Guidance

Surface water pollution risk assessment for your environmental permit

How to carry out a risk assessment if you are applying for a bespoke permit that includes discharging hazardous chemicals and elements to surface water.

From:
Environment Agency and Department for Environment, Food & Rural Affairs
Published
1 February 2016
Last updated
20 February 2026 — See all updates

Applies to England

You must carry out this risk assessment if you are applying for, or changing, a bespoke permit that includes discharging hazardous chemicals and elements to surface water from a water discharge activity. This discharge may be standalone or carried out as part of an installation or a waste operation.

Discharges (emissions) to surface waters include discharges to:

  • inland freshwaters, such as rivers, reservoirs, canals and lakes
  • estuarine waters
  • coastal waters
  • inland, estuarine or coastal surface waters from the foul sewer (indirect discharges)

Changes to your permit are often referred to as ‘permit variations’.

This assessment is sometimes referred to as a ‘specific substances assessment’.

When you need a surface water risk assessment

You will need a surface water risk assessment if you:

  • are applying for a new permit that includes discharging hazardous chemicals or elements to surface water
  • need to change (vary) your permit to cover an increase in the amount of hazardous chemicals and elements you plan to discharge
  • plan to discharge a new hazardous chemical or element 
  • are applying to change your permit for another reason and there are potentially hazardous chemicals and elements in your discharge, and they have not been assessed in the last 4 years 
  • take water from groundwater and discharge it to surface water 
  • use water in a process which concentrates hazardous chemicals or elements before they are discharged (for example, cooling water that partially evaporates)

You do not usually need to carry out this risk assessment if you: 

  • return water to the same body of water it was taken from, and the water has not been contaminated, evaporated significantly or had any hazardous chemicals or elements added (the quality of the receiving water body at the point of discharge must be comparable to the quality of the water from where and when it was taken)
  • have not added any hazardous chemicals or elements to the water and have not carried out a process which may add hazardous chemicals or elements to the water
  • discharge clean and uncontaminated water, such as rainwater from roofs, or surface water from uncontaminated areas of sites

When you need an environmental permit

For more information on when you need a permit, check: 

Overview of the risk assessment

This risk assessment has 4 parts: 

  1. Identifying the substances released from your site.

  2. Gathering and preparing data on your discharge.

  3. Carrying out screening tests on the data.

  4. Completing more detailed modelling (if needed following the screening steps).

Roles and responsibilities in screening and modelling for the risk assessment

Screening

For screening, you must:

  • carry out the screening tests for all discharges of hazardous chemicals and elements to inland freshwaters, estuarine and coastal waters
  • submit the results and raw data to the Environment Agency

Environment Agency staff will check your screening assessments.

Modelling

Screening may show that modelling is needed.

The Environment Agency will complete modelling for most discharges to inland freshwaters. 

You must complete modelling for discharges of hazardous chemicals and elements to:

  • inland freshwater lakes, canals and reservoirs
  • estuarine and coastal waters

You may need to employ a consultant to complete the modelling. Contact the Environment Agency if you need more advice on carrying out modelling.

Understanding Environmental Quality Standards (EQSs)

EQSs for surface waters are regulatory thresholds which intend to protect surface water ecosystems. We need to assess and regulate discharges using these thresholds to make sure the surface water environment is protected.

The screening and modelling tests assess hazardous chemicals and elements (substances) against EQSs. This helps determine if the substances pose a risk to the surface water environment.

There are several types of EQS:

  • maximum allowable concentration EQSs (MAC EQSs) – used to evaluate the short-term environmental impacts that the discharge could cause
  • annual average EQSs (AA EQSs) – used to evaluate the long-term environmental impacts that the discharge could cause
  • freshwater bioavailable EQSs – zinc, copper, manganese, nickel and lead have this type of EQS, which takes into account how much of the metal is in a form which could harm organisms

Not all substances have all types of EQS. Different types of statistics (for example, mean or maximum values) are used depending on which type of EQS is being assessed.

You may also need to risk assess substances which do not have EQSs. This includes substances which have ecotoxic properties and are released in concentrations high enough to risk harming the surface water environment. You should assess these substances using suitably derived ‘predicted no effect concentrations’ (PNECs).

PNECs are concentrations of a chemical below which no harmful effects are expected in the environment. Contact the Environment Agency for more information on using PNECs. 

Additional regulatory requirements for installations and waste operations 

For discharges from an installation or waste operation, Best Available Techniques –Associated Emission Levels (BAT-AELs) or equivalent emission limit values (ELVs), or both, apply in addition to EQSs.

These conditions and limits are set out in:

You must comply with the limits and conditions in your permit. If multiple limits apply, we will set the limits most protective of the surface water environment.

Check the guidance on using BAT in environmental permits.

