The first question you would probably ask is , “Why to use a ship sewage treatment plant if the waste is already biodegradable?”. The question definitely isn’t wrong but the problem is that not all sewage constituents are biodegradable and not all take the same time to break down. Also, untreated sewage has solid particles which take a lot of time to disintegrate. It is for this reason that a sewage treatment plant is used.
Raw sewage in water needs oxygen to break down naturally. This sewage when disposed to the sea absorbs excessive oxygen and thus reduce the requisite amount of oxygen needed by the fishes and marine plants. Bacterias present in the sewage produces hydrogen sulfide gas which produces acrid smell. Human waste has E. Coli bacterias which are found in the intestine. The amount of E.Coli bacteria in a water sample indicates the sewage content of that sample.
Sewage treatment plants on ships are of two types:
- Chemical sewage treatment plant
- Biological sewage treatment plant
Chemical Sewage Treatment plant
A chemical treatment plant consists of a big storage tank which collects, treates and stores the sewage for discharging it to the sea or to a shore receiving facility. The sewage is first collected in a tank and the liquid content is reduced. This can be done by flushing water from wash basins and bathroom drains directly into the sea. The liquid from any other sources is treated chemically to get rid of the color and smell and then it is reused as flushing water in toilets. The chemicals that are used assist in the process of breaking the solid constituents and also in sterilization.
A mechanical instrument known as comminutor is used to help breaking down of the solid particles to smaller ones. The liquid sewage remains at the top and the solid particles settle down, which are then discharged to a sullage tank. The liquid sewage is chemically treated and is used for toilet flushing purposes. The sewage from the sullage tank is discharged to the shore collecting facilities.
It is important to supply adequate quantity of chemical dosages to prevent odour and corrosion due to high level of alkalinity.
Biological Sewage treatment plant
Biological sewage treatment plant uses bacterias to facilitate the process of breaking down of solid constituents. It generates an oxygen rich atmosphere that aerobic bacterias utilizes to multiply and disintegrate the sewage waste to convert it into sludge. The treated sewage thus generated can be disposed off to any waters. The process that takes place inside the plant is known as aeration process.
The whole plant is divided into three compartments namely,
- aeration compartment
- settling compartment
- chlorine treatment compartment.
Sewage enters the sewage treatment plant first through the aeration compartment. Aerobic bacterias digests the sewage waste and reduce it to small particles. A continuous supply of atmospheric oxygen is provided to increase the rate of digestion process.The disintegrated solid waste is then transferred to the settling compartment where the solid constituents settle down under the effect of gravity. The liquid at the top is then passed to the chlorine treatment compartment. In this compartment the liquid water is treated with chlorine and other chemicals to kill any surviving bacterias. Once done the water is then discharged into the sea. The process of chlorination is facilitated with the help of chlorine tablets. The sludge that settles down in the settling compartment is removed and stored in a storage tank to later discharge it to shore facilities or decontrolled areas.