Statutory guidance

SR2025 No 7: generic risk assessment – domestic sewage effluent with a maximum daily volume of 5 cubic metres per day to surface water using non-British Standard equipment or where the watercourse does not always have flow, or both

Published 20 April 2026

Applies to England

SR2025 No 7: standard rules permit for discharge of secondary treated domestic sewage effluent with a maximum daily volume of 5 cubic metres per day to surface water where secondary treatment is achieved using equipment that does not conform to the British Standard or where the receiving watercourse does not always have flow, or both

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 7.

Check this generic risk assessment to understand:

• the potential environmental risks associated with discharges of secondary treated domestic sewage effluent with a maximum daily volume of 5 cubic metres per day to surface water where secondary treatment is achieved using equipment that does not conform to the British Standard or where the receiving watercourse does not always have flow, or both
• 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 water discharge activities under paragraph 3(1) of Schedule 21 to Environmental Permitting Regulations 2016 are the discharge of secondary treated domestic sewage effluent with a maximum daily volume of 5 cubic metres per day to surface water where secondary treatment is achieved using equipment that does not conform to the British Standard or where the receiving watercourse does not always have flow, or both

Parameter 2

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

Parameter 3

The discharge must have received secondary treatment.

Parameter 4

Discharges using works and equipment which are not British Standard must be an existing discharge to surface water and must be secondary treated. The sewage treatment plant shall be designed, operated and maintained so as to ensure that any effluent discharged does not contain concentrations in excess of 20 milligrams per litre of ATU-BOD as O₂, 30 milligrams per litre of suspended solids (measured after drying at 105°C), or 20 milligrams per litre of ammoniacal nitrogen (expressed as N). Prior to installation of the works and equipment, the operator shall obtain evidence from the installer that the sewage treatment system is capable of consistently treating the effluent to the required standards. This evidence must be kept with the management system documentation.

Parameter 5

Where the receiving watercourse does not always have flow, the discharge must be secondary treated, and the sewage treatment plant shall be designed, operated and maintained so as to ensure that any effluent discharged does not contain concentrations in excess of 20 milligrams per litre of ATU-BOD as O₂, 30 milligrams per litre of suspended solids (measured after drying at 105°C), or 20 milligrams per litre of ammoniacal nitrogen (expressed as N). If the discharge takes place on a principal or secondary A aquifer, then the sewage treatment plant shall be designed, operated and maintained so as to ensure that any effluent discharged does not contain concentrations in excess of 20 milligrams per litre of ATU-BOD as O₂, 30 milligrams per litre of suspended solids (measured after drying at 105°C), or 10 milligrams per litre of ammoniacal nitrogen (expressed as N).

Parameter 6

Where the receiving watercourse does not always have flow, the discharge must be made via a perforated pipe. The extent of the perforated pipe shall be sufficient to ensure that the discharge is only made when the receiving watercourse is flowing. Any section of perforated pipe must be located within 10 metres of the bank of the receiving watercourse.

Parameter 7

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 8

The discharge must not be made into surface waters within 500 metres upstream from a European site, Ramsar site, biological Site of Special Scientific Interest (SSSI), freshwater pearl mussel population, designated bathing water, Marine Conservation Zone, protected shellfish water, any body of water identified as containing a Protected Species, 200 metres of a water-based local nature reserve or National Nature Reserve or 50 metres of a chalk river, water-based local wildlife site or water-based protected habitat that the Environment Agency considers is at risk from this activity. For a site in tidal water ‘500m upstream’ means within 500 metres by the shortest distance over water in any direction from the nearest boundary of any of these sites. 

This restriction only applies to conservation sites that are water-based and hydraulically linked to the receiving water downstream of the discharge point.

Parameter 9

The discharge must not be made to a lake or pond, as defined in section 104(3) Water Resources Act 1991.

Parameter 10

Where the receiving watercourse does not always have flow, 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 11

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.

Parameter 12

For discharges in tidal waters, the discharge outlet must be below the mean spring low water mark.

