Off mains drainage guide. Includes advice on cesspools, septic tanks, sewage treatment plants, reed beds, soakaways and more.

As development takes us beyond the reach and capacity of the mains sewer systems, the number of people having to contemplate living off main drains is increasing. In addition, Building Regulations require that (in broad terms) the rainwater that falls on the site needs to stay on the site. There is an ecological benefit to treating waste water on site but generally it is still a choice forced upon us. 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 soakaway 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.

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 will have rotating discs which increase the surface area for the microorganisms to work on and speeds up the degradation of solid matter.

The liquid discharged is relatively clean and can be emptied into a water course (if there is one, or soakaway if not). The volume of solid matter is significantly reduced, and becomes non-toxic, 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 £2-6,000 but only need emptying every one to three years. They are fast becoming the preferred option as they’re cleaner and relatively low on maintenance.


This is a means of allowing water to slowly dissipate through the soil. There are two methods: a pit filled with stone or rubble into which the water is discharged, or a system of interlinked trenches containing a perforated pipe surrounded by shingle. 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 and thereby how big the pit, or how long the trench needs to be.

Soakaways are used to deal with liquid discharge from septic tanks and sewage treatment plants, and for rainwater.

Reed Bed

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 bacteria, fungi and 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.

There are a few things about reed beds that may come as a surprise:

Reed beds are quite small — a typical four bedroom house will need a reed bed of 8-10m2;

Reed beds are quite cheap — a typical reed bed will cost about the same as a biodigester (treatment plant);

Reed beds do not smell — the water is continuously flowing, and it tends to be stagnant water that produces unpleasant odours.

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.

Living Soakaway

Very similar to a reed bed and used where a conventional soakaway is difficult or not possible. A living soakaway is a shallow, gravel-filled pit planted with irises, reeds and willows that take up water that is then dissipated to atmosphere.


If you have reasonably good soil conditions, living off main drains should be a non-issue. A treatment plant and a soakaway (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.

Do You Need to Register?

It may soon be necessary to register septic tanks and treatment plants with the Environment Agency under EPP2 Regulations. The issue is currently under review by the Department for Environmental Food and Rural Affairs (DEFRA) and the Welsh Government. The argument is that registration will allow the EA control over the management of the system. Most septic tanks and treatment plants do not cause problems, but if they are poorly designed, installed or managed, the discharge from them can damage the environment. The EA feel they need better control to comply with EU requirements and registration is seen as being the way to do that.

If you are installing a septic tank or plant, check with the EA whether it needs registration.

Off Mains Drainage: Case Study

For one family, the septic tank they inherited with their home, was making life difficult. Choosing a sewage treatment plant proved to be a wise investment.

After more than 18 months of trying to cope with an overflowing septic tank situated halfway down the garden, Toby Sermon finally decided enough was enough.

The smell was so appalling that Toby and his family rarely used the garden. He and his wife constantly worried about the health risks to their two young children, as well as the dangers inherent in having, as Toby describes it, “essentially a 12ft hole with an aluminium cover,” close to where his daughter played with their family dog.

The old brick septic tank attached to his 1950s three bedroom property in Saffron Walden, Essex, had struggled for years to cope with the loading placed upon it by not one, but two houses — as it also served a neighbouring property. The situation was made worse because the sewage simply ran into a ditch, where it collected.

Cost Issues

Toby, a heating engineer with his own business, New Heat Ltd. in Saffron Walden, says: “My neighbour and I were both spending a great deal of money regularly emptying the septic tank to try and stay on top of the problem, but it was overwhelming and we needed a better solution. I just didn’t want raw sewage underneath my garden. My wife couldn’t cope when the emptying took place; the smell was unbearable.

“I went to my local Ridgeons branch in Saffron Walden and the staff put me in touch with Klargester. They explained that old brick septic tanks were once universal solutions for properties that could not be connected to the main sewer, but they don’t actually treat the waste, they just allow the solids to settle. Most of the treatment is done by the drainage field which is sited downstream of the tank. The consequences, unfortunately, were there for all to see (and smell!).

“Klargester recommended the existing tank should be removed and replaced with two sewage treatment systems to serve both properties.”

The two new Klargester BioDisc sewage treatment systems use microorganisms to break down the sewage, thus removing around 90% of the pollutants. What little remains is then filtered through drainage fields. The tanks also require emptying just once a year, saving Toby and his neighbour a great deal of money.

The BioDisc BA systems were installed in November 2012 and the difference to the two householders is already apparent. “Quite seriously, I feel so much cleaner knowing that sewage is now being treated and not sitting in my garden! I’m delighted,” says Toby.

New Technology

Dave Vincent, Commercial Director for Kingspan Klargester advises: “Septic tanks were once seen as the universal solution for properties that could not be connected to the main sewer. However, at best, septic tanks remove 50% of sewage pollutants, hence they are not suitable for sites that are likely to become waterlogged or have poorly draining soil.

“A sewage treatment plant by contrast can achieve an efficiency rating of up to 95%, producing a clear, odourless, overflow that is environmentally friendly and suitable for even the most sensitive sites. They also require a much smaller soakaway area than a septic tank.”

Running costs for the BioDisc are claimed to be 1.3kWh/day (about 15p). Expect to pay around £3,000 plus installation, for a system of this sort.

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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