A Quick Guide to Sewage Treatment Systems

Building regulations require foul drainage to be connected to a public sewer or where this is not practicable to one of the following:

  1. Cesspit (also termed Cesspool and Cess tank)
  2. Septic Tank
  3. Packaged Sewage Treatment Plant

The above options all have advantages and disadvantages, in order to decide which option is most appropriate to meet your requirements please study the following information.


What is a cesspit? – A cesspit is a sewage holding tank with no outlet.
How does it work? – Sewage flows into the tank and is stored; when the tank is full the waste requires emptying by a vacuum tanker.

Site Suitability:
Sites where the ground is unsuitable for the waste to soakaway to ground.
Sensitive sites e.g. SSSI’s and sites close to drinking water supplies.

Advantages & Disadvantages:
Low installation & maintenance cost, no treatment of sewage but regular emptying may prove costly subject to frequency, ease of emptying etc.


What is a septic tank? – A multi-chambered or singularly baffled tank with an outlet.
How does it work? – Primary tanks facilitate primary treatment to take place (the separation of liquids and solids by gravity).

Sewage flows into the tank and the heavy solids, “sludge” sinks to the bottom, lighter solids, grease, fats and oils or “scum” float to the surface. Some of the sludge is degraded by naturally occurring anaerobic, (without oxygen), bacteria. Liquid with solids etc now removed is now termed effluent or liquor and flows from the tank via gravity out to land by soakaway, (of various types including drainage fields).

Please note that some older septic tanks still discharge directly to watercourses, however this practice is becoming less common due to more stringent consent standards and consent is due to be removed in the near future for tanks still operating in this way.

Site Suitability:
Single domestic house or small developments – Where there is sufficient porosity in the ground to allow for a soakaway, (ground porosity determined via percolation tests, for information on how to carry out percolation tests please contact us).

Advantages & Disadvantages:
Relatively low installation cost – Requires emptying on an annual basis.
Some treatment – Can only discharge effectively where ground has sufficient porosity.


What is a treatment plant? – A treatment plant is a more sophisticated unit than a septic tank, there are different types of packaged sewage treatment plant but they all generally follow the same principles.

How does a treatment plant work? – A packaged sewage treatment plant creates an environment which facilitates the growth of bacteria which breaks down sewage into non- polluting end products. Treatment plants differ from septic tanks as not only does primary treatment take place but also secondary treatment. This requires an electricity supply which is used to artificially introduce air to the treatment plant; it is this oxygen transfer through the sewage which enables the growth of aerobic bacteria which are more effective in the breakdown of sewage than the bacteria present in a septic tank. This results in a higher quality effluent being produced, which can, (subject to Environment Agency Consent to Discharge), be discharged directly to a ditch, soakaway or watercourse.

Site Suitability:
Packaged sewage treatment plants are suitable for most sites from a single domestic house up to commercial and industrial applications.
Subject to Environment Agency or SEPA, i.e. whether they will grant you consent to discharge to land or to watercourse, it may prove prior consent is not required for your site/installation, please contact us for clarification.

Advantages & Disadvantages:
Sewage treated to higher standard – Requires electricity supply.
Suitable for larger developments – Requires regular maintenance to ensure efficient operation.