Optimization of coagulation-flocculation process for dye and COD removal from real dyeing wastewater and evaluation of effluent biodegradability in a carpet factory

Textile industry is one of the largest consumers of water in the world. The wastewater from the textile industry is known to have strong colour, chemical oxygen demand (COD), highly fluctuating pH and high temperature. The release of coloured wastewater represents a serious environmental and public health concern. Among the various physicochemical and biological methods of textile wastewater treatment, coagulationflocculation is considered as an attractive and favourable technique because of its low cost, easy operation and high efficiency.
The study was conducted in the laboratory scale using Jar test. Dye removal from real wastewater was investigated by the use of three mineral coagulants including poly aluminum chloride, ferric sulfate and ferric chloride. In order to optimize the process, parameters including pH, coagulant dose, time and speed of coagulation were considered. Treated samples were analysed to determine the residual color and COD.
According to the results of our experiments, under optimum conditions of process (pH 9, doses of 250 mg/lcoagulant and coagulation speed of 175 rpm with a duration of 5 minutes), the highest removal efficiencies (COD 44.4% and color 95%) were obtained by the use of poly aluminum chloride. The results showed that poly aluminum chloride enhanced the biodegradability of wastewater from 0.07 to 0.21 at optimized process conditions.
Although the coagulation process reduced a large percentage of pollution load in the wastewater, another process has to be followed to meet the wastewater discharge standards in receiving sources.