Waste Treatment in the Process Industries - Chapter 8 doc

The production of a textile requires several stages of mechanical processing such asspinning, weaving, knitting, and garment production, which seem to be insulated from the wettreatment

Treatment of Textile Wastes

Thomas Bechtold and Eduard Burtscher

Leopold Franzens University, Innsbruck, Austria

Yung-Tse Hung

Cleveland State University, Cleveland, Ohio, U.S.A

The production of textiles represents one of the big consumers of high water quality As a result

of various processes, considerable amounts of polluted water are released Representativemagnitudes for water consumption are 100 – 200 L of water per kilogram of textile product.Considering an annual production of 40 million tons of textile fibers, the release of wasted watercan be estimated to exceed 4 – 8 billion cubic meters per year

The production of a textile requires several stages of mechanical processing such asspinning, weaving, knitting, and garment production, which seem to be insulated from the wettreatment processes like pretreatment, dyeing, printing, and finishing operations, but there is astrong interrelation between treatment processes in the dry state and consecutive wet treatments.For a long time the toxicity of released wastewater was mainly determined by the detection

of biological effects from pollution, high bulks of foam, or intensively colored rivers near textileplants Times have changed and the identification and classification of wastewater currently arefixed by communal regulations [1,2]

General regulations define the most important substances to be observed critically by theapplicant, and propose general strategies to be applied for minimization of the release ofhazardous substances The proposed set of actions has to be integrated into processes and

strategic positions for actions to minimize ecological impact In this figure, the textile plant isdefined as a structure that changes the properties of a textile raw material to obtain a desiredproduct pattern The activities to treat hazardous wastes can range from legal prohibition to cost-saving recycling of chemicals Depending on the type of product and treatment, these steps canshow extreme variability

Normally the legal regulations are interpreted as a set of wastewater limits that have to bekept, but in fact the situation is more complex and at present a complex structure of actions hasbeen defined and has described useful strategies to improve an actual situation

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8.1.2 Strategies to Reach Existing Requirements

Figure 2 shows a general action path recommended to minimize a present problem in thewastewater released from a textile plant [3,4]

Replacement and Minimization

As a first step substances that are known to cause problems in the wastewater have to be replaced

by less hazardous chemicals or the process itself should be reconsidered; for example,

instead of carrier processes;

(EDTA), phosphonates

The implementation of these steps into a dyehouse reduces the chemical load of thereleased wastewater considerably In particular the replacement of substances that exhibit hightoxicity or very low biodegradability will facilitate the following efficient treatment of thewastewater

Figure 1 Flow structure of a textile plant (from Refs 2 and 3)

Optimization of Processes

The second general step recommended to improve an existing situation is the optimization oftreatment steps with regard to a lowering of the released amounts of hazardous substances [6,7]

In many cases this strategy is more intelligent and less expensive than a concentration of activities

on the final treatment of released effluents Typical examples for possible optimization are:

exhaustion, fixation, liquor ratio);

Separation and Recycling

Besides the replacement of substances, the improvement of processes on an optimization of thehandling of rather concentrated liquors, for example, used in sizing, caustic treatment likemercerization, dyeing, finishing processes, or in textile printing processes is the next step As adesired goal, a recycling of a main part of the substances should be attempted Examples that can

be mentioned include the recovery and regeneration of sizes and caustic soda solutions, and therecovery of lanolin from wool washing

Separation and Treatment for Disposal or Drain

If regeneration is impossible, a separate collection of a certain type of waste and anoptimized treatment of the concentrates is more efficient and cheaper than a treatment ofthe full waste stream Such treatments will concentrate on a minimization of costs fordisposal (e.g., disposal of sludge, printing pastes, chemical products) or reaching existinglimits defined for various parameters analyzed in the wastewater, for example, pH value,content of heavy metals, chemical oxygen demand (COD), adsorbable halogenated organiccompounds (AOX) [8]

Figure 2 Action path for consideration and improvement of an existing situation (from Refs 1 – 3 and 9)

General Wastewater Treatment

In any case the wastewater will finally be fed into rivers, lakes, or the sea; thus some wastewatertreatments have to be performed before the textile effluents are released either to the communalwastewater treatment plant (CWWT) or into the rivers, lakes, and so on Normally physical and(bio-) chemical treatments (e.g., adjustment of pH, temperature, sedimentation, flocculation) areperformed in the textile plant, while the following biological treatment (aerobic, anaerobicdegradation) is performed either in the textile plant or in a CWWT The site of the biologicaltreatment is dependent on the location of the textile plant; however, a biological treatment oftextile effluents preceding release into surface water is state of the art

