Connecting ship’s food waste to the sewage or greywater system and thus the sewage treatment plant of the vessel causes non-compliances to the MARPOL Convention and national biosecurity laws.

Recognising the pollution impact and the biosecurity risk of ship’s food waste, the IMO’s MARPOL Convention regulates its disposal, recording, and reporting requirements. MARPOL Annex V stipulates three legal food waste disposal routes. Ship’s sewage or greywater system is not one of them.

Connecting food waste to the sewage system of a vessel also violates Annex IV (sewage) of the MARPOL Convention. When waste streams of different MARPOL Annexes mix, the most stringent requirements shall apply. However, the discharges of the affected sewage and grey water systems are not labelled or managed as food waste. Food waste is made to disappear.

Food waste from international ships also carries biosecurity risks such as plant pests and diseases such as swine fever, rabies, and avian flu. Many countries have strict biosecurity regulations to protect their agricultural interests (table below). These national laws often prohibit the discharge of food waste into the territorial waters. They also stipulate strict procedures for storing, transporting and disposing such food waste when landed ashore. However, these national laws are violated when food waste is discharged into national waters and municipal sewers in disguise as effluent, grey water, sewage, or sewage sludge. Such non-compliant piping design is a non-compliance which can last the life of a ship. Wrongly approved by classification societies, it has been spreading across the shipping industry over the past decades.

The consequence can be far-reaching. The practices have violated the international and national laws, contributed to the poor performance status of sewage treatment plants onboard, undermined the credibility of the approval regimes, forced ship masters to authorise false entries in the Garbage Record Books, and damaged the reputation of the maritime industry especially when the aviation industry has managed to stay in compliance with the national biosecurity laws.

So far, no relevant authorities have aired their opinions or taken corrective actions. The shipping industry continues to be misguided by certain class rules. It may take an awful long time for the regulations to be effectively implemented. But there are signs of constructive change initiated by the industry.

The Cruise Lines International Association (CLIA) released an informative report in 2021 on the cruise industry’s commitment to responsible tourism practices and environmental technologies. It includes a case study of a liquid derivative generated from the food waste processing equipment. It stated that ‘CLIA members treat this liquid waste as food waste as governed by MARPOL Annex V Regulations 4 and 6 which set forth the maximum food particle size, discharge rates, and distance from the nearest land or ice-shelf when discharged’ [1]. This is a concrete step towards future-proofing compliant system design, installation, and operation.

EU - ‘International catering waste (ICW)’ or Category 1 material of animal by-products.To be stored, transported, processed and disposed in accordance with the requirements for Category 1 material.;
US - ‘Garbage regulated because of movements outside the United States or Canada… within the previous 2-year period’.Regulated garbage must be contained in tight, leak-proof covered receptacles during storage… and unloaded ‘under the direction of an APHIS inspector to an approved facility for incineration, sterilization, or grinding into an approved sewage system…’Title 7 CFR 330.400 -330.403; and
Canada - ‘Ship’s refuse’ from outside Canada and the US.To be stored in closed leakproof containers and removed at the first port of entry to approved facilities for incineration or sterilisation, or landfills for deep burial.,_c._296/page-5.html#docCont
New Zealand - ‘Risk goods’.While within New Zealand territorial waters, risk goods shall be secured onboard… and kept in a leakproof, vermin-, insect- and bird-proof container or room…Craft Risk Management Standards for Vessels, MPI, New Zealand Government, 2018. Guidance Document to the CRMS for Vessels, MPI, New Zealand Government, 2017
Australia - biosecurity wasteAustralia has requirements for the collection, storage, and transport of this waste for acceptable disposal methods.Australia Biosecurity Act and Management of biosecurity risks associated with transhipped ships stores. No. 2014–15/08, 2016.

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Having a sewage mixing tank with built-in aeration upstream of a sewage treatment plant onboard ships is not just a good practice, but a strategic move that ensures the future-proofing of new ships. The benefits of this setup are not only numerous but also significant, as we’ll delve into.


A mixing tank, positioned upstream of a sewage treatment plant, is a crucial component in the sewage treatment process. It balances different waste streams, buffers peak flows, minimizes health and safety risks, and ensures a stable treatment process. It also helps the sewage treatment plant meet its certified hydraulic capacity and comply with existing and future rules.

A mixing tank is distinct from a designated sewage holding tank, which is approved by the authority and stipulated in a ship’s International Sewage Pollution Prevention Certificate (ISPPC). The capacity of a designated sewage holding tank is determined by the Administration, taking into account the operation of the ship, the number of persons on board, and other relevant factors. Typically, a sewage holding tank provides black water a hydraulic retention time (HRT) of 8-10 days on cargo ships, and 1~3 days on large passenger ships. In comparison, a mixing tank is based on industry know-how (not by approval) and takes into account the sewage treatment plant hydraulic capacity and the daily flows of all waste streams (not just black water) to be treated. Typically, it provides a hydraulic retention time (HRT) of about 1 day on cargo ships or less when the sewage treatment plant has a higher peak flow factor or when the flows are less fluctuated.


One of the most crucial functions of an aerated mixing tank is managing sewage treatment plant influent flows (Qi). By capturing all sewage treatment plant influents in one place, the tank ensures that all flows and characteristics are accounted for, a fundamental aspect of effective sewage treatment.

It should be noted that gravity or vacuum (e.g. integrated ejectors) influent into a sewage treatment plant can be problematic because it is impractical to measure its flow or to take a representative sample. Likewise, on many ships, grey water is connected to the last stage of a sewage treatment plant, causing non-conformities and poor sewage treatment plant performances.

Therefore, it is a good practice to capture all waste streams in an adequately sized mixing tank to buffer the peak flow conditions and balance the different characteristics.


