Part C | 32.4
730 Part C Coastal Design
lower dilution. As such, plant processes and the feasibility of periodic marine construction works need to be
considered should this option be pursued.
Nozzle Sizes and Blanks
If it is not feasible to stage the installation of marine
outlet structures or sections of a line diffuser, another
option for managing low flows is to install blanking
plates at the nozzle openings. This solution provides
greater flexibility than staged construction because each
nozzle can be brought online or taken offline individually to suit the flow requirements. In addition, it allows
the total flow rate to be evenly distributed since nozzles
on multiple structures or at any location along a line diffuser can be blanked off. Near-field modeling is needed
to confirm the number and location of nozzles to be
blanked off for each long-term operating flow rate. Input from the plant operators should be sought when
considering this option, as making adjustments to nozzle blanks more than two or three times per year may
render this option infeasible.
A similar option is to install smaller diameter nozzles for the initial stages, then remove and replace with
larger diameter nozzles each time the plant is upgraded.
To minimize installation activities, each set of nozzles
should be designed to have the same entrance diameter such that modification to the manifold or rosette
structures is avoided. This option provides the same
degree of flexibility as nozzle blanking if demand or
process requirements lead to an extended period of low
flow; the smaller nozzles can be re-installed at any
time.
Although these options will also require marine
works each time the plant is upgraded, the scope and
duration of such works is likely to be considerably less
than for staged installation of marine structures or the
addition of more diffuser length. These options also
carry some operational risk due to the fact that the
plant could still be operated at any given flow rate, even
with an incorrect number or size of nozzles. A nozzle
configuration that is not aligned with the target operating flow could lead either to insufficient dilution (for
lower flow), or excessively high head losses through
the system (for higher flow). The operating procedures
for these two options must ensure that plant capacity is
linked to the installed nozzle configuration at any given
time.
Variable-Orifice Nozzles
Variable-orifice nozzles, also known as duckbill valves,
can be used instead of fixed diameter nozzles. These
valves are made from a variety of flexible materials such
as rubber or neoprene and are designed to close during
periods of no flow. Due to stiffness of the valve material, the valves gradually open in an elliptical shape as
the flow rate increases resulting in higher velocities at
lower flow rates as compared to fixed-diameter nozzles.
This eliminates or reduces the need for blanking nozzles
or introducing bypassed seawater into the outlet flows is
reduced or eliminated. The check valve feature of duckbill valves also prevents backflow from the sea into the
outfall system during periods of zero or low flow. Seawater intrusion into wastewater outfalls can therefore be
prevented with this option.
As material stiffness dictates the degree to which the
valves will open for any given flow rate, these valves
are usually custom designed for each particular application. Hydraulic testing of the valves is recommended
prior to selection and incorporation into the final design,
given the specificity of duckbill valves to each outfall
design. Hysteresis of the material should also be investigated to ensure that the valve will perform consistently
over time. This consideration is particularly important if
flows are expected to be highly variable and/or prevail
for extended periods (i. e., months or longer). The local
conditions in which the valves will be installed is another important aspect that should be taken into account
when deciding whether duckbill valves are an appropriate solution. If high levels of marine growth are expected, the valves may not close properly if organisms
attach themselves to the area near the valve opening.
32.4.4 Hydraulic Integration
Following a hydraulic assessment, the range of piezometric heads at the upstream boundary of an outfall system must be considered with respect to the hydraulics
through the entire treatment plant. Failure to properly
integrate the outfall system with other plant systems
could result in adverse operating conditions such as
decreased plant capacity, spillage and flooding, and inefficiencies for pumped systems. The choice between
gravity-driven or pumped outfalls depends on several
factors including site topography and plant elevation,
management of low flow scenarios and long-term operating or maintenance costs.
Gravity Systems
Gravity-driven outfall systems are often preferred because they are relatively simple to operate (few electrical and mechanical components) and have virtually no
long-term operating costs. Apart from periodic inspection and maintenance, the passive nature of a gravitydriven system ensures continuous reliability and minimal operator intervention. The free surface that exists
at an outfall shaft is convenient when outflows from
different parts of the treatment plant must be collected
and combined into a single outfall stream. In addition,
730 Part C Coastal Design
lower dilution. As such, plant processes and the feasibility of periodic marine construction works need to be
considered should this option be pursued.
