In this article we will discuss about the methods and techniques used to treat waste water from textile industry.
The textile industry actually represents a range of industries with operations and processes as diverse as its products. It is almost impossible to describe a typical textile effluent because of such diversity. After fabrics are manufactured, they are subjected to several wet processes collectively known as ‘finishing’ and it is in these finishing operations that the major waste effluents are produced.
These finishing processes are complex and ever-changing. This is a fact of life that is reflected in the variety of chemicals that find their way into textile finishing waste-waters.
Treatment of Waste Water from Textile Industry
Treatment of Waste Water – Water Pollution from Boilers:
Boilers meant for raising steam by combustion of fuels like coal, fuel oil, etc. contribute substantially in causing pollution in two ways:
1. Water pollution caused due to discharge of boiler blow down water.
2. Air pollution caused due to emission of chimney gases into the atmosphere.
In order to ensure that concentration of dissolved solids in the boiler water does not exceed the permissible limit, some amount of boiler water has to be discharged from the boilers either continuously or intermittently.
The boiler blowdown water so discharged contains very high amount of dissolved solids ranging from as low as 3500 mg/l for water tube boilers to as high as 10,000 mg/1 for Lancashire boilers.
In fact in some cases, the blowdown water from the Lancashire boilers are known to contain as much as 60,000 mg/l of dissolved solids. Whereas raw water with high carbonate alkalinity is fed into the boilers without softening, hardness precipitates inside the boilers giving rise to high level of suspended solids in the blowdown.
The alkalinity of the blowdown is also very high and is generally about 15 per cent of the dissolved solids. Likewise, pH of the blowdown is also about 10.5 to 11.0. Thus the blow-down adds to the water pollution in terms of dissolved solids, suspended solids, alkalinity and pH.
Treatment of Waste Water – Water Pollution from Textile Water-Treatment Plants:
It is required that the water used for processing of textiles and in generation of steam in the boilers should not be hard. The type of softening treatment generally imparted to the hard water is ion-exchange softening process. There is no water pollution from the softening process as such.
However, when the ion-exchange resin is regenerated with concentrated solution of common salt, its effluent will contain very high amount of sodium ions apart from high dissolved solids. In case of partial de-mineralisation process, the effluent contains weak acid. Some mills have water treatment plant of precipitation type.
Here lime is added to precipitate carbonate hardness and soda ash is added to remove non-carbonate hardness. Some coagulants like alum or sodium aluminate, etc. are added to accelerate the precipitation process.
In this type of treatment, some amount of water is drained out from the bottom of the reaction vessel to get rid of the precipitates. This waste-water contains high amount of suspended solids and adds to the water pollution when it joins the composite stream.
Treatment of Waste Water – Characteristics of Textile Waste-Waters:
The characteristics of the waste-waters from a textile plant will depend on the specific operations in the plant. It is misleading to speak of a typical textile effluent. The type of fibre involved and machinery employed are the main factors determining the type and quantities of chemicals present in the textile waste-waters.
Finishing processes may be either batch or continuous. For batch processes, the discharge is intermittent, with the interval between discharged depending on the operations.
All the waste-waters from a batch process are likely to come from the same operation, the first being the most heavily contaminated and the last rinse the most dilute. For continuous processes, a steady flow of effluents with moderate concentrations is expected.
Developing a database from sampling the textile effluent discharges at frequent intervals should lead to establishing reliable average values. In addition to frequent sampling, the other approach to determining the characteristics of textile waste-waters is to study the process, its waste components and volumes.
For example- the amount of organic matter that is removed from a fabric in the course of normal textile processing can be visualised when one considers that about 10 per cent of the gross weight of a cotton fabric consists of natural impurities that may be removed in processing.
For a firm that manufactures 20 tons of fabric per week, about 2 tons per week of impurities are discharged to the sewer. With known discharge volumes, the concentrations of the impurities in the effluent may be estimated.
