Types of Marine Oily Water Separators – Explained Simply
let’s talk about the different types of OWS you’ll come across onboard.
1. Gravity-Based Oily Water Separators
Simple in design, reliable in performance.
This is the most old-school type—based on the basic principle that oil floats on water. Bilge water enters a tank or chamber and is allowed to settle. Because oil is less dense, it naturally rises to the top, while the heavier water sinks.
Key Features:
- Uses coalescing plates or baffles to help tiny oil droplets stick together (coalesce) and float faster.
- Some systems have gentle heaters to help improve separation.
Pros:
- Easy to maintain
- Cost-effective
- No complicated moving parts
Cons:
- Struggles with emulsified oil (oil-water mix that doesn’t separate easily)
- May require polishing filters to meet the 15 ppm discharge limit
Used where?
Common on smaller vessels or where budget and space are tight.
2. Centrifugal Oily Water Separators
Spin it to win it.
These units work like a fuel oil purifier. Instead of waiting for gravity to do its job, they spin the bilge water at high speeds using centrifugal force. This accelerates the separation process.
How it works:
- Heavier water is thrown to the outside
- Lighter oil collects near the center
- Both are then extracted via separate outlets
Pros:
- Highly efficient, even with emulsified oil
- Compact, continuous operation
Cons:
- Requires power to operate
- More moving parts = higher maintenance
- Expensive upfront
Used where?
Popular on larger ships, new builds, or where high-performance separation is needed (especially in rough weather conditions).
3. Absorption and Adsorption Type
Like a sponge… but smarter.
These separators use special media (like organoclay or activated carbon) that either absorb oil or adsorb it onto their surface. The bilge water passes through the media, and the oil gets trapped.
Key Components:
- Reactor vessels or filter cartridges
- Organoclay or similar oil-attracting media
Pros:
- Excellent at removing emulsified oils and chemicals
- Can be used as a polishing stage after gravity separation
Cons:
- Spent media needs to be replaced (or regenerated)
- Disposal of used media is a concern
Used where?
Common in BOSS-type OWS systems; ideal when dealing with stubborn emulsions.
4. Chemical (Flocculation & Coagulation) Type
Breaking up is necessary.
These systems use chemicals to “break” emulsions. The added flocculants or coagulants cause the oil particles to bind together and form larger clumps (flocs), which are then separated.
How it works:
- Chemical is injected into the bilge stream
- Oil separates and is removed
- Cleaned water moves to the next stage or discharge
Pros:
- Effective on tough emulsions
- Works well as a secondary treatment
Cons:
- Sludge production is high
- Chemical cost and storage required
Used where?
Often found paired with other OWS types (e.g., JOWA 3SEP + EBU).
5. Biological Treatment Systems
Let bacteria do the dirty work.
Yes, you read that right—some OWS systems use living microorganisms to digest oil and convert it into harmless substances like CO₂ and water.
How it works:
- Bilge water enters a bioreactor
- Microbes consume the hydrocarbons
- Clean water exits
Pros:
- Environmentally friendly
- Can handle multiple contaminants
Cons:
- Slower process
- Sensitive to temperature and toxins
- High capital cost
Used where?
More common in offshore platforms and advanced wastewater systems.
6. Membrane Filtration Type
Technology meets tight limits.
These systems use ultra-fine filters or membranes to physically block oil from passing through.
Key Technologies:
- Reverse osmosis
- Ultrafiltration
- Microfiltration
Pros:
- Can consistently discharge <15 ppm (even <5 ppm)
- Great for vessels with strict environmental standards
Cons:
- High initial and maintenance costs
- Filters/membranes can clog easily if not maintained
Used where?
Seen on cruise ships, naval vessels, and newer eco-compliant merchant ships.
