What Is an Oil Water Separator: Working Principles, Types and Applications

Answering the core question: What is an oil water separator and how does it work? An oil water separator is a vessel or channel that removes free and dispersed oil from industrial wastewater by exploiting the density difference between the two immiscible phases. Separation follows Stokes law, where the rise velocity of an oil droplet is v = g x d2 x (rho_water - rho_oil) / (18 x mu), so a 150 micrometre droplet of 850 kg/m3 oil in water rises at roughly 3-4 mm/s. To capture it, the separator must slow the bulk liquid enough that the droplet reaches the surface before the water exits. A conventional API 421 gravity separator therefore limits horizontal velocity to 0.9 m/min (3 ft/min), keeps the Reynolds number below 2,000, and uses a depth-to-width ratio of 0.3-0.5. Properly sized units cut incoming oil from 100-1,000 mg/L down to 10-50 mg/L, while polishing stages reach the 10-15 ppm discharge limits set by regulators and by IMO MEPC.107(49) for shipboard bilge water.

Four physical principles decide whether a separator meets its outlet specification:

• Density Difference and Stokes Law: Everything starts with the density difference between the oil and water phases. Typical mineral oils sit at 820-880 kg/m3 against water at about 1,000 kg/m3, giving a driving difference of 0.12-0.18 g/cm3, but heavy crude, bitumen and some synthetic oils reach 950-1,010 kg/m3 and separate very slowly or not at all. Because the rise velocity scales with the square of the droplet diameter, halving the droplet size quarters the rise rate: a 300 micrometre droplet rises about 12 mm/s while a 75 micrometre droplet manages only 1 mm/s. This is precisely why separators are specified by their cut size, and why any upstream device that shears droplets smaller, such as a centrifugal pump or a throttling valve, silently destroys separator performance downstream.
• Residence Time and Quiescent Flow: A separator is a device for buying time. The liquid must remain in the vessel long enough for the design droplet to travel from the bottom of the flow path to the surface, which in an API separator means from the invert to the water surface, typically 1.5-3 m. With a rise velocity of 3 mm/s that requires 500-1,000 seconds of retention, or 8-17 minutes, and the vessel length follows from dividing the required retention by the allowable horizontal velocity. Turbulence destroys this calculation, so inlet distributors, perforated baffles and a long stilling zone are mandatory, and designers verify performance by checking that the Reynolds number stays below 2,000 and ideally below 500 in the separation zone.
• Coalescence and Droplet Growth: Gravity alone cannot remove droplets below about 50-60 micrometres in a reasonably sized basin, so most modern separators add a coalescing medium. In a corrugated plate interceptor the water flows through a stack of plates spaced 20-40 mm apart set at 45-60 degrees. Droplets rise only the short distance to the underside of the plate above, coalesce into a film, and the film then drains upward along the plate to the collection channel. Because the effective settling distance drops from metres to centimetres, a CPI achieves the same separation in 20-50% of the footprint of an open API basin. Oily solids slide down the plates into a sludge hopper, which must be drained regularly or the stack silts up and loses its advantage entirely.
• Flotation and Chemical Assistance: When the oil is emulsified or the droplets are stabilised by surfactants, gravity separation fails and the plant turns to flotation. Dissolved air flotation saturates a recycle stream with air at 300-600 kPa and releases it through a needle valve or eductor at atmospheric pressure, producing a cloud of 10-100 micrometre bubbles that attach to oil droplets and carry them to the surface, where a skimmer removes the float. Chemical assistance is almost always paired with it: a cationic or anionic polyelectrolyte breaks the emulsion, and a ferric or aluminium salt coagulant destabilises the charge. Correctly dosed DAF brings outlet oil to 10-30 mg/L, but overdosing restabilises the emulsion and makes the effluent worse than no treatment at all.

Selection is driven by inlet oil concentration, droplet size distribution, available plot space and the discharge limit that must be met:

