Guidance

Water companies: environmental permits for storm overflows and emergency overflows

Updated 13 September 2018

The Environment Agency regulates intermittent discharges from sewer overflows and waste water treatment works (WWTW) through environmental permits.

Classify storm overflows for improvement

As a water company, you must design, construct and maintain sewerage systems according to best technical knowledge not entailing excessive cost (BTKNEEC). You must also limit pollution from storm overflows. To make sure you do this, you must identify storm overflows that need improvement.

You must classify your storm overflows as either:

• unsatisfactory
• substandard
• satisfactory

Do this as part of your drainage strategy. Identify where investment is required. Read Ofwat’s guide on how to prepare a drainage strategy.

Unsatisfactory overflows

The Environment Agency classes storm overflows as unsatisfactory when they:

• operate in dry weather conditions
• operate in breach of permit conditions
• cause significant visual or aesthetic impact due to solids or sewage fungus
• cause or significantly contribute to a deterioration in the biological or chemical status of the receiving water
• cause or significantly contribute to failures in bathing water quality standards for identified bathing waters
• cause or significantly contribute to failures in shellfish quality standards for identified shellfish waters
• cause or significantly contribute to failures in water quality standards in coastal and transitional waters
• cause pollution of groundwater

You need to classify the status of your storm overflows. Methods and data you can use include:

• outfall aesthetics surveys that show dry weather operation, photographs of sewage litter and sewage fungus and proof of watercourse amenity
• biological surveys of combined sewer overflow (CSO) impacts on water quality
• justified public complaints to water companies, local authorities or the Environment Agency
• pollution incident data from the Environment Agency
• asset surveys that identify any reasons for unsatisfactory performance, like low weir settings or screen conditions
• water quality or sewer modelling to assess compliance against relevant water quality standards
• event duration monitoring (EDM) to show spill frequencies and support predicted performance

You need to classify your overflows to make sure they do not become unsatisfactory.

If an overflow becomes unsatisfactory the Environment Agency can review your permit or take enforcement action against you if you are in breach of your existing permit.

We will include an improvement condition in permits for unsatisfactory overflows to meet appropriate standards as soon as practicable. This is normally within 3 years.

The only exception is where an overflow becomes unsatisfactory due to new legal requirements. In this case, we’ll promote solutions to the affected overflows through the Water Industry National Environment Programme (WINEP).

Substandard overflows

Substandard overflows are overflows which are satisfactory in terms of environmental impact, but:

• do not meet modern standards of engineering and aesthetic control for storm overflow structures set out in the British standard BS EN 752:2017 drain and sewer systems outside buildings
• do not have sufficient hydraulic capacity compared to accepted minimum design standards
• risk becoming unsatisfactory because discharges have increased beyond the original design due to infiltration, growth and urban creep

Urban creep is where residents in urban areas install or enlarge patios, extensions and driveways, increasing the impermeable area.

If an overflow is substandard you must upgrade to modern standards of engineering. Do this in the medium to long-term through asset renewal under capital maintenance or supply demand balance programmes.

You must carry out routine maintenance and manage the substandard overflow using a drainage strategy to prevent it becoming unsatisfactory.

Satisfactory overflows

Satisfactory overflows meet the environmental criteria and modern standards of engineering for storm overflow structures, aesthetic control and have sufficient capacity.

You must carry out routine maintenance and manage the overflow to prevent it becoming unsatisfactory. You must maintain all previous improvements to meet environmental standards. To do this, develop a long-term drainage strategy that protects the overflow from deteriorating.

Use urban pollution management (UPM) and a partnership approach

Use UPM to plan all work affecting storm overflows. UPM is a procedure for managing wastewater discharges from sewer and sewage treatment systems in wet weather to meet the requirements of the receiving water in a cost effective way.

Following UPM procedures, including the Environment Agency’s requirement for no deterioration, will help you limit pollution from storm overflows.

Read the UPM manual on the Foundation for Water Research website. This guide shows how to model the impact of storm overflows to inland, estuarine and coastal waters. It includes guidance on selecting the level of detail and complexity of models for particular situations.

To improve the success of your permit application, work with the Environment Agency to agree the:

• scope
• modelling
• solution

UPM studies can take considerable time to complete and may be very complex. Working with us to agree these important stages can avoid delays in determining the permit at the end of the project.

Your scope includes:

• the reasons for the study and the problem it’s addressing
• the water quality and emissions standards that apply
• identifying technically appropriate modelling tools
• identifying existing data and models, and any new data and modelling requirements
• identifying a modelling and resource plan

Your modelling includes:

• developing fit for purpose models as identified in your scoping stage
• an understanding of model limitations and model use
• model audits

We may ask you to have your models independently audited to check they’re fit for purpose.

Your solution includes:

• model results – assessment of existing performance against compliance standards
• optioneering – development of solutions that achieve the compliance standards
• solution audits

For unsatisfactory storm overflows, use UPM to achieve the relevant environmental quality targets.

You must carry out modelling work according to the following modelling codes of practice. These codes supplement the UPM procedure and provide specific guidance on modelling and data collection:

• Chartered Institute of Water and Environmental Management (CIWEM) Urban Drainage Group (UDG) code of practice for the hydraulic modelling of urban drainage systems
• Wastewater Planning Users Group (WaPUG) guide to quality modelling of sewer systems
• WaPUG river data collection guide
• WaPUG river modelling guide
• WaPUG integrated urban drainage modelling guide
• CIWEM UDG rainfall modelling guide

These modelling codes of practice (and any updates) are published by the CIWEM UDG.

Design standards for storm overflows

The Environment Agency applies design standards when it issues permits for all new, improved or altered storm overflows. These include:

• minimum flow to full treatment settings at WWTW
• storm storage requirements for settled storm overflows at WWTW
• a minimum pass forward flow setting for storm overflows on the network and at WWTW inlets

Flow to full treatment settings at waste water treatment works

You must design WWTW to treat peak dry weather flow (DWF) and additional flows from light rainfall.

