2 Different systems for the disposal of sewage
may be found on ships and the following is a brief summary of the
systems likely to be found on board.
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.1 Discharges for the toilet bowls into common
sewage mains which are led to the ship's side and overboard through
storm valves.
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.2 Discharges to collecting and storage tanks,
with or without aeration facilities, in which the sewage is retained
for disposal ashore or at sea.
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.3 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.
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.4 Vacuum collecting systems where sewage, air
and water are drawn through piping to the holding/treatment tanks.
3 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.