The National Institute for Water Research of the South African Council for Scientific and Industrial Research has been engaged on extensive research into the conservation of water supplies and into the reclamation of industrial and sewage effluents. Within the framework of this programme, a comparative study of the efficiency in the reduction of bacterial numbers by existing conventional sewage treatment plants has been carried out. The numerical decreases in bacterial populations after primary sedimentation, biological filtration, the activated sludge process, secondary sedimentation (humus tank), and final sand filtration have been compared with the reduction in bacterial numbers attained by the treatment of sand-filter effluent, humus tank effluent, and primary biological filter effluent in units for further biological purification.

Various types of units for this purpose have been investigated, and from available results it appears perfectly feasible to consider the suitability of effectively purified sewage effluents as sources for raw water supplies for industrial and domestic use. Sewage effluents after final further biological purification could be admitted for unrestricted discharge into rivers. The possibility of objections from the point of view of public health and future use is completely ruled out by the fact that the quality of the final effluent is as good as that of river water.

A variety of experimental units for further biological purification of sewage effluents have been studied, and a comparison of the results obtained from grass-plots, artificial vleis (a natural vlei in South Africa is a marshy or swampy depression in the ground in which water collects and which supports a standing vegetation), algal beds, artificial model rivers, and maturation or oxidation ponds have shown that maturation ponds, possibly a number of ponds in series, with an established microflora and microfauna of their own, may provide the final solution of the problem. Further biological purification will play an important role in the conservation of water supplies and will contribute to the raw water sources available for industrial and domestic use.

In parallel with these studies, the effect of the discharge of sewage effluents from conventional treatment plants into a river system has been investigated together with the fate of the sewage bacteria after admission to a natural environment. The manifestations of self-purification have been recorded, and the additional or superimposed effect of run-off and drainage from populated and unpopulated areas, as well as the admission of storm-water, have been considered together with the effects of seasonal variations due to climatic conditions.

The author wishes to express his gratitude to his assistants who bravely bore the burden of the routine work involved, and wishes to thank the S.A. Council for Scientific and Industrial Research for permission to publish this paper.