of adsorptionand neutralisation or can work as coagulant aids by attaching thedestabilised particles with the functional groups by interparticlebridging. Here organic polymeric compounds are advantageousover inorganic materials, which posses several novel characteris-tics such as their ability to produce large, dense, compact ocsthat are stronger and have good settling characteristics (Renaultet al., 2009). In contrast to some traditionally used coagulantsuch as alum, organic polymers are benecial because of thelower coagulant dosage requirement, efciency at low tempera-ture and produce small volume of sludge whereas inorganicpolymers and chemical coagulants generally involve higher cost,less biodegradability and toxicity. For example, acrylamide is verymuch toxic and gives severe neurotoxic effects (Bratby, 2007).Toxicity effect due to cationic polymers to the plants has beenestablished long back (Gao et al., 2001). In this connection, Boltoand Gregory (2007) also reported that anionic and nonionicpolymers are generally less toxic as compared to cationic poly-mers especially to aquatic organisms. The major advantage ofnatural polymer is its non-toxicity to the environment andbiodegradability. Therefore, the efuent after natural polymertreatment can be treated by biological means, if required. Thisefuent will not pose any harm to the biological organisms, as isoffered, if it is treated by means of synthetic coagulants. Not onlythis, the sludge generated by the natural polymers can further be

Micro-organism basedsed

gulants

Xanthan gum

agulants with their examples.

tal Management 93 (2012) 154e168 163treated biologically or can be disposed off safely as soil condi-tioners because of their non-toxicity. Hence, there is an urgentneed to establish the use of natural low cost polymers for textilewastewater treatment.

In the view of this, many researchers have studied the effec-tiveness of various natural coagulants (Joshi and Nanoti, 1999)extracted from plants or animals (Christman, 1967) for the treat-ment of textile wastewater. These natural coagulants may alsoprove their effectiveness if used as coagulant aids along with thechemical coagulants. Most of the natural coagulants fall under thecategory of polysaccharides, hence also termed as polymericcoagulants. On the basis of the origin of production, natural coag-ulant can be divided in to three categories as shown in Fig. 5. Unlikesynthetic coagulants, natural coagulants generally exhibit twotypes of mechanism namely i) adsorption and charge neutralisa-tion, and ii) adsorption and interparticle bridging. As their molec-ular weight is high and contain long chained structure, thereforeoffers a large number of available adsorption sites. Adsorption andcharge neutralisation refers to the sorption of two oppositelycharged ions, while interparticle bridging occurs when poly-saccharide chain of coagulant sorbs the particulates (Miller et al.,2008). The existence of adsorption and interparticle bridgingbetween dye molecules and polysaccharides is due to the interac-tion of p- electron system of dyes and OH group of poly-saccharides (Fig. 6), which was rst suggested by Yoshida et al.

established. Although the mechanism of coagulation with natural

experimental results showed that, carboxymethyl chitosan inwastewater decolourisation and COD reduction, are superior overother commonly used polymer occulants. Szygula et al. (2009)reported approximately 99% colour removal from the simulatedtextile wastewater containing Acid Blue 92 at an optimum chitosandosage of 100 mg/L maintaining optimum pH of 9. In continuationof this, Mahmoodi et al. (2011) investigated the effectiveness of

mencoagulants has not been extensively investigated but the presenceof hydroxyl groups along the polysaccharide chain provides a large(1964) and then reviewed by Blackburn and Burkinshaw (2002)and Yin (2010).

5.1. Plant based polymers as coagulants

Various plant extracted polymers such as starch, guar gam, gumarabic, nirmali seeds, tannin, Moringa oleifera and cactus etc. aregenerally well known as coagulants within the scientic commu-nity. These polymers have large number of industrial application asthese are polysaccharides and possess various commercial appli-cations such as in paper industry, as food additives etc. By virtue ofthe effectiveness of natural polymer as coagulant, Sanghi et al.(2006) have investigated the use of Ipomeoa dasysperma seedgum and guar gum as coagulant aids alongwith PACl and found 86%and 87% removal of acid dye at the PACl dosage of 1 mg/L andI. dasysperma seed gum and guar gum dosage of 5 mg/L each, atoptimum pH of 9.5. Signicant removals of the order 73% and 80%were also been reported for direct dye at the same dosage ofcoagulant and coagulant aid and at the same pH of 9.5.

Adinol et al. (1994) have reported that polysaccharide extrac-ted from Strychnos potatorum (Nirmali) seeds can effectively reduceupto 80% turbidity of kaolin solution. M. oleifera, known as drum-stick tree is widely found throughout India, Asia, some parts ofAfrica and America. The trees bark, root, fruit, owers, leaves seedsand gum are also used as medicines. The seed of these trees is alsoused as coagulant and/or occulants in the water and wastewatertreatment. Beltrn-Heredia et al. (2009) have investigated the useofM. oleifera seed extract for the removal of anthraquinone dye andreported 95% dye removal at the coagulant dose of 100 mg/L and atpH 7. Further, Lea (2010) has investigated the effectiveness of M.oleifera seed extract for the treatment of turbid water and found99.5% turbidity removal at the dosage of 400 mg/L. Typically,increased dosage of seed extract does not enhance the dye removalafter maximum adsorption is reached. It might be due to the factthat no more new sites for adsorption remain available at thesurface of seed extract. M. oleifera seeds are also considered as anexcellent biofuel source for making biodiesel.

