Part C | 32.2
714 Part C Coastal Design
lution – oil, grease, rags, etc.? As noted under Public
Health, most countries have environmental guidelines.
Provided these guidelines are met, it is assumed that the
marine environment will be protected. The guidelines
are usually in the form of concentrations of substances
(e.g., metals, nutrients, and bacteria) that should be met
at a specific distance from the outfall (this distance defines a mixing zone). This implies that there will be
a region inside the mixing zone in which the guidelines
may not be met. The consequences are that the biological diversity inside the mixing zone may not be the same
as that in reference areas.
Regulation
What are the regulations that govern the discharge of
any substance to the marine environment? Regulations
are often in the form of licence conditions restricting
the types, concentrations, and/or loads of substances
that can be discharged to the marine environment. As
noted above, there is a tacit assumption that keeping
within these restrictions will ensure the safety of humans, and the protection of flora and fauna in the marine
environment.
Economic
Governments will invest a large amount of money for
the construction of a marine outfall. Ultimately, this
money is raised through taxes and governments are accountable for the wise use of the taxes they collect.
Outfall dollars will be competing with funding areas
as diverse as education, security, and care for the aged.
What does the community value? What is the community willing to pay to protect both humans and the environment? The marine outfall is just one of many options
that should be considered. Ultimately there is a balance
between the level of protection offered and the cost incurred by each option. It is the responsibility of the
engineer and scientist to evaluate each option and provide the government with the most effective solution.
32.2.2 Wastewater Treatment
Our main focus in this section is on municipal outfalls.
Critical to a marine outfall is knowledge of what is being discharged, particularly the types, concentrations,
and variability of contaminants in the wastewater. Discharge of contaminants from other sources including
private outfalls, rivers and estuaries, atmospheric inputs, discharges from vessels, and illegal dumping are
not considered. The reader is referred to Tchobanoglous
et al. [32.2], which provides considerable detail on
wastewater treatment.
Wastewater discharges from domestic, commercial,
and industrial sources. Often, the wastewater systems
are not isolated from the environment and infiltration
of water during storms may also occur. The composition of wastewater depends on the relative contribution
of these three main sources and on the type and size
of industry and/or commercial activity. Each wastewater system is unique and treatment plants are designed
to deal with the quantity and quality of wastewater produced by a specific system.
Wastewater comprises particulate matter, pathogens, nutrients, organic, and inorganic material. Severe
environmental damage can result if wastewater is discharged undiluted or without treatment. Therefore the
main objective of sewage treatment is the elimination
or reduction in concentration of these materials.
Different concentrations of substances will invoke
different responses in different species. Metals may be
adsorbed onto particulates that may be ingested by fish
and shellfish. Organics are often adsorbed by the fatty
tissues in aquatic animals. Reducing the concentrations
of suspended solids, oil, and grease during the wastewater treatment process, reduces the quantity of metals
and organics that may affect marine organisms.
Wastewater treatment can be broadly divided into
three levels: primary, secondary and tertiary (or advanced). The levels are modular, subsequent treatments
being bolted onto lower levels of treatment. Within
each level of treatment there are multiple options that
produce wastewater of similar quality. The distinction among the treatment levels themselves is blurred
and will depend on how individual levels are operated
and maintained. Usually, concentrations of suspended
solids, biochemical oxygen demand (BOD), and indicator bacteria in the effluent are used to distinguish the
levels of treatment. The type of wastewater treatment
plant adopted is often based on the collective experience of the engineers and process workers within an
organization.
Primary Treatment
Primary treatment removes debris that could damage
the wastewater treatment system. This is done by passing the sewage through trash racks and screens. Sewage
then flows through sedimentation tanks at low velocities ensuring residence times of 23 h or more [32.2].
This allows sufficient time for negatively buoyant solids
to settle at the bottom of the tank and positively buoyant oils and greases to rise to the surface of the tank.
Chemicals can be added to the sewage to accelerate the
settling process. Both the solids and oil and grease can
then be easily removed. Primary treatment also helps
regulate the flow of sewage to subsequent levels of
treatment. Primary treatment may be used in isolation,
but this usually depends on the environment into which
the wastewater is discharged.
