treatment and reuse of wastewater from the textile wet-processing industry review of emerging...

J. Chem. T echnol. Biotechnol. 1998, 72, 289302

Review

Treatment and Reuse of Wastewater from theTextile Wet-Processing Industry : Review ofEmerging TechnologiesPhilippe C. Vandevivere,1 Roberto Bianchi2 & Willy Verstraete1*1 Laboratory of Microbial Ecology, Coupure L 653, University of Gent, 9000 Gent, Belgium2 Centro Imprese Depurazione Acque s.r.1. (CIDA), Via Laghetto 1, 22073 Fino Mornasco (Co), Italy(Received 28 July 1997 ; revised version received 10 December 1997 ; accepted 6 April 1998)

Abstract : New ecolabels for textile products and tighter restrictions on waste-water discharges are forcing textile wet processors to reuse process water andchemicals. This challenge has prompted intensive research in new advancedtreatment technologies, some of which currently making their way to full-scaleinstallations. These comprise polishing treatments such as ltration, chemicaloxidation and specialized occulation techniques and pre-treatment steps includ-ing anaerobic digestion, xed-lm bioreactors, Fentons reagent oxidation, elec-trolysis, or foam otation. Though several of these new technologies arepromising in terms of cost and performance, they all suer limitations whichrequire further research and/or need broader validation. A segment of theresearch deals with the separate handling of specic sub-streams such as dyebatheffluents to which membrane ltration is sometimes applied. The main limitationof this approach is the treatment of the concentrate stream. The spectrum ofavailable technologies may, in the future, be further broadened to include

oxidation, specialized bio-sorptive processes, solvent extrac-fungi/H2O2-drivention, or photocatalysis. 1998 SCI(

J. Chem. T echnol. Biotechnol. 72, 289302 (1998)

Key words : textile ; wastewater ; dyes ; azo ; full-scale ; activated sludge ; ltration ;coagulation ; ozonation

NOTATION

AOX Adsorbable organic halogensBOD Biological oxygen demandBv Volumetric loading rateCOD Chemical oxygen demandHRT Hydraulic retention timeIC

xConcentration causing x% inhibition

LCx

Concentration causing x% lethalityMLVSS Mixed liquor volatile SSNF NanoltrationNPE Nonyl phenol ethoxylate

* To whom correspondence should be addressed.E-mail address : Willy.Verstraete=rug.ac.beContract/grant sponsor : European Commission ; Contract/grant number : ENV4-CT95-0064.

RO Reverse osmosisSRT Sludge retention timeSS Suspended solidsTS Total solidsTSS Total suspended solidsUASB Upow anaerobic sludge blanketUF UltraltrationWWTP Wastewater treatment plant

1 INTRODUCTION

Considering both volume discharged and effluent com-position, the wastewater generated by the textileindustry is rated as the most polluting among all indus-trial sectors.1 In Flanders in 1994, four textile wet pro-cessors were among the six factories ranked with the

2891998 SCI. J. Chem. T echnol. Biotechnol. 0268-2575/98/$17.50. Printed in Great Britain(

290 P. C. V andevivere, R. Bianchi, W . V erstraete

largest annual BOD discharges.2 During the last fewyears, new and/or tighter regulations coupled withincreased enforcement concerning wastewater dis-charges have been established in many countries. Thisnew legislation, in conjunction with international tradepressures such as increasing competition and the intro-duction of ecolabels for textile products on the Euro-pean and US markets, is threatening the very survivalof the textile industry in many industrialized countries.3The textile industry swiftly responded to these new con-straints with a broad range of drastic changes and inno-vations in the generation, treatment, and reuse of textilewastewaters (Fig. 1). This review article is an attempt tosummarize and compare the diverse innovative treat-ment technologies which have been used to treat andreuse textile effluents. The focus is placed on full-scaleplants in order to be able to assess the new technologiesin real situations.

2 THE NATURE OF TEXTILE EFFLUENTS

The main sources of wastewater generated by the textilewet-processing industry originate from the washing (orscouring) and bleaching of natural bers and from thedyeing and nishing steps. Given the great variety ofbers, dyes, process aids and nishing products in use,these processes generate wastewaters of great chemicalcomplexity and diversity which are not adequately

treated in conventional WWTP. The chemical composi-tion of textile mill effluents is also changing rapidly as aresult of shifting consumers preferences. Most signi-cant is the current popularity of cotton fabrics andbright colors leading to greater usage of reactive andazo dyes,4 respectively. An even more important causeof shifting wastewater composition are the new andtighter restrictions on discharged effluents and con-sumer goods5 as, for example, the 1996 German ban onconsumer goods containing those azo dyes which cleaveto yield any one of 20 specied carcinogenic aromaticamines.1,6 Another example is the emergence of US andEuropean ecolabels which might include considerationsof emissions to the environment and chemicals usedduring the manufacture of textile products.7 In responseto these new restrictions, dye manufacturers aredeveloping new lines of dyeing auxiliaries in order tocomply with these ecologically-advanced labels, e.g.emulsiers not derived from alkyl phenols, pigments notcontaining halogeno substituents or heavy metals,chlorine-free bleaching agents, and synthetic thickeningagents which reduce the amount of dye wasted.8h10

The nature of textile wastewaters has already beenreviewed in terms of process chemicals used and interms of the classical parameters (BOD, COD, TSS, TS)and contents of N, P, and heavy metals.11h13 Thissection will therefore focus on those pollutants whichoften pose problems in conventional WWTP (as illus-

Fig. 1. Compilation of possible process technologies, both commercial and experimental, for the treatment and reuse of naleffluents of textile mills. 1 : Alto Lura WWTP managed by CIDA Srl in the Como area (Italy) and treating 22 000 m3 day~1 of amixture consisting of 17% domestic wastewater, 32% rain water and 51% equalized effluents from textile nishing factories

(cellulose, wool, silk, and synthetic ber). 2 : Levis Strauss WWTP, Wervik (Belgium).

T reatment and reuse of wastewater 291

trated in the following section), i.e. dyes, recalcitrantorganics, toxicants, AOX, and surfactants.

