What is an Interceptor?

What is a drainage interceptor?



They can also be called separators – because they separate contaminants from wastewater. However, the all perform much the same kind of tasks.

There are different designs for drainage interceptors, depending on the type and amounts of contaminants that need to be removed from wastewater before it is discharged.

It is important that drain interceptors are installed properly and well maintained to prevent pollution incidents that could damage the environment and result in heavy legal penalties for asset owners.

Why are drainage interceptors needed?

Drain interceptors must be installed in wastewater systems where the wastewater is known to contain, or there is a risk that it might contain, contaminants that:

  • Risk causing pollution in natural water courses or are a risk to public health
  • Cannot be effectively treated at sewage treatment works
  • May cause additional problems in sewers and sewage treatment works, such as blockages.

Where are drainage interceptors needed?

Drainage interceptors are commonly needed to capture and hold fuels, oils, and chemicals that either get into surface water in residual amounts or could be discharged accidently in large amounts if there was an untoward incident at a commercial or industrial premises.

Interceptors are often named according to this usage – for example, petrol interceptors, oil interceptors and chemical interceptors.

The equipment is installed where there is a higher risk of these pollutants being washed into surface water drains, escaping through industrial process pipes, or being discharged accidently through fuel, oil, or chemical spills.

Therefore, the facilities where drainage interceptors are commonly installed include:

  • Car parks
  • Logistics parks
  • Commercial garages
  • Petrol station forecourts
  • Roads
  • Oil and fuel storage depots
  • Factories, especially involving heavy engineering
  • Car washes.

How do drainage interceptors work?

Most standard drain interceptors comprise of a series of chambers, or tanks, which hold the water and separate contaminants in two ways:

  1. Heavy solids within the wastewater fall to the bottom of the tank
  2. Lighter oils, fuels, and chemicals float on the top of the water

A dip pipe in the chamber then allows the wastewater, minus the solids and lighter contaminants, to flow into a second chamber. The cleaned water then flows out into the wastewater system.

Interceptor & Septictank

This process may be repeated more than once. Most commonly, there are two-stage drain interceptors, or separators, with two tanks, or three-stage interceptors, with three tanks.

Some drain interceptors may have more sophisticated filtration systems to capture specific contaminants or ‘clean’ wastewater to a specific standard.

In all cases, the size of the interceptor system installed will depend on the expected flow rates through the system, especially at maximum volumes, the volume of contaminants that need to be separated, and the planned maintenance regime to empty the tanks.

Types of drainage interceptor

Full retention interceptors

These systems, also called full retention separators, that will always capture the full flow of water through drains. No wastewater can reach the external sewer system without contaminants being separated.

Full retention interceptors are fitted where its accepted that the contaminants pose too high risk to ever be discharged into a main sewer system.

Bypass interceptors

These systems, also called bypass separators, will capture and hold wastewater up to a certain flow rate. However, at higher flow rates, the wastewater will flow through the system without the contaminants being separated.

Bypass interceptors are installed where expected flow maximum flow rates, for example from heavy rain, are too high to allow water to be reasonably retained. Also, where the sewerage system can cope when contaminants are released on these infrequent occasions.

Drain interceptor alarm and monitoring systems

Where necessary, drain interceptors can be fitted with alarm and monitoring systems that remotely assess how full the system is and whether it is working effectively.

The asset owner will be alerted, for example, if too high a proportion of a contaminant is reaching the second or third tank within the separator, or if a tank contains a specified amount of contaminant.

How are drainage interceptors maintained?

The maintenance regime for a drainage interceptor will be determined by its design and size, the flow rates through it, the volume of contaminants in the wastewater (which may change over time) and external factors, such as changes in weather patterns.

Drain interceptors need to be emptied and cleaned regularly as part of a planned preventative maintenance programme (PPM).

The frequency will depend on the factors expressed above. The drain interceptor installer will advise on the optimum maintenance programme. However, this needs to be reviewed regularly.

Drainage interceptor emptying and cleaning

In most cases, drain interceptors can emptied and cleaned by a reputable drainage service specialist, using a jet vac tanker.

This is a specialist vehicle that allows the tank contents, including solid waste and lighter pollutants to be sucked up then taken for safe disposal at an authorised site.

The drainage specialist will then use the water jetting system on the jet vac tanker to clean the tank and ensure all pipework is clear and in good working order.

Drainage interceptor inspection

Drainage interceptors must also be regularly inspected to ensure they are in good working order. This will include making sure there are no leaks in the tank and pipework which would allow pollutants to seep into surrounding ground.

These drainage interceptor inspections can also be carried out be a qualified specialist drainage service provider. Inspections can be aided with the use of CCTV drainage camera that can be used to remotely capture video film inside the tank and its pipe system.