For a long time the treatment of textile effluents has concentrated mainly on two aspects:regeneration of concentrated effluents with regard to savings of chemicals and lowering ofchemical costs and treatment of effluents with high toxicity

Over the last decade the situation has changed and limits for a considerable number ofcompounds and parameters have been defined to avoid problems with regard to the following:

Table 1gives an extract of important parameters for wasted water from textile plants, asdefined by the Austrian Government [1] The table contains limits defined for both direct releaseinto surface water (rivers) and for release into a CWWT

Table 1 can be used as a guide to define “hazardous” wastes from textile plants Besidesthe direct toxicity of substances like chlorinated hydrocarbons, organo-Hg compounds, orconcentrated alkaline solutions, other parameters have been defined with regard to problemsduring biodegradation or accumulation in the sludge from CWWT A particular situation isfound with colored effluents, where limits for spectral absorption have been defined While thetoxicity of textile dyes is comparably low, these limits were derived from the visual aspect of thewater released from a textile plant because they look “unhealthy.”

As a result of these regulations, textile companies have to apply a strategic concept tolower both the daily load released into the wastewater stream and the concentrations of

improvement of the present situation of a plant has to be undertaken

Owing to the extreme diversity of the textile processes and products, it is impossible todevelop a realistic concept for an efficient wastewater treatment without detailed analysis of theparticular situation of a textile plant The more intelligently the applied technical concept hasbeen designed, the lower will be the expected costs for installation and working of theequipment

In the following sections techniques and technical solutions are given as examples that can

be adapted to a certain problem

To facilitate an overview and to consider the specific differences of textile fibers duringpretreatment, dyeing, and finishing, the sections have been focused on the most important types

of fibers: wool, cotton, and synthetic fibers Mixtures of fibers can be seen as systems ing problems of the single fiber types In Section 8.3 end-of-pipe technologies have beensummarized

combin-8.1.4 IPPC Directive of the European Community

In the legislation of different national governments, some limits were defined especially forwastewater and air The activities in Europe are covered by the Council Directive 96/61/ECconcerning Integrated Pollution Prevention and Control (IPPC) [9] This means that all

Table 1 Representative Limits Defined for Release of Textile Waste Water

Organic parameters (mg/L)

COD (chemical oxygen

Phenol index calculated as

environmental media (water, air, energy, ground) and a comprehensive description of theproduction have to be considered In addition a broad harmonization of requirements for theapproval of industrial plants can be reached.

The classification of a company as an IPPC plant is based on the definition of the workconcerning plants for the pretreatment (operations such as washing, bleaching, mercerization) ordyeing of fibers or textiles where the treatment capacity exceeds 10 tons per day As a firm basis

of reference the capacity will be calculated as the potential output a company could have in 24hours Capacity means what a plant is designed for and not what is really achieved (actualproduction) The treatment of fibers and textiles covers fibers, yarns, and fabric in the widersense of the word, that is, including knitted and woven materials and carpets As most textiles aretreated with continuous working machines with a very high theoretical maximum capacity, a lot

of companies have to fulfill the directions for IPPC plants

To reach the aim of the directive an efficient and progressive state of development isdefined by the best available techniques (BAT) In practice, this means precaution againstenvironmental pollution by the use of these techniques, special equipment and better way ofproduction, and an efficient use of energy for prevention of accidents and provisions for ashutdown of a production plant The term best available techniques is defined as the mosteffective and advanced stage in the development of activities and their methods of operation thatindicate the practical suitability of particular techniques for providing in principle the basis foremission limit values designed to prevent and, where it is not practicable, generally to reduceemissions and the impact on the environment as a whole These available techniquesare developed on a scale that allows implementation under economically and technically viableconditions, taking into consideration the costs and advantages when the techniques are used

In the best available technology reference document (BREF), particular attention is given

to the processes of fiber preparation, pretreatment, dyeing, printing, and finishing, but it alsoincludes upstream processes that may have a significant influence on the environmental impact

of textile processing The treatment of all main fiber types as natural fibers (cotton, linen,wool, and silk), man-made fibers derived from natural polymers, such as viscose andcelluloseacetate, as well as from synthetic polymers (such as polyester, polyamide,polyacrylnitrile, polyurethane, polypropylene) are described, including blends of these textilesubstrates Beside general information about the industrial sector and the industrial processes,the situation in the plants is described by data about current emission and consumption