2.1 Color

Dye molecules consist of a chromagen, i.e. an aromaticstructure absorbing visible light and which anchors thedye into or within the bers. There are about 12 classesof chromogenic groups, the most common being the azotype which makes up to 6070% of all textile dyestusproduced,14 followed by the anthraquinone type.13 Asecond classication of dyes is based on their mode ofapplication to textiles and distinguishes acid, reactive,metal-complex, disperse, vat, mordant, direct, basic andsulfur dyes. Research on textile effluent decolorizationhas often focused on reactive dyes for three reasons.First, reactive dyes represent an increasing marketshare, currently about 2030% of the total market fordyes, because they are used to dye cotton which makesup about half of the worlds ber consumption.5,15Second, a large fraction, typically around 30%, of theapplied reactive dyes is wasted because of dye hydro-lysis in the alkaline dyebath. As a result, dyehouse efflu-ents typically contain 0608 g dye dm~3.8,16,17 Third,conventional wastewater treatment plants, which relyon sorption and aerobic biodegradation, have a lowremoval efficiency for reactive and other anionic solubledyes, which leads to colored waterways, and publiccomplaints. As a result, new restrictions have beenestablished for the discharge of colored effluents, e.g. inGermany8 and in the UK,5 often forcing dyehouses todecolorize their effluents on-site.

Since dyes are intentionally designed to resist degra-dation, it is no surprise that little dye degradationoccurs in activated sludge systems.18h20 Among morethan 100 azo dyes tested, only a very few were degradedaerobically.21 The degree of stability of azo dyes underaerobic conditions is proportional to the structuralcomplexity of the molecule.22 While C.I. Acid Orange 7(Fig. 2) is aerobically biodegradable, the 2@-methylderivative (C.I. Acid Orange 8) is less biodegradable, arelated disulfonate dye (Acid Orange 10) is non-biodegradable. Under anoxic conditions, however, azodyes are readily cleaved to aromatic amines since theazo bond can serve as electron acceptor in the electrontransport chain provided a carbon source is availableand nitrates absent.14,23h26 The by-products of the azobond cleavage, aromatic amines, are not further metab-olized under anaerobic conditions but are readily bio-degraded in an aerobic environment.21 It is importantto note that most types of dyes are partially degradedunder anaerobic conditions, though less readily thanazo dyes.25,27,28 The rate of degradation may belowered by the dyes structural complexity, as celluptake was shown to be inversely proportional to thenumber of sulfonate groups,14,25 and moreover is sub-

Fig. 2. Chemical structures of 1-phenylazo-2-naphthol dyes.C. I. Acid Orange 7 is one of the rare dyes which is aerobically

biodegradable.

jected to inhibition in the presence of 15100 mg dm~3of certain acid azo dyes.14,29

2.2 Persistent organics

Due to a shift in process chemicals used, the biodegrad-ability of textile wastewaters has been increasing stead-ily during recent years. In Flanders, for example, the

ratio for the total wastewater dischargedBOD5/CODfrom textile wet processors increased from 018 in 1991to 029 in 1994.30 The latter value, which is representa-tive for the sector in other countries,31 points toward amoderately biodegradable organic load. The persistentmolecules present in textile effluents belong to verydiverse chemical classes, each used in relatively smallamounts. Aside from dye molecules (see above), theseinclude dyeing auxiliaries such as polyacrylates, phos-phonates, sequestering agents (e.g. EDTA), deoccula-tion agents (lignin or naphthalenesulfonates), antistaticagents for synthetic bers, carriers in disperse dyeing ofpolyester, xing agents in direct dyeing of cotton, pre-servatives (substituted phenol), and a large number ofnishing auxiliaries used for re-, moth-, and water-proong.4,11,32 EDTA may amount of 1 g dm~3 inreactive dyebath effluent.33 A particular case of persist-ence is the wash (scour) effluent of raw wool, consistingof a stable emulsion of detergents (08 g dm~3 nonylphenol ethoxylate), animal grease (15 g dm~3 lanolin)and suint salts (animal secretions) together with the


liposoluble pesticides sprayed on the sheep for parasitecontrol. Scouring of cotton also releases a great varietyof persistent pesticides used in the growing of cotton.Finally, carpet factory effluents may contain latex whichappears moderately biodegradable.

2.3 Toxicants

Textile effluents tend to inhibit only slightly, or not atall, the heterotrophic activity within activated sludges.This was illustrated by on-line toxicity measurements ofthe equalized effluent of a carpet factory in Flandersusing the Rodtox sensor.34 During the 2-month-longdata acquisition period, as part of the project nancingthis paper, the highest inhibition of the heterotrophicrespiration within sludge samples reached 1020%. A1020% decrease of the COD removal capacity wasalso detected upon addition of various acid azo dyes tolaboratory-scale activated sludge reactors35 or biolmreactors.22 Less biolm accumulation was also observedin the presence of the dyes.

Unlike heterotrophs, chemoautotrophic nitrifyingbacteria are often substantially inhibited in activatedsludges fed with textile effluents. Bo hm36 measuredoxygen uptake rate by nitriers in a xed-bed reactorupon feeding with wastewater from two textile plants.The values were 11 and 27% (v/v). VandevivereIC25and Verstraete (unpublished), using a short-term nitri-cation inhibition test, determined values in theIC50range of 1040% (v/v) for the equalized effluents fromtwo Flemish carpet factories. The inhibition was attrib-uted mainly to copper chelators because addition ofcopper to the wastewater eliminated the inhibition.Gru ttner et al.37 singled out a textile dyeing factory asthe most important source of nitrication inhibitors inan industrial catchment area in Denmark.

