Sewage from Ships – Rules and Realities

Sewage sample
effluent of an approved marine sanitation device, source: Inspectorate of a major shipping nation

Published Nov 3, 2018 6:29 PM by Wei Chen

Sewage pollution control was the first environmental initiative in history. Technologies and regulations for the land-based wastewater industries have been evolving for more than a century, bringing tangible improvements to our aqua environment.

The IMO’s MARPOL Annex IV Convention, which aims at achieving the prevention of pollution by sewage from ships, was developed in the 1970s, and entered into force since 2003. It contains a set of regulations prohibiting sewage discharge from ships, unless via an approved sewage treatment plant, via a comminuting and disinfecting unit (CDU) at a distance of more than three nautical miles from the nearest land, or to the open sea (> 12nm) while en-route at not less than four knots. Sewage can also be offloaded to port reception facilities. Although the U.S. is not a signatory country, U.S. Coast Guard rules are somewhat similar, with a sewage treatment plant being referred to as a marine sanitation device.

Discharging sewage to the open sea is simple and free, but it entails extra storage tanks, reduced flexibilities and it implies a somewhat negative image for the shipping line. CDU’s are primitive and rarely employed by ships sailing internationally. The harmful by-products from CDU’s have also prompted further reviews (MEPC 71/14/2). Sewage port reception facilities have gained regional attention, but issues such as availability, adequacy and fee structures have hindered their widespread adoption. It is easy to see, therefore, that sewage treatment plants have proven to be the popular option.

The Marine Environmental Protection Committee (MEPC) has developed guidelines on sewage treatment plant effluent standards and performance test specifications. These effluent standards are ambitious compared to those of equivalent coastal discharges ashore. For example, a sewage treatment plant for a 12-passenger ship trading in the Baltic Sea is given similar Total Nitrogen limits as that of a municipal wastewater treatment plant serving > 10,000 people, while sewage discharges from a decentralized 12-person community around the Baltic Sea does not have Total Nitrogen limits. In fact, a better-informed wastewater industry ashore considers applying such Total Nitrogen limits to a community of up to 300 people to be neither viable nor beneficial (BSAP, 2008).

At the same time, there is no shortage of approved sewage treatment plants that are low-cost and “care-free.” The marine industry seems to cope swimmingly with what are seemingly impossible targets for other industries. The MEPC has tightened the effluent standards twice in 10 years. In the meantime, sewage treatment plants often became smaller and cheaper. Why not then tighten the limits further? The sky is the limit.


Here is the catch. Unlike regulations ashore, MARPOL Annex IV has no compliance monitoring, and no enforcement protocols. How the sewage treatment plant is actually functioning, no one knows, nor needs to know. In fact, Annex IV does not request operational sewage treatment plants on board to meet any concentration limits.

Curiosity, however, reveals the facts. By year 2000, Alaskan regulators sampled 23 cruise ships. They were not impressed with what they found. Since 2001, they also surveyed five to 17 small ships each year, a sector plagued with marine sanitation devices using sea water for dilution. The Netherlands has taken samples from some 120 merchant ships since 2012, and “97 percent of the sewage treatment plants did not meet all the discharge standards.” Pollutants such as suspended solids and Faecal Coliforms often exceeded their limits by 10 and 10,000 times respectively. These ships were “discharging virtually untreated sewage” (MEPC 71/INF.22).

In 2018, Puget Sound has been designated as a sewage no-discharge-zone, including treated sewage from marine sanitation devices. The reality is clear, and it is not pretty. MARPOL Annex IV has not been effective.

In the absence of effective enforcement, while the marine sewage rules are becoming ever more restrictive, the gap between rules and realities is widening. The worst is still to come.


With MEPC Guidelines in place, one would hope that sewage treatment plant performance tests actually conform to the specifications, so that consistencies are assured. After seeing a few examples, however, one may be excused for feeling disappointed.

Gray water connection to disinfection stage

Incentivised by the classification societies’ green notations, many shipowners have taken the initiative to treat gray water, even if this is not required by the IMO. The sewage treatment plant should be suitably sized in order for gray water to receive the same treatment as sewage. However, some sewage treatment plants connect gray water to the last disinfection stage. This means there is non-conformity to MEPC 227(64).

Such sewage treatment plants gain a commercial advantage by “treating” gray water without increasing their sizing. The approved disinfection contact time is invalidated. Gray water pollution is disguised as sewage treatment plant effluent, thus causing poor performance. In an ironic twist, many new ships are awarded for carrying this non-conformity.

Marine Executive featured this issue back in 2015. The latest indication is that a group of Notified Bodies assigned by European Member States are starting to review the subject, with recommendations expected early next year.