Managing persistent organic pollutants (POPs)

POPs are substances that remain in the environment for many years and are a danger to human health and the environment. POPs are often associated with:

  • pesticides
  • industrial processes
  • chemicals manufacturing
  • some types of wastes (such as upholstered domestic seating, waste electricals and electronic equipment)

You must destroy or irreversibly transform POPs (where present above thresholds) to prevent them being transferred to surface waters or sewage sludge from your site.

Check the guidance on managing waste containing POPs.

Climate change resilience

Changes in environmental factors could affect the volume and quality of the discharges from your site, for example:

  • changes in rainfall
  • sea level change
  • variation in river flow
  • changes in temperature 

To improve resilience to future climate impacts, you may wish to account for climate impacts and extreme weather when planning, installing or improving treatment for your discharge. 

Check the guidance on climate change: risk assessment and adaptation planning in your management system.

Part 1: Identifying substances released from your site

Produce a list of substances to assess

You must declare and assess all the hazardous chemicals and elements which are likely to be in your discharge to surface waters. The term ‘hazardous chemicals and elements’ includes:

  • priority hazardous substances (PHSs)
  • priority substances (PSs)
  • specific pollutants (SPs)
  • substances with operational EQSs

These substances have been defined in UK legislation and have EQSs. They are sometimes referred to as ‘specific substances’.

Use the following links to download lists of substances with EQSs:

EQSs for inland freshwaters (ODS, 11.9 KB)

EQSs for estuarine and coastal waters (ODS, 11.6 KB)

Installations and waste operations are usually required to make an ‘inventory of emissions’ which may include additional substances to those listed in the EQS tables. For example, this could include substances set out in:

The term ‘hazardous chemicals and elements’ also includes substances which have ecotoxic properties and are released in concentrations high enough to risk harming the surface water environment. If these substances are shown to be present in the discharge they must also be assessed using this guidance, using PNEC thresholds in place of EQSs.

Contact the Environment Agency for more information on using PNECs.

When substances are likely to be in your discharge

Hazardous chemicals and elements are likely to be in the discharge if they:

  • have been measured in the discharge (detected using chemical analysis)
  • are discharged into the sewer network upstream of a sewage treatment works (for example, trade effluents)
  • are dosed into the effluent (follow the dosing guidance if this applies to your site)
  • are produced by, or known contaminants of, the processes on site and able to enter aqueous waste streams (for example, they are produced as waste products or concentrated in the effluent through evaporation or other losses)

Arranging chemical analysis

You should use chemical analysis to check which substances are in your discharge. You need to:

  • take samples of the discharge
  • send them to a United Kingdom Accreditation Service (UKAS) accredited laboratory

Make sure the laboratory:

  • tests for all substances which you expect to find in the discharge
  • uses an appropriate ‘minimum reporting value’ (MRV), often 10% of the EQS

MRVs are the lowest concentration of a substance that a laboratory can reliably detect when doing chemical analysis. Contact the Environment Agency for information on acceptable MRVs for each substance.

You must submit your chemical analysis with your application.

If you submit data which have not been analysed to an appropriate MRV, you must tell the Environment Agency why. For example:

  • samples may be from different sources (for example clean water, polluted water, sewer discharge or industrial discharge) and may need to be diluted before they can be analysed
  • the discharge may be diluted and analysis against the MRV may not be possible

Part 2: Gathering and preparing data on your discharge

Data you need for your discharge

For the discharge you will need:

For each substance you will need:

  • the chemical name and Chemical Abstracts Service (CAS) registry number – unique chemical identification number
  • the unit of measurement used (you might need to convert these for the application form)
  • the maximum, minimum and average recorded concentrations of the substance in the discharge
  • total and dissolved metal data for any metals in the discharge (total metal data is needed for screening of most metals, dissolved metal data is needed for screening of metals with bioavailable EQSs – zinc, copper, manganese, nickel and lead)
  • at least 12 representative samples of the substances in the discharge – these must be spread out across the time the discharge is flowing at different times of day and seasons of the year (as appropriate) – the ideal number of samples is 36
  • the MRV used for each substance and a justification where this is not 10% of the EQS
  • the relevant EQSs

For discharges to foul sewer, you need to calculate or apply sewage treatment reduction factors (STRFs) for each hazardous chemical or element in the discharge (see Calculating concentrations when discharging to a foul sewer).

Additional data for modelling

If you need to model any discharges of substances with freshwater bioavailable EQSs, you will also need to know:

  • the dissolved organic carbon (DOC) content in the receiving water
  • pH levels in the receiving water
  • calcium levels in the receiving water

The data you need for modelling of metals depends on the type of EQS the metal has:

  • for metals with bioavailable EQSs – you will need dissolved metal data
  • for metals with EQSs that represent the dissolved form – you will need both total and dissolved metal data

Consider collecting both dissolved and total metal data when you are collecting your data for screening in case any metals need modelling. If you cannot get dissolved data you can use total metal data but this may affect the accuracy of the modelling assessment.

