Off-grid sewerage

Rebecca Taylor from HD Services gives her insights into getting groundwork specifications right for your build, and explains the options available when no mains sewerage connection is possible

Building your own home can be one of the most rewarding things to do in your life, as well as one of the most daunting. There are many things to consider from building materials to groundworks, from roofing options to utilities. Connecting to services such as water, electricity, gas and sewerage can cost thousands if the connection points are not close to the chosen location of the build, but there are options available which can allow you to remain ‘off grid.’

If there is no mains connection available, you may be able to install a system to treat the sewage produced so that it can be safely disposed of. It is estimated that approximately 4 per cent of properties in the UK have off-mains drainage systems, and these properties are typically situated in more rural locations where a connection to a main sewer is not possible.

You must have Building Regulations approval if you have or are planning to install a new septic tank or small sewage treatment plant. You may also need planning permission.

There are three options for managing sewage from your property if you are not connected to the mains sewerage system – cesspits, septic tanks and sewage treatment plants.


A cesspit is a holding tank for the waste from your property. Waste goes in and stays there until the tank is emptied, as there is no ’outlet.’ The waste that enters does not undergo any treatment and only leaves the tank when emptied. Cesspits are generally seen as the last resort for a property’s drainage system, if the size or nature of the land available does not allow for a septic tank or sewage treatment plant to be installed. Cesspits should be at least 7 metres away from any habitable parts of the building, and located within 30 metres of an access point to allow for tank emptying.


A septic tank is the most common type of off-mains drainage tank. Its purpose is to treat wastewater from your property’s toilets, sinks, baths and showers, dishwashers and washing machines by a process of biological decomposition and drainage. The wastewater will usually pass through a number of inspection chambers. A septic tank does not have any mechanical parts and it doesn’t perform any direct ‘treatment’ of the waste. The term ‘septic’ means ‘infected with microorganisms’ – tanks contain bacteria which break down organic waste, separating the solids and the liquid. Once the wastewater reaches the level of the outlet pipe in the first chamber, it will overflow into the next chamber and so on.

The solids in the water break down and separate into three distinct zones or layers. The top layer is made up of less dense matter – fats, oils and solids that have not yet broken down. This layer is known as the crust. The second layer is mostly wastewater with solids ‘in suspension’. The third/bottom layer is known as sludge, and consists largely of more dense waste that builds up slowly over time. This, plus the top layer of crust, are what should be removed during a routine septic tank emptying.

A septic tank should only need emptying when the solids levels have built up within the tank. The settled liquid drains away through a soakaway or drainage field. The drainage field is designed to treat the waste effluent by the action of naturally occurring bacteria. For this process to work successfully the septic tank must be working efficiently and the subsoil free-draining.

The treated liquid effluent is commonly disposed of in a septic drain field or ‘shallow infiltration system.’ Nonetheless, groundwater pollution may occur and can be a problem.

Discharge regulations relating to wastewater from septic tanks were changed on 1 January 2020. Under the new rules, septic tanks can no longer discharge to surface water drains, rivers, canals, ditches, streams or any other type of waterway. If you have a septic tank that does, you are required to upgrade or replace your septic tank to a full sewage treatment plant.

Currently, all septic tanks that discharge into waterways must either be replaced with a sewage treatment system, or have the discharge to waterways impeded and redirected to a drainage field. Like cesspits, septic tanks must be at least 7 metres away from any habitable parts of the building and be located within 30 metres of an access point.


A sewage treatment plant system is the latest form of off-mains sewage treatment and drainage. Powered by an electricity supply, a sewage treatment plant provides treatment of the waste by physical, biological and sometimes chemical processes to remove pollutants. This means that the wastewater which leaves the tank is cleaner than that coming from a septic tank. For this reason, it is permitted for sewage treatment plants to discharge into a flowing ditch or watercourse.

Sewage treatment systems come in two different formats; package plant and bespoke design. A package plant is an ‘off the shelf,’ ready to go unit which can be purchased ready-made – like an all-inone stereo system which has an in-built radio, CD player, record player etc.

A bespoke design is akin to a ‘separates’ stereo system, where if you need only to replace the radio, you can. Bespoke systems can provide you with exactly what you require and may require in the future.

Rebecca Taylor is business development and compliance manager at HD Services