Sewage System
Analysis of the substance
Unpleasant smell
Nutritious for bacteria
Good environment for disease-causing
Natural break down in the water
• Biochemical oxygen demand (BOD)
measures the amount of dissolved oxygen
needed to decompose organic matter in
the water through aerobic processes. This
oxygen depletion can stress fish and other
aquatic organisms that depend on
dissolved oxygen to live.
Natural break down in the water
• Marine sewage contains nutrients such as
nitrogen that can cause algae to grow and
spread in the form of blooms. Algal blooms
can block the light needed by other marine
life, and if the algae decays it also uses up
Natural break down in the water
• Chemical products used in holding tanks
or in chemical toilets to disinfect and
deodorise the sewage are also discharged
when sewage is dumped into the sea.
These products have the potential to be
toxic to marine life.
Public health
Disease-causing bacteria spread by sewage:
Hepatitis A
Contact with polluted water can also cause skin
infections and respiratory problems.
Public health
• More importantly, people who eat shellfish
grown in contaminated water (especially
raw or poorly cooked oysters or mussels)
risk food poisoning.
• Viruses, in particular, can survive for a
long time in water, sediments and
shellfish, and are still able to infect people.
Millions of viruses are shed per gram of
The law
• In US and many UE countries – Discharge
of untreated raw sewage is illegal
anywhere within the US’ three-mile limit
• In NZ - No one can discharge untreated
sewage in the coastal marine area from a
ship or an offshore installation unless that
discharge is more than 500 metres from
shore, in waters deeper than 5 metres and
more than 500 metres from a marine farm
or gazetted mataitai reserve, or more than
200 metres from a marine reserve.
Regulations on treated sewage
The Regulations now provide for two
standards of treated sewage – Grade A
and Grade B. These systems produce a
different standard of effluent.
Grade A
These are the systems that have been approved
by the International Maritime Organisation (IMO)
as meeting the International Convention for the
Prevention of Pollution from Ships 1973/78
(MARPOL) standard of treated sewage. They
are the larger, more comprehensive sewage
treatment systems and they are usually fitted on
larger, commercial or recreational ships. They
also include systems that, when tested
according to IMO criteria, meet the standard set
out in schedule 6 of the Regulations.
Grade B
These are the systems that have been
approved by the United States Coast
Guard as a Type I system and are
available in New Zealand. These systems
are smaller, cheaper and can be installed
in recreational craft.
Regulations for Treated sewage
The discharge of treated sewage from
either a Grade A or Grade B system,
provided the systems are operated
correctly and maintained in good working
order, is allowed anywhere except within
100 metres of a marine farm (if you are a
Grade A system) and 500 metres of a
marine farm or a gazetted mataitai area (if
you are a Grade B system).
Ministry of Environment
• Website of the Ministry of Environment
You can find regulations and articles on the
Legal possibilities for discharge
• In the above described legal zone
• Pump-out facilities at marinas (p.42of Hold it. . .
We're Cleaning Up The Coast - Dealing With Sewage Safely and
Responsibly on Your Boat
)p.44 of the file
Pump-out facilities at marinas
• Standard deck connection is required
It is a Standard 38 mm pipe thread inside
diameter deck pump-out fitting.
( see p.45 and 30 of Hold it. . . We're Cleaning Up The Coast Dealing With Sewage Safely and Responsibly on Your Boat)
p.47 and 32 of the file
In our design we should provide possibilities
• Holding sewage and possibly to treat it
• Discharging the holding tank from deck
• Discharging the tank overboard
• Possibly Direct overboard discharging the
toilets for small crafts, when sailing in
unlimited zone
System design
Ignoring the portable chemical toilets
(p.40) for the smallest boats, where other
type facility can not be installed,
there are basically three methods by
which sewage can be handled and made
to flow along the soil pipes:
1. Gravity system
2. Pressure Pumping System
3. Vacuum Pumping System
Gravity system
Like common household the waste is flushed
along a relatively large diameter soil pipe by
quantity of water
• Each soil pipe run must have sufficient slope
(trim and heel)
• Water traps must be provided adjacent to the
• Small macerator may be fitted in each line
adjacent to the WC and pipe size may be
• A pressure pump may also be incorporated in
the unit which allows further reduction in piping
Gravity system
Pressure Pumping System
In these systems the soil is moved by pressure
created by a suitable displacement pump and
macerator (can run against gravity)
• It requires more flushing water than the gravity
• efficient non—return valves need to be fitted
• the system needs to be used frequently or
alternatively flushed through with clean water at
regular intervals
• requires periodic cleaning and dismantling of the
pump in the system
Vacuum Pumping System
Vacuum system uses air under vacuum pressures
to move the soil along the pipework to the
holding tanks
• The system includes a vacuum air pump which
maintains a constant vacuum pressure on the
holding tank (no special maintenance of the
• The system uses only about 1.5 litres of water
and reduces the sewage volume to
approximately 1/10th of that for conventional
gravity toilets
Vacuum Pumping System
Sewage treatment
Two particular types of sewage treatment
plant are in use
• employing chemical methods
• employing biological methods
Chemical Sewage Treatment
• The chemical method is basically a
storage tank which collected solid material
for disposal in permitted areas or to a
shore collection facility.
• Various chemicals are added at different
points for odour and colour removal and
also to assist breakdown and sterilisation.
• Tests must be performed daily to check the
chemical dosage rates. This is to prevent odours
developing and also to avoid corrosion as a
result of high levels of alkalinity.
• A chemical plant can be started up and shut
down at will as the ship enters and leaves
regulated waters.
• They may however require more attention while
they are in use as they are likely to use filters
and screens which would need regular cleaning,
and solids accumulate at a faster rate than in
biological plants. Waste entering the plant is first
screened to separate liquid from solids, which
then pass untreated into a collection tank. The
partially clarified liquid is treated with sodium
hypochlorite and a small amount of flocculent.
Purified effluent is then drawn through a weir in
the treatment tank and pumped overboard.
Treatment plant
Grade B sewage treatment system – available
in New Zealand. LectraSan is an electrochemical sewage treatment unit. (p.22)
Questions to be considered:
1. Does it suit the electrical balance of the boat at
any time?
2. Can the unit be acoustically insulated?
3. Can owner or crew meet maintenance
requirements given by manufacturer?
Electro-chemical sewage treatment
• It macerates solid sewage to indiscernible
• It oxidises it by hypochlorous acid
produced by an electrochemical action
with the aid of an electrode
This treatment destroys most of the bacteria
and the odour-causing agent
Biological sewage treatment
The biological system utilises bacteria to
completely break down the sewage into an
acceptable substance for discharge into
any waters.
• The extended aeration process provides a
climate in which oxygen-loving bacteria
multiply and digest the sewage, converting
it into sludge.
Biological sewage treatment
• The sewage then flows into the settling
compartment where the activated sludge
is settled out.
• The clear liquid flows to the chlorinator or
to ultra—violet light, and after treatment to
kill any remaining bacteria it is discharged.
• The sludge must be discharged only in a
decontrolled area ; [Incinerators (furnace,
for burning waste) can complete the
process. After use the ash box can be
emptied overboard ]
• Most popular head arrangement is manual one
Make a schematic for a single head
• What components should be included?
• What hazards can you foresee?
• How to prevent them?
Questions to decide
Material of the tank and pipes
Volume of the tank
Tank discharge pump type and size
Type of the toilet

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