The discharge of untreated sewage in controlled or territorial water is usually banned by National legislation.Aneex 4 of marpol 73/78 of IMO has aim of regulating the disposal of sewage from ships.1. untreated sewer should only be discharge outside the twelve mile limit.2.Cominuted particles and disinfected sewage could be discharged as near as four miles from land.3. Nothing should be discharged within 4 mile limit.
Why sewage treatment is done?
pungent smells from the sewage as a result of bacteria which produce hydrogen sulphide gas.in order to break down from naturally,raw sewage must absorb oxygen. oxygen in excess amount, it could reduce the oxygen content of water to the point where fish and plant life would die.particular bacteria present in human intestine known as coliforms are also to found in sewage.
They became harmful when in case of the dysentry,typhoid.Two specific kinds of wastewater treatment plants are in use:-1.Chemical sewage treatment2.Biological sewage treatment
1.Chemical sewage treatment plant
All sewage components are biodegradable and not all need to break down at the same time.
There are also strong particles in untreated sewage that take a lot of particular time to decompose. For this purpose, a treatment plant for sewage is being used.This system minimises the collected sewage,treats it and it retains it until it can be discharged in a de controlled area, usually well out at sea.
The sewage treatment plant of Eslan type, also known as the zero discharge system, has an original reception chamber in which liquid and solid sewage separation occurs.Waste drops onto a moving perforated rubber belt (powered by an electric motor) that passes through the liquid, but solids travel with the belt to fall into a tank for caustic treatment.Solids are then transported to the sullage or holding tank through a grinder pump. The liquid moves to treatment tanks containing chlorine and caustic-based compounds through the perforated belt.These chemicals create it acceptable to use liquid effluent as a flushing fluid.The Pneupress system that supplies liquid for flushing the toilets can supply recirculated fluid or seawater when the ship is on passage.The holding tank’s capacity is 2 liters per person per day. When the vessel is in port for a long time, the tank is pumped out at sea or to shore. The size of the tank is tiny as the liquid effluent primarily moves to the flushing system (excess over flow to the sullage tank )
2.Biological sewage treatment plant
The biological sewage treatment plant consists of four interconnected compartment.Rawsewage or effluent passes through a bar screen which breaks it up into smaller particles from where it passes into the primary compartment. When the level in this compartment rises sufficiently,over flow of the liquid takes place into the aeration compartment.This compartment contains bacteria which require oxygen and nutrients to propagate. The nutrient to solid waste material and oxygen is supplied by bubbling compressed air through the broken waste.The sewage is broken down here by aerobic activation.The liquid from the aeration compartment process to a settlement compartment where the activated sludge, as it is known settle and leaves a clear effluent.As there is no oxygen in this compartment,the sludge is continuously recycled to the aeration compartment where it mixes with incoming waste to assist in the treatment process.
the clear affluent discharged from a settlement compartment must be disinfected.
This infection is chi treating the clear effluent with the solution or tablet of sodium hydrochlorite in the chlorination compartment.The discharge pump lowers the level in the chlorine estanque automatically but pumping overboard。Automation is carried out by means of high or low level floats the discharge from cuts in at high levels and come out at the low level。