Types Of Sewage Treatment Systems

Here is a brief guide to the types of treatment systems available to domestic and commercial customers. We hope it helps you understand which type of tank might be suitable for you.

Much of the information here is taken from the Environment Agency (EA) Pollution Prevention Guidelines 4: 'Treatment and disposal of sewage where no foul sewer is available'.

Which Sewage Tank Do I Need?

We've put together a decision tree to help you decide. If you have plenty of land a septic tank and soakaway might be best. If space is of a premium and there are nearby surface water drains or water courses a sewage treatment plant might be best. Cesspits are a last resort!

Flow chart showing what sort of sewage treatment system you need

Package Sewage Treatment Plant

Picture of a sewage treatment plant

Sewage treatment plants - also known as package sewage treatment plants are more sophisticated than a septic tank, they are roughly the same size but there are a few key factors you should know about which are listed below:

  • Sewage treatment plants cost more (approx 2.5x) than septic tanks
  • Can outlet to surface water drains and water-courses (with EA approval)
  • No need for an expensive drainfield or soakaway usually
  • Treat sewage fully, unlike septic tanks
  • Require an electric power feed (approx 80W for a 3-bed house)
  • MUST have EN12566 - Part 3 Certification or you're wasting your money
  • Require Consent to Discharge (usually free) from the EA
  • Two main types, mechanical aeration (rotating disc) and air-blowers
  • Bacteria in the tank require oxygen to process sludge
  • 95% of material entering the tank exits the tank, just the sludge is collected
  • Require emptying annually usually
  • Suitable for 6 to 300 people
  • Running costs approx. £400pa for de-sludging, a service and power for a three bed-house

Disadvantages to these systems are that they require an electricity supply to power a small compressor or motor to help circulation.

An annual service is absolutely essential to ensure efficient operation and prevent major problems. We offer a cost-effective annual service that will keep your plant running smoothly all year round.

Septic Tanks

Picture of a septic tank

A septic tank system consists of the tank itself and drainage field or a water course discharge point. Raw sewage is fed to the tank, the tank performs primary filtering, then liquor leaves the tank and enters the drainfield (AKA soakaway) where naturally occurring bacteria in the soil perform secondary treatment. All septic tank outlets should have consent to discharge granted by the EA/SEPA. Key points about septic tanks are:

  • Cheaper to buy than sewage treatment plants (2.5x)
  • MUST outlet to a drainfield (AKA soakaway), NOT surface water
  • Drainfields can be expensive to install
  • Partial treatment only of sewage, bacteria in the drainfield do the rest
  • Do not require power usually, unless an uplift pump is necessary
  • New installs MUST have EN12566 - Part 1 Certification or you're wasting your money
  • Require Consent to Discharge (usually free) from the EA
  • Two main types, old brick tanks (usually 3 chambers) or newer plastic onion bulb tanks
  • Bacteria in the tank do not require oxygen to process sludge
  • 95% of material entering the tank exits the tank, just the sludge is collected
  • Require emptying annually usually
  • Suitable for upto 50 persons
  • Running costs approx. £300pa for de-sludging and a service for a 3 bed-house

It is essential that septic tanks are properly maintained if they are to work properly. Often this is not the case and owners leave it until they have an emergency (eg. toilets backing up, or the EA pays a visit). It is much better to have a regular maintenance plan to avoid this sort of problem in the first place.

There are many different types of septic tank systems on the market, but generally they all undertake the same task. Commercially manufactured septic tanks are usually made from GRP or other suitable stable materials. The tanks are commonly spherical in shape with a narrow shaft at the top to a manhole ground level. These tanks have built into them several baffles which perform the same function as dip pipes and separate out the heavier solids to the bottom and let the greases / scum, and effluent rise to the top.

Land Drains and Drain Fields

Picture of a drain field

After partial treatment by a septic tank, wastewater moves from the septic tank into perforated pipes, bedded on and surrounded in, gravel-filled trenches. To determine if your land will drain properly we can carry out a percolation test on your property to determine the suitability and size of your proposed drainfield.

From the pipeline nutrients and organic material are dispersed into the gravel where they are removed or neutralized as the liquid moves through into the soil. In a trench system, the effluent may flow into a series of drop boxes or manholes with another outlet to the next pipeline set at a higher level. As each set of trench lines reaches capacity, the drop boxes release liquids to the next set of pipes in the drainfield.


A cesspit (AKA cesspool) is a covered watertight tank used for storing sewage - EVERYTHING that goes in must get sucked out which will cost a fortune! It has no outlet, sewage flows in and is stored, when the tank is full the waste is tankered away. A cesspool requires regular emptying and must not be allowed to overflow. It is the least sustainable option for sewage disposal as no treatment occurs on site. You should consider a cesspit as a temporary measure pending a more satisfactory solution, such as the provision of mains drainage.