Working [Video Lesson]


The basic principle of the working of a biological treatment plant is the decomposition of raw sewage. This process is done by aerating the sewage chamber with fresh air.


The aerobic bacteria survive in this fresh air and decompose the raw sewage which can be disposed of in the sea. Air is a very important criterion in the functioning of the biological sewage plant because if the air is not present, it will lead to the growth of anaerobic bacteria, which produces toxic gasses that are hazardous to health.


Also, after the decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discolouration of water which is not acceptable for discharge. Thus in a biological sewage treatment plant, the main aim is to maintain the flow of fresh air.


Video Lesson:


Division of Processes The biological sewage plant is divided into three chambers: 

Aeration chamber:

This chamber is fed with raw sewage which has been grounded to form small particles. The advantage of breaking sewage in small particles is that it increases 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 important role in the decomposition of sewage. If the 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; that 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 the settling tank from the aeration chamber. In the settling tank, the sludge settles at the bottom and clear liquid is 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 which it will mix with the latter and assist in the breakdown of sewage. 


Chlorination and Collection:

In this chamber, the clear liquid produced 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 reduce these bacteria to an 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 done with the help of ultraviolet radiation.


The collected liquid is discharged to an overboard or settling tank depending on the geological position of the ship. If the ship is restricted or near the 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 a low-level switch activates.


Precautions for efficient operation of STP:

  • The aeration blower is installed to run continuously as it helps the microorganism to sustain and grow. Never switch off the blower as it will cause the death of microorganisms, which will, in turn, reduce its clarification efficiency and will take days to grow microorganisms again.
  • Never throw any foreign substances such as cigarette buds, paper, rags, etc. into toilets as it may choke the pipeline or filter hampering the STP operation
  • The toilet tissue used onboard should be free of vinyl components as it affects the growth of bacteria
  • Never use unauthorized chemicals or detergent to clean the toilet
  • The grey water inlet pipe must be placed lower than the water level of the inside of S.T.P to decrease the foam generation
  • The pH of the samples of effluent shall be in the range of 6 to 8.5
  • The Nitrite content is not to exceed 10 mg/ltr NO2.

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