Our off-mains drainage guide helps you understand how they work, how they must be maintained and how you can ensure they comply with regulations.
Off-mains drainage systems are installed where it is not practical to connect a property to the mains wastewater and sewage treatment system.
In most cases, each property will have its own wastewater storage and treatment system, with at least a proportion of the waste having to be collected at regular intervals for disposal at an authorised waste site.
Owners of properties with off-mains drainage systems are legally responsible for ensuring waste, including effluent and solid waste, is managed and disposed of in accordance with all relevant health and safety and environmental regulations.
These duties can be contractually passed to other parties, for example, those who lease or rent the property. Failure to fulfil these responsibilities can result in prosecution leading, potentially, to heavy fines and / or imprisonment.
Most off-mains drainage systems are located in more remote rural locations, away from the wastewater network that collects sewage for processing at treatment plants. In some cases, a property (which may be a dwelling, a public building or a commercial property) may be close to a main sewer but there could be specific technical and geographical reasons why it would be very costly to connect it to the network.
Some properties located in urban areas that have a mains drainage network retain their off-mains drainage systems because it is the choice of their owners to do so.
No, they do not. As the sewer network has developed, and expanded, properties with off-mains drainage systems will have had the opportunity to connect to it. However, as long as their off-mains system complies with regulations, they can choose to keep it, perhaps because they do not want to incur the connection cost.
There are three main types of off-mains drainage systems – cesspits (also known as cesspools), septic tanks, and sewage treatment plants, also known as small sewage treatment plants or mini sewage treatment plants.
A cesspit, also referred to as a cesspool, is a single underground holding tank with one opening in which wastewater (liquid waste and sewage) is temporarily stored.
The wastewater is not treated in a cesspool. Instead, all the liquid and sludge stored in one has to be regularly pumped out and taken away for safe treatment and disposal.
This increases the frequency with which the tank has to be emptied, compared with other off-mains tank systems (septic tanks and sewage treatment plants). In turn, it can make this off-mains drainage system more costly to maintain.
Septic tanks are underground holding tanks for wastewater (liquid waste that can include toilet waste, dirty kitchen water and water from clothes washing machines).
Unlike cesspools (see above) septic tanks are designed to process the wastewater by separating the liquids from solids. Wastewater is transported to the tank through a drainage pipe. An inlet pipe takes it into the tank. The wastewater is deposited in the tank through a dip pipe which has an opening below the inlet point.
As the wastewater builds up in the tank, heavy solids sink to the bottom of the tank, while lighter solids, including fats and oils, collect on the surface. This leaves liquid effluent in the middle.
Another dip pipe, usually located on the other side of the tank, then draws off the liquid effluent which is then dispersed into the ground via a drainage field.
The scum and the sludge are left inside the tank. At regular intervals, before the tank becomes too full, this waste must be pumped from the tank and taken by tanker to be disposed of at an authorised waste site.
Some septic tanks have just one chamber. Others are designed with two or, sometimes, three chambers to improve the filtration process. The chambers are separated by baffles. Effluent is passed from one chamber to the next through dip pipes. With each step, the effluent becomes cleaner.
The Public Health Act of 1848 made it a legal requirement for all dwellings to have a sanitary method for disposal of sewage. In most cases, this involved installing a cesspit. The first septic tank, which introduced a filtration and effluent discharge process, was invented by a Frenchman, called Jean-Louis Mouras in around 1860. He got around to patenting the design in 1881 and it was taken up across the rest of Europe, including in the UK, and also became popular in the USA.
Septic tanks are manufactured in different shapes and sizes, and with different internal designs to meet the specific needs of the property they serve.
Septic tanks can be made from concrete, plastic (HDPE) or glass reinforced plastic (GRP). Concrete septic tanks are heavier than plastic ones, so take more effort to lift into place. This may mean they are not suitable for some properties.
GRP septic tanks are lighter, but are more prone to damage. Plastic and GRP septic tanks are often cylindrical in shape. They can also be spherical. Another term for this shape is onion. Spherical septic tanks take up less space in a garden, but need a deeper hole.
Shallow-dig septic tanks, also known as low-profile tanks, can be installed in gardens where there is less depth available, for example due to ground conditions or land profile, but can take up more space.
The size of a septic tank is determined by the number of people who will be using the system on a regular basis. There are two key calculations: the number of people who will live in the property; and the volume of wastewater the property is likely to generate.
The basic calculation method, as stated in BS 6297 and the Flows and Loads code of practice issued by British Water, determines the minimum volume of a septic tank with the following formular:
C= (150 P + 2,000)
C is volume of the septic tank in litres. P is the number of people who could inhabit a property (not who actually are inhabiting it), and 2,000 is a base volume for the tank in litres.
