Reuse, Recovery & Stormwater
Stormwater Oil-Water Separator Companies
Oil-water separator suppliers for car parks, fueling stations, industrial yards, and transport hubs.
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Oil-Water Separators for Stormwater: Class I, Class II, and Coalescing Plate Technology
Oil-water separators (OWS) for stormwater treatment remove petroleum hydrocarbons, free oils, and light non-aqueous phase liquids (LNAPLs) from surface runoff before discharge to sewer or watercourse. Regulatory driver: UK: Environmental Permitting Regulations 2016 require Environmental Permit for contaminated surface water discharge; EA Pollution Prevention Guidelines (PPG) series (especially PPG2 Above Ground Oil Storage Tanks, PPG7 Refuelling, PPG22 Incident Response for Land Drainage) mandate OWS for high-risk land uses (petrol stations, vehicle maintenance, car parks, industrial sites). Standard: BS EN 858-1:2002 (Separator systems for light liquids, Part 1: Principles of product design, testing, marking and quality control) and BS EN 858-2:2003 (Part 2: Selection of nominal size, installation, operation and maintenance) define Class I (effluent less than 5 mg/L hydrocarbon) and Class II (effluent less than 100 mg/L hydrocarbon) separator performance, plus sediment trap and bypass provisions. US: EPA Stormwater Best Management Practice Design Guide; API Separator standard (API 421) defines gravity separation for oil densities less than 0.95 g/cm3. CIRIA Report C697 (Control of water pollution from construction sites) and CIRIA C532 (Control of pollution from highway drainage discharges) specify OWS for construction and highway applications.
Separator operating principles and design: Class I (bypass) separator: single or multi-compartment gravity separator with fixed orifice bypass weir; oil (density approximately 0.7 to 0.9 g/cm3) rises to surface by Stokes' Law and is retained in an oil storage compartment; sediment settles to base of silt trap; in high-flow conditions (greater than 10 times design flow) the bypass weir diverts untreated flow directly to outlet. Nominal size (NS) selection per BS EN 858-2: NS is the design flow rate in L/s; each separator has an NS determined by the maximum stormwater runoff rate from the contributing catchment at the 1 in 1 year storm event. Class I separator with coalescing plate pack: packed inclined parallel plates (45 to 60 degrees, spacing 25 to 40 mm) provide increased effective settling area (enlarged surface per API 421 enhanced gravity formula); achieves less than 5 mg/L TPH effluent under steady-state conditions. Class II separator: gravity only (no plate pack); less than 100 mg/L TPH effluent; lower cost, simpler; used for lower-risk catchments (car parks without fuel facilities).
Installation, maintenance, and performance monitoring: BS EN 858-2 specifies installation requirements: separator located on stable level ground with vehicle access for tanker emptying; access chambers at inlet and outlet for sampling; venting to prevent vapour buildup; frost protection for regions with sustained sub-zero temperatures (insulation or heating trace). Maintenance schedule: weekly: visual check of oil level in oil storage chamber; monthly: check sediment trap depth (desludge when greater than 50 percent full); 6-monthly: sample and test effluent quality (TPH less than 5 or 100 mg/L per BS EN 858 class); annually: full inspection of coalescing plate pack (clean if fouled), check structural integrity, update maintenance record. Oil disposal: collected oil (plus water phase, LNAPL mixture) removed by licensed waste contractor; classified as hazardous waste (EWC 13 07 03 - oily mixtures); manifest required (Hazardous Waste Regulations 2005). Full bypass events: during extreme rainfall, untreated bypass flow may reach the watercourse; Environmental Permit may require spill contingency plans for bypass events. For very high-risk sites (fuel tanker loading, bulk fuel storage), interceptors must be designed for credible spill scenarios (full tank truck discharge), not just routine stormwater.
Frequently Asked Questions
What is the difference between Class 1 and Class 2 oil separators?
BS EN 858 classifies oil-water separators by their effluent quality performance: Class I separators: achieve effluent less than or equal to 5 mg/L total hydrocarbons (as n-alkanes C10 to C40, measured by IR spectrophotometry); required for sensitive receiving waters, water protection zones, and high-risk installations (petrol stations with forecourt runoff, fuel storage areas, vehicle washing); typically incorporate coalescing plate packs to enhance gravity separation; more expensive and larger than Class II. Class II separators: achieve effluent less than or equal to 100 mg/L total hydrocarbons; gravity-only separation without plate packs; suitable for general car parks (over 200 spaces typically required per PPG2), vehicle access areas, road drainage where TPH risk is lower; more compact and lower cost. Both classes include: bypass provision for flows exceeding nominal size (NS) rated flow; silt trap (sediment interceptor) upstream; automatic closure device (hydrostatic float valve) that closes the oil storage chamber outlet when oil storage is full, preventing oil carryover. Selection: BS EN 858-2 and PPG guidance specify which class is required based on land use type, sensitivity of receiving water, and proximity to groundwater source protection zones. Class I is required near SPZ1 and SPZ2 groundwater protection zones; Class II may suffice for Class 3 and lower-risk locations.
