The persistent eutrophication (being too rich in nutrients) of the Baltic Sea prompted the IMO’s Baltic Member States to tackle shipping’s < 0.1% contribution by designating the Baltic Sea as the first Special Area under Annex IV of IMO’s MARPOL Convention . Passenger ships must give sewage to wastewater treatment works (WWTWs) ashore via the port reception facilities (PRFs) or operate a nutrient removal sewage treatment plant (STP) on board prior to discharge. It has been perceived as a positive move. But how special is the Special Area?
Rules at sea vs. rules on land – where do they meet, and clash
The coastal waters and the PRFs are where the local rules on land and the international rules at sea meet. As the IMO does not have jurisdictions over the local rules, the IMO’s Baltic Member States can, and should, reflect and integrate their local rules when creating the Special Area. BUT…
12 vs. 10,000. Total nitrogen (TN) removal is required for 12 or more passengers at sea, and for 10,000 or more population equivalents (p.e.) on land (see table). Low capacity TN removal applications are known to have their limitations and diminishing benefits [2, 3] on land. Such significant discrepancy in population threshold for TN removal leads maritime industry into uncharted waters. Are STPs for 12 passengers available and viable?
Availability and viability of small STPs. Ship sewage can be over 10 times more concentrated than urban wastewater  and has 500~1,000 mg N/l of ammonia [5,6] when collected by vacuum. Ships also have space and logistic constraints, they pitch and roll, and they lack the relevant skills and resources. These challenges are not reflected by the IMO’s approval regimes. Nutrient removal STPs have been tested and approved , and unfortunately, along with certified “magic boxes” [8,9]. These magic boxes give ‘availability’ a different meaning and become nutrient leakage for the Special Area. So, the lack of enforcement under MARPOL Annex IV, which has resulted in poor STP performance status over the decades, has now eroded the Special Area too. Furthermore, an available technology may not be viable when it entails excess costs, natural resources, and adverse impacts that outweigh its benefit. However, STP availability and viability cannot be assessed without proper enforcement. Fundamentally, the declared availability and viability for 12 passengers is at odds with the 10,000 p.e. threshold on land . They cannot both be correct when the Baltic eutrophication issue is at stake.
Open-ended percentage removal targets – a disservice. The EU Urban Wastewater Treatment Directive (UWWTD) caps the percentage TN removal target of WWTWs on land with a maximum concentration ceiling . But such a ceiling is absent for ships . As such, the EU TN standard can be exceeded by 8-15 times under the Special Area rules. The ammonia toxicity limits imbedded in the national regulations can be exceeded by 10~70 times, considering the typical ammonia discharge permit of around 4 to 30 mg N/l [10 -12]. Furthermore, the percentage removal targets are almost impossible to verify onboard.
Discharge bans in ports. Many ports have banned STP discharges in port waters, including the discharges from the so-called advanced wastewater treatment (AWT) systems with proven performance far more superior than that of equivalent coastal WWTWs. The ban contradicts with the intention of the IMO rules that allow STPs to discharge without restrictions. The creation of Special Area further promotes reliance on PRFs. For major PRFs that are served by mega-scale state-of-the-art municipal WWTWs with spare treatment capacities, it makes sense.
Adequacy of PRFs. Many major ports have invested in newer and better reception facilities. However, PRF adequacy is influenced by advanced planning of ship itineraries, PRF utilisations, connections at each relevant berth, the inclusion of grey water, and the fee systems, etc. The emerging aspirations to receive sewage from cargo ships  may spread the uncertainty of PRF adequacy to smaller and more remote ports that are served by smaller WWTWs. But PRF adequacy is not all.
Maximum permissible sewage concentrations. Wastewater entering local sewers can be restricted by maximum permissible concentration limits to protect the local infrastructures from corrosion, odour, toxicity, health&safety risks, and to safeguard the compliant performance of local WWTWs. Ship sewage readily exceeds these limits by a long way (see table), not to mention sewage sludge. The situation can be alleviated when ship grey water is one day regulated together with sewage – a necessity for having an effective MARPOL Annex IV .
