Combined Sewer Overflows

Thank you for contacting me recently regarding combined sewer overflows.

I have received the following response from Mr Steve Wilson, Managing Director for Waste Water Services with Dŵr Cymru. 

Thank you for your email regarding the operation of our wastewater assets that discharge in the vicinity of the Benllech, Benllech-St. Davids and Rhosneigr bathing waters in Anglesey. The details have been passed to me to respond as Managing Director for Waste Water Services.
Firstly, it is important to highlight the scale of investment we have made on behalf of our customers over the last 20 years, to improve and protect coastal waters. This has allowed Wales to stop continuous crude sewage discharges to our coastal waters and led to Wales having some of the best bathing waters in the UK. Benllech, Benllech-St. Davids and Rhosneigr were classified as “Excellent” in both 2018 and 2019 which is the highest standard available and sufficient to qualify for a Blue Flag award if the local authority chooses to apply for it. 
In your query you refer to concerns from your constituents regarding “discharges” in the vicinity of the bathing waters and the Surfers Against Sewage campaign. I believe you are referring to discharges from combined sewer overflows (CSOs) in the area rather than the continuous discharges from our local sewage treatment works. CSOs are an important feature of a combined sewer network which has to carry both foul sewage and rainfall run-off. During wet weather the volume to be carried by these sewers increases as rainfall runs off impermeable surfaces such as roofs and roads. If the rainfall is particularly heavy there is a risk that sewage could back up in the sewer and flood customers’ homes. CSOs help to prevent this by acting as “relief” points, allowing dilute sewage to discharge safely to the environment when it will do the least harm and they provide very important protection for our customers. 
However, CSOs can have a negative impact on bathing and shellfish waters. Consequently, our investment programmes have included measures to reduce the frequency with which our CSOs need to operate in locations where they have been shown to have an impact on water quality. We have also invested in measures to reduce their impact when they do operate, such as screening to remove gross solids. 
More recently we have become industry leaders in undertaking sustainable approaches to managing the amount of rainfall running off to our networks through our “RainScape” programme. We have also installed event duration monitoring (EDM) on virtually all our CSOs to measure how frequently they operate and we publish the results annually on our website (see We are also assessing the impact of our most frequently operating inland CSOs through a new storm overflow assessment framework and have secured more than £30m investment for improving the performance of the highest priority sites.
In addition to improvements to our assets we have undertaken an extensive programme of coastal modelling, to identify locations where further improvements to our sewage treatment works and CSOs could improve bathing or shellfish water quality sufficiently to justify the investment needed. The modelling has also highlighted locations where the main challenge to water quality comes from diffuse pollution sources, such as agriculture, allowing NRW to consider measures that would help to reduce those sources of pollution too.
Overall we are working towards minimising, as much as reasonably possible, discharges from our CSOs. However we have to balance the level of protection we provide our customers with those of the environment. This balance is becoming increasingly difficult to achieve as the combination of climate change and growth bring more intense rainfall events and greater flooding risks that our networks have to cope with. 
Turning to the bathing waters highlighted by your constituents, we have looked into the particulars of this query and checked the performance of the CSOs that discharge in the vicinity of the bathing waters above. These concluded that the only intermittent discharge that could have an impact on the water quality for Benllech and Benllech-St.Davids is the storm tank located at Pentraeth wastewater treatment works. As recommended in the recent investigation, its performance is monitored to make sure it continues to provide protection to the bathing water. Whilst this is the only CSO that could have a real impact on the quality of these bathing waters, we included two other assets, Benllech wastewater treatment works and Wendon Car Park Sewage pumping station in the investigation. The assessment concluded that the effect from these two sites on the bathing waters is negligible. With regards to Rhosneigr, there is only one CSO that discharges near the bathing water. This CSO discharged only once in 2020 after a period of very heavy rainfall. 
The degree of protection provided by these CSOs is backed up by NRW’s bathing water sampling programme. This shows that not only are all three waters classified as “Excellent”, the trend of annual mean concentration of indicator bacteria used to assess bathing water quality has been reducing consistently for the last five years.
In conclusion, I understand the concerns raised by your constituents and that is the reason why the company has an investment programme in place to minimise the frequency and impact of our most significant CSO discharges. However I have to balance this desire against the need to provide continued protection for customers from sewage flooding by maintaining and improving the resilience of our sewerage network.’ 

I am proud that our beautiful bathing waters are consistently classifying as ‘Excellent’ in the water sampling programme, and I am glad that the CSO’s are being constantly monitored to ensure the safety of our waters for both us to bathe and wildlife to strive. 

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