Assessment of the performance of wastewater treatment plants: A case study of Gammams And Otjomuise Wastewater Treatment Plants in Windhoek, Namibia.

Abstract

Wastewater Treatment Plants (WWTPs), if not operated properly, may cause undesirable effects such as eutrophication. Gammams and Otjomuise are two WWTPs, treating domestic wastewater in Windhoek, the capital city of Namibia. Both plants are alleged to cause negative impacts on the environment, through nutrients in their effluent, released into the environment. However, very few scientific studies have been done on the treatment plants to estimate their contribution to eutrophication. Therefore, the objectives of this study were to assess the performance of the two WWTPs, by analysing historical and primary water quality data on the plants’ performance. The variations of water quality and compliance with effluent standards were analysed using influent and effluent time series. The treatment efficiencies of the two plants were determined. The contribution to eutrophication of GWWTP was also estimated using a Life Cycle Assessment (LCA) Approach, using the Centre of Environmental Science of Leiden University (CML) method in the GaBi model. GWWTP complied with the Namibian water quality effluent standards of 2013, for total suspended solids, ammonia, Total Kjeldahl Nitrogen, Biological Oxygen Demand and Chemical Oxygen Demand, while for the same parameters; Otjomuise’s effluent did not comply. Temperature, pH, and orthophosphates effluent compliance were similar for both treatment plants. There were no significant monthly average variation trends in influent and effluent concentrations between the two treatment plants. Treatment efficiencies for TKN were 94% and 80%, Orthophosphates had 37% and 87% for Gammams and Otjomuise respectively. LCA Approach had the following results, for 2004-14 data, input/influent was 15.9kg while output/effluent was 0.960kg of phosphate loading per unit volume. For 2016 data, input/influent was 18.3kg, while output/ effluent was 3.6kg of phosphate loading per unit volume. The GaBi identified Orthophosphate as the parameter that significantly contributed to eutrophication. Similarly, overall results in assessing the performance of the wastewater treatment plants indicated that orthophosphates were the parameter that significantly contributed to eutrophication. Apart from recommending that the two wastewater treatment plants constantly upgrade the plants regularly, wastewater treatment should transfer enough phosphates to sludge to avoid eutrophication of receiving waters and communities should be sensitized on the use of adding phosphate containing detergents in washing.