1. Risk to local human population

1.1 Releases of pathogens

Pathogens within the treated sewage effluent enter surface water where people swim or harvest shellfish for consumption. 

We have assessed the potential harm as follows:

• illness

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 high

The reasons for giving the activity this rating is because there is potential for public health risk if people are swimming in water or harvesting shellfish downstream of the discharge.

Managing the risk

The following manages the risk:

• prior to the discharge, the sewage must have received secondary treatment
• the discharge shall not be made into surface waters within 500 metres upstream from the nearest boundary of a designated bathing water or protected shellfish water

• for discharges in tidal waters, the discharge outlet must be below the mean spring low water mark

• the discharge point to the receiving watercourse must be sited and constructed so that the discharge will not cause a nuisance or harm to human health

• in cases where the receiving watercourse does not always have flow, the discharge directly to the watercourse shall only be made when there is flow in the watercourse immediately upstream of that discharge point. At all other times the discharge shall be made indirectly to the watercourse via the ground
• 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
• the activities shall be managed and operated in accordance with a written management system
• any breaches shall be reported

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

2. Risks to local human population

2.1 Odour and amenity nuisance.

Sewage effluent discharges may create unpleasant odours or cause visible pollution of surface water resulting in a nuisance to the local population.

We have assessed the potential harm as follows:

• Odour or amenity nuisance

Judgement of risk

We have judged the:

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

The reasons for giving the activity this rating is because significant odour or pollution nuisance should only be likely from treatment systems that have a malfunction.

Managing the risk

The following manages the risk:

• prior to the discharge, the sewage must have received secondary treatment

• for discharges in tidal waters, the discharge outlet must be below the mean spring low water mark

• the discharge point to the receiving watercourse must be sited and constructed so that the discharge will not cause a nuisance or harm to human health

• 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
• in cases where the receiving watercourse does not always have flow, the discharge directly to the watercourse shall only be made when there is flow in the watercourse immediately upstream of that discharge point. At all other times the discharge shall be made indirectly to the watercourse via the ground
• the activities shall be managed and operated in accordance with a written management system
• the sewage treatment plant must be designed, operated and maintained to meet effluent quality standards that ensure secondary treatment is achieved by any non-British Standard equipment and that receiving surface waters are protected
• any breaches shall be reported

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

3. Risk to chemical and biological quality of receiving surface waters

3.1 Releases of organic matter, nutrients, suspended solids and toxic substances

Treated sewage effluent contains organic matter, nutrients, suspended solids, and toxic substances which will be discharged into the receiving surface water.

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

• organic matter removes oxygen from the water, which can cause harm to plants and animals
• suspended solids may blanket the bed of the watercourse, create a cloudy appearance in the watercourse and reduce the amount of light entering the watercourse
• ammonia can be directly toxic, causing harm to plants and animals
• toxic substances may cause harm to plants or animals
• phosphorous may stimulate plant growth which changes plant ecology or causes algal blooms which causes harm to plants and animals

Judgement of risk

We have judged the:

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

The reason for giving the activity this rating is that, although these pollutants are present in sewage effluent, the volumes are relatively small, and the sewage is only domestic.

Managing the risk

The following manages the risk:

• prior to the discharge, the sewage must have received secondary treatment
• the sewage treatment plant must be designed, operated and maintained to meet effluent quality standards that ensure secondary treatment is achieved by any non-British Standard equipment and that receiving surface waters are protected
• in cases where the receiving watercourse does not always have flow, the discharge directly to the watercourse shall only be made when there is flow in the watercourse immediately upstream of that discharge point. At all other times the discharge shall be made indirectly to the watercourse via the ground
• the sewage shall be solely domestic sewage and contain no trade effluent
• the activities shall be managed and operated in accordance with a written management system
• 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
• any breaches shall be reported

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

4. Risk to protected sites

4.1 Releases of pollutants and nutrients

Protected Sites are those set out in Parameter 8.

