Guidance

Surface water pollution risk assessment for your environmental permit

How to carry out a risk assessment if you're applying for a bespoke permit that includes discharging hazardous pollutants to surface water.

You must follow this guide if both of the following apply:

  • you’re applying for a permit that includes discharging hazardous pollutants to surface water under the Environmental Permitting Regulations
  • you need to carry out a bespoke risk assessment - bespoke permits are environmental permits customised to your own activities and are usually required if your site produces emissions that exceed certain levels or multiple types of emissions

Read the risk assessment overview before reading this guide.

Hazardous pollutants are the pollutants listed in the following tables:

Hazardous pollutants are also known as specific substances.

You must evaluate and assess any hazardous pollutants you plan to release into surface water. You must then carry out screening tests on the pollutants to check if they’re a risk to the environment. This is called a specific substances assessment.

You must carry out screening tests when:

  • you’re applying for a new permit
  • you need to change your permit to cover an increase in the amount of hazardous pollutants you plan to discharge or if you plan to discharge a new hazardous pollutant
  • you’re applying to change (vary) your permit for another reason and the hazardous pollutants in your discharge have not been assessed since 2010

Screening tests check the risk from hazardous pollutants to the environment.

If your screening tests show there’s a risk to the environment, the Environment Agency will tell you if more detailed tests need to be carried out. The detailed tests are known as ‘modelling’:

  • the Environment Agency may need to carry out modelling if you’re discharging to freshwater (you’ll need to carry out modelling if it’s required if you’re discharging to a lake or canal)
  • you may need to carry out modelling if you’re discharging to an estuary or coastal waters

When you do not need to carry out screening tests

You do not usually need to carry out screening tests if you discharge water taken from a river or groundwater to the same body of water you originally took from or if you have not added any additional hazardous pollutants to the water.

However, you do need to do screening tests if you:

  • take the water from groundwater and discharge it to surface water
  • use the water in a process which concentrates the existing pollutants before it’s discharged, for example water which is used for cooling and therefore partially evaporates
  • keep the water before you discharge it and you make the quality of river worse than its quality when the water was taken

How to do your screening tests

There are 3 stages to screening.

  1. Identify the pollutants released from your plant.
  2. Gather data on your pollutants before screening them.
  3. Carry out screening tests on the data.

Identify the pollutants released from your plant

You’ll need to produce a list of any hazardous pollutants that are likely to be in the discharge from your site. Find potentially hazardous pollutants in the following tables:

Pollutants are likely to be in the discharge if:

  • they’re allowed to be added to the discharge (for example water company trade effluent consent or discharges from installations)
  • you’ve added them to the discharge (for example iron or aluminium to remove phosphorus). Read the Environment Agency’s guidance on dosed substances for the rules on this
  • you’ve detected them using chemical analysis

To do chemical analysis you’ll need to take samples of the discharge and send them to a United Kingdom Accreditation Service (UKAS) accredited laboratory like the National Laboratory Service. Make sure the laboratory tests for all pollutants which you expect to find in the discharge and that they use an appropriate ‘limit of detection’ (LOD) (usually 10% of the environmental quality standards (EQS)).

You must submit the chemical analysis data that’s been analysed to an appropriate LOD by a UKAS-accredited laboratory with your application. If you submit data which has not been analysed to an appropriate LOD you must tell the Environment Agency why, for example:

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

Gather data on your pollutants before screening them

You need to measure your pollutants if you release hazardous pollutants into:

  • freshwaters
  • estuaries and coastal waters
  • sewers

For each pollutant you’ll need:

  • the chemical name of the pollutant being analysed
  • the unit of measurement, for example micrograms per litre
  • the maximum, minimum and average recorded concentrations of the pollutant in the discharge
  • the maximum and average recorded discharge flow
  • for freshwaters - the average concentration of the pollutants in the water upstream of the discharge if it’s available (if not, you can use estimated data) and the 95% exceedence river flow (you can request this from the Environment Agency)
  • for estuaries and coastal waters - the background concentrations at the discharge point
  • total metal data (collect dissolved metal data at the same time in case any of your pollutants need modelling)
  • a minimum of 12 samples (the ideal number is 36)
  • the LOD used
  • the relevant EQS

