Sewage treatment plant on a ship

Sewage Treatment Plant on a Ship
MARPOL ANNEX IV
Derleyen: Nejat Öztezcan
Chief Engineer
Nejat ÖZTEZCAN Chief Engineer

“Sewage” means:
1. Drainage and other wastes from any form of toilets, urinals, and WC
scuppers
2. Drainage from medical premises (dispensary, sick bay, etc.) via wash
basins, wash tubs and scuppers located in such premises
3. Drainage from spaces containing living animals
4. Other waste waters when mixed with the drainages defined above.
5. “discharge”, in relation to sewage, means any release howsoever
caused from a ship, and, includes any escape, disposal, spilling,
leaking, pumping, emitting or emptying.

The exact amount of sewage and waste water flow generated on board
ship is difficult to quantify.
European designers tend to work on the basis of 70 litres/person/day of
toilet waste (including flushing water) and about 130-150
litres/person/day of washing water (including baths, laundries, etc.).
US authorities suggest that the flow from toilet discharges is as high as
114 litres/person/day with twice this amount of washing water.

The breakdown of raw sewage in water is effected by aerobic bacteria if
there is a relatively ample presence of oxygen, but by anaerobic bacteria
if the oxygen has been depleted.
When the amount of sewage relative to water is small, dissolved oxygen
in the water will assist a bio-chemical (aerobic) action which breaks
down the sewage into simple, clean components and carbon dioxide.
This type of action is produced in biological sewage treatment plant in
which air (containing 21% oxygen) is bubbled through to sustain the
aerobic bacteria.
The final discharge from an aerobic treatment plant has a clean and
clear appearance.

The discharge of large quantities of raw sewage into restricted waters
such as those of inland waterways and enclosed docks, will cause rapid
depletion of any oxygen in the water so that aerobic bacteria are unable
to survive.
When the self-purification ability of the limited quantity of water is
overwhelmed in this way, breakdown by putrefaction occurs.
Anaerobic bacteria, not reliant on oxygen for survival are associated
with this action which results in the production of black colour water
and gases which are toxic and flammable.
The process is used deliberately in some shore sewage treatment works
to produce gas which is then used as fuel for internal combustion
engines on the site.

Effluent quality standards
To discharge sewage in territorial waters the effluent quality may have
to be within certain standards laid down by the local or national
authorities.
These will usually be based on one or more of three factors, namely the
• bio-chemical oxygen demand (BOD),
• suspended solids content and
• e coliform count of the discharce.

The sewage generated on the ship cannot be stored on the ship for
a very long time and it for this reason it has to be discharged into
the sea.
Though sewage can be discharged into the sea, we cannot
discharge it directly overboard as there are some regulations
regarding discharging of sewage that needs to be followed.
Sewage on sea is generally the waste produced from toilets, urinals
and WC scuppers.

Generally, ships prefer treating sewage before discharging to save
themselves from any type of embarrassment.
There are different methods of treating sewage available in the market,
but the most common of them is the biological type for it occupies less
space for holding tank, unlike those of the other methods. Moreover,
the discharge generated from this plant is eco friendly.
It is to not that each sewage treatment system installed onboard has to
be certified by classification society and should perform as per their
requirement and regulations.
The Marine Environment Protection Committee (MEPC) of the
International Maritime Organization (IMO) adopted resolution
MEPC.2(VI) Recommendation on International Effluent Standards and
Guidelines for Performance Tests for Sewage Treatment Plants in 1976.

The Annex entered into force on 27 September 2003.
A revised Annex IV was adopted on 1 April 2004 and entered into force
on 1 August 2005.
Special Areas
In July 2011, MEPC 62 adopted, by resolution MEPC.200(62), the most
recent amendment to MARPOL Annex IV, which entered into force on 1
January 2013.
The amendment introduced, inter alia, a definition for Special Area as
well as relevant requirements for the discharge of sewage from
passenger ships in Special Areas and for port reception facilities.

