Explaination on MARPOL ANNEX- IV, prevention of pollution by sewage from ships

As per Marpol Annex IV-Regulation for the prevention of pollution by sewage from ships, the discharge of sewage into the sea is prohibited, except in cases when the ship has in operation of  an approved sewage treatment plant. 

There are different types of sewage treatment plants, which are used to discharge biological sewage at sea. Almost all the sewage treatment plants work on the aeration process.

The aeration process, also known as the aerobic breakdown process, involves supplying oxygenated air through untreated sewage waste. This oxygenation helps the bacteria to thrive within the sewage waste and break it down into smaller particles. If oxygenated air is not available in the sewage plant, the bacteria cannot survive and this leads to the improper functioning of the sewage plant.

The Hazards and regulations regarding the Sewage Systems

Raw sewage discharged into restricted waters will eventually overwhelm the self purification ability of the limited quantity of water. In a closed dock the effect can be seen in a black sludgy water which when disturbed gives off an unpleasant smell possibly Hydrogen Sulphide.


Let see the how the sewage treatment plant works

  • The biological sewage treatment plant consists of four interconnected compartments, raw sewage or effluent passes through a bar screen, which breaks it up into smaller particles from where it passes into primary compartment.
  • When the level in this compartment rises sufficiently, overflow of the liquid takes place into the aeration compartment.
  • This compartment contains the bacteria, which require oxygen and a nutrient to propagate.
  • The nutrient is solids 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 passes to a settlement compartment where the activated sludge as it is known settles and leaves a clear affluent.



  • As there is no oxygen in the compartment, the sludge is continuously recycled to the aeration compartment where it mixes with the incoming waste to assist in the treatment process.
  • The clear affluent discharged from a settlement compartment must be disinfected.
  • Disinfection is achieved by treating the clear affluent with a solution or tablets of sodium hyppochloride in the chlorination compartment.
  • The discharge valve pump lowers the level in the chlorination tank automatically by pumping overboard.
  • Automation is carried out by means of high or low level floats. The discharge pump cuts in at high levels and cut out at low level.
Regulations according to MARPOL ANNEX-IV :

MARPOL ANNEX-IV regulates the disposal of waste from ships internationally. In addition certain countries have their own national and regional control.
In general, this means that untreated sewage can only be dumped outside 12 miles offshore, and treated disinfected waste outside four miles.
Note : sewage treatment plant, violin operation produces H2S gas, that is hazardous, therefore prior to carrying any repairs on the machinery of sewage plants, it has to be checked for this aspect.


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