Internationally, there is growing concern over the inappropriate and indiscriminate disposal of wastes into the sea. Recent United Nations (UN) reports highlight the increasing presence of “dead zones” in the marine environment. These threats are real and highly relevant to the sustainability of human health, biodiversity, ecosystem functioning, fisheries and tourism in the coastal zone.
The sea has been the ultimate “sink” for all terrestrial materials since the dawn of time. This is manifested in its saltiness and the fact that it contains traces of virtually all known substances. Marine organisms have evolved to cope and thrive in this chemically complex “soup”. Problems arise when, largely through the action of mankind, certain toxic or habitat-altering materials are concentrated in a confined area over a short period of time. These materials are generally derived from two sources. Firstly from diffuse sources and processes such as erosion and sediment deposition as well as storm water runoff from urban catchments and informal settlements; and secondly from deliberate point source discharges (drains and outfalls).
While mankind is clearly responsible for managing both sources of materials, it is important that we make a clear distinction between them. Diffuse sources are difficult to manage using current technology whereas point sources can simply be diverted to an alternative sink. It is important to realise that the latter cannot merely be switched off without much consideration. They can merely be diverted to another stream and sink that may or may not be more ecologically, socially or economically appropriate.
While the ideal is to have no discharges to sea over and above the natural background, this is clearly unattainable in the short term and a measure of compromise is essential. In some instances the direct disposal of selected effluents into the sea via deep sea outfalls emerges as the most appropriate solution from both ecological and socio-economic perspectives.
The City of Durban realised, from early days, that the open sea could be a potential viable disposal route for selected wastes and undertook pioneering work in both constructing and operating two deep-sea outfalls. The outfalls, which extend some four kilometres offshore, were designed on sound engineering principles and took local oceanographic circumstances into account. The aim was to maximise the dilution and dispersion of effluent and thus exert minimal impact on the environment. The Council for Scientific and Industrial Research (CSIR) has maintained a regular, stringent monitoring programme since the outfalls were commissioned in about 1960 to ensure that the design criteria are being achieved. The programme encompasses microbiology, ecology, chemistry and toxicity testing.
The microbiological surveys use a locally developed composite index to assess human health hazards to bathers and other recreational users of the sea.
The surveys have revealed a dramatic, and sustained, improvement in the microbiological health of bathing beaches following the introduction of the deep-sea outfalls. At the same time there has been no significant microbial degradation of offshore waters.
The ecological surveys focus on the health of the larger organisms (macrofauna) inhabiting the sea bed off Durban. Regular surveys evaluate the diversity and abundance of these communities and search for signs of impact. As would be expected close to a source of nutrients (sewage), there have been signs of organic enrichment. However these effects have, as a rule, been localised, transient and are not cause for ecological concern.
The chemical surveys monitor the coastal waters, sediments and mussels for a range of potential contaminants such as trace metals and chlorinated hydrocarbon pesticides. The results are compared with natural baseline values and against a set of internationally developed guidelines for recognising contamination. While there have been isolated instances of contamination, particularly in sediments close to the outfalls, there have been no signs of sustained degradation.
The toxicity of effluents is routinely measured using the fertilisation success of sea urchin gametes as a proxy for potential impact. This test is sensitive and provides a consistent basis for monitoring toxicity. It has proven to be a very useful tool in screening effluents destined for disposal and in revealing the more toxic components of composite effluents.
The CSIR surveys have demonstrated that the outfalls have not degraded the offshore environment and have, at the same time, resulted in a dramatic and significant improvement in the microbial and chemical quality of the inshore waters and bathing beaches. The controlled disposal of wastes to sea off Durban appears to be an appropriate strategy.
Given that there will be an ever increasing demand for effluent disposal, it is important that we keep a close watch on the capacity of the sea off Durban to assimilative wastes and to ensure that it is not exceeded. It is also vital to ensure that there are no long term cumulative impacts on the broader marine environment. The CSIR surveys are geared to monitor these aspects.
Despite the current success of the Durban outfalls, there is an ongoing imperative to reduce effluent loads wherever possible and to strive for the ideal of zero emissions. This concept is underpinned by the notion that all wastes (including the water carrying them) are potential resources and should not be discarded irretrievably. This will pose challenges for scientists and engineers.
|Samples of the sea-bed are taken near the deep sea outfalls off Durban using a self-triggering Grab. The retrieved material is used to assess the chemical and biological health of the environment."