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Use of Marine Outfalls in Wastewater Disposal

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Introduction:

Global experience has shown that disposing of properly treated domestic wastewater through effective ocean outfalls is an economical and reliable strategy for wastewater disposal with minimal environmental impact.

Better construction techniques, such as tunnelling, coupled with improved materials mean that the technology of marine wastewater disposal has advanced rapidly in recent years.

Oceanographic instrumentation is also rapidly advancing and this, alongside laboratory techniques such as laser-induced fluorescence, used to study the mixing of discharged flow close to the end of the outfall - near field plume mixing – means a now greater understanding of three dimensional coastal water dynamics and near field processes and the mechanisms that work to assimilate and render the discharged pollutants harmless.

Advances in mathematical modelling ensure effective and economical outfall designs that protect both the environment and human health.

 

Disposal System:

Outfalls typically range from 1 to 4km long and discharge into waters 20 to 70m deep.

The disposal system is a system comprising of the treatment plant, outfall, diffuser and the field region around the diffuser where rapid mixing and dilution occurs.

PhilipRobertsfigure1.JPG

Figure 1: A marine wastewater disposal system treatment plant, outfall pipe, diffuser and near field.

Environmental and Water Quality Aspects:

Environmental and human health impacts of marine discharges are held to certain standards by regulatory authorities in order to ensure that:

  • Concentrations of bacteria are at safe levels for human health
  • Concentrations of toxins and other contaminants are at safe levels for the local ecosystem
  • Ecosystem products of the effluent are kept within safe limits to prevent eutrophication
  • Dissolved oxygen concentrations and BOD (biochemical oxygen demand) are kept within allowed limits
  • The deposition of local particulate is not excessive
  • The wastefield is not visible on the water surface

Appropriate Treatment for Ocean Outfall Discharges:

According to the World Health Organization the level of wastewater treatment has little bearing on the risk to human health of discharge from an effective outfall.

Effluents discharged directly on to the beach or from a short outfall constitute a high health risk. Unfortunately this is a common occurrence in developing countries. Preliminary treatment alone will usually be sufficient with an effective outfall.

To understand why advanced treatment is often unnecessary we must consider an outfall with a diffuser that effects an initial dilution of 100:1 – usually easily accomplished. This corresponds to a 99% reduction in contaminant concentrations in the receiving water, which is far beyond the capabilities of even advanced conventional treatment processes.

Outfall Construction Techniques:

Advanced outfall construction techniques include materials such as large diameter HDPE (high density polyethylene) pipe; dredging technology; pipeline construction and installation methods; and trenchless construction, including tunnelling.

HDPE pipes are now available in diameters up to two metres or more and can be delivered almost anywhere in the world. They are extruded at the manufacturing plant then towed to the outfall location where ballast weights are attached and sunk into place.

Conclusions:

Experience shows that Wastewaters can be disposed of safely and economically into coastal waters by means of marine outfalls with minimal economic impact.

The combination of an effective outfall and preliminary treatment is crucial in developing countries where low cost and reliability are of the highest priority.

Rapid advances are being made in the science and technology of marine wastewater disposal: oceanographic instrumentation, mathematical modelling and construction techniques are all being developed in order to ensure reliable and economical systems.

As domestic sewage is degradable, potential problems are, at most, local and not regional or global.

References

This article is adapted from

Underwater and underused: The case for marine outfalls in wastewater disposal,  Philip Roberts, Water21 October 2010 (Issue 12.5) pp22-26 IWA Publishing

Oceanographic instrumentation

Clean Water Act Section 403

World Health Organization

Wiki Article on Marine Outfall

Marine Outfall Discharges

Publications

Marine Wastewater Outfalls and Treatment Systems (2010) IWA Publishing

Philip J. W. Roberts, Henry J. Salas, Fred M. Reiff, Menahem Libhaber, Alejandro Labbe, James C. Thomson

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