The recommended size of the sewage mixing tank cannot be calculated with a general rule. It depends on several factors, which have to be considered and weighted individually for each project. HAMANN AG takes into account the following aspects:

  • Expected total volume of black water and grey water per day
  • The recommended mixing ratio between black water and grey water of about 1:5
  • Peak volumes, e.g. in the mornings and evenings
  • Run time of the sewage treatment plant feed pump should be about 1 hour before it is stopped again to avoid excessive start/stop operation


A sewage mixing tank needs to be mixed to provide a homogenous influent to the sewage treatment plant. Mixing the tank using aeration is beneficial, if not essential.

Anaerobic conditions in sewage systems can lead to the production of toxic and flammable gases. This is an unacceptable hazard within the confined boundaries of a ship. Various safety features can be incorporated into the design and operation of a sewage system. Having barriers between the sewage gases such as the water traps, ventilation of the tanks, etc., should be considered as secondary measures of protection: the prime safety feature is to prevent the production of hazardous gasses within the system in the first place. The design of a tank holding sewage may include features for maintaining an adequate oxygen level in the liquid so as to eliminate anaerobic conditions. The IMO Maritime Safety Committee issued “Guidelines for the operation, inspection and maintenance of ship sewage systems” in MSC/Circ.648 (MEPC 53/16 contains MSC/Circ. 648 and can be downloaded from the IMO Docs Archive)

In addition, septic conditions of sewage treatment plant influent, represented by low oxidation-reduction potential (ORP), can negatively impact on the optimum coagulation and flocculation process setup and the efficacy of biological treatment processes, affecting treatment stability and performance.


Having an aerated mixing tank of adequate capacity is just a start. For a sewage treatment plant to perform, its influent flow (Qi) must not exceed its designed and certified average and maximum hydraulic capacities. The feed system connecting the mixing tank and the sewage treatment plant, which consists of feed pumps, level sensors, and control logic, must be suitably designed and commissioned for each installation.

An sewage treatment plant designed to take an even influent flow (peak factor = 1) is often fed by an integrated progressive cavity pump completed with suitable control logic as part of the approved sewage treatment plant. This, albeit at a higher cost, ensures the sewage treatment plant can perform within its approved hydraulic capacity.

However, not all sewage treatment plants are designed, approved, or supplied with such considerations in mind. A centrifugal pump can deliver an instantaneous flow of 1~2 magnitudes higher than the certified maximum sewage treatment plant hydraulic capacity. A mixing tank can be turned into a sewage reservoir, flushing the sewage treatment plant with huge batches on automatic level controls. These feed regimes do not represent the type of approved conditions of the sewage treatment plants and constitute serious non-conformities that rid the sewage treatment plants of the possibility to perform.

A sewage mixing tank itself does not necessarily warrant satisfactory sewage treatment plant influent conditions. It needs technical know-how.


Sewage treatment plant influent flow (Qi) is essential for the compliant operation of dilution machines that are certified with a Qi/Qe <1. Likewise, influent flow (Qi) and concentrations (Ci) are essential for sewage treatment plants approved with percentage nutrient removal standards.

The current IMO guideline for sewage treatment plants, IMO MEPC.227(64), also requests that ‘the sewage treatment plant influent (Qi and Ci) should be assessed without the contribution of any return liquors, wash water, or recirculates, etc., generated from the sewage treatment plant’. This means that a mixing tank should not receive sewage treatment plant sludge or wash water whenever assessments of Qi and CI are required, whether it is during a sewage treatment plant type-approval test or compliant operations.

These existing requirements are poorly implemented or enforced, leading to multiple type-approved non-conformities that are left unacknowledged or corrected. The situation might change in the near future when the IMO’s MARPOL Annex IV is under revision to ‘confirm the lifetime performance of the sewage treatment plants’.

An aerated mixing tank is set to become increasingly important in the compliant operation of sewage treatment plants on ships. Regulatory and technical know-how are essential to getting it right from now on.

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IMO and EU regulations and certification for sewage treatment plants continue to be a major source of confusion, misunderstanding and misinformation. In this guide, we try to give an overview of the facts and to clear some misconceptions.


  • IMO certificates and EC Type Examination (Module B, MED-B) certificates are two very different things: The IMO certificate is needed to prove the treatment performance of the plant; the MED-B certificate with the wheelmark is needed to be able to market the product within the EU.
  • The note on MED-B (Module B) certificates that a plant complies with IMO regulation only states that the manufacturer was able to convince the certifying body of a compliance. It does not state, that the plant has been tested by the certifying body to IMO standards.
  • MED-B certificates are not substitutes for IMO certificates
  • Within the EU, both MED-B and IMO certificates are needed, outside of the EU, only IMO certificates are needed
  • The weakness of the EC Type Examination (Module B, MED-B) certification process seems to be the lack of verification of information handed in by the applying manufacturer



IMO MEPC regulations aim to ensure that sewage treatment plants installed on ships meet certain performance standards in terms of effluent quality.


  • IMO certificates are valid worldwide. They confirm that the STP complies with the required effluent values.
  • To receive IMO certification, a 10-day test in operation has to be successfully completed, during which the influent and effluent quality is constantly monitored. In addition, the electrical test to MEPC.107(49) (vibration, temperature and humidity) has to be cleared.
  • An IMO certificate carries the following headline (may vary slightly):


Issued under the authority of the government of [member state]”

  • The underlying regulation are currently IMO Resolutions MEPC.227(64) excl. or incl. Section 4.2 (MARPOL Annex IV special areas) and MEPC.107(49).
  • Every IMO member state has one authorized certifying body that can issue IMO certificates, in Germany the BG Verkehr, in Norway is a state authority directly. The manufacturer has to use the certifying body in the state where he has his registered head office.