The discharge of sewage from passenger ships within a Special Area is
generally be prohibited under the new regulations, except when the
ship has in operation an approved sewage treatment plant which has
been certified by the Administration.
The sewage treatment plant installed on a passenger ship intending to
discharge sewage effluent in Special Areas should additionally meet the
nitrogen and phosphorus removal standard when tested for its
Certificate of Type Approval by the Administration.
Currently, the Baltic Sea area is the only Special Area under Annex IV.

Two particular types of sewage treatment plant are in use, employing
either chemical or biological methods.
The chemical method is basically a storage tank which collects solid
material for disposal in permitted areas or to a shore collection facility.
The biological method treats the sewage so that it is acceptable for
discharge inshore.

• “New ship” means a ship:
o For which the building contract is placed, or in the absence of a
building contract, the keel of which is laid, on or after 27
September 2003; or
o The delivery of which is on or after 27 September 2006.
• “Existing ship” means a ship which is not a new ship.
• “Holding tank” means a tank used for the collection and storage of
sewage.

“International voyage” means a voyage from a country to a port outside
such country, or conversely.
“Person” means member of the crew and passengers.
“Anniversary date” means the day and the month of each year which
will correspond to the date of expiry of the International Sewage
Pollution Prevention Certificate.
Greywater is drainage from dishwater, shower, laundry, bath and
washbasin drains.
Blackwater is any waste from toilets or urinals.

Application
The provisions of this chapter shall apply to the following ships engaged
in domestic and international voyages:
• New ships of 400 gross tonnage and above; and
• New ships of less than 400 gross tonnage which are certified to carry
more than 15 persons;
• Existing ships engaged in service in the Baltic Sea area and Danish
territorial waters with a gross tonnage of or above 400,
• Existing ships engaged in service in the Baltic Sea area and Danish
territorial waters with a gross tonnage below 400, approved for
carrying more than 15 persons,
• Existing ships of 400 gross tonnage and above, not later than on 27
September 2008.
• Existing ships of less than 400 gross tonnage which are certified to
carry more than 15 persons, not later than on 27 September 2008.Nejat ÖZTEZCAN Chief Engineer

Exceptions
Regulation shall not apply to:
The discharge of sewage from a ship necessary for the purpose of
securing the safety of a ship and those on board or saving life at sea.
The discharge of sewage resulting from damage to a ship or its
equipment if all reasonable precautions have been taken before and
after the occurrence of the damage, for the purpose of preventing or
minimizing the discharge.
The 2008 Regulations do not apply to any warship, naval auxiliary or
other ship owned or operated by a State and used, for the time being,
only on Government, noncommercial service.
However, as a matter of good practice these ships are recommended to
comply. Nejat ÖZTEZCAN Chief Engineer

The discharge of sewage into the sea is prohibited, except when:
The ship is discharging comminuted and disinfected sewage using a
system approved by the Administration in accordance with regulation
9.1.2 of this Annex at a distance of more than 3 nautical miles from the
nearest land,
or sewage which is not comminuted or disinfected at a distance of more
than 12 nautical miles from the nearest land, provided that, in any
case, the sewage that has been stored in holding tanks shall not be
discharged instantaneously but at a moderate rate when the ship is en
route and proceeding at not less than 4 knots; the rate of discharge
shall be approved by the Administration based upon standards
developed by the Organization.

The test results of the plant are laid down in the ship's International
Sewage Pollution Prevention Certificate; and
additionally, the effluent shall not produce visible floating solids nor
cause discoloration of the surrounding water.

What is shown on the International Sewage Pollution Prevention
Certificate (ISPP Certificate) ?
1. Ship details
o Name of ship
o Distinctive number or letters
o Port of registry
o Gross tonnage
o Number of persons which the ship is certified to carry
o IMO Number
o New/existing ship
o Build date

2. Equipment details
 Description of the sewage treatment plant:
o Type of sewage treatment plant
o Name of manufacturer
o Description of comminuter:
o Type of comminuter
o Name of manufacturer
o Standard of sewage after disinfection
 Description of holding tank:
o Total capacity of the holding tank
o Location

Passenger ships in the waters of Alaska are obliged to conduct all
transfers of sewage and graywater according to the US-CFR.
As of July first, 2019, according to MARPOL, it will be stepwise
mandatory for all passenger ships to comply to the rules for the Baltic
Sea Special Area.
The Sewage and Graywater Discharge Record Book meets the
requirements of US CFR for the record on discharge and transfer to
reception facilities and off board.
It serves the purpose of documentation according to MARPOL IV and
therefore may also be used by cargo ships.