Kremer and others reviewed the pollutants generated by the textile industry. They divided the pollutants into four general groups, i.e. sizes, detergents, dyes and priority pollutants. They reported that most of the priority pollutants contained in textile industry effluents are aromatics, halogenated hydrocarbons and heavy metals.
Cotton and linen contribute organic matter from the noncellulosic materials that are present in the natural fibres, whereas wool contains sand and grease that are removed during scouring. Synthetic fibres may contain spinning oils and antistatic dressings.
Textile wastes are generally coloured, highly alkaline and high in BOD, suspended solids and temperature. The raw waste-water (pH = 9) of a bleachery had 660 mg/l of BOD, 2080 mg/l of COD, 34 mg/l of oil and 2700 mg/l of TDS.
Randall and King reported that waste-waters from textile dyeing and finishing operations may be characterised as high in organic matter, both biodegradable and non-biodegradable. In addition, they tend to be high in surfactants and contain potentially significant concentrations of oils, phenols and heavy metals such as chromium, zinc and copper.
Randall and King reported the characteristics of raw waste-waters from three plants. The BODs varied between 260 and 560 mg/l. The colour (APHA units) varied between 1000 and 1335. The hue was brown, red to black or yellow to black. Kertell and Hill reported the characteristics of the waste-water from a dye and finishing company.
The average BOD was 371 mg/l and the average colour was 113. About 70 mg/l of oil and grease were present. The total solids concentration was about 480 mg/l. Troxler and Hopkins reported that the introduction of continuous dyeing machines had significantly increased the strength of carpet finishing waste-water.
This is due to the use of natural bean gum thickeners as a viscosity modifier for the dye solutions. The average BODs and COD from ‘beck dyeing’ wastewater were 232 and 943 mg/l, respectively. The average BODs and COD from continuous dyeing wastewater were 930 and 2912 mg/l, respectively.
Davis reported that the BODs of a dyehouse waste-water varied between 20 and 1250 mg/l with an average of 634 mg/l. The colour (APHA units) varied between 7,700 and 13,100 mg/l. The average flow of this plant was 1.58 mgd.
A wide variety of methods have been used for reporting results related to colour-removal processes. These include the use of American Public Health Association (APHA) colour units, transmittance, hue, intensity, etc. The current EPA standards are, however, based on a colour analysis procedure developed by the American dye manufacturer’s institute.
Treatment of Waste Water – Treatment of Textile Waste-Waters:
To solve the problems of treatment of waste-waters from a textile plant, several alternatives should be included.
The alternatives are the following:
1. In-plant control for waste reduction.
2. Treatment to ‘reuse standard’ on an external (end-of-line) basis or by closed-loop recycle systems.
3. Direct discharge to municipal waste treatment systems (i.e. POTWs).
4. ‘On-site treatment’ of textile waste-waters at POTWs before combining with municipal wastewaters.
5. ‘Pretreatment’ of textile waste-waters at the plant before discharging to sewer.
1. In-Plant Control:
A major portion of the waste load is inherent in the methods of textile processing and is independent of the efficiency of the processing plants. For example, the chemicals used for sizing to wrap yarns must be taken off before subsequent bleaching and dyeing.
In the industry the normal practice is washing the material with a good flow of running water. This washing operation is the most water-intensive in any textile mills and considerable quantity of water is consumed in this process.
Since the concentration level of pollutants in the combined waste-water are not generally high, at the cost of increase in concentration at the inlet level of the treatment plant, water can be saved by judicious use of water.
The biological treatment units are designed on the basis of organic loading and hence increase in concentration will not affect the size of the biological unit. On the contrary size of the units designed on hydraulic loading can be reduced apart from saving precious water.
Recently suitable enzymes have been developed which can act upon the starch applied during sizing rapidly at high temperatures ensuring faster degradation of the starch without affecting the material to be treated. With this the desizing operation can be made continuous using only a fraction of water originally required.
Chemicking is a treatment with a weak solution of sodium hypochlorite. Often excessive alkalinity of the bath detracts from bleaching. For satisfactory treatment, the bath alkalinity should not be more than 10 per cent of the available chlorine.