The EU Marine Equipment Directive (EU-MED) aims towards the harmonization of the implementation of standards set by international bodies like IMO among EU member states to ensure the smooth functioning of the EU internal market.


  • EC Type Examination Certificates (Module B) only confirm that a product may be placed on the EU internal market. They are of no value outside of the EU.
  • To receive a MED Module B certificate, has to document that the STP complies with IMO regulation, but no separate test in operation is carried out to validate the information given by the manufacturer.
  • A MED Module B certificate carries the following headline and carries the wheelmark sign:

(Module B)

  • The underlying regulation is currently Marine Equipment Directive (MED) 2014/90/EU, as amended by Commission Implementing Regulation (EU) 2018/773.
  • Every EU member state has authorized one or more notified bodies that can issue MED certificates, in Germany that is among others BG Verkehr and DNVGL. The manufacturer can use a certifying body of his choice that is authorized by the state where he has his registered head office.
  • The key certificate is MED Module B (MED-B, Type approval). It needs to be complemented with either one of the modules D, E, F – an MED-B alone is not enough to place a product on the market.


All HAMANN sewage treatment plants and advanced wastewater treatment systems are certified according to IMO and EU-MED regulations.

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This is part 2 of our short series on the ongoing discussion on conditioning water. Please click here to read part 1.

Sewage treatment plants (STPs) are tested and approved using an influent of 500 mg/l TSS. The concentration of ship’s vacuum sewage . It needs to be conditioned (or reduced) using freshwater or greywater to allow STPs to deliver their treatment performances proven during the tests. Currently, not many ships, except a few dozen cruise ships trading in Alaska, concern about STP discharge performances, because the regulations have not asked them to. But this will change when the IMO’s sewage rules are now being revised to confirm the lifetime performance of the STPs (see our four-part series on the revision of MARPOL Annex IV). Environmental rules need to be effective and practicable. Approving STPs at 500 mg/l TSS and using them at 2,000 mg/l is neither effective nor practicable. Conditioning water was introduced under this revision work in 2020 (PPR 8/7) so that its usage can be compliant, consistent, and prevented from abuse.

Conditioning water aims to reduce concentrations of vacuum sewage for compliant operation of STPs in accordance with their approved conditions. However, dilution also aims to reduce concentrations, but it aims to pollute in disguise. Tell them apart on paper and in practice is crucial.

The IMO tried to restrict dilution machines by tightening their performance standards by applying the dilution compensation factor, Qi/Qe, or influent flow over effluent flow, of less than 1 (MEPC 227(64), 2012). The intention is good, but the outcome is ineffective. Because there are no requirements to monitor and record real-time Qi/Qe factor to ensure compliance over the lifetime performance of these dilution machines. Flow meters, taken for granted in land-based wastewater industry, are perceived as burdensome for ships. Without flow meters, dilution machines can use far more dilution water than its approved conditions without anyone knowing.

IMO’s MEPC subcommittee PPR (Pollution Prevention and Response) tries to tell the difference between conditioning and dilution by their definitions. Dilution is defined as dilution water introduced after the STP influent sample point to reduce concentrations. Conditioning water is defined as freshwater introduced to reduce the concentrated sewage to the test conditions of, say, 500 mg/l TSS (PPR 10/12). These definitions are helpful for type tests but less so during STP operation because there are no regulatory requirements to mount influent sample points to dilution machines approved with Qi/Qe < 1. Also, to analyse influent TSS concentrations as a way of verifying genuine use of conditioning water can be burdensome if at all practicable.

A dilution ban was proposed in 2018 (PPR 6/14) by removing Qi/Qe <1. An allowance of up to 5% of Qe was introduced for essential STP services such as cleaning, polymer make-up, etc. The initiative could simplify implementations, enhance effectiveness, and reduce burdens. After all, dilution should not be the solution to pollution. However, dilution machines were proposed to be reinstated in 2021 (PPR 9/14), and again, with no requirements to monitor or record real-time Qi/Qe to confirm the lifetime performance. Making compromises while maintaining the effectiveness and practicality of the rules requires knowledge and skill. The revision work is ongoing.

HAMANN AG STPs do not use dilution (Qi/Qe = 1). HAMANN AG recommends the use of grey water to condition concentrated vacuum sewage on board because this provides added environmental credentials, and it is there already – so no reason to use precious fresh water. To size the STPs correctly to accommodate grey water is important, and we will discuss this in a separate article. We proactively engage with regulatory development and do our best to help our customers with compliant and future-proofing solutions.

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This is part 1 of our short series on the ongoing discussion on conditioning water. Please click here to read part 2.

The performance standards for treating ships’ sewage are comparable to that of municipal sewage on land. But ships’ sewage presents a more significant challenge because it can be far more concentrated.

The differences between sewage on ships and on land start with their definitions. Ships’ sewage, regulated by the IMO’s MARPOL Convention, refers to drainage from toilets and urinals. It can be highly concentrated when collected by vacuum systems. The less-concentrated greywater is collected separately from dishwater, showers, laundry, baths and washbasins. Unlike sewage, greywater is not regulated. Untreated greywater can be discharged from ships without restrictions. Hence, the concentration of ships’ sewage can be very high. When collected by the ships’ vacuum systems, it can be 10 times more concentrated than municipal sewage that contains greywater, groundwater infiltration, and rainwater.

However, the availability of sewage treatment plants (STPs) on ships is only proven using a mixture of municipal sewage and primary sludge with a target concentration much lower than that of vacuum sewage. For a ship choosing to treat vacuum sewage alone, a compliant STP operation is compromised because the STP cannot be operated in accordance with its approved conditions. The discrepancy is significant.

