How Does a Ship Sewage Treatment Plant Work?

Discarding ship wastewater produced onboard is among the tasks on a ship that should be taken care of only if one wants to save himself and his shipping company from heavy fines.

Ship wastewater cannot be stored on the ship for very long, so it has to be discharged into the sea.

Although sewage can be discharged into the sea, it cannot be discharged directly overboard, as some regulations regarding sewage discharging need to be followed.

Ship wastewater on the sea is generally the waste produced from toilets, urinals, and W.C. scuppers. The rules and regulations state that the ship wastewater can be discharged into the sea only after treatment. Also, the ship is four nautical miles from the nearest available piece of land.

However, if the ship wastewater is not treated, it can be discharged 12 nautical miles from the nearest land. Also, the discharged ship wastewater should not produce any visible floating solids. It should not cause any discoloration of the surrounding water.

Ships mostly prefer treating sewage before discharging to avoid embarrassment. Different methods of treating sewage are available in the market, but the most common is the biological type, which occupies less space for a holding tank.

 The favored type of sewage treatment plant involves aerobic bacteria. Anaerobic bacteria have the same capability of decomposing and breaking down the sludge. Still, during the process, they generate and release harmful gases such as H2S and methane gases.

Parts of Sewage Treatment Plant (S.T.P.)

Screen Filter

The screen filter mesh is installed on the first tank near the sewage entrance to the S.T.P. It helps remove non-sewage adulteration components, such as toilet paper, plastic paper, and other solids. If they go inside, these things can clog the complete system.


The biofilter is another part of the aeration chamber that treats the sewage from the screen filter. The biofilter reactor disperses the contaminated substance using the fine air bubbles supplied by the blower. It then diffuses and breaks down the organic matter the aerobic microorganism generates. The fine bubbles passing through the diffuser increase the oxygen transmission rate.  

Settling and Sedimentation Chamber

The processed sewage water from the biofilter reactor will enter the next chamber used for settling.

After settling in a sedimentation tank, the mixture is further separated into high-grade water and sediment. The clarification compartment is mostly of the hopper type with sloping sides, which prevent sludge from sticking and accumulating. It then sends the mixture to the suction side of the air lift tube.

The untreated sludge is settled in the bottom of the sedimentation tank. It then returns to the biofilter reactor to be broken up by microorganisms again.

Activated Carbon

Activated carbon is fitted after the settling chamber to remove Chemical Oxygen Demand (C.O.D.) by filtering and absorption. It also helps treat biological oxygen demand (B.O.D.) and suspended solids.


The chlorinator is installed in the last chamber. It is placed to treat the final stage water for discharging overboard. It can be tablet-dosing or chemical injection-based. Inside the tablet-based chlorinator, clean water comes in contact with the chlorine tablets directly, forming the chlorine solution. The chlorinator comprises cylinders for filling it with chlorine tablets.

In chemical pumps, a measured amount of NaOCl is injected into the sterilization and chlorination tank using a diaphragm-type reciprocating pump.

Two air blowers are usually installed, one of which acts as a stand-by. It supplies air (via air bubbles) to help form the microorganisms in the biofilter reactor. The blower also helps transfer the sludge from the sedimentation tank, supply air to the activated carbon tank, and back flush the sludge.

Discharge Pump

The discharge pump is given in a duplex. It is mounted on the last compartment of the S.T.P. It is a centrifugal pump of the non-clog type coupled to its respective motors. The pump is run in mostly auto mode. It is controlled by the level switches installed in the sterilization tank. It usually runs manually when removing sludge from the sludge compartments after cleaning the tank.


  • An inlet pipe that transfers the sewage to the plant is installed with the slope to avoid clotting and condensation.
  • The sewage pipe is placed so that the holes inside the pipe are accessible for cleaning during maintenance.
  • The overboard discharging outlet is to be placed 200~300mm lower than L.W.L.

Floats and Level Switches

  • There are three float switches: high-level, low-level, and high-alarm-level. They are usually fitted on the chlorination and sterilization chamber.
  • This chamber is also equipped with level switches to control the start and stop of the discharge pump.

Biological Sewage Plant

The basic principle of a biological treatment plant is the decomposition of raw sewage.

Aerobic bacteria live in this fresh air and decompose the raw sewage. Air is an important criterion for the functioning of the biological sewage plant if air is not present. It will lead to the formation and growth of anaerobic bacteria. This anaerobic bacteria produces toxic gasses that are hazardous to health.

Also, after the sewage is decomposed with anaerobic bacteria, a dark black liquid causes the water to discolor, which is unacceptable for discharge. Thus, the basic purpose of a biological sewage treatment plant is to maintain fresh air flow.

Division of Processes for Discharging Ship Wastewater

The biological sewage plant is split into three chambers:- 

Aeration Chamber

This chamber is loaded with raw sewage ground to form small particles. Breaking sewage into small particles increases the area, and many bacteria can simultaneously attack to decompose the sewage.

The sewage is then decomposed into:

  • carbon dioxide
  • water
  • Inorganic ship wastewater.

The pressure of air flow also has an essential role in the decomposition of the ship wastewater.

Settling Tank

The mixture of liquid and sludge is transferred to the settling tank from the aeration chamber. The sludge resides at the bottom of the settling tank. The clear liquid is on top. The sludge at the bottom cannot be kept inside the settling tank. This will lead to the formation and growth of anaerobic bacteria and produce foul gasses. The sludge formed is recycled with the sludge that is being transferred here. It will combine with the latter and assist in the breakdown of sewage. 

Chlorination and Collection

In this chamber, the clear liquid from the settling tank is overflown. Then, chlorine disinfects it because the liquid contains E. coli bacteria.

Chlorination reduces these bacteria to an acceptable level. Moreover, to reduce the e-Coli, the already treated liquid is kept for almost 60 minutes.

New Regulations for Ship Wastewater

The Baltic Sea area is the only Special Area under Annex IV. Dischargeable sewage from passenger ships within a Special Area is generally prohibited under the new regulations, except when the ship operates an approved sewage treatment plant and meets the nitrogen and phosphorus removal standard.

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