Oil separators and car park drainage design
Separators only remove free oil and they need maintaining. For most car parks the right answer is a treatment train, with the separator reserved for high hazard surfaces.
An oil separator is needed where a car park or service yard drains to a watercourse, a soakaway or a surface water sewer and the surface carries a real risk of oil, fuel or silt reaching that discharge. For most small commercial car parks the answer is not a separator at all but a treatment train of permeable paving or a filter strip, because separators only remove free oil and they need maintaining. Oil separator car park drainage design is a pollution control question first and an attenuation question second, and the two get conflated constantly.
Get the pollution hazard right and the component choice follows.
Which surfaces need treatment before discharge
Start by classifying the surface by what happens on it, not by its size.
Roofs are clean. They discharge without treatment in almost every case. Domestic driveways and small staff car parks with low turnover carry a low hazard: tyre wear, brake dust, occasional drips. A high-turnover retail or hospital car park, a coach park, a lorry park or a delivery yard carries a medium hazard. Fuel dispensing areas, vehicle washing bays, waste transfer areas and anywhere oils or chemicals are handled or stored carry a high hazard, and those need both treatment and containment.
The CIRIA SuDS Manual, C753, is the reference the Environment Agency and lead local flood authorities use for this, and it sets the pollution hazard indices for each land use type and the mitigation indices for each treatment component. The method is to match the hazard index for the surface against the mitigation index of the components in series. That is the calculation a drainage engineer should be showing in the Drainage Strategy, and its absence is the commonest gap in a submitted scheme.
Treatment trains and the sequence of components
A treatment train is a sequence of components, each removing something, arranged so the coarse material is intercepted first.
Interception comes before treatment. Capturing and losing the first few millimetres of rainfall removes a large proportion of the annual pollutant load, because the dirtiest water is the first flush off a dry surface. Then conveyance components that also treat: a filter strip or swale along the edge of the parking bays. Then the main treatment component: permeable paving over a sub-base, a bioretention area, or a pond with a permanent wet volume. Then attenuation, then discharge.
The sequence that gives trouble is a yard gully leading straight to a separator leading straight to a pipe. It treats one pollutant, has no interception, no silt removal beyond the separator's own sump, and offers nothing at all in a storm that exceeds the separator's flow rating.
Where space makes a surface component impossible, as on a tight urban yard, proprietary treatment devices and filter cartridges are the fallback. They work, they need a maintenance contract, and they should be the last choice rather than the first.
Separator classes and when each applies
Separators are classified by the quality of the effluent they produce and by whether they take the full flow or a bypass proportion.
A Class 1 separator achieves a lower residual oil concentration in the discharge and is specified where the receiving water is sensitive, which in practice means a discharge to a watercourse, to ground in a groundwater source protection zone, or anywhere the Environment Agency asks for it. A Class 2 separator achieves a higher residual concentration and is acceptable where the discharge goes to foul sewer, with the undertaker's consent, or to a surface water system where the regulator accepts it.
Full retention separators treat the whole flow from the drained area up to a design rate. Bypass separators treat the lower flows that carry most of the pollutant load and allow higher flows to pass around, which is accepted on car parks because a severe storm dilutes rather than concentrates oil. For fuel dispensing areas, a full retention Class 1 separator is the normal expectation, and the forecourt itself usually needs to be contained so that a spillage cannot leave the site.
Separators require an automatic closure device and an oil level alarm. Both get specified and neither gets commissioned, which is how a separator that has been full for two years keeps discharging.
Maintenance, alarms and record keeping
A separator that is not emptied is a storage tank for pollution with an open outlet.
The maintenance obligations are not complicated. Inspect regularly, measure the oil and silt depth, empty by licensed tanker before the oil layer reaches the trigger depth, service the alarm, and keep the waste transfer notes. Write all of that into the drainage strategy as a maintenance schedule naming the responsible party, because a planning condition will ask for one and a management company needs to inherit something intelligible.
Permeable paving has its own maintenance regime, which is sweeping and occasional vacuum cleaning of the surface to prevent clogging. It is less onerous than a separator and far more commonly neglected, mostly because the maintenance company does not know the car park is a drainage component.