The spent bleach can be further replenished at the required time thus saving about 2 to 3 litres of water per kg of fabric for each chemicking operation. Similarly the scouring bath also can be used 8 to 10 times after replenishing.
In mercerising 20 per cent caustic soda is used for treating the fabric and the treatment period ranges from 30 seconds for fabrics to 2 minutes for yarns. Caustic soda on cellulose material is a very tenacious chemical to wash and industries use larger quantity of water than required. This causes not only loss of water but also requires unduly long time to recover caustic from wash waters.
The measures by which water can be conserved in mercerising are:
1. Use higher temperature for the wash water.
2. Use counter-counter system of washing whereby caustic soda content in the fabric is progressively reduced.
3. Reduce the concentration of the caustic soda to the optimal limit of 20 per cent strength.
4. Increase the time of washing and the force of the wash water.
Most of the caustic used in textile mills comes from the kiering and mercerising section. Dialysis and evaporation methods have been employed to purify and reuse the caustic soda from these waste streams.
Groves and Buckley evaluated membrane separation technology for the reuse of textile effluents.
They studied two pilot-plant applications.
(i) High-temperature ultrafiltration of desizing effluent for polymer size recovery and water reuse, and
(ii) Hyperflltration (reverse osmosis) of mixed cotton/synthetic fibre dyehouse effluents for water reuse.
The membrane separation processes may offer potential for the recovery of various chemicals like sizing agents. Groves and Buckley concluded that the use of closed- loop recycle systems is technically feasible for textile waste-waters. They also discussed the ‘fouling’ problems and requirements of cleanings for the restoration of the design flux.
Davis reported that ultrafiltration has several applications for the recovery of textile sizes. Also, latex recovery can be accomplished using ultrafiltration membranes. Tinghul showed how the science of reverse osmosis offers a basis for the choice of membrane materials for use in reverse osmosis applications involving the separation of dyes in aqueous solutions.
The reuse of a textile effluent may be economic only if the plant faces an acute shortage of water. Complete reuse will probably be unrealistic under any circumstances for many more years.
For many textile plants, direct discharge to POTWs may be the best alternative. A mill’s waste-water may be clean enough or low enough in volume to be treated by the POTW at little or no extra cost. Even if preliminary or primary treatment is required, the cost to the mill may be much less than if a complete treatment facility were required.
Jones listed three advantages of combined treatment, i.e. the direct discharge of textile waters to POTWs. Potential economy of operation is the first advantage. Textile waste-waters may not contain enough nutrients (nitrogens and phosphorus) required for biological treatment.
Hence, combining such waste-waters with nutrient-rich domestic waste-waters appears to be another advantage. Finally, in combined treatment, the dilution of highly concentrated textile wastes can be achieved, which prevents shock loads of toxic materials from killing the bacteria in the treatment plant.
Newlin studied the economic feasibility of treating textile wastes in municipal treatment plants. The general conclusion, based on 26 municipalities serving some 100 textile mills, was that problems and conditions were so diverse that each case must be given individual attention.
Three of the six textile mills covered by the study would have saved money by direct discharges, as compared with the costs of treating their own wastes. However, three plants were saving money by treating their own waste-waters. In general, the mills with small amounts of waste-waters were better off paying their service charges to the POTWs.
Jones concluded that the findings regarding the cost of waste-water treatment in relation to total textile manufacturing costs in the southeastern United States raise doubts about the significance of waste-water treatment requirements as an important factor in a competitive strategy as long as the effluent standards do not change drastically. On the other hand, some old plants may face treatment costs that are high enough to create financial problems for them.
Figure 17.16 shows schematic diagrams for alternatives (a) and (b). It is important to note how ‘on-site treatment’ and ‘pretreatment’ are defined for this discussion.
On-site treatment refers to any additional treatment at the POTW before combining the textile effluents with the municipal waste-waters for the subsequent treatment. Such additional treatment of the textile waste-waters at the POTW may be physico-chemical or biological. On-site treatment appears to be feasible when several small, closely located textile plants discharge the waste-waters to the same POTW.
