Here is the British Water flows and loads PDF to help you with your caluculations when deciding which septic tank or sewage treatment plant to choose.
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Free Government Calculator
If you are looking for a simple and easy way to work out the size of your septic tank or sewage treatment plant, then help is at hand.
Here is a Government flows and loads calculator to help you work out the flows and loads.
Rules for existing and new discharges
Use the correct treatment system
You must use a septic tank or small sewage treatment plant to treat the sewage and then discharge the effluent (treated liquid) to ground via a drainage field.
A septic tank is an underground tank where the solids sink to the bottom, forming a sludge, and the liquid flows out to a drainage field.
A small sewage treatment plant, also known as a package treatment plant, works in a similar way to a septic tank but uses mechanical parts to treat the liquid to a higher standard before it goes to a drainage field.
A drainage field, also known as an infiltration system, is a series of pipes with holes placed in trenches and arranged so that the effluent can trickle through the ground for further treatment.
You cannot use a soakaway (designed for draining rainwater), well or borehole for discharging effluent to ground. Instead you must either upgrade to a drainage field or apply for a permit so that the Environment Agency can assess the risk of using this sort of system in your location.
Your treatment system must meet the right British Standard
Your treatment system must meet the relevant British Standard in force at the time of installation. The standards currently in force for new systems are:
- BS EN 12566 for septic tanks and small sewage treatment plants
- BS 6297:2007 for drainage fields
Your septic tank or treatment plant met the British Standard in place at the time of installation if:
- it has a CE mark
- the manual or other documentation that came with your tank or treatment plant has a certificate of compliance with a British Standard
- it’s on British Water’s list of approved equipment
You can also ask the company that installed your equipment to confirm that it complies with the British Standard that was in place at the time the equipment was installed.
If your treatment system was installed before 1983 there was no British Standard in place. You do not need to do anything to meet the British Standard requirement. You must still meet all the other general binding rules.
Your treatment system must be installed correctly and have enough capacity
Your treatment system must be large enough to handle the maximum amount of sewage it will need to treat. If you install a new septic tank, small sewage treatment plant or drainage field you must check with the installer that it meets the sizing requirements in British Water’s Flows and Loads 4 guidance.
If the amount of sewage the system needs to treat increases (for example, because you’ve extended your property or connected an additional property) you must make sure the treatment system is still big enough. You must also recalculate the maximum daily volume of your discharge and apply for a permit if it is more than 2 cubic metres (2,000 litres) a day.
Your treatment system must be installed in line with the manufacturer’s specification (the instruction manual or technical set of requirements that comes with the equipment).
Have your treatment system regularly emptied and maintained
You must get the sludge that builds up in your septic tank or small sewage treatment plant removed (desludged) before it exceeds the maximum capacity. As a minimum, you should have your treatment system desludged once a year or in line with the manufacturer’s instructions.
The company you use to dispose of your waste sludge must be a registered waste carrier. Ask the company to confirm this when you arrange to have your tank emptied or ask the tanker driver for a copy of the company’s waste carrier certificate.
You should have your treatment system regularly maintained in line with the manufacturer’s instructions. If these are not available, ask your local maintenance company for advice.
You must have your treatment system repaired or replaced if it is not in good working order, for example if it has:
- leaks
- cracks in tank walls or pipes
- blocked pipes
- signs that the effluent is not draining properly, pools of water around the drainage point
- sewage smells
- a failed motor
- a failed pump
- a failed electrical supply
Anyone who carries out maintenance on your system must be competent. Competent people include those on British Water’s list of accredited service engineers.
You sell your property: tell the new owner about the sewage treatment system
If you sell your property, you must tell the new operator (the owner or person responsible for the septic tank or small sewage treatment plant) in writing that a small sewage discharge is in place.
Include:
- a description of the treatment system and drainage system
- the location of the main parts of the treatment system, drainage system and discharge point
- details of any changes made to the treatment system and drainage system
- details of how the treatment system and drainage system should be maintained, and the maintenance manual if you have one
- maintenance records if you have them
You stop using your treatment system: make sure it’s properly decommissioned
You must remove anything that could cause pollution (for example, remaining sludge) when you stop using a septic tank or small sewage treatment plant.
This does not apply if you only stop using the equipment temporarily, for example if your property is empty.
You can ask a maintenance company for advice on how to decommission your septic tank or treatment plant properly.
