Oil Separator Alarms and Maintenance: The 90% Rule, the Six Month Check and the Fuel That Slips Through
TL;DR: An oil separator, also called an interceptor, stops oil in surface water runoff from reaching drains and rivers. UK guidance expects three things of its owner: an alarm that warns before the oil store is full, a check at least every six months, and records of every inspection, emptying and service. Since 1 January 2021 separators supplied in England, Wales and Scotland must come with that alarm. It only protects anyone if someone hears it.
Last updated: 16 September 2026
On this page
Key takeaways
- Government guidance for England says a separator is typically needed on car parks over 800 m² or with 50 or more spaces, vehicle maintenance areas, roads and refuelling facilities.
- The British Standard says an oil separator should have an automatic warning device or high level alarm. Since 1 January 2021 that has been a legal requirement for separators supplied in England, Wales and Scotland.
- GPP 3 sets the trigger: a visual and audible warning, relayed to a remote supervisory point where necessary, when oil reaches 90% of the oil storage volume.
- Check at least every six months, keep full service and maintenance records, and carry out an integrity test at least every five years.
- Detergent, poor sizing and hydrotreated vegetable oil (HVO) can all defeat a separator. HVO can mix with water and be released.
What is an oil separator?
An oil separator, also called an interceptor, is an underground tank on a surface water drain that lets rainwater through while holding back oil and silt. Oil is lighter than water, so it floats and stays in the tank. Silt is heavier, so it sinks. The cleaner water in between leaves through an outlet set below the oil layer.
Government guidance on pollution prevention for businesses says you may need an oil separator wherever oil can wash off a hard surface with the rain. It lists the typical cases: car parks larger than 800 m² or with 50 or more spaces, smaller car parks that drain to a sensitive environment, vehicle maintenance areas, roads and refuelling facilities.
The standard behind every oil separator is BS EN 858, in two parts. Part 1 covers the design, performance and testing of the product. Part 2 covers choosing its size, installing it, operating it and maintaining it. GPP 3, the guidance on oil separators published in 2022 by the Scottish and Northern Irish environmental regulators, is built largely on the same standard and gives the most detailed public account of how separators should be run.
Which type of oil separator do you need?
It depends on two questions: where the water goes, and how big a spill could be. The first decides the class of separator. The second decides whether it must treat every flow the drains can deliver, or can let the heaviest storms bypass it.
GPP 3 defines the classes by what they achieve under standard test conditions, and the types by how much flow they treat:
| Choice | Option | What it does | Typical use |
|---|---|---|---|
| Class | Class 1 | discharges under 5 mg of oil per litre | surface water drains and the water environment |
| Class | Class 2 | discharges under 100 mg of oil per litre | foul sewer, with the sewer provider’s agreement |
| Type | Full retention | treats the full flow, normally rain at 65 mm per hour | forecourts and vehicle maintenance areas |
| Type | Bypass | treats flows from rain up to 6.5 mm per hour | car parks where any spill would be small |
Any discharge to surface water needs a Class 1 oil separator. A forecourt separator must be a full retention unit, and where tanker deliveries are received, GPP 3 says an oil storage capacity of 7,600 litres, the typical size of a tanker compartment, meets the British Standard. The minimum working capacity of any separator should be 1,000 litres.
A foul sewer discharge also needs the sewer provider’s permission, usually as part of a trade effluent consent. Our guide to trade effluent monitoring covers what those consents tend to ask for. On low risk sites, sustainable drainage can replace an oil separator altogether, and our SuDS monitoring guide explains how those systems are checked.
Does an oil separator need an alarm?
Yes. The government guidance for England says the British Standard calls for an automatic warning device or high level alarm, and that since 1 January 2021 this has been a legal requirement for separators supplied in England, Wales and Scotland under the Construction Products Regulations.
GPP 3 is specific about what the alarm must do. It should give a visual and audible warning, if necessary at a remotely located supervisory point, when oil reaches 90% of the oil storage volume under static liquid level conditions. At that point the oil separator needs emptying straight away, because a full oil store is how oil escapes.
Full retention separators also carry an automatic closure device, which stops flow through the separator once the oil exceeds its storage volume. GPP 3 says a high level alarm should alert the operator when that device activates, because the drain upstream will start to back up. It also recommends a silt level alarm, particularly on quarries, builders’ yards and major trunk roads, and notes that oil and silt alarms can often be fitted to an existing separator.
Older installations deserve a look. The 2021 requirement applies to separators supplied since then, so an oil separator installed before it may have no alarm at all, or one that stopped working years ago and was never tested.
How often should an oil separator be checked and emptied?
At least every six months. Government guidance says the British Standards recommend checking at least every six months, looking at how much oil and silt has built up and whether all the parts are working. GPP 3 adds that heavily used or high risk sites may need more frequent inspections.
GPP 3 lists what each six monthly visit should cover, carried out by experienced people or in line with the manufacturer’s instructions:
- Inspect the integrity of the separator and all its mechanical parts.
- Measure the depth of accumulated oil and silt.
- Service the electrical equipment, including the alarms.
- Check the coalescer, if one is fitted, and replace it if needed.
- Clean the sampling shaft where there is one.
Empty and service the oil separator when a significant amount of oil or silt has built up, after an oil spill, and whenever an alarm activates. The work needs a waste company experienced with separators and holding a waste carriers licence. Trapped oil and sediment are hazardous waste, moved under a hazardous waste consignment note to a permitted facility.
