Sewage Treatment Plant on Ships Explained!

Sewage is a very smelly topic if left undiscussed. This article will help you understand how the maritime industry handles human waste aboard the ship without violating or harassing the environment.

Sewage Treatment Plant on Ships
Sewage Treatment Plant on Ships

How does a sewage treatment plant work on ships?

The sewage treatment plant on the ship begins by collecting and segregating the wastewater from each cabin space. There are two types of wastewater being treated aboard; Grey and Black water.

Grey water is wastewater coming from shower drains, lavatory spaces, and even galley spaces. Meanwhile, black water is wastewater that came from the heads – the toilet bowls.

On board, the grey water has a separate piping arrangement from the black water. The grey water goes to the grey water holding tank while the black water goes to the black water holding tank. Both are processed by the sewage treatment plant before pumping out to the sea.

At the port, sewage cannot be pumped out directly to the sea. The wastewater even treated can only be dumped at sea under certain rules which will be later discussed.

Upon arrival at any port, sewage discharge valves are sealed to make sure no one could tamper with any illegal discharge of wastewater. Furthermore, ships have MARPOL flanges which are the only discharge access from the ship to the port sewage facility.

Ship engineers use biological eating bacteria on the sewage pipelines to prevent the development of clogs and later on break down the wastewater components

Ship engineers make sure these rules are followed by properly maintaining the sewage systems and especially the sewage treatment plant aboard. The wastewater can only be dumped at sea when it is on an acceptable smell and appearance.

Why Are Sewage Treatment Plants Required?

In the simple context, sewage treatment plants are required not just on ships but should be on any establishments near the sea or on the sea.

STP is required to prevent pollution and destruction of marine life.

Sewage disposal on board is governed by several regulations internationally, that intend to protect the environment and marine life loss.

What Are the Stages of Sewage Treatment?

As the wastewater is collected, it is pumped into the sewage treatment plant, however, when at sea the waste water doesn’t go to the tank but directly to the STP.

The first stage or primary chamber is the beginning of the wastewater treatment on an STP. Here we can find a macerator pump that comminutes of breaks up the solid waste on the wastewater. In some arrangements, a filter is used to separate solids from the wastewater. The first chamber is overflowed to the second chamber or the aeration tank.

The aeration tank is assisted by an air compressor/ blower, the aeration process helps the bacteria to on stabilizing the wastewater by providing oxygen. The bacteria break down any organic matter in the wastewater.

The overflow from the aeration chamber is caught by the third chamber also known as the settling tank where the comminuted solids and biologically treated wastewater is separated. Waste water goes to the last chamber where chlorine is introduced.

The chlorine kills any biological component of the wastewater that could hurt marine life. The last chamber is also where the discharge pump sucks out to the sea, but, before it is pumped out dechlorination is induced to remove chlorine from the wastewater which could also hurt marine life when the treated wastewater is discharged.

Usually, the last chamber is fully automated by such level sensors are installed to automatically start the discharge pump when the chamber is full and stop the pump when it is empty.

Each chamber has a piping arrangement that can be lined up to the discharge pump to drain each chamber. Normally, a monthly routine of backflushing the STP is done by ship engineers aboard.

The sewage treatment plant is one machinery consisting of four different chambers each having a specific role in treating wastewater before it can be safely pumped out to the sea.

Sewage treatment plant components

  • Screen Filter – A screen filter is often installed on the first chamber to prevent solid waste such as tissues, napkins, cigarette butts and etc, which people usually dump on urinals, toilets, and shower drains.
  • Macerator pump – The macerator pump physically breaks the solid entering the first chamber into smaller pieces so the biological bacteria could easily decompose the organic components of the wastewater.
  • Chlorinator – The chlorinator pumps chlorine to the final chamber at a preset volume and chlorine percentage.
  • Discharge pump – The discharge pump sucks the treated water and releases it to the sea.
  • Blower/air compressor – The blower usually a rotary vane pump provides oxygen to the second chamber to help the organic bacteria to do its job of breaking carbon components into water and carbon dioxide
  • Primary chamber – The foremost tank on a sewage treatment plant where it collects the raw wastewater and comminuted solid wastes
  • Aeration chamber – The tank where decomposition takes place by the reaction caused by the organic eating bacteria and oxygen introduction
  • Settling chamber – Is where the separation of comminuted solid waste is separated from the partially treated wastewater
  • Collection chamber – The final stage of treatment where wastewater is chlorinated and dechlorinated before flushing into the ocean

Regulation Governing Sewage Treatment Plant

The main body which governs sewage disposal at sea is the MARPOL Annex IV: Prevention of Pollution by Sewage from Ships.

Enforced by the International Maritime Organization (IMO) the annex entered into force on September 27, 2003. A revision is adopted on April 1, 2004, which has been fully enforced by August 1, 2005.

Mainly, the discharge of sewage shall be;

  • Comminuted and disinfected sewage: at least 3 nautical miles from the nearest land, or
  • Sewage not comminuted or disinfected: at least 12 nautical miles from the nearest land. Sewage that has been stored in holding tanks shall be discharged at a moderate rate when the ship is en route and proceeding at not less than 4 knots (the discharge rate shall be approved by the Administration based upon standards developed by the Organization).
  • The ship has in operation an approved sewage treatment plant which has been certified by the Administration


Annex 4 of the MARPOL Convention has tons of rules and regulations, a law book, that has to be followed by ships to uniformly protect the environment from the hazards of sewage disposal at ports, open sea, and even special areas.  Violation of the said rules has it’s on weigh of consequence and punishment to the offender.

Nano Planted tank
Nano Planted tank