The last in our customer questions regarding Petrol/Oil Interceptors/Separators is; How do you calculate the size of a Petrol/Oil Separator needed for a site?

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Firstly you need to determine if you require a Bypass Interceptor or a Full Retention Interceptor; this is answered in a previous Customer Question answer; What are the different types of Petrol/Oil Interceptor (Separator) and what is the difference between them?  Secondly you need to determine if you require a Class 1 or Class 2 Interceptor; this is also answered in our last customer question answer; What is the difference between a Class 1 & Class 2 Separator?

After selecting the interceptor size and type, the correct size of interceptor is based on the flow into it based on rainfall onto a given surface area, this is known as nominal size or NS.

Each interceptor is allocated a nominal size based on EN858; the European Standard for Interceptor Design and Sizing.  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.

(Please note Petrol/Oil Separator and interceptor mean the same thing)

Example 1

An interceptor required for a 25,000 m2 car park discharging into a local authority surface water sewer.

As there is a low chance of oil spillage a Bypass Interceptor would be most appropriate.

As discharge is to a surface water sewer a Class 1 Interceptor, which is designed to achieve a concentration of 5 mg/l of petrol/oil at discharge under normal conditions and includes a coalescer and a closure mechanism is most appropriate. A Class 1 Interceptor is usually required for discharge to any surface water.

Based on EN858 a drainage are of 25,000 m2 gives a nominal size (NS) of 45 l/s.

So in this case BMS would select a BMS PB5 Class NSB 47 l/s Bypass Interceptor, which is the closest available in our range.

Example 2

An interceptor required for a 1,700 m2 fuelling area discharging direct to surface water.

Due to the high risk or fuel spillage a Full Retention Interceptor designed to contain the full contents of a full spill is required.

As discharge is to a surface water a Class 1 Interceptor, which is designed to achieve a concentration of 5 mg/l of petrol/oil at discharge under normal conditions and includes a coalescer and a closure mechanism is most appropriate. A Class 1 Interceptor is usually required for discharge to any surface water.

Based on EN858 a drainage area of 2,600 m2 gives a nominal size (NS) of 47 l/s.

So in this case BMS would select a BMS PB9 Class 2 NS 47 l/s Full Retention Interceptor, which is the closest available in our range.  Please note that the Nominal Size of a Full Retention Interceptor can also be denoted as NSF e.g. NSF; 47 l/s.

BMS manufacture produce a full range of ‘plug & play’ interceptors/separators from 1,000 – 100,000 litres manufactured in corrosion and rot proof GRP under ISO 9001:2008 to European Standards and are Irelands leading producer.

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.

Full Retention Interceptors

Bypass Interceptors

A bit about BMS

BMS is a quality manufacturer of package products for wastewater and stormwater treatment. BMS started in 1986 and has exported products to over 50 countries around the world.  It is a third generation family business with huge expertise in wastewater and stormwater.
BMS also manufacture a full range of products for surface water and wastewater treatment including our pioneering next generation RBC system the Aerotor, which forms part of our world leading Blivet package sewage treatment plant, a ‘one stop shop’ for all surface water products from design to installation including attenuation, interceptors, holding tanks (incl. chemical resistant), hydrobrakes, rainwater harvesting tanks & vortex silt/oil/debris separators, fully fitted pumping stations, our plug & play vehicle wash water recycling system; the Recyclone and much more.  Please see our enclosed Company & Product Profile for a full summary or View our full product/services catalogue at this link

A BMS PF10 Full Retention Interceptor arriving on site in Malta