Guidance

Water companies: control of chemicals used for dosing at waste water treatment works

Published 8 May 2018

Chemical dosing is used to treat sewage effluent at waste water treatment works (WWTW). It’s used to:

• reduce phosphorus concentration
• correct pH levels
• treat odour
• aid sludge and solids settlement

As a water company, you may be dosing continually or at regular intervals, for example seasonally to minimise the impact of spring sloughing in filter beds. Or you may be carrying out dosing as a one-off activity to maintain permit compliance due to operational issues.

Your environmental permit will include conditions that control the substance you’re using for dosing. Some substances must be controlled by numeric limits.

This guide explains:

• how the Environment Agency controls substances used for dosing
• how we set numeric limits
• when you need to notify us and apply to change your permit

Iron and aluminium dosing

Iron and aluminium salts are typically used to reduce phosphorus concentration in the discharge.

Iron is classed as a ‘specific pollutant’ for surface water and a non-hazardous pollutant for groundwater.

Aluminium is toxic to fish. It’s classed as a non-hazardous pollutant for groundwater.

You must notify the Environment Agency in writing at least 14 days before you start dosing using iron or aluminium salts.

You must apply to change your permit if you’re dosing:

• for more than 4 months in a year
• regularly for a shorter period – for example annually to deal with sloughing

If we change your permit to authorise dosing, we’ll add a numeric limit to your permit to control the use of iron or aluminium in the discharge.

You can apply to change your permit to allow dosing using either iron or aluminium salts to give you operational flexibility. We will then set limits for both iron and aluminium in your permit.

You do not need to notify us, or apply to change your permit, if your existing permit contains numeric limits for the chemical and your discharge will stay within those limits.

Dosing upstream of habitats designated sites

You must contact Natural England before you carry out chemical dosing if it may affect a habitats designated site. Habitats designated sites include sites of special scientific interest and Natura 2000 sites. Use Magic maps to check if your site is in or near a designated habitat.

If you’re applying to dose using aluminium upstream of a designated site, you must show why you cannot use iron as an alternative dosing chemical.

Dosing using iron: numeric discharge limits

When you dose with iron for phosphorus removal, we set the permit limit using a calculated 95-percentile total iron limit and 8mg/l as a maximum limit for total iron.

The 95-percentile total iron limit is calculated to meet the 1mg/l dissolved annual average environmental quality standard (EQS) for iron in the receiving surface water. This means the 95-percentile limit allows full use of the dissolved EQS for iron.

This differs to how we normally set limits for hazardous substances where we aim to prevent or minimise any deterioration in water quality. Here we allow a deterioration in the iron concentration in the receiving water up to its EQS because this supports our need to reduce the levels of phosphorus in receiving surface waters.

We apply the 95-percentile limit in permits as a look-up table (LUT) limit for total iron.

We set the 95-percentile limit at between 1mg/l and 4mg/l total iron.

If you’re dosing for any reason other than for phosphorus removal, we will set a site-specific limit. This is also called a ‘river needs’ limit. We’ll set this as a maximum limit in your permit. For discharges to ground we’ll set a maximum limit that meets our ‘prevent or limit’ objective. Read guidance on how to prevent hazardous substances and limit non-hazardous pollutants entering groundwater.

Dosing using aluminium: numeric discharge limits

For all dosing using aluminium we will set a site-specific limit. This is also called a ‘rivers needs’ limit.

For discharges to surface water, we’ll calculate the 95-percentile limit. We’ll set this as a maximum limit in your permit.

For discharges to ground we’ll set a maximum limit that meets our ‘prevent or limit’ objective. Read guidance on how to prevent hazardous substances and limit non-hazardous pollutants entering groundwater.

Dosing to correct pH levels

If you’re dosing to correct the pH of your discharge to protect the receiving water, the Environment Agency will include a pH limit in your permit.

Operational management conditions to control dosing

As well as setting numeric limits in permits, the Environment Agency will also set non-numeric conditions to control chemical dosing.

Choose low impurity dosing agents

Dosing salts may contain toxic impurities such as cadmium, cobalt or mercury. You must use salts that conform to the appropriate British Standards.

You must use the salt with the lowest level of impurities. If you choose to use a salt with more impurities, you must have good reasons why the alternative is not appropriate. For example, there’s a temporary shortage of the dosing agent with the least impurities.

Install telemetry

Where dosing is for phosphorus removal, you must install telemetry on the dosing equipment to warn of failure.

We do not normally include a dosing telemetry condition in permits. However, when you’re chemical dosing at a WWTW you must include adequate procedures in your management system to minimise the risk of pollution from dosing equipment failure.

Sludge blanket detectors

If you carry out second-stage dosing before final settlement, there’s a high risk of failure of the final settlement process. This could lead to a discharge of iron or aluminium-rich sludge into the environment.

We do not normally include a sludge-level detector telemetry condition in permits. However, you must include adequate procedures in your management system to minimise the risk of pollution from dosing.

Polyelectrolyte dosing

Polyelectrolyte dosing agents are used to:

• aid sludge and solids settlement
• remove colour

If you dose with polyelectrolytes, you 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.

Dosing using polyelectrolyte agents: EQS

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

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 the use of anionic and non-ionic polyelectrolytes to cationic polyelectrolytes. This is particularly where water hardness and pH are low, because polyelectrolyte activity will last longer in these conditions.

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 a pH 6 or less, and a total hardness of less than 20mg/l. For waters with a hardness of 20mg/l or more as calcium carbonate, we apply a 95-percentile limit EQS of 7.5mg/l.

Alternatively, we may apply a 0.05 or 0.1 times an actual 24h LC50 for the polyelectrolyte as a 95-percentile EQS. LC50 is the lethal concentration 50 – this is the concentration of the chemical that kills 50% of the test species population.

Compliance limits for other substances

Find out how the Environment Agency sets and assesses compliance against site-specific discharge quality numeric limits in environmental permits.

Read the guidance to find out how to assess the need for numeric limits and control other substances that are present in a WWTW discharge for:

• sanitary and other pollutants to surface water
• hazardous pollutants to surface water
• hazardous substances and non-hazardous pollutants to groundwater
• infiltration systems to groundwater

If you want to use herbicides to control plant growth at your site, such as on filter beds, you may need to apply to use herbicides in or near water.