Waste Treatment in the Process Industries - Chapter 8 doc

8 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. 8.1 IDENTIFICATION AND CLASSIFICATION OF TEXTILE WASTES 8.1.1 Textile Processes 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. Representative magnitudes 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 water can be estimated to exceed 4–8 billion cubic meters per year. The production of a textile requires several stages of mechanical processing such as spinning, weaving, knitting, and garment production, which seem to be insulated from the wet treatment processes like pretreatment, dyeing, printing, and finishing operations, but there is a strong 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 textile plants. Times have changed and the identification and classification of wastewater currently are fixed by communal regulations [1,2]. General regulations define the most important substances to be observed critically by the applicant, and propose general strategies to be applied for minimization of the release of hazardous substances. The proposed set of actions has to be integrated into processes and production steps [3]. Figure 1 gives a general overview of a textile plant and also indicates strategic positions for actions to minimize ecological impact. In this figure, the textile plant is defined as a structure that changes the properties of a textile raw material to obtain a desired product 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 can show extreme variability. Normally the legal regulations are interpreted as a set of wastewater limits that have to be kept, but in fact the situation is more complex and at present a complex structure of actions has been defined and has described useful strategies to improve an actual situation. 363 © 2006 by Taylor & Francis Group, LLC 8.1.2 Strategies to Reach Existing Requirements Figure 2 shows a general action path recommended to minimize a present problem in the wastewater 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, . use of high-temperature dyeing (HT-dyeing) processes for polyester fibers (PES) instead of carrier processes; . replacement of chloro-organic carriers; . replacement of preservatives containing As, Hg, or Sn organic compounds; . replacement of alkylphenolethoxylates (APEO) in surfactants [5]; . substitution of “chlorine” bleach for natural fibers by peroxide bleach processes; . substitution of sizes with poor biodegradability, e.g., carboxymethylcellulose (CMC); . replacement of “hard” complexing agents like ethylene-diamine-tetra-acetic acid (EDTA), phosphonates. The implementation of these steps into a dyehouse reduces the chemical load of the released wastewater considerably. In particular the replacement of substances that exhibit high toxicity or very low biodegradability will facilitate the following efficient treatment of the wastewater. Figure 1 Flow structure of a textile plant (from Refs. 2 and 3). 364 Bechtold et al. © 2006 by Taylor & Francis Group, LLC Optimization of Processes The second general step recommended to improve an existing situation is the optimization of treatment 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: . reconsideration of dyestuffs and machinery chosen in exhaust dyeing (degree of exhaustion, fixation, liquor ratio); . optimization of dyes and reducing agent in sulfur dyeing; . optimization of residual volumes of padders and printing machines; . optimization of water consumption. Separation and Recycling Besides the replacement of substances, the improvement of processes on an optimization of the handling of rather concentrated liquors, for example, used in sizing, caustic treatment like mercerization, dyeing, finishing processes, or in textile printing processes is the next step. As a desired 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 the recovery 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 an optimized treatment of the concentrates is more efficient and cheaper than a treatment of the full waste stream. Such treatments will concentrate on a minimization of costs for disposal (e.g., disposal of sludge, printing pastes, chemical products) or reaching existing limits defined for various parameters analyzed in the wastewater, for example, pH value, content of heavy metals, chemical oxygen demand (COD), adsorbable halogenated organic compounds (AOX) [8]. Figure 2 Action path for consideration and improvement of an existing situation (from Refs. 1–3 and 9). Treatment of Textile Wastes 365 © 2006 by Taylor & Francis Group, LLC General Wastewater Treatment In any case the wastewater will finally be fed into rivers, lakes, or the sea; thus some wastewater