IMO Regulations:
We have been talking about IMO regulations but what exactly are
they.
Well basically IMO says that you can discharge sewage directly
overboard if your distance from the nearest land is more than 12
nautical miles, provided it does not discolour the sea water or leads to
any floating debris.
Between 3 to 12 nautical miles from land, sewage can only be
disposed off after treatment.
Anything less than 3 nautical miles you cannot dispose overboard.

What Surveys are required?
• An initial survey before the ship is put in service or before the
Certificate is issued for the first time
• A renewal survey not exceeding five years
• An additional survey after any important repairs or renewals are
made.

Whenever an accident occurs to a ship or a defect is discovered, the
master or owner of the ship shall report at the earliest opportunity to
the Administration.
If the ship is in a port of another Party, the master or owner shall also
report immediately to the appropriate authorities of the Port State and
the nominated surveyor

Duration and validity of Certificate
1. An International Sewage Pollution Prevention Certificate shall be
issued for a period specified by the Administration which shall not
exceed five years.
2. When the renewal survey is completed within three months
before the expiry date of the existing Certificate, the new Certificate
shall be valid from the date of completion of the renewal survey to a
date not exceeding five years from the date of expiry of the existing
Certificate.
3. If a Certificate is issued for a period of less than five years, the
Administration may extend the validity of the Certificate beyond the
expiry date to the maximum period specified in paragraph 1 of this
regulation

4. If a ship at the time when a Certificate expires is not in a port,
the Administration may extend the period of validity of the
Certificate only for the purpose of allowing the ship to complete its
voyage to the port .
5. No Certificate shall be extended for a period longer than three
months, and a ship to which an extension is granted shall not, on its
arrival in the port in which it is to be surveyed, be entitled by virtue
of such extension to leave that port without having a new Certificate.
When the renewal survey is completed, the new Certificate shall be
valid to a date not exceeding five years from the date of expiry of the
existing Certificate before the extension was granted.

Standard discharge connections
To enable pipes of reception facilities to be connected with the ship's
discharge pipeline, both lines shall be fitted with a standard discharge
connection in accordance with the following table:

Reception facilities
Regulation 12 Reception facilities ;
The Government of each Party to the Convention, which requires ships
operating in waters under its jurisdiction and visiting ships while in its
waters to comply with the requirements of regulation 11.1, undertakes
to ensure the provision of facilities at ports and terminals for the
reception of sewage, without causing delay to ships, adequate to meet
the needs of the ships using them.
The Government of each Party shall notify the Organization, for
transmission to the Contracting Governments concerned, of all cases
where the facilities provided under this regulation are alleged to be
inadequate.

SEWAGE TREATMENT TECHNOLOGIES
ONBOARD SHIPS
• Mechanical sewage treatment
• Chemical sewage treatment
• Biological sewage treatment

Sewage can be processed with three principal methods:
• mechanical,
• chemical and
• biological.
The sewage treatment is usually a combination of the three principal
methods, such as mechanical-chemical, mechanical-biological and
chemical-biological.

