A population equivalent (PE-NHS) for the each WWTP catchment was estimated, by the number of people registered in the general practices (GPs) located inside each WWTP catchment zone. Individual GP information, such as postcode and number of people registered, were obtained from NHS Digital (). WWTPs catchment maps were provided by Wessex Water. Briefly, an R package was used, PrAna and WWTPs catchment maps to identify the GPs located inside each WWTPs catchments. The number of patients registered for these GPs were then calculated using R, an open-source software for statistical computing and graphics.