Press Release TOTAL WATER MANAGEMENT IN COTTON TEXTILE INDUSTRY Y.V.V. SATYANARAYANA HEAD- TECHNOLOGY ION EXCHANGE (INDIA) LTD.

Transcription

1 1 Press Release TOTAL WATER MANAGEMENT IN COTTON TEXTILE INDUSTRY INTRODUCTION: Y.V.V. SATYANARAYANA HEAD- TECHNOLOGY ION EXCHANGE (INDIA) LTD. The textile industry accounts for 14% of India s industrial production & around 27% of its export earnings. From growing it s own raw material (cotton, jute, silk &wool) to providing value-added products to consumers (fabrics & garments), the textile industry covers a wide range of economic activities, including employment generation in both organized & unorganized sectors. Manmade fibre accounts for about 40% share in the cotton dominated Indian textile industry. India accounts for 15% of the world s total cotton crop production & records the largest production of silk. Besides natural fibres such as cotton, jute & silk, synthetic products such as polyester staple fiber, polyester filament yarn, acrylic fibre & viscose fibre are produced in India. In India about 82% of the textile mills use cotton fiber & the remaining synthetic fibres. Textile mill operations consist of weaving, dyeing, printing & finishing. These processes are carried out in different steps, each employing various chemicals and dyes. The unused materials are discharged as waste water or effluent, most of the which contain colour, higher bio-chemical oxygen demand (BOD) values and toxic heavy metal concentrations. If these effluents are discharged untreated into surface water bodies, they create pollution and affect the native microflora. MANUFACTURING PROCESS The manufacture of cotton includes two major categories: i) Dry process ii) Wet process Dry processes include opening of cotton, blending, mixing, carding, combing, spinning, weaving & knitting. The dry cloth produced is not suitable for various unit operations in the process houses & the fabric is thus subjected to a series of operations, called wet process. In the wet process water is consumed & waste water is discharged as effluent. The wet process is of major importance from the environmental point of view. In the wet process, cloth passes sequentially from one process house to the next in order to transform the woven or knit goods into finished fabric. The basic processes consist of singeing, desizing, kiering, bleaching, mercerization, dyeing, printing, & finishing.

2 2 WATER USAGE IN TEXTILE INDUSTRY The overall water consumption in any textile mill depends on the quantity as well as the quality of the cloth produced, process of manufacture & sequences adopted for rinsing, washing and the availability of water. However, the water requirement for different purposes in a textile mill can be generalized as below. Water Consumption Pattern in Textile Mills Purpose Water Consumption % Steam production 5 Cooling water 6 Demineralised water for specific purposes 8 Wet process 72 Sanitary use 8 Fire fighting etc. 1 Water requirements vary from litre per kg of cloth produced with an average value of 235 litre per kg of cloth processed. Water Consumption Pattern in Wet Processes Process Total Water Consumption (%) Bleaching, Finishing 38 Desizing, Scouring 18 Dyeing 16 Printing 8 Boilers 14 Humidification in spinning 6 Total 100 Specific Water Consumption in Various Machines Machine Norms (l/kg of cloth) Desize saturator 2 Caustic saturator (J-box) 2 Peroxide saturator (J-box) 2 Kiers: a)steel open b)steel pressure c)cement 5 8 5

