Oil Separators and BS EN 858: What the Standard Actually Requires

An oil separator is a below-ground chamber that holds back petrol, diesel and oil from surface water run-off before it leaves a site. The standard behind it is BS EN 858, published in two parts: Part 1 covers how the product is designed and tested, Part 2 covers how you select a size, install it and maintain it. Almost every specification, planning condition and drainage drawing in the UK that calls for a "Class 1 full retention separator" is reaching back to those two documents, usually without saying so.

What the standard promises and what a separator delivers on a live site are different things, and the gap is written into the standard itself.

The Class 1 figure everyone quotes, 5 mg of oil per litre, is a laboratory result. BS EN 858-1 states it applies when the unit is tested to clause 8.3.3.1 and the samples are analysed for hydrocarbon content by infrared spectroscopy.

That distinction runs through everything below. The standard describes what a separator can achieve on a test rig. What it achieves on your site depends on how it was sized, what runs into it, and whether anyone has opened the lid in the last six months.

When does a site need an oil separator?

The Environment Agency's current guidance says a separator is typically needed on any site with a risk of oil contamination, and names five cases. That list is the practical trigger most drainage engineers and planning officers work to, and it has not changed since the guidance was last edited on this point in January 2024.

• Car parks over 800m², or with 50 or more spaces. The threshold is either/or, so a compact 50-space layout under 800m² still triggers it.
• Smaller car parks discharging to a sensitive environment. The Environment Agency's example is a marsh designated as a nature reserve.
• Vehicle maintenance areas. Workshops, service bays and anywhere oil is drained or handled over hardstanding.
• Roads. Adopted highway drainage and estate roads, and any site inside a critical drainage area, where the bar is higher.
• Refuelling facilities. Retail forecourts and private fuel islands alike, including diesel-only installations.

The regulatory function is narrower than most people assume. Under the Environment Agency's permitting guidance you do not need an environmental permit "to discharge uncontaminated water collected from public roads and small parking areas (that's been through a properly maintained oil separator) to surface water". In permitting terms the separator's job is to keep the discharge inside the no-permit category. There is no standard rules permit covering surface water run-off from car parks, forecourts or highways, so a discharge that falls outside that category needs a bespoke permit and a much longer determination.

In practice all of this has to be argued in the drainage submission that accompanies the planning application, which is where the specification, the sizing and the maintenance commitment are tested together. If a scheme has reached the point where a separator is on the drawing, it is worth having a surface water drainage strategy prepared by chartered drainage consultants rather than bolting the unit onto a layout and hoping the water quality case follows.

What does BS EN 858 actually cover?

BS EN 858 is a two-part standard with a clear division of labour. Part 1 is the product standard: it sets the classes, the nominal sizes, the design and material requirements, and the type test a manufacturer has to pass. Part 2 is the application standard: it tells a designer how to calculate the size, how to install the unit and what maintenance it needs. Part 1 carries a route to conformity marking. Part 2 does not.

• BS EN 858-1:2002+A1:2004. Principles of product design, performance and testing, marking and quality control. Prepared by CEN/TC 165 and published under UK committee B/505. The unamended 2002 edition is withdrawn, so citing "BS EN 858-1:2002" alone points at a superseded document.
• BS EN 858-2:2003. Selection of nominal size, installation, operation and maintenance. Published 15 April 2003 and never amended.

Both parts remain in their original editions. No revised edition of either part has been published, and no public draft has been issued.

Two details matter to a UK designer. The BSI national foreword disapplies the reaction-to-fire requirements at clauses 6.2.8 and 8.4, because there are no UK regulations dealing with reaction to fire of light liquid separators. And Part 2's normative references point at EN 752-2 and EN 752-4, both long since replaced by BS EN 752:2017, so a designer following Part 2 to the letter is being directed to withdrawn documents.

The standard also has a hard boundary that is easy to miss. It defines a light liquid as one with a density no greater than 0.95 g/cm³ that is "actually or practically insoluble and unsaponifiable". Above that density, or where the oil is emulsified or in solution, BS EN 858 does not apply at all. That single clause rules out most vehicle wash effluent, because detergent puts the oil into a stable emulsion the separator cannot break.

Is there a difference between a separator and an interceptor?

No. "Oil separator", "oil interceptor" and "petrol interceptor" all describe the same asset. "Separator" is the term BS EN 858 uses and the one that appears in current regulatory guidance. "Interceptor" is older UK usage that survives in Approved Document H, on drainage drawings, in older surface water drainage records and in the hazardous waste codes. Use "separator" in new documents and expect to read "interceptor" in older ones.

One naming difference does cause genuine confusion on site. BS EN 858-1 writes the classes in Roman numerals, class I and class II. UK regulatory guidance and Approved Document H write them in Arabic numerals, Class 1 and Class 2. They are the same two classes, and a specification that mixes both is not describing two different products.