Do all car parks need oil-water separators?
Not all car parks legally require oil-water separators, but the threshold and assessment depends on risk, receiving water sensitivity, and local regulatory requirements. UK Environment Agency guidance (PPG2, PPG7, Pollution Prevention guidance): OWS required for: petrol station forecourts (Class I separator mandatory); vehicle washing facilities (all types); car parks and hard standings over 800 m2 or greater than 50 car parking spaces where surface water could reach a watercourse, soakaway, or coastal water; lorry parks, coach parks, and vehicle maintenance areas. Not required by default: small car parks (less than 800 m2 / less than 50 spaces) draining to combined sewer (though check Environmental Permit); sealed roof-draining car parks where surface is not exposed to vehicle traffic. CIRIA C532 (Highway Drainage): OWS required for certain highway drainage categories. Planning conditions: local authorities increasingly impose OWS requirements on new car park planning consents regardless of size if within a sensitive catchment. Best practice: sustainable drainage systems (SuDS) - permeable paving, bioretention - provide hydrocarbon removal through adsorption and biodegradation and may eliminate need for separate OWS; a SuDS treatment train (permeable paving + bioretention) can achieve Class I equivalent hydrocarbon removal for car parks.
How often should an oil-water separator be emptied?
Oil-water separator maintenance and emptying frequency per BS EN 858-2 and EA PPG guidance: Oil storage chamber: inspect monthly by checking oil thickness (dipstick or sight glass); arrange emptying when oil layer exceeds 50 mm depth or approaches manufacturer-specified maximum storage capacity (typically 1.5 to 3.0 times NS in litres, e.g. NS15 Class I separator holds approximately 450 L maximum oil); for petrol stations with high spill risk: monthly inspection mandatory. Silt trap: inspect every 6 months; desludge when sediment depth exceeds 50 percent of silt trap volume (typically 100 to 300 mm in standard precast OWS); high-sediment sites (construction area drainage) may require monthly emptying. Annual service: full inspection by qualified contractor; clean coalescing plates (jet wash with low-pressure hot water if fouled with biofilm or wax); check automatic closure device (float valve) operation; sample outlet water and measure TPH (should be less than 5 mg/L for Class I; less than 100 mg/L for Class II). Maintenance record: keep records for 3 years minimum (good practice); required as evidence of compliance with Environmental Permit condition. Oil waste disposal: EA Waste Carrier licence required for contractor; EWC code 13 07 03 (oily waste water mixtures); consignment note required if over 500 kg.
What regulations govern stormwater discharge from industrial sites?
UK stormwater discharge regulation for industrial sites: (1) Environmental Permitting Regulations 2016 (EPR): most surface water discharges from industrial sites are 'water discharge activities' requiring either a Standard Rules Environmental Permit (SR) or bespoke permit; standard permits available for some lower-risk scenarios (e.g. SR2010 No.9 for stormwater from certain industrial sites); TPH limits typically 5 to 15 mg/L, suspended solids 30 to 100 mg/L; (2) Water Resources Act 1991, Section 85 (now implemented through EPR): causing or knowingly permitting polluting matter to enter controlled waters is an offence; EA can prosecute without prior warning for serious pollution events (unlimited fine in Crown Court, 2-year imprisonment for individuals); (3) Trade Effluent Consent (Water Industry Act 1991): if stormwater is discharged to the public sewer, trade effluent consent from the sewerage undertaker is required if it contains industrial contaminants above domestic thresholds; (4) Groundwater Environmental Permit: required for indirect discharge to groundwater via infiltration (soakaways on industrial sites); (5) Contaminated land (Part IIA, EPA 1990): if industrial activity has caused contamination of underlying soil or groundwater, a site investigation and remediation plan under Part IIA may be triggered regardless of stormwater permit compliance.
A large vehicle storage and preparation facility in the East Midlands with 12 ha of sealed hardstanding was discharging untreated stormwater to an adjacent ordinary watercourse under a pre-2010 consent. An EA inspection identified TPH exceedances of up to 38 mg/L (above the Environmental Permit limit of 5 mg/L) following vehicle washing and tyre-change operations, and issued a Compliance Assessment and an improvement requirement.