A historical lack of compliant discharge sampling and enforcement has masked the detrimental impact of the discrepancy to STP performances. But the situation is changing. With a vast majority of ships found to discharge ‘virtually raw sewage’ from their STPs, the IMO is determined to improve MARPOL Annex IV and its Guidelines to confirm the lifetime performance of the STPs.

To substantiate the availability of STPs their approval conditions shall be adhered to during compliant STP operations. Vacuum sewage must be conditioned by freshwater or greywater to bring the concentration closer to the STP test conditions.

The practice of conditioning the concentrated sewage for treatment has existed on ships for decades. The regulations are now trying to catch up. The Subcommittee on Pollution Prevention and Response has proposed a conditioning water definition since 2020. PPR 10/12 (2023) defines it as freshwater introduced to condition the concentrated sewage onboard to the similar concentrations as the type-test conditions. Whilst the concept is clear, the practicality of verifying conditioning water on board according to such a definition may need to be further reviewed.

It is also important to note that conditioning is not dilution. Conditioning water should not be confused with dilution water. The regulatory discussions on defining and differentiating the two are ongoing. This topic deserves separate updates and reviews.

HAMANN AG has always advised its customers to condition the concentrated sewage using greywater or fresh water. When using greywater, STPs are sized to accommodate the greywater flows according to customer specifications or industrial norms suggested by the classification societies. Treating greywater allows a ship to gain further environmental credentials under many class notations. When using fresh water for conditioning, the STPs are sized to accommodate the combined vacuum sewage and freshwater flow to match the typical sewage flow of a gravity system.

HAMANN STP systems are also complimented by auxiliary systems such as aerated mixing tanks and automated transfer systems for buffering and balancing sewage and greywater mixture for a safe, compact, stable, and effective treatment.

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MARPOL Annex IV is currently under revision by the IMO. We cover the most relevant topics in a multi-part article series:




The industry is getting familiar with record books that already exist for oily water separators (OWS), garbage, and ballast water management systems (BWMS), etc. The ongoing revisions of MARPOL Annex IV have recognised the need to introduce a sewage record book to ships’ sewage management. Under the newly proposed regulation 11A.3 (PPR 10/12), applicable ships shall be provided with a sewage record book approved by the administration. The proposed sewage record book is stipulated under the newly created implementation guideline.


In principle, the sewage record book should:

  • record each discharge (or each fixed 24-hour period for automatic discharges) into the sea or to a reception facility, including date, time, location and the estimated or measured amount discharged or incinerated, etc.;
  • record turbidity or other indicative parameters of the effluent from the sewage treatment plant;
  • record all maintenance, calibrations, shut-down/turn-on, and switching of nutrient removal processes, failures, overflow, laboratory analysis results, etc.
  • be signed by the master of the ship;
  • be kept on board and made readily available for inspection. The records and receipts obtained from reception facilities shall be preserved for at least two years.
  • be made available to authorised officers for inspection.
  • and so on…


Entries to a sewage record book should record the following proposed categories: A) sewage (Regulation 1.3), B) sewage sludge (Regulation 1.17), C) effluent (Regulation 1.16), D) comminuted and disinfected sewage, and E) other (to be specified by the ship, e.g, overflows).

It should be noted that the amount of sewage sludge and its disposals need to be recorded in the sewage record book, and in accordance with the ship-specific sewage management plan. The latest draft templates are available in PPR 10/12.


Whilst many details are to be finalised by the ongoing revision work, the latest development suggests that the sewage record book be introduced to all ships, including ships installed with sewage systems other than sewage treatment plants (PPR 10/WP.1/Rev.1), and all existing ships covered by MARPOL Annex IV (PPR 10/12).


Shipyards and ship owners should understand the potential implications for the existing ships, particularly the current new build specifications. For example, to comply with future MARPOL Annex IV, and to improve investment efficiency, it is plausible to choose sewage treatment plants that separate solids, are fitted with a sewage sludge discharge connection point, and have de-sludge provisions such as a sewage sludge holding tank to suit the intended trade patterns and itineraries. Certified “no-sludge” sewage treatment plants should be avoided or replaced.

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MARPOL Annex IV is currently under revision by the IMO. We cover the most relevant topics in a multi-part article series:




The ongoing revisions of MARPOL Annex IV have recognised the need for a sewage management plan (SMP) (PPR 7/16, MEPC 74/14) to assist the crew with handling, treating and storing sewage and other related wastewater by the sewage systems on board, taking into account the ship-specific conditions (PPR 7/INF.21).

The proposed SMP is stipulated under the newly created Implementation Guideline under the MARPOL Annex IV and covers different aspects of sewage management on board (PPR 10/12). It will be inspected and verified during the initial, annual, and renewal surveys.


A sewage management plan should include the following:

  • The designated person(s) in charge of carrying out the plan;
  • The Health & Safety (H&S) aspects;
  • The description of the sewage treatment plant installed on board, including its operation and maintenance manuals, system diagrams, sample points, hydraulic and organic load, all wastewater streams entering the sewage treatment plant, modification lists and a record of their approvals, etc.
  • A maintenance plan covering the instructions, schedule, spare parts and their service providers, service to the sewage treatment plant and its monitoring devices and flow meters, and guide to all maintenance work activities, including the prevention of overflows;
  • Procedures for monitoring the performance of the sewage treatment plant, such as sampling point and sampling procedures, and plans for indicative monitoring of STP effluent, performance testing, and sampling plan for other specific (analysis) checks;
  • Procedures for the handling and disposing of sewage sludge and related residuals of the sewage treatment process;
  • Procedures for handling, storing, and using chemicals and biological additives;
  • A plan for start-up, standby and shutdown of the sewage treatment plant;
  • Training and familiarisation with the use and processes of the STP;
  • And so on…


Yes, it looks like it. Whilst the introduction of the sewage management plan was initially aimed at future new ships and existing ships having a new sewage treatment plant, the latest discussions suggest the sewage management plan be introduced to all ships, including ships installed with sewage systems other than sewage treatment plants (PPR 10/WP.1/Rev.1), as well as all existing ships covered by MARPOL Annex IV (PPR 10/12).


