Occupational health risks and fate of chromium in sewage treatment co-treating tannery effluent

This study investigated chromium (Cr) concentrations and fate across processes at a sewage treatment plant (STP) co-treating tannery effluent and assessed health risks for STP workers and farmers using the effluent for irrigation. Two novel secondary treatments, an IPC membrane and a modified constructed wetland, were evaluated as a mitigation method. Methods combined key informant interviews (n = 4), observations (n = 4), Cr analyses (n = 132) and a mass balance approach to track the fate of Cr. Total Cr in the STP influent ranged from 2.49 to 14.95 mg/L, while hexavalent Cr was <100µg/L. Primary sludge contained high total Cr (8,672 to 17,525 mg/kgDM) and hexavalent Cr (0.07 to 0.67 mg/kgDM). Final effluents from the novel and conventional treatments met the Indian discharge standards for total Cr (2 mg/L) and hexavalent Cr (0.1 mg/L). Risk assessment identified three high-risk activities for STP workers, all related to skin contact during primary sludge handling. Farmers irrigated with mixed STP and common effluent treatment plant (CETP) effluent ;the Cr in the CETP effluent was 16 times higher than the STP effluent. The novel technologies did not lower the risks, as they did not address sludge management or CETP effluent quality. Targeted controls for sludge management and coordinated action on CETP discharges are therefore critical.