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Textile wastewater treatment by coagulation
process (A case study) Abstract Introduction Textile processing sector is one of the most important and largest industrial sectors of Pakistan with regard to production, source of foreign exchange and labour force employment. It alone accounts for 65% of the country's export, 46% of industrial Production, 38% of employed industrial manpower, and 9% of Gross National Product (GNP). Textile industry could be termed as the backbone of Pakistan's economy. It holds 31% of the total investment in the country and pays rupees 40 billion of salaries per year. The number of industries working in this sector is estimated to be around 670. Karachi has the major share with almost 300 industries and rests are located in Punjab. Wastewater is the major environmental issue of the textile industries besides other minor issues like solid waste, resource wastage and occupational health and safety. Pretreatment, coloration and after treatment of these fibers, usually require large amounts of water and variety of chemicals. Variations in the fabric quality and treatment process results into large fluctuation in daily flow rates and pollutants concentrations. Textile wastewater pollutant are generally caustic soda, detergents, starch, wax, urea, ammonia, pigments and dyes that increase its BOD, COD, solids contents and toxicity. Recent survey under ICPT project reveals that the majority of the textile mills discharge their untreated wastewater into municipal or industrial sewers, besides few directly discharge to the nearby drains and stagnant ponds. Environmental law in Pakistan is being strictly enforced and requires that the textile wastewater should be discharged after treatment and NEQS compliance. As wastewater treatment technology is new in Pakistan, textile management is facing problems in selecting the appropriate treatment technology that suits Pakistan environment. Some of the progressive industries are taking initiatives in this direction and various R&D institutions are carrying out research to find economical ways to combat wastewater pollution. Chemical coagulation process with settling or floatation is a simple technique, which may be applied to the combined wastewater or to the separated heavily contaminated wastewater stream, to remove, primarily the colloidal impurities including dyestuff and the associated biochemical and chemical oxygen demand (BOD & COD). As coagulation will not remove all the pollutants but it can be useful technique to initiate treatment in the first phase to reduce pollution level up to some extent. Coagulation Process Colloidal particles do not settle under gravity. The charges present on their surfaces result into repulsion and do not allow them to agglomerate and form flocs. Flocculation follows coagulation. Coagulation process neutralizes the charge present on the particles surfaces with the help of coagulants whereas flocculation makes them to come close to each other to make flocs by slow agitation. Settling follows coagulation and flocculation to remove resultant flocs from the wastewater. Designing of coagulation, flocculation and settling tank requires study regarding optimum dose of coagulants at suitable pH to give maximum removal and settling characteristics of resultant flocs. Study was carried out on composite wastewater sample of one local textile industry by the CPP team to find out design parameters for treatment plant. Experiment and result The objective of the experiment was as follows: Optimization of pH Mostly coagulation is dependent on wastewater pH. For that pH was optimized prior to selection of optimum coagulant dose. It was found that the optimum pH for better coagulation was 5-6 for textile wastewater. As the textile wastewater was alkaline so for pH reduction hydrochloric and sulfuric acid were applied. Sulfuric acid was found to be a better choice because bulky flocs were produced with high settling velocity. Dosage of coagulant aid The optimum dosage of coagulant depends on the amount and the nature of suspended solids present in the wastewater. Lime and polyelectrolyte (magnofloc) were used as coagulant aid to enhance the coagulation process. The selected optimum dosage of lime and polyelectrolyte was1500mg/l and 5mg/l respectively. Selection of suitable coagulant Different coagulants like ferric chloride, ferrous sulfates, alum, lime and their combinations were tried to select the suitable ones that provides higher removal efficiency. It was observed that the ferric chloride with or without lime/polyelectrolyte and ferrous sulfate without lime/polyelectrolyte were not able to coagulate the suspended particles in the effluent. Moreover alum and lime worked without coagulant aid. In case of ferrous sulfate and alum, used with lime and polyelectrolyte, heavy flocs were produced with good settling velocity. The results are presented in the following table. It was observed that the coagulation with alum was better than that of ferrous sulfate. In both cases, lime and magnofloc (as a coagulant aid) were employed to enhance the coagulation process. The flocs obtained with alum seemed to be more bulky and had high settling velocity than those obtained with ferrous sulfate. Within 60 minutes, about 90% of suspended solids were settled down in alum runs, besides removal of significant associated BOD & COD ranging from 20-50%. In case of ferrous sulfate and alum run, the settling time was much higher i.e. about 120 minutes to achieve the same removal efficiency. With alum runs, the removal efficiency was much less than those obtained with ferrous sulfate or alum with lime and magnofloc. From the above observation, it is clear that alum in combination with lime and magnofloc is the most suitable coagulant for the treatment of textile effluent. Following table shows the removal efficiency for different coagulants tested.
Cost analysis Alum is cheaper than other coagulants and, therefore, its application is favourable. Following table shows that the treatment cost of alum is about half of that with ferrous sulfate. Design parameters for coagulation system Curves are drawn with the help of above table that can be used in designing settling
tanks.
Conclusions Alum in combination with lime and a polyelectrolyte, such as magnofloc, is the best coagulant dose for the treatment. The pH within 5-6 is optimum in getting maximum coagulation. Cost-wise alum would be economical, besides its other advantages against ferrous sulfate. Chemical coagulation process is not only effective to remove the suspended solids and colour pigments up to 90-98% but it would be also beneficial for the reduction of associated wastewater BOD and COD up to 20 -50%. The design curves have been constructed in terms of percent removal of S.S. vs loading rates and detention times. These curves can, effectively, be applied in designing the settling tanks for the textile effluent treatment plants. Reference 1. APHA-AWWA-WPCF 1989, Standard Methods for the Examination of Water
and Wastewater, 17th edition, Washington, U.S.A. 1989.
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To know whether dyes can be coagulated and flocculated