Not all WWTWs remove nutrients. Those WWTWs serving <10,000 p.e. are not required to remove TN. They become a nutrient leakage when receiving ship sewage. Larger WWTWs can be overloaded by ship sewage during peak loading conditions especially in the absence of storage tanks in PRFs . Crucially, the timing, quantity, or quality of ship sewage are not visible to the receiving WWTWs. The disposal route of road tankers and barges may not be traceable. This lack of transparancy undermines the extended producer responsibilities (EPR) and risks potential nutrient leakages from facilities ashore.
Should a passenger ship rely on PRF or STP? Many ship owners choose to install STPs to gain flexibility and green credentials. But not many people have realised that treating sewage on some days and giving it to PRFs on other days will risk STP performance. This is simply because a proven biological TN removal process relies on daily sewage influent to perform – a point to be noted when MARPOL Annex IV is effectively implemented.
There is more…
Animal carriers. Animal carriers are not passenger ships, but animal sewage contains nutrients. A carrier with 22,500 cattle produces the same amount of phosphorus as a ship of 375,000 passengers . It can be a significant nutrient leakage for the Baltic Sea  as well as other waters of eutrophication concerns.
Diseases. The IMO introduced the most stringent Coliform standard for sewage discharges in order to minimise the spread of waterborne diseases by international shipping. But there is a loophole: untreated sewage received by the PRFs can carry international diseases into the local environment unchecked, because not all countries require their coastal WWTWS to disinfect the effluent, including those in the Europe. .
Comparing to Alaskan water, whereby the local laws have successfully regulated grey water and introduced compliance program to large passenger ships for 2 decades, the Baltic Sea Special Area has inherited the weaknesses of MARPOL Annex IV. This is evident by the certified magic boxes and a lack of visibility to STP discharge performances. On land, there is a lack of transparency to ship sewage disposal routes and the capabilities of the receiving WWTWs. Importantly, issues such as the conflicting TN removal population thresholds (12 vs. 10,000), the open-ended percentage TN target, the exceedances against the local permissible concentrations, and the nutrient contribution of animal carriers can have long-lasting implications.
The IMO needs to showcase effective implementation of its environmental rules. This well ringfenced Special Area can be a candidate. To address the <0.1% nutrient contribution from ships, money and natural resources are being invested twice, once in the PRFs, and once in the STPs. Industry is committed. The Baltic member states will no doubt be equally committed to showcase the effectiveness and the benefits of this Special Area. Time will tell if the Baltic Sea Special Area is really special.
 DE 55/17/2; MEPC.200(62); MEPC.227(64); MEPC.275(69).
 EU Urban Wastewater Treatment Directive (UWWTD), 91/271/EEC, 1991; Baltic Sea Action Plan, HELCOM, 2008
 Naar meer doelmatigheid bij IBA -systemen, STOWA, 2014
 Sewage discharge to port - what to expect, by E. Dorgeloh, and M. Joswig, SOWOS 9, 2015.
 MEPC 60/INF.4; Estimated nutrient load from waste waters originating from ships in the Baltic Sea area, VTT, 2009
 MEPC 67/8, submitted by Finland and Norway, 2014.
 Communications with Finnish Ministry of Environment, 2021
 GB 18918－2002, standard of pollutants for municipal wastewater treatment plant, 2002.
 Emission limit guidelines for sewage treatment plants, Department of Primary Industries, Water and Enviornment, Tasmania, 2001.
 GB/T 31962-2015, wastewater quality standards for discharge to municipal sewers.
 CP PRF 9-2021, HELCOM, June 2021; HELCOM Maritime meeting, document 4-2, 2017.
 Characteristics and Volume of Effluent Produced by Livestock Vessels, by Landline Consulting, 2003.