Part C | 32.3
716 Part C Coastal Design
Water quality in the treated effluent will be a function of the level of treatment. Estimates can be obtained
from other, similar treatment plants or from the indicative values provided in Table 32.1. Ongoing monitoring
of the effluent quality after construction will help ensure
maintenance of environmental standards.
Measuring water quality in the marine waters (into
which the wastewater is discharged) provides background concentrations of contaminants. The background concentrations must be added to the modeled
concentrations to estimate the total concentration of
contaminants in the marine waters. Background concentrations may already exceed environmental guidelines, in which case, they may need to be relaxed
or another outfall location sought. To obtain a representative picture of marine water quality, sampling
should take place over large spatial and temporal scales
and should include replication. Instruments that can be
moored in the field for long periods of time are increasingly being used to obtain water quality measurements,
although the accuracy of such results is less than can be
achieved in the laboratory.
Current speed and direction are important for plume
dilution. Moored current meters can provide detailed
temporal information at a point in space (or a profile throughout the water column). However, they are
expensive to deploy, maintain and retrieve, and careful consideration needs to be made in regard to the
number and location of such moorings. Spatial information can be obtained by profiling currents from
a vessel underway, drifters drogued at specific depths
and remotely sensed data (e.g., via satellites or airborne
scanners). The number and duration of moored current meters will depend on the size of the outfall under
consideration. For a moderately sized outfall, a single profiling current meter moored for 12 months and
serviced monthly, provides the minimum data requirements. A roving current meter (deployed at different
locations for one month at a time) may provide a compromise between the number of instruments and spatial
coverage.
Density stratification of the water column largely
governs the height of rise of the wastewater. (The effect is much reduced for brine discharges). In coastal
marine waters, density is a function of both temperature and salinity, both of which should be measured. For
shallow outfalls (outfalls in water depths less than about
10 m), stratification has little effect. However, for outfalls in deep waters, relatively small stratification may
produce a submerged plume resulting in lower dilutions
and a nonvisible plume (at least to a surface observer).
Moored temperature/salinity strings provide a profile
of density throughout the water column, although marine fouling will reduce the quality of data from salinity
sensors. Such data can also be collected during the servicing of moored instruments, which may be monthly
over a period of 12 months.
Other data such as surface waves and tides may also
be important, particularly for shallow outfalls where
the changes in water depth resulting from such processes may represent a significant proportion of the
water column.
32.3 Predicting Near-Field Dilutions
The design of a marine outfall centers on the dilution
required to meet the relevant guidelines. Occasionally,
guidelines may be met after an appropriate level of
sewage treatment. However, many substances will rely
on the dilution with marine waters to meet these guidelines. Dilution depends on:
Wastewater flowrate
Depth of water into which the wastewater is discharged
Length of the diffuser
Outlet diameter (and whether a single or multiple
outlets will be used)
Configuration of the diffuser (e.g., whether T-section outlets or gas-burner type rosettes are used,
whether nonreturn check valves are used)
Ocean conditions (e.g., currents, stratification of the
water column, tides, and ocean turbulence).
Together with cost, the above factors are used to
optimize the location and configuration of the marine
outfall. This is further discussed in Sect. 32.3.7
After discharge from the marine outfall, effluent rises (whereas brine descends) due to buoyancy
(Fig. 32.1). The wastewater (effluent or brine) then
mixes with the ambient currents and is diluted. Two
types of models are used to quantify this process: nearfield and far-field. This separation is made because the
time and space scales of the processes in each model
are substantially different.
In the near-field, the motion of the wastewater is
dominated by its initial momentum and buoyancy; the
velocities and rates of dilution are high. Up to 90% of
wastewater dilution takes place within the near-field at
the end of which most regulations apply. The engineer
can configure the outfall design to maximize dilution in
the near-field.
716 Part C Coastal Design
Water quality in the treated effluent will be a function of the level of treatment. Estimates can be obtained
from other, similar treatment plants or from the indicative values provided in Table 32.1. Ongoing monitoring
of the effluent quality after construction will help ensure
maintenance of environmental standards.
Measuring water quality in the marine waters (into
which the wastewater is discharged) provides background concentrations of contaminants. The background concentrations must be added to the modeled
concentrations to estimate the total concentration of
contaminants in the marine waters. Background concentrations may already exceed environmental guidelines, in which case, they may need to be relaxed
or another outfall location sought. To obtain a representative picture of marine water quality, sampling
should take place over large spatial and temporal scales
and should include replication. Instruments that can be
moored in the field for long periods of time are increasingly being used to obtain water quality measurements,
although the accuracy of such results is less than can be
achieved in the laboratory.
Current speed and direction are important for plume
dilution. Moored current meters can provide detailed
temporal information at a point in space (or a profile throughout the water column). However, they are
expensive to deploy, maintain and retrieve, and careful consideration needs to be made in regard to the
number and location of such moorings. Spatial information can be obtained by profiling currents from
a vessel underway, drifters drogued at specific depths
and remotely sensed data (e.g., via satellites or airborne
scanners). The number and duration of moored current meters will depend on the size of the outfall under
consideration. For a moderately sized outfall, a single profiling current meter moored for 12 months and
serviced monthly, provides the minimum data requirements. A roving current meter (deployed at different
locations for one month at a time) may provide a compromise between the number of instruments and spatial
coverage.
Density stratification of the water column largely
governs the height of rise of the wastewater. (The effect is much reduced for brine discharges). In coastal
marine waters, density is a function of both temperature and salinity, both of which should be measured. For
shallow outfalls (outfalls in water depths less than about
10 m), stratification has little effect. However, for outfalls in deep waters, relatively small stratification may
produce a submerged plume resulting in lower dilutions
and a nonvisible plume (at least to a surface observer).
Moored temperature/salinity strings provide a profile
of density throughout the water column, although marine fouling will reduce the quality of data from salinity
sensors. Such data can also be collected during the servicing of moored instruments, which may be monthly
over a period of 12 months.
Other data such as surface waves and tides may also
be important, particularly for shallow outfalls where
the changes in water depth resulting from such processes may represent a significant proportion of the
water column.
32.3 Predicting Near-Field Dilutions
The design of a marine outfall centers on the dilution
required to meet the relevant guidelines. Occasionally,
guidelines may be met after an appropriate level of
sewage treatment. However, many substances will rely
on the dilution with marine waters to meet these guidelines. Dilution depends on:
Wastewater flowrate
Depth of water into which the wastewater is discharged
Length of the diffuser
Outlet diameter (and whether a single or multiple
outlets will be used)
Configuration of the diffuser (e.g., whether T-section outlets or gas-burner type rosettes are used,
whether nonreturn check valves are used)
Ocean conditions (e.g., currents, stratification of the
water column, tides, and ocean turbulence).
Together with cost, the above factors are used to
optimize the location and configuration of the marine
outfall. This is further discussed in Sect. 32.3.7
After discharge from the marine outfall, effluent rises (whereas brine descends) due to buoyancy
(Fig. 32.1). The wastewater (effluent or brine) then
mixes with the ambient currents and is diluted. Two
types of models are used to quantify this process: nearfield and far-field. This separation is made because the
time and space scales of the processes in each model
are substantially different.
In the near-field, the motion of the wastewater is
dominated by its initial momentum and buoyancy; the
velocities and rates of dilution are high. Up to 90% of
wastewater dilution takes place within the near-field at
the end of which most regulations apply. The engineer
can configure the outfall design to maximize dilution in
the near-field.