714 Part C Coastal Design
lution – oil, grease, rags, etc.? As noted under Public
Health, most countries have environmental guidelines.
Provided these guidelines are met, it is assumed that the
marine environment will be protected. The guidelines
are usually in the form of concentrations of substances
(e.g., metals, nutrients, and bacteria) that should be met
at a specific distance from the outfall (this distance defines a mixing zone). This implies that there will be
a region inside the mixing zone in which the guidelines
may not be met. The consequences are that the biological diversity inside the mixing zone may not be the same
as that in reference areas.
Regulation
What are the regulations that govern the discharge of
any substance to the marine environment? Regulations
are often in the form of licence conditions restricting
the types, concentrations, and/or loads of substances
that can be discharged to the marine environment. As
noted above, there is a tacit assumption that keeping
within these restrictions will ensure the safety of humans, and the protection of flora and fauna in the marine
environment.
Economic
Governments will invest a large amount of money for
the construction of a marine outfall. Ultimately, this
money is raised through taxes and governments are accountable for the wise use of the taxes they collect.
Outfall dollars will be competing with funding areas
as diverse as education, security, and care for the aged.
What does the community value? What is the community willing to pay to protect both humans and the environment? The marine outfall is just one of many options
that should be considered. Ultimately there is a balance
between the level of protection offered and the cost incurred by each option. It is the responsibility of the
engineer and scientist to evaluate each option and provide the government with the most effective solution.
32.2.2 Wastewater Treatment
Our main focus in this section is on municipal outfalls.
Critical to a marine outfall is knowledge of what is being discharged, particularly the types, concentrations,
and variability of contaminants in the wastewater. Discharge of contaminants from other sources including
private outfalls, rivers and estuaries, atmospheric inputs, discharges from vessels, and illegal dumping are
not considered. The reader is referred to Tchobanoglous
et al. [32.2], which provides considerable detail on
wastewater treatment.
Wastewater discharges from domestic, commercial,
and industrial sources. Often, the wastewater systems
are not isolated from the environment and infiltration
of water during storms may also occur. The composition of wastewater depends on the relative contribution
of these three main sources and on the type and size
of industry and/or commercial activity. Each wastewater system is unique and treatment plants are designed
to deal with the quantity and quality of wastewater produced by a specific system.
Wastewater comprises particulate matter, pathogens, nutrients, organic, and inorganic material. Severe
environmental damage can result if wastewater is discharged undiluted or without treatment. Therefore the
main objective of sewage treatment is the elimination
or reduction in concentration of these materials.
Different concentrations of substances will invoke
different responses in different species. Metals may be
adsorbed onto particulates that may be ingested by fish
and shellfish. Organics are often adsorbed by the fatty
tissues in aquatic animals. Reducing the concentrations
of suspended solids, oil, and grease during the wastewater treatment process, reduces the quantity of metals
and organics that may affect marine organisms.
Wastewater treatment can be broadly divided into
three levels: primary, secondary and tertiary (or advanced). The levels are modular, subsequent treatments
being bolted onto lower levels of treatment. Within
each level of treatment there are multiple options that
produce wastewater of similar quality. The distinction among the treatment levels themselves is blurred
and will depend on how individual levels are operated
and maintained. Usually, concentrations of suspended
solids, biochemical oxygen demand (BOD), and indicator bacteria in the effluent are used to distinguish the
levels of treatment. The type of wastewater treatment
plant adopted is often based on the collective experience of the engineers and process workers within an
organization.
Primary Treatment
Primary treatment removes debris that could damage
the wastewater treatment system. This is done by passing the sewage through trash racks and screens. Sewage
then flows through sedimentation tanks at low velocities ensuring residence times of 23 h or more [32.2].
This allows sufficient time for negatively buoyant solids
to settle at the bottom of the tank and positively buoyant oils and greases to rise to the surface of the tank.
Chemicals can be added to the sewage to accelerate the
settling process. Both the solids and oil and grease can
then be easily removed. Primary treatment also helps
regulate the flow of sewage to subsequent levels of
treatment. Primary treatment may be used in isolation,
but this usually depends on the environment into which
the wastewater is discharged.