A Step-By-Step Introduction to Off-Mains Drainage Systems

This step-by-step, guide is aimed at owners of off-mains properties who want to understand more about their existing (private) drainage system and/or to install a new system.  

It is intended to give you an introduction to off-mains drainage to help you make informed choices.  If you need to know more, please do not hesitate to contact one of our expert team for further advice. 

The General Binding Rules stipulate the legal requirements for property owners of off-mains drainage systems in England (regional variations apply for Scotland, Wales and NI), all of which are designed to protect public health and the environment. 

These Rules are published on the Government website and regularly updated, the onus being on the property owner to stay abreast of them. 

For example, since January 2020, a septic tank is no longer allowed to discharge directly to surface water (stream, ditch, river etc.).  If you are planning to buy a property with a non-compliant off-mains drainage system, then you will need to agree a plan with the seller as a condition of sale.

To comply with the General Binding Rules and Building Regulations you must, if possible, connect to a mains sewer – you can use a pump station to facilitate the connection.  Only install a private drainage system if a connection is not possible. 

An off-mains drainage system typically consists of two elements: 

        1) The septic tank/sewage treatment plant which collects and starts the process of cleaning the wastewater.

        2) An attached drainage field that finishes the job of cleansing the wastewater effluent once it is emitted from the tank.  If you have a septic tank, the drainage field should also have an attached reedbed so that it can work effectively.

Find out more about the difference between a sewage treatment plant and a septic tank.

If you have bought a property with an off-mains drainage system, you should have been advised in writing by the previous owner:

a)       Which system has been installed

b)       Where it is on your property

c)       Its service and maintenance record

Note: if you are installing a new tank, make sure it has a EN 12566 certificate.  This provides comprehensive information about its quality and cost-effectiveness, as well as its durability and reliability.  Look on the certificate for the test results of BOD5, ammonia and suspended solids. The lower the number, the better the quality of treatment.

The age of your treatment or septic tank could have a bearing on its effectiveness and efficiency.

A septic tank has a lifespan which can range from 15 to 40 years - steel tanks may be affected by corrosion and so have a relatively short lifespan, while plastic tanks usually have a longer life.

A properly maintained domestic sewage treatment plant should have a lifespan of around 40-50 years. Klargester’s treatment plants are known for their longevity.

Well-maintained drainage fields can last up to 50 years, but if they haven’t been looked after then they can fail.  

Questions to consider when deciding whether to get your system checked by an expert drainage engineer:

• Is it working effectively?  Is there a smell, or has the noise level changed?  Are your toilets backing up?  Is the drainage field soggy, or is there a white scum on top?  Has the property flooded?  All of these are indications that you need to bring in expert help ASAP.

• Are your written maintenance records in-line with the manufacturer’s recommendations?

You can find further information in our complete, easy-to-read, guide to drainage fields.

Any drainage field installed after January 2000 will fall under H2 Building Regulations. It must be designed in accordance with British Standard BS 6297:2007 + A1 2008 and be signed off by a building inspector. 

If it was installed before then, it still needs to comply with the General Binding Rules. 

Details around where a drainage field can be located are set out in Building Regulations Document H.

Your tank needs to be desludged once a year, depending on usage.

It’s also important to care for your treatment or septic tank between services.  For example, do not allow solids – such as wet wipes, sanitary towels, kitchen paper - to enter it.  You should also keep oil, grease and large volumes of milk, alcohol, food, bleaches, and cleaners out of your system.

Find out more about annual servicing in our guide to servicing and maintenance.

Drainage systems are complex, and potentially highly polluting, so you should only agree to your system being reviewed or maintained by a suitably qualified engineer – ideally a British Water accredited drainage engineer.

To find the right drainage expert for you:

• Make sure they are certified and protected (insured).  The British Water website has a list of accredited engineers in your area
• Ask for references and reviews
• Get a detailed proposal and maintenance plan, and ensure this is  in line with your system manufacturer’s recommendations

You can reach out for support via our Klargester Team.