Assessment of Hybrid Wastewater Treatment Systems: Integrating Constructed Wetlands for Improved Effluent Quality in Lusaka, Zambia

Abstract

Urban wastewater treatment systems in rapidly growing African cities often face operational and capacity constraints, resulting in the discharge of inadequately treated effluent into receiving watercourses. This study assessed the operational performance of the Manchinchi Wastewater Treatment Plant (WWTP) in Lusaka, Zambia, characterised its impact on the Ngwerere Stream, and designed a horizontal subsurface-flow constructed wetland (HSSF-CW) as a downstream polishing stage. Effluent and stream water quality were sampled during the 2024/2025 dry season at three WWTP stations and ten stream stations, and analysed for BOD₅, TSS, dissolved oxygen and nutrients against Zambia Environmental Management Agency (ZEMA) discharge standards. Results showed severe organic pollution, with WWTP effluent BOD₅ of 600-629 mg/L against a ZEMA limit of 50 mg/L, near-zero dissolved oxygen at the discharge points, and BOD₅ concentrations of 240-290 mg/L immediately downstream in the Ngwerere Stream. Using a temperature-corrected first-order areal kinetic model, a 24.7-ha HSSF-CW was designed, with a predicted BOD₅ removal efficiency exceeding 91%, reducing effluent BOD₅ from 600 mg/L to below 54 mg/L within a hydraulic retention time of approximately 2.0 days. The 24.7-ha land requirement is identified as the principal feasibility constraint for application in peri-urban Lusaka. Modular cell construction intensified (vertical-flow or aerated) wetland configurations, and restoration of the WWTP's existing secondary treatment units are recommended to reduce the land footprint. The findings support the integration of nature-based polishing systems with conventional infrastructure as a practical pathway toward ZEMA compliance and Zambia's Vision 2030 sanitation targets.