Statutory guidance

SR2025 No 5: generic risk assessment – domestic sewage effluent with a daily volume of greater than 2 but no more than 5 cubic metres per day to ground

Published 20 April 2026

Applies to England

SR2025 No 5: standard rules permit for discharge of secondary treated domestic sewage effluent with a daily volume of greater than 2 but no more than 5 cubic metres per day to ground via an infiltration system

The Environment Agency produces the generic risk assessments for all standard rules permits. These list the potential risks and how to manage them.

This generic risk assessment covers activities set out in standard rules set SR2025 No 5.

Check this generic risk assessment to understand:

• the potential environmental risks associated with discharges of domestic sewage effluent with a daily volume of greater than 2 but no more than 5 cubic metres to ground via an infiltration system
• how to manage the risks effectively

Each risk comprises:

• information about the source, pathway and receptor – and the potential harm to that receptor
• a judgement of the level of risk and justification of that judgement
• actions for managing the risk (through permitting) and a residual risk rating after managing it

Risk management involves breaking or limiting the source-pathway-receptor linkage to reduce the risk. We will control the residual risk (after risk management) when we assess compliance.

If you need to check the meaning of any terms we have used (in the context of this risk assessment), see the explanation of terms at the end of this document. To see the definition of technical terms, see the “Interpretation” section of the Standard Rules associated with this risk assessment.

Risk assessment parameters

This generic risk assessment is based on the following parameters.

Parameter 1

The permitted groundwater activities under paragraph 3(1) of Schedule 22 to Environmental Permitting Regulations 2016 are the discharge of secondary treated domestic sewage effluent with a daily volume of greater than 2 but no more than 5 cubic metres to ground via an infiltration system.

Parameter 2

The total daily volume of the discharge calculated by the method specified in the current edition of “Flows and Loads” shall be greater than 2 but no more than 5 cubic metres per day.

Parameter 3

All works and equipment used for the treatment of sewage effluent and its discharge must comply with the relevant design and manufacturing standards, meaning the British Standard that was in force at the time of the installation, and guidance issued by the appropriate authority on the capacity and installation of the equipment.

Parameter 4

The discharge must not be made within a groundwater source protection zone 1 (SPZ1) or within 125 metres of any well, spring or borehole that is used to supply water for domestic drinking or food production purposes.

Parameter 5

The sewage must be secondary treated and, if the discharge takes place on a principal or secondary A aquifer, then any effluent discharged must not exceed a limit of 10 milligrams per litre (mg/l) of ammoniacal nitrogen (expressed as N).

Parameter 6

The discharge must be a minimum distance from certain types of sensitive receptor. It cannot be in or within:

• 50 metres of a European site
• 50 metres of a Ramsar site
• 50 metres of a biological Site of Special Scientific Interest (SSSI)
• 50 metres of a Marine Conservation Zone
• 50 metres of a National Nature Reserve
• 50 metres of a Local Nature Reserve
• 50 metres of a Local Wildlife Site
• 50 metres of a water-based protected habitat or body of water known to contain protected species that the Environment Agency considers is at risk from this activity

Parameter 7

The discharge must not be within an Ancient Woodland.

Parameter 8

No part of the infiltration system constructed shall be more than 2 metres below ground level.

Parameter 9

No part of the infiltration system shall be less than 1.2 metres above the highest predicted annual groundwater level.

Parameter 10

The infiltration system shall not connect to any land drainage system.

Parameter 11

The infiltration system shall not be situated within 10 metres of any watercourse (including any ditch that runs dry for part of the year), or any other surface water.

Parameter 12

At the time the discharge is first made, it must not be made to a discharge point within 50 metres of any other exempt or permitted sewage effluent discharge to ground or surface water. 

Parameter 13

The boundary of the premises that the sewage treatment system serves must not be within 30 metres of a public foul sewer. If the sewage treatment system serves more than one property, then it must not be within a distance that is less than 30 metres multiplied by the number of properties served.

If some or all of the discharge is from non-domestic properties, then the maximum volume in cubic metres that is being discharged from those other premises should be divided by 0.75 and the result multiplied by 30. The result is a distance in metres and no boundary of the premises that the sewage treatment system serves can be within that distance of a public foul sewer.

Parameter 14

The works and equipment shall be serviced a minimum of once every 12 months (or more if required by the manufacturer’s specifications) by a competent and suitably qualified person. Records demonstrating compliance with this rule shall be maintained by the operator.

1. Risk to groundwater in underlying aquifers

1.1 Releases of hazardous substances, non-hazardous pollutants and pathogens

Hazardous substances, non-hazardous pollutants and pathogens within the treated sewage effluent may migrate from the drainage field through the unsaturated zone and into groundwater.

