Estuary Processes and Managment - River Dynamics and Integrated River Management - page 510

8.5 Waste Disposal and Impact Assessment
8.5.1 Waste Disposal
The ocean has long been used as the ultimate sink of most pollutants owing to its enormous assimilative
capacity. Nonetheless significant water quality problems may occur on a local or regional scale, as high
concentrations of released substances can cause considerable harm and impact. Natural passive turbulent
mixing can be rather slow, and visible “pollution belts” along a river bank can often be observed for many
kilometers downstream of an industrial wastewater discharge (Fig. 8.50). In the coastal environment,
domestic wastes issuing from the open end of a submerged discharge pipe may be poorly diluted,
resulting in foul odors, visible sewage slicks, and high bacterial concentrations at the nearby shoreline.
Typically, environmental regulations specify concentration limits or water quality standards that may not
be exceeded outside of a “mixing zone”. For example, for protection of public health the maximum
E.coli. concentration at a bathing beach should not exceed 180 counts/100 mL and the average dissolved
oxygen (DO) level should exceed 4 mg/L for protection of marine life (Hong Kong standards).
In general, the scientific management of water resources requires the ability to predict pollutant
concentrations resulting from a given set of discharge and ambient conditions. Environmental impact
assessment studies are required for any proposed infrastructure development and the pollution source
must be controlled in such a way that the water quality objectives are achieved. There are two major
sewage strategies: ķ land-based sewage treatment; and ĸ advanced effluent diffusion technology to
discharge partially-treated effluent through a submerged outfall diffuser system.
Fig. 8.50 Pollution belt resulting from discharge from a paper mill on the Yangtze River (See color figure at the
end of this topic)
For municipal discharges into inland rivers and lakes, a high degree of treatment is usually required;
on the other hand, the use of sea outfalls in combination with a certain degree of land-based treatment
may be appropriate for coastal or marine waters. Ultimately a rational decision on the appropriate level
of treatment must be based on a consideration of environmental risks (damage to the ecosystem) and
economic and public health costs. In many densely populated coastal cities, wastewater outfalls are often
located in relatively shallow waters (e.g., 5-2 m depth), and not far (e.g., 5-10 km) from sensitive areas
such as beaches or shellfisheries (Choi and Lee, 2007). The complex and highly variable hydrographic
conditions present significant technical challenges for environmental impact assessment. In recent years,
advances in environmental hydraulics and numerical modeling have enabled the proper determination of
the risks associated with different sewage treatment and disposal schemes.