The treatment of sewage includes the following stages:
Waste water accumulation and management: In this stage of proces
waste water (black water, grey water and galley water has been
collected in holding tanks before processing planet.
Waste water pre-treatment: Wastewater pre-treatment protects the
other phases of the purification process.
Sewage contains a lot of solid waste and grease that may cause
problems in the later stages of the process.
The pre-treatment process reduces the amount of solids in the waste
water.
Effective waste-water pre-treatment also reduces the need for
oxidation. The pre-treatment is mechanical and consists of sieving and
sedimentation units. The large particles pass through a shredding pump
before sieving. Nejat ÖZTEZCAN Chief Engineer

Waste water oxidation: The mechanical filtering results in a maximum
of 50% reduction in organic load. The remaining organic compounds
have to be oxidized, either chemically or biologically.
Certain chemicals, e.g. ozone, chlorine, hydrogen peroxide, are added to
the sewage in the chemical oxidation. The chemicals oxidize the organic
impurities in the sewage water. When compared to the ozone and
hydrogen peroxide, chlorine is not a very environmental friendly oxidant
because of the carcinogenic compounds that develop as a by-product of
the reaction.
The added chemicals have an impact on the organic matter that has
dissoluted slightly and the BOD reduction remains small.
The estimated treatment results for reduction in BOD and phosphorus
are good. “Over-chlorination” results in high levels of residual chlorine
in the discharge, which is lethal to marine organisms.

In the biological treatment the micro-organisms use the impurities in
the sewage as their nourishment.
There are several types of bioprocesses and the most common
biological process is the active sludge treatment plant, where the
sewage is mixed in a continuous-action aeration tank with active
sludge.
Biological filters and biorotors are also used as biological treatment
plants. In these devices the bacteria that destroy the impurities attach
to the filtering material.
The biological treatment system is the most efficient way of reducing
the BOD load. The estimated reduction in BOD is 80–95% and the
reduction in phosphorus is 20–40%.
The effectiveness of the bioprocess depends on the amount of active
biomass and the bacteria living conditions.Nejat ÖZTEZCAN Chief Engineer

The disadvantages of biological treatment are the long starting period
and its sensitivity to external disturbances.
The reasons for malfunction of the biological sewage treatment system
are the following:
• Strong chemicals that have got into the plant are destroying the
bacteria.
• Bacteria die due to the lack of oxygen when the ventilation does not
work.
• The return of active sludge does not work.

Waste water clarification and filtration: After oxidation, the sludge is
separated in a sedimentation tank and returned to the aeration tank.
Separating the active biomass, sediment particles and bacteria from
the water is a critical phase in the wastewater purification process.
The clarification and filtration processes used in the ships are
membrane filtration, dissolved air flotation (DAF) and settling.
It is useful when treating waters that are high in total suspended solids
(TSS) or have highly variable suspended solids content.

Waste water disinfection:
The last phase in the wastewater purification process is disinfection.
Depending on the previous treatment method, the disinfection
enhances the quality of the wastewater or is an essential part of the
purification process.
When the membrane clarification and filtration is used, the disinfection
is performed with UV-light.
If the water is very turbid, the UV-light is not suitable for disinfection.
The other potential disinfectants are, for example, chlorine, radicals
and ozone.

Chemical sewage treatment
This system minimises the collected sewage, treats it and retains it until
it can be discharged in a decontrolled area, usually well out to sea.
Shore receiving facilities may be available in some ports to take this
retained sewage.
This system must therefore collect and store sewage produced while the
ship is in a controlled area. The liquid content of the system is reduced,
where legislation permits, by discharging wash basins, bath and shower
drains straight overboard.
Any liquid from water closets is treated and used as flushing water for
toilets.
The liquid must be treated such that it is acceptable in terms of smell
and appearance. Nejat ÖZTEZCAN Chief Engineer

Various chemicals are added at different points for odour and colour
removal and also to assist breakdown and sterilisation.
A comminutor is used to physically break up the sewage and assist
the chemical breakdown process.
Solid material settles out in the tank and is stored prior to discharge
into the sullage tank.
The liquid is recycled for flushing use. 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.