What are Class 1 and Class 2 oil separators?

Class 1 separators are designed to discharge less than 5 mg of oil per litre under standard test conditions. Class 2 separators are designed to discharge less than 100 mg per litre. Any discharge to surface water, to a watercourse or to ground requires Class 1. Class 2 is only suitable where the discharge goes to foul sewer, and then only with the sewerage undertaker's agreement, which makes it a foul drainage decision as much as a surface water one.

The class is not defined by what sits inside the unit. Table 1 lists coalescing and gravity separation as the "typical separating technique (for example)" for each class, which is illustrative rather than definitional. Part 2 makes the point explicit by supplying a density factor for "class I separators operating by gravity only". A Class 1 separator is simply one that met 5 mg/l on the test, however it got there.

Approved Document H is blunt about the choice: "Separators discharging to infiltration devices or surface water sewers should be Class I." If the scheme relies on infiltration and the testing to support it, or on a soakaway designed to BRE Digest 365, Class 2 is not an option.

Full retention, bypass or forecourt: which type?

A full retention oil separator treats the whole flow the drainage system can deliver, conventionally taken as the run-off from 65 mm/hour of rainfall. A bypass separator fully treats flows up to 6.5 mm/hour and allows anything above that to pass untreated. A forecourt separator is a full retention unit sized for a tanker spillage as well as for the catchment.

"Full retention" is not a term in BS EN 858. The standard recognises a separator and a bypass separator, the latter defined as one "with a device, which allows a flow in excess of the maximum permissible effluent flow to bypass the separator". Full retention is UK shorthand for a separator without a bypass. The 65 mm/hour and 6.5 mm/hour intensities are UK guidance conventions as well; Part 2 says only that rainfall intensity "mainly depends on the analysis of local rainfall data and shall be adopted according to local regulations".

The standard is restrictive about bypass units. They "shall be used only in locations where it is unlikely that there will be significant hydrocarbon contamination during times of heavy rainfall", and they are ruled out for trade effluent altogether.

On a forecourt the sizing driver is the delivery tanker rather than the rainfall. Regulatory guidance treats a separator with 7,600 litres of oil storage, the typical tanker compartment size, as meeting BS EN 858-2 clause 4.3.6. Anything smaller needs a risk assessment behind it covering catchment size and foreseeable delivery spillage. If the tankers serving the site carry larger compartments, the separator has to be sized to them.

How is an oil separator sized?

Two oil separator sizing methods are in circulation and they are not the same calculation. BS EN 858-2 gives a flow-based equation. UK guidance gives a shortcut based on catchment area alone: NS = 0.018 × A for a full retention separator and NS = 0.0018 × A for a bypass, with A in square metres. The shortcut is a simplification of the equation that happens to hold for petrol and diesel, and stops holding elsewhere.

Nominal size is worth pinning down first, because it is routinely read as a tank volume. BS EN 858-1 defines NS as a "number, without units, approximately equivalent to the maximum effluent flow in litres per second from the separator when tested in accordance with 8.3.3". It is a designation derived from the type test. The standard's preferred nominal sizes run 1.5, 3, 6, 10, 15, 20, 30, 40, 50, 65, 80, 100, 125, 150, 200, 300, 400 and 500.

The standard's own method, at clause 4.3.1, is NS = (Qr + fx × Qs) × fd:

1. Calculate the maximum rainwater flow Qr in litres per second, from Qr = Ψ × i × A, where i is rainfall intensity, A is the area receiving rainfall measured horizontally, and the run-off coefficient Ψ can be taken as 1 in most cases.
2. Calculate the maximum wastewater flow Qs by adding the flows from draw-off points, car washes and high pressure cleaning units. Part 2 sets 2 l/s for each high pressure unit and 2 l/s for a car wash. For a car park with no process water, Qs is zero.
3. Apply the impediment factor fx to Qs only. It is 2 for trade effluent such as washing and process water, where detergents are expected, and 1 where the separator's duty is purely to retain spillage.
4. Apply the density factor fd to the whole bracket, taken from the table below according to the light liquid's density and the configuration of the system.
5. Round the result up to the next preferred nominal size.

Petrol and diesel sit at or below 0.85 g/cm³, so fd = 1 in every configuration. A car park has no trade effluent, so Qs is zero and fx drops out. The equation collapses to the rainfall term, which is exactly what the UK area-based shortcut encodes.

