CURRENT TECHNOLOGIES FOR BIOLOGICAL TREATMENT

OF TEXTILE WASTEWATER–A REVIEW

TEXTILE WASTEWATER With the increased demand for textile

products, there is increase in the textile industry and its wastewater proportionally, making it one of the main sources of severe pollution problems worldwide. In particular, the release of colored effluents into the environment is undesirable, not only because of their color but also because of the breakdown products. The release of colored wastewater in the ecosystem is a remarkable source of esthetic pollution, eutrophication, and perturbations in aquatic life.Dyes can absorb light with wavelength in visible region (350–700 nm); they are colored and are detectable even in the concentration of 1 mg/l. Furthermore, the absorption of light due to textile dyes creates problem to photosynthetic aquatic plants and algae.

Processes Used in Fabric Production

1. Sizing and Desizing The sizing process is the process where the yarn is wrapped for easy working in weaving, knitting, and the tufting process. Sizing is carried out by the application of the chemicals like polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), and polycyclic acids. PVA and CMC are resistant to biological degradation, but starch is easily biodegradable. As such, there is little or no waste from this stage of textile processing2. Scouring In this process, cotton wax and other non cellulosic components of cotton are removed by hot alkali, detergent, or soap solution like glycerol ethers and scouring solvents. The pH of the wastewater is highly alkaline ranging from 10 to 11.3. Bleaching Mostly sodium hypochlorite, sodium silicate, hydrogen peroxide, and organic stabilizers like enzymes are used as bleaching agents. High levels of chlorides or peroxide might cause inhibition problem, and this contributes to little or no residual waste with high pH-containing wastewater.

5. Dyeing Dyes Depending on the dyeing process, many chemicals like metals, salts, surfactants, organic processing assistants, sulfide, and formaldehyde may be added to improve dye adsorption onto the fibers, which are the major pollutants in the wastewater.6. Finishing Finishing is the final process of the fabric preparation, which is done to get some desired properties of the fabric. Cotton fabrics are given temporary starch finishing. In this, various fabrics are used, and small amount of which can enter into wastes.

4. Mercerizing It improves luster of the fabric and dye uptake. For mercerizing, the fabric is padded through 25 % caustic soda wash. This increases the pH of the wastewater.

TOXICITY Textille Wastewater Components and Treatment Difficulties

Wastewater from textile industries constitutes a threat to the environment in a large part of the world. The characteristics of wastewater generated by composite, processing, and woolen industries are given in Table 1. There are extreme fluctuations in different parameters such as chemical oxygen demand (COD), BOD, pH, color, and salinity with textile processing. The wastewater composition will depend on the different organic-based compounds, chemicals, and dyes used in the industrial dry- and wet-processing steps.

The main pollutants of textile wastewater originates from the dyeing and finishing steps that involve dying of the man-made or natural fibers to the desired permanent color and processing of those fibers into final commercial products.

PROBLEM POLLUTANTS Major Pollutants in Textille Wastewater and it’s Impact of Textille Chemicals in Biological Treatment

BIODEGRADATION

1. Organic load Scouring and desizing effluents are a major contribution to the organic load in textile effluents. Traditional sizes such as starches and their derivatives are readily biodegradable under aerobic and anaerobic conditions. However, bulking of activated sludge occurs frequently if a large proportion of the wastewater consists of desizing wastewater. Anaerobic treatment produces little sludge when compared with aerobic treatment.2. Color Reduction of the chromophore can be achieved under reducing conditions prevailing in anaerobic bioreactor. The amines produced by the reduction of the azo dyes are colorless, but they are very resistant to further degradation under anaerobic conditions. Under aerobic conditions, the mineralization of these amines can be accomplished. Complete treatment can thus be obtained by a sequenced anaerobic–aerobic treatment.

3. Toxicity The anaerobic microbial community that maintains the reducing conditions for the decolorization of the textile dyes is known to be sensitive to toxic shocks and is one of the reasons for the retardation in color removal.4. Co-substrates Glucose, raw municipal wastewater,

and yeast extract, among others, have been reported as examples of an essential co-substrate needed to obtain good color removal.5. Redox potential The redox potential has been reported to be below −450 and −500 mV for azo dye reduction to occur.6. Nutrients Additives containing nitrogen and phosphorus (e.g., urea, ammonium acetate, ammonium sulfate, and ammonium phosphate) are the main sources of nutrients in the textile effluent. Textile effluents were found to be major inhibitors of the nitrifying bacteria in aerobic treatment systems, thus hampering nitrogen removal. Importance should be given in the COD/N ratio in considering anaerobic treatment of textile effluents. Effort should be put into the reduction of the amounts of dye bath additives applied and the selection of nitrogen- and phosphorus-free alternative auxiliaries in the production process so as to minimize the need for nutrient removal in wastewater treatment.

TREATMENT Anaerobic Treatment of Textile WastewaterUnder anaerobic conditions, dyes are readily cleaved via a four electron at cleavagegenerating aromatic amines, which is a stable biotransformation product of metabolism. Dye decolorization under methanogenic condition requires an organic carbon/energy source. Simple substrates like glucose, starch, acetate, ethanol, whey, and tapioca have been used as dye-decolorizing substrate.Decolorization under Anoxic ConditionAlthough many of these microorganisms were able to grow aerobically, decolorization was achieved only under anoxic condition. Decolorization of azo dyes under anoxic conditions also required co-substrates such as yeast extract, peptone, or carbohydrates Sequential DegradationIt has been repeatedly suggested that aromatic amines formed during anaerobic cleavage of the azo dyes could be further degraded in an aerobic treatment. The feasibility of this strategy was first demonstrated for sulfonated azo dyes Mordant Yellow. After aeration, the amines formed were completely mineralized by the microorganisms.

THANK YOU