A catalogue of emission reduction or other environmentally beneficial techniques that areconsidered to be most relevant in the determination of BAT (both generally and in specific cases)

is given as a pool of possible techniques including both process integrated and end-of-pipetechniques, thus covering pollution prevention and pollution control measures Techniquespresented may apply to the improvement of existing installations, or to new installations, or

a combination of both, considering various cost/benefit situations including both lower andhigher cost techniques To obtain a limitation of emission impact, different techniques areproposed corresponding to the basic possibilities for pollution prevention:

dye liquors from semicontinuous and continuous dyeing, residual printing pastes,residual finishing liquors, residues of prepared but not applied dyestuffs, textileauxiliaries, and so on;

toxic compounds to water by substituting harmful substances with less harmfulalternatives;

reduction of releases to air from thermal treatment installations like stenter frames;

consi-dering the available options for wastewater treatment; wastewater treatment includingpretreatment onsite before discharge to the sewer as well as treatment of effluent onsite

in case of discharge to rivers; efficiency of treatment of textile wastewater togetherwith municipal wastewater;

pretreatment, fixation of dyes, finishing operation, and drying

General

The annual production of wool is approximately 1.2 million tons, which corresponds to a share

of 2% of the total production of textile fibers A simplified route for the preparation, dyeing, and

Table 2 Annual Production of

Besides more general strategies of process optimization, three representative steps will bediscussed in more detail because of their particular importance with regard to wastewater Themain problem resulting from these steps is given in parentheses:

Washing of Raw Wool

The high content of impurities in raw wool has to be removed before further processing, forexample, in carbonization, spinning, and weaving As a considerable part of the raw material(approx 30%) is removed and released into the wastewater, washing of raw wool can causeheavy pollution problems These difficulties are not due to the toxicity of the released compo-nents, but result from the high concentrations and the load of organic material released in the

techniques that can be applied to lower the initial COD in the effluent from approximately80,000 mg/L to a final value of 12,000 mg/L [11,12]

The lanolin extracted from the wool is purified further for use in cosmetics, hand cream,boot-polish, and so on Part of the permeate from the ultrafiltration is recycled to save freshwater A particular advantage arises from the fact that the dissolved sweat components exhibitFigure 3 General processing route of woolen textiles (from Ref 3)

Table 3 Average Composition of Raw Wool

Soil, plant material (cellulose) 13

Source: Refs 3,11,12.

distinct washing properties for raw wool and thus a certain content of dissolved sweat isfavorable to improve the washing effect.

Various treatment concepts have been presented in the literature [11 – 13] Besides therelease of the pre-treated wastewater into the CWWT and aerobic biodegradation, in some casesevaporation of the wastewater and incineration of the residue are performed

Antifelt Finishing of Wool

The surface of a wool hair is covered by keratin sheds, which cause a distinct tendency toshrinkage and formation of felts This behavior is usually undesirable and thus an antifeltfinishing is the most important treatment during the processing of woolen textiles One of themost important standard procedures, the Hercosett finish, is based on the oxidative treatment ofwool by application of compounds that release chlorine Examples for applied chemicals are

Such processes lead to the formation of adsorbable halogenated organic compounds(AOX) in high concentrations Typical concentrations found in a continuous antifelt

the baths is one of the sources for the formation of high concentrations of chlorinatedcompounds The formation of chlorinated products is the result of chemical reactionsdirectly with the fiber, with organic compounds released from the fibers, and with addedauxiliaries

An average size of continuous treatment plant for antifelt treatment of wool releasesapproximately 140 g/hour AOX As an optimization of the process is possible only withincertain limits, alternative processes for an antifelt treatment have to be chosen to substitute

persulfate), or corona or plasma treatment In many cases combinations with resin treatments areproposed

Figure 4 General scheme for the treatment of effluents from wool washing (from Refs 11 – 13)

Chromium in Wool Dyeing

A considerable part of the wool dyes contains Cr complexes The average consumption of dyesused in 1992 is shown in Table 5 At this time approximately 70% of all dyes used containchromium

0.1 mg/L While conventional 1 : 2 and 1 : 1 dyes permit chromium concentrations in thedyebath at the end of the dyeing process of 3.0 – 13.0 mg/L Cr, the application of moderndyestuffs and optimized processes permits final concentrations to approximately 1 ppm Bygeneral optimization of the process (e.g., dosage of acid), use of dyes with a high degree ofexhaustion, and minimal concentration of free chromium [15], final bath concentrations below

