Oily Water Separators: How to Size A Bilge Water Treatment System


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An oily water separator is a component of your vessel’s bilge water treatment system responsible for removing oil from bilge water. This type of system is crucial for the function of your vessel—and crucial for your bottom line because failing to discharge water safely can result in steep fines. 

Maritime regulations state that the oil content of your discharged water must be less than 15 ppm. Your oily water separator can help you achieve that goal, but only if it’s up to the task and properly sized for your system and your needs.

Risks of Sizing Your Oily Water Separator Incorrectly

The proper sizing of the oily water separator on any vessel is critical to its performance. Consider the following: 

  • If you size your oil water separator too small, the system will work continuously but won’t be able to keep up with the production of bilge water. As a result, you may face a possible shutdown of operations or costly storage and shipping of bilge water to shore. 
  • If you size the equipment too large, it will certainly keep up with bilge water production. However, it will consume more capital than is required, and you won’t be maximizing your system’s capabilities.

Considerations for Sizing Your Oily Water Separator Strategically

Wondering how to know what size your oily water separator should be? Here are the factors you’ll need to take into account. 

The Size of Your Vessel 

Sizing your separator may be as simple as following the standard for your vessel’s size. However, these are not absolute, and it’s still a good idea to reach out to an expert for further assessment. The standards are:

  • 400-1,600 GT: BOSS 2T-107 (0..5m3hr), BOSS 2T-107Y and MiniBOSS
  • 1,600-4,000 GT: BOSS 5T-107 (1.1m3hr)
  • 4,000-15,000 GT: BOSS 11T-107 (2.5m3hr)
  • 15,000 GT+: BOSS 25T-107 or BOSS 45T-107. This is typically a rig/FPSO or platform application

We’ll provide more information about each of these systems below. 

Engineering Specifications 

Most vessels are designed by an engineering team. Your vessel’s specifications should match with a required standard flow rate for your oily water separator. You can easily select this range from our wide variety of BOSS oily water separator sizes. 

The Available Space on Your Vessel

Your vessel may have a limited amount of space available for your oily water separator. If you have a smaller vessel, consider selecting one of the more efficiently sized BOSS systems. 

The Type of Contamination in the Bilge

Consider the bilge that needs treating in your area. Is it mostly floating free oil? Or is it emulsified, mixed with the water without separation? Your answer will influence how much oil-absorbing media you will consume. Each BOSS oily water separator has a specific amount of media.  If you’re in an area where there is a high amount of emulsified oils, you may consume more media. This is information that is important to know prior to selecting your oily water separator. 

The Proposed Run Time of Your System

Consider this question: How long will you be using your oily water separator at each given time? Our various separators are designed for different frequencies of use. For example, the BOSS oily water separator is designed for continuous 24/7 use. The MiniBOSS oily water separator is designed for shorter run times and lower volumes.

Specifications of the Various Oily Water Separators You Could Select

These are the BOSS separators we offer. Based on the above factors, one of these systems should fit the needs of your vessel nicely. 

BOSS 25T-107 & BOSS 45T-107

Designed for rig, FPSO, and production platform applications. Can be designed for hazardous areas and harsh weather applications. Flow rates from 25-45 gpm (5.6-10.2m3hr). USCG, ABS, BV, and MED approvals per IMO MEPC 107(49).

BOSS 2.2T-107YO 

Features color options, easy-to-change dual cartridges, or longer-lasting bulk media tank.

BOSS 2.2T-107 (cartridges) 

Has an adjustable flow rate for optimal efficiency, easy-to-change cartridges, or longer-lasting bulk media tank. USCG, ABS, BV, and MED approvals per IMO MEPC 107(49).

BOSS 5T-107 (cartridges) 

Designed for vessels 1,600-4,000 GT with an adjustable flow rate for optimal efficiency, easy-to-change cartridges, or longer-lasting bulk media tank. USCG, ABS, BV, and MED approvals per IMO MEPC 107(49).

BOSS 11T-107 (cartridges)

Designed for vessels greater than 4,000GT with the ability to utilize cartridges or a longer-lasting bulk meida tank. IMO certified with an adjustable flow control valve to always obtain the targeted flow rate.


IMO certified oily water separator but Designed for small vessels not subject to IMO regulations. Certified flow up to 2 GPM. Small footprint and easy installation makes this an economical choice to oil water separators.

H2O LLC Is Here to Help You Size Your Separator

Clearly, sizing your oily water separator properly is key to the functionality of your system and your ability to avoid steep fines. Fortunately, at H2O LLC, we’re able to provide a range of systems perfectly suited for various vessels of varying sizes. Our technicians are more than happy to help you assess your needs and recommend an oily water separator for your system. Give our friendly team a call today for a timely response and the information you need. 

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