Guidance

Water companies: water treatment works discharge limits for environmental permits

Published 8 May 2018

Water treatment works (WTW) discharge effluent from a range of activities associated with the production of drinking water. Discharges include:

• filter backwash water
• settled sludge supernatant liquors
• lagoon discharges
• raw water
• treated water that cannot be put into supply – such as during treatment start up or if it’s outside specification

These discharges may be continuous or intermittent.

The Environment Agency sets numeric limits on effluent discharges by taking into account the:

• process involved and the substances the discharge is likely to contain
• environmental quality standard (EQS) of substances in the receiving water
• sensitivity of the receiving water – such as nearby drinking water abstractions
• use of the receiving water
• cost

We apply permit conditions to limit the impact of relevant substances, such as:

• chlorine
• aluminium
• polyelectrolytes
• suspended solids – including granulated activated carbon (GAC) fines
• iron

We may apply permit limits for other substances to reflect changes in process, technology or site-specific issues. For example, to protect surface waters designated as nitrate-sensitive areas or groundwater within Safeguard Zones, we may use site-specific nitrate and chloride standards to control effluent from nitrate removal plants.

Limits on discharge volume

Your permit will include an effluent flow limit.

Together the flow and the quality limits control the load of substances that you can discharge in the effluent.

Volume limits are expressed as total daily discharge volumes in cubic metres per day or as maximum discharge flow rates as litres per second, or both.

Set limits to prevent deterioration

We do not normally permit a new or increased discharge if it’s likely to cause a significant deterioration in water quality.

When we assess permit applications and calculate permit limits we check whether the discharge is ‘liable to cause pollution’. A discharge is classed as being ‘liable to cause pollution’ if it:

• risks breaching an EQS in the receiving water
• results in a deterioration of more than 10% of the EQS in receiving water quality

Find out how to carry out screening tests to check the risk from hazardous pollutants to the environment.

For discharges to surface water we take into account the EQS when we set numeric discharge quality limits. Where there are no statutory EQS, we use the targets set out in this guide to protect aquatic life and groundwater quality. Read the guide on how we assess compliance against site-specific quality numeric permit limits.

For discharges to groundwater we take into account our ‘prevent or limit’ objective when we set numeric discharge quality limits. We will only issue a permit if your discharge meets our ‘prevent or limit’ objective. Read the guide on how to prevent hazardous substances and limit non-hazardous pollutants entering groundwater.

Chlorine discharge limits to surface water

Chlorine is classed as a ‘specific pollutant’.

We only include chlorine limits in permits for discharges that are likely to contain chlorine. Discharges that are likely to contain chlorine include:

• discharges of water treated for supply
• filter backwash that uses chlorinated water
• discharges mixed with another discharge that may include chlorinated water

For inland surface waters the EQS is:

• an annual average limit of 2 micrograms per litre (μg/l)
• a 95-percentile limit of 5μg/l

We set limits as maximum limits for total available chlorine.

We assume there’s no chlorine in the upstream receiving water, unless there’s another input immediately upstream of your discharge. If appropriate, we also take the rapid breakdown of chlorine into account.

The current detection range of chlorine is 20 to 50μg/l. Where at least 10 times dilution of the effluent is available, we may be able to set a discharge limit concentration that can be measured.

Where less dilution is available, we set a limit based on the sensitivity and uses of the receiving water, not on the detection range of chlorine.

If you treat the effluent with chemical de-chlorination before discharge, you must avoid overdosing. These reducing agents can have significant adverse impacts on the receiving water such as de-oxygenation or lowering the pH.

Aluminium discharge limits to surface water

Aluminium is acutely toxic to fish.

For discharges to receiving waters with a lower 95-percentile pH value greater than or equal to 6, the EQS is a maximum allowable concentration (MAC) of 1mg/l (total).

However, we will also apply an emission standard of 10mg/l (dissolved) as a maximum concentration in the effluent if dilution is greater than 1:10.

These standards reduce the possibility of active aluminium occurring downstream of the discharge. They also minimise deposition of particulate aluminium on the bed of a watercourse. At dilutions of less than 10:1 the total aluminium standard also limits dissolved aluminium in the discharge.

Aluminium limits for receiving waters with low pH

Aluminium in receiving waters with a low pH may remain in solution or re‑dissolve. These waters are likely to already contain some dissolved aluminium, so the capacity to accept more will be limited.

We therefore apply different limits for aluminium in discharges to receiving waters that have a lower 95-percentile pH value of less than 6.

For soft waters, where the calcium carbonate concentration is less than 20mg/l annual average, the EQS is a:

• 95-percentile limit of 75μg/l (dissolved)
• MAC of 100μg/l (dissolved)

We will apply an emission standard of 500μg/l (total) as a maximum concentration in the effluent.

