Petrol/Oil Bypass, Full Retention & Forecourt Interceptors

BMS manufacture a full range of Petrol & Oil Interceptors (Separators) from 1,000 - 100,000 L including Full Retention, By-Pass & Forecourt Interceptors for all requirements.  All BMS Interceptors are manufactured under an NSAI verified ISO 9001:2008 Quality Management System & to the new European Standard; PR EN 858 1&2.


Why do you need a Petrol/Oil Interceptor?

Surface water from sealed surfaces flows away via drainage to receiving waters, sewers or to soakaways.  If there is potential for petrol/oil spills onto these surfaces e.g. car parks, forecourts, roads, garages and depots, it is vital to intercept/separate any petrol/oil contaminants to prevent damage to the surrounding environment.  The petrol/oil is then held within the interceptor for safe removal.  BMS Petrol/Oil Interceptors perform this role.  BMS manufacture a full range of interceptors for all applications.  All BMS Interceptors are manufactured in rot proof and durable Glass Reinforced Plastic (GRP)  It is critical that the correct interceptor is selected depending on a sites particular requirements to ensure that they function correctly. A number of different factors need to be taken into account when selecting the correct interceptor. 

Case Studies on previous interceptor and surface water installation can be found in our Attenuation/Surface Water Case Studies Section

Please contact us for free advice on to the best interceptor to suit your needs.

BMS have introduced a full set of Petrol Interceptor/Holding Tank Sizing & Selection Charts to help you easily size and select an Interceptor/Holding Tank suitable for your requirements. They can be downloaded at this link.

Petrol/Oil Interceptor Terminology

  • Full Retention Interceptor: Designed to treat the full flow delivered by a drainage system up to 50 mm/hr.
  • Bypass Interceptor: Designed to treat flows of up to 5 mm/hr.  This covers 99% of rainfall.
  • Class 1 Interceptor: Designed to achieve a concentration of 5 mg/l of petrol/oil at discharge under normal conditions, includes a coalescer and a closure mechanism.  A Class 1 Interceptor is usually required for discharge to any surface water.
  • Class 2 Interceptor: Designed to achieve a concentration of 100 mg/l of petrol/oil at discharge under normal conditions.  Normally used when discharge is to a foul sewer.
  • Petrol/Oil Separator: Another word used to describe an interceptor.
  • NS: Nominal Size. Each interceptor is allocated a nominal size based on EN858.  This will indicate the nominal flow that the interceptor is sized for e.g. 4 l/s.  A full retention interceptor with a capacity of 4 l/s would be designated NS04 while a By-Pass interceptor with the same capacity would be designated NSB04.

Types of BMS Petrol/Oil Interceptor

Full Retention Petrol/Oil Interceptor

Used when there is a risk of regular contamination of surface water run off with petrol/oil and/or the risk of larger spills e.g. vehicle maintenance areas, heavy goods parking areas or where frequent vehicle manoeuvuring takes place.

By-Pass          Petrol/Oil Interceptor

Used when there is a risk of infrequent light contamination of surface water run off with petrol/oil and/or the risk of only small spills e.g. car parks, airports, industrial estates, roads and other low risk areas.                                                                  

Petrol/Oil       Forecourt Interceptors

The BMS Filterceptor range of Forecourt Interceptors is used when there is a significant risk of contamination of surface water run off with petrol or oil and/or a large risk of of spills e.g Petrol Stations, Haulage Fueling Yards & Fuel Depots.

Three Chamber Full Retention Interceptors

BMS Three Chamber Full Retention interceptors are available in a wide range of sizes varying from 1,100 L up to 100,000 L. Larger sizes can be accommodated on order. Full Retention interceptors are used in areas where there is a high risk of petrol/oil contaminating the on-site drainage e.g. Fuel Distribution Depots, Scrap Yards, Car/ Vehicle Workshops and Washdown areas. 


Under normal flow conditions the BMS Full Retention Interceptor is designed to retain the full contents of an oil spill or contamination up to flows of 50 mm/hr.

By-Pass Interceptors

A requirement of full retention interceptors is that they must have a large enough capacity to prevent pollutants being flushed through the unit during storm conditions.

In the case of larger drainage areas such as car parks, airports and industrial estates, Full Retention Interceptors are not practical, as they would require very large full retention petrol interceptors, making production, installation and handling more costly. A practical and economical solution to this problem is the BMS Bypass Interceptor range.


Under normal flow conditions the BMS Bypass Interceptor will separate and retain silt and pollutants such as oil and petrol prior to discharge.

In storm conditions, when periods of peak flow are experienced, it will treat a certain percentage of the maximum flow whilst at the same time allowing the excess to by-pass the retention chamber of the unit.

As contaminated water enters the first chamber, pollutants float to the surface. Due to the unique internal design; vortex formation occurs at the by-pass pipe resulting in the pollutants being skimmed off the surface and become trapped allowing the uncontaminated water to discharge from the interceptor.

When storm flow intensity increases, the vortex pipe floods thus restricting the flow from the first chamber to the retention chamber. As a result of the increase in water level in the first chamber, hydraulic pressure increases, forcing the majority of the water through the by-pass weir and into the chamber to discharge.


BMS have developed their 'Filterceptor' range of Forecourt Separators in line with the proposed new CEN European Standard for light liquid separators. It doubles both as an interceptor and separator of pollutants on the petrol station forecourt where there is significant risk of petrol or oil spillage. The Filterceptor is designed to contain the maximum contents of a spillage from a fuel delivery tanker compartment.

BMS Filterceptors Forecourt Interceptors

Doubles as both an Interceptor and a Separator of pollutants on the petrol forecourt, where there is significant risk of petrol or oil spillage. They are designed to contain the maximum contents of a spillage from a fuel tanker compartment.


The Filterceptor unit contains an automatic shut-off valve and coalescer and is specifically designed for applications where the requirement for hydrocarbon pollutant concentration from the unit is 5 mg/l or less. When a change in the specific gravity of the liquid present in the unit occurs or when water present is replaced by less dense hydrocarbon pollutants a specially calibrated ball will drop and seal the outlet from the Filterceptor. The coalescer filter consists of a polyurethane open cell foam enclosed in a perforated PVC housing. Untreated flow cannot leave the Filterceptor without first passing through the coalescer filter. An optional extra is an oil level probe and alarm system.


To ensure that BMS Interceptors continue to operate at maximum efficiency, regular maintenance should be carried out. Coalescer filters and automatic shut off units are easily removable to facilitate general inspection, cleansing, and maintenance.  If you have any queries regarding your interceptor please contact BMS.


Full installation instructions are provided with each BMS Interceptor.  They are easily connected to any size of pipework.  Access shafts and vents are available in order to accommodate varying depths of pipework.  BMS can supply of full range of oil level alarm systems complying with EN858.  Please contact us for more information.

Design, Construction & Quality

All BMS Interceptors are manufactured in GRP and each finished unit undergoes rigorous testing (including individual hydraulic tests) by our Quality Control Department before leaving our premises.  BMS operate an NSAI verified ISO 9001:2008 Quality Management System.