treatments have to be performed before the textile effluents are released either to the communal wastewater 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) are performed in the textile plant, while the following biological treatment (aerobic, anaerobic degradation) is performed either in the textile plant or in a CWWT. The site of the biological treatment is dependent on the location of the textile plant; however, a biological treatment of textile effluents preceding release into surface water is state of the art. 8.1.3 Definitions and Limits 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 of chemical costs and treatment of effluents with high toxicity. Over the last decade the situation has changed and limits for a considerable number of compounds and parameters have been defined to avoid problems with regard to the following: . biotoxicity (e.g., disturbance of biodegradation processes); . heavy metal content (accumulation in sludge of CWWT); . corrosion problems (e.g., sulfate can cause corrosion of concrete tubes); . total COD/BOD load in the released effluents (capacity of the CWWT). Table 1 gives an extract of important parameters for wasted water from textile plants, as defined by the Austrian Government [1]. The table contains limits defined for both direct release into surface water (rivers) and for release into a CWWT. Table 1 can be used as a guide to define “hazardous” wastes from textile plants. Besides the direct toxicity of substances like chlorinated hydrocarbons, organo-Hg compounds, or concentrated alkaline solutions, other parameters have been defined with regard to problems during biodegradation or accumulation in the sludge from CWWT. A particular situation is found with colored effluents, where limits for spectral absorption have been defined. While the toxicity of textile dyes is comparably low, these limits were derived from the visual aspect of the water released from a textile plant because they look “unhealthy.” As a result of these regulations, textile companies have to apply a strategic concept to lower both the daily load released into the wastewater stream and the concentrations of hazardous substances therein. On the basis of the action plan given in Figure 2, a stepwise 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 to develop a realistic concept for an efficient wastewater treatment without detailed analysis of the particular situation of a textile plant. The more intelligently the applied technical concept has been designed, the lower will be the expected costs for installation and working of the equipment. 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 during pretreatment, 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 combin- ing problems of the single fiber types. In Section 8.3 end-of-pipe technologies have been summarized. 366 Bechtold et al. © 2006 by Taylor & Francis Group, LLC 8.1.4 IPPC Directive of the European Community In the legislation of different national governments, some limits were defined especially for wastewater and air. The activities in Europe are covered by the Council Directive 96/61/EC concerning Integrated Pollution Prevention and Control (IPPC) [9]. This means that all Table 1 Representative Limits Defined for Release of Textile Waste Water Limits for emission Release into river Release into CWWT General parameters Temperature (8C) 30 40 Toxicity ,2 No hindrance of biodegradation Filter residue (mg/L) 30 500 Sediments (mL/L) ,0.3 — pH 6.5–8.5 6.5–9.5 Color, spectral coefficient of extinction: 436 nm (yellow) (m 21 ) 7.0 28.0 525 nm (red) (m 21 ) 5.0 24.0 620 nm (blue) (m 21 ) 3.0 20.0 Inorganic parameters (mg/L) Aluminum 3 Limited by filter residue Lead 0.5 0.5 Cadmium 0.1 0.1 Chromium total 0.5 1 Chromium-VI 0.1 0.1 Iron 2 Limited by filter residue Cobalt 0.5 0.5 Copper 0.5 0.5 Zinc 2 2 Tin 1 1 Free chlorine (as Cl 2 ) 0.2 0.5 Chlorine total (as Cl 2 ) 0.4 1 Ammonium (as N) 5 — Total phosphor (as P) 1 No problems in P elimination Sulfate (as SO 4 ) — 200 Organic parameters (mg/L) TOC (total organic carbon as C) 50 .70% biodegradation COD (chemical oxygen demand as O 2 ) 150 .70% biodegradation BOD 5 (biological oxygen demand as O 2 ) 20 — AOX (adsorbable organic halogen as Cl) 0.5 0.5 Total hydrocarbon 5 15 VOX (volatile organic halogen) 0.1 0.2 Phenol index calculated as phenol 0.1 10 Total anionic and nonionic surfactants 1 No problems in sewer and CWWT Source: Ref. 1. Treatment of Textile Wastes 367 © 2006 by Taylor & Francis Group, LLC environmental media (water, air, energy, ground) and a comprehensive description of the production have to be considered. In addition a broad harmonization of requirements for the approval