5. Pretreatment of Textile Waste-Water:
Pretreatment refers to the treatment of waste-waters by the textile plants before discharging to the sewer (Fig. 17.16). Again, such treatments may be physico-chemical or biological. A combination of physico-chemical and biological (both anaerobic and aerobic) processes may also be feasible. Such pre-treatment appears to be feasible for large textile plants.
i. Physical/Chemical Treatment:
Textile waste-waters may be treated using physical/chemical processes either at the POTW (on-site) or at the plant (pre-treatment). Experience has demonstrated that chemical processes remove biodegradable organic matter. Some of the physical/chemical processes are coagulation clarification, multimedia filtration, granular carbon adoption, dissolved air flotation and ozonation.
Coagulation/clarification is an effective process for textile waste-water treatment. This method may be, especially effective for colour removal. Typical coagulants are alum, ferrous sulphate, ferric sulphate and ferric chloride with lime or sulphuric acid for pH control.
Other widely used coagulants are cationic, anionic and nonionic organic polymers. For effective coagulation, the experimental determination of the optimum dosage is required. Some waste-waters may require very high coagulant dosage. Chemical dosages in the range of 500 to 1000 mg/l are not uncommon for textile waste-waters.
The addition of large amounts of chemicals result in the production of significant quantities of waste solids. The ultimate disposal of these wastes may be very expensive. Hence, the cost calculations of coagulation/clarification should include the additional costs for ultimate disposal.
Stahr reported that the most economical approach to colour removal by coagulation appears to involve the use of a cationic polymer coagulant aid with alum as the primary coagulant. The resulting sludges from this process are reported to be more easily dewatered and conditioned than the sludges produced through the use of alum alone.
Stahr also reported that the critical parametres defining the optimum polymer dose for colour removal include the waste solution pH, the concentration and types of dyes present and the charge density of the polymer being added.
Abo-Elela reported that coagulation using a lime-ferrous sulphate combination was effective in removing the organic contaminants of textile waste-waters. Davis reported that the optimum ferric chloride coagulant dose for a composite dyehouse waste-water was 400 mg/l.
Brower and Reed studied the treatment of textile dye wastes with sodium hydroxide and ferric chloride. Sodium hydroxide was used to minimise additional sludge production at the design pH of 7. Adding sodium hydroxide before coagulation produced a weak floe with little colour removed, whereas adding sodium hydroxide after coagulation removed more than 85 per cent of the colour.
Granular carbon adsorption worked well for some textile waste-waters, whereas for others it was found that a portion of the organic removal occurred from physical filtering rather than an adsorption mechanism. McKay evaluated a model to explain the adsorption of selected dyes on activated carbon.
The feasibility of activated carbon treatment of dye house waste-waters frequently depends on costs associated with the regeneration of spent carbon. Thermal regeneration has been the primary means of regenerating granular activated carbon.
Posey and Kim studied the feasibility of solvent regeneration of exhausted activated carbon using methanol as the organic solvent. They found that for the three dye compounds tested, solvent regeneration was not cost-competitive with thermal regeneration because of the large amounts of methanol required.
Overcash and Rendall studied the feasibility of land treatment of textile waste-waters. They concluded that from an investment perspective, wool scouring with dyeing and finishing and woven fabric dyeing and finishing require the greatest expenditure for land treatment.
At the other extreme, knit fabric dyeing and finishing involve the smallest investment. To sum up due to the variability of textile waste-water characteristics, treatability studies should be conducted on a case-by-case basis in order to identify and confirm the required design parametres.
For textile waste-water treatment, physical/chemical, biological or a combination of both processes may be suitable. A combined aerobic/anaerobic treatment may be shown to be a feasible process for the treatment of dye waste-waters. Land treatment may be suitable for some dye wastes. After identifying the technically feasible processes, the final selection should be based on the economics of the processes.