The normal minimum setting is:

Flow to full treatment (FFT) = 3PG + I_max + 3E

Where:
P = catchment population (number)
G = per capita domestic flow (l/head/day)
E = trade effluent flow (l/d)

This FFT setting is also known as 3DWF. I_max is the maximum infiltration rate over the whole year. In certain circumstances you will need to consider the infiltration for summer and winter separately.

To find I_max, calculate infiltration for every dry day as:

I_{dry day} = measured TDV − PG − E

A dry day is a day when rainfall does not exceed 0.25mm.

Do not calculate infiltration for the first day after it has rained or after there has been significant snow melt (the flows measured may contain significant runoff or snow melt from the previous day). Calculate infiltration for all the remaining days.

Your analysis must use data from at least 12 consecutive months. Ideally you should use flow data over several years.

The infiltration rate required is the maximum calculated value.

We may allow changes from the normal minimum FFT requirement if you show there is cost and environmental benefit. For example, attenuation in some sewer systems may result in a dry weather maximum daily peak flow (MDPF) at the sewage treatment works (STW), which is less than 3DWF. In these situations, for example STW with large flat catchments, the receiving water may be adequately protected at a lower FFT. In these cases, you will need to:

• provide us with influent flow records or sewerage modelling on a catchment basis to show the MDPF is significantly less than 3PG + I_max + 3E
• use the UPM procedure to show the impact of the final effluent and any storm discharges on the receiving water will not be significantly less effective than that provided by an FFT of 3PG + I_max + 3E

The need for a high FFT setting in catchments with a high MDPF may be offset by inlet flow balancing. This may be the case for small catchments or catchments where there are pumped inflows. You must not use storm storage to balance flows in dry weather.

You must justify all proposed new or altered storm discharges using the UPM procedure and show that they meet our no deterioration objective.

Flow to full treatment: consider seasonal population variation

If your catchment has a significant population increase in summer, you must calculate FFT separately for summer and winter. The required FFT will be the highest figure.

Use the relevant populations and a different I_max for each of the summer and winter periods. To find the different seasonal maximum infiltration rates, assess the flow data separately. You may need to use data for several years. If data is limited, use estimates or reasonable assumptions of I_max based on similar catchments.

Flow to full treatment: consider future population increases

When you design treatment capacity for future increases in population, you must make an allowance for future levels of infiltration.

The infiltration allowance for the increase in population is normally at 50% of the per capita rate of infiltration in the existing sewerage system. However, you should consider existing levels of infiltration and the type of development. For example, population increases via infill or high-rise developments may not increase sewer lengths and infiltration significantly.

Storm storage requirements

Your storm tank must settle out solids and have a minimum capacity of 68l/head served or a storage equivalent of 2 hours at the maximum flow rate to the storm tanks.

Formula A

A minimum retained flow in the sewer of formula A is the normal minimum requirement for storm overflows on the sewer network and for unsettled storm overflows at the inlet to WWTW. It’s calculated as:

Formula A (l/d) = DWF + 1360P + 2E

Where:
DWF = total dry weather flow (l/d) calculated from PG + I + E
P = catchment population (number)
G = per capita domestic flow (l/head/d)
I = infiltration (l/d)
E = trade effluent flow (l/d)

The formula A value is the minimum retained continuation flow rate. It must be maintained in the downstream sewer during the full duration of a spill, not just at first spill.

Where storage is provided at a CSO and in very large sewerage systems where significant smoothing of flows occur, this will be taken into account in defining the performance equivalent to formula A.

Where there are significant areas within the catchment that were designed, and remain, separately drained, an allowance for separately drained areas may be made. You will need to provide evidence that the storm response in these sewers is minimal (less than or equal to 3DWF). Consequently, the minimum pass forward flow (PFF) required from those populations served by separately drained areas is:

3PsG + Is + 3Es

Where:
Ps = population in areas served by a separate system
G = per capita domestic flow (l/head/d)
Is = infiltration flow from separately drained areas (l/d)
Es = trade flow from separately drained areas (l/d)

Formula A becomes:

Formula A (l/d) = DWF + 2PsG + 1360 Pc + 2Et
and:
DWF (l/d) = PtG + It + Et

Where:
Pc = population in areas served by combined and partially separate sewers
Pt = total population
It = total infiltration (l/d)
Et = total trade flow (l/d)

Freshwaters: water quality standards

The Environment Agency applies water quality design standards when it issues permits for all new, improved or altered storm overflows.

You should agree the design standards that apply to your overflow with us via the partnership approach to UPM.

There are 3 standards relevant to protecting freshwaters from the impact of storm overflows:

Guidance on which standard you apply is based on both the:

• significance of the discharge
• environmental problems being addressed by the scheme

The significance of the discharge depends on the characteristics of the receiving water and the characteristics of the sewerage system.

The Environment Agency will confirm the standards, level of planning and modelling approach when we agree the scope for the project.

99 - percentile standards

The 99-percentile river quality standards are based on the Water Framework Directive (WFD) water body type. They set limits for:

• biochemical oxygen demand (BOD)
• total ammonia
• un-ionised ammonia

The 99-percentile standards are detailed in section 2 of the UPM manual. This manual is published on the Foundation for Water Research website.

Fundamental intermittent standards (FIS)

FIS are based on concentration-duration-frequency thresholds. They set limits for:

• dissolved oxygen (DO)
• un-ionised ammonia

FIS are detailed in section 2 of the UPM manual. This manual is published on the Foundation for Water Research website.

We will only apply FIS for sustainable salmonid ecosystems to discharges affecting established salmonid fisheries and spawning grounds (designated and undesignated). For all other discharges, you must apply the standards for a sustainable cyprinid fishery.

You must also demonstrate that the proposed scheme will allow the receiving water to achieve the required water quality at any other percentile standards relevant to its classification, as well as any other relevant objectives and uses.

