Ships over 400 gross tonnage now need to have an oily water separator with an oil content monitor and a bilge alarm to make sure polluted bilge water is not discharged overboard. Yet, the progress has been dogged by issues such as weak guidelines, non-conformities, inadequate equipment, illegal dumping, and the unintended publicity of oil slicks found during rescue missions [1]. Dr Wei Chen, consultant at EN Decision, along with several co-authors, explains where we are now.

This article was submitted to SWZ|Maritime by:

  • Dr Wei Chen, Consultant, EN Decision Ltd, UK
  • Markus Joswig, Head of Marine Department, PIA GmbH, Germany
  • Benny Carlson, Chairman and owner, Marinfloc, Sweden
  • Gillian Lovering, Business Development Manager, Rivertrace Limited, UK
  • Göran Fransson, Global Business Manager, Alfa Laval, Sweden
  • Gavin Gao Yuan, Technical Manager, Maritec PTE. LTD., a member of CTI Group, China
  • Merlin Gehrt, Technical & Sales Development Manager, Deckma Hamburg GmbH, Germany

Bilge water is a mixture of oily water, emulsion, lubricants, grease, detergents, condensate, and spills that accumulate in the lower machinery spaces of a vessel, and it is harmful to the environment [2]. At 0.01 to 13 m3 per day [3], bilge water is low in volume and high in complexity when compared to the so-called produced water of the offshore industry.

The IMO’s MARPOL Annex I Convention requires ships of over 400 gross tonnage to have an oily water separator (OWS) with an oil content monitor (OCM) and a bilge alarm. Overboard discharge shall be stopped when the OCM reading exceeds 15 ppm. OWSs and OCMs are tested and approved to the MEPC.107(49) guideline, which superseded MEPC.60(33) in 2005.

Discharge standard has uncertainties

In the offshore industry, the discharge of treated produced water is subject to compliance monitoring. Routine samples are required to be sent to a lab for regulatory reporting, and field instruments are not used for regulatory purposes. The discharge standard of 30 ppm has proven to be practicable and sustainable.

In comparison, the bilge water discharge standard of 15 ppm is far more stringent, considering its low volume and highly diversified and fluctuating characteristics. Motion, tight spaces, and a lack of skills and man-hours pose further challenges to compliant discharges. Yet, some have been pushing for a tougher performance standard of 5 ppm to echo that imposed on the Canadian Great Lakes. However, in the absence of performance verifications, it is uncertain if the bilge water standard is practicable and proven on most ships. In fact, the uncertainties go back further to the laboratory analytical methods.

Also read: Why grey water on board ships needs to be regulated

Analytical uncertainties

Whilst many lab results indicate largely compliance performance, they are method dependent. The test fluids used to test OCMs are analysed using the ISO 9377-2 method in the labs. The ISO method is more suitable for applications with known oil types. But its minimum detection limit (MDL) remains undefined.

USEPA’s Vessel General Permit (VGP, 2013) stipulates a somewhat different EPA 1664A method. It can detect a wider range of compounds and gives higher results than the ISO method. The differences of the results by these analysis methods are startling [5].

Whilst the ISO method produces perfectly compliant results, the EPA 1664A method produces results that exceed the 15-ppm limit (figure below). What is equally startling is the poor correlations between the OCM readings and the lab results, as indicated by the low R2 values.

Bilge water

The EPA’s self-sampling program has revealed the same findings (see the following figures) [6].

Bilge water

OCM test fluids and procedures also carry uncertainties. But side-by-side comparisons of OCMs from different makers do not seem to have existed on land or on ships.

OCM accuracy is an issue too. The typical accuracy is ± 5 ppm, or at best ± 3 ppm using a loose testing standard (TP12301), may not warrant their sensible applications for a typically more robust 5-ppm OWS approved under cleaner class notations.

These analytical uncertainties are no small matters. They readily undermine the existing performance standard and the aspiration of a tougher one. But in the absence of performance verifications, they are a moot point.

Certified uncertainties

Approved marine environmental products often contain features that do not conform to the requirements of the guidelines or the objectives of the regulations. For example, MEPC 107(49) requires OCMs be designed and constructed ‘to avoid wilful manipulation’, but multiple OCMs are found with vulnerabilities that make them readily manipulatable [7].

The guideline also demands OCMs receive ‘a truly representative’ sample with ‘adequate pressure and flow’, but OCMs on as many as thirty per cent of ships surveyed by one authority fail to raise an alarm when there is no sample flow [8].

The situation of certified non-conformities is not unique to MARPOL Annex I. Under Annex IV, some approved sewage treatment plants are found to defy science [9,10]; under Annex V, international food waste is made to somehow vanish from the approved piping diagrammes [11-13].

