user's manual for corhyd: an internal diffuser hydraulics model - IfH

2 Background
2.1 Multiport diffusers
Waste water treatment plants commonly discharge treated effluents through outfalls into
rivers or coastal waters. These plants are designed to minimize environmental impacts by
reducing the pollutant concentrations of the effluent. Nevertheless, even discharges of stateof-the-art
treatment plants may cause local pollution of the receiving waters, if the effluent
contains persistent substances and especially if discharge flowrates are high, which is the case
for most large metropolitan areas (like Buenos Aires, New York, Rio de Janeiro, HongKong,
Boston, Istanbul, …). To prevent local pollution and to protect ecologically sensitive regions,
persistent substances have to be reduced directly at the source and the large discharges have to
be distributed over a wider area. For the latter purpose, long outfall pipes with multiport
diffuser installations are used to disperse the effluent to non-critical levels (Jirka and Lee,
1994), aided by the natural pollution degradation rates of the receiving water bodies.
An optimized combination of on-land treatment and receiving water capacities, especially for
nutrient inputs from municipal sources, may positively affect the world’s severe health
problems often directly caused by sanitation problems (UNEP, 2004). New water quality
regulations (e.g., US: EPA, 1994; Europe: EC-Water framework directive, 2000; Brazil:
CONAMA, 2000; Argentina / Uruguay: Guarga et al. 1992) account for that combined
approach and therefore also result in a worldwide increasing utilization of treatment plants
with multiport diffuser outfalls (e.g., Australia: Philip and Pritchard, 1996; USA: Signell et
al., 2000).
An outfall is a pipe system between the dry land and the receiving water. It consists of three
components (Fig. 1): the onshore headwork (e.g. gravity or pumping basin); the feeder
pipeline which conveys the effluent to the disposal area; and the diffuser section, where a set
of ports releases and disperses the effluent into the environment to minimize the impacts on
the quality of the receiving water body. Diffusers can be single branched or double branched
systems (T- or Y-shaped, Fig. 2). If the the diffuser section is simply laid on the sea bed it is
composed of port orifices in the wall of the diffuser pipe (simple port configuration, Fig. 3a),
which may carry additional elements like elastic, variable area orifices (duckbill valves, Fig.
3b). If diffusers are covered with ballast, laid in a trench or even tunneled in the ocean floor
vertical risers (riser/port configuration, Fig. 3c) are connected to the diffuser to convey the
effluent to the water body. For deep tunneled solutions often rosette-like port arrangements
(similar to a gas burner device, Fig. 3d) are used to save the number of risers and allow for
increased dispersion. Also risers may carry duckbill valves, which change their effective open
port area related to the pressure difference between inside and outside the valve. They avoid
salt water intrusion during low flow periods and allow high discharges during peak flow
periods.
The flow in multiport diffusers is controlled by two boundary conditions: first, the entrance
boundary (flow rate or head), and, second, the ambient/disposal boundary, where the effluent
physical properties differ from the ambient fluid. Both conditions vary in time due to
discharge variations (diurnal changes, storm water events and long-term changes due to
increased sanitation coverage) and pressure variations, density variations, tides or waves.
Institut für Hydromechanik, Universität Karlsruhe 5