Estimated data

You need to use estimated data for the screening tests if you do not have any discharge monitoring data, for example for new discharges that you are not yet discharging. For AA (annual average) EQS you need average discharge concentrations, with a minimum of 12 individual sample results from on-site tests or a proxy site (a similar sized site and manufacturing process which is likely to have a similar discharge). You’ll need to average your results - if you have a less than figure you must round it up to the nearest whole number (for example less than 10 is assumed to be 10).

Intermittent discharge

If you discharge pollutants intermittently (for example you operate 12 hours a day, 5 days a week or you operate seasonally) you’ll need to work out the average flow rate to use in the screening tests.

If your discharge starts and stops regularly, for example once a day, you’ll need to work out the average flow rate during discharge and then multiply it by the proportion of the year that the discharge takes place. 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 x 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 both of these examples.

Background data for estuaries or coastal waters

You’ll need the following data on background pollutant concentrations to carry out the screening tests:

  • for new pollutants that you have not discharged before - a sample of data from the planned discharge point
  • for existing pollutants - a sample of data taken from a point away from the discharge point (so it’s not affected by the discharge plume)
  • for estuaries where the background quality can vary depending on the tide - use the maximum background value for the pollutant concentration rather than the average value

If you do not have upstream or background data for estuaries or coastal waters and freshwaters

Assume that the concentration of each pollutant is 10% of the EQS in clean water (for example where there’s no other discharges of the pollutant) and 50% of the EQS in polluted water (where there are other discharges of the pollutant). If you’re unsure, assume that the upstream concentration is 50% of the EQS.

Calculate the concentration of pollutant in the discharge when discharging to a sewer

Sewerage treatment works will remove a proportion of the pollutant in the discharge before it’s discharged to a freshwater or estuary or coastal water. You must take this into account when you calculate the concentration of the hazardous pollutant which you’ll discharge to the river or coastal water through the sewer.

The amount of each pollutant which will be removed by a sewerage works is known as the sewage treatment reduction factor. These factors have been calculated for each individual pollutant.

Use the following calculation to work out the concentration of the pollutant after sewage treatment:

Multiply the concentration of the pollutant in your discharge released to the sewer in milligrams per litre by the sewage treatment reduction factor in milligrams per litre.

You’ll need to average your results - if you have a less than figure you must round it up to the nearest whole number (for example less than 10 is assumed to be 10). For AA EQS, you should use the average release concentrations and for MAC EQS you should use the maximum release concentration.

Download the sewage treatment reduction factors:

Carry out screening tests on the data from the pollutants

You must compare the measurements of pollutants from your discharge to the following:

  • environmental quality standards (EQS) maximum allowable concentrations (MAC) - to evaluate the short term environmental impacts that your emissions can cause
  • environmental quality standards (EQS) annual average (AA) - to evaluate the long term environmental impacts that your emissions can cause

The MAC and AA EQS for the pollutants are listed in the following tables:

Not all pollutants have both types of EQS.

When a pollutant has both an AA and MAC EQS, compare the measurements of pollutants from your discharge to the AA, apart from test 4 for freshwaters where you must use the MAC EQS as well.

You must compare the measurements of pollutants from your discharge to the MAC as well as the AA if you release it in batches rather than continuously.

Screening tests: freshwaters

Carry out the following tests to check whether your discharge of hazardous pollutants to freshwaters is a risk to the environment. For each test you’ll need to provide more data than for the previous one. If your pollutant is screened out by a test, you do not need to complete the rest of the tests.

Contact the Environment Agency to get access to the screening tool to help you carry out these tests.

Test 1

Check whether the concentration of the pollutant in the discharge is more than 10% of the environmental quality standard (EQS).