Numerous large-scale studies have been conducted toassess the toxicity of textile effluents toward aquatic life.Typical (96 h) values amount to 56% (v/v) withLC50bleaching, dyeing, or mixed textile effluents, using afreshwater sh as test organism.38 Costan et al.39 foundthat a textile effluent ranked second in toxicity, amongeight industrial sectors represented, by using a series ofbioassays assessing the acute, sublethal and chronic tox-icity at various trophic levels. Using the Microtox assay,Unde n40 ranked textile industries as heavy pollutersamong 49 Swedish industries tested. The actual processchemicals causing such harm remain unidentied.Unlawful discharge of organic solvents used in printingprocesses has at times been linked to effluent toxicitytoward activated sludge (Bianchi, unpublished) oraquatic life.3 Extensive work has been dedicated to thepotential toxicity of commercial dyes toward aquaticlife. Since the exposure level in rivers is always muchlower than the values of the vast majority of dyes,LC50these are not thought to cause acute toxicity to aquatic

life.41,42 Data at chronic-level exposure are lacking formost of the commercial dyes and their derivatives17,43and the potential toxicity of degradation intermediatesremains therefore undocumented. Numerous dyes,together with the aromatic amines formed by reductivecleavage of azo bonds, are mutagenic.44

2.4 Surfactants

Most textile wet processes, such as sizing, spinning,weaving, desizing, dyeing, scouring and washingconsume large amounts of surfactants. Wool scour efflu-ents may contain up to 800 mg dm~3 NPE.45 Com-bined municipal and textile effluents in the Como areacontain peak concentrations of 11 and 67 mg dm~3anionic and non-ionic surfactants respectively (Bianchi,unpublished) and the effluent of a silk and Lycra dyeingfactory contained typically 3040 mg dm~3 anionic sur-factants.46 A large fraction of the non-ionic surfactantsused in textile processes are alkyl phenol ethoxylates asthe textile industry is the largest consumer of these sur-factants.47 The issue is rather controversial in view ofthe fact that the discharge of alkyl phenol surfactants tosewers is often restricted, e.g. 05 mg dm~3 of phenolequivalents in Portugal, or even banned, e.g. inGermany.8 These restrictions were established becausealkyl phenol polyethoxylate surfactants are biodegradedto alkyl phenols which tend to sorb onto and accumu-late in sewage sludge wherein average concentrations upto 1000 ppm have been recorded.48,49 Alkyl phenolsare, however, much more toxic than the ethoxylatedforms, with maximum accepted concentration in thelow ppb range.47 The discharge limit for other sur-factants (in natural waters) is typically set at2 mg dm~3 (Italy, Portugal).

2.5 AOX and heavy metals

Traditionally, sodium hypochlorite was usually pre-ferred to for the bleaching of cotton and linenH2O2because of superior whiteness, lower cost and possibleber damage by radicals arising from decomposi-H2O2tion.50 Hypochlorite bleaching effluents contain up to100 mg dm~3 AOX including considerable amounts ofcarcinogenic chloroform.9 Hypochlorite is being super-seded by alkaline peroxide bleaching. Spent liquorsfrom shrinkproong with chlorine and mothproong ofwool may also contain AOX, up to 39 and12 mg dm~3, respectively.50 Finally, certain reactivedyes are AOX. Given their carcinogenic nature, the dis-charge of AOX is restricted in a number of countriesincluding Belgium, Sweden, and Germany, with a dis-charge limit of 05 mg dm~3 in the latter country.9,50Gru ttner et al.37 measured an average of 075 mg dm~3AOX in the effluent of a textile dyeing factory.


Heavy metals concentrations in dyebath effluents,typically in the 110 mg dm~3 range, were reviewed byCorreia et al.11 The use and discharge of heavy metalsin the textile wet processing industry has decreased byan average of 50% in Flanders during the period 19911994.30 In 1994, textile wastewaters in Flanders stilltypically contained about 015 mg dm~3 of Cr and Cu(used as metal-complex dyes) and 08 mg Zn dm~3.30Carpet factory effluents have higher concentrations ofCr and Ni (0609 mg dm~3) and Zn (19 mg dm~3).

3 PERFORMANCE OF FULL-SCALETREATMENT PLANTS

3.1 Full-scale plants relying solely on conventionalactivated sludge systems

The performance of activated sludge treatment is sum-marized in Table 1, together with other treatment tech-nologies. In 1994, 27% of the wastewater generated bythe textile mills located in Flanders was treated on-siteand most of it subsequently discharged to surfacewaters.30 Average effluent COD and BOD were 163and 19 mg dm~3, respectively, while total N and Pamounted to 22 and 1 mg dm~3, respectively. Suchvalues exceed the tighter discharge norms established

recently in many countries, with maximum permittedlevels of COD and total N typically around 80 and10 mg dm~3, respectively. Because the above per-formance data apply to both conventional andadvanced treatment, they probably overestimate theperformance of conventional activated sludge systems.Other reports mention a 6080% COD reduction inactivated sludge plants31,51 or laboratory-scale reac-tors.52 Considering a typical COD content of700 mg dm~3 for textile effluents, these values corre-spond to c. 200 mg COD dm~3 effluent. The per-formance of an activated sludge plant treating theeffluent of a carpet factory in Flanders was followed for4 months. Average effluent COD usually fell in therange 150220 mg dm~3 but peak values above250 mg dm~3 were frequently observed (Fig. 3 ; P. VanMeenen and B. Vanderhaegen, private communication).

Conventional activated sludge systems are clearlyineective in decolorizing textile effluents even in caseswhere these are mixed and treated together with sewage.At the Pixton works (UK), the discharged effluent hasan absorbance three times higher than the new dis-charge consent (though it should be noted that some ofthe new discharge consents in the UK specify extremelysmall absorbance values).53 This leads to colored water-courses at the points of discharge and public com-plaints.3,5,54 Primary settling removes relatively high

TABLE 1Evaluation of Various Technologies for the Treatment of Textile Effluents

Process Stage Status Performance L imitations

Fenton Pre-treatment Several full-scale Full decolorization ; lowoxidation plants in S. Africa capital and running costs

Electro Pre-treatment Pilot-scale Full decolorization ; cheap Foaming and electrodelysis lifespan

Foam Pre-treatment Laboratory-scale Removes 90% color andotation 40% COD; cheap, compact

Filtration Main- or Extensive use in High performance ; reuse of Handling and disposalpost-treatment S. Africa water, salts, and heat of concentrate stream

BiodegradationActivated Main treatment Widely used Removes bulk COD, N High residual COD, N,sludge color, surfactants.

