Interceptors; out of sight out of mind.
On a recent training course we encountered a common issue regarding interceptors. Out of sight out of mind.
All of our training sessions are tailored to the site and company we are working with, and in the process of preparing the course we were sent a copy of the site drainage plan and had a discussion regarding the spill control devices on site.
The drain plan did not show that there was an interceptor on site, and showed that all rainwater drains merged with the foul line before leaving site. However the facilities team said they believed there was an interceptor on site and believed the storm and foul lines were 2 separate systems which did not combine.
In order to offer the most comprehensive and accurate training, we arrived on site earlier than usual and met with the facilities team and EHS team to try and get some clarity on the situation.
On the site boundary there were 3 partially buried covers in a row marked in both red and blue, and a vent pipe which had been snapped off at knee level. According to the facilities staff the manhole covers had not been lifted in over a decade.
Once the plant growth had been removed the covers were all lifted and it was immediately evident that this was indeed an interceptor, and that it required servicing immediately.
Fortunately for the site the interceptor was still functioning despite its lack of maintenance. Chamber 1 had captured a significant amount of thick black oil and a significant amount of solids, chamber 2 was free from oil but had collected over 100cms of solids. The image above shows nuts and screws visible on top of a bed of dense solids in chamber 2. The site uses polymer beads in their manufacturing and evidence in the upstream drainage was that these were being allowed to enter the storm drains in the service yard explaining the high solid levels trapped in the interceptor.
To the untrained eye GRP interceptors like the SPEL Stormceptor which had been installed can be mistaken for foul drain features e.g. a cess pit, which may be why the previous environmental manager had mistaken this as combined drainage. However they are completely different systems, with different purposes (see the FAQs below).
We gave the site recommendations to:
Have a 5 yearly integrity inspection undertaken to remove the oil and solids (which were both significantly past compliance levels) and to ensure the lack of maintenance had not lead to structural damage
Add the unit onto a 6 monthly PPM regime.
Repair the broken vent pipe.
Undertake a drainage survey and produce a new drain plan to ensure the site drain plan was accurate going forward.
Correctly colour code the manhole covers.
Ensure loose polymer beads were prevented from entering storm drains in the future.
Fortunately we caught these issues before the interceptor started discharging pollutants to the environment. When it comes to interceptors ignorance is not a defense, if you have an interceptor which is not maintained because no one on site is aware it exists you will be held liable if it starts to discharge pollutants in contravention of the Water Resources Act or Water Industries Act as can be seen in HERE.
Spill Training UK do not offer supplementary services, but we always try to advise our clients when we see gaps in the sites spill control preparedness to help them protect their staff and the environment.
Frequently Asked Questions on Oil Interceptors
This FAQ aims to provide a thorough understanding of oil interceptors, covering everything from their purpose and function to maintenance and regulatory considerations.
What are Interceptors?
Oil interceptors, also commonly known as oil-water separators or oil traps, are critical environmental controls used to remove silt, oil and other LNAPLs (Light Non-Aqueous Phase Liquids) from water before it enters sewage systems or the environment.
Interceptors work on the basis that some pollutants float on water and others sink in water. Oils such as petrol and diesel rise to the surface and are trapped, solids such as silt sink to the bottom and are trapped preventing harmful substances from entering sewers, watercourses, or treatment plants.
Where are oil interceptors commonly used?
Oil interceptors are typically installed in:
· Vehicle maintenance shops and garages
· Car washes
· Industrial manufacturing plants
· Car Parks
· Petrol stations
· Adjacent to carriageways
Why are oil interceptors necessary?
Oil interceptors protect the environment and public health by removing oil and hydrocarbons from wastewater. Without them, oil can contaminate water systems, harm aquatic life, and damage sewage infrastructure. Most jurisdictions also require their use to comply with environmental regulations.
How does an oil interceptor work?
Oil interceptors operate on the principle of gravity separation. Wastewater enters the interceptor, where slower flow allows oil (which is less dense than water) to float to the surface and silt (more dense than water) to sink to the bottom. The separated pollutants accumulate in a chamber and are later removed, while the cleaner water exits to the drainage system.
Design and Installation
What sizes and capacities are available?
Oil interceptors come in various sizes, from compact units suitable for small workshops to large industrial models handling thousands of litres per hour. The appropriate size depends on expected flow rates, volume of effluent, and the specific application.
How do I determine the right size for my facility?
Sizing depends on several factors:
· Maximum expected flow rate of wastewater
· Type and amount of oil in the catchment area
· Type of system (Bypass or Full Retention)
· Local regulations and standards
Many manufacturers provide sizing charts or calculators to assist in selecting the correct model. In addition section 5 of GPP 3 Use and design of oil separators in surface water drainage systems provides calculations to assist in the specification.
Where should an oil interceptor be installed?