4 ppm can be reached, even for black shades By application of such procedures the exhaustion

of the chromium should reach values of better than 95% of the initial value

Owing to the low limits for concentrations of chromium the proposed processes forwastewater treatment concentrate on the removal, for example, by flocculation and precipitation,but as a result chromium-containing sludge/precipitate or concentrates are obtained that needfurther treatment

General

Cellulose fibers (Co, CV, CMD, CLY) represent the main group of textile fibers used [10] In thissection cotton will be considered as a representative type of fiber because the treatments forother cellulose fibers are similar in many cases, and often milder conditions are applied for othercellulose fibers

Table 4 Concentrations for AOX

Determined in the Chlorination

Bath of the Chlorine-Hercosett Process

Sources for textile effluents that need further treatment are found in all steps of processing.Table 6 shows a list of important parameters and wastes that require further treatment.

Sizing – Desizing

Before weaving, the warp is covered with a layer of polymer to withstand the mechanical stress(abrasion, tension) during weaving These polymer coatings are so-called sizes Normally nativestarch, modified starch like carboxymethyl-starch (CMS), carboxymethyl-cellulose (CMC),polyvinylalcohols (PVA), polyacrylates, and proteins can be used The amount of addedpolymer for staple yarns like Co is between 8 and 20% of the weight of the warp As a result, inmany cases the final amount of polymer to be removed in the desizing step is approximately

5 – 10% of the weight of the fabric

Sizing is not necessary in the case of knitted material, and much lower amounts arerequired for filament yarn (2 – 10% of the weight of the warp) The main problem resulting from

summarizes the COD and biological oxygen demand (BOD) values determined for various sizes

To estimate the COD/BOD load released from a desizing step, Eqs (1) and (2) can be used:

Table 6 Processing of Cotton: Process Steps and Selected Parameters

polyacrylates

and added auxiliaries

Bleach

Dyeing

Thickener, dyestuff

Source: Ref 3.

Desizing of m ¼ 1000 kg of goods, which contain 5% of weight starch size ( p ¼ 0.05) cause a

a result, a COD value of 5000 mg/L can be calculated for the effluent

Two different paths can be followed to describe the behavior of sizes released in effluents:

high values of COD are coupled to high BOD

In such cases the polymer is removed from the waste stream in the WWT/CWWT byflocculation, adsorption, hydrolysis, and, to a certain degree, by biodegradation.Representatives are PVA, CMC, and acrylate sizes [16,17]

The strategies to handle size-containing wastes are dependent on the type of size andparticularly on the technique of desizing (Fig 5) In the case of starch, the desizing step isusually performed by enzymatic degradation, and in some cases oxidative degradation is used.However, the starch is degraded and a reuse is not possible in such cases The disadvantage

of a high COD caused by the released partially degraded starch is accompanied by easybiodegradation, thus the effluents can be treated in a WWT/CWWT with sufficient capacity forbiodegradation with no further problems

Table 7 COD and BOD per Mass of Size Released

Water-soluble sizes permit a recycling of the polymer for further weaving processes.Various techniques have been proposed to regenerate sizes released from the fabric Generalrequirements that have to be considered as fundamentals for possible reuse of sizes aresummarized as the following:

In practice, a recycling of sizes is hindered for a number of reasons In many casesvarious qualities of fabric containing different sizes are treated in a dyehouse and the type

of size is often not known The selection of sizes with regard to easy nation is necessary When a regeneration is intended a direct interaction between the selection

biodegradation/bioelimi-of size, desizing procedure, recycling processes, and the sizing/weaving process have to beconsidered

Two general technological strategies have been developed and proposed:

overview of these two techniques

Washing techniques have been proposed for PVA and acrylate sizes [18] When applyingwashing techniques the volume of concentrated washwater for each size is limited by the volumeactually spent in the following up sizing process (e.g., 900 L in Fig 6) [19 – 21] The use ofhigher amounts of water would increase the mass of recovered size, but the dilution of theregenerate is too much and hinders a reuse without reconcentration A typical balance for a full

