Evaluation of waste stabilisation pond units for treating domestic wastewater

Abstract

AbstractThe purpose of this study was to evaluate the performance of individual units of waste stabilisation ponds (WSPs) system treating domestic wastewater. The study was conducted by measuring the concentrations of physiochemical parameters and bacteriological indictors in the influent and effluent samples of each pond. The thickness of sludge was determined by measuring depths of sludge layer in each pond. Knowing the performance of each pond will speed up targeted intervention to improve performance. The influent was mainly characterised by 224, 500, 270 mg L−1, 258 Nephelometric Turbidity Unit (NTU) respectively, of biochemical oxygen demand (BOD), chemical oxygen demand (COD), total suspended solids (TSS), turbidity and 35600 coliform forming units/100 ml of faecal coliforms. Bacteriological counts were fluctuating and effluent faecal coliforms counts were 1.22 × 104 ± 2.0 × 102 coliform forming units/100 ml. The hydraulic retention times (HRTs) decreased by 2.0, 13.6 and 7.2 days in the first three ponds and by 3.3 days in the last three ponds. Biochemical oxygen demand in the final effluent was 40 mg L−1 more than the design value of f 25 mg L−1. Chemical oxygen demand in the final effluent was 190 mg L−1, higher than the prescribed limit of 150 mg L−1 of discharge into rivers. Total suspended solids in the final effluent was 150 ± 127 mg L−1 more than the national limit of 25 mg L−1. Total suspended solids and turbidity correlated positively with bacteriological indicators, but there was no correlation with temperature except with faecal coliforms. There was also no correlation observed between suspended solids and pH. The treatment system did not meet the design treatment criteria because of overloading as a result of increased population which exceeded the design value. The final effluent from the system did not meet the discharge limits, therefore, it is unsuitable for discharge into the environment as it can pollute ground and surface waters as well as the open environment. Knowledge on the performance of each pond will help in proper interventions to be undertaken at the targeted pond or unit. It is therefore eminent to desludge each pond as a short‐term solution and expand the system for a long‐term solution.