3 1 Washing machines: a)tight rope b)slack rope c)tensitrol Open width bleaching range 50 Mercerizing machine 25 Jigger 30-50* Open width soaper 15 Continuous dyeing range 25 Yarn dyeing plant 60** Ager 2.5 Water mangle 2 Starch padding mangle 1.5 Felt calendar 1.5 Shrinking range 5 * Depends on the type of dye ** Litre per kg of yarn [Note: - These values are for general guidance only & not absolute minimum possible consumption. Depending on any special features of the machines in any mill or any special processing parameters to fit the conditions in that mill.] Quality of water required in various cotton textile processing operations are: Quality of Water for Wet Processing of Textiles. Characteristics Tolerance for bleaching, dyeing & subsequent processing Colors (Hazen unit) Max. 5* Turbidity (silica scale units) Max. 5 ph value Total hardness expressed as CaCO3 50 Iron as Fe, ppm 0.25 Manganese as Mn, ppm 0.10 * When dark or medium dark dyeing is done. Waste Water Characteristics of Textiles The pollution features of textile waste water differ widely among various segments of the industry, each type presenting a special treatment problem. Organic substances such as dyes, starches & detergents in textile waste undergo chemical & biological changes which consume dissolved oxygen from the receiving stream & so destroy fish life. Colours from dyes may not be toxic but are aesthetically objectionable, particularly in recreational waters. Certain carrier chemicals used in dyeing, such as phenol may add taste & odour. A composite waste from an integrated cotton textile plant consists of the following materials: starches, dextrin, gums, glucose, waxes, pectin, alcohol, fatty acid, acetic acid, soaps, detergents, sodium hydroxide, carbonates, sulfides, sulfates, chorides, dyes & pigments, CMC, gelatin, dye carriers (phenols & benzoic acid), peroxides & chlorine bleach compounds. The chemicals used in various unit operations of textile processing and the pollution effects are as follows

4 2 Unit Processes, Chemicals Used and Pollution Caused Unit Process Composition of waste water Characteristics Desizing Starch, glucose, CMC, PVC, resin, fats & wax. High BOD 35%-50% of total CMC & PVA don t exert a high BOD. Kiering Caustic soda, wax, grease, soda ash, soda silicate & cloth pieces. Strongly alkaline, dark colored, high BOD (30% of total) Bleaching Hypochlorite, chlorine, caustic soda, H 2O 2, acids. Alkaline & contributes fairly high BOD. Mercerizing Caustic soda Strongly alkaline, low BOD, (<1% of total), high sodium content. Dyeing Various dyes, mordant & Strongly coloured, high BOD & Printing reducing agent. Various colors, starch, gums, oils, china clay, mordants, and acid metallic salts. Finishing Traces of starch, tallow common salt, Na 2SO 4. COD. Highly coloured, fairly high BOD & oil. Slightly alkaline, low BOD. The characteristics of waste water generated from various stages of cotton processing is shown in the following tables : Table: Characteristics of Textile Effluents: Characteristic Desizing Kiering Bleaching Mercirizing Dyeing Printing Combi ned ph Alkalinity, mg/l Total solids, mg/l TDS, mg/l Suspended solids, mg/l BOD 20 o C, mg/l COD, mg/l

5 3 POLLUTION REDUCTION IN COTTON TEXTILE INDUSTRY For efficient control of water pollution a practical approach is to take following steps. 1) Reduction in the waste volume. 2) Reduction in concentration of chemicals used, thereby reducing their harmful effects. 3) Reduction of waste concentration by recovery & reuse. 4) Reduction of waste concentration by chemical substitution. 5) Reduction of waste concentration by process modification. 6) Segregation of drains. 7) Devising suitable treatment based on point of ultimate disposal, to meet the requirement. 8) Exploring the economics related to reuse of treated effluents Reduction in waste water volume Proper water management in the mills can achieve this. Of the water used for various operations, a large portion is consumed in wet processing. Water required for wet processing varies from mill to mill and depends upon: i) Source(s) of water ii) Availability of water iii) Quality & quantity of fabric processed iv) Processing sequence adopted v) Number of washings in the processing sequence vi) Type of processing machinery used The reduction in water consumption does not reduce gross pollution load. Handling of a correspondingly smaller volume of concentrated effluent is more economical, than handling large volumes of diluted effluent. Reduction in waste water volume can be achieved by: i) Reducing the number of washings in the sequence & use of hot water in washings. ii) Reutilisation of wash water from subsequent washings for the previous washings.i.e. recycling of less contaminated wash water in the preceding washing operations. iii) Use of counter-current system of washing at maximum places. iv) Use of standing bath in dyeing. v) Use of low material to liquor ratio systems. vi) Good house keeping and prevention of leakages & spillages. In this manner, textile mills may be able to achieve a reduction of 20%-40% in the volume of waste water generated. Reduction in concentration of chemicals It has been observed that invariably higher concentration of acids, alkalies, hydrosulphite, bleaching chemicals & various auxiliaries are used in textile processing as per manufacturer s recommendations to provide a large margin of safety. A critical study of chemical recepes used for each step is necessary to reduce the pollution load. This will help not only in reducing the pollution load but also in effecting considerable economy. Any extra residual chemical even from sweeping etc., entering the drains will exhibit its harmful effect & make the effluent highly vulnerable