Selecting standards and modelling approaches

The guidance assumes that simple models and standards produce conservative solutions. Therefore formula A will provide a more protective (conservative) solution than 99-percentiles. Modelling to achieve 99-percentile standards will produce a more protective solution than complex impact modelling to meet FIS. However, this will not apply in all circumstances. When we’re not confident that a simpler approach is conservative, we’ll ask you to provide more complex modelling to confirm this.

You can use more sophisticated modelling if you believe you can find a more cost effective solution. We will accept this where sensitivity analysis demonstrates confidence in the solution and the additional cost of a conservative solution outweighs any risk of failing to achieve the standards.

Low significance discharges: standards and modelling

Discharge significance is classed as low when:

• dilution is greater than 8:1 (low 5-percentile river flow : DWF)
• there is no interaction with other discharges

When both of these criteria are met, use formula A standards.

Use a verified sewer model to predict CSO performance.

This approach is acceptable where it’s expected that formula A will protect 99-percentiles and FIS. It’s subject to a check on proposed spill frequency and volume where there are other considerations such as amenity.

Medium significance discharges: standards and modelling

Discharge significance is classed as medium when:

• dilution is greater than 2:1 (low 5-percentile river flow : DWF)
• there is little or no interaction with other discharges
• population is less than 10,000
• the discharge is to cyprinid (non salmonid) waters – designated or undesignated

In these cases use 99-percentile standards.

Use verified sewer modelling and simple stochastic river modelling.

This approach is acceptable where it’s expected that 99-percentile standards will protect FIS.

High significance discharges: standards and modelling

Discharge significance is classed as high when:

• dilution is less than 2:1 (low 5-percentile river flow : DWF)
• there is interaction with other discharges
• population is greater than 10,000
• the discharge is to cyprinid or salmonid waters – designated or undesignated

In these cases use FIS.

Use detailed, verified sewer hydraulic and quality modelling and calibrated river quality impact modelling.

The minimum standards for FIS solutions are those for a sustainable cyprinid ecosystem.

You must do checks against percentile standards when designing to FIS risks compliance with WFD and 99-percentile standards. For example, where well aerated rivers can tolerate frequent large spills. 99-percentile standards also provide an alternative approach where confidence in FIS modelling is low.

Freshwater modelling approaches

Whether formula A or 99-percentile standards will protect FIS is related to the nature of the river. A shallow, fast flowing, turbulent stream will have high rates of re-aeration and be able to tolerate a higher BOD load than a deep, slow flowing one. Therefore 99-percentile standards are likely to protect fast flowing, shallow, well aerated rivers, and detailed FIS modelling might be needed for deep sluggish water bodies. The Environment Agency will require FIS modelling where we believe formula A or 99-percentiles will not provide adequate protection.

If FIS are routinely exceeded in receiving waters due to background or natural causes, we may accept 99-percentile design standards. Alternatively, we may ask you to assess and limit cases where FIS are exceeded due to storm discharges. This may be the case in slow flowing eutrophic rivers.

Where unsatisfactory storm discharges do not routinely cause FIS to be exceeded, we may require 99-percentile standards or formula A to resolve the problem. This may be the case in shallow, fast flowing rivers.

For all assessments you must demonstrate that the scheme will be sufficient to protect:

• the river’s uses
• our water quality objectives
• target water quality standards

Estuaries and coastal waters: water quality standards

Coastal waters are within one nautical mile of the coast, or have a salinity of 30 to 34.5 parts per thousand (ppt).

Estuaries (transitional waters) have a salinity of less than 30ppt.

As a minimum, formula A will normally apply.

Where discharges are made into the freshwater end of an estuary, we may require FIS or 99-percentile standards modelled using the flow series for the river. This is likely to be the case for high significance discharges. Alternatively, we may require impact modelling to demonstrate compliance with the relevant WFD standards for transitional and coastal waters. You can see the WFD standards in tables 14 and 15 of the WFD standards and classification directions.

We may ask for site-specific environmental quality standards (EQSs), such as those developed for the Thames Estuary. You may also need to assess other substances under the WFD or Habitats Regulations. We’ll confirm the standards and modelling approach with you at the scoping stage of the project.

For discharges to coastal waters, the soffit of the outfall should be located below the mean low water of spring tides (MLWS). If this is not possible, the location must be based on local factors, such as:

• outfall visibility
• public access
• amenity
• practicality of construction

Bathing waters: water quality standards

There are 2 approaches to improve overflows to protect designated bathing waters:

• spill frequency emission standards
• environmental quality design standards

Bathing waters: spill frequency emission standards

For improvements at designated bathing waters the Environment Agency will set the minimum standard depending on the target status of the waters.

Storm overflows that discharge directly into or impact on bathing waters with a target of good or sufficient status, must have no more than 3 significant spills per bathing season on average. The bathing season is from May to September.

Where more than one discharge affects the bathing water, you must aggregate the spills. The aggregated spills must be no more than 3 significant spills on average per bathing season.

For storm overflows that discharge to bathing waters with a target of excellent status, the emission standard is for no more than 2 significant spills per bathing season on average.

Whether a spill is significant will be considered on a site-specific basis. In general, for design purposes, a spill greater than 50m³ is significant. Guidance on spill definition and how to aggregate spills for frequency and volume.

For discharges into coastal waters, the soffit of the outfall should be located below the mean low water of spring tides (MLWS). If this is not possible, we will adjust the spill frequency standard.

When the soffit is above MLWS but below the mean high water level of spring tides (MHWS), the spill frequency standard is between 3 spills per bathing season and one spill in every 5 bathing seasons. We will base the frequency on local considerations and the actual location of the CSO soffit.

When the soffit is at MHWS, the spill frequency standard is no more than one spill in every 5 bathing seasons.

When the soffit is above MHWS, the spill frequency standard is no more than one spill in every 5 years.

Bathing waters: environmental quality design standards

You can use marine impact modelling to design a solution to meet risk based environmental quality standards. These standards are designed to ensure a low (less than or equal to 5%) risk of non-compliance.

EQSs are set for faecal indicator organisms.