The environmental consultant designed a stormwater treatment train comprising a 4,500 L Class I NS 30 separator (BS EN 858-1, achieving below 5 mg/L TPH) preceded by a 3,000 L silt trap, installed in-ground at the site's lowest point. The concrete chamber was manufactured from sulphate-resistant Portland cement. A hydrocarbon sensor alarm with automatic closure valve was fitted. Monthly inspection records and quarterly TPH sampling were added to the site's Environmental Permit compliance monitoring schedule.
Post-installation TPH sampling over four quarterly monitoring events showed outlet concentrations of 1.2 to 3.8 mg/L, all below the 5 mg/L permit limit. EA confirmed compliance at a follow-up inspection 10 months after installation. The site avoided the threatened enforcement notice and associated GBP 45,000 penalty. Annual maintenance cost (oil chamber pumping, silt trap desludging) is GBP 3,200.
Questions to Ask Shortlisted Providers
- 1
What is the receiving water type (ordinary watercourse, main river, coastal water, groundwater via soakaway) and is it within a Source Protection Zone?
Receiving water sensitivity determines whether a Class I (below 5 mg/L TPH) or Class II (below 100 mg/L TPH) separator is required; SPZ1 and SPZ2 require Class I and may require additional treatment such as bioretention or activated carbon polishing.
- 2
What is the peak stormwater flow rate (m3/hr) from the contributing catchment at the 1:10 year return period event?
BS EN 858-2 nominal size (NS) selection is based on peak throughput flow rate; an undersized separator allows short-circuit flow above the separator's coalescence design velocity, causing hydrocarbon carryover.
- 3
What types of hydrocarbons are present (mineral oil, diesel, petrol, PAHs) and is the separator surface water discharge to a groundwater source protection zone?
The presence of volatile components (petrol) affects separator design and requires vapour-tight covers; petrol stations require Class I plus additional venting to prevent explosion risk; SPZ1 may require activated carbon polishing.
- 4
Is there a current Environmental Permit for the surface water discharge and when was the last TPH compliance monitoring conducted?
EPR permits include specific conditions on separator maintenance and monitoring frequency; failure to maintain records or conduct sampling is a permit breach regardless of whether effluent quality limits are being met.
- 5
What is the separator maintenance regime and who holds the oil waste carrier licence for removal of separated hydrocarbons?
BS EN 858-2 requires monthly inspection and emptying when oil storage reaches 50 mm depth; oil waste removal requires an EA-registered waste carrier; failure to maintain the separator causes compliance failure and potential pollution event.
What Drives Cost in This Category
Class II precast concrete separators (NS 6 to NS 30) cost GBP 2,500 to 12,000 supply-only; Class I coalescing plate units cost GBP 8,000 to 35,000; GRP units cost 15 to 25% less than concrete but require WRAS/EA acceptance for the specific installation.
Below-ground separator installation typically costs GBP 5,000 to 20,000 depending on access, ground conditions, depth, and inlet/outlet pipework complexity; high-groundwater sites require sheet piling or dewatering.
Automatic closure (hydrostatic float valve) is standard on BS EN 858 separators at no significant premium; remote hydrocarbon sensors with SMS alarm add GBP 800 to 3,500 per separator but are required for unattended sites under some EA permits.
Annual maintenance (inspection, oil pumping, silt desludge) costs GBP 1,500 to 5,000 per separator depending on size and oil accumulation rate; oil waste disposal by licensed contractor costs GBP 100 to 300 per visit.
Key Regulations & Standards
BS EN 858-1 specifies design, testing, and performance requirements for Class I (below 5 mg/L TPH) and Class II (below 100 mg/L) separators; BS EN 858-2 covers selection of nominal size, installation, operation, and maintenance; compliance is required for EA-regulated surface water discharges.
Surface water discharges from industrial sites containing hydrocarbons are regulated water discharge activities requiring an Environmental Permit; Standard Rules Permit SR2010 No.9 is available for some lower-risk scenarios; bespoke permits specify TPH limits, monitoring frequency, and maintenance requirements.
Causing or knowingly permitting polluting matter to enter controlled waters is a criminal offence; the EA can prosecute directly without a prior warning; unlimited fine in Crown Court; incidents involving petroleum products entering watercourses are a prosecution priority.
EA Pollution Prevention Guidelines specify OWS requirements for petrol stations, vehicle washdown areas, and other high-risk land uses; PPG26 recommends spill containment bunds and stormwater management plans as part of site environmental management.
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