Data you need for the receiving water

For inland freshwaters and riverine estuaries or low water channels (where the water is predominantly fresh), you need:

  • the average background concentration (BC) of each substance in the receiving water upstream of the discharge (if available) – if not available, you can use estimated data (see section on ‘Using estimates when you cannot collect data’)
  • the 95% exceedance river flow (the flow rate which is equalled or exceeded 95% of the time – also known as the Q95)

Riverine estuaries are the upper reaches of an estuary where the water is predominantly fresh and flow forms a straight line (rectilinear) which may reverse with the tide.

Low water channels are the route taken by a river at low water where the estuary bed is exposed on either side and flow continues even at low water.

For other estuaries and coastal waters, you need the following background data:

  • for new substances that you have not discharged before – a water sample from the planned discharge point, analysed for all the substances that may be in the discharge
  • for existing substances – a water sample taken from a point away from the discharge point (where it is not affected by the discharge plume)
  • for estuaries where the background quality can vary depending on the tide – use the maximum BC of the substance rather than the average value

Using estimates when you cannot collect data

If you do not have discharge monitoring data (for example for new discharges that are not yet discharging), use estimated data for the screening tests.

You could also use manufacturer specification data or data from a proxy site (a similar sized site with a process which is likely to have a similar discharge). For AA EQSs you need average discharge concentrations. For MAC EQSs you need maximum discharge concentrations taken from a minimum of 12 individual sample results (ideally 36) from on-site tests at the proxy site.

When you calculate your average results, you must round up any ‘less than’ figures to the nearest whole number (for example, assume ‘less than’ 10 is 10).

If you do not have background data for inland freshwaters, estuaries and coastal waters, assume the concentration of each substance is 50% of the EQS.

Calculating flow rates for intermittent discharges

If you discharge substances intermittently, you will need to work out the average flow rate to use in the screening tests. Examples of intermittent discharges include those which operate seasonally or vary throughout the day (for example, 12 hours a day, 5 days a week).

If your discharge starts and stops frequently (for example, once a day):

  1. Work out the average flow rate during discharge.

  2. Multiply it by the proportion of the year that the discharge occurs.

For example, if you discharge at 100 litres per second for 12 hours every day of the year, the average flow rate would be 100 × 0.5 = 50 litres per second.

If your discharge runs for only part of the year (for example, continuously for 6 months and then stops for 6 months) use the average flow rate during the 6 months you operate.

For example, if you discharge at 100 litres per second continuously for 6 months and then stop discharging for the other 6 months, the average flow rate would be 100 litres per second.

Check with the Environment Agency if your discharge pattern is different to these examples, for example rainfall-dependent discharges from a site.

Calculating concentrations when discharging to a foul sewer

This section applies to installations and waste operations that discharge to a foul sewer (as indirect emissions).

Discharges to foul sewer will go to the sewage treatment works before being discharged to surface water. Processes at the sewage treatment works may remove a proportion of the hazardous chemicals and elements in the discharge. To account for this, you should use STRFs when calculating the concentration of each substance you will discharge to surface water via the foul sewer.

STRFs have already been calculated for several substances: 

Sewage treatment reduction factors (ODS, 15.7 KB)

To work out the concentration after sewage treatment, multiply the release concentration (RC) of the substance in your discharge released to the foul sewer by the sewage treatment reduction factor:

  • when using AA EQSs – use the average RCs
  • when using MAC EQSs –use the maximum RC
  • if you have any ‘less than’ figures when calculating the average concentration, round them up to the nearest whole number (for example, less than 10 is assumed to be 10)

If you cannot find a STRF for a substance, assume none of the substance is removed when assessing the discharge.

If your discharge contains POPs above thresholds, or the discharge is from a POPs waste treatment operation, you should not discharge to the foul sewer. You must destroy or irreversibly transform the POPs to prevent transfer to surface waters or sewage sludge. STRFs should not be considered as treatment of POPs-contaminated discharges.

If your discharge to foul sewer is via offsite wastewater treatment, you must consider whether this treatment guarantees protection of the environment as a whole. For example, transferring substances from effluent to sewage sludge which is spread to land is not considered adequate treatment or abatement and onsite pre-treatment may be necessary.

Part 3: Screening tests

Before you start screening

Identify whether you need to complete the screening tests for:

  • discharges to inland freshwaters – rivers, canals, lakes and reservoirs
  • discharges to estuarine and coastal waters
  • discharges of cooling waters (to inland freshwaters or estuarine and coastal waters)

You need to follow the screening tests relevant to your discharge environment.

Use raw data for the screening tests.

You can complete screening manually or by using the automated H1 screening tool. Contact the Environment Agency for access to the H1 screening tool.

You must complete each screening test for all the substances that are in or likely to be in the discharge. If you have 2 or more discharges of the same substance from different parts of your site, these need to be tested separately.