Only use marine impact models to predict compliance for coastal situations where we are confident that the predictions are accurate. You will need extensive investigations, including model calibration from field exercises.

To reduce the risk of failure, you must meet the modelled EQSs for 98.2% of the time.

Do not use it for estuaries or discharges into rivers.

Do not use it for discharges made directly into, or close to the bathing water. Only use this approach where the discharge is made from a long sea outfall at least 200m beyond MLWS.

You may use impact modelling to design solutions allowing more than 3 spills per bathing season. However, do not use marine impact models to justify increasing spills above existing levels.

For complex situations such as estuaries, you can use marine impact modelling to help prioritise overflows for investment to meet the ‘3 spills’ design standard. Do not use it to design a solution to meet the EQS.

EQS for faecal bacteria in coastal bathing waters

Bathing water class (2006 Bathing Water Directive)	E. coli standard (colony forming unit (CFU) per 100ml)	Intestinal enterococci standard (CFU per 100ml)	Standard type	Maximum exceedance threshold (% of bathing season duration)
Sufficient	500	185	90-percentile	1.8
Good	500	200	95-percentile	1.8
Excellent	250	100	95-percentile	1.8

Shellfish waters: water quality standards

There are 2 approaches to improve unsatisfactory overflows that impact on designated shellfish waters. You can use:

• spill frequency emission standards
• environmental quality design standards

Shellfish waters: spill frequency emission standards

Storm overflows affecting a designated shellfish water must have no more than 10 significant spills on average per year. This assumes each significant spill impacts for up to 24 hours. If the impact lasts for more than 24 hours, the standard may be limited to less than 10 per year.

Where more than one discharge affects the shellfish water, you must aggregate the spills.

Whether a spill is significant depends on a number of factors. It will be considered on a site-specific basis. In general, for design purposes a spill greater than 50m³ is significant. Guidance on spill definition and how to aggregate spills for frequency and volume.

Shellfish waters: environmental quality design standards

You can use marine impact modelling to design a solution to meet:

• the water quality standard of 1,500 faecal coliforms/100ml for 97% of the time
• a geomean of 110 E. coli/100ml – to make sure shellfish flesh samples comply with the WFD shellfish flesh standard the Environment Agency may require a geomean design standard of 5 E. coli/100ml

These EQS’s apply to any location within the shellfish water, not just the monitoring point.

These approaches may allow spill frequency solutions greater than the ‘10 spills’ standard, which are likely to be conservative in most cases. However, do not use these approaches to justify increasing spills above existing levels.

Only use marine impact modelling for coastal situations where the Environment Agency is confident the predictions are accurate. You will need extensive investigations, including model calibration from field exercises.

Only use these approaches where the discharge is made from a long sea outfall at least 200m from MLWS.

Do not use these for estuaries or discharges into rivers.

For complex situations such as estuaries, you can use marine impact modelling to help prioritise overflows for investment to meet the ‘10 spills’ design standard. Do not use it to design a solution to meet the EQS.

Counting spills: bathing and shellfish waters

We will agree spills, in terms of significance and counting method, at the scoping stage of the project. This will be on a site-specific basis.

In general, a spill greater than 50m³ is considered significant.

Count spills using the 12/24 counting method:

1. Start counting when the first discharge occurs.
2. Any discharge (or discharges) in the first 12-hour block are counted as one spill.
3. Any discharge (or discharges) in the next, and subsequent 24-hour blocks, are each counted as one additional spill per block.
4. Continue counting until there’s a 24-hour block with no discharge.

For the next discharge after the 24-hour block with no discharge, you begin again with the 12-hour and 24-hour block spill counting sequence.

Aggregate spills

You must aggregate (combine) spills that affect the same bathing or shellfish water. This makes sure the designated water is not impacted more frequently than the average design spill frequency.

It can be difficult to determine aggregations, especially where there are multiple sewerage catchments and designated waters. Different approaches may be needed depending on site-specific considerations. Our fall back position is for full determination of the aggregations across the whole shellfish water catchment area.

You must agree the method you adopt with us at the scoping stage of the project. This will be on a site-specific basis.

Where a number of designated waters are next to each other (contiguous), consider them as one single large designated water.

Where all the storm discharges are within a single sewerage catchment, aggregate all intermittent discharges in terms of spill frequency and volume. This applies regardless of the size of the sewerage catchment.

Where the storm discharges are spread across a number of small sewerage catchments, aggregate spills if the area covered by the intermittents is no more than 100km².

Where the storm discharges are spread across a number of sewerage catchments, and the total area covered by the intermittents is greater than 100km², divide the area into a number of suitable subcatchments. Aggregate the spills within each subcatchment and consider each aggregation separately.

Where more than one designated water is impacted by the same storm discharges, consider them as one designated water for the purposes of determining aggregations.

You may need to use marine impact modelling where the:

• catchment is so large and complex that it’s too difficult to determine aggregations
• standard aggregation method results in solutions that require very low spill frequencies at individual assets to meet the spill frequency standard in aggregation

Meet aesthetic control standards

You must meet aesthetic controls for all new storm overflows and for existing unsatisfactory storm overflows.

The Environment Agency will base the solids separation requirements in your permit on amenity and spill frequency.

For amenity, we take account of uses for a reasonable distance downstream of the discharge.

You must estimate spill frequency using a sewer model constructed and verified in accordance with the CIWEM UDG code of practice for the hydraulic modelling of urban drainage systems. The model must be confirmed as fit for purpose and be available for external audit.

Standards for solids control are:

• 6mm solids separation: separation from the effluent, of a significant quantity of persistent material, and faecal and organic solids, greater than 6mm in any 2 dimensions
• 10mm solids separation: separation from the effluent, of a significant quantity of persistent material, and faecal and organic solids, giving a performance equivalent to that of a 10mm bar screen

These standards are usually achieved through the use of screens.

This guide sets out the minimum solids separation required. You may choose to fit 6mm screens regardless of amenity and spill frequency.

