Estuary Processes and Managment - River Dynamics and Integrated River Management - page 511

Sewage treatment —Normally there are three levels of sewage treatment: ķ primary treatment typically
removes at least 60% of suspended solids (SS) and 30% of biochemical oxygen demand (BOD) by means
of sedimentation and filtration, but removes no nitrates, phosphates, salts, pesticides; ĸ secondary treatment ,
which removes at least 90% of SS and BOD, 70% of phosphorus (mostly as phosphates), 50% of nitrogen
(mostly as nitrates), and 5% of dissolved salts through biological processes—mainly the activated sludge
process (e.g., aeration of bacterial flocs) and bacterial biofilm process (e.g., biofilter tank); Ĺ tertiary
treatment , which reduces the concentrations of inorganic nutrients and products of biological oxidation
during secondary treatment (Miller, 2005). In the European Union, all communities with more than
15,000 people have been required to have secondary treatment since 2000, with all urban centers slated
to have tertiary treatment by 2010. In developing countries like China, however, only about 10% (up to
2002) of municipal sewage receives secondary treatment (Ministry of Environmental Protection, China,
http://www.sepa.gov.cn/cont/city)—the construction and operational costs of secondary treatment is 3-4
times higher than that of primary treatment. For small coastal communities located next to well-flushed
waters, well-designed sea outfalls can provide an environmentally sound and economically attractive
alternative to secondary treatment (Sharp, 1991).
Outfall optimization —As even state-of-the-art treatment plants cannot completely prevent pollution
near coastal cities, there is a genuine need to develop better techniques for optimization of sewage
discharge. There are two principal means to prevent, or at least mitigate, any harmful local effects of waste
disposal impact (Jirka and Lee, 1994). First, through the choice and design of a discharge structure , we
may control the amount of initial mixing and dilution that take place within a short time (typically within
minutes to an hour) after the effluent enters the sea. Wastewater is commonly discharged via a submerged
outfall system which consists of three sections (Fig. 8.51): onshore headwork (e.g., for gravity or pumped
flow), a feeder pipeline section and the diffuser section that serves as the disposal terminal to release the
wastewater. Second, through proper siting strategies and ambient transport predictions, we may assure
that the discharged material is effectively and continuously removed from the discharge locality. From a
fluid mechanics viewpoint, these two means involve the analysis of near-field processes and of far-field
processes that affect the mixing and transport of the pollutant effluents.
Fig. 8.51 Schematic diagram of sewage outfall diffuser
8.5.2 Initial Mixing
Mixing zones —The effluent discharge through a diffuser is mixed by the turbulent vortices in the
environment, leading to a continuous and rapid reduction in pollutant concentration. Conceptually the
mixing processes extend over two regions (Fig. 8.52). In the near-field (also called active dispersal
region or initial mixing region), the region close to the discharge, the mixing is dynamically affected by