Evaluation of indicators for assessment of human and animal faecal pollution of surface run-off

Abstract

The value of selected indicators for assessment of faecal pollution, as well as the distinction of faecal pollution of human or animal origin, has been investigated. The following indicators were included: faecal coliform bacteria, faecal streptococci, sorbitol-fermenting bifidobacteria. Rhodococcus coprophilus, somatic and male-specific coliphages, phages of Bacteroides fragilis, and cytopathogenic viruses. Comparative tests were carried out on samples collected from a stream and river exposed to predominantly faecal pollution of domestic animal origin, and the same stream and river after downstream exposure to run-off from a low socio-economic informal settlement with restricted sanitation. Samples were collected from perennial flow during the dry season and from storimwater run-off after thundershowers. Stormwater run-off from the settlement reached faecal coliform counts of up to 4 400 000 per 100 ml, which is equivalent to that of many raw sewage effluents. Faecal pollution was less during the dry season. Sorbitol-fermenting bifidobacteria were identifiable with faecal pollution of human origin, and R coprophilus bacteria with that of animal origin. The ratio of faecal coliforms to faecal streptococci was in the order of 3.5 to 4.7 immediately after exposure to sewage pollution of human origin. In water exposed to faecal pollution predominantly of animal origin, and downstream from pollution of human origin, this ratio varied from 0.8 to 1.7, which indicates that under circumstances the ratio may also distinguish between faecal pollution of human and animal origin. Phages of B fragilis and cytopathogenic viruses were not detected by direct titration in any of the samples, which implies that their application in this situation would require more sensitive techniques. The results show that the run-off from the informal settlement constituted a major source of pollution for a river catchment which downstream is used as a source of water for human consumption, and that faecal pollution of human and animal origin can reliably be distinguished by means of combinations of appropriate indicators.