The screening tests are progressive – if a substance is screened out, you do not need to carry out the remaining tests (unless it is a PHS). PHSs need additional screening tests even if screened out by earlier tests.

Installations and waste operations may also need to use BAT-AELs and ELVs in the screening assessment (as appropriate).

Substances that do not screen out will need modelling.

Screening tests for discharges to inland freshwaters

Use the EQSs for inland freshwaters to complete these screening tests as follows:

  • where a substance has an AA and a MAC EQS, focus the initial screening process on the AA standard, but also carry out test 4 against the MAC EQS
  • any assessment against the MAC EQS should take the MAC standard at face value (even if the standard is a 95 percentile, assume that it is an absolute value)
  • if a substance is released in batches rather than continuously, or if there is a large variation in the concentration of the substance released over time, use both the MAC and the AA EQSs for all screening tests
  • if a substance only has a MAC EQS, use this standard for the screening tests
  • bioavailable EQSs should be used for screening metals where available (for zinc, copper, manganese, nickel and lead)
  • the measured concentration for substances should have the same unit of measurement as the EQS (you may need to convert units)

A substance can be screened out and no further assessment is needed, if at least one of the following criteria are met:

  • test 1 is passed
  • test 2 is passed
  • both test 3 and test 4 are passed

PHSs must also pass the significant load test.

Discharges to designated protected areas – inland freshwaters

Modelling may be required for any substances which exceed the EQS if your discharge is direct to a designated protected area (for example a Site of Special Scientific Interest – SSSI).

Test 1: Initial concentration check

This test checks how the concentration of the substance in the discharge compares with the EQS (whether it is more than 10% of the EQS).

To complete the test, you need to compare the substance concentration in your discharge to 10% of the EQS.

If the concentration is:

  • less than 10% of EQS – stop here, no further tests needed (unless the substance is a PHS)
  • greater than 10% of EQS – go to test 2

Test 2: Compare the process contribution (PC) with the EQS

This test checks whether your discharge exceeds acceptable levels after accounting for dilution in the receiving watercourse.

To complete this test, you will need to collect:

The PC is the concentration of a discharged substance in the receiving water after dilution. To calculate the PC, complete these formulas:

  1. [Effluent flow rate (EFR, m³/s)] x [RC (µg/l) of the substance in the effluent]

  2. [EFR, m³/s] + [the river flow rate (RFR, m³/s)]

  3. [Result of step 1] ÷ [result of step 2] = PC (µg/l).

For AA EQS use the average EFR and RC. For MAC EQS use the maximum EFR and RC.

RFR represents the Q95 RFR (equalled or exceeding 95% of the time and represents low flow conditions).

If the PC value is:

  • equal to or less than 4% of the EQS – stop here, no further tests needed (unless the substance is a PHS)
  • greater than 4% of the EQS – go to tests 3 and 4

Test 3: Check how the difference between background water quality and the predicted environmental concentration (PEC) compares with the EQS

This test checks whether your discharge increases the concentration of the substance downstream by more than 10% of the EQS.

Before completing this test, you need to know the BC data for the water you are discharging to.

If BC data is not available, use estimated data.

Calculate the PEC

To complete this test, you need to work out the PEC of the substance in the water downstream of your discharge.

If the RFR is more than ten times the EFR, the PEC should be calculated as follows with the PC taken from test 2, step 3: [PC (µg/l)] + [BC (µg/l)] = PEC (µg/l).

If the RFR is equal to or less than ten times the EFR, the PEC should be calculated as follows:

  1. [EFR (m³/s)] x [RC (µg/l)]

  2. [RFR (m³/s)] x [BC (µg/l)]

  3. [Result of step 1] + [Result of step 2]

  4. [EFR (m³/s)] + [RFR (m³/s)]

  5. [Result from step 3] ÷ [Result from step 4] = PEC (µg/l)

For AA EQS use the average EFR and RC. For MAC EQS use the maximum EFR and RC.

If the difference between background water quality and the PEC:

  • is greater than 10% of the EQS – modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests)
  • does not exceed 10% of the EQS – go to test 4

Test 4: Check if the PEC exceeds the EQS in the receiving water downstream of the discharge

This test checks if the EQS is complied with in the receiving water by checking whether the PEC (calculated in test 3) is higher than the EQS.

You need to complete this check against the AA EQS and the MAC EQSs:

  1. Compare the average PEC with the AA EQS.

  2. Compare the maximum PEC with the MAC EQS.

If the PEC exceeds either EQS, modelling is needed. If the substance is a PHS, you also need to complete the PHS screening tests.

If neither EQS is exceeded and the substance passed test 3, stop here – no further tests needed (unless the substance is a PHS).

Screening tests for discharges to coastal and estuarine waters

Use the EQSs for coastal and estuarine waters to complete these screening tests.

The Environment Agency will check that the correct tests have been carried out depending on whether the discharge is to a low-water channel, a riverine estuary or to other coastal waters. Your application may be delayed if the screening assessment is not representative of the discharge environment.

