Performance evaluation of desludged and upgraded waste stabilisation ponds: A case study, Botswana

Abstract

The scarcity of fresh water for agricultural purposes requires the reuse of wastewater, if the effluent is of acceptable quality and is regularly monitored. The quality of the wastewater in the individual treatment units of waste stabilisation ponds was monitored after desludging and upgrading with a trickling filter unit. The overall hydraulic retention time had increased to 80% of the design value compared to 30% before desludging and upgrading. The biochemical oxygen demand, the chemical oxygen demand, the turbidity, and total suspended solids effluent concentrations were 41 ± 8 mgL−1, 83 ± 9.3 mgL−1, 43 ± 37 NTU, and 45 ± 38 mgL−1 respectively, compared to 40 mgL−1, 190 mgL−1, 145 ± 66 NTU, and 150 ± 127 mgL−1 before desludging and upgrading.The corresponding national thresholds for biochemical oxygen demand, turbidity and total suspended solids are 30, mgL−1, 30 NTU, and 25 mgL−1 respectively. Effluent log counts after upgrading were 2.4 ± 2.32 2.96 ± 2.3, 2.82 ± 2.42, and 2.44 ± 2.37 respectively, for E. coli, and the total coliforms and faecal coliforms compared to 5.4 ± 4.1, 5.1 ± 4.1, and 4.1 ± 23 log counts before upgrading. Helminth eggs were greater than 1 egg L−1 in the final effluent. The pHosphate effluent concentration was greater than the national threshold value (1.5 mgL−1) for discharge into the environment. Desludging and the addition of the trickling filter improved the removal efficiency of some of the contaminants. Operation and maintenance of the system needs to be improved and monitored to find out insights into poor performance even after desludging and addition of a trickling filter unit.