Environment Agency investigation into sewage treatment works

The Environment Agency’s national investigation into potential breaches of permit conditions at wastewater treatment works by water and sewerage companies.

On 18 November 2021, the Environment Agency and Ofwat announced separate major investigations into potential widespread non-compliance by water and sewerage companies at wastewater treatment works (WWTW).

The Environment Agency is now investigating more than 2,200 WWTW that discharge into English waters. This relates to all water and sewage companies.

The Environment Agency’s initial assessment indicates that there may have been widespread and serious non-compliance with the relevant regulations by all water and sewerage companies.

When the Environment Agency completes their investigation, they will consider all the options available under their enforcement and sanctions policy. As with all criminal investigations, they need to obtain and secure evidence to make sure they can prove their case beyond reasonable doubt if they begin court proceedings. This takes time and care.

Published information about the investigation will be limited to avoid prejudicing the investigation or any potential legal proceedings.

Updates about the investigation

The Environment Agency published updates in:

The Ofwat website gives information about their investigation.

Flow to full treatment and how storm overflows at WWTWs work

What are storm overflows?

Flow to full treatment refers to the level of rain and wastewater, or flow, that a WWTW must treat before it is permitted to discharge excess flows to storm tanks or the environment. A WWTW must treat all flows up to a threshold specified in permits.

Storm overflows at WWTWs are permitted to discharge excess flows either:

  • directly to the environment
  • through storm tanks when the volume of rain and wastewater is greater than the treatment works can cope with

This is necessary to prevent flooding of properties and infrastructure by allowing excess flows to be discharged in wet weather. Discharges are permitted during storm conditions because the receiving waters will be in enhanced flow due to the rainfall. This ensures that there is enough dilution in any overflow to minimise the environmental impact of a discharge.

Storm overflows must only be used under strict permitted conditions that control their environmental impact.

It is the responsibility of water and sewerage companies to comply with their permit conditions for storm overflows. These permits make it clear that they should only use storm overflows when the flow to full treatment threshold has been reached due to rainfall or snow melt, where the capacity of sewer system could be overwhelmed.

If there is a breach, water companies should tell the Environment Agency. Failure to do this, or to comply with the other conditions of the permit, is a breach of permit and potentially an offence.

Ofwat also explain flow to full treatment on their website.

Storm overflows discharge reduction plan

In August 2022, the government released the storm overflows discharge reduction plan which set new targets for water companies to improve all storm overflows. This will require companies to deliver the largest infrastructure programme in water company history.

This means that water companies will have to ensure a progressive reduction in the adverse impacts of discharges from storm overflows to protect people and the environment.

This plan does not impact the Environment Agency’s ongoing criminal investigation.

Water and sewerage company performance

Annual reporting on the environmental performance of the 9 water and sewerage companies, including the Environmental Performance Assessment (EPA).

Updates to this page

Published 17 November 2022
Last updated 10 June 2024 + show all updates
  1. In the 'updates' section, added a link to the blog post 'Anglian Water Services Ltd convicted in case brought by Environment Agency'.

  2. Added a link to the November 2023 update on Environment Agency investigation.

  3. Added link to 'Water and sewerage companies in England: environmental performance report 2022'.

  4. Added a link to June 2023 blog post, 'Environment Agency investigation into sewage treatment works moves to next phase'.

  5. Added link to November 2022 update on Environment Agency investigation.

  6. First published.