of industrial plants can be reached. The classification of a company as an IPPC plant is based on the definition of the work concerning plants for the pretreatment (operations such as washing, bleaching, mercerization) or dyeing 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 24 hours. Capacity means what a plant is designed for and not what is really achieved (actual production). The treatment of fibers and textiles covers fibers, yarns, and fabric in the wider sense of the word, that is, including knitted and woven materials and carpets. As most textiles are treated 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 is defined by the best available techniques (BAT). In practice, this means precaution against environmental pollution by the use of these techniques, special equipment and better way of production, and an efficient use of energy for prevention of accidents and provisions for a shutdown of a production plant. The term best available techniques is defined as the most effective and advanced stage in the development of activities and their methods of operation that indicate the practical suitability of particular techniques for providing in principle the basis for emission limit values designed to prevent and, where it is not practicable, generally to reduce emissions and the impact on the environment as a whole. These available techniques are developed on a scale that allows implementation under economically and technically viable conditions, 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 also includes 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 and celluloseacetate, as well as from synthetic polymers (such as polyester, polyamide, polyacrylnitrile, polyurethane, polypropylene) are described, including blends of these textile substrates. 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 are considered 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-pipe techniques, thus covering pollution prevention and pollution control measures. Techniques presented 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 and higher cost techniques. To obtain a limitation of emission impact, different techniques are proposed corresponding to the basic possibilities for pollution prevention: . handling of concentrates from various processes such as textile pretreatment, residual dye liquors from semicontinuous and continuous dyeing, residual printing pastes, residual finishing liquors, residues of prepared but not applied dyestuffs, textile auxiliaries, and so on; . recovery of chemicals such as NaOH, sizing agents, indigo; . assessment of textile auxiliaries aiming at a reduction of emissions of refractory and toxic compounds to water by substituting harmful substances with less harmful alternatives; 368 Bechtold et al. © 2006 by Taylor & Francis Group, LLC . reduction of releases to air from thermal treatment installations like stenter frames; . reduction of releases to water by applying process-integrated measures and consi- dering the available options for wastewater treatment; wastewater treatment including pretreatment 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 together with municipal wastewater; . options for handling and treatment of residues and waste from different sources; . minimizing of energy consumption used in energy-intensive processes such as pretreatment, fixation of dyes, finishing operation, and drying. 8.2 FIBER-SPECIFIC PROCESSES The activities described in this section intend to minimize or avoid the release of chemicals into the stream wastewater by substitution, optimization, reuse, and recycling. Besides a lowering of the costs for following up general wastewater treatment, benefits due to minimization of chemical consumption are intended. As there are various specific problems arising from the particular treatment steps applied for different fibers, this section concentrates on the most important problems. Table 2 gives an overview of the annual production of textile fibers [10]. 