Interceptors are typically installed:
· Underground, near the source of oily wastewater discharge
· Outside the building to allow easy access for maintenance
· In locations where gravity can assist in flow, avoiding the need for pumps
The specification and install location should be factored in to the design of a site and considered when changes to site would dictate the need for an interceptor to be installed.
Who should install an oil interceptor?
Installs are ideally undertaken during the construction of a site, by the drainage contractors. Unfortunately, units are often not installed correctly with common problems such as interceptor alarms not being installed. Correct installation ensures compliance with safety and environmental regulations. Units should be checked before the site is handed over to the client.
Operation and Maintenance
How often should an oil interceptor be cleaned?
Cleaning frequency varies based on usage, but regular inspection is crucial. BS EN 858-2:2003 - Separator systems for light liquids (e.g. oil and petrol) - Selection of nominal size, installation, operation and maintenance states “maintenance of the system has to be carried out at least every six months by experienced personnel” and “In intervals of at maximum five years the separator system shall be emptied and then submitted to general inspection”.
What happens if an oil interceptor is not properly maintained?
Neglecting maintenance can lead to:
· Blockages and overflows
· Reduced separation efficiency
· Environmental contamination
· Fines or penalties for non-compliance
Can the intercepted oil be reused or recycled?
Yes, collected oil can often be sent to recycling facilities where it is processed and reused, reducing waste and environmental impact.
Are there any signs that indicate problems with an oil interceptor?
Signs of trouble include:
· Slow drainage
· Inconsistent water levels between chambers
· Visible oil in discharge water
· Frequent blockages
· Alarms triggering
If any of these issues arise, inspection and remedial actions are required.
Regulations and Compliance
Are oil interceptors required by law?
On certain sites oil interceptors are legally required however in most cases their install is only recommended. Discharge of pollutants such as oils into controlled waters is illegal so if a site chooses not to install an interceptor the risk of discharging oil will be higher.
GPP3 recommends units are installed
“Anywhere there is a risk of oil entering surface waters or sewers from rainfall runoff.
• Low risk sites such as:
o car parks, typically larger than 800m2 in area or for 50 or more car parking spaces
o smaller car parks discharging to a sensitive environment
o roads
• High risk sites: such as:
o vehicle maintenance areas
o industrial sites where oil is stored or used
• Petrol station forecourts or vehicle refuelling sites.
• Any other site where there is a risk of oil contamination.”
What standards apply to oil interceptors?
Common standards include:
· BS EN 858 (Europe and UK): Specifies design, performance, and testing requirements for separators
· GPP 3 Use and design of oil separators in surface water drainage systems
· Each manufacturer will also have their own guidelines for maintenance of their units.
How do I demonstrate compliance?
Maintaining service records, inspection logs, and waste disposal documentation is essential for demonstrating compliance during official inspections. Where integrity inspections are undertaken these should include video or photographic evidence of the condition.
Waste notes alone do not provide evidence that sufficient maintenance has been undertaken.
Technical and Operational Details
What are the main components of an oil interceptor?
Typical components include:
· Inlet and outlet pipes
· Settling chamber(s)
· Oil collection and storage area
· Coalescing filters
· Automatic Closure Devices (ACDs)
· Access covers for inspection and cleaning
· Alarms or monitoring systems
Are there different types of oil interceptors?
There are 2 main differentials between interceptors full retention OR bypass and Class 1 OR Class 2.
Class 1 interceptors are designed to achieve a discharge concentration of less than 5 PPM of oil. These units are required for discharges to surface water drains and the water environment.
Class 2 interceptors are designed to achieve a discharge concentration of less than 100 PPM of oil. These units are acceptable for discharges to foul water drains subject to permission from the Sewerage Undertaker.
Full retention interceptors treat the full flow that can be delivered by the drainage system. Full retention separators are used where there is a risk of regular contamination with oil and a foreseeable risk of significant spillages.
Bypass interceptors treat all flows in dry conditions and generated by rainfall rates of up to 6.5 mm/hour. Flows above this rate are allowed to bypass the interceptor. These units are used in low risk catchment areas when it is considered an acceptable risk not to provide full treatment for high flows e.g. car parks.
Can oil interceptors handle other contaminants?
As described above interceptors work on the basis they catch contaminants which float or sink in water, water soluble contaminants will not be captured. It is worth noting that viscous liquids may initially sink in water however if they are water soluble they will break down and pass through the unit causing pollution.
Troubleshooting and Support
Where can I find support or spare parts?
Most manufacturers offer technical support, spare parts, and guidance on system upgrades
What should I do if my oil interceptor fails?
First, isolate the problem and prevent further discharge of oily water. Contact a specialist firm or the manufacturer for assessment and repair. Some specialist companies can even perform structural repair broken units.
Spill Training UK are happy to provide support and advice if you have any concerns regarding your interceptor(s).