For example, take a four-bed house. This is considered to have a maximum occupancy of six people. Therefore, the formula looks like this:
C = (6×150) + 2000. So C = 900 + 2000. This means C, the minimum volume of the septic tank is 2,900 litres.
The formula assumes the septic tank is de-sludged every year and that children count as adults.
At present, the smallest septic tank size for a domestic property is 2700 litres. This is stipulated in building regulations 2010 H2 which state that the tank must be big enough for four users.
Sewage treatments plants are off-mains drainage systems that incorporate additional technology for processing wastewater effluent that makes it cleaner and, therefore, less of a risk to the environment.
They work in a similar way to septic tanks in that sewage goes through a gravity fed filtration system, with sludge falling to the bottom of the tank. The effluent then goes through a final process in which bacteria digests the harmful organic matter in the effluent, which greatly reduces the environmental risk it poses.
The tank is charged with the bacteria when the tank is installed and then maintained. The digestion process is then encouraged by the compressed air being pumped into the chamber in a process called aeration.
Another way effluent is cleaned in a sewage treatment plant is through the installation of an ultraviolet light disinfection unit. The wastewater is passed over a UV light array, which kills bacteria and other bugs.
Once effluent has been processed in either of these ways, it is clean enough to be discharged directly into a water course or into the ground through a drainage field.
In certain circumstances, a property owner must apply for a discharge licence. For example, if the tank discharges over 2,000 litres of effluent per day, or it is located in a groundwater source protection zone.
A septic tank or sewage treatment plant should, ideally, be located at least seven metres away from the home it is serving. Also, it should be at least 15 metres away from the next nearest home.
One of the biggest mistakes made during installation, is placing tanks in locations that are difficult to access for emptying. The easier it is for a tanker to get as close as possible to the tank, the lower the emptying charge is likely to be.
There are a number of tell-tale signs of problems owners of septic tanks or sewage treatment plants should look out for. These include:
However, if the septic tank installation process is planned and implemented properly, and the tank is then well maintained, these tell-tale signs are much less likely to be spotted because there will be no underlying faults.
As long as you have an off-mains drainage system that is the right size for your property, you will not have to ration water. However, people with such systems are often much more aware of the advantages of using water with care.
Useful actions to take are:
Interestingly, a well maintained septic tank or sewage treatment plant should not smell of much. Therefore, if there are foul smells coming from a tank, it could mean it is not working optimally.
This could be because it is full and needs to be emptied as a soon as possible. Or, in the case of a sewage treatment plant, it could be because the aeration system is faulty. It could also mean unprocessed effluent is leaking from the tank.
If the smell persists, you should call in a reliable off-mains tank maintenance specialist the investigate.
It depends on the size of the septic tank. As a rule, a standard septic tank (4,500 litres or smaller) should be emptied every year. However, it depends on how intensively it is used. Larger tanks may only need to be emptied every three to five years.
Septic tanks should not be left until they are full before they are emptied. This runs the risk of them failing and puts extra strain on the tank structure.
It is very important that all waste, including sludge be removed and disposed of at an authorised site by a registered, experienced waste carrier in accordance with the tank’s recommendations.
Septic tanks should also be cleaned regularly. This is done by water jetting, to remove waste from all surfaces and dislodge compacted sludge from the tank floor.
No you cannot. Not unless you are a registered waste carrier with experience of tank emptying and cleaning using specialist equipment. Effluent waste is hazardous.
Waste held in septic tanks give off noxious, potentially deadly, gases. There are many recorded incidences of people being killed while trying to maintain, empty or clean their own septic tanks.
Septic tanks also contains harmful organic matter and micro-organisms. This is why off-mains tanks should be maintained by experienced and qualified operatives.
It is recommended to that you seek expert advice from drainage contractor or consultant is experienced at installing and maintaining off-mains systems.
Key factors to consider will be the:
It is recommended to that you seek expert advice from drainage contractor or consultant is experienced at installing and maintaining off-mains systems.
Key factors to consider will be the:
In England, you are not required to register a septic tank with the Environment Agency. However, in certain circumstances, you may need to apply for a permit to discharge. In Wales and Scotland, septic tanks need to be registered with the relevant environment authority.
There are some circumstances where you will need to apply to the relevant environment agency for a licence or a permit to operate or install a septic tank. These are if you:
Seek advice from your relevant environment agency.
As has just been said, septic tanks and sewage treatment plants generate noxious, potentially deadly, gases. If a person comes into contact with these gases they can quickly become overcome. There are regularly instances where people are killed in this way.
The effluent also contains bacteria, viruses and parasites that can cause a range of serious illnesses which can be deadly. This is why it is important that off-mains tanks are maintained in way that prevents sewage from leaching out into the ground.