6 4 Reduction of waste concentration by recovery & reuse From the viewpoint of possible reduction of waste load the following points need attention: (a) Recovery of caustic soda from mercerizing wash waters and its reuse after evaporation & concentration is a vital factor. Most composite mills, realizing its importance & related economics, have successfully adopted this advantage. (b) Use of standing bath in dyeing is possible for several baths & should be practised. (c) Finishing leftovers should be collected & reused for finishing of either the same or other sorts after sufficient adjustment. (d) Possibility of reusing spent acids from carbonizing should be explored. (e) Synthetic sizing agents can be recovered from desizing both by suitable method & can be reused for sizing along with fresh size. Reduction of waste concentration by chemical substitution Following chemical substitutions are recommended to reduce pollution load: i) Use of carboxymethyl cellulose (CMC) & polyvinyl alcohol (PVA) in place of starch & gelatin used in sizing. The former have only 1%-3% BOD against 50% BOD of starch & 100% of gelatin. ii) Use of mineral acid in place of acetic acid. Mineral acids have zero BOD against about 60% BOD of acetic acid. iii) Use of synthetic detergents in place of soaps. Synthetic detergents exhibit 0-20% BOD iv) against 140% BOD for soaps. Use of mineral oils with nonionic emulsifier in place of traditional carding oils for woolens. The former have about 20% BOD against 100% BOD of the latter. v) Use of reactive colours in place of traditional vats & azoics. Reactive colours have very low BOD compared to corresponding vats & azoic colors. Reduction of waste concentration by process modification Some of the modifications in textile processing, which are capable of reducing the water pollution load are: Use of foam technology Use of transfer paper printing. Partial or complete replacement of printing gums by suitable emulsions. Use of mineral acid in desizing.

7 5 Segregation of drains This is an important factor and needs considerable consideration. The textile industry is an age-old industry and the drainage system is equally old. Most mills do not have separate drains for industrial effluent, domestic effluent and rainwater. Laying of new drains involves huge expenditure. If the effluent treatment to be finally adopted is based on biological methods, segregation of domestic effluent is not necessary. On the contrary domestic effluent is needed for the purpose of dilution of industrial waste. If preliminary and primary treatment only becomes necessary for industrial waste, it is advisable to segregate domestic waste from industrial waste to reduce the volume to be handled in the effluent treatment plant. Treatment of textile effluents Treatment of textile processing effluents involves many stages and consequently much expenditure. A good treatment system includes primary, secondary & tertiary treatments. Primary Treatment i) Screening: - Screening and grit removal are essential steps in waste water treatment as they remove coarse matter like lint and fibers in addition to heavy and readily settlable grit & dirt. ii) Equalization: - This is an essential step in treatment, as different streams of effluent possess different ph values & characteristics, and further treatment needs an effluent of uniform quality. Equalization helps in providing such effluent and prevents shock loads & sudden increase or decrease in ph values due to mercerising or kier boiling & other effluents. These effluent streams should be segregated and stored in separate holding tanks, and discharged to equalization tanks at a regular and uniform rate. iii) Neutralization: - After equalization of waste, its ph correction is necessary for the efficiency of subsequent treatment. For biological treatment the optimum ph range is 6-9, for coagulation by alum it is & for coagulation by ferrous sulfate the optimum ph is 9.5. Depending upon the ph of the waste, automatic dosing of mineral acid or lime solution can be made to get the desired ph. iv) Chemical Coagulation: - To remove colour, suspended impurities, and colloidal particles (starch & gums), the effluents are treated with coagulants (alum, ferrous sulfate, ferric chloride & chlorinated copperas). Coagulation efficiency can be improved by coagulation aids like polyelectrolytes. After adding the coagulant the waste water is clarified in a clariflocculator. The sludge is separated and dried on sand beds. Chemical coagulation by lime & ferrous sulfate affects colour removal and reduces the BOD & COD to a considerable extent. About 50% BOD reduction of chemical waste can be achieved.