Sewage treatment plant recirculation during a performance test

MEPC.227(64) requires sewage treatment plant influent to represent raw sewage and prohibits recirculates generated from the sewage treatment plant to be returned to its influent. Otherwise, the influent no longer represents raw sewage, and the plant’s capacity would be overrated. Yet, some conformity assessment bodies have approved precisely these plants. As of now, one market surveillance administration has mobilised its internal procedures to review one possible candidate. 

In racing to the lowest level of functionalities and cost, some approved sewage treatment plants do not even conform to basic environmental science and engineering principles.

Chlorination disinfection without de-chlorination

Chlorination relies on a time-concentration relationship, which has for decades been well understood and documented. With a typical contact time of 30 minutes, a chlorine dose of 5-15 mg/l can effectively disinfect biologically treated effluent (Wastewater Engineering Treatment and Reuse, 4th edition, Metcalf & Eddy). For this concentration to be reduced to below the 0.5 mg/l limit, a de-chlorination step is a must. Yet, the de-chlorination step is absent in some chlorine-based sewage treatment plants. Considering that almost all chlorine-based ballast water treatment plants incorporate a de-chlorination step for less arduous disinfection duty, the inconsistencies between approvals for sewage treatment plants and ballast water treatment systems are beyond comprehension. 

“No-sludge” production

Some sewage treatment plants do not have provisions to discharge sewage sludge. Such no-sludge claims do not conform to environmental science, and are simply untrue. One such sewage treatment plant was even certified to remove Total Phosphorus, in which case the “removed” Total Phosphorus has to disappear into nowhere. Recently, one Notified Body announced suspension of the MED approval certificate of a sewage treatment plant, subject to further reviews. Despite this development, there are still other magic boxes with no-sludge claims, approved by other Notified Bodies, and the issue may still persist beyond the European efforts.
These non-conformities and magic boxes have been finding their ways into new ships in their hundreds. Ship yards and owners are entitled to gain further insights to these issues. The credibility and accountability of the approval regime is at stake.

Silver lining

Having seen the realities, Alaska regulators took actions starting in the early 2000s. They introduced new rules and the Commercial Passenger Vessel Environmental Compliance (CPVEC) program to the large cruise ships trading in Alaska water each year. Gray water, being more polluted than sewage, was also brought under the same rules.

Some vendors, including Wärtsilä Water Systems Ltd, developed a new generation of technologies known as Advanced Wastewater Treatment Systems (AWTS). AWTS carry the same certificates as sewage treatment plants and marine sanitation devices do, but differ from them in that the ships’ discharges are sampled and monitored under CPVEC, with the results published in the public domain. The success of this program has been praised by all stakeholders, in particular by the cruise industry.

Alaska waters remain to this day as the only marine environment in the world whereby enforcement is not only in place, but is also effective.

The way forward

During MEPC 71, under pressure from Norway and the Netherlands, the Committee agreed to address inconsistencies in the application of its sewage Guidelines and “mal-functioning” sewage treatment plants. However, enforcement of the environmental regulations - as has happened in Alaska and the rest of our society - is still out of reach for MARPOL Annex IV. It has a long way to go.

Nevertheless, improving the sewage Guidelines can still be a very positive and significant step. It may be important to introduce process monitoring requirements, including on-line monitoring for sewage treatment plant effluent turbidity and effluent flow rates. This would provide good visibilities for the marine industry to get a handle on sewage pollution control. Such visibilities could pave the way for a future regulatory framework capable of closing the gap between the rules and the realities, that will be evidence based, practicable, and sustainable.

However, improving the Guidelines is meaningful only when they are conformed to. Non-conformities cause inconsistencies and poor performance status. To understand the root causes is essential in order to prevent reoccurring. To address these non-conformities is critical, even though the issue may be thorny since they are already approved by the conformity assessment bodies. But without corrective actions, tougher Guidelines would merely incentivise more non-conformities, in particular on new build ships.

At present, shipowners and yards have vessels carrying non-conformities and poor sewage treatment plant performance status. At the same time, the conformity assessment bodies burden themselves with these magic boxes that can be easily challenged, equipment vendors continue their race to the lowest levels of functionalities and cost, and regulators end up with regulations that are not effective.

Most importantly, the marine environment continues to be polluted by discharges of sewage from ships under the IMO’s type approval regime. No one wins. While improving the Guidelines is a constructive step, ultimately effective enforcement is the key to making MARPOL Annex IV a win-win for all. And only the collective efforts of the Member States can make it happen.

Wei Chen is Future Programme Development Manager at Wärtsilä.

The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.