By: Dr. Harshit A. Patel
Fiber (especially wool) scouring involves the use of hot water and detergents to remove soil, vegetable impurities, grease and other contaminants from fibers. Wool scouring typically uses water and alkali, although scouring with an organic solvent is also possible. Scouring with alkali breaks down natural oils and surfactants and suspends impurities in the bath. The scouring effluent is strongly alkaline, and a significant portion of BOD and COD loads from textile manufacturing arises from scouring processes. The recommended pollution prevention and control techniques include the following:
- Design of scouring systems to remove heavy settleable solids continuously; increase recovery of grease; recover heat from the final facility effluent; and control water usage.
- Use of readily biodegradable detergents / surfactants that do not give rise to toxic metabolites. (e.g. APEO should be replaced with alcohol ethoxylates);
- Optimization of mechanical removal of water prior to the drying process;
- Adoption of low volatile organic compound (VOC) emitting solvent wash for removal of water insoluble oils.
Wet processing includes the main processes of fabric preparation, namely desizing, bleaching, mercerizing, dyeing, printing, and other specific treatments. These phases treat fabrics with chemical and liquor baths and often require several washing, rinsing, and drying steps, generating significant wastewater effluents. The recommended pollution prevention and control techniques for the finishing pretreatment steps include the following:
- Selection of water soluble and biodegradable lubricants for knitted fabrics instead of mineral oil and wash them with water;
- Use of organic solvent washing for non water soluble lubricants;
- The thermofixing step may be performed before the washing step. Air emissions generated from the stenter should be treated by dry electrofiltration. The oil separated should be collected to limit effluent contamination;
- Residual liquor should be minimized through reduced application, reduced tank volumes and padding liquor recycling;
- Using mechanical dewatering equipment to reduce water content of the incoming fabric and reduce energy consumption in stenter frame.
Desizing operations may generate effluents with significant concentrations of organic matters and solids. BOD and COD loads from desizing may be significant (35 to 50 percent of the total load), and COD concentrations up to 20,000 mg/I may be generated. Recommended pollution prevention and control techniques include:
- Selection of raw material with low add-on techniques (e.g. pre-wetting of the warp yarn);
- Selection of more bio-eliminable sizing agents (e.g.-modified starches, certain galactomannans, polyvinyl alcohol, and certain polyacrylates);
- Application of enzymatic or oxidative desizing with starch and modified starch sizing agents, followed by washing systems;
- Integration of de-sizing / scouring and bleaching in a single step to reduce effluent generation (e.g. reuse of bleach rinse water in desizing);
- Recovery and reuse of specific water-soluble synthetic sizing agents (e.g. PVA, poly-acrylates, and carboxy-methyl cellulose) by ultra-filtration.