These guidelines apply to all ships. It is recommended that shipowners, masters and crew
members should examine the sewage systems installed in their vessels, to consider the potential
risks of sewage gases gaining access to working and living spaces.
Different systems for the disposal of sewage may be found on ships and the following is e
brief summary of the systems likely to be found on board:
Discharges from the toilet bowls into common sewage mains which are led to the ship's
side and overboard through storm valves.
Discharges to collecting and storage tanks, with or without aeration facilities, in which
the sewage is retained for disposal ashore or at sea.
Sewage treatment systems incorporating a combination of collecting and treatment
tanks, with processes designed to break down the sewage into effluent suitable for
discharge into the sea without harmful effects.
Vacuum collecting systems where sewage, air and water are drawn through piping to
the holding/treatment tanks.
Sewage flushed from the toilet bowls enters the sewage mains and storage tanks where it is
broken down by naturally occurring bacteria. This is an aerobic process which strips oxygen from
the water. producing more water. carbon dioxide and new bacteria. If insufficient oxygen is
present, alternative bacteria become dominant and the process becomes anaerobic with the
production of gases, including hydrogen sulphide, methane, ammonia, etc. These gases have highly
toxic and flammable properties, in particular hydrogen sulphide is toxic to humans in
concentrations as low as 10 parts per million and its flammable vapours are heavier than air so that
potentially lethal pockets of gas may accumulate in enclosed spaces.
The generation of toxic and explosive gases in the event of anaerobic conditions is present to
varying degrees in each system. It is evident from the foregoing that anaerobic conditions in
sewage systems leading to the production of toxic and flammable gases is an unacceptable hazard
within the confined boundaries of a ship.
In general sewage systems should be of a design which will avoid the problems outlined in
these guidelines. The primary safety features should be incorporated in the design and operation of
a sewage system and the barriers between the sewage gases such as water traps of the toilet bowls,
ventilation of the pipework and tanks should only be considered a secondary means of protection,
The primary objective should be the prevention of the production of hazardous gases within the
Storage and treatment tanks6
Sewage may be collected into storage tanks, either for holding prior to transfer to a treatment
unit, or for later discharge. Any tank used for holding sewage is a potential source of anaerobic
activity, and the resultant production of toxic and flammable gas. The design of a tank may include
features to maintain an adequate oxygen level in the liquid, thereby eliminating anaerobic
conditions. This may be achieved by direct air injection, or by air entrainment into the liquid whilst
pumping through an ejector nozzle, etc.
When not equipped with an active aeration feature, the conditions within the storage tanks
should be especially monitored. These tanks should be completely emptied and flushed through at
intervals which will ensure that satisfactory conditions are maintained. The design and
configuration of such tanks should be such as to facilitate the effective drainage and flushing of the
Treatment systems may use the aerobic digestion process, or may use other means to purify
the effluent. The manufacturer's recommendations for the operation and maintenance should be
followed to ensure satisfactory operation at all times. All tanks and associated systems should be
subjected to a thorough inspection at least every year or whenever unsatisfactory conditions are
detected. Such inspections should include the following:
Removal of tank inspection covers, and the cleaning out of any deposits, paying
particular attention to areas behind internal tank divisions.
Inspections of the tank structure, internal divisions, pipework, etc.
Checking of sensing instruments, level measuring devices and valves.
Confirmation that air distribution systems are free from leaks and any nozzles or
diffuser elements used to introduce air are in satisfactory condition.
Checking that any internal systems used to transfer tank contents are in a satisfactory
Inspecting air blowers to check discharge pressure is within allowable limits.
It is recommended that any alarms fitted to air blower systems operate on sensing of
unsatisfactory air pressure rather than only monitoring the fans and their prime movers.