‘Significant quantity’ for solids separation is defined in terms of screen performance criteria set out in Appendix C of the UPM3 manual.

You can use novel or unconventional devices and alternatives to screening. However, they must meet the relevant performance criteria. You can demonstrate this by following the test procedure in Appendix C of the UPM3 manual.

All solid separation controls must meet good engineering design. You can meet good engineering design by following the recommendation in the guide to the design of CSO structures (FR0488) on the Foundation for Water Research website.

You must design new chambers for 6mm and 10mm solids separation following the principles in the WaPUG guide for the design of CSO chambers to incorporate screens on the CIWEM website.

We do not usually ask you to screen overflows predicted to spill less often than 1 in 5 years on average.

High amenity: aesthetic control standards

Discharges are classed as being to high amenity when the receiving waters:

• influence an area where bathing and water contact sport (immersion) is regularly practised, such as windsurfing or canoeing
• pass through formal public parks
• pass through formal picnic sites
• are designated shellfish waters
• are designated bathing waters
• are designated under the Birds and Habitats Directives as Special Protection Areas (SPAs) or Special Areas of Conservation (SACs)
• are designated Marine Conservation Zones

Where discharges to high amenity sites occur:

• more often than once per year, you need 6mm solids control
• less than or equal to once per year, you need 10mm solids control

Moderate amenity: aesthetic control standards

Discharges are classed as being to moderate amenity when the receiving waters:

• are used for boating
• are used for recreation and contact sport (non immersion)
• have a popular footpath next to the watercourse
• pass through a housing development or frequently used town centre area, such as a bridge, pedestrian area or shopping area
• are linked through substantiated reasons for failure in WFD classification as being less than good

Where discharges to moderate amenity sites occur:

• more often than 30 times per year, you need 6mm solids control
• less than or equal to 30 times per year, you need 10mm solids control

Low and none amenity: aesthetic control standards

Discharges are classed as being to low amenity when the receiving waters have:

• basic amenity use only
• casual riverside access on a limited or infrequent basis, such as a road bridge in a rural area or footpath next to a watercourse

Discharges are classed as being to none amenity when the receiving waters are:

• seldom or never used for recreation
• in a remote or inaccessible area

Discharges to low and none amenity sites must achieve solids separation through good engineering design.

Screen design

For moderate and high amenity watercourses, screen spill flows up to and including the 1 in 5 year return period storm of duration are critical to screen performance. You must provide a bypass weir to prevent flooding due to flows greater than the 5-year screen design flow, or if the screen becomes fully blinded. Screens and chambers must not increase flood risk. The 5-year return period design should take into account forecast development and make a suitable allowance for partial screen blinding during spills (for example 50%).

Screen designs must meet the no deterioration objective.

Screens must not increase spill frequency or volume, for example due to reducing an existing weir level to incorporate the screen. Replacement screens must achieve at least the same performance in solids separation.

Where you use static screens, you should provide additional screen area, for example 50%, to account for blinding. This should take into account how often you plan to clean the screen and the solids content of the storm sewage. The solids content will be higher where the catchment is partially combined, or where the upstream sewers are laid at slack gradients and solids are mobilised during wet weather. You must confirm loading rates quoted by manufacturers using realistic field trials for prolonged and repeated use.

Where spill volumes are high, you should use automatically cleaned screens to reduce the risk of blinding during a spill.

You should only use sac screens at sites that spill infrequently, and where spill volumes are very low.

Only retrofit screens to existing chambers where you are confident the solution will work. Provide evidence to support the design with your permit application. You may need to carry out monitoring to confirm retrofitted screens are not bypassed more than once every 5 years on average.

Monitoring and telemetry

Mechanically cleaned screens must have telemetry to notify you if the screen cleaning mechanism fails (unless otherwise agreed with us in writing). Return the screen cleaning mechanism to normal operation as soon as reasonably practicable after receiving the failure notification.

You may need to monitor screen operation, performance and bypass. There will be details about this in your permit.

You may need to provide telemetry about screen operation or bypass, for example, when you:

• carry out post scheme appraisal monitoring to confirm the screen is not bypassed more than once every 5 years on average
• have a sensitive receiving water
• are uncertain about the capacity of a proposed screen
• are uncertain about model predictions
• are uncertain about the performance of a retrofitted screen due to screen or chamber design
• have previous poor performance
• trial new screening or alternatives
• need to be notified for clean up

Maintenance

You should plan and adjust screen maintenance and cleaning based on experience to make sure the screen retains its required capacity. You can base the frequency of routine maintenance visits on the expected spill frequency. Alternatively, use telemetry to inform you that a spill or screen bypass has occurred.

You should record the degree of screen blinding before cleaning and compare to design expectations.

If you have repeated excess blinding, you should review your cleaning programme and check the aesthetic impact on the environment. Where the design proves inadequate, you may need to redesign the screen and chamber.

Meet the no deterioration objective

When the Environment Agency reviews permits for existing discharges, or issues permits for new ones, we apply our no deterioration objective.

Our no deterioration objective for storm overflows is to avoid any increase in pollution discharged to the water body. You must demonstrate that your proposed scheme achieves no deterioration. You can do this by showing no increase in the frequency and volume of storm discharges where the discharge location remains unchanged.

You must demonstrate no deterioration for individual and aggregated discharges.

We may need you to show no deterioration in the wet weather quality of the receiving water. In this case, you must demonstrate no deterioration at any percentile (including percentiles higher than 99) and no increase in the number of times FIS are exceeded.

Relocating or rationalising storm discharges because of a water quality improvement, flooding or growth scheme could cause localised deteriorations outweighed by improvements elsewhere in the same catchment. Contact us to see if this is likely to be acceptable. It’s not acceptable to improve one catchment by causing deterioration or a breach of standards in another catchment.

Event duration monitoring (EDM)

The Environment Agency sets monitoring requirements using a risk-based approach.

Monitoring and reporting requirements depend on the significance of the discharge. The significance of the discharge is based on spill frequency and amenity.

