MARPOL Annex 4 Explained

MARPOL Annex 4: Regulations for the prevention of Pollution by Sewage from ships

The discharge of raw sewage into the sea can create a health hazard. Sewage can also lead to oxygen depletion and can be obvious visual pollution in coastal areas – a major problem for countries with tourist industries. The discharge of sewage into the sea from ships also contributes to marine pollution.

Legal Requirements

  • MARPOL Annex 4 contains a set of regulations regarding the discharge of sewage into the sea from ships, including regulations regarding the ships’ equipment and systems for the control of sewage discharge, the provision of facilities at ports and terminals for the reception of sewage, and requirements for survey and certification. It also includes a model International Sewage Pollution Prevention Certificate to be issued by national shipping administrations to ships under their jurisdiction. 
  • It is generally considered that on the high seas, the oceans are capable of assimilating and dealing with raw sewage through natural bacterial action. Therefore, the regulations in Annex IV of MARPOL prohibit the discharge of sewage into the sea within a specified distance of the nearest land, unless they have in operation an approved sewage treatment plant. 
  • Governments are required to ensure the provision of adequate reception facilities at ports and terminals for the reception of sewage. 

Entry into force

The MARPOL Annex IV entered into force on 27 September 2003. A revised Annex IV was adopted on 1 April 2004 and entered into force on 1 August 2005.

Shipboard Sewage Pollution Sources

  • The drainage and other wastes from any form of toilets and urinals; 
  • The drainage from medical premises (dispensary, sickbay, etc.) via washbasins, washtubs, and scuppers located in such premises; 
  • The drainage from spaces containing living animals; 
  • Other wastewaters when mixed with the drainages defined above. (Regulations not applicable to the disposal of greywater – e.g. drainage from the dishwasher, shower, laundry, bath, and washbasin drains).

Applicability of MARPOL Annex 4

  • Ships engaged in international voyages and of 400 GRT and above; or 
  • Ships engaged in international voyages and of less than 400 GRT, which are certified to carry more than 15 persons.

Exceptions from Annex IV of MARPOL

The discharge of sewage into the sea is allowed when: 

  • securing the safety of a ship and those on board or saving life at sea; or
  • the discharge of sewage resulting from damage to a ship or its equipment if all reasonable precautions have been taken before and after the occurrence of the damage, for the purpose of preventing or minimizing the discharge. (Regulation 11 of MARPOL Annex 4)

Discharging of sewage from ships other than passenger ships in all areas and discharge of sewage from passenger ships outside special areas

The Annex requires ships to be equipped with either an approved sewage treatment plant or approved sewage comminuting and disinfecting system or a sewage holding tank. The discharge of sewage into the sea is prohibited unless the following requirements are fulfilled:


Special Areas for MARPOL Annex 4

As per the most recent amendments to MARPOL Annex 4 which entered into force on 1 January 2013, the Baltic Sea is considered as a special area and adds new discharge requirements for passenger ships while operating in the special area.

Discharging of sewage from passenger ships within a special area

  • Discharge of sewage into the sea is forbidden for:
    • new passenger ships as of Jan 01, 2016 
    • existing passenger ships as of Jan 01, 2018, 
  • Except when the following conditions are satisfied: 
    • a certified sewage treatment plant that meets the operational requirements of the Regulations is available aboard and 
    • no visible floating solids and no discoloration in surrounding waters.

Exceptions from Regulation 11 of MARPOL Annex 4

The discharge of sewage into the sea is allowed when: 

  • securing the safety of life or the ship; or 
  • the discharge of sewage is a result of damage to a ship or its equipment if all reasonable precautions have been taken before and after the occurrence of the damage, for the purpose of preventing or minimizing the discharge.

Equipment requirements

  • discharge pipeline fitted with the standard discharge connection * and 
  • approved sewage treatment plant or 
  • comminutor/disinfection system with temporary means of storage or 
  • holding tank.

* applies to all new ships contracted for construction on or after January 1, 2007.

Sewage Treatment Plant

There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for the holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco-friendly.

Working of a Biological Sewage Plant

The basic principle of the working of a biological treatment plant is decomposition of the raw sewage. But first it is important to understand what is aerobic and anaerobic bacteria.  Aerobic bacteria inhales oxygen to remain alive, whereas anaerobic bacteria dies in the presence of oxygen and hence avoids oxygen.

The decomposition process involves aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be then disposed into the sea. Air is a very important criterion in the functioning of the biological sewage plant because if air is not present, it will lead to the growth of anaerobic bacteria, which produce 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 acceptable for discharging at sea. Thus, in a biological sewage treatment plant, the main aim is to maintain the flow of fresh air.

The biological sewage plant is divided into four chambers: 

Primary chamber

This chamber is fed with raw untreated sewage and this sewage is screened and thereafter ground to form small particles. The advantage of breaking sewage in small particles is that it increases the area and a higher number of bacteria can attack simultaneously to decompose the sewage.

Aeration chamber

The sewage which arrives into the next chamber (aeration chamber) is decomposed into carbon dioxide, water, and inorganic sewage. The air is forced through the diffuser (an attachment or a duct for broadening air flow and reducing its speed) into the air chamber. The pressure of air flow also plays an important role in decomposition of the sewage.

If 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; 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. 

MARPOL Annex 4 Explained 2
Courtesy of Marine Engineering Study Materials

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 clears 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 back into the aeration chamber where 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 overboard or stored in a settling tank depending on the 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.


International Convention for the Prevention of Pollution from Ships (MARPOL)

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