EVALUATION OF THE PERFORMANCE OF AN ONSITE WASTEWATER TREATMENT PLANT

ACABSTRACT

In areas where there are no sewerage systems, wastewater requires to be treated onsite. This study evaluated the performance of onsite wastewater treatment plants located at Maasai Mall at Rongai, Kajiado County, and at Kabete Flats in the Lower Kabete area of Nairobi. The two onsite wastewater treatment systems worked on the principle of aerobic digestion through aeration. This OWTS has one flow equalization chamber, one sludge-holding chamber, two aeration chambers, two settlement tank, stand-by pump system, chlorinator housing and a storage chamber. The OWTS treats approximately 4,000 litres per day in the Maasai Mall system and 2,700 litres per day in the Kabete system. These systems were designed by the JET Inc.®, and sized, installed and maintained by CESP Kenya. Samples were collected from the inlet and outlet, and analyzed for the major water quality parameters pH, COD, BOD5, TSS and TDS to evaluate the efficiency of the OWTS. The OWTS at Maasai Mall reduced initial concentrations of BOD5, COD, TSS, TDS of 674, 1088, 200 and 1410 mg/L respectively to approximately 19, 56, 60 and 810 mg/L respectively. Percentage removal of BOD5, COD, TSS and TDS of up to 95.6, 94.9, 70.0 and 42.6% respectively was achieved. Similar removals were observed for the Kabete OWTS. The ratio ofBOD5to COD in the samples tested was in the range 0.4 to 0.7 for untreated effluent and 0.1 to 0.2 for the final treated effluent. This is typical of domestic/municipal sewage wastes where the BOD5 to COD ratio is between 0.3 to 0.8 and 0.1 to 0.3 for untreated and treated effluent, respectively. The effluent was within standards for treated wastewater re-use in flushing of toilets and gardening.

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