What types of sewage do ships generate?

Maritime and cruise ships generate two main types of sewage wastewater:

Grey water

This is the wastewater produced by activities such as handwashing, showering, laundry, and the use of sinks. It may contain soap residues, detergents, food waste, grease, and small quantities of organic matter. Although grey water is less hazardous than black water, it can still have an environmental impact due to its nutrient content and potential to reduce oxygen levels in receiving waters if released untreated.

Black water

This is the more hazardous form of sewage generated on board. It primarily consists of human waste from toilets and urinals, as well as wastewater flushed through the ship’s sanitary systems. Black water contains higher levels of harmful bacteria, organic matter and nutrients, and requires specialist treatment to reduce contamination risks before storage or discharge.

What happens to sewage from ships?

Waste produced on board ships undergoes a controlled treatment process designed to reduce contaminants and limit environmental impact before any storage or discharge takes place. Treating sewage via these integrated treatment systems removes pollutants and produces water that meets strict maritime discharge standards. The stages of this treatment are:

Bioreactor aeration chamber

Raw sewage first enters the bioreactor aeration chamber. Here, naturally occurring bacteria are introduced or cultivated to break down organic contaminants dissolved in the liquid. Aeration, the continuous supply of oxygen, is used to keep these bacteria active and to promote the decomposition of biodegradable material. This step is essential for reducing the biological oxygen demand of the wastewater, which in turn lowers the impact on marine ecosystems if discharge is permitted.

Membrane filtration system

Once the initial aeration is complete, the wastewater passes into a membrane filtration system to further filter impurities. This stage uses specialist membranes to separate out larger solids, suspended particles and residual organic matter that were not removed in the aeration chamber. The filtration process is highly effective at improving clarity and reducing the volume of contaminants. After filtration, the liquid is typically recirculated back into the aeration chamber. This closed-loop approach allows the treatment cycle to be repeated as many times as necessary to achieve the required effluent quality.

Settlement chamber

When filtration has reduced the volume of suspended solids, the liquid is transferred to a settlement chamber, sometimes called a sedimentation tank. In this stage, any remaining dense substances sink to the bottom of the tank and form sludge. The clarified water remains in the upper part of the chamber and is ready to progress to final treatment. The sludge collected at the base is stored separately on board in dedicated containment areas for later removal and disposal at an approved on-shore facility.

Disinfection chamber

The clarified water then moves into the disinfection chamber. Here, ultraviolet (UV) radiation is applied to damage the genetic structure of microorganisms and reduce bacterial content to safe levels. Unlike chemical disinfectants such as chlorine, UV treatment does not introduce additional compounds into the treated water, which makes it an effective and widely used method in maritime wastewater systems.

Once disinfection is complete, the treated water is either kept in storage tanks on board or discharged overboard, depending on the ship’s location and applicable regulations. For example, the right to dump treated waste is strictly controlled in coastal areas and prohibited in designated special protection zones.

These protocols enable ships to comply with national and international environmental standards that govern the discharge of treated sewage water.

What regulations must ships follow when handling or disposing of sewage?

Ships must comply with a detailed framework of environmental and safety regulations to manage sewage responsibly. Breaching these requirements can lead to unlimited financial penalties and, in some cases, imprisonment for those in charge. The main obligations are set out under the International Convention for the Prevention of Pollution from Ships (MARPOL) and the Environmental Protection Act 1990:

MARPOL

Annex IV of MARPOL governs the prevention of pollution by sewage from ships. The requirements apply to:

• All vessels of 400 gross tonnage or more.
• Ships certified to carry more than 15 persons.

These vessels must be equipped with one of the following systems to manage sewage effectively:

• An approved sewage treatment plant designed to treat waste to the standards specified in the regulations.
• An approved comminuting and disinfecting system that grinds and disinfects sewage before discharge.
• A holding tank capable of storing sewage on board for later disposal at port facilities.

Where untreated sewage has not been comminuted or disinfected, discharge is only permitted when strict conditions are met:

• The ship must be situated more than 12 nautical miles away from the nearest landmass.
• The vessel must be underway, not stationary or anchored.
• The ship must maintain a minimum speed of 4 knots during the discharge operation.

When calculating the permitted discharge rate, no dilution with additional water or grey water is taken into account. This means:

• The discharge limits are set at conservative levels to help protect water quality and marine organisms.
• The movement of the ship through the water, including the mixing action of the wake, assists in dispersing the treated effluent.

Where a holding tank is used in combination with a pump operating at a fixed discharge rate, the pump must be:

• Calibrated to match the maximum discharge volume permitted at 4 knots; or
• Calibrated for a higher minimum vessel speed if that is the intended mode of compliance.

If a planned discharge exceeds the allowable rate, operators must reduce the volume or increase the ship’s speed to remain within the limits set out in Annex IV.

Environmental Protection Act 1990

The Environmental Protection Act 1990 imposes further obligations on ship owners and operators handling controlled waste.

Section 34: Duty of care

• Take all reasonable measures to stop unauthorised or harmful disposal of sewage or other controlled waste.
• Transfer waste only to registered carriers or authorised waste management companies.
• Provide accurate written information describing the waste whenever it changes hands.
• Store, transport and handle waste in a safe and secure manner to prevent leaks, spills or contamination.
• Maintain all necessary records, such as waste transfer notes and supporting documentation, for a minimum of two years to aid traceability.

Section 33: Prohibited conduct

• Do not deposit, store, treat or dispose of controlled waste without the correct environmental permit in place.
• Do not cause or knowingly permit waste to be managed in any way that could pose a risk to human health or the environment.

These combined regulations are intended to protect the marine environment, promote responsible waste management and uphold rigorous standards across the shipping industry. Ship operators are expected to document their processes, keep all necessary certifications up to date and carry out regular checks to confirm compliance at all times.

How can you remain compliant with waste handling regulations?

Ships can meet their legal obligations and manage waste safely by applying structured procedures and working with qualified partners. The following measures help ship operators to meet their regulatory requirements and protect the marine environment:

Create a waste management plan

A waste management plan sets out clear procedures for handling all types of waste generated on board. It should specify what waste will be produced, how it will be stored, recycled or treated, and which individuals are responsible for each part of the process. The plan must also include details of the volumes expected, how waste will be transferred or disposed of, and which licensed carriers will remove or discharge the treated material. A robust waste management plan provides clarity for crew members and a framework for inspections.

Use licensed waste handlers

Licensed waste handlers have the necessary training and authorisation to manage sewage and wastewater in line with regulatory requirements. By appointing a reputable provider, ship owners can be confident that all waste is collected, treated and disposed of lawfully.

Universal Tankers has extensive experience supporting maritime businesses with wastewater management. We handle all forms of ship wastewater, including grey water, black water and bilge water. Our services include:

• Guaranteed next-day servicing anywhere in England and Scotland, supported by our nationwide depot network and modern fleet of tankers.
• Emergency call-outs to manage urgent issues quickly and professionally.
• Scheduled waste removal services tailored to your operational needs.
• Drain jetting for ship drainage systems and pipework, to reduce blockages and maintain sanitary conditions on board.

By putting effective plans in place and partnering with an experienced waste management specialist like Universal Tankers, you can operate with confidence that your vessel remains compliant with all regulations. Our dedicated support helps you maintain high standards, protect the environment and keep your operations running smoothly.