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Off-mains drainage guide
Many thousands of properties – domestic homes and public or commercial buildings – rely on off-mains drainage systems to store, process and disperse wastewater.
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.
Click on one of the boxes to see a specific answer or scroll down to see all questions and answers in this guide.
What is off-mains drainage?
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.
In what areas are off-mains drainage systems most commonly used?
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.
Do property owners have to connect to the main sewer network if they can?
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.
What are the main types of off-mains drainage systems
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.
What are cesspits?
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.
What are septic tanks?
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.
When were septic tanks first introduced?
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.
What are the different types of septic tanks?
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.
How big does my septic tank need to be?
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.
What are sewage treatment plants? (also referred to as small STPs and mini STPs)
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.
What is the best position for an off-mains drainage tank?
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.
What are the common signs of a problem with a septic tank or sewage treatment plant?
There are a number of tell-tale signs of problems owners of septic tanks or sewage treatment plants should look out for. These include:
- Slow drainage of wastewater from toilets, baths and sinks
- Effluent backing up in drains
- Bad smells around the tank drain covers
- Pooling of water on hard surfaces or lawns
- Lush patches on lawns
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.
What are the main ways septic tanks and sewage treatment plants can develop faults?
- A common problem is a drainage field being too small to cope with the amount of water being generated. This can result in effluent backing up in the tank and associated drainage system. It can also result in water not seeping into the ground quickly enough and so pooling near or on the surface, creating boggy ground.
- Likewise, if the tank is too small for the amount of water entering it, the filtration processes within the tank can fail to work properly and water can also backup in the supply drains.
- If plastic or GRP tanks are not bedded into the ground properly, with the right support, they can flex and crack, allowing effluent to leak out.
- Tanks can also become damaged during their installation.
- User abuse – items not suitable for disposal in septic tanks being put down toilets or sinks – for example wet wipes, sanitary products and large volumes of fats, oils or grease.
- Septic tanks and sewage treatment plants will stop working optimally if they are not emptied regularly and at the right times. Sludge and fats/oil waste can build up and clog the system.
- Sewage treatment plants will stop working properly if the aeration system fails.
- If tanks become overfull, pressure on their walls can cause cracks that can cause water leaks or may even result in the tank collapsing.
- If service pipes leading to tanks are located under drives, the weight of vehicles may cause them to crack or collapse, cause water leaks of blockages.
- Vehicles movements or heavy items stored over drainage fields (see below) can cause similar problems.
If I have a septic tank or sewage treatment plant will I have to ration water?
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:
- Fit water-saving devices to taps, shower heads and toilets;
- Fix leaks in taps, toilets, showers and appliances as quickly as possible;
- If you are replacing washing machines or dishwashers, buy models that are best at conserving water;
- If you have a choice between taking a shower or a bath, take a shower as it uses a fraction of the water, or invest in a reduced-capacity bath.
What else can I do to make sure my septic tank or sewage treatment plant always works optimally?
- Only flush poo, pee, water rand toilet paper down your toilet. Do not use it for disposing of sanitary or dental products, such as wet wipes (including baby wipes), tampons or dental floss, or cosmetics;
- Prevent food waste from escaping down sinks by using plughole filters and wiping kitchen utensils, cutlery, plates and dishes before putting them in the dishwasher;
- Do not dispose of other forms of fats, oils or grease (FOG) down sinks or drains;
- Minimise the use of powerful detergents and other cleaning fluids, for example bleaches, in the home. This is not to say they cannot be used. Use biodegradable cleaning products, where possible.
Should I worry if my septic tank smells?
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.
How often should a septic tank be emptied?
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.
Can I empty my own cesspit, septic tank or sewage treatment plant?
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.
How do I choose the best type of off-mains drainage system for my property?
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:
- Size and nature of the grounds encompassed by property;
- Nature and structure of the ground/soil;
- Proximity of water courses and sensitive sites;
- Volume of effluent that needs to be treated and dispersed.
Do I need planning permission to install a septic tank?
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:
- Size and nature of the grounds encompassed by property;
- Nature and structure of the ground/soil;
- Proximity of water courses and sensitive sites;
- Volume of effluent that needs to be treated and dispersed.
Do I need to register a septic tank with an environment agency?
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.
Do I need a licence or permit for a septic tank?
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:
- Discharge more than 2,000 litres of effluent per day;
- Discharge in a groundwater source protection zone 1 (SPZ1);
- Discharge within 50 metres of a designated sensitive area.
Seek advice from your relevant environment agency.
What hazards are associated with off-mains drainage systems?
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.
What is a drainage field?
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.
What is a soakaway?
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.
What are the rules about the design and location of a drainage field?
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.
What is a percolation test?
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:
- Dig a 300mm square hole to a depth of 300mm below the proposed invert level (bottom of the pipe) of the infiltration pipes in the drainage field;
- Fill the hole with water and leave it to seep away overnight;
- Refill the hole with water the next day and time how long it takes for the water to seep away from 75% full to 25% full.
- Divide this time by 150. Drainage fields are only suitable if the resulting number is between 12 and 100.
- Repeat this test at least three times with at least two different holes. Do not carry out a percolation test during heavy rain, severe frost, or a period of drought.
What are General Binding Rules?
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.
Why have General Binding Rules been introduced?
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.
What off-mains systems are governed by General Binding Rules?
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.
How do I ensure my system complies with general binding rules?
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.
What are the options for upgrading septic tanks under General Binding Rules?
Property owners who use septic tanks can consider a number of options to ensure they stay withing the Environment Agency’s water discharge regulations.
- Connect to a mains sewer. Septic tanks have, conventionally, been installed where access to mains sewers is difficult. However, over time, the mains sewers may have been extended. This means connecting the property to a main sewer may now be the most cost-effective solution.
- Install a drainage field. It may be possible to create a drainage field within the property’s boundary. However, there are regulations governing the location and design of drainage fields which may prevent this from happening.
- Install a small sewage treatment plant These systems are also referred to as package sewage treatment plants.
- Install a secondary sewage treatment unit. In most cases, this would be an ultra-violet filter, which kills the bacteria before it is discharged to a water course.
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.
Who is liable for water discharge under General Binding Rules?
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:
- The owner of a property that has an operational septic tank.
- The owner of a property that shares a septic tank with a neighbouring owner.
- Any person responsible for the maintenance of the septic tank under a written agreement, such as a lease, or a tenancy agreement.
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.
What penalties could I face if GBRs are not followed?
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.
What is a secondary sewage treatment unit?
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.
What must I do if I sell a house with an off-mains drainage system, for example a septic tank?
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.
What do I need to look out for when buying a property with an off-mains drainage system?
A property buyer should ensure they have clear information about:
- The type of off-mains drainage system;
- The age of the system;
- When the tank and drainage field were installed;
- Whether it complies with General Binding Rules;
- Whether it has been modified and upgraded;
- If so, when and by whom;
- How often it is emptied;
- How much it costs to empty;
- What condition the tank, drainage field and associated assets are in;
- Whether there have been any operational problems with the system;
- Whether there are permits in place to operate it;
- Whether any regulatory action has been taken related to it, or whether such action is likely.
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.
Find out more
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.
Talk to Catalyst today. Telephone 0800 870 8080. Email: [email protected].
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