ASSESSMENT OF HEAVY METAL PERSISTENCE FOLLOWING CONVENTIONAL MUNICIPAL WASTEWATER TREATMENT IN NIGERIA: IMPLICATIONS FOR ADVANCED TREATMENT

Abstract

Conventional municipal wastewater treatment plants are primary designed to remove organic pollutants and nutrients rather than dissolved heavy metals. Consequently, toxic pollutants may persist in treated effluents and be discharged into receiving water bodies, posing potential risks to aquatic ecosystems and public health. This underscores the need to evaluate the effectiveness of conventional treatment for heavy metal removal. The performance of a conventional activated sludge municipal wastewater treatment plan in Nigeria was evaluated by assessing physicochemical parameters and heavy metal concentrations at the influent, post-aeration chamber, and final effluent. Grab samples were collected during three sampling campaigns and analysed for selected physicochemical parameters (pH, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, nitrate, nitrite, and phosphate) and heavy metals using standard analytical methods. The activated sludge process effectively reduced organic pollutants and nutrients, with the effluent complying with the National Environmental Standards and Regulations Enforcement Agency (NESREA) discharge limits for conventional wastewater quality parameters. In contrast, heavy metal removal was limited. Final effluent concentrations of Chromium (0.58-1.24 mg/L), iron (1.10-10.21 mg/L), and lead (0.19-0.48 mg/L) remained above the NESREA permissible limits, exceeding the regulatory thresholds by approximately 82-91%, 72-97%, and 47-79%, respectively. These findings demonstrate that satisfactory performance for conventional wastewater quality parameters does not necessarily ensure compliance for dissolved heavy metals. The results support the incorporation of an appropriate advanced polishing process to remove residual heavy metals before effluent discharge, thereby improving environmental protection and promoting sustainable municipal wastewater management.