Survival of faecal coliforms in four different types of sludge-amended soils in Botswana

Abstract

This study aimed at identifying the factors that affected the survival of faecal coliforms as measured by E. coli in four types of soils in Botswana which were amended with sewage sludge. Physico-chemical and biochemical properties and coliform population in the different soils, sludge, and soil/sludge mixtures were determined after sampling, on composing the soil/sludge mixtures, and 90 days after composing. Coliform population in the different soil/sludge mixtures decreased by about 90% after 90 days. The age of the sludge used and the rate at which it was applied to the soils initially determined the population of E. coli in each soil/sludge mixture, but after 90 days, differences inherent in the different soil types were the main determinants of the E. coli population. Percentage reduction of coliforms in Type 1 sludge mixtures were lower (38%) than in Type 2 sludge mixtures (57%). Up to 79.8% of the reduction observed in E. coli population in the Barolong luvisol-sludge mixtures after 90 days was caused by reduction in pH and moisture content, while 72.6%, 84.5% and 55.1% of the reduction in E. coli population in Tuli luvisol-, arenosol- and vertisol-sludge mixtures, respectively, was accounted for by the reduction in moisture content and Olsen P concentration. Coliform survival rates varied with soil types being 12%, 6.4%, 5.3% and 5.8% for the vertisol, arenosol, Barolong luvisol and Tuli luvisol, respectively. A minimum period of 90 days should be allowed between when sludge is applied to similar soils and when seeds are sown. The exact period should, however, be determined by the properties of the soil with fine-textured soils requiring a longer period than coarse-textured soils.