Rules for discharges in a groundwater source protection zone 1 (SPZ1)
Check if the discharge point is in a groundwater SPZ1
A groundwater SPZ1 can be either:
- the area around a commercial water supply (used for drinking water or food production) shown on the map of protected zones – check if your discharge is in the inner zone (zone 1) or ask the Environment Agency
- any area within 50 metres of a private water supply for human consumption – ask your neighbours if they have one and if so how far their spring, well or borehole is from your drainage field
You must apply for a permit if you have an existing discharge or are planning to start a new discharge to the ground in a SPZ1. The permit costs £125. A permit will include additional conditions to the general binding rules.
The Environment Agency will grant the permit if there’s:
- no evidence of pollution
- the risk of pollution is acceptable
If there is evidence of pollution or the risk of pollution is unacceptable the Environment Agency will ask you to make changes to your system and may issue a permit with improvement conditions.
The Environment Agency regularly checks:
- surface and groundwater quality
- permit compliance
If they find your system may be causing pollution to surface or groundwater they will contact you to discuss the issues. This may result in your permit being reviewed or revoked. See the guidance on how to comply with your permit.
See the guidance on how to apply for a permit. Contact the Environment Agency if you need help with your application.
Additional rules for new discharges from treatment systems installed and in use on or after 1 January 2015
You must follow these additional rules if you:
- started a new discharge from a septic tank or small sewage treatment plant on or after 1 January 2015
- had a discharge to a surface water before 1 January 2015 which you now want to change to discharge to ground, or the other way round
- had a discharge to ground before 1 January 2015 and you want to install a new drainage field more than 10 metres away from the existing one
These are called new discharges.
Check if there’s a public sewer nearby
If any part of the building your treatment system serves is within 30 metres of a public sewer, the Environment Agency will not allow you to start a new discharge from a septic tank or small sewage treatment plant under the general binding rules.
If you are building a development of more than one property, this distance must be multiplied by the number of properties – for example, if there are 3 properties then the distance will be 3 x 30 metres = 90 metres.
To find out if there is a public sewer near your property, contact your local water company.
If there is a good reason why you cannot connect to the sewer (for example, if there is a river or a hill in the way) then you must apply for a permit so that the Environment Agency can decide whether to allow you to use a small sewage treatment plant instead. Contact the Environment Agency to find out what information you will need to put in your application.
Building regulations and planning approval
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.
Check if the discharge point is in or near a designated sensitive area
If you have or are planning to start a new discharge to ground in or near a designated sensitive area, you must apply for a permit.
You will need a permit if the new discharge will be in an ancient woodland or in or within 50 metres of any:
- special areas of conservation
- special protection areas
- Ramsar sites
- biological sites of special scientific interest
Contact the Environment Agency to check if you’re in or near a designated sensitive area and to find out if you need a permit.
See the full list of general binding rules published by the government.
If there are any rules you cannot comply with contact the Environment Agency to discuss what you need to do.
Overview
If your house or business is not connected to the mains sewer, your sewage will go to one of the following:
- a septic tank – an underground tank where the solids sink to the bottom and the liquid flows out and soaks through the ground
- a small sewage treatment plant (also known as a package treatment plant) – a part-mechanical system that treats the liquid so it’s clean enough to go into a river or stream
- a cesspool (also called a cesspit) – a sealed tank that collects the sewage
- a non-standard system, such as a reed bed or trench arch system
There are different rules for septic tanks and treatment plants in Northern Ireland, Scotland and Wales.
You have a non-standard system
You must contact the Environment Agency to find out if you need a permit.
Building regulations approval
How to apply
Contact a ‘building control body’ (BCB) to check the building regulations or apply for approval.
There are different rules in Scotland and Northern Ireland.
Where to apply
There are 2 types of BCB. It’s up to you which you use.
Local authority BCBs
You can apply for approval from your council.
Private BCBs
You can apply through a private approved inspector.
They’ll tell your local authority about the work. This is called giving an ‘initial notice’.
Choose a type of application
You must decide on the type of application for your planned build, extension or alteration work.
Full plans
This is the most thorough option. You can expect a decision within 5 weeks, or 2 months with your consent.
You’ll get a completion certificate within 8 weeks of completion of the building work as long as it complies.
Building notice
This type of application is only for smaller projects. You can start work 2 days after your notice has been submitted to your BCB. You do not get formal approval like you do with full plans.
Regularisation
You can apply for ‘regularisation’ – retrospective approval for work already carried out without consent – from a local authority BCB only.
Only work carried out after 11 November 1985 can be approved in this way.
You might need to make alterations before your BCB can agree the work complies and give you a regularisation certificate.
