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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The green conscience is one reason for using high quality and efficient solutions for processing sewage. The other not negligible part is the high risk of being caught with no or false records and losing high value of the vessel. A guide for owners and shipyards on IMO certificates.



This will not become a general discussion about the development of standards in the maritime industry. Even though it is not a discussion about technical aspects or the efficiency of sewage treatment plants in detail. It is also well known to us, that specific STP’s are just on board without any relevant usage over years. Nevertheless, from our point of view it is an absolute must to protect the (maritime) environment with appropriate measures. (more…)

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