These certified non-conformities are often driven by cost saving initiatives. Sadly, once a bad product has gained its unfair competitive advantage, other products are forced to become copycats, whilst other approval authorities are forced to approve them. In a race to the bottom, certified non-conformities under the IMO’s type approval regimes spread across the shipping industry like a pandemic. And there is no vaccine for it.

OWSs that are conforming to the rules can become inadequate too, thanks to the weak guidelines. The eight-hour test period, the stationary test conditions, and the constant and stable test fluids all fail to represent real challenges on board [4,14]. Calls to improve the regulation [14-16] have been considered unnecessary [17]. Unacknowledged and uncorrected, these approved non-conformities and inadequacies continue to pollute the oceans and destroy the IMO’s environmental aspirations.

Also read: ‘Ships discharge 10,000 m3 of marine litter a year under MARPOL Annex IV’

Operational uncertainties

With so many operational problems being experienced on ships, technical guidance notes have been compiled to supplement the vendors’ operation and maintenance instructions [18-20]. But a decade on, operators are still afraid to use inadequate OWSs. They keep such OWSs in a working-but-unused condition, ready for the inspectors [21,23].

Desperate attempts to by-pass OWSs using “magic pipes” (image below) have led to fines and prosecutions [24-28], tipped off by crew members incentivised by life-changing prize money. Unfortunately, these high-profile cases shed no light on why people risk their careers and reputations to by-pass an OWS. On the other hand, the readiness of operators to resort to ‘practical solutions’ in evading the challenges of not their own making has only served to sustain these challenges. Two decades on, magic-pipe cases remain persistent [29-32].

Bilge water
“Magic pipes”.

Other tricks are more discreet. ‘Simple jumpers, swapped wires, disconnected wires, the addition of internal switches’ have been used within OCM cabinetry [7]. The use of flushing water during operations, additional clean water connections to the discharge sample line, additional powering capabilities to the three-way valves or discharge control valves, mechanical overrides limiting the action of electro-pneumatic operated control valves, the use of screwdrivers to override interlocks and switches, insufficient battery life for data logging, and the use of OCMs that fail to alarm at no sample flow, have been reported [7,33,34]. OWS duty capacities are found to exceed the certified limits [18]. The line between the certified non-conformities by design, and the ‘deliberate manipulations’ in operation, is often blurred into a tangled mess.

Enforcement uncertainties

Enforcement exists, probably more so than that of other MARPOL Annexes. But the disparity is evident. The US Coast Guard has been the one authority following up on almost all magic pipe cases, as if the violations had never happened elsewhere.

Crucially, certified inadequacies and non-conformities have not been effectively challenged by the enforcement regimes. The introduction of guidance notes, the recommendation of being ‘alert’ to manipulations [7], and the harmonised surveys under the IMO’s resolution A.1104(29) have not led to tangible improvements [8].

After decades of technological advances, such as tamper-proof monitoring solutions featuring a flow switch to the OCM sample flow, the OWS flow meter and GPS, etc., they remain an ‘optional’ choice trailing behind certified non-conformities that facilitate unchecked OWS discharges.

Illegal dumping and regional aspirations

With these uncertainties, it may not be a surprise that legal dumping of bilge water to sea continues [35, 36]. A waste gap of 31,000 m3 [37] has shifted the mood of the EU policy makers from encouraging bilge-to-ports to forcing it [38]. However, such regional aspirations to patch up the holes of the MARPOL rules can further diminish OWS utilisation on board, inflicting negative impacts on OWS familiarisation, operation, and maintenance.

They also create increasingly fragmented requirements which move further away from the universal environmental rules craved by international shipping, and by the IMO. Time will tell if they can better serve the goal of MARPOL. But how do we end up where we are, and what can be done next?

Also read: ‘Marine sewage treatment plants require compliance monitoring’

What’s next?

Are vendors to blame for cutting every corner afforded by the approval authorities? Are operators to blame for navigating their own ways around the certified yet inadequate OWSs that are beyond help? Are approval authorities to blame when a guideline can never be exhaustive in blocking off all tricks? Are enforcement bodies to blame for ticking boxes under a regulatory framework that requires no performance verifications? How can the certified inadequacies and non-conformities that carry the weight of sovereign administrations be challenged? Can certified mistakes ever be acknowledged and corrected under the IMO’s approval regimes, considering the entailing political, legal, and commercial implications?