In brief, the introduction of a sewage management plan is a positive step forward, and it should provide better visibility to all stakeholders on how ship-specific sewage systems should be managed on board. Shipyards and ship owners should understand the potential implications to existing ships, including the current new build ships in the pipeline. Those wrongly certified sewage treatment plants that “pretend” not do produce the inevitable sewage sludge or do not even have a sewage sludge discharge connection point will not be compatible with the new requirements, even on paper.

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MARPOL Annex IV is currently under revision by the IMO. We cover the most relevant topics in a multi-part article series:




Land-based wastewater treatment works (WWTW) serving our cities, towns, and villages turn raw sewage into clear effluent. They also produce an inevitable by-product called sewage sludge. All treatment methods produce sludge at some point, even if it is purely natural degradation. A major part of wastewater science and engineering concerns the characteristics, management, and sustainable recycling and disposal of sewage sludge.


But when it comes to sewage treatment onboard ships, the notion of sewage sludge does not exist in the IMO’s regulations – the MARPOL Annex IV. Without effective compliance monitoring, sewage sludge produced by the sewage treatment plants (STPs) has mostly been discharged into the STP effluent, making it ‘virtually raw sewage’ (MEPC 64/23, MEPC 67/8/1, MEPC 71/INF.22). So, to recognise, define and manage sewage sludge is essential.


Relevant proposals under the revisions of MARPOL Annex IV and its guidelines include:

  • the introduction of a sewage sludge definition,
  • recording of sludge production during STP type approval tests,
  • entries on sludge in the Sewage Record Book (SRB),
  • management practices regarding sludge in the Sewage Management Plan (SMP),
  • the introduction of a sludge holding tank and its moderate discharge rate,
  • and alas, the Baltic countries can prohibit the discharge of sludge in MARPOL Annex IV Special Areas,
  • and so on …

The scene is set, but the debate is far from over.

The necessity of a dedicated sludge holding tank reflected in the IMO sub-committee Pollution Prevention and Response (PPR 8/7 and PPR 9/14) has now been rolled back in PPR 10/12, without offering a compliant solution. Many hope the old ways of managing a sewage treatment plant are still OK for the future. This is expected when the root cause of the poor performance status of sewage treatment plants is left unexamined.


Some sewage treatment plants are designed to return their sludge to their feed tank. This does not conform to the IMO’s guidelines and mounts to cheating. Multiple sewage treatment plants are wrongly approved. Taking the non-conformity to the extreme, a biological sewage treatment plant is approved without a bioreactor. For ships without a dedicated sludge holding tank, sludge can only go to the sewage holding tank feeding the STP, making a compliant assessment of STP influent without the influence of STP return flows difficult if not impossible. But unrecognised and uncorrected, these certified mistakes can affect the decision-making of new rules.

There are also ‘no-sludge’ STPs which do not have features to separate solids. These science-defying ‘no-sludge’ STPs become a symbol of type-approved ‘magic boxes’ and a tipping point for the rule change. In 2017, Norway declared ‘all sewage treatment systems produce sewage sludge’ (MEPC 71/14/2). In 2019, in a courageous move, the UK MCA withdrew the MED certificate of a sewage treatment plant on the market. In 2020, the affected ships were advised to take corrective actions. But none of these has resonated with the type-approval regimes. Other ‘no-sludge’ STPs remain certified. No one has asked how a science-defying STP can be successfully approved in the first place to prevent reoccurrence. For example, if an STP has its effluent sampled during its freshwater flushing cycle, how can a guideline stop it? When a batch-operated STP completes its ‘no-sludge’ cycle in 30 minutes, how can a prolonged test period from 10 to 30 days help?


For shipyards and shipowners wanting to be risk-avert and future-proofing, the best advice is to avoid sewage treatment plants which claim not to produce sludge and to have a dedicated sludge holding tank on board.

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MARPOL Annex IV is currently under revision by the IMO. We cover the most relevant topics in a multi-part article series:




Controlling ships’ sewage pollution is one of the first environmental initiatives stipulated under the MARPOL Convention. 50 years after its creation MARPOL Annex IV and its guidelines are undergoing a major overhaul, including

  • the introduction of the Sewage Management Plan (SMP) and Sewage Record Book (SRB),
  • the introduction of commissioning tests for sewage treatment plants during initial surveys,
  • the introduction of performance tests for sewage treatment plants during renewal surveys,
  • the removal of comminuting and disinfecting systems (CDS) as an alternative to proper sewage treatment plants,
  • the requirement to integrate an effluent monitoring unit,
  • the revisions of type-test guidelines for sewage treatment plants,
  • the revisions of discharge standards during type tests for and operations of sewage treatment plants,
  • the introduction of a definition of sewage sludge and its management practices,
  • the review of the moderate discharge rate,
  • the reception of sewage by the port facilities,
  • and so on …

Whilst most of the proposals are targeted towards future new ships, the Sewage Management Plan (SMP) and the Sewage Record Book (SRB) could be made applicable for all ships, including existing ones, under the revised MARPOL Annex IV.

Considering the breadth and depth of the revisions, the work is a mammoth task. But how did it start and why is it necessary?


