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Marine Sewage Treatment Plant

The basic principle of working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which is dispose off in the sea. Air is a very important criteria in the functioning of the biological sewage plant because if air is not present, it will lead to growth of anaerobic bacteria which produces toxic gases that are hazardous to health. Also after decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discoloration of water which is not accepted for discharging. Thus in a biological sewage treatment plant the main aim is to maintain the flow of fresh air.

Division of processes:The biological sewage plant is divides into three chambers

  1. Aeration Chamber
  2. Settling tank
  3. Chlorination and Collection tank
Aeration Chamber: This chamber is fed with raw sewage which has been grinded to form small particles. The advantage of breaking sewage in small particles is that it increases the area and high number of bacteria can attack simultaneously to decompose the sewage. The sewage is decomposed into carbon dioxide, water and inorganic sewage. The air is forced through diffuser into the air chamber. The pressure of air flow also plays an important role in decomposition of the sewage. If pressure is kept high then the mixture of air and sewage will not take place properly and it will escape.
Without doing any work required for decomposition. It is for this reason, controlled pressure is important inside the sewage treatment plant as this will help in proper mixing and decomposition by the agitation caused by air bubbles. Generally the pressure is kept around 0.3-0.4 bars.

Settling tank: The mixture of liquid and sludge is passed to settling tank from the aeration chamber. In the settling tank the sludge settles at the bottom and clear liquid on the top. The sludge present at the bottom is not allowed to be kept inside the settling tank as this will lead to growth anaerobic bacteria and foul gases will be produced. The sludge formed is recycled with the incoming sludge where it will mixes with the later and assist in the breakage of sewage.

Chlorination and Collection: In this chamber the clear liquid produced from the settling tank is over flown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-coil bacteria present in the liquid. To reduce these bacteria to acceptable levels chlorination is done. Moreover to reduce the e-coil, the treated liquid is kept for a period of at least 60 minutes. In some plants disinfection is also done with the help of ultra violet radiation. The collected liquid is discharged to overboard or settling tank depending on the geological position of the ship. If the ship is in restricted or near coastline then the sewage will be discharged into the holding tank, otherwise, the sewage is discharge directly into the sea when high level is reached and is disposed automatically until low level switch activates.


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