The forms of oil in an aqueous stream
Oil is not present in water as a single population. It is distributed across several fractions that behave very differently inside a separator, and a visual assessment reveals only the coarsest of them.
- Free oil. Buoyant droplets, broadly above about 150 micrometre, that rise readily and form a visible film. This fraction is removed efficiently by gravity.
- Dispersed oil. Finer droplets, commonly in the range of roughly 20 to 150 micrometre, that rise slowly and tend to be carried out with the water phase.
- Emulsified oil. Very fine droplets, frequently stabilised by surfactants or by shear across pumps and control valves, which for practical purposes do not separate under gravity.
- Dissolved oil. Hydrocarbon in true solution, which no mechanical separator can retain.
These bands are indicative rather than sharp. The true droplet size distribution depends on the upstream process, the temperature, and the degree of mechanical shear the stream has experienced.
What a discharge consent actually measures
A permitted limit is defined on a laboratory determination, typically by gravimetric extraction, infrared absorption or gas chromatography, and that determination integrates every oil fraction present. Visual clarity is not a valid proxy for compliance, because the dispersed and dissolved fractions that dominate a marginal result are not visible. Analytical methods also carry an appreciable measurement uncertainty, so a sound design targets a working margin below the nominal consent rather than the consent itself.
Indicative discharge limits
Permitted oil in water concentrations vary widely by jurisdiction, receiving environment and industry. The values below are broad and approximate, given only to convey the order of magnitude. The governing permit always takes precedence, and local limits are frequently set lower.
| Setting | Indicative oil in water target |
|---|---|
| Discharge to municipal sewer or trade effluent | Commonly of the order of 15 to 30 mg/L, or as set by the local authority |
| Industrial discharge to surface water | Often reduced to the low tens of ppm or below |
| Offshore produced water (OSPAR region) | Typically about 30 mg/L as a monthly average, with lower site specific targets common |
| Marine bilge water (IMO framework) | Commonly about 15 ppm |
| Produced water for reinjection | Often specified in the low tens of ppm, according to the formation |
Values are approximate and for orientation only. They are not a compliance standard, and the applicable permit governs.
Separation mechanism as a function of droplet size
In the laminar settling regime the terminal rise velocity of an oil droplet increases with the square of its diameter and with the density difference between the phases, and falls as the continuous phase viscosity rises. The practical consequence is direct. Coarse droplets rise quickly and are removed by gravity, whereas fine droplets rise so slowly that gravity alone becomes ineffective within any realistic residence time. As the droplet population becomes finer, the correct response is not a longer residence time but a change of mechanism, first by promoting coalescence and then, for the dissolved remainder, by adsorption.
The staged separation train
A dependable configuration combines two or three complementary stages, each addressing the fraction that the preceding stage leaves behind.
- Enhanced gravity separation. Closely spaced inclined plates shorten the vertical distance a droplet must travel and greatly increase the effective settling area, so a compact vessel performs the duty of a much larger open basin. A corrugated plate interceptor is the usual selection at this stage. It removes free oil and settleable solids and reduces the bulk oil load toward the low ppm range, subject to the inlet droplet size distribution and hydraulic loading.
- Coalescing polishing. Dispersed droplets that survive the first stage are brought into contact with oleophilic media, on which they coalesce until they are large enough to rise. This duty is met by a coalescing filtration skid, and it is frequently the stage that distinguishes a visually clear effluent from a genuinely compliant one.
- Adsorptive polishing. Dissolved and residual hydrocarbon that remains after coalescence is retained on activated carbon as a final guard stage, applied where the consent is stringent or where the treated water is to be reused.
Where the feed contains a stabilised emulsion, a chemical demulsification or dissolved air flotation step is introduced ahead of polishing, since neither gravity nor coalescence can resolve an emulsion that has not first been destabilised. The correct sequence is a function of the feed characterisation, and the arrangement above should be read as a framework rather than a fixed recipe.
Application to site runoff and produced water
The same staged logic recurs across a wide range of duties. On paved industrial areas and fuelling forecourts, a compact CPI separator for storm water runoff provides the primary hydrocarbon removal stage ahead of any coalescing polish, and is commonly combined with a sludge collection sump for the solids that accompany surface water. The related API separator and stormwater interceptor duty follows the same principles at larger hydraulic scale.
In upstream oil and gas, produced water treatment applies enhanced gravity separation followed by coalescing polishing for reinjection or overboard discharge, while refinery effluent treatment and marine bilge water treatment add adsorptive polishing to satisfy stricter consents. These duties sit within the broader field of oil water phase separation and are most prevalent across oil and gas and heavy industry.
Frequently asked questions
Why does an oil water separator pass on visual inspection yet fail the laboratory test?
Visual inspection detects only free oil. The dispersed and dissolved fractions reported by a laboratory can remain well above a low ppm consent while the surface appears clear. Closing that gap generally requires a coalescing or adsorptive stage downstream of the separator.
Can a larger separator resolve a discharge limit problem on its own?
Increasing the size of a gravity separator assists with free oil but does little for fine dispersed or emulsified droplets, which rise negligibly regardless of residence time. Introducing a stage that changes the separation mechanism is generally more effective than adding volume.
Is coalescing or adsorption required, or both?
The answer depends on the stringency of the consent and on the droplet size distribution of the feed. As a general rule, coalescing addresses dispersed oil and adsorption addresses dissolved or residual traces. A representative water sample and the target consent are the sound basis for the decision.
Need to reach a specific discharge limit?
Share your inlet oil content and target consent, and our engineers will propose a suitable train.