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Up to 3% of properties in the UK have off-mains drainage and sewage treatment systems. These properties are typically homes situated in more remote, rural locations where a connection to the main sewer is not possible.

Wastewater is taken away from a property in two ways:

  • The house is connected to a pipe network leading to a main sewer and centralised treatment works.

  • The building has its own private pipes and treatment system, known as off-mains drainage and sewage treatment.

Choosing off-mains treatment for your property

Owners of a property with off-mains drainage have two options:

  • Pump to the nearest main sewer if it is close enough to the property and according to building regulations requirements.

  • Install private sewage treatment in the form of a package sewage treatment plant or a septic tank.

A package sewage treatment plant is a small-scale wastewater processing unit, where flows from a property pass through three treatment stages. In the first chamber, gravity separates out the water from the solids, which sink to the floor of the tank.

The liquid then travels into a second chamber fitted with an air pump or rotating discs that circulate air to encourage the growth of bacteria, helping to break down contaminants in the water, effectively cleaning it prior to discharge to the environment.

Systems with a third chamber make it possible for any remaining solids to settle on the floor of this tank before the treated effluent is discharged to a drainage field or watercourse.

What is a septic tank?

A septic tank is an underground storage container that collects wastewater flowing from a property and uses gravity to separate the water from the solids within the tank. The solids sink to the bottom of the tank, while the liquid flows out nearer the top, and soaks into the ground.

As a householder, you are responsible for a septic tank or sewage treatment plant where your property uses an off-mains system. Depending on whether you select a package sewage treatment, or a septic tank, different planning rules and environmental regulations apply.

In England, you must make sure your system meets the Government’s general binding rules for small sewage treatment plants and septic tanks. Different rules apply in Wales, Scotland and Northern Ireland.

Read more - New domestic sewage treatment plants – 2023 rule updates.

Planning a package sewage treatment plant


Tank discharge outlet must be at least 10m from any watercourse or building


If the plant needs electricity to operate, it should be able to function without power for up to six hours, or have a power supply that cannot be interrupted


Drainage field – or soakaway - should be at least 15m from any building and 50m from a water supply, such as a well


Must meet the EN12566-3 standard for small wastewater treatment systems


Can discharge to either a watercourse or drainage field

Planning a septic tank


Should be at least 7m away from any habitable parts of a building


Should be at least 10m from a watercourse


Drainage field should be at least at least 15m from any building and 50m from a water supply, such as a well


Should be located within 30m of an access point for tank emptying


Can only discharge to a drainage field, not a watercourse


No access roads, driveways or paved areas should be located within the area

What else should I consider?

For householders looking to install or replace an off-mains treatment system, the process can seem daunting, but decisions generally come down to five considerations: size of property; cost of purchase, installation and maintenance; environmental impact; landscaping and gardening potential; and risk of odour.

Read more - Domestic sewage treatment plants: The dos and don'ts of everyday use

Sizing a package sewage treatment plant generally depends on a calculation based on the number of bedrooms in the property. This assumes an average of two people per bedroom, plus one additional person for the household. So, for a three-bedroom house, the population estimate (PE) is seven.

Package sewage treatment plant should be sized at least 1.5 to 2 times the predicted total daily flow, to handle peak flows and ensure efficient treatment.

British Water’s Best Practice Guide to Flows and Loads provides a table which allows the total daily sewage load from properties to be calculated and it is recommended that all designers use this table when sizing and designing a non-mains sewage treatment system.

Septic tank size is usually based on the predicted daily flow rate. Generally, septic tanks are sized larger than a sewage treatment plant. A common rule is to have a tank capacity of at least 2.5 to 3 times the daily flow rate. This allows for adequate settlement and digestion of solids.

Consider unusual water uses such as spa baths and multiple bathrooms or utility rooms, as these can increase the biological load on the system.

The calculated size provides the minimum requirement, so it is advisable to round-up to the nearest available size of package sewage treatment plant or septic tank, to ensure adequate capacity.

Read more - Do you need a bigger septic tank or sewage treatment plant for your home extension?

One advantage of having an off-mains drainage and sewage treatment system is that you will not pay water company sewerage rates. However, you will need to have the solids collected in your package sewage treatment plant or septic tank removed regularly.

The price of the sewage treatment system and ongoing maintenance costs should be taken into consideration before you make an investment. Consider the whole-life cost of different systems - those with fewer mechanical moving parts and electrical connections are less likely to break down and will require less maintenance over their lifespan.

Maintenance requirements for domestic sewage treatment plants vary depending on the size, type, and age of the equipment. When selecting equipment, compare the length and scope of warranties for different systems.

When undertaking an installation, costs can vary depending on the site location, quality of the ground, the model chosen, and how long installation takes. Where an existing system needs to be removed, this would also need to be factored into estimates from installation contractors.

Wastewater treatment systems eliminate the harmful microorganisms and pollutants in sewage that can cause illness and disease. Both package sewage treatment plants and septic tanks separate out many contaminants in the solids that are later removed by tanker.

The additional stages of treatment in a package sewage treatment plant means a higher quality of water is achieved before the treated effluent is discharged from the tank and enters the ground or watercourse. This gives greater protection to the environment, plants and wildlife.

There are tight regulations around discharging wastewater near watercourses and these are even stricter at Sites of Specific Scientific Interest (SSSIs). Discharges from septic tanks directly to a watercourse are not permitted under current environmental regulations. In this scenario, septic tanks should be replaced with a package sewage treatment plant.

Read more - 9 reasons to install a domestic sewage treatment plant

Landscaping and gardening

Gardening over a package sewage treatment plants and drainage field with care can enhance a property aesthetically, while ensuring the all-important functioning of the wastewater system. For modern systems, odours are no longer an issue, and package sewage treatment plants can be located below-ground.

Because drainage fields are moist and nutrient rich, with careful landscaping and plant selection, it is possible to create a space that is both beautiful and functional.

Read more - Can you garden over a domestic sewage treatment plant and drainage field?