Testing methods to estimate population size for wastewater treatment plants using census data: Implications for...

Area of Science:

• Environmental science
• Public health
• Geospatial analysis

Background:

• Wastewater-based epidemiology (WBE) relies on per capita wastewater load calculations.
• Accurate population normalization is essential for WBE, often using census data.
• Methods for calculating WWTP population size from census data are inconsistently reported in WBE literature.

Purpose of the Study:

• To compare four geospatial methods for calculating WWTP population size using census data.
• To assess the impact of different methods on population estimates across New Zealand.
• To identify the most reliable method for population normalization in WBE.

Main Methods:

• Applied four geospatial methods to combine wastewater catchment areas and census units nationally.
• Disaggregated census area units by individual properties for a refined population calculation.
• Compared census-derived population estimates with those from wastewater operators.

Main Results:

• Population estimates varied by up to 73% between methods, potentially skewing WBE comparisons.
• Variability in population estimates was significantly higher in smaller catchments.
• Census estimates showed broad agreement with operator data but with notable variations for some sites.

Conclusions:

• Method selection significantly impacts population estimates in WBE, particularly for smaller WWTPs.
• A property-disaggregation method offers a widely applicable approach for accurate population normalization.
• Transparent reporting of population calculation methods is vital for WBE study comparability and reliability.