CHEMICAL SYSTEM

Biological sewage treatment
•Traditional Type II Marine Sanitation Devices (MSD)
Most of a cargo and cruise ships with traditional Type II Marine
Sanitation Devices (MSD), sewage is treated using biological treatment
and chlorination. Some cruise ships do not treat their sewage
biologically, but instead use maceration and chlorination.
The treatment system typically includes aerobic biological treatment to
remove biochemical oxygen demand and some nutrients, clarification
and filtration to remove solids, and final chlorine disinfection to
destroy pathogens.
The system also may include screening to remove grit and debris.
Cruise ships typically install up to four systems, allowing one or two to
be placed off-line for maintenance at any one time. Cargo ships uses
one unit only. Nejat ÖZTEZCAN Chief Engineer

• Advanced Wastewater Treatmant Systems (AWT):
To improve environmental performance, cruise lines are testing and
installing wastewater purification systems that utilize advanced
technologies.
These onboard wastewater treatment systems are designed to result in
effluent discharges that are of a high quality and purity; for example,
meeting or surpassing standards for secondary and tertiary effluents
and reclaimed water.
Effluents meeting these high standards would not be subjected to the
strict discharge limitations.

AWT systems are still at the development stage. Generally advanced
treatment systems utilize enhanced aerobic digestion with physical
filtration to clean shipboard waste water.
On some cruise vessels, sewage and often graywater are treated using
AWTs.
AWTs generally provide improved screening, biological treatment,
solids separation (using filtration or flotation), and disinfection (using
ultraviolet light) as compared to traditional Type II MSDs.

Some manufacturers of AWT mostly installed on cruise vessels are
described below:
1. HAMWORTHY'S Membrane Bioreactor (MBR) system uses aerobic
biological treatment followed by ultrafiltration and ultraviolet (UV)
disinfection.
Hamworthy MBR system treats wastewater from accommodations and
sewage.
Wastewater is first treated in screen presses to remove paper and
other coarse solids.
Next, the wastewater enters a two stage bioreactor, where bacteria
digest the organic matter in the waste. Following biological treatment,
the wastewater is filtered through tubular ultrafiltration membranes to
remove particulate matter and biological mass, which are returned to
the bioreactors.
In the final stage of treatment, the wastewater undergoes UV
disinfection to reduce pathogens.Nejat ÖZTEZCAN Chief Engineer

2. ROCHEM’s ROCHEM LPRO and ROCHEM Bio-Filt system treats high
concentration and low concentration waste streams with different
processes.
ROCHEM LPRO part of the system treats wastewater from laundry and
accommodations while the ROCHEM Bio-Filt treats wastewater from
galley and sewage, as well as the membrane concentrate from the
ROCHEM LPRO system.
The ROCHEM LPRO system uses screens to remove fibers and hair,
reverse osmosis membranes to remove particulates and dissolved
solids, and UV disinfection to reduce pathogens.

3. The Zenon ZeeWeed MBR system uses aerobic biological oxidation
followed by ultrafiltration and UV disinfection.
Graywater from the laundry, galley, accommodations, and food pulper
combines with sewage and flows through two coarse screens into a
collection tank.
From the collection tank, the wastewater is pumped to an aerated
bioreactor. After the bioreactor, the wastewater flows through the
proprietary ZeeWeed hollow-fiber ultrafiltration membrane system
under a vacuum.
In the final stage of treatment, the combined wastewater from the
membranes undergoes UV disinfection to reduce pathogens.
The Zenon system is the only system that EPA sampled that treats all
graywater and sewage sources .

SCANSHIP AWP (Advanced Wastedwater Purification) system uses
aerobic biological oxidation followed by dissolved air flotation and UV
disinfection.
Sewage and graywater from the galley, accommodations, and laundry
combine in one graywater and sewage holding tank.
The combined wastewater is pumped through a coarse drum filter and
then through two separate aerated bioreactors.
After aeration, the wastewater is pumped to two dissolved air flotation
(DAF) units to separate solids.
From the DAF units, the wastewater is pumped to polishing screen
filters.
In the final stage of treatment, the wastewater undergoes UV
disinfection to reduce pathogens.Nejat ÖZTEZCAN Chief Engineer

The Hydroxyl CleanSea system uses aerobic biological oxidation
followed by dissolved air flotation and UV disinfection.
Sewage and graywater are combined and pumped to a fine
wedgewire screen for coarse solids removal.
Next, the wastewater enters the ACTIVECELL biological reactors where
free-floating plastic beads support biological growth without the need
for recycled biological mass.
The wastewater then enters the ACTIVEFLOAT dissolved air flotation
units for solids separation.
Final treatment steps include polishing filters and UV disinfection to
reduce pathogens.