8 6 Biological Treatment Textile processing effluents are amenable to biological treatment. Combined treatment with domestic sewage is advantageous in terms of dilution, economy and provision of microorganisms. Followings are some of the aerobic oxidation methods. i) Activated sludge process ii) Cyclic activated sludge process (C-Tech) iii) Submerged or floating fixed film biological system iv) Membrane bio -reactors v) Trickling filter vi) Aerated lagoon vii) Oxidation pond viii) Oxidation ditch Biological treatment is termed as secondary treatment. Microorganisms convert colloidal and dissolved carbonaceous organic matters into various gases and cell tissues. Development of bacteria, specificity of bacterial action, effect of water toxicity on bacteria leading to their killing and the need for efficient ph control are main drawbacks of biological treatment. Hydroxyl alkaline is detrimental to process performance and hence must be corrected. Bacterial treatment removes colour to an appreciable extent and can reduce BOD by more than 90%. Dyes have to be increasingly resistant to ozone, nitric oxides, light, hydrolysis, and other degrading environments to be successful in the commercial market. It is not surprising that most studies on the biological degradation of dyestuffs yield negative results when dyes are designed to resist this type of treatment. Of those dyes, which are known to undergo biodegradation, the azo dyes are perhaps most commonly studied, although they tend not to be readily biodegradable in sewage treatment works. It is reported that some azo dyes (e.g., acid yellow 151 & acid red 337) are moderately adsorbed but not biodegraded by the activated sludge process. Azo dyes like acid blue 113 appeared to be strongly adsorbed & possibly biodegraded; acid orange 7 & acid red 88 were moderately adsorbed & significantly biodegraded. It is also reported that of the various dye removal mechanisms in an activated sludge process, adsorption and/or biodegradation appeared to be the only two removal mechanisms. Because of the limitations of biological treatment in removing dyes and also in reducing dissolved salts, further treatment of textile effluents are needed to meet the pollution control limits using some tertiary treatment techniques. These are important because they serve as polishing of effluent treatment. They include filtration, adsorption, ion exchange, and chemical oxidation. Recycling of textile effluents using membrane technology As water is becoming a scarce commodity, sustainable development of the textile industry needs recycling of waste water generated to reduce the water requirement and also dependency on other water sources. As the cost of water supplied to industry keeps increasing, recycle schemes become more attractive with good payback periods. Already many textile industries in water scarce areas are installing water recycle plants using advanced membrane based treatment techniques.

9 7 Membrane processes cover an astonishingly wide range of separations, using different types of membranes. For treatment of textile waste water, membrane technologies are especially suitable because they operate at ambient temperature and thus consume low energy compared to other separation processes. Membrane bio-reactor (MBR), ultra filtration (UF), nano filtration (NF) and reverse osmosis (RO) are the frequently used membrane technologies for recycle of effluents in the textile industry. Membrane bio-reactor (MBR) : MBR technology is one of the latest technologies in biological treatment, which can be applied to textile waste water. Submerged membranes are used in place of clarifiers to separate sludge from the waste water so as to produce high quality permeate (usable water). These MBRs can handle very high sludge concentrations in the aeration tank because of which the size of the aeration tank reduces four to five fold. As the membrane acts as a fine filter, it does not require any further treatment using sand filters, activated carbon filters, etc. MBR has the following advantages over conventional treatment processes: 1. It requires less area 2. It does not require clarifiers and hence all problems related to clarifiers such as sludge bulking, sludge rising, etc. are eliminated. 3. It does not require sludge recycle 4. It does not require costly tertiary treatment units to make the effluents suitable for recycle. It makes the treatment scheme short and compact. 5. MBR produces high quality treated water, which can be directly fed into the RO system. 6. Because of high concentrations of sludge and long sludge age periods, it generates low volume of highly stabilised sludge. Ultra filtration (UF) : UF is mainly used as a pretreatment to NF and RO so as to reduce the silt density index (SDI), a parameter important to avoid NF/RO fouling. The following advantages accrue by for using UF as pretreatment to RO: 1. Increased flux in RO 2. Reduced cleaning frequencies of RO thereby reducing RO downtime and chemical cleaning costs 3. Minimising cartridge filter consumption. The cartridge filter is used only as safety a barrier after UF to take care of any possible contamination resulting from chemical dosing. 4. Increase life of RO membranes UF also selectively rejects some of the sparingly soluble dyes such as indigo. Though many soluble dyes may pass through the UF system. In some cases, charged UF membranes are employed successfully to remove soluble dyes. Nano filtration (NF) : NF has a much smaller pore size than UF, hence it can reject many colour causing elements. NF can very effectively separate the dye and concentrate it too. This mrthod of concentration and purification reduces the loss of dyes. Also, when dyes are removed from the concentrated salt solution, they can be reused in the process thereby reducing the pollution load and also saving water and salt.