Storm overflows with less than one spill per year are classed as ‘D – low significance’. They do not require any monitoring.

High amenity: monitoring and reporting

Discharges are classed as being to high amenity when the receiving waters:

• include areas where bathing and water contact sport (immersion) is regularly practised, for example windsurfing and canoeing
• are designated shellfish waters
• are designated bathing waters

Discharges to high amenity waters with one or more spills per year are classed as ‘A – high significance’.

For high significance discharges you must:

• carry out EDM with telemetry in real, or near real, time
• carry out spill monitoring at 2 minute intervals
• submit an annual or bathing water season report summarising the total number and duration of spills
• provide a detailed report of start and end times for individual spill events if we ask for it

You may also provide:

• volume measurement – the Environment Agency MCERTS monitoring certification scheme is not required for this
• live warnings

These are optional. We will agree these with you.

Moderate amenity: monitoring and reporting

Discharges are classed as being to moderate amenity when the receiving waters:

• pass through formal public parks
• pass through a formal picnic site
• are designated as SPAs or SACs under the Birds and Habitats Directives
• are designated Marine Conservation Zones
• are used for boating
• have a popular footpath next to the watercourse
• are used for recreation and contact sport (non immersion)
• pass through a housing development or frequently used town centre area, for example a bridge, pedestrian area or shopping area
• are linked through substantiated reasons for failure in WFD classification as being less than good

Discharges to moderate amenity waters with one or more spills per year are classed as ‘B – medium significance’.

For ‘B – medium significance’ discharges you must:

• carry out EDM with spill monitoring at 15 minute intervals – telemetry in real, or near real, time monitoring is optional
• submit an annual spill report summarising the total number and duration of spills
• provide a detailed report of start and end times for individual spill events if we ask for it

Low and none amenity: monitoring and reporting

Discharges are classed as being to low amenity when the receiving waters have:

• basic amenity use only
• casual riverside access on a limited or infrequent basis, such as a road bridge in a rural area or footpath next to a watercourse

Discharges are classed as being to none amenity when the receiving waters are:

• seldom or never used for amenity purposes
• in remote or inaccessible areas

Discharges to low or none amenity waters with 20 or more spills per year are classed as ‘C – medium significance’.

For ‘C – medium significance’ discharges you must:

• carry out EDM with spill monitoring at 15 minute intervals
• submit an annual spill report summarising the total number and duration of spills
• provide a detailed report of start and end times for individual spill events if we ask for it

Discharges to low or no amenity waters with less than 20 spills per year are classed as ‘D – low significance’.

For ‘D – low significance’ discharges no monitoring is required.

We expect EDM reliability testing and actions to be part of your management system. This should be to a standard such as set out in the EDM good practice guide on the CIWEM website.

Reporting requirements

For reporting, use the 12/24 spill counting method. Keep spill records for at least 6 years from the date you made the records.

When you renew your monitors you must apply the risk-based approach. This is subject to any existing permit requirements.

We may also ask you to monitor sites not identified for monitoring under the EDM drivers of the 2014 and 2019 price reviews (PR14 and PR19), as a condition of your permit. Where we require this, we’ll explain why. For example, we may require monitoring at previously unknown or unpermitted sites or at overflows that have caused repeated pollution incidents.

Your annual report of overflow operation must cover the 12-month period from 1 January to 31 December. You must submit your annual report to us by the end of February each year.

Bathing water reports must cover the 5-month period from 1 May to 30 September. You must submit your bathing season report to us by 31 October each year.

If we request a more detailed report you must submit the report to us within 28 days, unless we tell you otherwise in writing.

Flow monitoring

Most permits for storm overflows include pass forward flow (PFF) conditions. This is to make sure a minimum flow is passed forward to treatment while the overflow discharges to the environment.

Where the overflow setting is fixed and cannot be adjusted, carry out occasional spot checks to confirm the overflow is complying with the permit.

Where the PFF is adjustable, for example controlled by actuated penstocks or pumps, we may ask you to continuously monitor the PFF and storm separating device.

We may require PFF monitoring where the performance of the overflow is uncertain, or we think it’s not complying with the permit.

Solutions for unsatisfactory overflows

You must design, construct and maintain solution schemes, and the drainage system as a whole, according to BTKNEEC.

Sewer flooding and pollution from storm overflows are expected to increase as a result of:

• population growth
• climate change
• urban creep

Your solution schemes must account for these factors.

You can assess the impact of urban creep in various ways. Find approaches to predict the impact of urban creep on sewerage systems. This report is published on the UKWIR website.

Find approaches to account for climate change in the CIWEM urban drainage group’s Rainfall Modelling Guide 2015.

Sustainable drainage systems (SUDS) reduce the pressure on the sewerage network and can support wider environmental objectives such as local amenity and biodiversity. Your solution schemes should consider the potential for surface water separation and SUDS to meet the scheme’s objectives. Where feasible, you should retrofit SUDS solutions to address flood risk problems and CSO drivers in existing catchments.

Permit applications: supporting information

To apply for a bespoke permit you need to complete application form parts A, B2, B6 and F1.

Your application for an environmental permit for a storm overflow must be supported by the documented results of the work needed to justify the permit. The supporting documents depend on the type of application.

Follow the UPM partnership approach so that the Environment Agency will have already approved your modelling and investigation work when you submit your permit application. We may need further documented evidence of the UPM modelling and engineering details of the agreed solution.

Examples of the information we may ask for include:

• model build and verification reports
• model audit reports
• verified and option models and associated files
• options or study reports
• manhole and ancillary survey data, including photographs
• sewer record plans
• CCTV survey data
• impermeable area survey data
• flow survey data, monitor location plans and flow survey report
• event sampling data such as sewer and river quality
• ancillary calculations
• design drawings
• screen design details

Permits for existing unpermitted storm overflows

Historic unpermitted overflows refer to ‘legacy’ overflows that were unknown at the time of privatisation.

We’ll permit overflows ‘as is’ where you can provide evidence that they are satisfactory. We may be able to help provide your evidence and agree the overflow’s status.

