We have learnt in our articles related to marine pollution that the governing bodies such as the International Maritime Organisation and other agencies are getting more strict over time. No longer can ships even discharge sewage as and when they like, let alone discharging of oil which a mariner cannot even think of thesedays without having to face dire consequences.
In the following sections we will learn about such a modified marine sewage treatment plant which goes by the name of Elsan Holding & Recirculation Sewage System used in marine vessels. We will see how it is different from the conventional system in that it has holding tanks and will study its arrangement using its pictorial representation.
Sewage treatment plants have been in use for fairly long time now and we also took a detailed look at such a marine sewage treatment plant in our previous article. A conventional sewage treatment plant on ship is used to treat the sewage before discharging it to the sea. However, with the environmental regulations getting stricter day by day, even the treated sewage is not allowed to be disposed in the territorial waters of any country. In such situations, sewage plant holding tanks are required on marine vessels to store the sewage. This sewage is pumped out to the shore reception facilities when the vessel it at a port or discharged overboard when the ship is beyond 12 nautical miles from the shore
The size of these tanks depends on the type of the ship and the sewage system of the particular ship. A conventional flushing system of a toilet passes around 5 liters of water in one flush. Thus all those ships having automatic flushing systems should have high capacity retention tanks. Passenger ships and ferries generally have large capacity tanks. Just imagine the size of such tanks required for some of the biggest cruise liners which have the capacity to carry literally thousands of people at one time.
Size is certainly not a problem, since big tanks can be accommodated on big ships, but the main problem with retaining untreated waste is that they become a thriving place for anaerobic bacteria as there is not enough oxygen for the aerobic bacteria to break down the sewage waste. Anaerobic bacteria thrive in absence of oxygen, which leads to putrefaction, corrosion of tank and production of toxic and flammable gases.
So let us see how to tackle this problem
Elsan Sewage System
The Elsan Sewage system is just like a conventional sewage system but consists of retention tanks. A basic arrangement of Elsan type sewage plant is shown in the figure.
- The separation of the solid and liquid sewage takes place in the reception chamber.
- The solid and liquid waste then drops on a perforated moving rubber belt which leads to a caustic treatment tank.
- From there the solids are transferred to a sullage or holding tank with the help of a girder pump.
- The liquid passes through the perforated rubber belt.
- The liquid waste is treated with chlorine and other caustic based compounds.
- The chemical treatment makes the liquid acceptable to be used again as a flushing fluid.
- The rubber belt moves with the help of an electric motor.
- The system which continuously supplies liquid for flushing toilets is known as pneupress arrangement.
A general capacity of the holding tank in general cargo and tanker ships is around 2 liters per person per day. The capacity in such ships is less because all the liquid affluent is continuously used by the flushing system of toilets.
Sewage Treatment, Purification and Utilization: A Practical Manual - J.W. Slater