Environmental Engineering Reference
In-Depth Information
to the surface are more easily transported to the shore
by wind currents. An illustration of a discharge port and
a plume rising (and being trapped) in a stratified envi-
ronment is given in Figure 9.2. Density stratification in
coastal waters may be caused by temperature differ-
ences (due to heating of the upper layers) and/or by
salinity differences (due to river inflows). An internal
hydraulic jump sometimes occurs near the boil, causing
dilutions that are three to five times higher than at the
center of the boil. Ocean currents advect the plume
away from the outfall, and spatial variations in these
currents result in further mixing of the effluent plume.
The region in the immediate vicinity of the outfall,
where mixing is dominated by buoyancy effects, is called
the near field, , and the region farther away from the
outfall, where mixing is dominated by spatial variations
in ocean currents, is called the far field. .
Ocean outfalls must generally be designed to mini-
mize the adverse impacts of wastewater discharges on
both benthic and pelagic marine communities. Benthic
communities live on or within the bottom substrate of
the ocean, while pelagic communities live within the
water column with little or no association with the
bottom. Benthic organisms are sometimes called
benthos .
If the effluent discharged from an ocean outfall does
not meet the ambient water-quality standards, which is
usually the case, regulatory agencies usually allow the
delineation of a mixing zone surrounding the outfall,
within which there is sufficient dilution that the ambient
water-quality criteria are met on and beyond the bound-
ary of the mixing zone. There are usually statutory limits
for the maximum size of a mixing zone surrounding an
ocean outfall. In the state of Florida, mixing zones in
open-ocean waters are required to have areas less than
or equal to 50.3 ha (124 ac) (Florida Department of
Environmental Protection, 2012), which is equal to the
area enclosed by a circle of radius 400 m (1300 ft),
whereas New Jersey defines a mixing zone as the area
within 100 m (330 ft) of the effluent discharge. Contami-
nants (sometimes called stressors) of concern in domes-
tic wastewater discharges to oceans can have either
ecological or human health impacts. Ecological stressors
include nutrients, metals, and volatile and synthetic
compounds, while human health stressors include
pathogenic microorganisms, metals, organic compounds,
and endocrine-disrupting compounds. Human health
water-quality criteria must be considered in addition to
aquatic life water-quality criteria, with the most restric-
tive criteria taking precedence. The primary human
health criteria associated with the discharge of treated
Figure 9.1. Mawddach estuary, UK. Source : Newbould (2005).
(a)
(b)
Figure 9.2. (a) Discharge port; and (b) plume rising in a stratified environment. Source of (b) : Fan and Brooks (1969).
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