Page 160 - Introduction to Marine Engineering

146 Auxiliaries

plant life would die. Pungent smells are also associated with sewage as a
result of bacteria which produce hydrogen sulphide gas. Particular
bacteria present in the human intestine known as E, coli are also to be
found in sewage. The E. coli count in a measured sample of water
indicates the amount of sewage present.
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.

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.
A treatment plant is shown diagrammatically in Figure 7.9. 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.

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 (Figure 7.10). The sewage enters the