EVAC is a company that designs, manufactures and markets
environmentally friendly waste and wastewater collection and
treatment solutions for the marine industry worldwide.
The Evac MBR is a single stream Advanced Waste Water Treatment
system where all the waste streams are treated in one process.
The Evac MBR is based on effective equalizing and mixing of the
incoming waste streams, pre-treatment by screens, an aerated biotank
and a membrane bioreactor.
In this proposal, a nutrient removal step is added to the basic process.
The Evac MBR process is fully automated and controlled through a PLC
by vacuum/pressure switches, level switches, DO, TSS and pH sensors,
flow meters and foam detectors.
Membranes are of submerged type, supplied by Japanese company
Kubota.

According to international regulations, sewage treatmant plant has to
satisfy the effluent standards for its certificate of type test.
Most of traditional (Type II MSD) in practise operate on the vessels
with certain errors due to
• late start of the system before arrival in the port,
• reduced bio sludge,
• overclorination,
• disrupted by intermittent flow common to shipboard life.

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 a sludge. These oxygen-loving bacteria are known as aerobic.
The treatment plant uses a tank which is divided into three
watertight compartments: an aeration compartment, settling
compartment and a chlorine contact compartment .

The sewage enters the aeration compartment where it is digested by
aerobic bacteria and micro-organisms, whose existence is aided by
atmospheric oxygen which is pumped in.
The sewage then flows into the settling compartment where the
activated sludge is settled out. The clear liquid flows to the chlorinator
and after treatment to kill any remaining bacteria it is discharged.
Tablets are placed in the chlorinator and require replacement as they
are used up.
The activated sludge in the settling tank is continuously recycled and
builds up, so that every two to three months it must be partially
removed.
This sludge must be discharged only in a decontrolled area.

Working of a Biological Sewage Plant
The basic principle of the working of a biological treatment plant is
decomposition of the raw sewage.
This process is done by aerating the sewage chamber with fresh air.
The aerobic bacteria survive on this fresh air and decompose the raw
sewage which can be disposed off in the sea.
Air is a very important criterion in the functioning of the biological
sewage plant because if air is not present, it will lead to growth of
anaerobic bacteria, which produces toxic gases that are hazardous to
health.
Also, after decomposition of the sewage with anaerobic bacteria, a
dark black liquid causes discoloration of water which is not accepted
for discharging. Thus in a biological sewage treatment plant the main
aim is to maintain the flow of fresh air.Nejat ÖZTEZCAN Chief Engineer

Division of Processes
The biological sewage plant is divides into three chambers:-
Aeration chamber
This chamber is fed with raw sewage which has been grinded to form
small particles. The advantage of breaking sewage in small particles is
that it increases the area and high number of bacteria can attack
simultaneously to decompose the sewage.
The sewage is decomposed into carbon dioxide, water and inorganic
sewage. The air is forced through diffuser into the air chamber. The
pressure of air flow also plays an important role in decomposition of
the sewage.
If pressure is kept high then the mixture of air and sewage will not
take place properly and it will escape without doing any work required
for decomposition.

It is for this reason; controlled pressure is important inside the sewage
treatment plant as this will help in proper mixing and decomposition
by the agitation caused by air bubbles.
Generally the pressure is kept around 0.3-0.4 bars.
Settling tank
The mixture of liquid and sludge is passed to settling tank from the
aeration chamber.
In the settling tank the sludge settles at the bottom and clear liquid
on the top.
The sludge present at the bottom is not allowed to be kept inside the
settling tank as this will lead to growth of anaerobic bacteria and foul
gases will be produced.The sludge formed is recycled with the
incoming sludge where it will mixes with the later and assist in the
breakdown of sewage. Nejat ÖZTEZCAN Chief Engineer

Chlorination and Collection
In this chamber the clear liquid produced from the settling tank is over
flown and the liquid is disinfected with the help of chlorine.
This is done because of the presence of the e-coli bacteria present in
the liquid. To reduce these bacteria to acceptable level chlorination is
done.
Moreover, to reduce the e-coli, the treated liquid is kept for a period of
at least 60 minutes. In some plants disinfection is also done with the
help of ultra violet radiation. The collected liquid is discharged to
overboard or settling tank depending on the geological position of the
ship. If the ship is in restricted or near coastline then the sewage will
be discharged into the holding tank; otherwise, the sewage is
discharged directly into the sea.