The general binding rules
You must follow the general binding rules if you’re the operator of a septic tank or small sewage treatment plant.
The sewage must:
- be domestic in nature, for example from a toilet, bathroom, shower or kitchen of a house, flat or business (such as a pub, hotel or office) – contact the Environment Agency if you’re not sure if the sewage is domestic in nature
- not cause pollution – find out how to check for pollution
There are other rules depending on whether you’re releasing this sewage:
- to the ground, for example in your back garden
- to a surface water, for example a river or stream
Ask your local installation or maintenance company if you’re not sure what sort of system you have.
Releasing to the ground
You must use a septic tank or a small sewage treatment plant and a drainage field (infiltration system).
You must apply for a permit if you release (‘discharge’):
- to a well, borehole or other deep structure
- more than 2 cubic metres (2,000 litres) per day
- in a groundwater source protection zone (SPZ1)
You must also read the additional rules for discharging sewage to the ground.
Calculate how much sewage you discharge
For sewage from a residential property, use the calculator to work out how much you discharge per day.
For commercial properties (such as a hotel, restaurant or office) or holiday accommodation (such as a cottage or chalet), use British Water’s flows and loads guidance or contact the Environment Agency for advice.
Work out if you’re in a groundwater source protection zone 1 (SPZ1)
An SPZ1 can be either:
- the area around a commercial water supply shown on the map of protected zones – check whether your discharge is in the inner zone (Zone 1) or ask the Environment Agency
- any area within 50 metres of a private water supply for human consumption – ask your neighbours if they have a spring, well or borehole, and how far it is from your drainage field
Releasing to a surface water
You must use a small sewage treatment plant. You must apply for a permit if you’re discharging more than 5 cubic metres (5,000 litres) per day. Use the calculator to work out how much you discharge per day.
You must also read the additional rules for discharging sewage to a surface water.
For commercial properties (such as a hotel, restaurant or office) or holiday accommodation (such as a cottage or chalet), use British Water’s flows and loads guidance or contact the Environment Agency for advice.
Installing a new system
You must have building regulations approval. You may also need planning permission.
Check with your local council.
If you did not get permission and approval
You must apply retrospectively for planning permission. You may also need building regulations approval.
If your system was installed before 1 January 2015, you should contact your local council for advice.
You must read septic tanks and treatment plants: permits and general binding rules before you follow this guidance.
New rules came into force on 1 January 2015. If your system was installed and discharging before 31 December 2014 you have an ‘existing discharge’. If your system was installed and discharging on or after 1 January 2015 you have a ‘new discharge’.
Rules for existing and new discharges
Use the correct treatment system
You must use a septic tank or small sewage treatment plant to treat the sewage and then discharge the effluent (treated liquid) to ground via a drainage field.
A septic tank is an underground tank where the solids sink to the bottom, forming a sludge, and the liquid flows out to a drainage field.
A small sewage treatment plant, also known as a package treatment plant, works in a similar way to a septic tank but uses mechanical parts to treat the liquid to a higher standard before it goes to a drainage field.
A drainage field, also known as an infiltration system, is a series of pipes with holes placed in trenches and arranged so that the effluent can trickle through the ground for further treatment.
You cannot use a soakaway (designed for draining rainwater), well or borehole for discharging effluent to ground. Instead you must either upgrade to a drainage field or apply for a permit so that the Environment Agency can assess the risk of using this sort of system in your location.
Your treatment system must meet the right British Standard
Your treatment system must meet the relevant British Standard in force at the time of installation. The standards currently in force for new systems are:
- BS EN 12566 for septic tanks and small sewage treatment plants
- BS 6297:2007 for drainage fields
Your septic tank or treatment plant met the British Standard in place at the time of installation if:
- it has a CE mark
- the manual or other documentation that came with your tank or treatment plant has a certificate of compliance with a British Standard
- it’s on British Water’s list of approved equipment
You can also ask the company that installed your equipment to confirm that it complies with the British Standard that was in place at the time the equipment was installed.
If your treatment system was installed before 1983 there was no British Standard in place. You do not need to do anything to meet the British Standard requirement. You must still meet all the other general binding rules.
Your treatment system must be installed correctly and have enough capacity
Your treatment system must be large enough to handle the maximum amount of sewage it will need to treat. If you install a new septic tank, small sewage treatment plant or drainage field you must check with the installer that it meets the sizing requirements in British Water’s Flows and Loads 4 guidance.