Two details are easy to miss. The separator must be refilled with clean water before it goes back into use, not the settled water that came out of it, or it will not hold oil in the next spill. And the British Standard expects an integrity test at least every five years, with full service and maintenance records available for inspection.
What can stop an oil separator working?
Five things, and none of them is visible from the car park. The government guidance names three directly: an oil separator does not work if it is the wrong size, if it is not maintained, or if there is detergent in the water. The other two are newer fuels and alarms that nobody hears.
- Detergent. Detergent from vehicle washing keeps oil mixed into the water, so it cannot separate. GPP 3 says drainage containing detergent should not enter a separator that discharges to surface water.
- Hydrotreated vegetable oil. HVO is a renewable alternative to diesel, and the guidance warns that it can mix with water and be released from the separator. As it breaks down in a river it strips oxygen from the water. If HVO is stored or used on the drained area, speak to your supplier about additional filters.
- Silt. Silt takes up the volume the separator needs to work. GPP 3 notes a separator may not work to full capacity when it is full of silt.
- The wrong refill. A separator refilled with its own settled water, or left part empty, cannot hold oil.
- An alarm nobody hears. A beacon on a depot wall at 2am protects no one. The alarm needs to reach a person who can act on it.
How do you monitor an oil separator remotely?
Send its alarms to people rather than to a beacon, and add a level reading that shows what the chamber is doing. GPP 3 already allows for this: its warning requirement includes relaying the alarm to a remotely located supervisory point where necessary, which on an unstaffed site means almost always.
Start with the alarm you already have. Keep the maker’s certified probe in the oil separator, and ask what remote output its alarm panel offers, typically a relay or a data connection. That signal can be brought into a telemetry system, so a 90% alarm becomes a message to the site manager and the waste contractor, with the time recorded.
Then add liquid level. The AQUAIOT Radar measures level without contact, to within ±2 mm, and reads reliably through foam, vapour and condensation in enclosed underground chambers. Mounted above the separator or its inlet chamber, it shows a rising level when the closure device has shut or the outlet has blocked, which is the high level condition GPP 3 asks you to alarm.
Anything electrical in a separator must be safe for an explosive atmosphere. GPP 3 says any electrical device used in a separator, or used to monitor sensors placed in one, must be certified to a suitable explosion protection standard and conform to BS EN 60079-10-1, the standard for classifying explosive gas atmospheres. The AQUAIOT Radar comes in ATEX variants for hazardous environments, and our guide to ATEX radar level sensors explains the checks.
The full AQUAIOT oil separator capability stack
A separator is one asset in a drainage system, and the same monitoring covers the rest of it. These are the layers AQUAIOT brings to separators and the drains around them.
Chamber level
The AQUAIOT Radar reads liquid level in the separator, the inlet chamber or the outfall, so a closure device that has shut, a blocked outlet or a surcharging drain shows up as a trend line rather than a flooded yard.
Existing alarm panels
Oil and silt alarms keep their certified probes. Panels with a data output join the same dashboard over RS485 and Modbus or an open API, with alerts routed to the right people by role.
Drainage around the separator
Separators also fail from upstream. Level monitoring on sumps, chambers and outfalls across the site sits in the same view, and the sewer monitoring service brings the layers together.
Records for the six month check
Alarms and level readings are time stamped in role based dashboards with trend lines and exports, which makes the service log GPP 3 asks for easier to keep and easier to defend.
Frequently asked questions
Do oil separators need an alarm by law?
Separators supplied in England, Wales and Scotland since 1 January 2021 must be fitted with an automatic warning device or high level alarm, under the Construction Products Regulations. GPP 3 says the warning should sound when oil reaches 90% of the oil storage volume.
How often should an oil separator be emptied?
Whenever a significant amount of oil or silt has built up, after any spill, and whenever an alarm activates. It should be checked at least every six months, and emptied for an integrity test at least every five years.
What is the difference between a Class 1 and a Class 2 separator?
A Class 1 separator is designed to discharge less than 5 mg of oil per litre and is required for discharges to surface water. A Class 2 separator is designed for less than 100 mg per litre and suits discharges to the foul sewer, with the sewer provider’s agreement.
What is the difference between full retention and bypass separators?
A full retention separator treats the full flow from the drained area, normally rain at 65 mm per hour, and suits high risk sites such as forecourts. A bypass separator treats flows from rain up to 6.5 mm per hour and lets heavier flows bypass it, so it suits low risk areas such as car parks.
Can HVO pass through an oil separator?
It can. Government guidance warns that HVO can mix with water and be released from a separator. If HVO is stored or used on the area the separator drains, speak to your supplier about the additional filters that may be needed.
Is waste from a separator hazardous?
Yes. Trapped oil and sediment removed from a separator is hazardous waste. It must be moved under a hazardous waste consignment note, by a company holding a waste carriers licence, to a permitted facility that can accept it.
Where to start
Find the separator first. GPP 3 asks for every separator to be shown on the drainage plans and for its manhole cover to be marked Separator, but on older sites it is often an unlabelled lid in the corner of the yard. Then read its label, which should state its class, nominal size, oil storage capacity and alarm details.
Next, test the alarm and ask who would hear it at 2am. If the answer is nobody, that is the gap to close first, and it is usually the cheapest part of looking after an oil separator properly.
Speak to an AQUAIOT engineer about bringing your separator alarms and chamber levels into one dashboard, alongside the rest of your drainage.