The advantage of UF techniques is the higher rate of size recovery, because a reduction ofvolume is possible In some cases an evaporation step is used as final concentration step becausethe viscosity of the sizes increases and the permeate flow is reduced substantially Problems canresult from a change in the composition of the size due to changes in the molecular weightdistribution as a result of the cutoff of the UF membrane Attention has to be paid to avoidbiodegradation of the recovered sizes, which changes the properties of the polymer and causesintensive odor of the regenerates

Figure 6 Recycling of sizes (from Refs 18 – 24)

In general, for a recovery of sizes, the following points have to be defined:

Scouring, Alkaline Pretreatment, and Peroxide Bleach

A central step of pretreatment of natural cellulose fibers like cotton or linen for dyeing and

pretreatment of cotton Besides the destruction of the natural yellow-gray color of the fibers bythe bleach chemicals, a considerable part of the organic compounds is removed from the fibersduring the alkaline scouring step [3] Average values of the compounds present in raw cotton aregiven inTable 8

liquor ratio of 1 : 10, 1 kg of cotton is extracted with 10 L of water, thus a COD of 2000 mg O2/Lcan be estimated without consideration of the COD resulting from added auxiliaries orcomplexing agents At present auxiliaries are usually in use that are easily biodegradable; thusafter neutralization no problems should appear during the treatment in a CWWT The mainproblem arising from alkaline scouring is therefore due to the considerable load in COD

A typical recipe for alkaline scouring processes (liquor ratio 1 : 10) is as follows:

The total water consumption of the treatment including the rinsing step is approximately

up to 50 L/kg When the composition of an auxiliary is known, an estimation of the COD can bemade by calculation of the oxygen demand for total oxidation Examples are given below for

Na – polyacrylate (22CH222CHCO2Na22) and for Na – gluconate Basing on Eqs (3) and (4), theoxygen demand for 1 g of compound can be calculated

For the oxidation of 1 g of Na – gluconate, 810 mg of O2are required, and for the oxidation

formulations and the actual composition is given very rarely, but on the basis of the content ofactive compounds and an assumption of the chemical structure, an estimation of the contribution

of the auxiliaries to the COD can be made

The COD contribution of a recipe using 2 g/L of an auxiliary that contains 50%polyacrylate to the total COD in the wastewater will be approximately COD ¼ 2

Generally the treatment of waste water from alkaline scouring/bleaching (peroxide)processes will require an adjustment of pH and temperature, which is normally made by mixingwith wastewater from other treatment steps When surfactants, complexing agents, and so on,

Table 8 Average Composition of Raw Cotton

with good biodegradability/bioelimination have been selected, the COD load is removed inCWWT without problems The main load of the COD is due to the substances released from thefibers and added auxiliaries, thus an optimization of the load of COD released is limited to theauxiliaries only; however, these components will represent only a minor part of the total COD.The application of chlorine bleach on the basis of hypochlorite/chlorite for the preparation

of cotton/linen results in considerable formation of AOX in the effluents Such processes should

be replaced by bleach processes on the basis of peroxide To obtain a sufficient degree ofwhiteness during the bleach, a two-step bleach (peracetic acid/peroxide) process has beenproposed in the literature [25 – 27] Such processes avoid the formation of chlorinated organiccompounds (AOX)

Mercerization

Depending on conditions applied, the treatment of cotton textiles in concentrated alkalinesolutions, for example, 300 g/L NaOH, leads to increased luster, improved dimensionalstability, high uptake of dyes, and changes in strength and hand Usually a continuous treatmentprocess is applied As a result enormous amounts of concentrated caustic soda solution have to

be removed during the washing step As a typical value approximately 300 g of NaOH aretransported per 1 kg of cotton into the following up stabilization/washing baths In thestabilization step the caustic soda is rapidly removed by washing with diluted caustic sodasolutions The effluents from the stabilization step contain approximately 40 – 60 g/L NaOH.Figure 9 gives an overview of the steps during mercerization of cotton

The high costs for the consumed NaOH and the costs for neutralization of the NaOH inwastewater favor the recycling of NaOH by reconcentration procedures Normally a re-concentration is made up to at least 400 g/L NaOH Starting from a diluted NaOH containing

50 g/L NaOH, 7.8 L of water has to be removed to obtain 1 L NaOH with 440 g/L Thereconcentration is usually made by reboiling For this purpose evaporation plants with severalevaporation stages are in use The use of several stages (normally at least three stages) is of