Nozzle Sizes and Blanks
If it is not feasible to stage the installation of marine
outlet structures or sections of a line diffuser, another
option for managing low flows is to install blanking
plates at the nozzle openings. This solution provides
greater flexibility than staged construction because each
nozzle can be brought online or taken offline individually to suit the flow requirements. In addition, it allows
the total flow rate to be evenly distributed since nozzles
on multiple structures or at any location along a line diffuser can be blanked off. Near-field modeling is needed
to confirm the number and location of nozzles to be
blanked off for each long-term operating flow rate. Input from the plant operators should be sought when
considering this option, as making adjustments to nozzle blanks more than two or three times per year may
render this option infeasible.
A similar option is to install smaller diameter nozzles for the initial stages, then remove and replace with
larger diameter nozzles each time the plant is upgraded.
To minimize installation activities, each set of nozzles
should be designed to have the same entrance diameter such that modification to the manifold or rosette
structures is avoided. This option provides the same
degree of flexibility as nozzle blanking if demand or
process requirements lead to an extended period of low
flow; the smaller nozzles can be re-installed at any
time.
Although these options will also require marine
works each time the plant is upgraded, the scope and
duration of such works is likely to be considerably less
than for staged installation of marine structures or the
addition of more diffuser length. These options also
carry some operational risk due to the fact that the
plant could still be operated at any given flow rate, even
with an incorrect number or size of nozzles. A nozzle
configuration that is not aligned with the target operating flow could lead either to insufficient dilution (for
lower flow), or excessively high head losses through
the system (for higher flow). The operating procedures
for these two options must ensure that plant capacity is
linked to the installed nozzle configuration at any given
time.
Variable-Orifice Nozzles
Variable-orifice nozzles, also known as duckbill valves,
can be used instead of fixed diameter nozzles. These
valves are made from a variety of flexible materials such
as rubber or neoprene and are designed to close during
periods of no flow. Due to stiffness of the valve material, the valves gradually open in an elliptical shape as
the flow rate increases resulting in higher velocities at
lower flow rates as compared to fixed-diameter nozzles.
This eliminates or reduces the need for blanking nozzles
or introducing bypassed seawater into the outlet flows is
reduced or eliminated. The check valve feature of duckbill valves also prevents backflow from the sea into the
outfall system during periods of zero or low flow. Seawater intrusion into wastewater outfalls can therefore be
prevented with this option.
As material stiffness dictates the degree to which the
valves will open for any given flow rate, these valves
are usually custom designed for each particular application. Hydraulic testing of the valves is recommended
prior to selection and incorporation into the final design,
given the specificity of duckbill valves to each outfall
design. Hysteresis of the material should also be investigated to ensure that the valve will perform consistently
over time. This consideration is particularly important if
flows are expected to be highly variable and/or prevail
for extended periods (i. e., months or longer). The local
conditions in which the valves will be installed is another important aspect that should be taken into account
when deciding whether duckbill valves are an appropriate solution. If high levels of marine growth are expected, the valves may not close properly if organisms
attach themselves to the area near the valve opening.
32.4.4 Hydraulic Integration
Following a hydraulic assessment, the range of piezometric heads at the upstream boundary of an outfall system must be considered with respect to the hydraulics
through the entire treatment plant. Failure to properly
integrate the outfall system with other plant systems
could result in adverse operating conditions such as
decreased plant capacity, spillage and flooding, and inefficiencies for pumped systems. The choice between
gravity-driven or pumped outfalls depends on several
factors including site topography and plant elevation,
management of low flow scenarios and long-term operating or maintenance costs.
Gravity Systems
Gravity-driven outfall systems are often preferred because they are relatively simple to operate (few electrical and mechanical components) and have virtually no
long-term operating costs. Apart from periodic inspection and maintenance, the passive nature of a gravitydriven system ensures continuous reliability and minimal operator intervention. The free surface that exists
at an outfall shaft is convenient when outflows from
different parts of the treatment plant must be collected
and combined into a single outfall stream. In addition,