Starting of a Sewage Plant
Sewage plant is generally running all the time during sailing, but it
might need to be started when the ship is installed with a new sewage
treat plant which needs to be stopped at regular interval of time for
improving its performance and maintenance procedures.
Below are the points that need to be followed for starting a sewage
treatment plant.

1. Make sure if any maintenance is carried out on the sewage
treatment system, all the openings have been closed properly before
starting.
2. The sewage plant is be filled with fresh water inside the chamber.
3. At this stage, there are no aerobic bacteria inside the chamber, but
the sewage has started coming to the plant. Thus, in order to increase
efficiency and starting rate of the plant bio pac is added to the plant
by flushing the amount specified in the manual. This bio pac is mixed
with warm water which helps in growth of these bacteria and also
efficient functioning of the plant.
4. If the bio pac is not added, the plant might take up to 5 to 7 days to
be completely functional. However, with the bio pac it becomes
functional within 24 hours.

5. Start the air compressor or open the air valve as per the design of
the plant. The pressure is maintained as per the manual. Generally 0.3-
0.4 bars.
6. Open the sewage overboard valve and close holding tank valve
when the ship is out of restricted waters.
7. The plant is continuously monitored and checked for the flow
through the transparent plastic tubes.
8. The sample is taken for checking for suspended solids and chlorine
content.

Stopping of the plant
Stopping of the sewage treatment plant is generally done either
before entering the dry dock or in case some maintenance has to be
carried out inside the treatment plant.
1. For stopping the system, close the inlet valve to the sewage plant
and close the overboard valve and let the sewage go overboard.
2. Empty all the three chambers of the plant i.e. aeration, settling
and chlorination chambers. If the chambers are not emptied, it will
lead to growth of anaerobic bacteria which forms the toxic H2S gas.
3. If entry has to be made inside the tank, the later should be
checked for hydrogen sulphide gas H2S with the help of dragor tube
by taking a continuous sample from the plant. Entry is made with the
help of mask and rubber gloves should be put on.
4. In case the ship is going to dry dock the overboard should be
connected to shore reception facilities.

While operating the sewage plant, engineer must know:
• Procedure for starting and stopping sewage treatment plant
• Maintenance and checks for sewage treatment plant
However, apart from the above mentioned aspects, marine
engineers should also know four important terms while dealing
with sewage treatment plants on ships. They are:
1.Biochemical Oxygen Demand (BOD)
2.Coliform Count
3.Recommended levels of pumping out solids
4.Bio-chemical digestion of sewageNejat ÖZTEZCAN Chief Engineer

1. Biochemical Oxygen Demand
Biochemical oxygen demand is a test to identify biological
decomposable substances and to test the strength of the sewage.
BOD depends on the activity of bacteria in the sewage. These bacteria
feed on and consume organic matter in the presence of oxygen.
BOD can also be defined as the amount of oxygen required by the
micro-organisms in the stabilization of organic matter.
BOD of raw sewage is 300-600 mg/litre.
IMO recommends BOD of less than 50 mg/litre after treatment
through sewage treatment plant.

2. Coliform Count
Coliform is a type of organism which is present in human intestine and
is recognized as indicator organisms of sewage pollution. Presence of
these organisms in water is an indication of pathogen, which are
diseases causing bacteria responsible for cholera, dysentery, typhoid
etc.
The number of coliform organisms present in sewage on ship is very
large, with each person contributing around 125 billion in winters and
400 billion in summer.
IMO recommends faecal coliform count of less than 250 faecal/100
ml. of affluent after treatment.