There is no compliance monitoring requirement under MARPOL Annex IV. In 2012, the Dutch authorities commissioned a Rotterdam-based laboratory to take effluent samples from sewage treatment plants from a few dozen visiting ships. The laboratory collected more samples up to 2016. The results showed that 97% of sewage treatment plants did not meet the required standards and discharged ‘virtually raw sewage’. The Netherlands introduced the findings to the IMO’s Marine Environment Protection Committee MEPC (MEPC 64/23, MEPC 67/8/1, MEPC 71/INF.22). The poor performance status of sewage treatment plants and the huge gap between the rules and realities have reached the tipping point for a rule change.  Issues related to sewage treatment plants drove Norway to propose a revision of the guidelines for sewage treatment plants (MEPC 71/14/2), which the MEPC approved in 2017.  This marked the start of the revision work undertaken by the MEPC sub-committee Pollution Prevention and Response (PPR). In 2018, the PPR’s Correspondence Group (CG) proposed expanding the work to include the revisions of MARPOL Annex IV (PPR 6/14) to confirm the lifetime performance of sewage treatment plants. It received overwhelming support and was approved in 2019 (MEPC 74/14). The revision work continued by the subsequent Correspondence Groups (PPR 7/16 in 2019, PPR 8/7 in 2020, PPR 9/14 in 2021, and PPR 10/12 in 2023), which is now scheduled to complete in 2025.


The Netherlands are not the first to learn about the poor performance status of sewage treatment plants. Many knew but decided to look away because MARPOL Annex IV in its current version does not require discharges from sewage treatment plants on ships to meet any numeric standards. Until now, IMO regulations solely specify numeric standards for the type approval tests of sewage treatment plants. Only the State of Alaska introduced effective State Laws after finding the real performance status of sewage treatment plants on board cruise ships in 2000. There are valuable lessons to be learnt.

So, are decision-makers at the IMO moving closer to a more effective MARPOL Annex IV to confirm the lifetime performance of STPs, and to narrow the gap between rules and realities for its shipping industry?

Time will tell.

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Certificates for sewage treatment plants: How to recognise non-conformities and magic boxes


Certification of sewage treatment plants without conformity to the rules is a real problem as the certification process does not always live up to the required standards. As regrettable as it is, it is a fact. We take a closer look at the problem and show you how to determine whether a plant is legitimately carrying its certificate or not.

Many ships have a sewage treatment plant (or marine sanitation device in U.S. terminology) that is approved under the inter-related regulatory regimes such as the IMO’s MEPC Guidelines (IMO certificates), the European Marine equipment Directive (EU-MED certificates), and the Code of Federal Regulations (CFR) of the US (USCG certificates), who is not party to the IMO’s sewage rules. The approval authorities are the gate keepers responsible for testing and evaluating candidate technologies of various features and claims. A successfully type approved sewage treatment plant is the basis for a series of models with the same design principles covering a wide range of treatment capacities.

However, some sewage treatment plants on the market are certified to the current standards of IMO MEPC.227(64) and/or the U.S. 33 CFR 159 despite obvious non-conformities or even scientific impossibilities. Ship owners and operators should be aware of this fact. Having a sewage treatment plant on board which is not only certified, but which actually complies with the rules and performs as required by the rules can save them a lot of trouble if their pollution prevention equipment is inspected and tested by the authorities.

Non-conformities and “magic box” issues are related to these four areas:

  • Handling of sewage sludge
  • Use of chlorine
  • Recirculation
  • Handling of greywater

The good news is that it is fairly easy to determine whether a particular plant is in breach of the rules. By asking the following four questions, you will get straight to the heart of the matter.


A sewage treatment plant that is claimed not to produce sludge is the classic example for what we call a “magic box”. This is not only a certification without conformity to the rules, it is a scientific impossibility, as sludge is an inevitable by-product of sewage treatment (further reading here). These magic boxes do not have the technologies to separate the sludge that develops in the treatment process. They cannot perform no matter how well they are operated. To identify the magic box is easy: Simply check if there are any de-sludge instructions and provisions, e.g. that a dedicated sludge holding tank is required.


Chlorination can be effective in disinfecting biologically treated sewage. But to meet the residual chlorine limit of 0.5mg/l, a de-chlorination step is necessary. However, many sewage treatment plants have been certified without de-chlorination –another scientific impossibility, given the level of chlorination and the short contact time (further reading here). To identify this magic box, check whether de-chlorination is provided for in the treatment process.


Recirculating sewage sludge back to a treatment plant’s inlet is explicitly prohibited by the Guideline for type approval tests (see IMO MEPC.227(64) Paragraph 5.2.1 and 5.2.3, and Figure 1  and 33 CFR 159.121 Paragraph (c) and (d). Also further reading here) It can artificially inflate treatment capacity, and, by replacing raw sewage with sewage sludge that has already been through the treatment process, invalidate the challenging characteristics of raw sewage. To identify such a certification without conformity to the rules, ask if the sewage sludge is required to be returned to the sewage treatment plant’s feed tank.


For ecological and economic reasons, it makes a lot of sense to treat grey water together with sewage (black water) in one process. It this is to be done, it inevitably leads to an increased size and capacity of the sewage treatment plant to accommodate the additional wastewater volume. However, in a few cases, certification has been awarded to sewage treatment plants having the grey water connected to the final sewage disinfection stage. This not only renders the disinfection stage ineffective, resulting in poor performance, it is a non-conformity (further reading here). IMO MEPC.227(64) introduced a Dilution Compensation Factor with the intention to rule out sewage treatment plants relying on dilution (e.g. with seawater) as a “treatment principle”. This factor Qi/Qe =1 means that influent volume to the plant must equal effluent volume from the plant. If greywater is sent to the sewage treatment plant downstream of the influent sample point Qi, then the factor is Qi/Qe <1. To identify such a certification without conformity to the rules is easy: If the sewage treatment plant treats grey water but does not receive it at the beginning of the treatment process, it is in breach of the rules.