If it’s less than 10% you do not need to collect the data for the next 3 tests - you do not need to anything more as your hazardous pollutant is not a risk to the environment.

If it’s more than 10%, carry out test 2.

If you have 2 or more discharges of the same pollutant from different parts of your plant you’ll need to test them separately.

Environmental quality standards (EQS) for freshwaters

Download the EQS for freshwaters:

Test 2

This test introduces the dilution available in the receiving water. You’ll need the river flow data and daily discharge volume for this test.

Check whether the process contribution (PC) of your pollutant is more than 4% of the EQS. PC is the concentration of a discharged pollutant in the water after it’s been diluted.

Contact the Environment Agency to get the river flow data for the water you’re discharging to.

Do the following steps to work out the PC.

  1. Multiply the effluent flow rate (EFR) by the release concentration of the pollutant in the effluent (RC).
  2. Add your value for the EFR to the river flow rate (RFR).
  3. Divide the result of step 1 by the result of step 2.

If your value for PC is 4% or less of the EQS, you do not need to carry out tests 3 and 4.

If the PC is more than 4% of the EQS you’ll need to carry out tests 3 and 4.

Test 3

You need the background concentration data (BC) for this test. Contact the Environment Agency to get the BC for the water you’re discharging to or use estimated data.

Check whether your discharge increases the concentration of the pollutant in the river downstream of the discharge by more than 10% of the pollutant’s EQS value.

The predicted environmental concentration (PEC) in the water downstream of the discharge is a combination of the PC and background concentration.

To work out the PEC add the PC to the average BC.

If the result of step 2 in test 2 shows that the river flow rate is less than 10 times the effluent discharge flow rate you should also do the following calculation.

  1. Multiply EFR by RC.
  2. Multiply RFR by BC.
  3. Add the results of step 1 and 2 together.
  4. Add EFR to RFR.
  5. Divide the result from step 3 by the result from step 4.

If the difference between BC and PEC is more than 10% of the EQS the Environment Agency will need to carry out modelling (if you’re discharging to a canal or lake you’ll need to do the modelling). If it is not, carry out test 4.

Test 4

Check whether the PEC is higher than the EQS.

If it is, the Environment Agency will need to carry out modelling (you’ll need to do the modelling if you’re discharging to a canal or lake).

Screening tests: estuaries and coastal waters

Carry out the following tests to check if your plant will discharge potentially hazardous pollutants to estuaries and coastal waters. For each test you’ll need you to provide more data than for the previous one.

If your process involves using cooling water you must carry out these alternative tests.

Contact the Environment Agency to get access to the screening tool to help you carry out these tests.

Test 1

Check whether the level of pollutant in the discharge is more than the EQS limits. You need to test for both annual average limits and maximum allowable concentration if the pollutant has both types of EQS.

If the pollutant is more than EQS limits, carry out test 2. If it’s below EQS limits you do not need to anything more as your pollutant is not a risk to the environment.

Environmental quality standards (EQS) for estuaries and coastal waters

Download the EQS for estuaries and coastal waters:

Test 2

Check whether you’re discharging to the low water channel (if the water does not flow across the estuary bed at any stage of the tide) in the upper parts of an estuary where the water is mainly fresh.

If the discharge is direct to the low water channel, do the screening tests for freshwater starting at test 2.

In the calculations for freshwater tests 2 to 4 use the freshwater flow rate and upstream quality but use the EQSs for estuaries and coastal waters.

If these do not apply to your discharge site, carry out test 3.

Test 3

The Environment Agency will do test 3 for you if the water you discharge into has restricted dilution or dispersion like a small coastal bay (for example Lulworth Cove) or a creek in an estuary. Contact the Environment Agency to find out if your discharge site has restricted dilution or dispersion.

If you’re discharging to water where the dilution or dispersion is restricted you’ll need to carry out modelling.

