Estimating population served by sewage treatment works from readily available GIS data

Environmental risk assessment of household chemicals at the catchment scale requires an estimate of the load at individual Sewage Treatment Works (STWs). This can be achieved based upon population served and market consumption data. Although the population served is difficult to obtain, this paper shows that reasonable estimates can be made using readily available spatially referenced data. A new method is developed using STW data from the Exe and the Aire and Calder catchments and validated using 193 STWs within the Environment Agency's Anglian region. The estimated populations served were compared with available estimates of Population Equivalents (PEs). The population estimates were broadly similar to PEs for small works but agreement was lower for larger plants. The discrepancy for larger works is consistent with trade influent inclusion in the PE. The method is suitable for application to both rural areas and large urbanised areas, although the interpretation of corroborating data becomes increasingly difficult in very large urban areas serving more than one STW.