As development takes us beyond the reach and capacity of the mains sewer systems, the number of people having to contemplate living off main drainage is increasing. That said, there are options and choices to be made, and understanding the terminology is a good place to start.

Cesspit or Cesspool

A cesspit (often called a cesspool — there’s no difference) is essentially a holding tank. There is no outlet and no intent to treat or discharge the sewage. It is merely collected in the tank then periodically pumped into a lorry for disposal. The tank is generally sized to hold six to eight weeks of sewage and vented to allow gas build-up to escape but is otherwise sealed. Cesspits are banned in Scotland and are considered a last-resort option in the rest of the UK.

Emptying a cesspit will cost between £100 and £300 each time and it may need doing eight times each year.

Septic Tank

This construction is very similar to a cesspit but compartmentalised to allow the separation of solid and liquid waste. Solids are retained in the tank and liquid discharged to a drainage field or mould to be cleaned by percolation through soil. Solids are emptied from the tank in the same way as a cesspit but typically this will only be done once or twice each year.

Such a system relies on the subsoil being capable of receiving the final discharge without backing up and preventing the unit from working. This is determined by a percolation test. In the event that the ground is unsuitable then, then it is usual to employ a sewage treatment plant.

Septic tanks are a common solution for off mains properties. Capital cost is low, running cost is low and it is a well understood system.

Sewage Treatment Plant

Don’t be put off by the grand name — many are domestic in scale and modestly priced. In most models, compressed air blown into the bottom of the tank accelerates the activity of the microorganisms which break down the waste. The tank may have rotating discs which increase the surface area for the microorganisms to work on and speeds up the degradation of solid matter.

The liquid is treated and can be discharged to a drainage field or mould (and in some circumstances, discharge to a water course). The volume of solid matter is significantly reduced, but still needs pumping into a lorry for disposal.

Treatment plants are more expensive than septic tanks but this is soon recovered in lower running costs (some, such as WTE’s FilterPod, don’t need electricity at all). Often they cost between £2,000-£6,000 but only need emptying every one to three years — four years in some cases.

They are fast becoming the preferred option as they’re cleaner and relatively low on maintenance and running costs.

Drainage Fields and Mounds

Both are a means of allowing water to slowly dissipate through the soil. The choice and design will be determined by the amount of space available and by porosity or percolation tests. These tests will establish how quickly water can dissipate.

Drainage fields and mounds are used to deal with liquid discharge from septic tanks and sewage treatment plants. (They are often incorrectly referred to as ‘soakways’, which are different and instead used to deal with surface water — ie rainwater run off from roofs.)

Reed Beds

Reed beds are not usually a complete sewage treatment system on their own but are generally used with a septic tank or treatment plant. They allow microorganisms to digest the sewage and clean the water. There are two basic types of reed bed – vertical flow and horizontal flow – and the best system often results from combining the two.

Reed beds are the ecological solution. They do largely the same job as a treatment plant, but produce cleaner water and need a septic tank as well. They’re prettier to look at and provide a rich habitat, but are difficult to justify on any other grounds.


If you have reasonably good soil conditions, living off main drains should be a non-issue. A treatment plant and a drainage field or mound (with or without a reed bed) is a simple and effective solution. Even in poor soil conditions living off mains is still possible, but there is a capital cost issue.

Articles like this Comments

    Sustainable Sewage Treatment
    The article is now incorrect as recent developments in the sewage treatment industry have resulted in the emergence of completely non-electric sewage treatment plants.
    BIOROCK sewage treatment plants are cheaper to run than septic tanks.
    Benefits of the BIOROCK sewage treatment plant
    They use no electricity and have a 4 year emptying interval – 3 years longer than a septic tank.
    They save over 0.5 tonnes of Carbon Dioxide emissions per house per year.
    The Filter media is self-cleaning and does not require anything more than a hose-down once a year.
    As the effluent is cleaner than electric sewage plants, this has to be the way forward for sustainable building.

  • Renovator

    We have just installed a biorock system. It was a fairly involved process and I must admit the help that we received from the supplier could have been better.

    Once it is up and running I will post again to give an update


    If you feel that we could have helped you more, then please let us know how.
    We try to give the best support we can and would rather that you discussed any problems with us first.
    We are exhibiting at the Homebuilding and Renovating Show at the NEC in March and would be glad to offer you 2 free tickets in order for you to let us know how we can improve our service.

  • Anonymous

    i would ask many other people who have brought this system, its not the best, has issues and does require electric unless you live either on a hill or have a 3m deep ditch, so green?? only IF you have a house on a hill, BUT have a look at other sites and call other companies, you may be supprised what you will hear!!

  • Simply Green Buildings

    Aren’t new cess pools banned in Scotland and only allowed as a last resort in England? As I understand it, you need to demonstrate to the Environment Agency that connecting to the mains sewage system is unfeasible and septic tanks/treatment plants cannot be done.

    The EA are pretty helpful so give your local office a call if you think connecting to the main system is not feasible – they will be able to advise on next steps and can usually let you know what you’ll need.

    You can also check the maps on the EA website which show whether you’re over an aquifer (this affects how much treatment you need to do before soaking away).

    I definitely wouldn’t recommend going down the cesspool route without first checking with the environment agency. They are likely to insist on a septic tank or treatment plant instead.

  • J Wilde

    Hi we are thinking of a Biorock for our selfbuild project and would love some feed back on what you think, also I would like to install it myself, we do have a slope down to a stream so our situation is good its just are they any good?

    Kind Regards

    J Wilde

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