A comprehensive analysis of combined stormwater overflow (CSO) discharges to coastal waters and data communications by a UK water company between 2021 and 2023

Sewage discharges to aquatic environments present a real danger to human and ecosystem health. Event duration monitors (EDMs) from combined stormwater overflows (CSOs) are now fitted to over 90% of storm overflows in England and Wales. These have transformed our understanding of consented and non-consented discharges of sewage and wastewater from UK water companies. In 2018, Southern Water Services Ltd launched “Beachbuoy” which is an online ‘near’ real-time platform to inform customers when EDMs have been activated at particular CSOs and bathing water sites. Since April 2022, this water company categoried CSO discharges as genuine, genuine but non-impacting, and not genuine (false alarms by EDMs). We analyzed Beachbuoy data to provide an overview of CSO discharges and EDM activity and performance in the region. Across all assets, between December 2020 and February 2023 there were 7,164,656 genuine (impacting and non-impacting) minutes of discharges of which 19% overall were regarded as non-impacting of bathing water locations. Non-impacting discharges from all assets often persisted beyond multiple tidal cycles suggesting the impacts on bathing waters may need to be reevaluated. Discharges classed as ‘not genuine’ (false alarms) were highly variable between CSO for which some recorded false discharges 100% of the time. There were very strong correlations between the triggering of genuine and not genuine discharges and time of the day. Overall, 39% of all total minutes discharged and 14% if discharge events were classified as not genuine. Sewage releases from CSOs were more likely to happen between 7-10 am indicating that earlier morning patterns in human behaviours are substantially impacting the infrastructure's ability to tackle increased capacity in the system through precipitation. We discuss the appropriateness of classifying sewage discharges as ‘non-impacted’ and whether data should also be obtained on false negative discharges (EDMs not activating) as well as false positive discharges (not genuine). We call for better transparency of the data and models used by the water industry and regulation of how this information is presented to the public.