The difference between a sewage treatment plant and a septic tank

 Is a septic tank the right choice for you?

If you need a non-mains drainage system, choosing which type of system is right for you may be a challenge. There are three main sewage and wastewater treatment systems utilised for the removal of sewage and wastewater in both domestic and commercial settings; septic tanks, cesspools and sewage treatment plants.


A cesspool is simply an underground tank that collects and stores sewage upon its removal from the property. The sewage and wastewater is not processed at all, and no effluent is passed into the environment. As a result, it fills up quickly and requires regular emptying, usually on a monthly basis.

These are no longer in common use in the UK, as it is a costly choice due to the need for it to be emptied so frequently. Cesspools are therefore only used as a last resort these days, in settings where it is impractical or even impossible for a septic tank or sewage treatment plant to be used.

Septic Tanks

 A septic tank does not simply store waste, but partially processes it, producing an effluent which is then digested further using natural biological processes. In a typical septic tank installation, sewage and wastewater is fed through from the property to the septic tank. The solids settle at the bottom of the tank, while the liquid components are pumped out into a soakaway.

A soakaway, also known as a drainage field, is a series of underground pipework which disperses the liquid effluent from the septic tank into the soil, where naturally occurring aerobic bacteria breaks down the remaining waste. There are stringent rules and regulations regarding the use of drainage fields, and the land in question must pass a series of tests in order for a soakaway to be installed. These tests are to ascertain whether there is a risk of contamination to watercourses, as well as ensuring the soil is the right type for drainage purposes.

Even if these tests are passed, installing a soakaway can be a challenge due to regulations on where they can be placed. They must not be too close to buildings, boundaries or watercourses, be away from electrical cables or pipework and not be part of a road, driveway or other area where vehicles could pass over it and cause damage.

While septic tanks are a suitable method of waste removal, the stringent guidelines mean it can be very difficult to install. Where a septic tank is not an option, a sewage treatment plant should instead be used where possible, with a cesspool being the last resort.

Sewage Treatment Plants

Sewage treatment plants are a highly efficient, low polluting method of removing waste where a property is not connected to mains drainage. They work by mechanically breaking down waste before pumping out a roughly 95% clean effluent which is deemed safe to be discharged into a ditch, stream or other watercourse.

There are a wide range of sewage treatment plants on the market, which all work in slightly different ways, however the main principle is the same. Unlike a septic tank, sewage treatment plants break down the solid matter and bacteria within the tank itself without relying on naturally occurring bacteria to process the waste. This means that less potentially hazardous waste enters the environment making it the safer and more environmentally friendly option.

One other benefit of choosing a sewage treatment plant over a cesspool or septic tank is that they do not need to be emptied as often, although regular servicing is advised in order to maintain optimum efficiency.

Which system you opt for will depend on a range of factors. Still not sure which to choose? Contact MPC Services today, we will be happy to help.