8.2.1 Protein Fibers: Wool 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 finishing of woolen textiles is shown in Figure 3. Table 2 Annual Production of Textile Fibers 2001 Type of fiber Mt/year Man-made fibers Synthetics 31.6 Polyester 19.2 Polypropylene 5.8 Polyamide 3.7 Acrylics 2.6 Others 0.3 Cellulosics 2.7 Natural fibers Cotton 19.8 Jute 3.1 Ramie 0.2 Linen 0.6 Wool 1.2 Silk 0.1 Total 59.2 Mt, million tons. Source: Ref. 10. Treatment of Textile Wastes 369 © 2006 by Taylor & Francis Group, LLC Besides more general strategies of process optimization, three representative steps will be discussed in more detail because of their particular importance with regard to wastewater. The main problem resulting from these steps is given in parentheses: . washing of raw wool (COD); . antifelt treatment of wool (AOX); . dyeing processes (chromium). Washing of Raw Wool The high content of impurities in raw wool has to be removed before further processing, for example, 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 cause heavy 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 form of dispersed and dissolved substances. Figure 4 gives an overview of a general set of techniques that can be applied to lower the initial COD in the effluent from approximately 80,000 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 fresh water. A particular advantage arises from the fact that the dissolved sweat components exhibit Figure 3 General processing route of woolen textiles (from Ref. 3). Table 3 Average Composition of Raw Wool Component % Fiber, protein 58 Wool-fat, lanolin, waxes 14 Soil, plant material (cellulose) 13 Sweat/salt, water soluble 5 Humidity 10 Source: Refs. 3,11,12. 370 Bechtold et al. © 2006 by Taylor & Francis Group, LLC distinct washing properties for raw wool and thus a certain content of dissolved sweat is favorable to improve the washing effect. Various treatment concepts have been presented in the literature [11–13]. Besides the release of the pre-treated wastewater into the CWWT and aerobic biodegradation, in some cases evaporation 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 to shrinkage and formation of felts. This behavior is usually undesirable and thus an antifelt finishing is the most important treatment during the processing of woolen textiles. One of the most important standard procedures, the Hercosett finish, is based on the oxidative treatment of wool by application of compounds that release chlorine. Examples for applied chemicals are NaOCl, Cl 2 gas, and dichloroisocyanuric acid (DCCA) [14]. Such processes lead to the formation of adsorbable halogenated organic compounds (AOX) in high concentrations. Typical concentrations found in a continuous antifelt treatment are shown in Table 4. The high dissolved organic carbon (DOC) determined in the baths is one of the sources for the formation of high concentrations of chlorinated compounds. The formation of chlorinated products is the result of chemical reactions directly with the fiber, with organic compounds released from the fibers, and with added auxiliaries. An average size of continuous treatment plant for antifelt treatment of wool releases approximately 140 g/hour AOX. As an optimization of the process is possible only within certain limits, alternative processes for an antifelt treatment have to be chosen to substitute the chlorination process, for example, enzymatic processes, oxidative processes (KMnO 4 , persulfate), or corona or plasma treatment. In many cases combinations with resin treatments are proposed. Figure 4 General scheme for the treatment of effluents from wool washing (from Refs. 11–13). Treatment of Textile Wastes 371 © 2006 by Taylor & Francis Group, LLC Chromium in Wool Dyeing A considerable part of the wool dyes contains Cr complexes. The average consumption of dyes used in 1992 is shown in Table 5. At this time approximately 70% of all dyes used contain chromium. As shown in Table 1 the wastewater limit for chromium is 0.5–1 mg/L and Cr VI is 0.1 mg/L. While conventional 1 : 2 and 1 : 1 dyes permit chromium concentrations in the dyebath at the end of the dyeing process of 3.0–13.0 mg/L Cr, the application of modern dyestuffs and optimized processes permits final concentrations to approximately 1 ppm. By general optimization of the process (e.g., dosage of acid), use of dyes with a high degree of exhaustion, 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 for wastewater treatment concentrate on the removal, for example, by flocculation and precipitation, but as a result chromium-containing sludge/precipitate or concentrates are obtained that need further treatment. 8.2.2 Cellulose Fibers: Cotton General Cellulose fibers (Co, CV, CMD, CLY) represent the main group of textile fibers used [10]. In this section cotton will be considered as a representative type of fiber because the treatments for other cellulose fibers are similar in many cases, and often milder conditions are applied for other cellulose fibers. Table 4 Concentrations for AOX Determined in the Chlorination Bath of the Chlorine-Hercosett Process Parameter Concentration AOX 20 mg/L CHCl 3 160–1200 mg/L CCl 4 25–50 mg/L DOC 1110 mg/L Source: Refs. 3,14. Table 5 Dyestuff Consumption in Wool Dyeing Dyestuff % 1 : 2 Metal-complex a 35 Chromium dyes a 30 Acid dyes 28 1 : 1 Metal complex dyes a 4 Reactive dyes 3 a Contain Cr or Cr-salts are added. Source: Refs. 3,15. 