As thorny and difficult as these issues may be, the IMO’s aspiration on controlling bilge water pollution is not wavering. Some well-equipped, robust, and adequate OWS systems have survived and even thrived, thanks to blue-chip ship owners who go beyond type approval certificates and commit to environmental protection. But to make OWS regulations effective, more careful considerations and competent inputs are needed:

  • Lab analysis methods need to reflect the complexity of the oil contents in bilge water, the phasing-out of heavy fuels, and the advance of biofuels and other greener fuels.
  • Good correlations between lab results and OCM readings are essential during tests and operations.
  • An OWS system should encompass the OCM, bilge alarm, sample points, and the associated control valves. Its type-approval should focus on the integrity and robustness of the design and construction. A ten-day independent test including the simulation of pitching and rolling on board a ship, is plausible. Test fluids need to reflect the advance of fuels [39] and the changing detergents and other chemicals. OWS effluent flow should be metered to stop excess dilution. A period of onboard operational experiences should form part of the OWS approval process.
  • A ship-specific management plan is needed to illustrate a conforming and robust OWS system installation against manipulations. It should also facilitate optimum configurations and best practices to improve interface conditions, to minimise interferences, and to reduce OWS loadings where possible. Common interferences to OWS performance should be managed, such as to eliminate sewage overflow into bilge (PPR 9/14).
  • A guideline can never be exhaustive enough to prevent all mistakes from being certified. The IMO needs to create an effective channel for reporting non-conformities and inadequacies without fear or repercussions. The IMO also needs to create a mechanism to acknowledge, investigate and correct certified mistakes with good accountability and transparency.
  • A credible type-approval regime is important for the maritime industry. But it is not enough. A discharge should be permitted only when its compliant performance is maintained under on-going compliance sampling, assessment, and enforcement, by undertaking, for example, one independent sample every year. Such transparency will allow regulators to establish the best available technologies and to arrive at evidence-based and practicable performance standards.
Bilge water

With such a “regulatory cycle” (figure above) that has been proven by the environmental endeavours of other industry sectors for decades, we can hope that the maritime industry can join the rest of our society to arrive at a level playing field, which encourages the best, but not the worst technologies, and an effective regulatory framework that aims for the real goal of protecting our environment with compliant discharge performances.


  2. Oil in the sea III: inputs, fates, and effects, Washington, National Research Council (NRC), 2003
  3. The Management of Ship-Generated Waste On-board Ships, EMSA/OP/02/2016, Delft, CE Delft, January 2017
  4. Oily Bilgewater Separators, USEPA, 2011
  5. Characterizing shipboard bilge water effluent before and after treatment, by C. McLaughlin, Env. Sci. and Pollution Res., 2014
  6. VGP self-sampling results, USEPA, 2021
  7. Technical Advisory – Oil Content Monitors, USCG Assistant Commandant for Marine Safety, Security and Stewardship, 2008
  8. MEPC 77/14/2, by China, 2021
  14. Bilge water treatment – centrifugal or gravity separation? Tanker Operator, 2009
  15. BIMCO discusses oily water at the IMO, BIMCO bulletin Vol 102, 2007
  16. Ferry oily wastewater treatment, Separation and Purification Technology, by R. Ghidossi, et. al., 2009
  17. MAX1 studies final report, Martin Ottaway, 2015
  18. Guidelines for the inspection of oily water monitor and separator systems, policy letter 04-13, USCG, 2005
  19. Guidance for the enforcement of MARPOL Annex I during port state control examinations, policy letter 06-01, USCG, 2006
  20. MEPC.1/Circ.677, 2009
  21., 2010
  22. A Guide to Operating Oily Water Separator on Ships, by Mohit Sanguri, 2013
  23., 2015
  24.—bulletin-482.pdf, 2008
  25. US v. Dalnave Navigation, Inc, Stamatakis P, Papadakis D. A case in the US District Court, D. New Jersey, 2009
  26. US v. Ilios Shipping Company. A case in the US District Court, Eastern District of Louisiana, New Orleans, 2011
  27. US v. Kun Yun Jho, 534 F. 3d 398—Court of Appeals, 5th Circuit, 2008
  28. A prosecutor’s perspective, by M. Fink.
  29., 2016
  30., 2020
  31., 2020
  32., 2021
  33. Guide to oil in water monitoring & discharge, Rivertrace, 2021
  34. Marine Safety Lessons Learned, 01-19, Inspections and Compliance Directorate, USCG, 2019
  37. Impact Assessment, Proposal for PRF Directive for the delivery of waste from ships, 2018.
  38. Commission implementing regulation, (EU) 2022/89, 2022
  39. Call to change bilge water regulations, by B. Carlson, Motorship, 2022.