If the amount of sewage the system needs to treat increases (for example, because you’ve extended your property or connected an additional property) you must make sure the treatment system is still big enough. You must also recalculate the maximum daily volume of your discharge and apply for a permit if it is more than 2 cubic metres (2,000 litres) a day.
Your treatment system must be installed in line with the manufacturer’s specification (the instruction manual or technical set of requirements that comes with the equipment).
Have your treatment system regularly emptied and maintained
You must get the sludge that builds up in your septic tank or small sewage treatment plant removed (desludged) before it exceeds the maximum capacity. As a minimum, you should have your treatment system desludged once a year or in line with the manufacturer’s instructions.
The company you use to dispose of your waste sludge must be a registered waste carrier. Ask the company to confirm this when you arrange to have your tank emptied or ask the tanker driver for a copy of the company’s waste carrier certificate.
You should have your treatment system regularly maintained in line with the manufacturer’s instructions. If these are not available, ask your local maintenance company for advice.
You must have your treatment system repaired or replaced if it is not in good working order, for example if it has:
- leaks
- cracks in tank walls or pipes
- blocked pipes
- signs that the effluent is not draining properly, pools of water around the drainage point
- sewage smells
- a failed motor
- a failed pump
- a failed electrical supply
Anyone who carries out maintenance on your system must be competent. Competent people include those on British Water’s list of accredited service engineers.
You sell your property: tell the new owner about the sewage treatment system
If you sell your property, you must tell the new operator (the owner or person responsible for the septic tank or small sewage treatment plant) in writing that a small sewage discharge is in place.
Include:
- a description of the treatment system and drainage system
- the location of the main parts of the treatment system, drainage system and discharge point
- details of any changes made to the treatment system and drainage system
- details of how the treatment system and drainage system should be maintained, and the maintenance manual if you have one
- maintenance records if you have them
You stop using your treatment system: make sure it’s properly decommissioned
You must remove anything that could cause pollution (for example, remaining sludge) when you stop using a septic tank or small sewage treatment plant.
This does not apply if you only stop using the equipment temporarily, for example if your property is empty.
You can ask a maintenance company for advice on how to decommission your septic tank or treatment plant properly.
Rules for discharges in a groundwater source protection zone 1 (SPZ1)
Check if the discharge point is in a groundwater SPZ1
A groundwater SPZ1 can be either:
- the area around a commercial water supply (used for drinking water or food production) shown on the map of protected zones – check if your discharge is in the inner zone (zone 1) or ask the Environment Agency
- any area within 50 metres of a private water supply for human consumption – ask your neighbours if they have one and if so how far their spring, well or borehole is from your drainage field
You must apply for a permit if you have an existing discharge or are planning to start a new discharge to the ground in a SPZ1. The permit costs £125. A permit will include additional conditions to the general binding rules.
The Environment Agency will grant the permit if there’s:
- no evidence of pollution
- the risk of pollution is acceptable
If there is evidence of pollution or the risk of pollution is unacceptable the Environment Agency will ask you to make changes to your system and may issue a permit with improvement conditions.
The Environment Agency regularly checks:
- surface and groundwater quality
- permit compliance
If they find your system may be causing pollution to surface or groundwater they will contact you to discuss the issues. This may result in your permit being reviewed or revoked. See the guidance on how to comply with your permit.
See the guidance on how to apply for a permit. Contact the Environment Agency if you need help with your application.
Additional rules for new discharges from treatment systems installed and in use on or after 1 January 2015
You must follow these additional rules if you:
- started a new discharge from a septic tank or small sewage treatment plant on or after 1 January 2015
- had a discharge to a surface water before 1 January 2015 which you now want to change to discharge to ground, or the other way round
- had a discharge to ground before 1 January 2015 and you want to install a new drainage field more than 10 metres away from the existing one
These are called new discharges.
Check if there’s a public sewer nearby
If any part of the building your treatment system serves is within 30 metres of a public sewer, the Environment Agency will not allow you to start a new discharge from a septic tank or small sewage treatment plant under the general binding rules.
If you are building a development of more than one property, this distance must be multiplied by the number of properties – for example, if there are 3 properties then the distance will be 3 x 30 metres = 90 metres.
To find out if there is a public sewer near your property, contact your local water company.
If there is a good reason why you cannot connect to the sewer (for example, if there is a river or a hill in the way) then you must apply for a permit so that the Environment Agency can decide whether to allow you to use a small sewage treatment plant instead. Contact the Environment Agency to find out what information you will need to put in your application.
Building regulations and planning approval
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.
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