• API Gravity Separator: The reference design for refinery and petrochemical oily water, standardised in API 421 and long used as the first stage of treatment. It is a long rectangular concrete basin with a chain-driven flight skimmer sweeping floating oil to a slotted pipe at the downstream end, and a sludge scraper moving settled solids to a hopper at the inlet. Its strengths are simplicity, tolerance of slug loads and solids, and a predictable 60-99% removal of free oil. Its weakness is footprint: treating 1,000 m3/h to a 150 micrometre cut requires a basin roughly 40-60 m long. It handles free oil only and does nothing for emulsified or dissolved oil.
• Corrugated Plate Interceptor and Tilted Plate Interceptor: A CPI or TPI packs the same separation principle into a steel tank a fraction of the size, using the parallel plate stack described above. Plate spacing of 20-40 mm and an inclination of 45-60 degrees let solids migrate down while oil films migrate up, so the unit self-cleans to a degree. These units are factory-built, skid-mounted, and typically treat 5-500 m3/h, which makes them the standard choice for offshore platforms, terminals, and plant revamps where a new concrete basin cannot be poured. They are more prone to plugging by solids and heavy greases than an open basin, so upstream screening and regular plate cleaning are non-negotiable.
• Dissolved Air Flotation and Induced Gas Flotation: Flotation units treat what gravity cannot: emulsified oil and fine dispersed droplets in the 5-50 micrometre range. DAF uses the pressurised recycle stream described earlier and is common in refinery and petrochemical effluent treatment as a secondary stage after the API separator. IGF instead disperses gas mechanically with an impeller-induced eductor or a sparged rotor, avoiding the saturator vessel and pump and tolerating higher solids. Both need chemical conditioning, both produce a float skimmings stream of 1-5% oil that must be routed back for recovery or disposal, and both are sensitive to flow surges, so equalisation upstream is a design requirement rather than an option.
• Hydrocyclones, Coalescers and Membrane Polishing: Where space is at a premium, as on offshore platforms, liquid-liquid hydrocyclones spin the stream at high velocity so that the heavier water is thrown to the wall and the lighter oil core is drawn off through the vortex finder. They are compact and have no moving parts, but they need a pressure drop of 200-500 kPa, are intolerant of solids and gas, and their performance falls off sharply at turndown below 40-50% of design flow. For polishing to very low levels, a coalescing cartridge filter or an ultrafiltration or ceramic membrane unit can bring oil below 5-10 ppm, at the cost of regular media replacement and a concentrate stream that must be handled.

Q: What is the difference between an oil water separator and an API separator?

A: An oil water separator is the general category covering any device that splits oil from water, while an API separator is one specific gravity design standardised by the American Petroleum Institute in API 421. An API separator is a long rectangular basin with a flight skimmer and sludge scraper sized to remove droplets above 150 micrometres, typically holding horizontal velocity to 0.9 m/min and the Reynolds number below 2,000. Because it is an open basin it tolerates slug loads and solids well, but it needs a large plot area. In practice an API separator is the primary treatment stage, and CPI units, DAF, hydrocyclones and coalescers are the secondary or polishing stages that follow it.

Q: What oil concentration can an oil water separator achieve?

A: It depends on the technology and on the inlet droplet size distribution. A well-run API gravity separator typically delivers 50-150 mg/L from an inlet of several hundred to a few thousand mg/L, since it removes free oil only. A corrugated plate interceptor improves this to 20-60 mg/L. Adding chemically assisted DAF brings the effluent into the 10-30 mg/L range, and a coalescer or membrane polishing stage can reach below 5-10 mg/L. For shipboard bilge water the regulatory limit is 15 ppm under IMO MEPC.107(49), which is met with a dedicated oily water separator using a coalescing and filter stage followed by an oil content monitor and automatic overboard valve.

Q: Why does an oil water separator suddenly stop performing?

A: Four causes account for most failures. Shear upstream: a new centrifugal pump, a throttling control valve or a partially closed restriction emulsifies the oil into droplets below the design cut size, and no gravity device can recover them. Solids accumulation: silt, scale and sludge fill the basin or bridge across the plate pack, cutting effective volume and short-circuiting flow. Temperature drop: water viscosity rises sharply as it cools, and since rise velocity is inversely proportional to viscosity, a unit that performs in summer can fail in winter. Finally chemical changes: a new surfactant, cleaning agent or corrosion inhibitor in the waste stream stabilises the emulsion and defeats gravity separation entirely.

Q: How should an oil water separator be sized?

A: Start from the design droplet cut size, usually 150 micrometres for API duty, and compute the rise velocity with Stokes law using the actual oil and water densities at the operating temperature rather than at ambient. Set the horizontal velocity to no more than 0.9 m/min and the depth-to-width ratio between 0.3 and 0.5, then check that the Reynolds number is below 2,000. Vessel length follows from the horizontal velocity multiplied by the required retention time, which is the separation depth divided by the droplet rise velocity. Always apply a safety factor of 1.5-2.0 for inlet turbulence, confirm turndown behaviour down to 30-40% of design flow, and add an oil level alarm and automatic skim control to prevent oil carryover during slug loads.