Sequential anaerobic Main treatment Very few reports Better removal of COD, High residual coloraerobic color, and toxicants and COD

Fixed-bed Main treatment Some pilot trials in Better removal of COD,China color

Fungi/H2O2 Main treatment Laboratory-scale Full-decolorization

Coagulation Pre-, main or Extensive use Full decolorization ; water Not always eective ;occulation post-treatment reuse sludge disposal

O3 Post-treatment Full-scale Full decolorization ; water Expensive ; aldehydesreuse formed

Sorption Pre- or post- Laboratory or full-scale, New sorbents are eective High disposal or(carbon, treatment depending on and cheap ; water reuse regeneration costsclay, biomas) sorbent type

Photocatalysis Post-treatment Pilot-scale Near-complete color Only as nal polishingremoval ; detoxication step


Fig. 3. Time courses of the COD and ammonium concentra-tions (daily averages) in the nal effluent of a conventionalactivated sludge plant treating the wastewater of a carpet

factory in Flanders, Belgium.

proportions of insoluble disperse and vat dyes, whileactivated sludge removes medium to high proportionsof soluble basic and direct dyes, principally by adsorp-tion. On the other hand, the widely-used reactive andacid dyes are poorly removed.4,18,19,20,54,55

Aside from COD and color removal, nitrication isoccasionally impaired in sewage works receivinguntreated textile effluents.37,40 At the Alto Lura WWTP(Italy), which treats a mixture of domestic and textilemills wastewaters, the content in the nal efflu-NH4`-Nent exceeded the discharge limit (118 mg dm~3) on 47days out of the 217 days monitored (Bianchi,unpublished). Similar problems were noted in a WWTPtreating carpet effluents in Flanders, with levelNH4`-Namounting to 92 ^ 97 mg dm~3 in the treated effluent(Fig. 3). Bortone et al.52 found that long SRT (up to3040 days) were necessary to overcome the nitrica-tion inhibitory eect in bench-scale reactors treating amixture of municipal and textile effluents. With respectto heavy metals, conventional WWTP designs do notachieve the discharge limits for surface waters, as Crand Ni still amount to about 01 mg dm~3 and Zn to07 mg dm~3 in the nal effluent of the industrial treat-ment plants in Flanders.30

Another problem with a seemingly high incidence inWWTP treating textile wastes are Nocardia foamingand lamentous bulking.47,51,55h57 While these arecomplex and poorly-understood phenomena, their fre-quent occurrence in WWTP receiving textile effluentsmay possibly be linked to high levels of starch and sur-factants. NPE, a class of surfactants extensively used inthe textile industry, together with their degradationintermediate nonyl phenol, are readily removed in acti-vated sludge systems, with a removal efficiency lyingbetween 925 and 998%.47 In a survey encompassing alarge number of sewage works, treated effluents con-tained up to 261 ppb NPE and up to 94 ppb nonylphenol.47,58 These low values do not necessarily reect

near-complete degradation since it was estimated that6065% of all NPE entering sewage works are in factremoved with the excess sludge.

3.2 Full-scale plants using sequential anaerobic/aerobicreactors or xed lm reactors

Several laboratory-scale investigations have illustratedthe potential of sequential anaerobic/aerobic bio-treatment steps for textile wastewaters. Anaerobic pre-treatment oers several potential advantages such asbetter removal of color, AOX, and heavy metals.Improved heavy metal removal may follow sulde pro-duction,46 while the improvement of the color andAOX removal stems from the rapid reduction andcleavage under anaerobic conditions of the azo groupsin arylazo pollutants and of electron-withdrawingchloro or nitro substituents.21,59 Jianrong et al.60achieved 90% COD reduction and 96% color reductionin a laboratory-scale UASB reactor (HRT\ 8 h) fol-lowed by an activated sludge reactor (HRT\ 6 h) fedwith a deeply-colored high-strength effluent of a dyemanufacturing plant. The greatest fraction of the colorand COD reduction occurred in the UASB reactor.Other studies have shown, however, that meth-anogenesis, and hence COD removal, is easily inhibitedby textile effluents.61 The inhibition of methanogenesiscan be avoided by inserting granular activated carbonin UASB reactors treating textile effluents. This wasdemonstrated with a bench-scale set-up that achievedfull decolorization and effluent COD level below100 mg dm~3 when fed with deeply-colored carpetfactory effluents (Verstraete, not published). In caseswhere the inhibition of methanogenesis cannot be cir-cumvented, anaerobic pre-treatment may remain usefulas, for example, a decolorizing step, which seems toproceed optimally around a redox potential of[250 mV,62 or as a bioocculation step. The lattermay achieve high removal rates of poorly degradableorganics such as the grease and detergents present inthe very high-strength wool scouring effluents.45,56,63

The effluent from a silk and Lycra printing factory inComo (250 m3 day~1) is being treated in a full-scaledenitrication/nitrication single-sludge system.46Operating conditions included the dosing of22 mg dm~3 sodium sulde to remove Zn, the additionof 330 mg dm~3 pharmaceutical waste in the denitri-cation reactor as a source of reducing equivalents, andthe addition of 1 mg dm~3 lyophilized bacteria to thenitrication plant. A tertiary treatment, consisting ofquartz bed ltration and UV sterilization, has allowed3040% water reuse without deleterious eects ongarment quality during 10 months of full-scale oper-ation. Effluent COD (less than 120 mg dm~3), color,and (less than 10 mg dm~3) all met Italian dis-NH4`-Ncharge standards. The nitrication rate (003005 g N


g~1 MLVSS, day~1) was much lower than that whichcan be obtained with the municipal sewage. This lowrate was attributed to inhibition caused by the presenceof 3040 mg dm~3 anionic surfactants in the waste-water. Both the Seveso and Alto Lura WWTP, locatedin Como (Italy), treat sewage containing 80% textileeffluent. The Seveso plant, in which two-thirds of theinuent passes through an anaerobic selector (2 gCOD g~1 MLVSS day~1 with an HRT of 45 min) per-forms better than the Alto Lura plant, which does notinclude such a selector.57 Increased performance wasreected in the elimination of the foaming and bulkingproblems, better P removal, and low effluent TSS(average of 20 mg dm~3 instead of more than 100 inAlto Lura).