10 8 In general it may be stated that this separation technology depends both on size separation as well as the charges on the membranes. It can be economically applied to separate organics, higher valency cations or anions and associated salts from monovalent salt. Thus softening of any aqueous stream is possible by separating out Ca ++ or Mg ++ or SO 4 - -, CO from Na +, Cl - etc. Reverse Osmosis (RO) : RO is a membrane technology used for separation of salts from textile waste water so as to make it reusable in the process. To understand RO it is necessary to first understand osmosis. When more concentrated solution is separated by a semi-permeable membrane, the flow of the less concentrated solution towards the more concentrated solution side takes place due to the difference in the osmotic pressure of the two solutions. Now, if by some means, we are able to apply external pressure which is equivalent to this difference in pressures on the more concentrated solution, the flow of water will stop and the systems will be in equilibrium. Beyond this, when the pressure is further increased, water will start flowing in the reverse direction i.e. from the concentrated solution side to the less concentrated solution side. In a typical RO system, the solution is first filtered through a rough filter like sand or active carbon or a dual media filter etc. If the solution contains (a) calcium, magnesium salts (b) iron (c) carbonates like calcium or magnesium, then acid dosing is introduced. The ph is adjusted and the solution is then filtered through a micro cartridge filter (usually 5-10 micron pore size). The pretreated water is then pumped into the RO tank with a high-pressure pump. The membrane separates the pollutants in concentrated form in the reject stream and pure water is collected as permeate. Since the membranes do not filter out (reject) the CO 2 generated by addition of acid in pretreatment, is physically removed in a degassifier system. The pressure range for RO systems varies from 10 kg/ cm 2 to 65 kg / cm 2. RO is more useful to separate salts and organic compounds from the textile effluents which are pretreated for removal of suspended/colloidal matter, and certain pollutants which are likely to foul the RO membranes. As textile effluents contain high amount of dissolved salts, RO is a suitable technology for separation of these salts and for producing permeate which can be used in the process. Regulatory Requirement for Discharge of Effluents from Textile Industry in India Standards for Effluents from Textile Industry Parameter ph Total suspended solids 100 Bio-chemical oxygen demand (BOD) 30 Chemical oxygen demand (COD) 250 Total residual chlorine 1 Oil and grease 10 Total chromium as Cr 2 Sulfide as S 2 Phenolic compounds as C 6H 5OH 1 Concentration not to exceed, milligram per litre (mg/l), except ph

11 9 Note:1. Where the treated effluent is discharged into a municipal sewer leading to terminal treatment plant, the BOD may be relaxed to 100 mg/l and COD to 400 mg/l 2. The quantity of effluent (litre per kilogram of product) shall not exceed 100, 250 and 80 in composite cotton textile industry, composite woolen textile industry and textile processing industry, respectively. REFERENCES: 1. Advanced Waste Waster Treatment by Trivedi 2. Water & effluents in textile mills by P.B. Jhala, M.M.Vyas, K.Subhramanyum 3. Pollution control in textile industry by H.R.Jones 4. Textile Effluent by Padma Vankar 5. IS: Guide for treatment and disposal of effluents of cotton and synthetic textile industry