Discharges to coastal and estuarine protected areas

You will need modelling for any substances which exceed the EQS if your discharge is direct to a designated protected area (for example a SSSI), even if the substance would screen out in subsequent screening tests. This is because the discharge conditions will need to be considered in the context of site-specific conservation objectives.

Test 1: Compare the discharge concentration with the EQS

This test checks whether the concentration of a substance in the discharge is more than the EQS.

To complete this test, you need to compare the concentration of each substance in your discharge against the relevant EQS. Test both the AA and MAC EQSs if the substance has both types of EQS.

If the concentration of the substance:

  • is more than the EQS (or EQSs) – go to test 2
  • is below the EQS (or EQSs) – stop here, no further tests needed (unless the substance is a PHS)

Test 2: Check if the discharge is to a riverine estuary or low water channel

This test checks whether your discharge location requires freshwater screening methods.

You will need to check if you are discharging to parts of the estuary where the water is mainly fresh (riverine estuaries or low water channels).

Riverine estuaries are the upper reaches of an estuary where the water is predominantly fresh and flow forms a straight line (rectilinear) which may reverse with the tide.

Low water channels are the route taken by a river at low water where the estuary bed is exposed on either side and flow continues even at low water.

If discharging to a riverine estuary or low water channel:

  • use the screening test for inland waters (freshwaters), starting at test 2
  • use freshwater flow rate and background quality in your calculations
  • continue to use the EQSs for estuaries and coastal waters (not the freshwater EQSs)

If you are not discharging to a riverine estuary or low water channel, go to test 3.

Test 3: Check if the discharge location has restricted dilution or dispersion

This test checks whether your discharge location has limited water exchange that would affect dilution.

You need to check if your discharge is to a location with restricted dilution or dispersion. For example, enclosed bays (such as Lulworth Cove in Dorset), docks or ports.

If your discharge:

  • has restricted dilution or dispersion – modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests)
  • does not have restricted dilution or dispersion – go to test 4

If you need help checking whether your discharge is to a location with restricted dilution or dispersion, contact the Environment Agency at marine_modelling_requests@environment-agency.gov.uk.

Test 4: Compare the discharge location with chart datum

This test checks whether your discharge is too close to shore or in water that is too shallow.

To complete this test, you need to:

  1. Use online navigation charts to check the discharge location.

  2. Measure the minimum distance between your discharge point and where the seabed is at chart datum (shown as 0 on nautical charts).

  3. Check the seabed depth at your discharge location.

If either of the following apply, you need to stop screening and modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests):

  • the discharge location is less than 50 metres offshore from chart datum
  • the seabed at the discharge location is less than 1 metre below chart datum

If these do not apply, go to test 5.

Test 5: Assess the effective volume flux of the discharge

This test checks whether the discharge volume and substance concentration are within allowable limits for the water depth.

Step 1: Check if discharge is buoyant

Most discharges to estuaries and coastal waters are buoyant, as they are mainly freshwater discharges to a saltwater environment.

If the discharge:

  • is not buoyant (for example an effluent from evaporative cooling or desalination) – stop screening, modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests)
  • is buoyant – continue to step 2

Step 2: Determine allowable effective volume flux

The allowable limit depends on water depth below chart datum:

  • for depths from 1m, up to 3.5m below chart datum – allowable effective volume flux is proportional to the water depth (for example, if water depth is 2m, allowable effective volume flux is 2m³/s)
  • for water depths more than 3.5m below chart datum – allowable effective volume flux is 3.5m³/s

Step 3: Calculate your discharge’s effective volume flux

Complete the following calculation using the AA EQS and the MAC EQS (if available):

  1. [Effluent discharge rate (m³/s)] x [RC (µg/l)]

  2. [EQS] − [Average BC at the discharge] (make sure units are the same)

  3. [Result from step 1] ÷ [result from step 2]

Step 4: Compare and decide

If the effective volume flux is:

  • more than the allowable effective volume flux – modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests)
  • less than the allowable effective volume flux – stop here, no further tests needed (unless the substance is a PHS)

Screening tests for discharges of cooling water

This section covers how to assess substances diluted by cooling water at your site before they reach the receiving water. This includes discharges of cooling waters to inland freshwaters and estuarine and coastal waters.