Despite the numerous laboratory-scale trials showingthe potential of anaerobic treatment for color removal,large-scale installations equipped with an anaerobicpre-treatment do not generally achieve full decoloriza-tion. Zaoyan et al.64 operated a large pilot plant (24 m3day~1) for 9 months at a textile dyeing mill which usedreactive and other dye classes on polyester/cottonfabric. The plant consisted of two multi-stage rotatingbiological contactors, a rst anaerobic one (HRT \ 7 h ;

COD day~1 dm~3) followed by a secondBv \ 2 gaerobic one (HRT \ 5 h ; COD day~1Bv\ 23 gdm~3). The sequential anaerobic/aerobic systemachieved a higher color removal (72%) than the aerobicsystem alone (\60%) but the nal effluent was stilldeeply colored. Both systems achieved 78% CODremoval (155 mg dm~3), 95% removalBOD5(13 mg dm~3) and 70% anionic surfactants removal(07 mg dm~3). Another pilot two-stage anaerobic/aerobic plant achieved [70% color removal fromdyeing wastewater.65 The Hammarsdale (South Africa)sewage works consists of a conventional ve-stage bard-enpho nutrient removal process treating deeply-coloredtextile effluents.62 Though the overall system achieves95% decolorization, of which 60% takes place in therst anaerobic state (HRT of 1 h and redox potential of[135 mV), the nal effluent is still excessively colored.Since decolorization is fastest at [250 mV (24 h), thecolor removal at the Hammarsdale works may beimproved by increasing the HRT in the anaerobicstage.62

Several experiments have been carried out in SouthAfrica to investigate the possibility of feeding dyebatheffluents directly to the anaerobic digesters treatingprimary sludge from sewage works. Carliell et al.33mixed spent reactive dyebath effluent with the primarysludge (6% v/v) entering the digester at Umbilo sewageworks. Color was completely eliminated as shown bycomparison with a control digester. Laboratory-scaleside trials indicated, however, that methanogens maybecome inhibited after a few days operation at a higherloading (18%) due to the build up of sulde originatingfrom the 42 g dm~3 sulfate in the dyebath effluent. Pro-

posed measures to overcome the inhibition were thesubstitution of NaCl for sodium sulfate, the addition ofheavy metals (e.g. from electroplating effluents) to pre-cipitate the suldes, or molybdate to block sulfatereduction. Similar full-scale trials were carried out byGravelet-Blondin et al.3 for a period of 5 months. Nocolor was present in the overow of the digester buttransport costs were considered prohibitive.

The combined activity of anaerobic/aerobic bacteriacan also be obtained in a single step if the bacteria areimmobilized in biolms since penetration seldomO2exceeds several hundred micrometers.21 In addition toproviding anaerobic/aerobic zones, xed lm reactorsoer the advantages of higher SRT necessary to preventwashout of adapted microorganisms, protection againsttoxicants such as azo dye Acid Orange 7 (Fig. 2), andlow sludge production.35 Laboratory-scale studies havedemonstrated that acid azo dyes which are notdegraded in activated sludge were removed up to 60%in xed lm reactors provided the dissolved concen-O2tration is kept below 1 mg dm~3 and the loading rate ishigh.22,35 Jian et al.66 could treat the effluent from adye manufacturing plant very eectively using multi-stage percolation columns with immobilized decolo-rizing bacteria. The pilot scale installation, whichtreated 20 m3 day~1, removed consistently 97% of bothcolor and COD during a 7-month trial.

3.3 Full-scale plants using a combination of activatedsludge and coagulation/occulation

Chemical coagulation is applied as either a pre-, post-or main process for the treatment of dyeing mill efflu-ents (Fig. 1) and is, according to Ga hr et al.,8 the treat-ment most widely used in Germany for these effluents.This author reports, however, the drawback associatedwith the large amounts of toxic sludge requiring com-bustion. Many French textile manufacturers likewiserely on a physico-chemical post-treatment, followingactivated sludge, to treat their colored effluents.31 Mar-magne and Coste31 contend that the nal effluentusually contains from 150 to 300 mg dm~3 COD (85%removal) and color levels above the maximum dischargestandard of 100 mg Pt.Co dm~3 which is expected to beenforced in France in the near future. Insufficient colorremoval is a result of the poor occulation of certaintypes of dyes, e.g. reactive or certain acid dyes.31 Forexample, a mere 20% color removal was achieved inpilot experiments in which polyaluminum chloride(10 mg Al dm~3) was dosed in the effluent of an activat-ed sludge treating a mixture of municipal and textilewastewaters.67

New occulants have, however, been developed witha high affinity for reactive dyes.68,69 Several reportsdetail, for example, the successful use of cationic poly-mers allowing a few major UK textile processors to


meet their color consent for direct discharge torivers.17,54 Dissolved air otation can eectivelyseparate the occulated dyes with little sludge produc-ed. At Livescia WWTP (Como, Italy) treating a mixtureof municipal and textile wastewaters, a 35% colorremoval was obtained by dosing 20 mg dm~3 ofcationic polymers in the effluent of the biological oxida-tion treatment (Bianchi, unpublished). Adequate colorremoval may be achieved by dosing cationic polymersdirectly to the activated sludge. The Wanlip sewageworks (UK), receiving 11% of its dry weather ow fromtextile mills, could meet the discharge consent followingthe addition of 5 ppm Magnaoc 368 directly to theactivated sludge tank and a concomitant halving of theeffluent absorbance.53 In cases where higher polymerconcentrations were necessary, however, severe prob-lems with inhibition of nitrication were observed.17The toxicity of polymers toward nitriers is highly vari-able, with values ranging from 8 to [100 ppm.IC50

Heathcoat & Co. Ltd (UK) discharges 2000 m3day~1 from reactive dyeing and nishing operations.They achieve their direct discharge consent with achemical pre-treatment using lime (pH 113), FeSO4and polyelectrolyte followed by percolation throughbeds of expanded porous shales against a ow ofair.70 Together with color, the physico-chemicalpre-treatment removes anionic surfactants (but notthe non-ionic ones). Many process chemicals, includingseveral non-ionic surfactants and one complete rangeof dyestus, had to be replaced by more biocompatibleones.