Gum Arabic, also known as Gum Acacia is highly branched withbeta-Galactose backbone having high molecular weight of250,000e750,000 Da, water and fat soluble polysaccharide. Thecoagulation studies with this novel natural coagulant are yet to be

Fig. 6. Schematic representation of intermolecular interaction between p- electronfrom dye molecule and hydroxyl group of polysaccharide (Yoshida et al., 1964).

A.K. Verma et al. / Journal of Environ164number of available adsorption sites that might lead to the inter-particle bridging between polysaccharide and dye molecule asshown in Fig. 7.

Paulino et al. (2006) studied the removal of methylene blue withthe help of hydrogel formed by modied Gum Arabic, polyacrylateand polyacrylamide and reported that 98% of dye removal can beachieved at pH 8 with maximum adsorption capacity of 48 mg ofthe dye per gram of hydrogel. However, use of gum arabic and guargum for colour removal due to the widely used dyes in the textileindustries are yet to be established. In connection with applicationof natural coagulants, various researchers (Kumar, 2000; No andMeyers, 2000; Kurita, 2006; Renault et al., 2009) have studied theeffectiveness of animal extracted polymer as coagulants forindustrial wastewater treatment.

5.2. Animal based polymers as coagulants

Chitosan is a linear copolymer of D-glucosamine (deacetylatedunit) and N-acetyl-D-glucosamine (acetylated unit) produced bythe deacetylation of chitin, a natural polymer of major importance(Roberts, 1992; Kurita, 2006). The degree of deacetylation can bedetermined by NMR spectroscopy. Chitin is the structural elementin the exoskeleton of crustaceans (crabs, shrimps etc.) and in theendoskeleton of other invertebrate.

Chitosan possesses several intrinsic properties such as non-toxicity, its biodegradability and its outstanding chelationbehavior that make it an effective coagulant and/or occulant forremoval of contaminant in the dissolved state. Various studies fortreatment of industrial wastewater using chitosan have beencarried out during late 70s by Bough and coworkers (Bough, 1975,1976; Bough et al., 1978). They have investigated the effectivenessof chitosan for coagulation and recovery of suspended solids (SS) inprocessing of waste from variety of food processing industries andfound that this novel coagulant is very much effective for efcientreduction of COD as well as removal of SS and turbidity. Numerousworks claim that chitosan involved in a dual mechanism includingcoagulation by charge neutralisation and occulation by bridgingmechanism (No and Meyers, 2000; Guibal and Roussy, 2007). Thepossible interactions between dye molecules and chitosan havebeen shown in Fig. 8. Zhang et al. (1995) have used carboxymethylchitosan for printing and dyeing wastewater treatment. The

Fig. 7. Schematic representation of the interaction of dye molecule with (a) Guar Gumand (b) Gum Arabic.

tal Management 93 (2012) 154e168chitosan for removal of Acid Green 25 and Direct Red 23 and re-ported approximately 75% and 95% dye removal respectively in10 min at optimum pH 2maintaining the stirring speed of 200 rpm.

5.3. Microorganism based polymer as coagulant

Xanthan gum is a polysaccharide, derived from the bacterialcoat of Xanthomonas campestris, used as food additive and rheologymodier (Davidson, 1980). It is produced by the fermentation ofglucose by the X. campestris bacterium. After fermentation, thepolysaccharide is separated from the growthmediumwith the help

of solvent separation technique, dried and ground into a nepowder (Cohan, 2010). The use of xanthan gum for the treatment of

Fig. 8. Schematic representation of the formation of chitosan from

A.K. Verma et al. / Journal of Environmentextile wastewater is not been reported in the literature yet. Due tocomplex structure and higher molecular weight of xanthan gum(250,000e750,000 Da), as compared to guar gum (about220,000e250,000 Da), it may also be considered as one of thepromising coagulant and/or coagulant aids for the treatment oftextile wastewater. Hence, extensive study is required to be con-ducted to establish the various facts about effectiveness of xanthangum for the treatment of textile wastewater. The possible mecha-nism of coagulation by interparticle bridging as observed for guargum can also be observed for xanthan gum (Fig. 9).

It can be summarised from the above discussions that plantextracted coagulant may be encouraged over animal extractedcoagulant for the treatment of textile wastewater due to the factthat non plant sources possess limited potential for the massproduction as compared to the plant sources. Extra involvement ofFig. 9. Schematic representation of possible interaction between xanthan gum and dyemolecule.ourisation of the waste stream due to the removal of dye moleculesfrom the dyebath efuents, and not due to a partial decompositionof dyes, which can lead to an even more potentially harmful andtoxic aromatic compound. The major disadvantage of coagulation/occulation processes is the production of sludge. However, thesludge amount could be minimised if only a low volume of thehighly coloured dyebath could be eliminated by chemical treat-ment directly after the dyeing process (Golob et al., 2005).