For cooling waters discharging to:

  • a freshwater river, riverine estuary or direct to a low water channel in the upper reaches of an estuary – use the screening or modelling approach for discharges to freshwater
  • coastal or estuarine areas with restricted dilution or dispersion characteristics, or to the middle reaches of estuaries – dilution will need to be considered on a site-specific basis
  • lower estuaries and coastal waters – follow the method for predicting concentrations in cooling water

Predicting concentrations in cooling water

Calculate predicted concentrations in cooling water

For these tests you need to know the:

  • average and maximum substance load in the waste stream
  • average process waste stream flow rate
  • process waste stream flow relevant to the maximum substance load in the waste stream
  • average and minimum operational cooling water flow rate
  • average and maximum BC in abstracted cooling water

Use the following calculations to work out the predicted concentration in the cooling water:

  1. [Average BC] x [average cooling water flow]

  2. [Average substance load in waste stream] + [result from step 1]

  3. [Average process waste stream flow] + [average cooling water flow]

  4. [Result from step 2] ÷ [result from step 3] = predicted average concentration

Calculate predicted maximum concentrations in the cooling water

Where the EQS is marked as a ‘95th percentile’ in the EQS tables, the maximum BC can also be a 95th percentile value.

Complete the following calculations:

  1. [Maximum BC] x [minimum cooling water flow]

  2. [Maximum substance load in waste stream] + [result from step 1]

  3. [Process waste stream flow relevant to maximum substance load in waste stream] + [minimum cooling water flow]

  4. [Result of step 2] ÷ [result of step 3] = predicted maximum concentration

Compare concentrations with EQSs

You will need to compare:

  1. The predicted average concentration in the cooling water with the AA EQS.

  2. The predicted maximum concentration in the cooling water with the MAC EQS (if available).

If the concentration of the substance in the cooling water is:

  • more than the relevant AA or MAC EQS – modelling is needed (if the substance is a PHS, you also need to complete the PHS screening tests)
  • less than the relevant AA or MAC EQS – stop here, no further tests needed (unless the substance is a PHS)

Additional screening tests for PHSs

You will need to carry out this additional screening step for PHSs in discharges to inland freshwaters and to coastal waters and estuaries (including via the foul sewer or for discharges of cooling waters).

Complete this test even if the substance has been screened out by the previous screening tests.

You can identify which PHSs can be used in this test using Table 1 ‘Significant load limits for PHSs’.

Significant load test

This test checks whether the annual load discharged for the PHS is greater than the significant load limit set for the PHS.

Calculate the annual load

Use the following calculation to work out the annual load: 

  1. [Average discharge concentration (µg/l)] x [average flow (litres per day)]

  2. [Result of step 1] ÷ by 1,000 = result in mg per day

  3. [Result of step 2] ÷ by 1,000 = result in g per day

  4. [Result of step 3] ÷ by 1,000 = result in kg per day

  5. [Result in kg per day] x [number of discharge days per year] = annual load (kg per year)

If the discharge is continuous throughout the year, the number of days will be 365 days.

If the discharge is not constant, multiply the result of step 4 by the number of days per year the discharge is active.

If there are 2 or more discharges of the same substance on the same permit from the same site, combine the loads for the annual load calculations.

Compare with significant load limits

Check your result against the relevant significant load in Table 1.

Table 1: Significant load limits for PHSs 

Hazardous chemical or element Annual significant load limit (kg)
Anthracene 1
Brominated diphenyl ether 1
Cadmium 5
Chloroalkanes C10-13 1
Dioxins 0.0001
Endosulphan 1
Hexachlorobenzene 1
Heptachlor 1
Hexachlorobutadiene 1
Hexachloro-cyclohexane 1
Mercury and its compounds 1
Nonylphenol (4-Nonylphenol) 1
Pentachlorobenzene 1
Polycyclic aromatic hydrocarbons (PAHs) 5
Tributyltin compounds (Tributylin-cation) 1

If the annual load is less than the significant load limit, the outcome will depend on the previous screening results:

  • if the initial screening suggested no modelling is needed – no modelling is needed
  • if the initial screening suggested modelling was needed – modelling is needed

If the annual load is more than the significant load limit, go to ‘check you have enough samples above the MRV’.

Check you have enough samples above the MRV

This test identifies whether a PHS is likely to be in the discharge by comparing your sample results with the MRV. This check can only be completed if your MRV is 10% of the EQS or less. Complete this test separately for each PHS in your discharge.

  1. Count how many samples you have in the assessment period.

  2. Count how many of your sample results are equal to or above the MRV (do not count ‘less than’ results).

  3. Compare the number of sample results from step 2 with the minimum number of samples given in the table below.

Number of samples in assessment period Minimum number of samples that need to be equal to or greater than the MRV
12 to 14 4
15 to 20 5
21 to 27 6
28 to 34 7
35 to 41 8
42 to 48 9
49 to 56 10
57 to 63 11
64 to 71 12
72 to 79 13
80 to 86 14
87 to 94 15
95 to 102 16

If your count from step 2 is equal to or less than the minimum number in the table, stop here – you do not need to continue PHS screening or data clean up.

