OEP publishes reports on investigation into regulation of Combined Sewer Overflows

The Office for Environmental Protection (OEP) has today published reports on its investigation into the regulation of combined sewer overflows by Defra, the Environment Agency and Ofwat.  

After announcing back in July that it was to conclude its investigation after securing significant progress in tackling the issues it identified, the OEP has now published detailed reports setting out its findings. These reports also include additional recommendations to further strengthen regulation and prevent reoccurrence of the failures identified.  

The OEP investigation found that there had been failures to comply with environmental law by each of the three public authorities relating to the regulation of network CSOs. Decision notices were issued to them setting out the actions required to address the non-compliances. The OEP welcomed responses from them which showed positive action to address the concerns raised and significant progress being made.  

Helen Venn, the OEP’s Chief Regulatory Officer, said: “We are confident that as a result of our investigation future reforms relating to regulation of untreated sewage discharges can be based on a clearer understanding of what the law requires and the current roles and responsibilities of the different authorities.

“There is now clarity that the regulatory system should only allow untreated sewage discharges to take place in exceptional circumstances, unless the cost of preventing or mitigating them would be excessive.

“Progress has been demonstrated on cross-cutting issues, such as the clarification of roles and responsibilities between the authorities and improvements to the assessment of cost-effective solutions for reducing CSO discharges.

"Many of the actions that we said were necessary have now been taken, are being progressed, or commitments are in place to do so. Significant progress has been made.  

“We set out the detail of our work and what we found in the three reports – one for each of the authorities - published today. This does not mean that our interest in this important issue has now ended. We take this opportunity to make a number of further recommendations, and we will continue to keep watch to ensure that promised actions are delivered.”

Steps taken by the authorities in response to the OEP’s recommendations include:  

• Defra has replaced outdated guidance with a new policy and guidance document that better reflects the legal duties of sewerage undertakers.
• Ofwat has revised its enforcement guidance and taken enforcement action against several water companies.  
• The Environment Agency has updated its Storm Overflow Assessment Framework and is progressing with plans to modernise permits and introduce spill limits.

The investigation was launched in response to a complaint received by Salmon & Trout Conservation UK, now known as WildFish.  

ENDS

The OEP press release announcing the conclusion of the investigation in July 2025 can be found here.

Additional recommendations for the public authorities included in the reports published today include:  

Defra should:  

1. Establish a process to monitor implementation of the new Storm Overflows Guidance and incorporate this into the five yearly statutory review of the Storms Overflow Discharge Reduction Plan (SODRP).  
2. Monitor the implementation of the SODRP requirements through Environment Agency permitting.  
3. Together with the Environment Agency and Ofwat, review the MoU in line with the SODRP review cycle and any reforms arising from the Independent Water Commission.

The Environment Agency should:

1. Update its document “Water companies: environmental permits for storm overflows and emergency overflows” 13 September 2018 (the ‘September 2018 Guidance’) to properly and adequately set out the revised approach to regulating CSOs following updates to the Storms Overflow Assessment Framework (SOAF), spill frequency threshold permitting and Defra’s 1997 Guidance.  
2. Review the SOAF 2025 every five years, or earlier if appropriate.  
3. Together with Defra and Ofwat, review the MoU 12 months after the date of signature, or earlier if required.
4. Establish a robust and transparent methodology for consistent data collection and reporting. This framework should enable clear tracking of progress for each CSO and provide structured summaries at each stage of the SOAF process. It must also support accurate reporting of BTKNEEC assessment outcomes, ensuring that data is both reliable and comparable across submissions.
5. Use better data to regularly monitor and review the proportion of CSOs that are not progressed for improvement following BTKNEEC assessments.  
6. Strengthen its regulatory oversight of BTKNEEC assessments and their outcomes. Greater scrutiny is needed to ensure that, in cases where CSOs are not improved, the concept of excessive costs is being correctly applied through robust cost-benefit analysis.

Ofwat (or any subsequent authority) should:

1. Together with Defra and the Environment Agency, review the storm overflows MoU 12 months after the date of signature, or earlier if required.
2. Collaborate with the Environment Agency on the above data collection and reporting methodology. 

The investigation reports can be found here.