Decomposition of wastewater sources in England

The flow arriving at a wastewater treatment plant mixes municipal sewage, rainfall, soil moisture and groundwater, yet is routinely interpreted as if its composition were fixed. Despite the uncertainty this introduces to any case where the signal is used, from combined sewer overflow management to wastewater-based epidemiology, source-resolved composition is relatively underexplored. Here we introduce WWCompose, an empirical-conceptual model that recovers the dynamic contributions of rainfall, soil moisture and groundwater to daily influent, and apply it to approximately 2900 treatment plants across England using open national data. Predicted flows match observations out of sample (median held-out Kling–Gupta efficiency 0.64; 60% of plants above 0.6), with fitted parameters organising around hydrogeology, permeability and urbanisation. Non-municipal water is widespread: municipal flow averages 61% nationally, and at individual works may vary from 100% in dry weather to <5% during the wet conditions that often dominate periods of interest.