We have assessed the potential harm to groundwater resources as impact upon the chemical or biological quality of groundwater in the aquifer.

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because treated sewage effluent typically contains a range of substances and pathogens at different concentrations.

Managing the risk

The following manages the risk:

• the sewage must be secondary treated and, if the discharge takes place on a principal or secondary A aquifer, then any effluent discharged must not exceed a limit of 10 milligrams per litre of ammoniacal nitrogen (expressed as N)
• a minimum of 1.2 metres of unsaturated zone between the base of the drainage field and the seasonally highest groundwater level to ensure attenuation of the effluent prior to it entering groundwater
• the permitted activity shall be managed and operated in accordance with a written management system
• the sewage must only be domestic effluent, which does not typically contain significant concentrations of hazardous substances
• all works and equipment used for the treatment of sewage effluent and its discharge must comply with the relevant design and manufacturing standards, meaning the British Standard that was in force at the time of the installation, and guidance issued by the appropriate authority on the capacity and installation of the equipment
• the equipment used for the treatment and discharge of the sewage effluent must be serviced by a competent person a minimum of once every 12 months
• no other emissions to groundwater are allowed other than the activities described within the permit
• any breaches shall be reported

Taking these actions will further control the risk and we rate it as ‘low’.

2. Risk to public and private drinking water supplies

2.1 Releases of hazardous substances, non-hazardous pollutants and pathogens

Hazardous substances, non-hazardous pollutants and pathogens within sewage effluent may enter groundwater and migrate towards boreholes used to supply water for human consumption.

We have assessed the potential harm to drinking water supplies as follows:

• impact upon the chemical or biological quality of groundwater
• impact upon public or private groundwater boreholes which supply water for domestic drinking or food production purposes

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as medium
• overall severity of potential consequences as high
• overall risk rating as medium-high

The reasons for giving the activity this rating is because treated sewage effluent typically contains a range of substances and pathogens. These may enter groundwater and migrate to boreholes used to supply water for domestic drinking or food production purposes.

Managing the risk

The following manages the risk:

• discharges are not allowed within a groundwater source protection zone 1 or within 125 metres of any well, spring or borehole that is used to supply water for domestic drinking or food production purposes
• the sewage must be secondary treated and, if the discharge takes place on a principal or secondary A aquifer, then any effluent discharged must not exceed a limit of 10 milligrams per litre of ammoniacal nitrogen (expressed as N)
• a minimum of 1.2 metres of unsaturated zone between the base of the drainage field and the seasonally highest groundwater level to ensure attenuation of the effluent prior to it entering groundwater
• the permitted activity shall be managed and operated in accordance with a written management system
• the sewage must only be domestic effluent, which does not typically contain significant concentrations of hazardous substances
• all works and equipment used for the treatment of sewage effluent and its discharge must comply with the relevant design and manufacturing standards, meaning the British Standard that was in force at the time of the installation, and guidance issued by the appropriate authority on the capacity and installation of the equipment
• the equipment used for the treatment and discharge of the sewage effluent must be serviced by a competent person a minimum of once every 12 months
• no other emissions to groundwater are allowed other than the activities described within the permit
• any breaches shall be reported

Taking these actions will further control the risk and we rate it as ‘low’.

3. Risk to surface waters

3.1 Releases of hazardous substances, non-hazardous pollutants and pathogens

Hazardous substances, non-hazardous pollutants and pathogens within the treated sewage effluent may enter groundwater that discharges into surface water features such as rivers, springs or ponds.

We have assessed the potential harm to surface waters as follows:

• impact upon the chemical or biological quality of surface water

• harm to surface water ecology

• visible signs of pollution to surface waters (for example discolouration)

Judgement of risk

We have judged the:

• likelihood of the hazard affecting these receptors as low
• overall severity of potential consequences as low
• overall risk rating as low

The reason for giving the activity this rating is that groundwater is often hydraulically connected to surface water, and contaminated groundwater could discharge into surface waters.

