Discarding sewage created onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to save him and his shipping company from heavy fine. The sewage created on the ship cannot be stored on the ship for a very long time and it, for this reason, it has to be discharged into the sea.
However sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some rules regarding discharging of sewage that needs to be followed. Sewage on sea is generally the waste made from toilets, urinals, and WC scuppers. The rules say that the sewage can be discharged into the sea water only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.
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On the other hand, if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also, the discharged sewage should not create any visible floating solids nor should it cause any discoloration of surrounding water.
In general, ships fancy treating sewage before discharging to save themselves from any type of embarrassment. There are different methods of treating sewage available in the market, but the most regular of them is the biological type for it occupies less space for holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco-friendly. It is to not that each sewage treatment system fixed onboard has to be certified by classification society and should perform as per their requirement and regulations.
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How a Biological Sewage Plant works on a Ship
The elementary principle of the working of a biological treatment plant is decomposition of the raw sewage. This process is carried out by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed of in the sea. Air is a very vital criterion in the working of the biological sewage plant because if air is not present, it will lead to the growth of anaerobic bacteria, which produces toxic gasses 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 purpose is to maintain the flow of fresh air.
Division of Processes
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The biological sewage plant is divided into three chambers:-
This chamber is fed with raw sewage which has been grinded to form small particles. The benefit of breaking sewage in small particles is that it raises the area and a 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 the diffuser into the air chamber. The pressure of air flow also plays an essential role in decomposition of the sewage. If pressure is kept high then the mixture of air and sewage will not take place correctly and it will escape without doing any work needed for decomposition. It is for this reason; controlled pressure is vital 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.
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 the growth of anaerobic bacteria and foul gasses will be produced. The sludge formed is recycled with the incoming sludge where it will mix with the later and assist in the breakdown of sewage.
Chlorination and Collection
In this compartment, the clear liquid manufactured from the settling tank is overflown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-Coli bacteria present in the liquid. To decrease these bacteria to acceptable level chlorination is done. Moreover, to reduce the e-Coli, the treated liquid is kept for a period of at least 60 minutes. In some plants, disinfection is also carried with the help of ultraviolet 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 discharged directly into the sea when a high level is reached and is disposed of automatically until low-level switch activates.