SALMONELLA CONTAMINATION OF RECYCLED EFFLUENT OF TREATED SEWAGE AND URBAN WASTE-WATER

This study was conducted to determine the effectiveness of the removal of enteric pathogens such as Salmonella in recycle sewage effluent an d urban waste water after the treatment process at the Harare Firle Se wage Works. A total of 633 samples were collected from raw sewage, tan k effluent, filter effluent, humus, sludge, maturation pond, final eff luent, river upstream and river downstream at the point of effluent di scharge. These samples were processed and colonised on nitrocellulose filters to form dot-blots. The colonised filters were prepared for DNA -DNA hybridisation, using a Salmonella-specific DNA probe. When the re sults were analysed, an overall Salmonella contamination frequency of 23.7% was obtained. Further analysis at each stage in the sewage treat ment process showed that in raw sewage 65% of the samples were positiv e for Salmonella while 52% of tank effluent and 4% of maturation pond- sample were positively respectively. In filter effluent 7.3% of the sa mples were positive, while the humus effluent and the sludge samples w ere 2% and 3.12% positive respectively. In the final effluent 1.8% of the samples were positive for Salmonella. Thirty four percent (34) of samples from the river upstream of effluent discharge and 36% of sampl es from the river downstream of effluent discharge were positive for S almonella. The bacterial concentrations in the positive samples varied from greater than 1 x 10(8) cells (++++) cells per tested sample volu me in raw sewage which is higher than the clinically infective dose of 1 x 10(5) cells, to less than 1 x 10(2) (+) cells per tested sample v olume in the final effluent. This indicated that the sewage treatment process at Firle Sewage Works does not effect total removal of Salmone lla. Other pathogens, which were not tested, could be higher. The fina l effluent, which is recycled from the Harare Firle Sewage Works for p otable water supply and also used directly in agriculture for irrigati on, probably supplies Salmonella and other pathogens at a contaminatio n rate of 1.8%. The methods employed in this study provide a rapid, se nsitive, specific and cost-effective routine detection procedure for t he monitoring of Salmonella in environmental samples and waste-water t reatment works.