Socio-spatial characterization of sub-sewersheds for wastewater-based epidemiology (WBE): Developing and evaluating two estimators for population-related variables

Wastewater-based epidemiology (WBE) offers valuable population-level health insights. Sampling within sewer systems enables small-scale differentiation, but requires accurate local population data. Allocating population data to sewersheds is complicated by mismatched administrative and sewershed boundaries, for which no established method exists. This study introduces and evaluates two methods for estimating sub-sewershed populations: Proportional Building-based Population Estimation (PBPE) and the Spatial Grid Population Estimation (SGPE). Both were compared with the reported population figures and the result of a simple overlay procedure. The methods were then applied to characterize populations in 195 sub-sewersheds in a large German metropolitan area. PBPE reallocates population data by weighting intersecting city sub-units based on the number of residential buildings, while SGPE uses inverse-distance weighting to interpolate population density based on residential land use across a hexagonal lattice. Despite different data requirements and methodologies, both estimators produced highly concordant results with strong correlations between estimated and reported population numbers (Pearson r: 0.97 for both) and between estimates for all population-related parameters (Pearson r range: 0.89-0.95). Median deviations were -0.6% (PBPE) and +3.3% (SGPE) for total population, and under 5% across age and socio-economic strata. The strong agreement confirms both estimators are reliable. PBPE shows lower deviations but requires detailed building data and is sensitive to missing structures and uniform-occupancy assumptions. SGPE works without exhaustive building footprints but depends on user-selected grid parameters. This study provides an empirical basis for improving small-scale WBE data use and interpretability in densely populated urban areas, facilitating an improved public health database.