Oil-water separators for oily wastewater · Gravity and coalescing-plate units with forebay, separator and afterbay, 150 µm versus 20–30 µm droplet removal, polyethylene plates at 60°, and centrifugal separators as an alternative
An oil-water separator (OWS) is a multi-chamber tank that lets free oil float out of wastewater and solids settle out before the water is discharged or treated further. Gravity separators rely on retention time alone; coalescing separators add plates or media on which small droplets join and rise faster. Ask three things first: what flow and oil, solids and detergent load the wastewater carries, what discharge limit applies, and how sediment and oil will be removed and the unit cleaned.
Oil-water separators for oily wastewater and runoff from vehicle washracks, workshops, fuelling and industrial areas: simple gravity separators, coalescing-plate and media separators, and aboveground or below-ground units, with centrifugal oil-water separators where compared. Compressed-air line filters and coalescing filter elements (see sterile air filter for one type), compressor air/oil separators (see oil separator), fuel-water separators and tramp-oil skimmers (see oil skimmer) are outside this page.
Start with your task
01
Envirosoil's oil-water separator rationale, filed with a Nova Scotia environmental assessment, describes OWS as multi-chambered devices that remove oil from water in industrial processes, bilge water, food industry fats, oils and grease, pretreatment and stormwater from vehicle wash areas or fuelling stations, and that also collect sediment. It names gravity and coalescing-plate types, each with a forebay, separator section and afterbay. Gravity units give enough retention for droplets to rise, with the outlet below the oil layer and diffusion baffles at the inlet, and are described as providing a discharge of about 100 mg/L based on a 150 µm droplet size. Coalescing units pass water through oleophilic polyethylene or polypropylene media where droplets join and rise, and are described as providing 15 mg/L or less with 20–30 µm droplets. [2]
The U.S. Army Corps of Engineers study ERDC/CERL TR-00-40 explains that the Army's older washrack separators are below-ground, cast-in-place concrete gravity units, while off-the-shelf coalescing-type gravity separators are now installed because they are smaller, less expensive and need less site work; coalescers shorten the distance oil must rise, so small droplets collect on the plates and coalesce while heavier soil particles slide to the bottom. It notes that dirt attaches to oil droplets, that Stokes' law predicts separation only for simple mixtures of free oil in water, and that vendor literature based on it can therefore mislead buyers treating washrack water. [1]
Centrifugal oil-water separators, including disc-type units, are listed as an alternative by Juneng. [3]
Give the supplier the source of the wastewater, peak and average flow, oil type and specific gravity, oil and suspended solids concentrations, detergents or emulsifiers present, temperature, the discharge limit, installation (aboveground or buried, traffic loads), access for cleaning, and how oil and sludge will be removed.
02
ERDC/CERL TR-00-40 tested coalescer plates and recommends, for off-the-shelf coalescing separators treating washrack water: polyethylene plates, a maximum surface loading rate of 0.37 gpm/ft², a 60° plate angle and downward flow at 60° to the horizontal, together with storage for the solids removed in the coalescer; it found no evidence that sediment would build up on plates enough to block flow if plate spacing is at least 0.75 in, and cites the Corps' ETL 1110-3-466 requirement that coalescer packs be easy to remove and clean. [1]
The Envirosoil filing lists the selection factors it used, including temperature 5–75 °C, oil specific gravity 0.7–0.94, removal of over 99% of free oil above 20 µm, maximum horizontal flow below 1 m/min, maximum Reynolds number 500, minimum residence times of 30 minutes at average flow and 15 minutes at peak flow, coalescing plate incline of 40–60° and a minimum plate area of 300 ft². [2]
Editorial calculation: 0.37 gpm/ft² is about 15 L/min per m² (0.015 m/min); 0.75 in is about 19 mm; 300 ft² is about 27.9 m². [1] [2]
03
For ships, IMO Resolution MEPC.107(49) sets the type-approval specification for 15 ppm bilge separators and 15 ppm bilge alarms: effluent discharged overboard must not exceed 15 ppm of oil by volume, and the separator must cope with emulsified bilge water and with cleaning agents and emulsifiers present. The bilge alarm must show a ppm display, respond within 5 seconds of a change in sample, trigger automatically above 15 ppm or if it fails, record date, time, alarm and separator status for at least 18 months, and be sealed against tampering, with its accuracy checked at IOPP certificate renewal. An automatic stopping device must divert discharge, and the whole chain from an exceedance to stopping overboard discharge must take no more than 20 seconds. Type tests use three fluids: Test Fluid A, a residual fuel oil (ISO 8217 RMG 35, density at least 980 kg/m³); Test Fluid B, a distillate (DMA, at least 830 kg/m³); and Test Fluid C, an emulsion of 947.8 g fresh water, 25 g each of A and B, 0.5 g surfactant and 1.7 g iron oxides per kilogram. [4]
Editorial calculation: 15 ppm by volume is 15 mL of oil per cubic metre of water, roughly 15 mg/L for oil near water density, so the marine limit sits at the same level as the 15 mg/L or less the Envirosoil filing gives for coalescing separators and well below the 100 mg/L for gravity units. Test Fluid C is about 5% oil by mass with a surfactant, which is why approval for this fluid matters if your wastewater contains detergents. [4] [2]
| Requirement | MEPC.107(49) value | Why it matters |
|---|---|---|
| Effluent limit | 15 ppm oil by volume [4] | Discharge compliance. |
| Alarm response | ≤ 5 s [4] | Detects exceedance quickly. |
| Exceedance to stop | ≤ 20 s overall [4] | Limits oily discharge. |
| Data record | ≥ 18 months [4] | Inspection evidence. |
| Test fluids | Residual (≥980 kg/m³), distillate (≥830 kg/m³), emulsion with surfactant [4] | Covers heavy oil and emulsions. |
04
Taizhou Weicai Plastic Industry Co., Ltd. lists polyethylene grease traps and buried multi-compartment separation wells; Luoyang Reclea Eco Environmental Protection Technologies Co., Ltd. and Chungho Environmental Protection Technology Chengdu Co., Ltd. list oil-water separators within wastewater and liquid purification ranges; JUNENG MACHINERY (CHINA) CO., LTD. and Henan Gems Machinery Co.,Ltd list centrifugal units.