Managing the risk

The following manages the risk:

• the sewage must be secondary treated and, if the discharge takes place on a principal or secondary A aquifer, then any effluent discharged must not exceed a limit of 10 milligrams per litre of ammoniacal nitrogen (expressed as N)
• the infiltration system shall not connect to any land drainage system
• the infiltration system shall not be situated within 10 metres of any watercourse (including any ditch that runs dry for part of the year), or any other surface water
• a minimum of 1.2 metres of unsaturated zone between the base of the drainage field and the seasonally highest groundwater level to ensure attenuation of the effluent prior to it entering groundwater
• the permitted activity shall be managed and operated in accordance with a written management system
• the sewage must only be domestic effluent, which does not typically contain significant concentrations of hazardous substances
• all works and equipment used for the treatment of sewage effluent and its discharge must comply with the relevant design and manufacturing standards, meaning the British Standard that was in force at the time of the installation, and guidance issued by the appropriate authority on the capacity and installation of the equipment
• the equipment used for the treatment and discharge of the sewage effluent must be serviced by a competent person a minimum of once every 12 months
• no other emissions to groundwater are allowed other than the activities described within the permit
• any breaches shall be reported

Taking this action will control the risk and we rate it as ‘very low’

4. Risk to protected sites and ancient woodlands

4.1 Releases of hazardous substances and non-hazardous pollutants

Protected Sites are those set out in Parameter 6.

Hazardous substances and non-hazardous pollutants within the treated sewage effluent may enter groundwater and migrate towards protected sites or enter surface waters associated with those sites.

We have assessed the potential harm to protected sites as follows:

• impact upon the chemical quality of water at water-dependent protected sites
• impact upon habitats or species at protected sites as a result of changes in groundwater chemistry (for example nutrient enrichment)

Judgement of risk

We have judged the:

• likelihood of the hazard affecting the receptor as low
• overall severity of potential consequences as medium
• overall risk rating as medium

The reasons for giving the activity this rating is because some protected sites are sensitive to changes in water chemistry.

Managing the risk

The following manages the risk:

• the discharge must not be made in, or within, 50 metres of a European site, Ramsar site, biological SSSI, Marine Conservation Zone, National Nature Reserve, Local Nature Reserve, Local Wildlife Site, water-based protected habitat or body of water known to contain protected species that the Environment Agency considers is at risk from this activity and must not be in an Ancient Woodland
• the sewage must be secondary treated and, if the discharge takes place on a principal or secondary A aquifer, then any effluent discharged must not exceed a limit of 10 milligrams per litre of ammoniacal nitrogen (expressed as N)
• the infiltration system shall not connect to any land drainage system
• the infiltration system shall not be situated within 10 metres of any watercourse (including any ditch that runs dry for part of the year), or any other surface water
• a minimum of 1.2 metres of unsaturated zone between the base of the drainage field and the seasonally highest groundwater level to ensure attenuation of the effluent prior to it entering groundwater
• the permitted activity shall be managed and operated in accordance with a written management system
• the sewage must only be domestic effluent, which does not typically contain significant concentrations of hazardous substances
• all works and equipment used for the treatment of sewage effluent and its discharge must comply with the relevant design and manufacturing standards, meaning the British Standard that was in force at the time of the installation, and guidance issued by the appropriate authority on the capacity and installation of the equipment
• the equipment used for the treatment and discharge of the sewage effluent must be serviced by a competent person a minimum of once every 12 months
• no other emissions to groundwater are allowed other than the activities described within the permit
• any breaches shall be reported

Taking this action will control the risk and we rate it as ‘low’.

Explanation of terms

Receptor

The things at risk and that need protecting.

Receptors considered include surface waters, groundwater, humans, ecosystems and their habitats.

A single receptor may be at risk from several different sources and all must be addressed.

Source

The agent or process that has the potential to cause harm.

A contaminant or pollutant (a hazard) that has the potential to cause harm. For example, the activity or operation taking place for which a particular hazard may arise.

Harm

The harmful consequence to the receptor if the hazard is realised.

Pathways

The route or means by which a defined hazard may affect a receptor.

Source-pathway-receptor linkage

There has to be a link between the source, pathway and receptor for there to be a risk.

Likelihood of exposure

This is the likelihood of the receptors being exposed to the hazard. The meaning of the definitions are:

• high – exposure is probable – direct exposure is likely with no or few barriers between the hazard source and the receptor
• medium – exposure is fairly probable – feasible exposure is possible as the barriers to exposure are less controllable
• low – exposure is unlikely – several barriers exist between the hazard source and receptor to reduce exposure
• very low – exposure is very unlikely – effective, multiple barriers are in place to reduce exposure

Overall magnitude of potential consequence

This is the severity of the consequence if the hazard is realised and may cause actual or potential harm.

This will have a high, medium, low or very low rating using attributes and scaling to consider ‘harm’.

Risk rating

We work out the risk rating by combining the likelihood of exposure with the magnitude of the potential consequences.

We assign these ratings:

• high risk – requires additional assessment and active management
• medium risk – requires additional assessment and may need active management and, or monitoring (or both)
• low and very low risks will require a periodic review