Where there is evidence that the overflow is currently unsatisfactory, you will need to improve it to meet water quality, aesthetic and design standards. We’ll include an improvement condition in your permit to reflect the required standards and timing. You’ll need to make improvements as soon as reasonably practicable – normally within 3 years.

We may refuse a permit or require spills to be reduced where the overflow is substandard. Examples include where the overflow:

• is not properly constructed, such as a broken open pipe
• appears to have been constructed recently
• is in a separately drained area
• operates more frequently than designed, for example due to upstream growth

Permits for improvement or alteration of existing permitted storm overflows

Proposals to improve unsatisfactory overflows, or modify existing ones, should be agreed with us following the partnership approach to UPM.

Improvement schemes must:

• meet the relevant water quality standards
• not increase flood risk

Alterations to existing overflows must meet our no deterioration requirements.

Permits for new storm overflows as part of rationalisation or overall improvement scheme

New storm overflows that are part of a rationalisation or overall improvement scheme must demonstrate no deterioration in wet weather impacts. They must also meet water quality, aesthetic and design standards. Some localised deteriorations may be allowed if they are outweighed by improvements elsewhere in the same catchment.

Discuss your proposals with us at an early stage. The UPM procedure should show the net overall improvement and any localised deterioration.

Permits to limit flows to treatment

In exceptional circumstances, we may consider proposals to create new or increased overflows at sewage inlet works to improve final effluent quality by limiting flow to treatment as part of an overall improvement scheme.

You need to demonstrate no deterioration in the wet weather quality of the receiving water. To achieve no deterioration, the impact of the storm overflow will need to be outweighed by the improvement in wet weather quality of the fully treated effluent.

You must also meet water quality, aesthetic and design standards.

Permits for new storm overflows to stop flooding

We normally refuse permits for new overflows to stop sewers flooding.

Your long-term drainage strategies should manage the flood risks posed by population growth, urban creep and climate change. New or increased discharges from storm overflows are not an acceptable solution to inadequate planning, design or maintenance of the sewer network.

We expect the need for new storm overflows to be kept to an absolute minimum. Before you make an application for a new storm overflow to stop flooding, you must complete the following steps:

1. Investigate the catchment to identify and quantify the causes of flooding, for example connected impermeable area, infiltration, land drainage connections, misconnections, system throttles.
2. Construct a verified hydraulic model that reproduces the observed flooding problem.
3. Develop a costed plan for removing excessive infiltration or inflows that you are directly responsible for.
4. Develop a costed plan to make sure the public sewers have design capacity to convey dry and wet weather flows according to BTKNEEC. Include any scope for surface water separation to alleviate pressure on the foul or combined system with source control via SUDS principles.
5. Develop a costed plan to resolve inflows that you are not directly responsible for. Produce an action plan to implement this work together with relevant interested groups such as local authorities, householders and landowners.
6. Use the verified hydraulic model to predict any remaining flood risk following the BTKNEEC improvements to the collection system. Where the residual flood risk remains unacceptable, a new storm overflow may be considered together with the BTKNEEC improvements.
7. Appraise options to resolve the residual flood risk including alternatives to a new CSO.
8. Where a CSO is identified as the best option, provide evidence that pollution from the new overflow will be limited according to BTKNEEC by following UPM procedure. You must make sure you meet all relevant water quality and aesthetic standards and minimise any deterioration in quality.
9. Provide evidence that the new overflow will not significantly increase flood risk in the receiving water.

Permits for existing emergency overflows to operate during storms

We normally refuse permits for existing emergency overflows to operate during storms.

You must complete the following investigation and improvement work to support an application to allow emergency overflows to operate during storms:

1. Provide details of the catchment’s sewerage history and development. Include the permitting history of the pumping station and the history of storm discharges.
2. Investigate the catchment to identify and quantify the causes of the storm discharges. For example connected impermeable area, infiltration, land drainage connections and shortfalls in pumping capacity.
3. Construct a verified hydraulic model that predicts the observed storm discharges.
4. Assess whether the sewerage system, including the pumping station, has been designed, constructed and maintained according to BTKNEEC. Develop a costed plan to deal with any shortfalls contributing to storm discharges for which you are responsible.
5. Develop a costed plan to resolve inflows for which you are not directly responsible. Produce an action plan to implement this work together with relevant interested groups such as local authorities, householders and landowners.
6. Use the verified hydraulic model to predict the wet weather performance of the pumping station following the BTKNEEC improvements. Where storm discharges are predicted to continue, we may consider permitting a new storm overflow together with the BTKNEEC improvements.
7. Appraise options to resolve the need for residual storm discharges following the BTKNEEC improvements, including alternatives to a new storm overflow.
8. Provide evidence that pollution from the new overflow will be limited according to BTKNEEC by following UPM procedure. You must meet all relevant water quality and aesthetic standards and minimise any deterioration in quality.
9. Provide evidence that the new overflow will not significantly impact flood risk in the receiving water.

These steps help us to understand the problems that have led to the emergency overflow operating in storms. Where the problems are due to a failure to maintain the system, following the procedure will solve the problem.

Permits to relieve surcharge due to groundwater infiltration

We’ll normally refuse permits for overflows to relieve surcharge due to groundwater infiltration. You must prevent leaks in accordance with BTKNEEC.

Under certain circumstances we may allow you to make planned discharges to protect people and property. In these situations we will not take enforcement action where you comply with the requirements of our regulatory position statement (RPS). Contact us to request our RPS for discharges made from groundwater surcharged sewers.

Permits for new emergency overflows

The Environment Agency will normally permit emergency overflows at sewage pumping stations on the network up to and including the WWTW inlet. The permit will include conditions to minimise the risk of failures. We’ll only permit where the reduced risk of a discharge outweighs the risk of legitimising a potentially polluting discharge.