Illustrative example: a 30-space car park serving a trade counter, draining to a ditch. Permeable paving over an attenuating sub-base handles interception, treatment and attenuation in one layer, with a flow control at the outfall. No separator. The high-hazard element, a small bunded oil store beside the building, drains to its own contained area with no connection to the surface water system at all. That split, treating the hazard where it arises rather than at the end of the pipe, is usually the better scheme.
Combining treatment with attenuation features
Attenuation and treatment can share the same structure, and the planning advantage of doing so is land take.
Permeable paving with a tanked or lined sub-base provides storage and treatment together. A detention basin with a shallow wet margin treats as it attenuates. A bioretention strip between parking rows treats, intercepts and takes up a strip that would otherwise be a kerb and a planting bed. The non-statutory technical standards for sustainable drainage systems and the Flood and Water Management Act 2010 regime sit behind what the lead local flood authority will ask for, and a scheme that stacks functions reads better than one with a separate device for each.
Where a site is in Flood Zone 2 or 3 the discharge design has to sit alongside the Flood Risk Assessment, because an outfall that cannot discharge during a flood event needs storage behind it. The surface water position is easy to overstate, and reading the surface water flood risk map without overstating it is worth doing before the strategy is written.
Discharge consents and undertaker requirements
Three permissions can bite, and they are separate from planning permission.
Discharging to an ordinary watercourse needs land drainage consent from the lead local flood authority for the structure in the channel, covered in land drainage consent for works to an ordinary watercourse. Discharging to a main river needs an environmental permit for a flood risk activity from the Environment Agency. Discharging to a public sewer needs the sewerage undertaker's agreement under section 106 of the Water Industry Act 1991, and the undertaker will set the permitted rate and may require a Class 2 separator or better.
Discharging to ground in a source protection zone is the one that changes the whole design. Infiltration of car park runoff may be refused outright, which pushes the scheme towards a lined system and a different outfall.
Specification in the drainage strategy
Write the pollution control case as its own short section rather than a sentence. State the surface types and their hazard indices, the components proposed and their mitigation indices, the resulting comparison, the separator class and type where one is used, the discharge destination and the consent route, and the maintenance schedule with the responsible party named.
Then attach the drawings that prove it: a catchment plan showing which surface drains where, and a section through each treatment component. Officers and lead local flood authority engineers read the drawings before the text.
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Frequently asked questions
Does every car park need an oil separator?
No. Separators are for medium and high pollution hazard surfaces such as fuel dispensing areas, lorry parks and vehicle washing bays. A small low-turnover car park is usually better served by permeable paving or a filter strip, which intercepts the first flush and removes silt as well as oil. The CIRIA SuDS Manual hazard and mitigation indices settle the question.
What is the difference between a Class 1 and Class 2 separator?
The classes describe the residual oil concentration in the discharge, with Class 1 achieving the lower figure. Class 1 is expected where the receiving water is sensitive, including discharges to a watercourse or to ground in a source protection zone. Class 2 may be accepted for a discharge to foul sewer with the undertaker's consent or to some surface water systems.
What is a bypass separator?
A bypass separator treats lower flows, which carry most of the pollutant load, and lets higher flows pass around it. It is accepted on car parks because a severe storm dilutes rather than concentrates oil. A full retention separator treats the entire flow up to its design rate and is the normal expectation for fuel dispensing areas and similar high hazard surfaces.
Who maintains an oil separator after the development is built?
Whoever the drainage strategy names, which is why the strategy has to name someone. Maintenance means regular inspection, measuring oil and silt depths, emptying by licensed tanker before the trigger depth is reached, servicing the oil level alarm and keeping waste transfer notes. A planning condition will usually require the schedule to be submitted and approved.
Do I need a permit as well as planning permission to discharge?
Often, yes. Works in an ordinary watercourse need land drainage consent from the lead local flood authority, works affecting a main river need an environmental permit for a flood risk activity, and a connection to a public sewer needs the undertaker's agreement under the Water Industry Act 1991. These run separately from the planning application and have their own timescales.
Can I infiltrate car park runoff into the ground?
Sometimes, but not in a groundwater source protection zone without careful justification, and sometimes not at all. Infiltration of runoff from a trafficked surface risks a direct route to groundwater, so the Environment Agency position and the zone designation have to be checked before the strategy assumes soakaways. If infiltration is ruled out, the scheme needs a lined system and another outfall.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.