Abstract

The objective of the research is to evaluate the performance of aerated lagoon for biological treatment of textile wastewater and to determine kinetic coefficients for the design of treatment facilities. For this purpose, a bench scale model of aerated lagoon was set up and was operated continuously for 92 days by varying aeration times from 5 to 15 days. Primary treated wastewater effluent collected from Nishat Textile Mills (Lahore, Pakistan) was fed as an influent to aerated lagoon. To evaluate the performance of aerated lagoon, the suspended solids (SS), biological oxygen demand (BOD) and chemical oxygen demand (COD) for both the influent and effluent were measured at each aeration time after ensuring steady state conditions. The influent BOD was observed to vary from 540 to 355 mg/L while the effluent BOD varied from 121 to 32 mg/L during the study period. The corresponding range of influent COD was between 1813 and 1116 mg/L and for effluent COD was between 592 and 224 mg/L. The results show that the average effluent BOD at aeration time of 7 days was lower than 80 mg/L. However, the effluent COD did not meet the National Environmental Quality Standards (NEQS) limit of 150 mg/L even at the aeration time of 15 days. This indicated the presence of large amount of chemicals in the textile wastewater, for instance bleaching agents like hydrogen peroxide and sodium hypochlorite; complex dyes and pigments like chromium based dyes used for imparting color to the fabric. The fate of these chemicals varies ranging from 100% retention on the fabric to 100% discharge with effluent. Chemical coagulation, prior to primary sedimentation, by coagulants like lime, aluminum salt (especially aluminum sulphate or alum), ferric chloride and ferrous sulphate, is most commonly employed for significant removal of COD. Overall removal rate constant (K) for the present study was calculated to have a mean value of 0.79 day^-1 based on BOD. Based on COD, the overall removal rate constant (K) was found out to have a mean value of 0.39 day^-1.

Key words: Biological treatment, wastewater, kinetic coefficient, biological oxygen demand (BOD), chemical oxygen demand (COD).