Prevention of Pollution by Sewage from Ships – Rules and Realities

Prevention of Pollution by Sewage from Ships – Rules and Realities

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 (STP), via a comminuting and disinfecting unit (CDU) at a distance of more than 3nm from the nearest land, or to the open sea (> 12nm) while en-route at not less than 4 knots. Sewage can also be offloaded to port reception facilities (PRFs). Although the USA is not a signatory country, USCG rules are somewhat similar, with an STP being referred to as a Marine Sanitation Device (MSD).

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 PRFs 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 STP’s have proven to be the popular option.

The Marine Environmental Protection Committee (MEPC) has developed Guidelines on STP effluent standards and performance test specifications (Table 1).

Table 1. MEPC Guidelines on STP effluent standards.

The effluent standards are ambitious compared to those of equivalent coastal discharges ashore. For example, in the Baltic Sea, a ship with 12 passengers is given a Total Nitrogen (TN) target that is the equivalent of a town of > 10,000 people. In addition, sewage from ships can be far more concentrated that from ashore. A better-informed wastewater industry might consider this target to be neither viable nor beneficial.

At the same time, there is no shortage of approved STPs 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, STPs 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 (Figure 1). How the STP is actually functioning, no one knows, nor needs to know. In fact, Annex IV does not request operational STPs on board to meet any concentration limits.

Figure 1. Cycle of environmental regulations, and a lack of it for ships’ sewage rules.

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 5 to 17 small ships each year, a sector plagued with MSDs using sea water for dilution. The Netherlands has taken samples from some 120 merchant ships since 2012. These STP/MSD effluent qualities are shown in data series 1, 2 and 3 of Figure 2. 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). Some STP effluent samples are shown in Figure 3. Washington State in the USA petitioned to designate Puget Sound a no-discharge-zone (NDZ), including treated sewage from MSDs. 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.

Figure 2. Realities of STP/MSD discharges - suspended solids and F. Coliform.


With MEPC Guidelines in place, one would hope that STP 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.

Grey water connection to disinfection stage

Incentivised by the Classification Societies’ green notations, many ship owners have taken the initiative to treat grey water, even if this is not required by the IMO. The STP should be suitably sized in order for grey water to receive the same treatment as sewage. However, some STPs connect grey water to the last disinfection stage (Figure 4), giving a higher Qe than Qi, and yet STP certificates state that the Qi/Qe factor = 1. This means there is non-conformity.

Figure 4. Grey water to the STP’s last stage awarded with ‘greener’ notations.

Such STPs gain a commercial advantage by ‘treating’ grey water without increasing their sizing. The approved disinfection contact time is invalidated. Grey water pollution is disguised as STP effluent, thus causing poor performance. In an ironic twist, many new ships are awarded for carrying this non-conformity.

STP recirculation during a performance test

MEPC.227(64) requires STP influent to represent raw sewage, and prohibits recirculates generated from the STP to be returned to its influent. Otherwise, the influent no longer represents raw sewage, and the STP’s capacity would be overrated. Yet, some conformity assessment bodies have approved precisely these STPs (Figure 5).

Figure 5. STP recirculation during performance tests

In racing to the lowest level of functionalities and cost, some approved STPs 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 STPs. Considering that almost all chlorine-based BWMS incorporate a de-chlorination step for less arduous disinfection duty, the inconsistencies between approvals for STPs and BWMS are beyond comprehension.

‘No-sludge’ production

Some STPs do not have provisions to discharge sewage sludge. Such ‘no-sludge’ claim does not conform to environmental science, and is simply untrue. One such STP was even certified to remove Total Phosphorus (TP), in which case the ‘removed’ TP has to disappear into nowhere. Such magic boxes will never be compliant, no matter how they are operated or maintained. Most ‘no-sludge’ STPs use seawater, and the demand has been fuelled by certain sectors of the marine industry which benefit from this claim.

These non-conformities and magic boxes have found their ways into new ships in their hundreds. 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. Grey water, being more polluted than sewage, was also brought under the same rules. Some vendors, including Wartsila Water Systems Ltd, developed a new generation of technologies known as Advanced Wastewater Treatment Systems (AWTS). AWTS carry the same certificates as STPs/MSDs do, but differ from them in that the ships’ discharges are sampled and monitored under CPVEC, with the results published in the public domain (data series 4 of Figure 2). 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’ STPs. 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 STP 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, ship owners and yards have vessels carrying non-conformities and poor STP 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.

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