OCEANS AND ESTUARIES - Water Quality Engineering in Natural Systems - page 308

near field far field

ambient
current, V

waste field

effluent plume

merged plume

single plumes

flow

diffuser section

Figure 9.3. Wastewater discharge from an ocean outfall diffuser.

(a) (b)

Figure 9.4. Outfall construction: (a) laying an outfall pipe; (b) lowering a diffuser to the sea floor. Source : Christchurch City
Council (2005).

domestic wastewater into the oceans are usually associ-
ated with human exposure to bacteria-contaminated
seawater at public sea-bathing beaches.
The design of ocean outfalls and the analysis of
plume dilution have been the subject of research for
many years and are now fairly mature fields. However,
the analysis of far-field mixing is far less certain. Far-
field mixing processes determine the impact of ocean
discharges on beaches and are important in selecting
acceptable outfall locations, since the wastewater dis-
charged must continuously be removed from where the
wastewater is discharged.
A typical ocean outfall diffuser is illustrated in
Figure 9.3. Diffusers contain multiple ports, with each
port designed to discharge effluent at the same rate.
Outfall pipelines that transport wastewater from
onshore treatment plants to the diffusers are generally
buried to the point where the water is deep enough to
protect them from wave action, usually about 10 m
(30 ft). Beyond the buried portion, the outfall pipe rests
on the bottom of the ocean, with a flanking of rock to
prevent currents from undercutting it where the bottom
is soft. The construction of an ocean outfall and the
lowering of a diffuser to the seafloor are illustrated in

Figure 9.4. In cases where the outfall pipe and diffuser
are entirely buried under the ocean bottom, riser nozzle
assemblies are used to discharge the effluent into the
ocean. A typical riser nozzle assembly consists of a ver-
tical pipe (riser) with 4-8 nozzles located at the top of
the riser; where nozzles are also commonly referred to
as ports. Most ocean outfalls are installed at depths of
30-70 m (100-230 ft) and at distances from shore of
1-8 km (0.6-5 mi), and the characteristics of several
ocean outfalls are shown in Table 9.1. The discharge
per unit length of most outfalls is on the order of
0.01 m 2 /s (0.03 ft 2 /s).
In shallow waters, plumes originating from individual
ports in multiport diffusers tend to reach the ocean
surface prior to merging, in which case dilution of the
effluent is determined by the dynamics of the individual
plumes discharged from each of the ports. The effects of
surface waves on near-field dilution at shallow water
outfalls are usually neglected; in certain cases, however,
these effects can be significant (Chin, 1987, 1988). In
deepwater outfalls, plumes originating from individual
ports typically merge together well in advance of reach-
ing the ocean surface or being trapped due to density
stratification.