Performance evaluation and modeling of faecal coliforms removal in waste stabilisation ponds: a case study of Buterere Waste stabilisation plant - Burundi

The performance evaluation and faecal coliforms removal in the waste stabilization ponds (WSPs) in urban area was carried out. The Buterere WSPs comprised of anaerobic, facultative and maturation ponds in two parallel series. The effluents of WSP which are discharged in the Lake Tanganyika via Kinyankonge river had average BOD, COD, TSS, and FC reductions of 93.67%, 82.81%, 88.55% and 99.98% respectively. The average effluent BOD, COD, TSS, and FC concentrations were 57.63 mg/l, 251.375 mg/l, 67.88 mg/l and 1.97E+04MPN/100 ml for BOD, COD, TSS and FC respectively. From these results, it has been found out that the Buterere WSPs in is not discharging effluent, which meets WHO guidelines for discharge to receiving water body. The design analysis has shown that the design of the WSPs was based on the assumed waste water temperature instead of the mean minimum of the coldest month. Also, the design faecal coliforms removal in the anaerobic has been overestimated by taking a higher Kb (5.08 d-1) instead of 2.62 d-1 which lead to an error in the estimation of the effluent FC found as 7E+03 MPN/100 ml instead of 1.21E+04 MPN/100 ml. Also, the design analysis has showed that the actual efficiencies for BOD and FC were below the design values for all ponds even if the retention time is higher than the design one. The modeling of the faecal coliforms removal had show that the predicted Kb for CSTR model present a wide distribution (minimum = 0.92 d-1 and maximum = 2.7 d-1) while the predicted Kb for DF model present a narrower distribution (minimum = 0.174 d-1 and maximum =0.262 d-1) which prove the adequacy of the DF model. In addition the predicted average FC removals were closed to observed values for CSTR model (98.62 %, 99.51 %) than the DF model (99.84%, 99.75%).