Pollutants and nutrients within the treated sewage effluent may alter water chemistry and adversely affect a conservation site or species that relies on surface water or water dependent ecosystem.

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 at protected sites as a result of changes in surface water chemistry (for example nutrient enrichment)

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 some protected sites are sensitive to changes in water chemistry.

Managing the risk

The following manages the risk:

• the discharge must not be made into surface waters within 500 metres upstream from a European site, Ramsar site, biological SSSI, freshwater pearl mussel population, designated bathing water, Marine Conservation Zone, protected shellfish water, any body of water identified as containing a Protected Species, 200 metres of a water-based local nature reserve or National Nature Reserve or 50 metres of a chalk river, water-based local wildlife site or water-based protected habitat that the Environment Agency considers is at risk from this activity. For a site in tidal water ‘500m upstream’ means within 500 metres by the shortest distance over water in any direction from the nearest boundary of any of these sites
• prior to the discharge, the sewage must have received secondary treatment
• the sewage treatment plant must be designed, operated and maintained to meet effluent quality standards that ensure secondary treatment is achieved by any non-British Standard equipment and that receiving surface waters are protected
• the sewage shall be solely domestic sewage and contain no trade effluent
• the activities shall be managed and operated in accordance with a written management system
• 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
• any breaches shall be reported

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

5. Risk to groundwater in underlying aquifers

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

If the receiving watercourse does not always have flow, then at any time that flow is not present immediately upstream of the discharge point, the discharge must be made indirectly to the watercourse via a drainage system. Consequently, hazardous substances, non-hazardous pollutants and pathogens within the treated sewage effluent may migrate from the discharge area through the unsaturated zone and into groundwater.

We have assessed the potential harm to groundwater resources as follows:

• 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 low
• overall severity of potential consequences as low
• overall risk rating as low

The reasons for giving the activity this rating is because treated sewage effluent typically contains a range of substances and pathogens at different concentrations, but the discharge will not continuously be made to ground.

Managing the risk

The following manages the risk:

• because the discharge will be made directly to a watercourse for some of the time, it must be secondary treated
• because the discharge may be made to ground some of the time, the Standard Rules Permit also requires that the sewage treatment plant is designed, operated and maintained to meet effluent quality standards that are protective of groundwater
• the maximum daily volume is restricted to no greater than 5 cubic metres per day
• no part of the infiltration system constructed shall be more than 2 metres below ground level or below the level of the bed of the receiving watercourse. This will maximise the amount of unsaturated zone available for additional attenuation of the treated effluent.
• 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
• 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
• any breaches shall be reported

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

6. Risk to public and private drinking water supplies from groundwater sources

6.1 Releases of hazardous substances, non-hazardous pollutants and pathogens.

If the receiving watercourse does not always have flow, then at any time that flow is not present immediately upstream of the discharge point, the discharge must be made indirectly to the watercourse via a drainage system. Consequently, hazardous substances, non-hazardous pollutants and pathogens within the treated sewage effluent may migrate from the discharge area through the unsaturated zone and into groundwater and travel 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 low
• overall severity of potential consequences as medium
• overall risk rating as low-medium

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:

• where the receiving watercourse does not always have flow, the discharge must not be made within a groundwater SPZ1 or within 125 metres of any well, spring or borehole that is used to supply water for domestic drinking or food production purposes
• because the discharge will be made directly to a watercourse for some of the time, it must be secondary treated
• because the discharge may be made to ground some of the time, the Standard Rules Permit also requires that the sewage treatment plant is designed, operated and maintained to meet effluent quality standards that are protective of groundwater
• the maximum daily volume is restricted to no greater than 5 cubic metres per day
• no part of the infiltration system constructed shall be more than 2 metres below ground level or below the level of the bed of the receiving watercourse. This will maximise the amount of unsaturated zone available for additional attenuation of the treated effluent
• 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
• 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
• any breaches shall be reported

Taking these actions will further 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