Test 4

If the water you discharge to does not have restricted dilution or dispersion and the discharge is submerged at all states of the tide, you need to measure the minimum distance between the point you discharge waste water from and the point (or line) where the water depths are shown on nautical charts as zero (which is known as chart datum).

Use online navigation charts to check the location you’re discharging to.

You will need to carry out modelling if the discharge contains pollutants at concentrations above EQS and either of the following apply:

  • the discharge location is less than 50m offshore from where the sea bed is at chart datum
  • the sea bed at the discharge location is less than 1m below chart datum

If these do not apply to your discharge site, carry out test 5.

Test 5

You can only carry out this test if the discharge is buoyant.

Most discharges to estuaries and coastal waters are buoyant, as they are mainly freshwater discharges to a salty environment. If the discharge is not buoyant, for example if the receiving water is fresh, then you’ll need to carry out modelling.

Check if the effective volume flux of the discharge is within the allowable limits.

The maximum effective volume flux you can work with is proportional to the water depth, for depths up to 3.5m below the depth of water at the point where chartered water depths are shown on nautical charts (chart datum). For example, if the water depth below chart datum is 2m the allowable effective volume flux is 2 cubic metres per second. For water depths more than 3.5m below chart datum, the allowable effective volume flux is fixed at 3.5 cubic metres per second.

Work out the effective volume flux of the discharge.

  1. Multiply the effluent discharge rate (in cubic metres per second) by the release concentration of the pollutant (in micrograms per litre).
  2. Subtract the average background concentration of the discharge location from the EQS.
  3. Divide the result of step 1 by the result of step 2.

Check your pollutants against the maximum annual concentration (MAC) EQS as well as against the AA.

If the effective volume flux is more than the allowable effective volume flux for the discharge location you will need to carry out modelling. If it’s less you do not need to do anything further.

Screening tests: discharges into cooling water which are then discharged to estuaries or coastal waters

You need to check how much the pollutants are diluted by the cooling water in your plant before the discharge reaches the receiving water, to check whether modelling is needed.

Work out the predicted average concentration in the cooling water.

  1. Multiply the average background concentration by the average cooling water flow.
  2. Add the average load of the pollutant in your waste stream to the result from step 1.
  3. Add the average process waste stream flow to the average cooling water flow.
  4. Divide the result of step 2 by the result of step 4.

Work out the predicted maximum concentration in the cooling water.

  1. Multiply the maximum background concentration by the minimum cooling water flow.
  2. Add the maximum load of the pollutant in your waste stream to the result from step 1.
  3. Add the average process waste stream flow to the minimum cooling water flow.
  4. Divide the result of step 2 by the result of step 4.

You’ll need to carry out modelling if the concentration of the pollutant in the cooling water is more than the relevant EQS AA or MAC. If it’s less you do not need to do anything further.

Screening test: ‘priority hazardous pollutants’

You must carry out additional screening for all priority hazardous pollutants for freshwaters and priority hazardous pollutants for coastal waters and estuaries even if the pollutants did not need modelling as a result of screening tests 1 to 4 for freshwaters or tests 1 to 6 for estuaries and coastal waters.

The priority hazardous pollutants are listed in the following tables:

You need to find out whether the annual limit of pollutants you discharge is more than the significant load limit (an annual load limit that has been set for priority hazardous pollutants).

Significant load limits

Pollutant Annual significant load limit in kg
Anthracene 1
Brominated diphenyl ether 1
Cadmium 5
C10-13 Chloroalkanes 1
Endosulphan 1
Hexachlorobenzene 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

Calculate the significant load

  1. Multiply the average discharge concentration by the average flow (litres a day).
  2. Divide the result by 1,000 to give you mg a day.
  3. Divide the result by 1,000 to give you g a day.
  4. Divide the result by 1,000 to give you kg a day.
  5. Multiply the result by 365 to give you kg a year.

Check your result against the relevant significant load in significant load limits table.

If the load you calculate is less than the significant load for the pollutant and the pollutant did not need modelling as a result of screening tests 1 to 4 for freshwaters or tests 1 to 6 for estuaries and coastal waters, the pollutant is insignificant and you do not need to do anything further.

