Evac Co-authored: Discharge of screenings in the high seas

Co-authored by

  • Dr Wei Chen, Future Program Development Manager, Wartsila Water Systems, UK
  • Felix von Bredow, Board of Hamann AG, Hamann AG, Germany
  • Oliver Jost, Maritime Environmental Affairs, Wasserschutzpolizei (Water Police) Hamburg, Germany
  • Markus Joswig, Head of Marine Department, PIA GmbH, Germany
  • Mats Riska, Head of Concept & Process Development, Evac Oy, Finland

Endorsed by

  • Niclas Karlsson, Managing Director, Clean Ship Scandinavia AB, Sweden
  • Ed White, Environmental Consultant, former Alaska DEC cruise ship compliance manager, USA
  • Antony Chan, MEng CEng MIMarEST, General Manager, Victor Marine Ltd., UK
  • Ronni Palmqvist, Director and Owner, Dancompliance ApS, Denmark
  • Dr. Theodora Alexakis, Vice President Business Development, Terragon Environmental Technologies Inc., Canada
  • Alex Myers, Chief Engineer, Sea Education Association, USA
  • Benny Carlson, Chairman and owner, Marinfloc, Sweden
  • Helge Østby, Senior Technical Advisor, Jets Vacuum AS, Norway
  • Mark Beavis IEng IMarEng FIMarEST, Sales Director, ACO Marine s.r.o., Czech Republic, ACO Marine Systems GmbH, Germany


Let’s first of all understand what screenings material is and how it is managed on land.

Wastewater treatment plants (WWTP) use biological treatment to purify the wastewater from our cities, towns and villages. These WWTPs generate a by-product called sewage sludge – a mixture of surplus natural bacteria and inert particles. WWTPs have another by-product called screenings. This consists of large debris carried down the sewer, such as rags, paper, plastics, and debris, etc.

The screening material is mostly intercepted by inlet screens positioned at the beginning of a WWTP (photo), or further upstream at sewage pumping stations and storm bypasses. The aim is to prevent clogging or interference with downstream operations and minimise any unsightly floating plastics in receiving waters. Due to marked increase of plastics in sewage since the 1980s, comminutors and grinders used in some small WWTPs have been replaced by screens.

Coarse screens, such as bar screens, typically have openings > 6 mm, and fine screens from 1.5 to 6 mm. Smaller openings result in more screenings of higher water content and organic contents such as faecal matter, and hence worse odour nuisance. Openings of 38 mm and 13 mm intercept about 10 and 60 litres/1,000 m3 of screenings respectively [1,2,3]. Finer openings often result in a liquid screenings flow. Environmental protection can be a dirty job. The handling of screenings, or merely the sighting and smell of it, can make a few novice operators throw up.

While sewage sludge is often recycled for agriculture, soil conditioning and landscaping under the strict biosolids regulations, the screenings material it is incinerated or landfilled. Under the European Landfill Directive, screenings material is washed and de-watered to reduce its organic and water content, before it is disposed as a non-hazardous waste (European Waste Code 190801) [4].

Now, what has been happening in the maritime sector?

Screenings material exists in ship’s sewage too [5], which also contains rags, plastics, paper wipes, and other spent sanitation product, etc. A close description of screenings material under the IMO’s MARPOL Annex IV Convention is ‘visible floating solids’. Unfortunately, discharge of visible floating solids is not prohibited in the high seas.

Discharge of macerated and chlorinated sewage from the notorious [6] comminuting and disinfecting systems (CDS) is permitted at a distance of > 3 nm from the nearest land, and discharge of untreated sewage is permitted > 12nm, along with sewage screenings.

Regulation 11 does prohibit visible floating solids from effluent of sewage treatment plant (STP), but this has not stopped screenings from being discharged in the high seas. Far from it:

Thousands of physical-chemical STPs are designed and promoted to be switched off when outside 12 nm from the nearest land. Several STPs use ‘fine maceration’ instead of screens to reduce operational input and hygiene risks. While many STPs have screens, their effectiveness may be questionable due to the use of cutter pumps in vacuum collection systems. At times, intercepted screenings may be re-mixed with sewage sludge for discharge.

But when a sewage holding tank is used to feed a STP, the above contributing factors become trivial.

Ships often have designated sewage holding tank(s) of 1-10 days of holding capacity. Once sewage enters these tanks, much of its screenings material tends to form a thick layer of floating scum (photos). This scum layer is not readily broken up by a mixing device, nor is it removed by the transfer pumps serving the STPs or the onshore connections (just as well, because it may block the sewer). The trapped screenings can only be flushed out in the high seas during tank cleaning.

The wastewater industry on land has avoided holding unscreened sewage for a good reason.

So, despite STPs, the majority of sewage screenings material ends up in the high seas.

This practice should stop. This is because the existing marine rules have prohibit the discharge of plastics and garbage from all ships in all waters, and the rest of our society has been trying hard to keep sewage screenings away from our natural environment.

There are some discussions at the IMO (PPR 7/16), but they are confined to the STPs. It will do nothing to stop screenings discharge from holding tanks and CDS. In fact, the situation could be worsened by it, when ships are given easier solutions by evading STP altogether.

The solutions can be readily available, i.e., to intercept and remove screenings material before it enters the sewage holding tank, the CDS, and the STP, and then dispose it by thermo destruction technologies onboard as already happened on some ships or by the port reception facilities.

It can bring integrated and consistent regulatory approach to manage ship’s wastes and bring maritime sector on a par with the rest of our society. Then, the shipping industry may justify and sustain the practice of discharging sewage and sewage sludge, free of screenings, in the high seas [7].

Such initiative requires support by the IMO member states and the industry. Are we ready?


[1] Wastewater Treatment Manuals – Preliminary Treatment, EPA, Ireland, 1995

[2] Wastewater Technology Fact Sheet Screening and Grit Removal, USEPA, 2003

[3] Unit Operations and Processes in Environmental Engineering.  PWS Publishing Company. Boston, Massachusetts, Reynolds, T. and P. Richards, 1996.   

[4] The treatment and disposal of sewage screenings and grit, by B. Thompson, 2012. 

[5] Sampling Episode Report, Sampling Episode 6503, 6504, 6505, 6506, USEPA, 2006. 

[6] Sewage Comminuting and Disinfecting Systems – A Disservice to MARPOL Annex IV, Maritime Executive, 2020. https://www.maritime-executive.com/editorials/support-for-major-reshape-of-marpol-annex-iv 

[7] https://www.youtube.com/watch?v=H6gEnj4_lGk