Coagulation/occulation, alone or in combinationwith biological processes, can sometimes allow waterreuse. A production plant of Courtaulds Textile, thelargest UK manufacturer of socks, has been using, since1995, a full-scale purely physico-chemical process totreat the mixed effluent from their dyeing, printing andnishing units. The process consists of dye adsorptiononto synthetic organic clays which are separated follow-ing the addition of alkali and a coagulant with highrelative molecular mass. Even though cotton is the mainber being processed, and thus reactive dyes the maindye class used ; 50% water reuse is possible in thedyeing units.71 This new technology is being applied atseveral locations. Similarly, a leading denim condition-ing and washing mill in Yorkshire recently built a full-scale occulation plant. The effluent quality isconsistently within the consent limit and half of it isreused.72 Finally, a textile processing plant located inCyprus recycles its effluent (200 m3 day~1) on irrigatedland.51 Equalized wastewater is rst treated with lime,

and polyelectrolyte before entering the activatedFeSO4sludge tank which is equipped with an oxic selector inorder to eliminate bulking. Tertiary treatment consistsof disinfection with occulation with alum andCl2 ,polyelectrolyte, and nally ltration on anthracite,quartz sand and barytes. The nal colorless effluent

(518 nephelometric turbidity units) had a andBOD5COD of 10 and 100310 mg dm~3, respectively. Totaloperating costs amount to 02 US$ m~3.

While most coagulation/occulation systems rely onlime, one laboratory study reported efficient colorremoval (90%) from spent reactive dyebath with50 ppm Fe3` or Al3` under acid conditions.73 Finalclarication allowed reuse of water and salt with no lossof fastness to light or washing.

3.4 Full-scale plants using ozonation

Ozone decolorizes all dyes, except non-soluble disperseand vat dyes which react slowly.31,74h77 Because isO3less efficient with high-strength raw textile wastewater,it is advisable to use only as a nal treatment or atO3least following chemical coagulation.74,78,79 Aside fromcolor, ozonation also removes a large proportion ofAOX8,80 and surfactants.46,81 Typical ozonation pro-ducts are dicarboxylic acids and aldehydes, whichexplains the little reduction of COD (020%) but sub-stantial increase of BOD during ozonation.8,81

Ozonation is becoming increasingly popular as a naltreatment to eliminate color and other persistent sub-stances. The sewage works at Leek receives 60% of itstotal load (76 000 inhabitant equivalents) from sevendyehouses. Considerable extensions to the works wererequired following tighter discharge consent since1992.5 Following pilot-scale investigations showing thatrequired color removal could be achieved with 95 ppm

with an HRT of 20 min, a tertiary treatment involv-O3ing lagoons, sand lters and an ozonation plant wasbuilt at a capital cost of 5 million. In about 1 year,only four samples out of 39 analyzed have failed thecolor consent. It is considered that may be used onO3treated sewage effluent where the proportion of coloreddyewaste to domestic sewage is high enough to justifythe capital cost. In one case, the installation of an ozon-ator at a sewage works doubled the discharge leviesbilled to the textile companies, from 04 to 08 m~3.54The Alto Lura WWTP (Como, Italy) treats a mixture of75% textile and 25% municipal wastewater with asequence of pre-denitrication, activated sludge, sandltration and ozonation. The ozonator was built in1992 in order to reduce surfactants and color in thenal effluent. These goals are being achieved with 20 mg

dm~3 but unwanted by-products are formed, espe-O3cially aldehydes in the range 052 mg HCHO dm~3(Bianchi, unpublished). Plans are made to reduce alde-hyde formation by adding a tertiary occulation unitbefore the sand lters (Fig. 1).

Numerous textile wet-processors also use ozonationin order to reuse the water. Capital costs are, however,high. A dyeing mill, producing 100 m3 h~1 wastewater,was recently built in Italy at a total cost of 13 millionUS$, of which 20% was allocated to the wastewater


treatment plant (clarication, equalization, activatedsludge, decanter, ltration, and ozonation).80 In the caseof the above-mentioned silk and Lycra printing factoryin Como, laboratory trials have demonstrated that upto 65% reuse, in washing and printing units, will bepossible by integrating an ozonation reactor(20 mg dm~3 which eliminates the residual colorO3)and non-ionic surfactants.46 The Levi Strauss nishingplant at Wervik, Belgium, was recently facing limi-tations in groundwater pumping and stringent dis-charge standards. Use of tap or river water wasinadequate. Reuse trials in pilot plants withcoagulation/occulation or ultraltration showed dete-rioration of garment quality.82 Activated carbon l-tration yielded a better COD and color removal andgarment quality but sulde formation in the lteryielded odor problems. Finally, a solution was found bytreating the mixed stream with a sequence ofcoagulation/occulation, activated sludge, and a 10-minnal ozonation step (Fig. 1). Three years of pilot testingdemonstrated the feasibility of 70% water reuse withoutdeterioration of garment quality.

Numerous laboratory-scale experiments have docu-mented the feasibility of ozonating dyebaths effluentsseparately and reusing the decolorized effluent severaltimes with excellent color reproducibility.75h77,83,84 Forexample, spent reactive dyebath can be renovated andreused by coarse ltration followed by ozonation. Costsavings associated with the reuse of salts (80 g dm~3 inreactive dyebaths) are up to 30 times larger than thesavings associated with the reuse of water.75 Eventhough considerable amounts of (up to 1 g dm~3)O3may be required to achieve nearly complete colorremoval due to high dye concentration (severalhundreds ppm) and interference by dyeing auxiliaries,theoretical payback periods as short as 13 years areclaimed.75 Reports on full-scale application of a reusestrategy involving separate ozonation of dyebath efflu-ents are, however, apparently lacking. The reason couldbe the sometimes considerably lower decolorizationrate, up to 20 times,76,77 in the presence of dyebathadditives such as EDTA, silicone defoamers, eth-oxylated alcohol surfactants, leveling agents, butyl ben-zoate carriers, guar gum and salts.5,8,74 As aconsequence, the amount of necessary for near com-O3plete decolorization may become prohibitively expen-sive.