If the load you calculate is less than the significant load for the pollutant and the pollutant did need modelling as a result of screening tests 1 to 4 for freshwaters or tests 1 to 6 for estuaries and coastal waters:

  • the Environment Agency will need to carry out modelling if you’re discharging to freshwater (except lakes and canals)
  • you’ll need to carry out modelling if you’re discharging to an estuary or coastal waters

If the load you calculate is more than the significant load for the pollutant, you must repeat the test with cleaned-up data (this means making some changes to make sure your data is accurate).

Calculate the significant load using cleaned-up data

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

If your calculations with cleaned-up data are more than the significant load for the pollutant, the Environment Agency will include an emission limit in your permit which will tell you how to control the pollutant.

If the pollutant did not pass one or more of the screening tests:

  • the Environment Agency will need to carry out modelling if you’re discharging to freshwater
  • you’ll need to carry out modelling if you’re discharging to an estuary or coastal waters

You do not need to do anything further if the load you calculate is less than the significant load for the pollutant and it passed tests 1 to 4 for freshwaters or 1 to 6 for estuaries and coastal waters.

Cleaning up your data

Check that a minimum number of your samples exceed the limit of detection (LOD).

Number of samples in assessment period Minimum number of samples which need to be equal to or above the required LOD
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 a minimum number of your samples do not exceed the LOD, the pollutant is not a risk to the environment and you do not need to carry out modelling (this only applies if your LOD was 10% of the EQS or less).

Follow these steps to check that your data represent the current situation at your plant.

  1. Check whether there are significant changes (changes that are obviously different when you look at the data particularly if you know the reasons for the change) in your data over a period of time, for example changes in effluent treatment or changes in trade inputs to a sewerage works.
  2. If there are significant changes, select a time period which reflects current quality even if this means using less than 3 years’ data. Your data must include a minimum of 12 samples.
  3. If your data is not evenly distributed, for example if there is seasonal variation, it can still be used, but you should 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.
  4. Check your data is current, for example, you should include any recent treatment processes that might have altered the discharge.
  5. Check your data for very high or low values (‘outliers’) as these may not be accurate and could distort your data. For example, a value may be correct but relate to exceptional circumstances such as treatment failure, so you should not include it in your assessment. Or a value might be correct and a normal part of your activity, so you should include it in your assessment.

Modelling

You need to find a consultant to carry out detailed tests called modelling if your screening tests for estuaries and coastal waters did not screen out the pollutants. Consultants should contact the Environment Agency to find out about modelling and read the guidance on carrying out modelling.

The Environment Agency will carry out the modelling tests for you if your screening tests for freshwaters showed that your discharge is a risk to the environment, unless you’re discharging into a lake or canal, then you must carry out modelling.

Modelling tests will show whether your discharge will cause pollution or not. If the modelling tests show that your discharge will cause pollution, the Environment Agency will include conditions to control the hazardous pollutant in your permit or they may refuse your permit application if the impact on the environment is unacceptable.

Submit your results

Submit the results of your screening tests to the Environment Agency along with your application for a new permit or to change an existing permit. You can submit your results in the screening tool if you have used this to carry out the tests.

You must also include the raw data you used for screening.

Contact

Contact the Environment Agency if you have queries.

General enquiries

National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY

Monday to Friday, 8am to 6pm

Published 1 February 2016
Last updated 3 April 2018 + show all updates
  1. Hazardous pollutants are known as specific substances. If your discharge includes specific substances your risk assessment will need to include a specific substances assessment.
  2. New files for: 'Estuaries and coastal waters specific pollutants and operational environmental quality standards' and 'Freshwaters priority hazardous substances, priority substances and other pollutants'.
  3. Updated tables for: 'Sewage treatment reduction factors' and the 'Estuaries and coastal waters priority hazardous substances, priority substances and other pollutants'.
  4. First published.