Drivers of wastewater dynamics: a statistical analysis of England's large wastewater treatment works

The availability of geospatial and hydrological datasets is creating new opportunities for large-sample analyses of urban drainage and wastewater systems. In parallel, wastewater networks face increasing pressure from limited capacity and the rising frequencies of combined sewer overflow spills. To address these challenges, this study develops the first national-scale dataset of wastewater signatures and sewershed characteristics for England, comprising 951 sewersheds associated with large wastewater treatment works. Multiple linear regression models and random forest models were applied for the first time to identify the dominant influences on daily per capita effluent flow and spills at a national scale. Precipitation emerged as having the dominant impact on daily effluent flow variability and spill occurrence, influencing both high-flow and baseflow conditions. However, precipitation's signal is diminished at a water company-level due to lower spatial variability. In comparison, larger proportions of utilised treatment capacity decreased per capita effluent discharges consistently across regions and scales. The results emphasise the value of combining national and regional approaches to identify more overarching trends and consistent patterns. The dataset and findings provide a foundation for the development of a unique framework for wastewater systems, enabling transferable analyses of sewershed behaviour and the creation of wastewater systems models.