3.5 Full-scale plants using ltration processes

Ultraltration, nanoltration and reverse osmosis havebeen used for the full-scale treatment and reuse of waterand chemicals by South African textile wet processorsfor nearly two decades. Specic lter congurationshave been developed for the reuse of wool scour andbleaching effluents, spent sizing and desizing liquors,

and spent dyebaths.84h86 The high cost of ltrationtechniques may be outweighed by the signicant costssavings achieved through the reuse of permeate, saltsand sizes (i.e. polymers applied to warp yarns to facili-tate weaving). While careful choice of the membranesystem, use of pre-lters and regular cleaning eliminatemembrane fouling problems, economic viability of con-centrated waste treatment and disposal remains uncer-tain. In South Africa, where ltration techniques arewidely used in order to reuse water, the concentrates areoften dumped in the ocean or disposed to a sewer.3,13

Desizing effluents typically contain 2550% of thetotal organic load in 510% of total effluent volume. Asearly as 1978, UF was used commercially to recoverpolyvinyl alcohol sizes from desizing effluents.87 Boththe retained sizes and the permeate are reused.84,87,88Polyacrylate sizes were also successfully recovered andreused from desizing effluents using high temperatureUF pilot plants. Hot permeate reuse yields savings inheat and water.87,89

Wool scouring liquors can also be treated by l-tration techniques.86 Norways largest yarn factory gen-erates hot wool scouring liquors containing high pHdetergents and up to 100 g COD dm~3. A full-scaleultraltration treatment plant has been operating since1989 with [ 80% removal of COD, fats and suspendedsolids.90 The permeate is disposed to a sewer and theconcentrate (10% v/v) must be trucked to lagoons athigh cost. Membrane lifetime is 1 year with an averageow of 60 dm3 m~2 h~1. AOX, of which 50% are insolid form in wool effluent, are also signicantlyreduced by ltration techniques.50 Wool scouringliquors are also being reused in full-scale plants usingdynamic UF membranes.91 The latter oer the advan-tage of not requiring membrane replacement since it isformed in situ by deposition of a colloidal suspension ofhydrated zirconium oxide on a porous support. Themembranes, which are reformed every other month,achieve up to 90% TS rejection and allow 85% waterrecovery.

Dyehouse effluents are also being reused successfullyfollowing reverse osmosis.85,89 In this case, only waterreuse is possible. Coagulation and microltration arenecessary to prevent membrane fouling. Trery-Goatleyet al.92 reported the successful treatment and reuse ofdyehouse effluents in a pilot plant (50 m3 day~1) inte-grating the sequence of alum coagulation, micro-ltration and reverse osmosis. Concentrate volume wasonly 510% and RO membrane lifespan 2 years. Simi-larly, Buckley13 reports on a alum coagulationmicroltrationRO sequence treating 40 m3 day~1 ofcotton/polyester dyehouse effluent. A UF plantequipped with dynamic membranes has been achieving,since 1985, 85% water recovery with 80% ionic rejec-tion and 95% color rejection at a textile mill dyeingpolyester/viscose bers.13,91 The application of l-tration technologies remains, however, severely limited


by the problem of disposal of the concentrate streamsince the currently-used schemes, such as evaporationor ocean discharge,3 are either economically or environ-mentally unacceptable.

Because dyebath salts may have a 10-fold greatervalue than water, it may be preferable to treat and reusedyebath and rinse effluents separately with membranesthat let pass the salts but retain the dyes. Such a reusestrategy is described by Erswell et al.93 for a SouthAfrican cotton dyehouse. In this case, a charged UFmembrane was used for dyebath effluents that allowed90% reuse of salt and water, while a 10% (v/v) concen-trate was disposed of. The volume of the concentratestream was reduced to only 2% in an elegant reusescheme described by Woerner et al.94 in the case of acotton mill (160 tons per month). The rinse liquors arereused following a sequence of UF and RO. The con-centrate is mixed with the dyebath effluent which is sub-jected to the sequence UF and NF enabling both waterand electrolytes reuse. Buckley13 reports on an NFplant treating reactive dye liquors from a cotton dye-house (200 tons per month). Reuse of permeate withelectrolytes yielded a payback period of less than 3years. Even microltration has been used in the case ofpolyester dyebaths using insoluble disperse dyes.13 Theslight residual color in the permeate did not prejudiceits reuse in dyeing or rinsing units.

3.6 Full-scale plants using the Fentons reagentiron)(H

2O

2+ ferrous

Several full-scale Fentons reagent plants were builtrecently in South Africa to treat textile mill effluents.3This technology is eective in decolorizing a wide rangeof dyes.95,96 According to Lin and Peng,97 the Fentonsreagent works by oxidizing ferrous to ferric iron withsimultaneous splitting of into hydroxide ion andH2O2hydroxyl radical. The latter oxidizes the dye while theformer precipitates with ferric iron together withorganics. The reagent is used preferably at pH valuesaround 34.96h98 Ferric seems as eective as ferrousiron, and only a few ppm are required if high tem-peratures are used.95 For example, 1 g dm~3 of a directdye was completely decolorized in 30 min with15 g dm~3 of in the presence of 2 mg dm~3 FeH2O2at 98C. At normal temperatures, 50100 mg Fe2`dm~3 are required. Lin and Peng97 obtained very goodperformance in a pilot scale installation treating actualtextile wastewaters with the sequence of chemical pre-cipitation, Fentons reagent and activated sludge. Com-plete decolorization was obtained after the Fentonsreagent stage and nal COD, after activated sludge, wasonly 80 mg dm~3. Estimated total running costs, notincluding sludge disposal, was 04 US $ m~3 which wascheaper than that of the conventional treatment.

