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Oil & Grease Removal Companies
Oil/water separation experts, API separators, DAF, coalescers, and membranes for free, emulsified, and dissolved oil.
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Oil and Grease Removal from Water: DAF, API Separators, and Coalescing Plate Interceptors
Oil and grease (O&G) in industrial wastewater includes free oil (droplets above 150 microns, easily settled or floated), dispersed oil (droplets 20 to 150 microns, requiring coalescing or flotation), emulsified oil (droplets below 20 microns stabilised by surfactants, requiring chemical emulsion-breaking), and dissolved hydrocarbons (require biological or activated carbon treatment). US EPA effluent guidelines specify O&G limits by industry: offshore oil and gas 29 mg per L (30-day average); petroleum refining 9.9 mg per L (30-day average, 40 CFR Part 419); and food processing varies by product. EU IED BAT-AEL for refineries: 1 to 5 mg per L O&G in treated effluent. Environment Agency UK trade effluent consent typical limit: 5 to 30 mg per L O&G.
API (American Petroleum Institute) oil-water separators (API 421) use gravity separation for free oil removal: rectangular vessel with 30 to 60-minute HRT, horizontal flow velocity below 0.003 m per s (to prevent turbulence resuspending settled solids), surface loading 1.5 to 3.0 m3 per m2 per hour, oil collection by skimmer. API separators remove droplets above 150 microns at 90 percent efficiency. Coalescing plate interceptors (CPI, parallel plate or corrugated pack media) enhance API performance: plates at 45 degrees (chevron) with 15 to 50 mm spacing provide enhanced surface area for droplet coalescence and collection, removing droplets above 50 to 60 microns at 90 percent efficiency within a fraction of the footprint of an API separator. Dissolved Air Flotation (DAF) generates 20 to 80 micron bubbles (by dissolving air at 4 to 6 bar and releasing at atmospheric pressure) that attach to oil droplets and float them to the surface; removes droplets above 10 to 20 microns at greater than 95 percent efficiency.
Chemical demulsification is required for stabilised oil-water emulsions (O/W emulsions from cooling lubricants, cutting oils, metal working fluids, food processing). Emulsion-breaking chemicals (cationic coagulants: polyDADMAC, Al/Fe salts; pH adjustment to ISO point of emulsifying agent; acid cracking of soap-type emulsifiers at pH 2 to 3) destabilise the emulsion and allow the oil phase to coalesce and separate. Acid cracking: adjust pH to 2 to 3 with H2SO4, heat to 50 to 70 degrees C, settle or DAF. Coagulation-based breaking: add 200 to 500 mg per L alum or ferric at pH 6 to 8, 2 to 5 minutes rapid mix, 15 to 20 minutes flocculation, then DAF removes floated oil-sludge complex. Ultrafiltration (UF) or microfiltration is an emerging replacement for chemical demulsification in cutting oil treatment, avoiding chemical waste.
Frequently Asked Questions
What is the difference between an API separator and a DAF unit for oil removal?
An API oil-water separator (API 421) uses gravity separation alone: oil rises to the surface (density difference) in a quiescent chamber, removed by skimmer; heavy solids settle to floor and are removed by scraper. Effective for free oil above 150 microns. Simple, low-cost, no chemicals needed. Limitations: large footprint (30 to 60 min HRT), ineffective for dispersed or emulsified oil below 150 microns. Dissolved Air Flotation (DAF) uses micro-bubbles (20 to 80 microns diameter, generated by air dissolution under pressure) that adhere to oil droplets and float them rapidly to the surface as a froth layer, removed by skimmer paddle. Effective for oil droplets above 10 to 20 microns. Chemical coagulant and flocculant addition (10 to 50 mg per L FeCl3 plus 1 to 5 mg per L polyelectrolyte) improves emulsified oil removal by breaking the emulsion before flotation. DAF achieves 95 to 99 percent O&G removal in a compact footprint (HRT 15 to 30 minutes). Capital cost: API separator $20,000 to $200,000; DAF $50,000 to $500,000.
What are the discharge limits for oil and grease in industrial wastewater?
US EPA effluent guidelines (sector-specific): oil and gas extraction (offshore): 29 mg per L monthly average total O&G (40 CFR Part 435); petroleum refining: 9.9 mg per L (30-day average), 15 mg per L (daily maximum) total O&G (40 CFR Part 419); food processing (meat, poultry): 10 mg per L (40 CFR Part 432); metal finishing: no explicit O&G limit but TSS and pH limits indirectly control emulsified oil. EU Industrial Emissions Directive (2010/75/EU) BAT-AEL for refineries: 1 to 5 mg per L O&G in treated effluent per BAT Conclusions published 2015. UK Environment Agency: site-specific consent limits typically 5 to 30 mg per L total O&G for discharge to sewer; 5 to 10 mg per L for direct discharge to surface water. Petroleum-contaminated stormwater from petrol station forecourts: EN 858-1 Class 1 separator required (effluent below 5 mg per L free oil).
How is emulsified oil removed from wastewater?