6. Future scope of research

Very limitedwork has been carried out on the decolourisation oftextile wastewater containing multiple dyes of different classesalong with the various chemical additives which are used duringcost in the processing of microorganism based coagulant may notbe an attractive option. Application of plant based coagulants willbecome more attractive if the coagulants producing plants areindigenous.

The main advantage of coagulation and occulation is decol-

chitin polysaccharide and its interaction with dye molecules.tal Management 93 (2012) 154e168 165textile processing. Also the effectiveness of most of the pre-hydrolysed coagulants for decolourisation of textile wastewatercontaining multiple dyes is yet to be established. Considering theindustries dependencies on the cost effective chemical coagulationand occulation technologies for their coloured wastewater treat-ment, it is required to conduct more and more future research tocome up with best coagulants or combinations of coagulants alongwith coagulant aids which can produce very promising results evenat a wider variations of pH and other interfering agents of thetextile wastewaters. Investigation of the effectiveness of morenumber of natural coagulants is also need to be assessed. Thisassessment may be carried out using natural polymers as coagulantaids as well as coagulant itself. Effectiveness of natural coagulantsalso required to be carried out against simulated as well as rawtextile wastewater.

7. Conclusion

All decolourisation methods described in this review have someadvantages as well as some drawbacks, and their selection willmostly governed by the textile wastewater characteristics like classand concentration of dyes, pH, organic contents, heavy metals, etc.Among different physical, chemical, biological, and advanced

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Al-Malack, M.H., Abuzaid, N.S., El-Mubarak, A.H., 1999. Coagulation of polymericwastewater discharged by a chemical factory. Water Research 33, 521e529.

Alaton, I.A., Balcioglu, I.A., Bahnemann, D.W., 2002. Advanced oxidation of a reactivedye bath efuent: comparison of O3, H2O2/UV-C and TiO2/UV-A processes.Water Research 36, 1143e1154.

Anjaneyulu, Y., Chary, N.S., Raj, D.S.S., 2005. Decolourization of industrial efuents-available methods and emerging technologies-a review. Reviews in Environ-mental Science and Biotechnology 4, 245e273.

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Anouzla, A., Abrouki, Y., Souabi, S., Sa, M., Rhbal, H., 2009. Colour and COD removalof disperse dye solution by a novel coagulant: application of statistical designfor the optimization and regression analysis. Journal of Hazardous Materials166 (2e3), 1302e1306.

Arslan, I., Balcioglu, A., 2001. Degradation of Remazol Black B dye and its simulateddyebath wastewater by advancedoxidation processes in heterogenous andhomogeneous media. Coloration Technology 117, 38e42.

Arslan-Alaton, I., 2003. A review of the effects of dye-assisting chemicals onadvanced oxidation of reactive dyes in wastewater. Coloration Technology 119,345e353.advanced oxidation processes: fenton, fenton-like, photo-fenton and photo-Fenton-Like. Journal of American Science 6 (10), 128e142.

Aboulhassan, M.A., Souabi, S., Yaacoubi, A., Baudu, M., 2006. Improvement of paintefuents coagulation using natural and synthetic coagulant aids. Journal ofHazardous Materials B138, 40e45.

Adewuyi, Y.G., 2001. Sonochemistry: environmental science and engineeringapplications. Industrial & Engineering Chemistry Research 40, 4681e4715.

Adinol, M., Corsaro, M.M., Lanzetta, R., Parrilli, M., Folkard, G., Grant, W.,Sutherland, J., 1994. Composition of the coagulant polysaccharide fraction fromStrychnos potatorum seeds. Carbohydrate Research 263, 103e110.chemical oxidation technologies, chemical coagulation and oc-culation is still a cost-comparative alternative for the treatment ofindustrial textile wastewaters and is widely practiced by the smallto large scale industries. Among chemical coagulation and occu-lation technologies, comparatively, pre-hydrolysed coagulants suchas PACl, PFCl, PFS and PAFCl may be considered as the bettercoagulants because of their superior colour removal even at smalldosage and affectivity at wider pH range of wastewater. Ferroussulfate may also be considered as a better coagulant over otherhydrolysing metallic salts. Additionally, due to some novel prop-erties of natural coagulants such as non-toxic, biodegradability,environment friendly, ability to encapsulate etc., these may also beconsidered as the promising coagulants as well as coagulant aidsfor textile wastewater treatment specially at rst stage, which willnot hinder the biological treatment (if required) because theresidual coagulant may act as nutrient for the microorganisms.However, till date the applicability of these natural coagulants forthe textile wastewater is very limited. More and more studiesrequired to evaluate their application for colour removal of textilewastewaters particularly, their behaviour at high pH of textilewastewaters.

Acknowledgement

The authors wish to thank all the reviewers for their valuablesuggestions for improving the quality of the manuscript andDepartment of Civil Engineering, School of Infrastructure, IndianInstitute of Technology Bhubaneswar, India, for providing facilitiesfor carrying out research work in the related area.

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