4 EMERGING TECHNOLOGIES AWAITINGFULL-SCALE TRIALS

4.1 Electrolysis

Through production with sacricial iron elec-Fe(OH)2trodes, electrolysis can be used to remove acid dyes veryeectively via sorption onto the precipitated iron andvia Fe(II)-driven reduction of azo dyes to arylamines.99Solutions containing 50 mg dm~3 dyes were 100%decolorized with 100150 mg Fe dm~3. Greater than80% decolorization of a real textile effluent wasachieved in laboratory-scale trials by applying 2 kWh/m~3.100 The process was used very successfully byLin and Peng101 in a pilot scale installation treating theraw effluent from a cotton/polyester dyeing and nish-ing mill in Taiwan. The effluent, which contained 15 dif-ferent dyes and 8001600 mg COD dm~3, was fullydecolorized in an electrolyte cell (HRT\ 18 min) with aconcomitant 50% COD reduction. The total process,consisting of coagulationelectrolysisactivated sludge,was cheaper (running costs, not including sludge dis-posal, was 03 US$ m~3) and more eective (COD innal effluent\ 80 mg dm~3) than the conventionaltreatment. Other pilot-scale trials have proven equallysuccessful.65

4.2 Photocatalysis

UV light has been tested in combination with orH2O2solid catalysts such as for the decolorization ofTiO2dye solutions. While the process appearedUV/H2O2too slow, costly and little eective for potential full-scaleapplication,102,103 the combination seemsUV/TiO2more promising.104h106 Because UV penetration in dyesolutions is an inherent limitation, Huang et al.107found the best use of UV technology as a post-treatment after ozonation. Not only was the residualcolor completely removed, close to 90% of residualtotal organic carbon was also removed from the efflu-ents of two textile mills. The German rm Wedeco isusing such a combination of in a mobileO3/UV/H2O2pilot plant.80

4.3 Sorption

Despite the fact that eective decolorization has oftenbeen shown to be feasible via sorption onto activatedcarbon as a nal step,5,31,54,82,108 full-scale use of thistechnology has apparently not been reported yet, alikely consequence of high capital and regenerationcosts and possible foul odors.82 CIDA Srl is currentlytesting full-scale GAC lters at the Alto Lura WWTP(Fig. 1) as part of the research project supporting this


paper. The nal effluent seems suitable for reuse in silkdesizing units but not in silk dyeing units. Full-scalebiologically-activated GAC lters are also under inves-tigation at the site. Though certain microbial speciesmay sorb impressive quantities of dyes (up to 3 g sulfurdye and 045 g reactive dye per g biomass),109 estimatedtreatment costs remain prohibitive (26 US$ m~3).Sorptive capacity is almost two-fold higher with par-ticles of polyamideepichlorohydrincellulose but thereaction rate seems too low.110 Several authors testedthe use of industrial or agricultural wastes as a low-costalternative for developing countries. Results obtainedwith corn cobs, chrome sludge from the electroplatingindustry, yash or biogas residual slurry were disap-pointing.111h114 Lazlo115 reviewed the eectiveness ofall biological adsorbents for removing anionic dyes. Thehighest affinity adsorbent was cross-linked chitosan at apH of 34. The best choice was quaternized lignocellu-losic biomass which oers the advantages of high sorp-tion capacity, low cost, rapid kinetics and possibleregeneration.

4.4 Flotation and others

Lin and Lo116 demonstrated the potential of foam o-tation as an eective and simple technology for thedecolorization of dyehouse effluents. Mixing the waste-water with 70 mg dm~3 of a surfactant and compressedair yielded, after a few minutes, 90% color removal and40% COD removal. Color and COD were recovered ina highly concentrated stream after the foam, leaving thecontact chamber at the top, had passed through a foambreaker. Solvent extraction can also be applied for dyerecovery from dyebath effluents. For example, sul-fonated dyes can be rendered hydrophobic with longchain amines at pH 25 and concentrated up to 1000-fold in a solvent phase.16 Dyes, solvent and surfactantsare reused.

Phanerochate chrysosporium and other fungal speciescan mineralize all types of dyes at a very rapid pace viaa peroxidation pathway.43,117,118 Van der Waarde etal.119 developed a decolorization technique based onthe combined use of fungal degradation and in situ elec-trochemical production, an essential substrate forH2O2fungal activity. They achieved complete decolorizationof several spent dye liquids (reactive and disperse dyes)in an air lift continuous reactor.

At 150250C and partial pressures of 035O214 MPa, concentrated solutions of dyes are fullydecolorized and almost completely mineralized in 14 h.120 Because no external energy is required when theCOD exceeds 20 g dm~3, wet air oxidation may oer asolution for the problematic disposal of the concentratestreams produced by ltration or foam otation tech-nologies.

5 CONCLUSIONS

Conventional WWTP relying on activated sludgesystems are not adequate for the treatment of textilemill effluents, neither on site nor after dilution withdomestic wastewater at the sewage works. Activatedsludge and other types of bioreactor fail to remove suffi-cient color, COD, N, surfactants and other micro-pollutants present in the textile effluents. Tertiarycoagulation/occulation is often used with variableresults but at times near-complete decolorization andwater reuse is possible ; sludge disposal remains,however, a problem. Ozone is increasingly used as anal step but cost and aldehyde formation prevent itswider acceptance. Membrane ltration of process sub-streams may yield substantial cost savings by allowingwater, chemicals and heat reuse. Handling and disposalof the concentrate stream remains, however, a severelimitation to ltration processes. In view of the need fora technically- and economically-satisfying treatmenttechnology, a urry of emerging technologies are beingproposed and tested at dierent stages of commer-cialization. Promising among these are biologically-activated GAC ltration, foam otation, electrolysis,photocatalysis, (bio)sorption and Fenton oxidation.Broader validation of these new technologies and inte-gration in the current treatment schemes will, mostlikely in the near future, render these both efficient andeconomically viable.

ACKNOWLEDGEMENTS

The writing of this paper is part of the project Inte-grated water reuse and emission abatement in the textileindustry, supported by a grant from the Environmentand Climate program of the European Commission(ENV4-CT95-0064).

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