Effects of population pressures on the design capacity and effluent quality of the Nkowankowa wastewater treatment works in South Africa

This study investigated the effects of population growth on the performance of Nkowankowa Wastewater Treatment Works (WWTW) in Mopani District Municipality, South Africa. Four-year (2020–2023) effluent quality data obtained from the Nkowankowa WWTW was compared with the Department of Water and Sanitation (DWS) general limit standards for effluent discharge. Performance of the WWTWs was assessed using three (Compliance Monitoring, Wastewater Treatment, and Effluent Quality Compliance) key performance areas (KPAs) as outlined in the South African Green Drop standards. Numerical data was analysed using descriptive statistics (Microsoft Excel). The results revealed that Nkowankowa population has more than doubled (from ~ 42 859 to ~ 110 963 people) since the construction of the WWTW, and this has put enormous pressure on the plant’s design operational capacity (from 4.5 to 17.75 ML/day). During this period, various effluent parameters were non-compliant and exceeded the limits with the following maximum values: E. coli 2,233.85 counts per 100 ml, Ammonia 137.00 mg/L, Orthophosphate 38.25 mg/L, and chemical oxygen demand (COD) 115.62 mg/L. Nkowankowa WWTW was found to be non-compliant in all three KPAs with an overall score of 32%, which is classified as a high risk as per the Green Drop standards. Moreover, the effluent quality improved over the years with the increase in the population. This study concluded that there were other factors which contributed to the non-performance of WWTW other than population increase.