53-Toxicity of composite tannery effluent and extent of amelioration by traditional method of effluent stabilization

Abstract

Throughout the year composite industrial and municipal wastewater contaminate the East Calcutta Wetlands a special conservation site (Ramsar Site No.1208), at the eastern fringe of Kolkata city, West Bengal, India. The aim of the present study was to characterize the raw tannery effluent and its amelioration through stabilization when it is productively used in pisciculture. Physico-chemical features of raw composite tannery effluent at the tannery discharge site, unmixed tannery wastewater collected from wastewater-carrying canal, 1 km away from the vicinity of Dhapa tannery agglomerate and the stabilized tannery wastewater from the selected fishpond were compared to study the extent of amelioration by natural processes. Acute toxicity tests were also performed with a fish model (Guppy; Poecilia reticulata) to study the impact of physico- chemical characters of tannery effluent on fish biosystem. Natural stabilization of the composite tannery effluents using traditional method was observed to be very efficient in ameliorating critical pollutants like TDS (74.18% reduction), TSS (59.62% reduction), total hardness (22.65% reduction), P043- (increment by 64.37% allowing nutrient support of live protoplasm building), BOD5 (70.25% reduction), COD (56.53% reduction), and the heavy metals like Cr (98.33% reduction and Pb (34.29% reduction). Interesting to note that the stabilized tannery wastewater-fed fishpond exhibited on an average 70 times better mixing condition than the 96hr-LC50 mixing ratio of 1:7 for composite tannery effluent and freshwater respectively as the 96hr-LC50 value of the composite tannery effluent was 137.5 ml L-1. The result of the present study amply justifies the reasons for sustainability of this traditional resource recovery system practiced in ECW for the last ten decades or so.