South East Rivers Trust (& the Wandle Trust)

A beginner’s guide to off-mains drainage

Septic tanks and package treatment works

Around 95% of properties in the UK are connected to the main drainage system which is processed at a local sewage treatment works. But that leaves 5%, mostly in rural areas, that have their own self-contained system to take wastewater from the property.

Whether you already have a home or business that relies on off-mains drainage, or you’re considering buying one, there are things you should be aware of.

Why am I reading about this on a Rivers Trust site?

Well maintained septic tanks will remove solids from domestic waste. Small sewage (or package) treatment plants will also reduce nitrogen in the form of nitrate and ammonia.

However, neither of these systems is efficient at removing phosphate, a common ingredient of many cleaning products. Some dishwasher tablets contain nearly 1/3 of their weight in phosphate! Across the SERT working area, 39% of rivers fail to meet standards for phosphate concentration. Switching to brands with no, or low phosphate is a really easy way to help our rivers.

When they enter the river, nutrients like phosphate encourage algal and plant growth. Excessive growth clogs the river channel (altering the balance of habitats and possibly increasing flood risk), and when algal blooms die they can reduce dissolved oxygen in rivers, sometimes killing fish and other aquatic life.

F. lamiot (own work) (cc-by-sa/2.5)
Photo © N Chadwick (cc-by-sa/2.0)

Some algal blooms (known as toxic blooms) can harm people, pets and livestock using the river. Inefficient, or failing, small sewage plants may also introduce pathogens and solid wastes to watercourses as well as the wider environment, thus becoming a health hazard in themselves.

You can help to minimise the problems in rivers associated with small sewage discharges by:

  • using cleaning products that have a low phosphate content;
  • making sure your system is regularly checked and maintained;
  • having your tank emptied when needed;
  • not using harsh cleaning products like bleach or caustic soda that can harm the microbes that treat your waste;
  • not disposing of oils or solvents down the drain;
  • not overloading your drainage system by doing repeated loads of washing
  • not connecting rainwater drains to your system.

Why are some people not connected to mains drainage?

In an urban environment, nearly all properties are connected to the mains drainage (or sewerage) system. In more rural areas, connecting to the mains network can be technically difficult, or very expensive, depending on the distance to the nearest connection point, or the landscape in between.

Non-mains drainage is often thought to be associated with older properties. However, increasingly, wastewater treatment works are already working at or over capacity. Alongside this, more development is taking place so that more new properties, even those in urban/suburban locations, are relying on non-mains drainage for their sewage provision.

Off-mains drainage options are independent systems, owned and maintained by the property owner. This means that the owner is liable for any pollution or health and safety issues arising from failures in the system.

For this reason, it’s important to know you have one and how to look after it. There are rules that govern these systems, so it’s important to be aware of them and to make sure you comply with the requirements under the law, administered by the Environment Agency.

Check out the government website for more information

Septic tanks and small sewage (‘package’) treatment plants: What’s the difference?

Septic tanks are essentially settling tanks, where liquid and solid wastes are separated under anaerobic conditions, and the liquid phase is then discharged through a drainage field, where microbial processes treat it, before entering the wider environment.

Discharges from septic tanks directly to a surface water are not allowed under the General Binding Rules (see below). If you have an existing septic tank that discharges directly to a surface water, you will need to replace or upgrade your treatment system by 1 January 2020 at the latest. 

© USGS [Public domain]

Small sewage treatment works (or package treatment plants) have an additional stage that aerates the sewage, allowing microbial processes to reduce some damaging nutrients before being discharged to the environment. These systems require a power source. Because they include an additional processing stage, small sewage (or package) treatment works can discharge into a drainage field or directly to watercourses.

Although older septic tanks are known to discharge into watercourses, current rules prohibit this means of disposal. Despite a lack of information on exactly how many, and where these tanks discharge to streams, the Environment Agency is working to eliminate this practice and ensure all septic tanks discharge into a drainage field that provides additional processing via soil biota.

Do these systems require a permit?

As of January 2015, the requirement to apply for a permit for a small sewage discharge has been reviewed. Systems must be installed and operated under what are known as General Binding Rules.

When discharging the following volumes, no permit is required with:

  • septic tanks discharging less than 2m3 per day to a drainage field
  • package treatment plants discharging less than 2 m3 per day to a drainage field
  • package treatment plants discharging less than 5 m3 per day to a watercourse

You must also apply for a permit if you are discharging into Designated Sensitive Areas as follows:

Designated Sensitive Areas list from January 2015

Discharges to ground:
• Groundwater Source Protection Zone 1s (SPZ1) for discharges to ground
Discharges to ground or surface water:
• Special Areas of Conservation (SAC)
• Special Protection Areas (SPA)
• Ramsar sites
• Sites of Special Scientific Interest (SSSIs) designated for biological reasons
• Designated Bathing Waters
• Shellfish Protected Waters
• Ancient Woodlands
• Selected protected species and protected habitats
• Local nature reserves and local wildlife sites which are aquatic in nature

BUT small sewage works that discharge more than these limits, or are near sensitive areas MUST apply for a permit and be registered with the Environment Agency.

What can go wrong?

Off-mains drainage solutions must be individually designed for the property(ies) they serve. They must also be installed correctly, with a large enough drainage field to provide the additional treatment by soil biota, and in soil that has the correct degree of permeability.

Tests must be carried out by a competent engineer to ensure the conditions will allow the waste to soak away at an appropriate rate. This design is based on the amount of wastewater that will be discharged on a daily basis (you can calculate daily flows using the tools found on the government website).

Overloading the system, even over a short period, can lead to the ground becoming unable to deal with the effluent, and that’s when problems arise, with the whole site becoming unpleasant and possibly a health hazard. That means repairs, re-siting of the drainage network and possibly legal proceedings.

Regular checks and maintenance, being careful not to overload your system, using sympathetic cleaning materials and following guidance about what not to dispose of, will reduce problems and the costs associated with inefficient systems.

The drainage field for even a well maintained and suitable system has a finite life; around 20 years for a septic tank and longer for a package plant. The microbes that treat the waste form a mat around the drainage pipes and they become clogged, possibly backing up waste and causing slow drainage and / or unpleasant odours. The drainage field may need to be moved to another site, or new pipes may be laid between the existing ones if there is sufficient distance between them. That’s why maintenance is so important as it will maximise the lifetime of the drainage field.

To find accredited engineers to help you design, install and maintain your system visit

We have more detailed information and links to resources that can help you manage your non-main drainage system – saving our rivers and your money.

Please contact us if you would like to hear more.