If your count from step 2 is greater than the minimum number in the table:

  1. Follow the data clean-up steps in ‘Data clean up steps for PHS screening’.

  2. Repeat the significant load screening test using cleaned-up data.

  3. Compare the new results with the significant load limit.

If the cleaned-up annual load is still more than the significant load limit:

  • the Environment Agency will assess the discharge
  • an emission limit may be included in your permit to control the substance

If the cleaned-up annual load is less than significant load limit, you will need to check if the substance passed the other screening tests:

  • if substance failed one or more other screening tests – modelling is needed
  • if substance passed all other screening tests – no modelling is needed

Data clean up steps for PHS screening

Check data is current

Check whether your data is current and reflects any recent treatment processes that might have altered the discharge.

If your data does not represent current discharge conditions, collect more data. You may need to repeat the screening tests.

If your data does represent current discharge conditions, continue with the data clean up steps.

Check for step changes (significant changes)

There may be step changes in your data over time. You may be aware of these, or they may only show up when looking at the data. For example:

  • changes in effluent treatment
  • changes in trade inputs to sewerage works

If there are step changes:

  • select a time period reflecting current discharge quality (even if this means using less than 3 years’ worth of data) – you will need a minimum of 12 samples
  • continue with the data clean up steps

If there are no step changes, continue with the data clean up steps.

Check data distribution

If your data are not evenly distributed (for example if there is seasonal variation):

  • data can still be used – continue with the data clean up steps
  • consider the uneven spread when you interpret the results – you can analyse seasonal variation using a statistics package
  • you may need to contact the Environment Agency to help you interpret the data

If your data is evenly distributed, continue with the data clean up steps.

Check for outliers

Check your data for very high or low values ‘outliers’ as these may not be accurate and could distort your data.

You can identify high values resulting from exceptional circumstances or normal performance by looking at other substances sampled on the same date or results from the same time period in different years.

A value may be considerably higher or lower than the rest of the dataset because it is:

  • incorrect due to sampling, recording or coding errors – exclude these values from your assessment
  • correct but relates to exceptional circumstances (such as treatment failure) – exclude these values from your assessment
  • correct and is a high value but part of the normal performance of the permitted activity – include this value in your assessment

If you have fewer than 12 samples, do not remove outliers.

You can use statistical analysis to help identify if the outlier is a ‘true outlier’.

If it is not a true outlier, leave it in your data and repeat the significant load screening test using cleaned-up data.

If it is a true outlier, remove it from your data and repeat the significant load screening test using cleaned-up data.

Adjust ‘less than’ and low values

You need to adjust all ‘less than’ values to half their face value (by dividing by 2). For example:

  • < 10 (less than 10) becomes 5
  • < 50 (less than 50) becomes 25

If positive values (not less than values) are reported below the MRV, take them at face value.

Part 4: Modelling

Modelling will need to be completed for:

  • any substances that were not screened out by the screening tests
  • discharges that are direct to a designated protected area of estuarine or coastal waters where a substance exceeds the EQS
  • discharges directly to a designated protected area of inland freshwaters where a substance exceeds the EQS

The Environment Agency will complete modelling for most discharges to inland freshwaters.

You must complete modelling for discharges of hazardous chemicals and elements to:

  • freshwater lakes, canals and reservoirs
  • estuarine and coastal waters

You may need to employ a consultant to complete the modelling. The best modelling approach to take depends on the specific characteristics of the discharge, the substances in the discharge and the receiving water environment. Before starting any modelling work, you can discuss which approach to modelling is most appropriate with the Environment Agency. 

Modelling uses ‘cleaned-up’ data and assesses each substance in more detail. Follow the steps in ‘Preparing data for modelling’ before to prepare your data for modelling.

We are updating our guidance on carrying out modelling. For further details, contact the Environment Agency. 

If the modelling tests show that your discharge could cause harm, the Environment Agency will include conditions to control the hazardous chemical or element in your permit.

If the impact on the surface water environment is unacceptable, the Environment Agency may refuse your permit application.

Preparing data for modelling

You need to clean up your data in preparation for modelling. These steps apply to all discharge environments. You should complete these steps before generating summary statistics to use in modelling.

1. Check you have enough samples above the MRV

This test identifies whether a substance is likely to be in the discharge by comparing your sample results with the MRV. This check can only be completed if your MRV is 10% of the EQS or less. Complete this test separately for all substances in your discharge which did not screen out:

  1. Count how many samples you have in the assessment period.

  2. Count how many of your sample results are equal to or above the MRV (do not count ‘less than’ results).

  3. Compare the number of sample results from step 2 with the minimum number of samples given in Table 2.

Table 2: Minimum number of samples equal to or above the MRV according to sample size

Number of samples in assessment period Minimum number of samples which need to be equal to or above the required MRV
12 to 14 4
15 to 20 5
21 to 27 6
28 to 34 7
35 to 41 8
42 to 48 9
49 to 56 10
57 to 63 11
64 to 71 12
72 to 79 13
80 to 86 14
87 to 94 15
95 to 102 16

If your count from step 2 is:

  • equal to or less than the minimum number in the table – stop here, modelling is unlikely to be needed (unless any of the positive samples exceeded the MAC EQS – then modelling may be needed)
  • greater than the minimum number in the table – continue to step 2

2. Check the data represent the current conditions

Check data is current

Check whether your data is current and reflects any recent treatment processes that might have altered the discharge.

