The treatability of chromium tannery wastes

Leather processing industry wastewaters contain several conventional pollutants as well as micropollutants. Conventional pollutants can be effectively treated although considerable treatment comprised of several stages is required. However, the potential of conventional methods to remove COD and micropollutants has not been fully explored. The purpose of this paper is to investigate the extent of COD removal by conventional methods. Experimental studies are conducted using raw wastewaters of industries applying chromium tanning to cattlehides and sheepskins. Biological treatability after pretreatment by chemical coagulation is assessed using a lab-scale continuous activated sludge system. Results indicate that chemical coagulation produces wastewaters with a COD of around 2000 mg/l almost independent of raw wastewater quality. Activated sludge operated at 0.17 g BOD₅/g MLSS.d loading is found to provide limited nitrification and almost complete BOD₅ removal while the COD could only be reduced to around 500 mg/l. The residual COD identified using a newly developed experimental approach shows that it cannot be reduced lower than 300-550 mg/l.