EW/FS/WW/AT3
Waste water treatment
Ocean outfalls: Initial dilution process - Overview
The behaviour of a fresh water effluent jetted into the sea depends on the dynamics and stratification
of the receiving water. Initial dilution is brought about by the entrainment of surrounding ‘clean’
seawater into the effluent jet as it leaves the diffuser port and rises to the surface. The typical
conditions and the behaviour of a wastewater plume are:
Stagnant unstratified water (no currents and no stratification), in which the wastewater plume will rise
vertically to the surface and laterally disperse in a surface field.
Stagnant stratified water (no currents with a density gradient between the surface and the bottom), in
which the rising of the wastewater plume can be inhibited and a submerged waste field can be formed
below the surface.
Moving, unstratified water (currents and no stratification), in which the current component will
lengthen the time and the path of the rising plume, thereby enabling it to entrain seawater and
resulting in increased initial dilutions
Moving, stratified water (currents and stratification), in which the current component will lengthen the
time and the path of the rising plume, but the wastewater plume will become trapped below the
surface because of the inhibitory effect which stratification has on the rising of the plume.
Sea surface
Sea surface
Temperature
No currents
Stratification
No currents
No stratification
Stagnant unstratified water
Temperature
Current
Sea surface
Stagnant stratified water
Temperature
Current
Sea surface
Stratification
Stratification
Weak currents with stratification
Moderate currents with stratification
Reference: Republic of South Africa, Department of Water Affairs and Forestry. 2004. Water Quality Management Series
Sub-Series No. MS 13.3. Operational policy for the disposal of land-derived water containing waste to the marine
environment of South Africa: Guidance on Implementation. Edition 1. Pretoria.
A surfacing effluent plume. Rhodamine dye
was injected to facilitate the recording of
the plume behaviour.
Contact: ewisa@wamsys.co.za URL: www.waterinfo.co.za
Prepared by: eWISA
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