Figure 9 Mercerization of cotton (from Refs 28 – 31)

importance to keep the energy consumption of the process within reasonable dimensions.Typical values for energy consumption are 0.2 – 0.3 kWh/kg of evaporated water Largeamounts of waste energy are released from the condensation of the evaporated water and have to

be used in the form of warm water Care has to be taken to achieve a reuse of the warm waterbecause the degree of heat recovery is essential to obtain an acceptable return on investment(ROI) of the unit [28]

Purification of the reboiled caustic soda is important to remove sizes mercerization), dyes (mercerization of dyed materials), fibers, and impurities released fromthe fibers Important techniques are filtration, centrifugation, flotation processes, andoxidative processes [29 – 31] The application of membrane processes for reconcentration islimited to low concentrations of NaOH because of the insufficient chemical stability of themembranes

(raw-The reuse of the diluted caustic soda from the first stabilization compartment in otherprocesses, for example, alkaline scouring, has been recommended Problems can arise fromvariations in concentration and impurities present in the reused lye, so the recycling of thediluted NaOH for other treatment processes is not used widely As the amount of caustic sodathat can be reused for other processes is low compared to the amount of NaOH released from themercerization step, regeneration by evaporation is normally the favored process

Dyeing of Cellulose Fibers

Dyeing of cellulose textiles can be performed at all stages of textile processing, for example,fibers, yarn, fabric, or garment dyeing Depending on the desired final properties of the dyedmaterial, various classes of dyes are used, which are collected in gamuts of common application.Important classes of dyes are direct dyes, reactive dyes, and vat dyes, including indigo and sulfurdyes

Wastewater problems mainly arise from three different sources:

soda, silicates), COD (glucose, hydroxyacetone), N-content (urea)

literature [33 – 35] When optimized dyes and processes with a high degree of fixation areimplemented into a dyehouse, problems of colored wastewater can be minimized As heavymetal ions are mainly present in complexed form in the dyestuff, a lowering of the Cu and Nicontent in the wastewater goes in parallel with an increase in dyestuff fixation A similarsituation is found with AOX values, which result from the halogen bound in the dyestuffmolecules In dyehouses where chlorine bleach has been substituted by other bleachchemicals, halogens bound in dyes can cause a main contribution to the AOX value found inthe wastewater

phthalo-cyanine dyes) and AOX from covalently bound halogen is comparable with direct dyes.Selection of processes with a high fixation of dyestuff yields a considerable decrease in Cu/

Ni concentrations and AOX For the fixation process certain amounts of alkaline are added tothe dyebath As the total amount of alkali used is low compared to the consumption of alkaliduring mercerization, scouring, and bleach, high pH due to the alkali from reactive dyeing is

of minor relevance Two main problems have to be mentioned in connection with reactivedyeing [36]:

High load of soluble salt (NaCl, Na2SO4) For acceptable exhaustion of dyes,considerable concentrations of salt (up to 50 g/L) are required in exhaust dyeingprocesses The release of the used dyebath transports a rather high load of salt into thewastewater stream When a liquor ratio of 1 : 10 is applied, 10 L of dyebath are usedfor dyeing of 1 kg of goods, thus at a salt concentration of 50 g/L an amount of 0.5 kgsalt is released for dyeing of 1 kg of goods.

wastewater particularly arises when dyestuff exhaustion and fixation proceed only to alimited degree, typically only 70 – 80%, so that between 30 and 20% of the dye isreleased with the spent dyebath and the washing baths that follow Such a situation isobserved particularly with reactive dyeing processes where a covalent reaction of thedye with the fiber takes place but some of the reactive groups become hydrolyzedduring dyeing and thus some dye remains unfixed in the dyebath Depending on thegeneral method of dyeing, two different qualities of colored wastewater can beidentified (Fig 10)

Particularly in the case of dyes with a limited degree of fixation the dyestuff content in thewasted water leads to intensively colored wastewater As the reactive group of the unfixeddyestuff is hydrolyzed into an inactive form, a reuse is not possible On the basis of an exhaustdyeing with 5% color depth, a liquor ratio of 1 : 10, and a degree of dyestuff fixation of 70 – 80%corresponding to 3.5 – 4 g/L of dye are fixed on the goods and 1.5 – 1 g/L of hydrolyzed dyes arereleased with the dyebath

For exhaust dyeing processes a reduction of the liquor ratio leads to significant ments When the dyestuff fixation is known for a certain liquor ratio, the lowering of the amount

improve-of unfixed dye released into the wasted water can be estimated as a function improve-of the liquor ratio

Figure 10 Sources for colored wastes from textile dyeing operations (from Ref 55)