Be critical! Certification without conformity to the rules is more common than one might think. While it is very unfortunate that an IMO, MED or USCG certificate cannot be trusted blindly, it is fairly easy to check whether a sewage treatment plant is legitimately carrying its certificate or whether it should not have been certified in the first place. When considering the costs that can be imposed on owners and operators if plants fail official tests in real operation, only few arguments remain in favor of installing such a plant, however small or low-priced it may be.

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Food waste and wastewater handling onboard ships

Food waste and wastewater: Don’t mix and match!

Food waste does not belong in the wastewater system of a ship, not only regarding the efficiency of the system but also from a legal point of view. The handling of food waste is strictly regulated for good reasons, and making it disappear with the help of the sewage system is at best non-compliance. Disposing of ship sewage mixed with food waste on land is in breach of national biosafety regulations and poses a potential biohazard to local communities and the environment. Mixing food waste with wastewater on board a ship already violates Annex V of IMO’s MARPOL convention as such.

Food waste can spread plant pests, as well as livestock and poultry diseases such as swine fever, rabies, foot and mouth disease or avian flu. Some countries have strict biosecurity regulations falling under the jurisdiction of the national agricultural authorities, while others have ad-hoc measures.

In short, these rules demand that food waste be either discharged to the sea beyond 3 nm or 12 nm from the nearest land or transferred to the approved facilities for disposal. In the meantime, it must be managed in a way that is leakproof.

Annex V of IMO’s MARPOL Convention permits three routes for a ship’s food waste: to the sea (beyond 3 or 12 nm from the nearest land); to an onboard incinerator; and to port reception facilities. The actual disposal route must be recorded in a Garbage Record Book.

However, on many ships, food waste and its derivatives are sent, in whole or in part, to the grey water system, where it vanishes without a trace. The technical standards of some classification societies permit food waste to be sent to the sewage treatment plant instead of a food waste holding tank as stipulated in section 2.9.2 of IMO MEPC.259(71). Such standards create further non-conformity because sewage treatment plant is not one of the three permitted routes in the Garbage Record Book, which is completed and signed by the ship Master.

The diagram below illustrates the extent of potential non-compliances, looking at a food waste disposer as one of many potential examples.Non-conform food waste handling through wastewater system

Food waste entering the system from the top left corner of the diagram never comes out from the system; it simply vanishes. Tracing the lines to the port reception facilities (PRF), it is clear how food waste gets ashore in disguise, escaping the ‘approved facilities’ intended by the local biosecurity rules. From there it can end up on the agricultural land, carrying biosecurity risks with it.

Read the full article by Dr. Wei Chen on The Martime Executive

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Wastewater treatment terms: Wastewater, sewage, blackwater, greywater, graywater. Which term means what?


What is wastewater, sewage, black water, grey water? There are a multitude of definitions of these terms, which sometimes differ more, sometimes less. These definitions are given by the INTERNATIONAL MARITIME ORGANISATION (IMO), the U.S. ENVIRONMENTAL PROTECTION AGENCY or classification societies. Here you can find out what is meant and what it is all about!


Wastewater is to be understood as a generic term that can include many different substances. Ballast water, bilge water, wash water from exhaust gas cleaning systems, residual water from food waste, drainage from toilets, showers, wash basins, etc. – all this basically falls under the term wastewater.  The term wastewater is therefore not very selective.

When we speak of wastewater in our field of activity, we mean sewage. Both terms are often used as synonyms, although this is not very accurate.


HAMANN AG defines Sewage as follows:

  • Drainage and other wastes from any form of toilets, urinals and WC scuppers
  • Drainage from medical premises (dispensary, sick bay, etc.) via wash basins, wash tubs and scuppers located in such premises
  • Drainage from spaces containing living animals
  • Grey water when mixed with the drainages defined above

Sewage therefore contains black water in any case and can also contain grey water.  The technology of HAMANN sewage treatment plants is designed to always process a mixture of black water and grey water – i.e. sewage in its entirety.

Different definitions of the term sewage come from e.g. the IMO, the EPA and classification societies.  Our definition above is largely congruent with that of the IMO. The IMO, however, formulates the last point more openly and speaks of “other waste waters when mixed with the drainages defined above”. We are more specific here by limiting this point to grey water. One reason for this is, for example, that mixing black water with residual water from food waste (which would fall under “other waste water”) would mean that the rules of MARPOL Annex IV (Sewage) would no longer apply to this mixture, but those of MARPOL Annex V (Food Waste).


The term blackwater is not officially used by regulatory authorities such as the IMO, but it is used by the industry and classification societies. As with the term sewage, the definitions of black water also differ depending on the source.

HAMANN AG defines blackwater as follows:

  • Drainage containing faecal matter, e.g. from toilets
  • Drainage from medical premises if present

Black water is therefore a subcategory of sewerage. The terms blackwater and sewage are often used synonymously, as they both contain faecal matter.  However, HAMANN AG distinguishes between the two due to the higher concentration of faeces, pathogens and other potentially hazardous substances in black water.


First of all this term is spelled in two different ways: Graywater with an “a” is mostly used in American English and greywater with an “e” is used in British English.

HAMANN AG defines greywater as follows:

  • Drainage free from faecal matter or fat, e.g. from dishwashers, showers, wash basins, laundry, etc.
  • Galley water after it has been processed in a grease separator.

The IMO (International Maritime Organization) resolution MEPC.227(64) point 2.7 also includes galley water in its definition of grey water, without making the treatment in a grease separator a requirement. However, due to the high fat content in galley water, separate treatment in a grease trap is important not only for the performance of sewage treatment plants, but e.g. for the entire piping system for sewage and greywater on board, which can otherwise quickly become clogged by fat deposits.


At HAMANN AG, we distinguish between grey water and galley water – unlike IMO, for example. The reason for this is the high fat content in galley water. Fat impairs the performance of sewage treatment plants and can, for example, clog the piping system for sewage and greywater on board due to deposits. That is why we require galley water to be treated in a grease separator so that it can subsequently be considered greywater and processed further in our wastewater treatment plants.