Tank ventilation arrangements9
Ventilation pipes to collection, storage and treatment tanks should be in good condition, clear
from internal obstruction, and of adequate size to minimize pressure drop and to ensure satisfactory
clearance of gases. The size of the vent pipes should also be sufficient to vent any air from blowers
or from vacuum collection system discharges. Ventilation pipes should be arranged to be
self-draining at all angles of heel and trim, to eliminate any water traps that may otherwise form
and cause blockage. Such pipes should not terminate in areas to which personnel have frequent
access and should be clear of any sources of ignition. Any flame gauze or other fittings on the vent
terminal should be checked for cleanliness.
Toilets showers, washbasins, etc.10
Check all drain pipes for satisfactory water/gas tightness and adequate water seals and traps
to prevent backflow of sewage gases into the toilet compartments. Check that all sanitary fittings
are securely fastened to prevent relative movement at pipe joints. Toilet bowls incorporate a water
trap and are often fitted with vacuum breaking arrangements at the back of the trap such as
individual air pipes or patented backflow prevention valves. Such items should be checked for
satisfactory condition and operation. There should be an adequate supply of flushing water to clear
the toilet bowl and to replenish the water seal.
Drainage and ventilation pipe systems11
Check that drains and their air vents are clear of obstruction and are in sound water/gas tight
condition throughout their length and are self-draining at all expected angles of heel or trim.
Adequate air vents should be fitted to the piping network, paying special attention to the
extremities of the system. These should ensure an adequate supply of air and obviate any tendency
for plugs of water to form within the system, tending to syphon or create vacuums thus removing
water seals, when moving through the pipes especially under the action of violent rolling or
Accommodation ventilation arrangements12
Particular attention should be paid to the exhaust and supply systems in toilet and washing
areas. Ventilation systems to all compartments of a ship should be designed, installed and balanced
to ensure satisfactory distribution of air. They should be maintained in a clean and efficient
condition to achieve the designed air changes throughout the service life of the vessel. In general,
attention should be paid to the following,
Examination of extraction grills, louvres, ducts, etc., in order to ensure that they are
clear an-d free of accumulations of dirt, fluff, etc.
Checking the adjustment of extraction louvres or cones to ensure an even extraction rate
from all spaces. However, when satisfactorily adjusted, there should be no interference
with these units as the adjustment of one unit can seriously affect the rate of extraction
through other units. The design of these fittings should be such that they are not capable
of manual adjustment without the use of special tools or equipment. Ensure that
extraction units are never blocked off for any reason.
Ensuring that air extraction from alleyways is operating efficiently and that essential
air-gaps under cabin doors, etc., have not been blocked off.
Ensuring that the forced ventilation of cabin spaces is operating efficiently.
Inspecting the associated elements of ventilation systems, including:
Fans: Checking the direction of rotation, condition of impellers, etc.
Flaps and dampers: Ensuring that they have not become detached from the
operating mechanisms and that they are actually open when indicating open.
Goosenecks and vents: Ensuring that they are clear of any obstructions, that grilles
or meshes are not painted over, etc.
Piping or ducting: Ensuring that these are intact and free from leaks throughout
their length, especially where they pass through accommodation areas.
The use of large quantities of disinfectant toilet cleaners may destroy the bacteria which are
essential to the operation of aerobic sewage treatment plants. The manufacturers recommendations
should be followed in relation to the type and quantities of cleaners to be used.
All officers and crew should be informed of the dangers of sewage gases being generated and
finding their way into working and living spaces. Their attention should be drawn to the hazards
identified in these guidelines. There should be an operational procedure on board ship for reporting
and recording the inspection and maintenance of sewage systems and the action taken to deal with
complaints of foul or musty smells which may be due to toxic, flammable or oxygen-depleted
The contents of these guidelines should be brought to the attention of all shipboard personnel
engaged in the day-to-day operation of vessels to ensure they are fully understood. This is
particularly important where multi-lingual crews are employed.
The attention of all personnel engaged in maintenance work on sewage systems should be
drawn to the hazards of encountering oxygen-depleted, toxic and flammable gases when entering
sewage tanks or working on the system. Appropriate safety procedures should be observed before
entering enclosed, hazardous or suspect spaces.