The effluent can also be an environmental hazard. The organic matter in it can quickly strip oxygen from water, killing plants, insects, fish and mammals. This is why leaks from septic tanks may have to be treated as a pollution incident.
A drainage field a system of pipes used to disperse wastewater into the ground. It must conform to BS 6297: 2007. The pipes, which are commonly called infiltration pipes, can be made from clay or plastic. They have perforations (holes) along their lengths, through which wastewater escapes into the ground.
The pipes are laid in trenches bedded in gravel. A geotextile membrane is laid on top of the gravel before it is covered with soil. A drainage field is laid so it gently slopes away from the tank (at a gradient of no more than 200:1), allowing water to gradually seep into the ground under the effects of gravity.
Unlike a drainage field, which is used to disperse wastewater, a soakaway is designed to disperse rain water. This does not stop people from often wrongly using the terms interchangeably.
The design and location of drainage fields are laid out in statutory guidance called Drainage and Waste Disposal: Approved Document H.
https://www.gov.uk/government/publications/drainage-and-waste-disposal-approved-document-h
This guidance lays out rules for locating a drainage field. It should be at least 10 metres from watercourse, 15 metres from buildings, 50 metres from a water supply, separated from any other water supply pipes or other underground services, and set away from access roads, driveways or paved areas. Drainage fields also should not be located near property boundaries and other drainage fields or soakaways.
A percolation test is carried out to make sure the ground you plan to lay your drainage field in can manage the volume of water that will be discharged from the drainage field.
A standard percolation test is carried out in the following steps:
Since January 1st 2020, regulations called General Binding Rules: Small Sewage Discharge to the Ground, have been in force. They do not allow wastewater from a standard septic tank to be discharged directly into water courses.
The regulations have been brought in to improve protection of water courses, ground water and the general environment from pollution risks. Wastewater discharges can adversely affect wild plants, fish and animals. They also represent a potential health hazard, for example for people who use waterways for recreation. Prevention of environmental pollution is the responsibility of everyone, not just water companies.
The two off-mains drainage systems the regulations are aimed at controlling are septic tanks and sewage treatment plants. Cesspits are not covered because all wastewater is contained and, at regular intervals, is removed from site.
In a nutshell, if you have a septic tank that discharges effluent into a water course, it is not compliant with general binding rules and should be upgraded as a matter of urgency.
You must ensure the wastewater treatment system you have, or plan to install, meets the British Standard in force at the time of installation. The current standards for new systems are BS EN 12566 for small sewage treatment plants and BS 6297:2007 for drainage fields.
It is advised that you obtain the advice of an off-mains drainage contractor or consultant to establish the most sustainable and cost-effective treatment system for your property.
Property owners who use septic tanks can consider a number of options to ensure they stay withing the Environment Agency’s water discharge regulations.
In the unlikely situation that none of the options referred to above are viable, the property owner can apply to the Environment Agency for a permit to continue to discharge the effluent into the water course.
The Environment Agency, which regulates water discharge in England. identifies the individual or organisation responsible for the off-mains drainage systems as being its “operator”.
It is the operator who will be deemed guilty of an offence if found to be in breach of the regulations. An operator of a septic tank includes:
General binding rules are also enforced in Wales, where they are overseen by Natural Resources Wales. In Scotland, they are applied under regulations overseen by the Scottish Environmental Protection Agency. In Northern Ireland, discharges into water courses are regulated by the Northern Ireland Environment Agency.
The Environment Agency has powers to enforce General Binding Rules through civils and criminal courts to ensure compliance. Property owners found not to be compliant face fines that, in general, range from £200 to £5,000. They will also be required to make their off-mains drainage system compliant.
However, the Environment Agency now has powers to seek financial penalties up to a maximum of £50,000. Fines for offences tried in Crown Courts can, theoretically, be unlimited.
A secondary sewage treatment unit can be fitted to an off-mains drainage system to provide additional cleaning of effluent before it is discharged. Such a unit is most commonly added to a septic tank to raise the quality of the wastewater to a level that allows it to be discharged into a water course. A common type of secondary device uses an ultraviolet light source to kill harmful bacteria.
Operators of off-mains drainage systems are legally obliged to inform a buyer in writing about off-main sewage systems. They must also provide a detailed description of its use, location, required maintenance and up to seven years of maintenance history.
A property buyer should ensure they have clear information about:
If you are buying a property served by an off-mains drainage system, it is recommended that you commission a suitably qualified drainage contractor or consultant to carry out a home buyer’s drainage survey to establish the answers to these questions.
Catalyst Services UK provides off-mains drainage installation and maintenance services for the insurance industry, plus commercial and domestic customers.
Its other services include: CCTV drainage surveys; drain and sewer cleaning; drain repair and rehabilitation; water leak detection; water supply pipe maintenance and renewal; subsidence investigation; and home emergency services.
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