For hard waters, where the calcium carbonate concentration is greater than or equal to 20mg/l annual average, the EQS is a:

• 95-percentile limit of 500μg/l (dissolved)
• MAC of 1mg/l (dissolved)

We will apply an emission standard of 1mg/l (total) as a maximum concentration in the effluent.

For receiving waters that already contain aluminium, we consider discharge limits on a detailed, site-specific basis.

Polyelectrolyte discharge limits to surface water

Polyacrylamide polyelectrolytes are used as secondary and sometimes primary coagulants. If your discharge contains polyelectrolytes, it must not exceed the EQS in the receiving water.

Polyelectrolytes are normally kept at concentrations below the EQS by operational control through your management system. The Environment Agency will only set numeric permit limits in exceptional circumstances.

Cationic polyelectrolytes are highly toxic to fish. Acute toxicity occurs at concentrations from 300μg/l. This is lower than the practical chemical analytical detection limit of approximately 1mg/l. If you want to use cationic polyacrylamides, you must provide us with evidence to show why you cannot use alternative coagulants.

For discharges to receiving waters with a lower 95-percentile pH value greater than or equal to 6, the EQS for cationic polyelectrolytes is 0.1 x 24h LC50 for salmonid and cyprinid fish species, as a 95-percentile limit. LC50 is the lethal concentration 50 – this is the concentration of the chemical that kills 50% of the test species population.

Where dilutions are greater than x30, we consider the standards on a case-specific basis.

Anionic and non‑ionic polyacrylamide polyelectrolytes are significantly less toxic than cationic forms. Acute toxicity occurs at concentrations from 50mg/l to 100mg/l. They have a detection limit of approximately 1mg/l. We prefer anionic and non‑ionic polyelectrolytes particularly where water hardness and pH are low, because polyelectrolyte activity will last longer in these conditions.

For discharges to receiving waters with a lower 95-percentile pH value greater than or equal to 6, for anionic and non-ionic polyelectrolytes the EQS is a:

• 95-percentile limit of 7.5mg/l
• 0.1 x 24h LC50 for salmonid or cyprinid fish

We may relax the EQS on a case-by-case basis. You will need to provide us with toxicity data and information on analytical methods.

Polyelectrolyte limits for receiving waters with low pH

We apply different limits for polyelectrolytes in discharges to receiving waters that have a lower 95-percentile pH value of less than 6.

For cationic polyelectrolytes we adopt a site-specific approach while the analytical detection limit remains greater than 0.1x 24h LC50 for salmonid and cyprinid fish.

For anionic and non-ionic polyacrylamide polyelectrolytes, we apply an EQS of 3.5mg/l as a 95-percentile limit in soft acid waters. Soft acid waters are waters with pH 6 or less, and a total calcium carbonate concentration of less than 20mg/l annual average.

For discharges to hard waters with a calcium carbonate concentration of equal to or greater than 20mg/l annual average, we set a 95-percentile limit EQS of 7.5mg/l.

Alternatively, we may use an LC50 limit for salmonid or cyprinid fish. For soft waters we set 0.05 times an actual 24h LC50 for the polyelectrolyte as a 95-percentile EQS. For hard waters we set 0.1 times an actual 24h LC50 for the polyelectrolyte as a 95-percentile EQS.

Suspended solids discharge limits to surface water

The Environment Agency has a default permit standard for suspended solids of 100mg/l.

We may set tighter limits when the receiving waters have poor dilution and there are aesthetic or environmental problems.

We may relax the standard if there are no environmental or aesthetic needs, and achieving the 100mg/l standard would incur additional cost. You will need to provide evidence to show why we should consider a higher suspended solids limit for a discharge.

Remove granulated activated carbon (GAC) fines and residues

You must take steps to remove fines and residues of GAC from discharges to protect the receiving water and prevent pollution. To do this, you can collect initial backwash waters and:

• treat it on site
• dispose of it off site
• discharge it to the foul sewer

You can agree other approaches with us to make sure the effluent is an acceptable quality.

Your process backwash waters are subject to routine permit controls (such as management conditions, flow and sampling requirements) and the appropriate numeric discharge quality limits.

Iron discharge limits to surface water

The EQS for dissolved iron is 1mg/l as an annual average.

For new discharges, we set maximum limits for total iron that prevent deterioration.

For existing discharges, we set the maximum total iron limit so that the receiving waters meet the EQS.

Sediment concentrations above 90mg/kg dry weight total iron can have a negative effect on the environment. Where iron particle deposition is a problem we may apply a MAC of 90mg/kg iron, dry weight in sediment.