The Effect of Sewage Sludge Disposal to Sea through Pipelines Previously Discharging only Settled Effluent

Abstract

The City of Durban has two submarine outfall sewers designed for the disposal of approximately 365 000 m3/d of domestic and industrial effluents. Until recently these outfall sewers were only permitted to discharge settled effluents, the resultant sludge having to be treated and disposed of on land in the one instance and incinerated in the other. As the result of a two year programme of research the city has been permitted to discharge settled effluent from both outfalls to which the underflow from the sedimentation basins has been reintroduced. Retention of the sedimentation basins has permitted effective removal of floatable material. The pipelines were operated during the research period by discharging only settled effluent from the one whilst from the other the discharge consisted of the settled effluent stream to which the sludge had been reintroduced. Halfway through the programme the roles of the two pipelines were reversed. This paper briefly reviews marine disposal in South Africa, the Durban submarine outfalls, the events which preceded the research project, the design of the research programme and the results of the study in terms of bacteriological, biological and chemical monitoring, current measurements in the immediate vicinity of the pipelines and dilution calculations with the aid of more rigorous mathematical treatment.