Emulsified oil (droplets below 20 microns stabilised by surfactants or mechanical emulsification) requires chemical or physical emulsion breaking before separation. Methods: (1) Acid cracking: reduce pH to 2 to 3 with H2SO4 (breaks soap-type emulsifiers by protonating carboxylate surfactants); heat to 60 to 80 degrees C to accelerate coalescence; settle or DAF to separate broken emulsion. Cost-effective for high-pH, soap-emulsified cutting fluid waste. (2) Coagulation: add alum (200 to 500 mg per L) or polyDADMAC (10 to 50 mg per L) to neutralise surface charge on oil droplets, allowing coagulation and DAF removal. Achieves O&G below 10 mg per L. (3) Ultrafiltration (UF, 0.01 to 0.1 micron): rejects oil droplets mechanically regardless of emulsion type; effective for waste cutting oils and coolants; permeate at below 10 mg per L O&G; generates concentrated reject (10 to 30 percent oil content) requiring further disposal. (4) Biological treatment: dissolved and emulsified hydrocarbons are biodegradable; activated sludge or biofilm systems after physical oil removal achieve effluent O&G below 5 mg per L.
Can oil recovered from wastewater be reused?
Oil recovery and reuse feasibility depends on oil purity and concentration. High-purity free oil skimmed from API separators at petroleum facilities: can be returned to the refinery crude oil feed or sold as slop oil at $150 to $400 per tonne (well below crude oil price due to water content and contamination). Edible oil recovered from food processing wastewater (DAF float): subject to food safety regulations, cannot be returned to food production; may be used as animal feed ingredient if clean, or as biodiesel feedstock. Mineral oil from metal working coolant waste: concentrated waste coolant (5 to 30 percent oil) from UF treatment can be burned as a fuel in cement kilns or co-fired industrial boilers under waste fuel permit conditions. Oil-water mixture skimmings from refinery API separators (3 to 10 percent oil): typically mixed with crude oil or burned as fuel. Recovery economics improve when oil commodity price is high; at oil prices below $50 per barrel, recovery from wastewater may be net-cost neutral versus disposal.
A machining facility generating 180 m3 per day of mixed cutting fluid and rinse water with oil and grease concentrations ranging from 150 to 600 mg per L was discharging to sewer under a trade effluent consent specifying below 30 mg per L O&G. Consent exceedances were triggering consent review proceedings with the sewerage undertaker.
The existing gravity interceptor was replaced with a two-stage system: a coalescing plate interceptor removed free oil above 60 microns, followed by a DAF unit (with 20 mg per L ferric dosing and 2 mg per L polyelectrolyte) treating the plate interceptor overflow. A weekly oil skimmer servicing programme was implemented on both units. UF (0.05 micron PVDF) was added as polishing for the emulsified fraction from the grinding bays.
Combined O&G in the DAF/UF effluent averaged 6.8 mg per L over the first 12 months of operation, well within the 30 mg per L consent limit. The sewerage undertaker formally withdrew the consent review. Recovered oil from the skimmers (3 to 4 tonnes per month) was collected by a licensed waste oil contractor and co-processed as secondary fuel.
Questions to Ask Shortlisted Providers
- 1
What is the oil and grease concentration, type (free, dispersed, emulsified), and flow rate of the wastewater stream?
Technology selection depends entirely on droplet size distribution; misidentifying emulsified oil as free oil will result in a system that cannot meet the consent limit.
- 2
What is the trade effluent consent O&G limit and how is it measured (total O&G, free oil, or n-hexane extract)?
Measurement methodology matters; EN ISO 9377-2 (n-hexane extractable) and gravimetric methods give different results for the same sample, affecting compliance assessment.
- 3
Are any surfactants, detergents, or emulsifying agents used in the process that could stabilise oil-water emulsions?
Surfactant type and concentration determine what emulsion-breaking chemistry is required; some synthetic surfactants resist standard coagulation and require specific demulsification agents.
- 4
What is the proposed oil skimmings disposal route and is there a licensed waste oil contractor with collection capacity?
Recovered oil is classified as hazardous waste; disposal must be by a licensed contractor and tracked under waste duty of care; disposal cost affects the O&M budget.
- 5
What are the ATEX zone classifications in areas where oil mist concentrations could reach flammable levels?
Cutting oil mists can form flammable concentrations; DSEAR 2002 zone classification affects equipment specification for fans, motors, and control panels in the treatment enclosure.
What Drives Cost in This Category
DAF unit capacity (and therefore capital cost) scales with flow; high O&G loads increase chemical consumption and skimmate volume, raising OPEX proportionally.
Highly stabilised emulsions requiring specialist coagulants (polyDADMAC, polymer blends) can increase chemical OPEX by 3 to 8 times compared with simple ferric-based DAF for free and dispersed oil.
UF or biological polishing adds 100,000 to 400,000 GBP capital for systems treating mixed free and emulsified oil streams; justified only where the consent limit is below 10 mg per L O&G.
Licensed waste oil collection costs 50 to 150 GBP per tonne; high-volume generators with oil quality suitable for re-refining may negotiate zero-cost or credit collection contracts.
Key Regulations & Standards
Governs all industrial discharges to public sewer; O&G consent limits (typically 30 to 50 mg per L) are enforced by the sewerage undertaker, with consent review and surcharge for persistent exceedances.
European standard for the design, performance, and testing of oil separators; Class 1 separators (below 5 mg per L free oil in effluent) are required for petrol station forecourts and vehicle servicing areas.
Requires ATEX zone classification where flammable oil mist or vapour may be present; affects equipment selection for fans, motors, and electrical installations in oil treatment enclosures.
Recovered oil and oily sludges are classified as hazardous waste (EWC 13 02); disposal must be by licensed carrier with Hazardous Waste Consignment Notes and registered waste site; retained for 3 years.

