New developments should be drained by separate foul and surface water systems constructed to standards set out in Sewers for Adoption 7th edition. The foul flow is usually small and predictable enough to be taken away by tanker if there is a plant or power failure. The developer must provide sufficient storage and telemetry warning systems to allow time for a tanker to attend in an emergency. The design for these pumping stations, including minimum storage requirements, is described in Sewers for Adoption 7th edition. We only permit emergency overflows from these pumping stations in exceptional circumstances.

We do not permit emergency overflows between process stages on WWTW (interstage emergency overflows). You must identify and minimise pollution risks associated with the operation of the works via your management system.

Types of emergency covered by the permit are:

• electrical power failure
• mechanical breakdown of duty and standby pumps
• rising main failure
• blockage of the downstream sewer

You must demonstrate that the emergency was beyond your control or action, and not due to lack of routine maintenance.

You must take all reasonable steps to mitigate the impact and return the pumping station to normal operation.

Emergency overflow: key protection measures

When we permit a new or altered emergency overflow we set key protection measures to avoid the likelihood of an emergency discharge due to power or plant failure. We’ll include these in your permit as requirements under the operating techniques condition and table S1.2.

We will send you a key protection measures form to complete as part of your permit application.

Minimum requirements: standby pumps

You must have at least one standby pump. This must be equivalent to the duty pump, and must lift the DWF as a minimum.

The standby pump must run automatically in the event of the duty pump failing for reasons other than power failure.

Minimum requirements: standby power

You must have a connection for a mobile generator.

Where storage is below our normal minimum requirements, you may also need permanent standby generation or a duplicate power supply.

Minimum requirements: storage

The storage must be above the level of the high-level alarm and below the level of the overflow.

Storage must be at least 6 hours at DWF or 2 hours at 3DWF where:

DWF = PG + I + E

3DWF = 3PG + I + 3E

DWF = total dry weather flow (l/d)
P = catchment population (number)
G = per capita domestic flow (l/head/d)
I = infiltration (l/d)
E = trade effluent flow (l/d)

We may relax the storage requirements to 1 hour at 3DWF if it’s impractical or too expensive to provide and the environmental consequences of a discharge are low. Where the environmental consequences are high, we may relax the storage requirement if you provide redundancy measures such as:

• a duplicate power supply or permanent standby generator
• additional standby pumps
• a duplicate rising main
• raised alarm response levels

Emergency overflows discharging to shellfish waters always require at least 2 hours at 3DWF.

Where pumping stations exist in series, we may agree to reduced storage where you have controls in place to switch off upstream pumping stations.

We’ll include all redundancy measures used to relax the normal minimum storage requirements in your permit as key protection measures.

Minimum requirements: telemetry warning systems

You must provide telemetry to warn of:

• power failure
• pump failure
• high levels in the wet well
• discharges from the emergency overflow

Minimum requirements: tanker access

You must provide enough access where an emergency dry weather discharge could be realistically reduced by using tankers to empty the wet well.

Minimum requirements: over pumping

We may require a facility to allow a mobile pump to be connected in the event of duty and standby pump failures.

Minimum requirements: screening

Where emergency overflows are also permitted as storm overflows, we’ll normally require screening in line with our amenity and spill frequency guidance. We may also require screens on emergency only overflows.

You must provide a bypass to allow relief if the screen becomes fully blinded during an emergency discharge. The bypass level must not increase flood risk.

Environmental consequence

Environmental consequences depend on how sensitive the receiving water is and the potential impact of a discharge based on dilution. Use the environmental consequence matrix table to estimate the environmental consequence of a discharge.

The sensitivity rating of the receiving waters is high for:

• bathing waters
• shellfish waters
• still or slow moving water bodies
• high amenity, for example public parks, immersion sports
• sustainable fisheries
• commercial fishery abstraction
• high strength trade discharge or dangerous substances 10 × EQS in sewage
• downstream of Sites of Special Scientific Interest, SACs, SPAs, Ramsar sites or Marine Conservation Zones – within 3km

The sensitivity rating of the receiving waters is moderate for:

• moderate amenity, for example public footpaths
• downstream abstractions
• watercourses at the back of properties

The potential impact of an emergency overflow depends on the dilution. The impact is:

• low when the watercourse 5-percentile flow : sewer dry weather flow is greater than 8:1
• medium when the watercourse mean flow : sewer dry weather flow is greater than 8:1
• high when the watercourse mean flow : sewer dry weather flow is less than 8:1

Potential impact	High sensitivity	Moderate sensitivity	Low sensitivity	Matrix score	Environmental consequence
High	9	6	3	≥3	High
Medium	6	4	2	2	Medium
Low	3	2	1	1	Low

Management system

You must have a written management system that identifies and minimises the risks of pollution. Your management system must set out how you:

• will maintain your site
• plan to respond in the event of a failure

Find out how to develop and maintain a management system.

Keep records

You must keep records of all routine and non-routine cleaning and maintenance of the pumping station in accordance with your management system. Give staff access to these records to help manage the pumping station efficiently.

You must keep records of all telemetry alarms, breakdowns, failures, blockages and emergency discharges, and what you did to resolve those problems. You must keep these records for at least 6 years.

You must make records available to the Environment Agency on request.

Reporting

Where routine spill event duration monitoring is required, you should provide this using an agreed format. In all cases, you must provide the Environment Agency with a written report on the operation of the emergency overflow, which should provide all available information requested.

Where the emergency overflow affects designated shellfish waters, you must produce an annual report on the operation of the overflow. You must send the report to us and the local food authority or environmental health authority. The report must cover the 12-month period 1 January to 31 December. You must submit the report by the end of the following February.

If we ask for it, you must provide us with written details of individual spill events.

Notify failures

You must notify the Environment Agency, as soon as reasonably practicable, if you detect a pumping station failure that is likely to cause significant pollution.

Where the overflow discharges to designated shellfish waters, you must notify us of any emergency discharge of sewage. You must also notify the local food authority or environmental health authority. You must notify them as soon as reasonably practicable, and no later than 24 hours after the event.

You must give the reasons for the emergency discharge and the actions you took.