Influence of wastewater treatment plant discharge on the water quality of the Veldwachters river, Western Cape

Abstract

The importance of the Veldwachters River for ecosystem function in our climate-changing world cannot be overemphasized. The Veldwachters River is a non-perennial river, and it is completely dry in some parts during the summer months. It is recharged with the effluent discharge from a wastewater treatment plant (WWTP) that receives domestic and municipal wastewater. The aim of the study is to investigate the influence of a wastewater treatment plant discharge on the water quality of the Veldwachters River, as well as possible associated ecological risks of the WWTP effluent. The temporal and spatial physico-chemical characteristics of the Veldwachters River water samples and ecological risks of discharged effluents were assessed. Laboratory measurements using standard methods and bioassay experiments were carried out over four seasons - summer, autumn, winter, and spring. Effluent samples were classified using the hazard classification system for wastewaters discharges into aquatic environments. The Veldwachters River water samples, WWTP’s influent and effluent samples were further analysed for Microplastics (MPs). Influent and effluent samples were characterised to determine the WWTP’s removal efficiency for MPs. Values of samples pH, dissolved oxygen (DO), temperature, total dissolved solids (TDS), biochemical oxygen demand (BOD) and chemical oxygen demand (COD) ranged between 4.7 – 9.75; 1.7 – 9.5 mg/L; 14.2 – 29.5 oC; 376 – 840 ppm, 0.8 – 175.58 mg/L and 0.83 – 912.15 mg/L respectively. The ecotoxicological results showed that crustaceans Daphnia magna was classified as Class III (acute toxicity) for all sampled seasons, meanwhile, Tetrahymena thermophila was more sensitive to the effluent compared to D. magna and Raphidocelis subcapitata. The ecotoxicological results indicated that the use of ecotoxicity assessment methods for municipal WWTP effluent is beneficial and may contribute positively to existing water monitoring strategies. The most prominent MP forms found in the water samples were fibres, with the most common colours being black/grey. Observations during our reconnaissance survey suggest that the discharged effluent contributes to the river health downstream. There is a need for consistent monitoring of the river system and effluent quality prior to discharge into the Veldwachters River.