Ask any supplier for separator type, rated and peak flow, design droplet size and discharge concentration with the test basis, coalescer material, angle, spacing and area, chamber volumes and residence time, sediment and oil storage, materials of construction, installation and load rating, access and cleaning method, and alarms or skimmers offered.
05
| Part | Role | Fix in the order |
|---|---|---|
| Forebay and inlet baffles | Calm flow; drop heavy solids. [2] | Volume. |
| Separator chamber | Gives oil time to rise. [2] | Residence time. |
| Coalescer plates or media | Join small droplets; shed solids. [1] [2] | Material; angle; spacing. |
| Sediment storage | Holds solids from the coalescer. [1] | Volume; access. |
| Oil layer and skimmer | Collects and removes oil. [2] | Removal method. |
| Afterbay and outlet | Draws water from below the oil. [2] | Outlet level. |
06
Values come from the Corps study, the Envirosoil filing and MEPC.107(49); tank material, weight and pipe connection sizes were not published there and must come from the supplier.
| Field | What it means and published values | Ask the supplier for |
|---|---|---|
| Separator type | Gravity; coalescing plate or media; centrifugal. [2] [3] | Type and test basis. |
| Design droplet size (µm) | 150 µm gravity; 20–30 µm coalescing. [2] | Droplet size basis. |
| Discharge oil concentration (mg/L) | About 100 mg/L gravity; 15 mg/L or less coalescing [2]; 15 ppm marine limit. [4] | Guaranteed value; test method. |
| Coalescer surface loading | Up to 0.37 gpm/ft² of plate (about 15 L/min per m²). [1] | Rated and peak flow per plate area. |
| Plate material, angle and spacing | Polyethylene at 60°, spacing at least 0.75 in [1]; 40–60° incline in one design basis. [2] | Material; angle; spacing. |
| Residence time | At least 30 minutes at average and 15 minutes at peak flow in one design basis. [2] | Chamber volumes. |
| Operating temperature (°C) | 5–75 °C in one design basis. [2] | Range. |
| Oil specific gravity | 0.7–0.94 in one design basis. [2] | Your oil. |
| Marine type approval | MEPC.107(49) 15 ppm separator and bilge alarm. [4] | Certificate. |
07
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Source: [washrack; workshop; process; stormwater] Flow: [average; peak] Load: [oil type; SG; mg/L; solids; detergents] Temperature: [range] Discharge: [limit; receiving system] Type: [gravity; coalescing; centrifugal] Installation: [aboveground; buried; traffic] Maintenance: [access; oil and sludge removal]
Five companies below list oil-water separators: buried polyethylene wells, units within wastewater treatment ranges, or centrifugal separators.
Lists polyethylene grease traps and buried multi-compartment oil-water separation wells.
Product scope checked 2026-10-11
Lists oil-water separators among wastewater treatment equipment.
Product scope checked 2026-10-11
Lists oil-water separators among liquid purification equipment.
Product scope checked 2026-10-11
Lists centrifugal oil-water separators.
Product scope checked 2026-10-11
Lists an oil-water separator machine among centrifuge machines.
Product scope checked 2026-10-11
Explore company information and manufacturing capabilities, then discuss the scope of your project with a relevant supplier.
The Envirosoil filing describes gravity units reaching about 100 mg/L for 150 µm droplets and coalescing units 15 mg/L or less for 20–30 µm droplets. [2]
The Corps study recommends polyethylene plates at 60°, downward flow and up to 0.37 gpm/ft². [1]
The Corps study notes Army separators need periodic cleaning for sediment, not just oil, and recommends solids storage. [1]
IMO MEPC.107(49) tests with residual fuel, distillate and a surfactant emulsion, requires effluent at or below 15 ppm, an alarm responding within 5 s and automatic stopping of discharge within 20 s. [4]
Sources checked 2026-10-12. The U.S. Army Corps of Engineers research lab tested coalescing separators; the Nova Scotia filing records one facility's separator selection; Juneng makes centrifugal separators.
Ask for use case, specification boundaries, supplier type, and RFQ preparation information for this product.
Compare manufacturer profiles with relevant product and process capability.