HAMANN AG defines kitchen wastewater as follows:

  • Drainage from kitchen premises via sinks and scuppers located in such premises as it likely contains fat


Sludge, or sewage sludge, is an unavoidable byproduct of sewage treatment. Sewage treatment plants always generate two output streams:

  • Treated sewage, also referred to as effluent
  • Sewage sludge

Treated sewage (effluent) can be discharged into the sea (local/regional discharge bans such as in no-discharge areas must be complied).

Sewage sludge mus first be kept on board in holding tanks. There are two options for the discharge of sludge:


Solid garbage is to be considered separately from wastewater. Garbage is defined in MARPOL Annex V as follows:

“1.1 Garbage means all kinds of victual, domestic and operational waste excluding fresh fish and parts thereof, generated during the normal operation of the ship and liable to be disposed of continuously or periodically except those substances which are defined or listed in other Annexes to the present Convention.”

Garbage also includes food waste. Important: If garbage as defined in MARPOL Annex V, e.g. food waste from the galley, is mixed with sewage as defined in MARPOL Annex IV, i.e. mainly from the toilets, the stricter requirements of MARPOL Annex V apply to the treatment of the mixture.

HAMANN sewage treatment plants are exclusively designed for the treatment of sewage according to MARPOL Annex IV and grey water according to IMO resolution MEPC.227(64) and are not suitable for the treatment of garbage according to MARPOL Annex V.

To find out more on HAMANN sewage and wastewater technology, click here!

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by Dipl. Ing. Olaf Hansen, Head of Technical Department at HAMANN AG

During the last months, the industry of wastewater and sewage treatment technology has seen some notable and partly confusing actions regarding the certification of products. Due to increasing pressure from both the industry and the legislative side, certificates for some sewage treatment plants on the market have been suspended or withdrawn by certifying bodies. This indicates that the certification system needs to be revised. A group of industry players – we are among them – has issued a call for action to improve the guidelines for the certification of sewage treatment plants.

Fortunately, it is the intention of many ship owners to reduce the environmental impact of their ships or just aiming to comply with existing regulations. As a matter of fact, these owners are exposed to a high financial risk when the sewage treatment systems on board do not comply with required effluent standards.

Public authorities around the world are getting more and more sensitive to marine pollution by sewage from ships. A recent example is the Maritime and Port Authority of Singapore (MPA Singapore) which has just issued a circular to all owners, managers and masters of Singapore-registered ships in which they point out that malfunctions of sewage treatment plants have been the most frequent reason for ships to be detained by Port State Control in the year 2019 so far.

If a certificate of an operating plant gets withdrawn because it does not meet the required effluent standards, the ship owner might be forced to have it replaced by another plant with a valid certificate and/or to pay a significant fine.

Considering this, we encourage all ship owners and shipyards to look beyond the certificates and to challenge us – the manufacturers – to deliver sewage treatment systems that perform in terms of effluent quality, ease of operation and reliability. We suggest to broaden the perspective and to take a look at what a sewage management system actually is – or should be.

Sewage management is more than having a sewage treatment plant. A sewage management system needs to be designed properly: From sewage collection via sludge handling to effluent discharge. Sewage treatment plants, black and grey water lines, grease separators, system separators preventing microbes from spreading against the flow from waste water lines into fresh water systems, holding tanks, tank aerations, transfer systems, sludge tanks, sludge processing facilities and performance monitoring are all part of the system. Off course, every project is different. Therefore a sewage management system has to be customized to cover the specific conditions on board.

There are a few key factors that influence how well a sewage treatment system performs. First and foremost, food waste and sewage lines strictly have to be separated. Food waste has a much higher BOD5 (Biological Oxygen Demand) value than raw sewage and therefore heavily increases the organic load on the sewage treatment plant. Grey water from galleys needs to run through a properly designed and dimensioned grease separator before being stored in a holding tank. Grease from galleys not only plugs up pipes and sensors, but also catalyzes the build-up process of hydrogen sulfide which is a toxic and extremely smelly gas inside the holding tanks. In the worst case, sulphuric acid may be formed inside the holding tanks which then attacks the piping and leads to a degradation of the effluent values.

Last but not least, the organic load of the sewage treatment plant should be monitored and controlled in order to maintain the organic design-load which was certified. For example, IMO resolution MEPC.227(64) requires a sewage treatment plant to produce the defined effluent values based on a TSS (Total Suspended Solids) value of ≥ 500 mg/l in the influent. Our sewage treatment plants are designed within these parameters but with a margin allowing for varying load situations. Operating the plant for a longer period above this margin leads to reduced treatment effectiveness and therefore a degradation of the effluent quality. In extreme cases, overloading can lead to a complete breakdown of the plant.

For us as a manufacturer it is evident that we cannot leave the planning and design of a sewage treatment system to the customers and just sell them sewage treatment plants. An important part of our business is consulting. The earlier we get involved in a project, the better the results can be. We often do workshops with ship design companies and naval architects before or at the very beginning of a project to improve the sewage management system as a whole. During the detail construction we closely work together with the shipyard engineers on creating the best possible conditions on board in order to operate our systems properly.

Sewage management is more than having a sewage treatment plant. It’s a properly designed wastewater management system. But even the best system is useless if operation and maintenance overtaxes the crew on board. It makes no sense to expect a ship’s crew to have expert knowledge in the field of sewage treatment like staff of most land based sewage treatment facilities have. Marine sewage treatment plants should be operated safely without years of prior training. We incorporate this aspect in the design of our systems and put a lot of effort in the automation of routine operations, high usability and low maintenance requirements. Only then, a well-designed sewage management system performs well.

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