372 Bechtold et al. © 2006 by Taylor & Francis Group, LLC [...]... expected in the wastewater, when 10 L of washing water is applied per 1 kg of goods The emission of colored wastewater here can be divided into two different sources, the wastewater from the washing of the dyed material and the residual filling of the padder Depending on the length of the dyed piece (80 0 – 5000 m) the contribution of the filling of the padder to the total dyestuff concentration in the wasted... during sample printing In particular for the production of very short lengths (e.g., 120 m), a considerable portion of the printing paste is required for the filling of the printing machine Depending on the coverage factor of a pattern, approximately 55 80 % of the paste is used for printing, while 45– 20% is spent for the filling of the printing machine, which is considerable with a mass of 5 kg in. .. aspects of textile processing J Soc Dyers Colour 1993, 109, 32 – 37 Huang, C.R.; Lin, Y.K.; Shu, H.Y Wastewater decolorization and toc-reduction by sequential treatment Am Dyest Rep 1994, 83 , 15 – 18 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 © 2006 by Taylor & Francis Group, LLC Treatment of Textile Wastes 98 99 100 101 102 103 104 105 106 107 1 08 109 110 111 112... fibers into them The pretreatment of elastomer-containing fibers can be regarded as representative for the pretreatment of other man-made fibers To improve the behavior of these fibers during spinning, winding, weaving, and knitting, considerable amounts of auxiliaries are added Typical examples for such compounds are: fatty amines; polyethylene glycols; hydrocarbons; silicone compounds In particular in. .. variations of textile printing processes are found in textile production depending on the type of fiber, applied dyes, desired effect, and fashion At present, flat screen printing and rotary screen printing are the main techniques used Here the dyestuff is dissolved/dispersed in a printing paste containing thickener and chemicals With every change of color, the filling of the dosing unit and of the screen has... (chlorine containing dyestuff) and from heavy metal content resulting from metal ions complexed in the dyes (e.g., Co, Cu, Ni) Attention also has to be given to the use of antimicrobial agents in the printing pastes, which are added to block the microbial growth that results in degradation of the thickener and lowering of the viscosity of the printing paste Generally, any release of printing pastes into... precipitation/flocculation, and reverse osmosis [120] 8. 3.5 Wastewater from Printing and Finishing Processes The main difference in the wastes from dyeing processes is identified in the presence of thickeners and, in some cases, additional difficulties can arise from the added auxiliaries and hydrotropes (e.g., urea) As a result, a high COD is found in the effluents and end-of-pipe technologies that form sludge have... into the wastewater should be avoided, and in many countries such action is forbidden Figure 13 gives an overview of the possible proceedings to minimize chemical load in the wasted water from the release of printing pastes [64,65] First the consumption of printing pastes has to be minimized by: Minimization of the required volumes to fill the equipment, e.g., printing screen, tubes, pumps, and container... for farming, these components are transported to farmland and thus get released there The reuse of bleach baths after catalase treatment has also been proposed in the literature [84 ] 8. 3.4 Treatment of Wastewater from Dyeing Processes The wastewater from dyeing processes contains a lot of components in various concentrations, for example, dyestuff, alkali, acid, salt, and auxiliaries [85 ] In a first... the weight of the warp) The main problem resulting from the desizing step is the high load in COD found in the polymer-containing effluent Table 7 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: LCOD ¼ Cpm  10À3 (1) À3 (2) LBOD ¼ Bpm  10 Table 6 Processing of Cotton: Process . mechanical processing such as spinning, weaving, knitting, and garment production, which seem to be insulated from the wet treatment processes like pretreatment, dyeing, printing, and finishing operations,. applying process- integrated measures and consi- dering the available options for wastewater treatment; wastewater treatment including pretreatment onsite before discharge to the sewer as well as treatment. should appear during the treatment in a CWWT. The main problem arising from alkaline scouring is therefore due to the considerable load in COD. A typical recipe for alkaline scouring processes (liquor