That is why the shortcut works, and it is also why it fails. Introduce a wash bay, a process discharge or a light liquid heavier than 0.85 g/cm³ and the factors stop cancelling. A Class 2 separator handling a liquid in the 0.90 to 0.95 band needs three times the nominal size the area formula would give. Minimum working capacity, excluding any silt provision, is 1,000 litres, and forecourts will normally need more than that. This is one of the calculations worth settling before layout is fixed, alongside the wider case for planning SuDS early.

The silt trap the UK shortcut under-sizes

UK guidance sets silt storage at C = NS × 100 litres for every separator, whatever the site does. BS EN 858-2 tiers it by expected sludge load. The UK figure matches the standard's lowest tier only, which is the tier the standard says should not be used at or below NS 10 except for covered car parks.

For an ordinary car park or forecourt draining petrol and diesel the two rules give the same answer, because fd = 1 and the load sits at the lower end. They diverge sharply on washing duties, where the standard demands two or three times the volume and imposes absolute floors the UK rule does not have. A designer who applies the area shortcut to a truck wash will under-size the silt trap against the very standard the specification cites.

Under BS EN 858-1 the sludge trap is not an accessory. A "separator system" is defined as an arrangement comprising a separator, a sludge trap and a sampling point, and the sludge trap has to have a flow-control device at the inlet to slow the incoming water and let sediment settle. On a bypass unit the silt capacity must sit upstream or in the bypass weir chamber, never in the main oil separating chamber. Getting sediment out before it reaches anything downstream is the same principle that governs the first flush of a storm and the sediment forebay on a pond.

Alarms, closure devices and sampling points

A full retention oil separator must have an automatic closure device that stops flow through the unit once the stored oil exceeds the oil storage volume. Every separator must have a device giving visual and audible warning when the oil reaches 90 per cent of that volume under static conditions. Since 1 January 2021 the alarm has been a legal requirement for separators supplied in England, Wales and Scotland under the Construction Products Regulations.

Oil storage volume follows from the nominal size: V = NS × 10 litres for a full retention separator, and V = NSB × 15 for a bypass. Do not fit a closure device to a bypass separator unless it was designed for one. Where the discharge is permitted, a sampling chamber is needed downstream of the separator. Where the unit retains flammable liquids, it needs ventilation.

Defra's National Standards go further on monitoring, and this is a requirement schemes routinely miss. Standard 7.18 says that where a drainage system includes "proprietary below ground products such as separators", provision "shall be made for automatic monitoring of the component's function and transmission of failure warnings to the body responsible for maintenance". A local alarm on a lid nobody visits does not satisfy that, and it is a point worth checking on any scheme carrying a SuDS proforma through validation.

Why an oil separator is not SuDS treatment

An oil separator carries no water quality credit in the CIRIA SuDS Manual, C753. Under the Simple Index Approach every generic SuDS component has published mitigation indices. "Proprietary treatment system" has none. The designer has to generate the performance evidence and persuade the regulator to accept it.

The design condition attached to that last row asks for a "detailed assessment of performance of designed component in reducing inflow concentrations of each pollutant type required as evidence of adopted indices". The Manual's proprietary treatment checklist goes further, asking for test data showing removal "for rainfall events up to the 1 year return period", and separate data showing the captured pollutants are not remobilised and washed through by later rainfall. That is a materially heavier burden than specifying permeable paving or a swale, which arrive with their indices already published.

Defra's National Standards for SuDS take the same position from the other side. Standard 4.7 accepts that higher-risk land uses "are likely to require proprietary treatment products as part of the management train", naming oil separators, but only where those products are "supported by sufficient evidence of their likely performance to the satisfaction of regulators". Part of the train, in other words, not instead of it. That is the same logic behind the four pillars of SuDS and the SuDS hierarchy: treatment is a sequence of components in series, not a single device at the outfall.

Two other regimes make the ranking explicit. On the strategic road network, National Highways requires a Departure from Standard application to use a bypass separator, while the standard vegetative solutions need no such application; its published treatment efficiencies give a bypass separator zero per cent for dissolved zinc and copper against a swale's 50 per cent. And for the highest-risk uses, National Standards 4.8 says a SuDS approach is unlikely to be appropriate at all: refuelling and washing operations should discharge to the public foul sewer under consent from the undertaker, or leave the site by tanker for disposal at a licensed facility. That is a foul drainage question rather than a surface water one.

None of this makes separators wrong. It makes them a component with a burden of proof attached, and that burden lands hardest on the high-hazard land uses where separators are most often specified: data centre and battery storage compounds, haulage yards and industrial estates. On sites where the pollution risk genuinely justifies one, such as an anaerobic digestion or biogas plant with segregated clean, grey and dirty water catchments, the full retention separator on the grey water stream is the right answer and the evidence is straightforward to assemble.

