Marine Outfalls 32.2 Governance 713
Part C | 32.2
figurations. (Risers are vertical structures used to
transfer the wastewater from an outfall tunnel to
the outlet nozzles. They may be tens of metres in
length).
The number of outlet nozzles attached to each riser
is usually restricted to eight or less. If more than
eight outlet nozzles are used, the plumes from adjacent nozzles interfere with each other and reduce
the effective dilution of the wastewater.
The outfall pipeline and diffuser may be tapered to
ensure the velocity of the wastewater remains sufficiently high to prevent sediments from settling in
the pipeline.
Outlet nozzles may be fitted with nonreturn check
valves (also called duckbill valves). These valves
are closed when the wastewater flow is zero and prevent the ingress of seawater into the pipeline. One
advantage of check valves is that they enhance dilution, compared with a round nozzle of the same
cross-sectional area [32.1]. However, they may be
fouled by biota or fishing nets, rendering them permanently open or closed.
32.2 Governance
There are many factors affecting the decision to build
a marine outfall.
Municipal wastewater collects at the bottom of the
catchment. For a coastal city, this is at the edge of the
marine environment. There are large costs associated
with the movement of wastewater to the top of a catchment for potable reuse, including construction of a pipe
network, pumps, and energy required to operate the
pumps. Furthermore, there may be high costs associated
with the conversion of wastewater to potable water. The
disposal of wastewater through a marine outfall may
be the best overall use of resources. Despite this, the
decision to proceed with a marine outfall should first
examine other options and maximize the beneficial uses
of recycled wastewater.
32.2.1 Drivers for a Marine Outfall
SPHERE is an acronym we use to describe the main
factors overlying the need for government sponsored
development (social, public health, environmental, regulation, economic). The first three elements of SPHERE
represent the main aspects in which a marine outfall has an impact (i. e., the community values). The
last two elements of SPHERE represent the constraints on the marine outfall – regulation tending
toward high treatment and consequential high cost and
economic tending toward low cost and consequential
low treatment. Below, some of the considerations of
SPHERE are described in the context of a marine
outfall.
Most countries have environmental guidelines that
need to be met during the design of a marine outfall.
These guidelines are unique to each country and all cannot be detailed here. Suffice to say that they include
meeting concentrations of contaminants which may include pathogens, nutrients, metals, and organics. These
guidelines usually apply at the boundary of a mixing
zone, which needs to be clearly defined prior to construction of the marine outfall.
Social
What does the community expect from a marine outfall? What are the values that are important to the
community? This will vary among and within different
geographical regions and cultural groups. Some communities will comprise a large number of beach users.
To them, the concept of a marine outfall may not be
palatable unless it can be clearly demonstrated that the
marine outfall poses minimal risk to their use of the marine environment.
Public Health
Is it safe to swim in the marine waters? What are
the types and concentrations of substances that will
be discharged to the marine environment? Will they
be of harm to us? Much information is available to
inform us about of the potential harm of substances
that may be discharged through a marine outfall. Most
countries synthesize this information into a set of guidelines applicable to their marine environment. There is
a tacit assumption that, provided the concentrations
of the substances are kept below the guideline levels, the health of the users of the marine environment
will be maintained. This does require knowledge of the
types, concentrations, and variability of the substances
in the wastewater. It should be noted here that all the
substances are potentially toxic given sufficiently high
concentrations and the environment into which they are
discharged.
Environmental
Will the discharge of substances through the marine
outfall cause harm to marine organisms? Will the marine environment be degraded into the future? Will the
beaches and marine waters be free from visible pol-
Part C | 32.2
figurations. (Risers are vertical structures used to
transfer the wastewater from an outfall tunnel to
the outlet nozzles. They may be tens of metres in
length).
The number of outlet nozzles attached to each riser
is usually restricted to eight or less. If more than
eight outlet nozzles are used, the plumes from adjacent nozzles interfere with each other and reduce
the effective dilution of the wastewater.
The outfall pipeline and diffuser may be tapered to
ensure the velocity of the wastewater remains sufficiently high to prevent sediments from settling in
the pipeline.
Outlet nozzles may be fitted with nonreturn check
valves (also called duckbill valves). These valves
are closed when the wastewater flow is zero and prevent the ingress of seawater into the pipeline. One
advantage of check valves is that they enhance dilution, compared with a round nozzle of the same
cross-sectional area [32.1]. However, they may be
fouled by biota or fishing nets, rendering them permanently open or closed.
32.2 Governance
There are many factors affecting the decision to build
a marine outfall.
Municipal wastewater collects at the bottom of the
catchment. For a coastal city, this is at the edge of the
marine environment. There are large costs associated
with the movement of wastewater to the top of a catchment for potable reuse, including construction of a pipe
network, pumps, and energy required to operate the
pumps. Furthermore, there may be high costs associated
with the conversion of wastewater to potable water. The
disposal of wastewater through a marine outfall may
be the best overall use of resources. Despite this, the
decision to proceed with a marine outfall should first
examine other options and maximize the beneficial uses
of recycled wastewater.
32.2.1 Drivers for a Marine Outfall
SPHERE is an acronym we use to describe the main
factors overlying the need for government sponsored
development (social, public health, environmental, regulation, economic). The first three elements of SPHERE
represent the main aspects in which a marine outfall has an impact (i. e., the community values). The
last two elements of SPHERE represent the constraints on the marine outfall – regulation tending
toward high treatment and consequential high cost and
economic tending toward low cost and consequential
low treatment. Below, some of the considerations of
SPHERE are described in the context of a marine
outfall.
Most countries have environmental guidelines that
need to be met during the design of a marine outfall.
These guidelines are unique to each country and all cannot be detailed here. Suffice to say that they include
meeting concentrations of contaminants which may include pathogens, nutrients, metals, and organics. These
guidelines usually apply at the boundary of a mixing
zone, which needs to be clearly defined prior to construction of the marine outfall.
Social
What does the community expect from a marine outfall? What are the values that are important to the
community? This will vary among and within different
geographical regions and cultural groups. Some communities will comprise a large number of beach users.
To them, the concept of a marine outfall may not be
palatable unless it can be clearly demonstrated that the
marine outfall poses minimal risk to their use of the marine environment.
Public Health
Is it safe to swim in the marine waters? What are
the types and concentrations of substances that will
be discharged to the marine environment? Will they
be of harm to us? Much information is available to
inform us about of the potential harm of substances
that may be discharged through a marine outfall. Most
countries synthesize this information into a set of guidelines applicable to their marine environment. There is
a tacit assumption that, provided the concentrations
of the substances are kept below the guideline levels, the health of the users of the marine environment
will be maintained. This does require knowledge of the
types, concentrations, and variability of the substances
in the wastewater. It should be noted here that all the
substances are potentially toxic given sufficiently high
concentrations and the environment into which they are
discharged.
Environmental
Will the discharge of substances through the marine
outfall cause harm to marine organisms? Will the marine environment be degraded into the future? Will the
beaches and marine waters be free from visible pol-