3. Recommended levels of pumping out solids
Dissolved solids – Solids which are dissolved in the solution
Suspended solids – Solids physically suspended in sewage that can
be removed by laboratory filtration and are relatively high in organic
matter.
Settleable solids – Suspended solids that will subside in quiescent
liquid in a reasonable period of time (usually around an hour)
Suspended level of raw sewage – Around 300-400 mg/litre; IMO
recommends 50 mg/ litre after treatment.
Residual disinfectant – After treatment residual disinfectant should
be as low as possible. IMO recommends use of ultra violet exposure
for chlorination method.

4. Biochemical digestion of sewage:
Anaerobic process
Anaerobic bacteria can only multiply in the absence of free oxygen
as they utilize chemically bound oxygen to survive. Anaerobic
bacteria break down the organic matter into water, carbon dioxide,
methane, hydrogen sulphide and ammonia. This process is called
putrefaction.
The products thus produced out of this process are noxious and
toxic. The effluent is of poor quality and by-products are highly
corrosive.
Aerobic process
Aerobic bacteria require free oxygen to survive. They break down
the organic matter to produce safe products such as water, carbon
dioxide, inert residue, and energy to synthesize new bacteria.

VACUUM SEWAGE SYSTEM

General Description
The system uses vacuum to transport sewage from toilets and
urinals to collecting units.
There is a vacuum only in the piping network and collecting units
(Toilets, urinals etc.) remain under atmospheric pressure unless
when the flush button is pushed; for a set value of time which is
usually 7-15 seconds.
Each toilet is connected to the vacuum piping.
The connection is shut all times, except during the toilet flushing.
When the toilet is flushed, its discharge valve opens the
connection to the vacuum piping network for a pre-set seconds
and the contents of the bowl will be evacuated into the sewer.
The evacuated sewage, very less water sprayed by spraying
nozzles, and few liters of air drawn during the flushing process,
which pushes the sewage plug forward in the piping system.Nejat ÖZTEZCAN Chief Engineer

Vacuum Pumps
The vacuum is created by specially designed jet pumps which are
connected to a vacuum manifold runs continuously until the vacuum set
point is reached on the vacuum manifold.
There are number of such pumps (Depending upon the number of
toilets installed in the ship) cut in / cut off sequentially by the vacuum
reading on the main manifold.
These vacuum creating pumps are cooled by water. Due to vacuum in
the pumping space the water starts to boil and the pump parts become
over heated to avoid this, a solenoid controlled valve activated by the
temperature sensor fitted on the body opens up cooling water and
closes when the temperature is under the limit.
The cooling water is eventually mixed with the sewage and pumped to
collecting tank

Main Advantage of the System
One of the main advantage of this system is that, very little flushing
water is required and the volume of sewage dealt with can be much
reduced with the downsizing of relevant equipment and cost saving.
This has made them very popular for passenger vessels. Lloyd’s
regulations state that the capacity of a sewage system for flushing water
with conventional plant is 115 liters/ person/ day and 15 liters for
vacuum systems.

Main Disadvantage
The main disadvantage is blockage due to drying and crystallization of
urea. Over a period of time this can be so severe as to completely close
the pipes.
Chemicals are on the market which can be added in very small doses
which help remove and prevent these deposits but there success is not
guaranteed.

Problems resulting from the retention of untreated sewage in a
holding tank.
1) The breakdown of raw sewage in water is effected by aerobic
bacteria, as long as there is a relatively ample presence of oxygen to
support the bacteria. In other words it’s effected by anaerobic bacteria
if the oxygen has been depleted.
2) When the amount of sewage relative to water is small, dissolved
oxygen in the water will assist a bio-chemical (aerobic) action which
breaks down the sewage into simple, clean components and carbon
dioxide.
3) In a holding tank if untreated sewage is stored then the bacteria turn
into an aerobic due to poor oxygen supply which creates black sludge
toxic and explosive gases which creates danger to the life of personnel
working around, in some cases they could explode as well due to
excessive accumulation of methane if a spark or heat source is
introduced.

Sewage treatment plant on a ship

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