Check for step changes (significant changes)

There may be step changes in your data over time. You may be aware of these, or they may only show up when looking at the data. For example:

  • changes in effluent treatment
  • changes in trade inputs to sewerage works

If there are step changes:

  • select a time period reflecting current discharge quality (even if this means using less than 3 years’ worth of data) – you will need a minimum of 12 samples
  • continue with the data clean up steps

If there are no step changes, continue with the data clean up steps.

Check data distribution

If your data are not evenly distributed (for example if there is seasonal variation):

  • data can still be used – continue with the data clean up steps
  • consider the uneven spread when you interpret the results – you can analyse seasonal variation using a statistics package.
  • you may need to contact the Environment Agency to help you interpret the data

If data is evenly distributed, continue with the data clean up steps.

Check for outliers

Check your data for very high or low values ‘outliers’ as these may not be accurate and could distort your data.

You can identify high values resulting from exceptional circumstances or normal performance by looking at other substances sampled on the same date or results from the same time period in different years.

A value may be considerably higher or lower than the rest of the dataset because it is:

  • incorrect due to sampling, recording or coding errors – exclude these values from your assessment
  • correct but relates to exceptional circumstances (such as treatment failure) – exclude these values from your assessment
  • correct and is a high value but part of the normal performance of the permitted activity – include this value in your assessment

If you have fewer than 12 samples, do not remove outliers.

You can use statistical analysis to help identify if the outlier is a ‘true outlier’.

If it is not a true outlier, leave it in your data.

If it is a true outlier, run the modelling tests with the outlier included in the dataset and then again without.

3. Adjust ‘less than’ and low values

You need to adjust all ‘less than’ values to half their face value (by dividing by 2) before calculating summary statistics for modelling. For example:

  • < 10 (less than 10) becomes 5
  • < 50 (less than 50) becomes 25

If positive values (not less than values) are reported below the MRV, take them at face value for modelling. For example, if the MRV for a substance is 10, but the reported value was 8.8, the value of 8.8 should be assumed to be accurate and used for modelling.

Once you have assessed and adjusted the data, calculate mean and standard deviation values to use in modelling.

Submit your results

Submit your screening or modelling results to the Environment Agency when you either:

You will need to include:

  • results of screening or modelling tests
  • raw data used for screening (in spreadsheet format where available)
  • cleaned up data used for modelling (where applicable)

You can submit your results in the screening tool if you have used this to carry out the screening tests.

If you need help

Contact the Environment Agency if you have questions or need more advice.

General enquiries

National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY

Email enquiries@environment-agency.gov.uk

Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm.

Updates to this page

Published 1 February 2016
Last updated 20 February 2026 show all updates

  1. Updated to provide more comprehensive guidance. This includes more information on preparing for and completing the screening tests, guidance on preparing data for modelling, clearer information on assessing discharges from installations and waste operations, and more links to related GOV.UK guidance.

  2. Updated the 'Screening tests: freshwaters' section, 'Test 2', to reflect that applicants can no longer request river flow data from the Environment Agency.

  3. Updated the 'Background data for estuaries or coastal waters' section to say that, if you do not have upstream or background data for estuaries or coastal waters and freshwaters, assume that the concentration of each pollutant is 50% of the EQS.

  4. Updated the 'Estuaries and coastal waters priority hazardous substances, priority substances and other pollutants environmental quality standards' and 'Freshwaters priority hazardous substances, priority substances and other pollutants environmental quality standards'.

  5. Updated the 'Estuaries and coastal waters priority hazardous substances, priority substances and other pollutants environmental quality standards' and 'Freshwaters priority hazardous substances, priority substances and other pollutants environmental quality standards'.

  6. Edited the section on 'Screening tests: estuaries and coastal waters' to clarify test 3. Updated the documents 'Estuaries and coastal waters specific pollutants and operational environmental quality standards', Freshwaters specific pollutants and operational environmental quality standards', 'Estuaries and coastal waters priority hazardous substances, priority substances and other pollutants' and 'Freshwaters priority hazardous substances, priority substances and other pollutants'.

  7. We have replaced all mentions of limit of detection (LOD) to minimum reporting value (MRV). Less than values should always be set at a MRV.

  8. Hazardous pollutants are known as specific substances. If your discharge includes specific substances your risk assessment will need to include a specific substances assessment.

  9. New files for: 'Estuaries and coastal waters specific pollutants and operational environmental quality standards' and 'Freshwaters priority hazardous substances, priority substances and other pollutants'.

  10. Updated tables for: 'Sewage treatment reduction factors' and the 'Estuaries and coastal waters priority hazardous substances, priority substances and other pollutants'.

  11. First published.

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