Occurrence and removal of Ascaris sp. ova by waste stabilisation ponds in Kenya

Abstract

A study was conducted to determine the occurrence of Ascaris sp. ova in municipal wastewater and their removal by a variety of waste stabilisation pond systems in the republic of Kenya. A total of 52 wastewater samples were examined for the presence of ova, comprising 10 raw wastewater and 42 pond effluent samples over a two month period during 1990. Ascaris sp. ova were detected in 6 out of 10 designated pond systems examined. In raw wastewater, ova concentrations ranged from 17.5 to 133.3 ova/l and in pond effluents from 0.7 to 88.9 ova/l. Although the majority of ova were removed in primary ponds that received raw wastewater, ova were detected in pond effluents with estimated hydraulic retention times far in excess of estimated settling rates for Ascaris. The occurrence of ova in pond effluent samples suggested that in these multi-pond systems, estimated cumulative retention periods of 12.4 (Eldoret), 26.2 (Dandora) and 17.8 (Nakuru) days, respectively, were required to achieve an effluent free of Ascaris sp. ova. Laboratory experiments were performed to assess physico-chemical and microbiological parameters to provide an indication of pond performance.