In recent years it has become increasingly apparent that there needs to be more investment in UK wastewater systems, as the frequency with which raw sewerage is released into the environment increases, due to growing populations and insufficient infrastructure spending [1]. The UK’s wastewater systems combine domestic sewage with surface runoff, so when it rains their capacity is often exceeded. To prevent wastewater backing-up into properties, sewer networks use Combined Sewer Overflows (CSOs), which release untreated wastewater into rivers or coastal waters during periods of high rainfall. The water utilities monitor when CSOs are active, but are unable to monitor the volume or dilution of the spills, so it is not known exactly how much untreated sewerage is being released.
From 2021-23 the Welsh Wastewater Programme used wastewater samples to estimate COVID-19 levels in Wales. This programme was funded by Welsh Government and jointly run by Bangor University, Cardiff University, Dŵr Cymru Welsh Water, and Public Health Wales. COVID-19 levels are estimated by measuring COVID-19 RNA levels in wastewater samples but need to be scaled by the wastewater dilution, which depends on the amount of rainfall runoff. To estimate the dilution we developed a Bayesian model based on auxiliary measurements of electrical conductivity, orthophosphate, and ammonia levels [2, 3].
We also developed a scheme for estimating the proportion of wastewater being released through CSOs. In the figure below, each point represents a day when CSOs were active in the Cardiff sewershed, during the period January 2022 to March 2023. The size of a point gives the total time CSOs were active that day (in seconds). For each point we have the wastewater dilution on the x-axis (from our estimate) and the average rate of flow recorded at the treatment plant on the y-axis (both on a log scale). Note that even when the average flow is below the threshold where CSOs become active, there will still be places and times in the sewer network where the threshold was exceeded, and CSOs were active locally.
From the plot the important features of CSO activity are readily apparent:
- The blue region is where CSOs were active nearly all day, and the observed flow is capped at the maximum capacity of the system.
- The green region is where the system is at maximum capacity, but the wastewater is more concentrated than usual due to the so called “first flush effect”, when accumulated waste is flushed from the system by heavy rainfall.
- The purple region also contains first flush events, but caused by short intra-day spikes in rainfall.
- The pink region shows events that could be partially ameliorated by storm tanks. Storm tanks temporarily store wastewater rather than releasing it through CSOs.
Last but not least, for the points in the blue region we can infer the flow from the dilution, which allows us to get an estimate for the proportion of wastewater released into the environment. The impact of untreated waste on the environment depends on its dilution, its volume compared to the body of water it is released into, and the local ecology. How to balance these to prioritise infrastructure spending to reduce CSO spills is an active problem for water utilities. The interested reader can learn more of the issues involved in the Dŵr Cymru Welsh Water funded project: Addressing the challenges of combined sewer overflows [4].
References
[2] Wilde, H.; Perry, W.B.; Jones, O.; Kille, P.; Weightman, A.; Jones, D.L.; Cross, G.; Durance, I. Accounting for Dilution of SARS-CoV-2 in Wastewater Samples Using Physico-Chemical Markers, 2022. Water 14, p.2885. https://doi.org/10.3390/w14182885
[3] Jones, O.D.; Baldwin, A.J.; Perry, W.B.; Wilde, H.; Durance, I.; Jones, D.L.; Weightman, A. Estimating Wastewater Dilution Using Chemical Markers and Incomplete Flow Measurements: Application to Normalisation of SARS-CoV-2 Measurements, 2024. https://doi.org/10.20944/preprints202402.1109.v1
[4] Perry, W.B.; Ahmadian, R.; Munday, M.; Jones, O.; Ormerod, S.J. and Durance, I., 2023. Addressing the challenges of combined sewer overflows. Environmental Pollution 343, p.123225. https://doi.org/10.1016/j.envpol.2023.123225