Vessel sewage: is Marpol Annex IV fit for purpose?

The GESAMP Working Group 43, a group of independent scientific experts, has completed a comprehensive review of various sea-based sources of marine litter and found quantifying the sources can be challenging due to a lack of information.

To contribute to this important initiative, Group 43 has attempted to quantify the levels of marine litter allowable under the current regulations.

Municipal wastewater on land contains litter such as plastics, rags, paper, wipes, etc. It is intercepted by the inlet screens of wastewater treatment works (WWTWs) to prevent it from entering downstream processes or mixing with sewage sludge. The intercepted sewage screenings material (solid material is often referred to as ’screenings’) is washed and dewatered on site before it is taken away to be incinerated or landfilled as a non-hazardous waste.

Importantly, the screenings-free sewage sludge can be recycled as a natural resource without being hampered by the inclusion of macro-plastics, the presence of which has increased dramatically in wastewater since the 1980s.

Inlet screens with a finer opening can intercept more screenings - a 12-mm opening is found to intercept 44 to 110 litres of screenings per 1,000 m³ of wastewater processed, which has a density close to 1 kg/m³. In other studies, 3- to 6-mm openings are found to intercept 0.5 to 3.5 kg of dewatered screenings per capita per year.

Recent research suggests that 73% of the mass of plastics in wastewater, which includes both macro and micro plastics, can be intercepted by screens with opening sizes down to 2 mm.

A ship’s black and grey water also contains litter such as plastics, wipes, paper, rags, etc. The use of cutter pumps on the vessel’s collection systems can render coarse screens ineffective.

Advanced wastewater treatment (AWT) systems often use fine screens or sieves of 0.5 to 2 mm and 2 mm screens can intercept 10 kg of dewatered screenings per person per year, which is greater than that intercepted by the 3- to 6-mm screens on land.

Plus the dewatered screenings are found to have higher levels of pollutants than that of raw sewage.

At any given time, there are some 2M people at sea. With a black and grey water generation rate of 200-300 litres per person per day, the amount of screenings contained in ship sewage and grey water can be estimated in the magnitude of 10,000 m³ per year.

Grey water is not regulated by the Marpol Convention, so litter in grey water can be discharged without restrictions.

Litter in a ship’s sewage is defined as ‘visible floating solids’ under Marpol Annex IV. These solids can be discharged to sea from sewage holding tanks when more than 12 nautical miles from the nearest land, or discharged from comminuting and disinfecting systems when beyond 3 nautical miles.

The discharge of visible floating solids from type-approved sewage treatment plants (STPs) is prohibited, even though the majority of STPs discharge ‘virtually raw sewage’.