What an oil separator costs to own

Every six months, or at whatever interval the manufacturer sets, experienced personnel have to inspect the separator's physical integrity and mechanical parts, assess the accumulated oil and silt depth, service the alarm and management system, check the coalescing device and replace it if needed, and clean the sampling shaft. Every five years the unit should be emptied and given a general inspection to test the integrity and performance of the system. Everything removed is hazardous waste.

There is no such thing as non-hazardous separator waste. Chapter 13 05 of the List of Wastes covers oil/water separator contents, and every entry in it carries an asterisk.

• 13 05 01* solids from grit chambers and oil/water separators
• 13 05 02* sludges from oil/water separators
• 13 05 03* interceptor sludges
• 13 05 06* oil from oil/water separators
• 13 05 07* oily water from oil/water separators
• 13 05 08* mixtures of wastes from grit chambers and oil/water separators

Each removal needs a hazardous waste consignment note and a permitted facility able to accept the waste type. After emptying, the separator has to be refilled with clean water, not with the settled water that came out of it. Different hazardous wastes must not be mixed, and mixing hazardous with non-hazardous waste makes the whole load hazardous. The duties sit under separate health and safety law once anyone has to enter the chamber.

The maintenance itself is not a caretaker task. A below-ground separator chamber meets the HSE's description of a confined space, an enclosed space carrying a risk of serious injury from hazardous substances, and the HSE names enclosed drains, sewers, pits connected to drainage systems and open-topped chambers among its examples. Add the explosion protection requirement above, which sits under the DSEAR approved code of practice, and the six-monthly inspection becomes a specialist, permit-controlled operation for the life of the asset.

A separator does not remove a pollution problem from a site. It concentrates a diffuse surface water pollutant into an absolute hazardous waste that has to be tankered, consigned and disposed of at the operator's cost, for as long as the site exists.

That is the honest whole-life picture, and it is the part of the specification most often left out of the maintenance plan. A SuDS proforma that lists a separator without naming who empties it, how often and at what cost is an incomplete document, and drainage officers increasingly say so.

Where the UK guidance has gone stale

The guidance a UK designer is pointed to is out of date, and in England part of it has disappeared. PPG3, the Environment Agency guidance on the use and design of oil separators, has been withdrawn. Since 22 April 2026 its GOV.UK address redirects to the National Archives, and no like-for-like replacement has been published. What remains in England is a section inside one general business guidance page.

Approved Document H is the sharper problem, because it is still in force and a building control body will still apply it. It remains the 2015 edition, unamended.

The practical consequence is that two regimes can pull the same asset in different directions. Approved Document H says fit a separator. The planning system, through the National Standards and the SuDS Manual, asks why a separator rather than a treatment train, and asks for evidence if the answer is a proprietary product.

That is a familiar gap on drainage schemes, and it is the same fault line that runs through the relationship between building control and planning on drainage. Resolving it early is cheaper than discovering it at the point of discharging a drainage condition, and the process and cost of getting conditions discharged is worth understanding before the specification is locked.

Guidance is also divided by nation. GPP 3, published in March 2022 as the replacement for PPG3, is current in Scotland and Northern Ireland and states plainly that it "is not endorsed by the Environment Agency as regulatory guidance in England". It also records that Welsh guidance "is currently being reviewed and will be updated shortly", which it has said since 2022.

What this means for a drainage strategy

Specify an oil separator where the pollution hazard genuinely warrants one, size it from the standard rather than the shortcut wherever the site has wash-down or process flow, and present it as one component of a treatment train with the evidence the approving body will ask for. A separator dropped onto a drawing as a substitute for water quality design is the version that attracts an objection. A surface water drainage strategy for planning should be able to show all of the following.

• Classify the pollution hazard first. Use the Simple Index Approach land use categories, and say which one the catchment falls into and why.
• State the class and the type, with reasons. Class 1 for anything reaching surface water or ground; full retention or bypass justified against the spillage risk, not against the cost.
• Show the sizing calculation. Give NS and the route you took to it, and use the BS EN 858-2 equation where there is any process or wash-down flow.
• Size the silt trap to the right tier. Name the expected sludge load and apply 100, 200 or 300 times NS/fd accordingly.
• Cover the alarm, the closure device and remote monitoring. Standard 7.18 wants failure warnings transmitted to whoever maintains the asset.
• Set out the maintenance and waste route. Six-monthly inspection, five-yearly integrity test, consignment notes and a named permitted facility.
• Show the rest of the treatment train. The separator is one stage; source control and the downstream components carry the water quality case with it.

If a scheme is at the point where any of this is in question, or an LLFA has asked for the water quality case to be evidenced properly, Unda can prepare a surface water drainage strategy that stands up to a drainage officer, or review a specification that has already been drawn. Terms used here are defined in Unda's flood risk and drainage glossary, and the wider service sits within Unda's drainage strategies for planning applications.

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