Oil and Grease Separators: Types, Sizing and Selection

How do oil and grease separators work?

Oil and grease separators work on the principle of gravity separation: because oil is less dense than water, oil droplets rise through the water column to form a surface layer that can be skimmed off, while clean water is drawn from below. The governing physics is Stokes' law, which states that a droplet's rise velocity is proportional to the square of its diameter and to the density difference between oil and water, and inversely proportional to the water's viscosity. The practical consequence is enormous: large free-oil droplets rise quickly and separate easily, whereas fine dispersed and emulsified droplets rise so slowly that simple gravity separation cannot remove them in any reasonable tank.

Every separator design is therefore an attempt to make the smallest possible droplet rise to a collection surface within the available residence time. A plain gravity tank gives droplets a long, unaided path; plate packs shorten that path dramatically; flotation actively enlarges the effective droplet by attaching air bubbles. Understanding the oil's droplet-size distribution — its state — is the first step in selecting the right separator.

Free, dispersed and emulsified oil

Oil exists in wastewater in three states, and which state dominates dictates which separator will work:

• Free oil — droplets larger than about 150 microns that rise readily under gravity. Removed easily by API separators and plate interceptors.
• Dispersed oil — droplets roughly 20–150 microns, mechanically broken up but not chemically stabilised. Plate interceptors and hydrocyclones handle the larger end; the finer fraction needs flotation.
• Emulsified oil — droplets below about 20 microns, stabilised by surfactants or vigorous shear so they will not coalesce. Gravity is useless here; the emulsion must first be chemically broken (de-emulsified) with acid, salts or a coagulant, then floated out by DAF.

Soluble (dissolved) hydrocarbons are a fourth fraction that no physical separator removes at all; they require biological treatment, adsorption or air stripping.

API separators

The API separator is the long-standing baseline for oily-water treatment, named after the American Petroleum Institute design method (API 421). It is a rectangular, gravity-flow basin sized so that the slowest free-oil droplet to be removed rises to the surface before the flow reaches the outlet. The basin contains flow-distribution baffles, a surface skimmer (often a rotating slotted pipe or a flight scraper) that collects the floated oil layer, and a sludge hopper for the heavy solids that settle to the floor.

API separators are robust, simple and tolerant of slugs of oil and solids, which is why they remain standard at refineries and large petrochemical sites as the primary de-oiling stage. Their limitation is droplet size: the API method is conventionally designed to remove free-oil droplets of 150 microns and larger, leaving an effluent still containing dispersed and emulsified oil. They are also large, because gravity separation of a 150-micron droplet demands considerable surface area and residence time. For that reason an API separator is almost always a roughing stage, followed by a finer separator or flotation unit for polishing.

Plate interceptors: CPI and TPI

Corrugated plate interceptors (CPI) and tilted plate interceptors (TPI) are the workhorse refinement of the gravity separator. They exploit a simple insight: a droplet does not have to rise the full depth of a tank — it only has to rise to the nearest surface. By packing the separation zone with closely spaced inclined plates (typically 20 mm apart at 45–60°), the vertical distance any droplet must travel to reach a collecting surface is cut to a few millimetres. Captured oil coalesces on the underside of the plates and slides up the corrugations to the surface; settled solids slide down to a hopper.

This “shallow-depth” or lamella principle lets a plate interceptor remove much smaller droplets — down to roughly 30–60 microns — in a fraction of the footprint of an API separator of equivalent duty. CPI and TPI describe variants of the same idea: corrugated plates promote coalescence in the corrugation troughs, while tilted flat-plate packs emphasise the inclined settling path. Plate packs are frequently retrofitted inside or downstream of an existing API basin to upgrade its performance. The limit remains droplet size — plate interceptors still cannot separate a stabilised emulsion.

Hydrocyclones, DAF and how to select a separator

Two further technologies extend the range. De-oiling hydrocyclones use centrifugal force rather than gravity: the oily water is injected tangentially into a tapering cylinder, generating a vortex in which the heavier water is forced to the wall and the lighter oil migrates to a central core that is drawn off through a reject port. Because the cyclone applies many times the force of gravity, it separates fine dispersed droplets in a very compact, no-moving-parts unit, which is why hydrocyclones dominate offshore produced-water treatment where space and weight are critical. They do not, however, break a true emulsion.

Dissolved air flotation (DAF) is where chemistry and physics combine for the hardest duties. After the emulsion is chemically broken with a coagulant and polymer, microbubbles attach to the freed oil and solids and float them rapidly to the surface. DAF is therefore the standard polishing stage for emulsified and finely dispersed oil that gravity and cyclones leave behind — our companion guide explains how DAF removes fats, oils and greases in detail.

Selecting a separator follows directly from the oil state and the discharge target. Characterise the influent — oil concentration, droplet-size distribution and whether it is free, dispersed or emulsified — and the consent limit (typically 5–40 mg/l for discharge to sewer or watercourse). Free oil only, with a loose limit? An API or plate interceptor alone may suffice. Tight limit or dispersed oil? Add a plate pack or hydrocyclone. Emulsified oil? A chemical de-emulsification stage followed by DAF is essential. In practice the most reliable oily-water plants are trains — a gravity or plate stage to strip the bulk free oil cheaply, protecting and reducing the load on a finer flotation polishing stage.

Materials of construction and ancillaries also shape the selection. Separators are built in coated steel, stainless steel, GRP or concrete depending on the effluent chemistry and the site's fire and ATEX requirements, since refinery and forecourt duties handle flammable hydrocarbons and often demand vapour-tight covers and bunding. A separator is rarely specified in isolation: upstream balancing protects it from flow and oil-concentration slugs that would otherwise wash oil through to the outlet, while recovered oil needs a holding tank and the de-oiled water frequently passes to a downstream biological or adsorption stage to strip the residual soluble fraction. Sizing every stage against the characterised peak load — rather than the average — is what keeps an oily-water plant inside consent during the inevitable shock loads of a wash-down or a tank-bottoms drain.