Assessing long-term specific pollutant loads in municipal wastewater plants influent using monitoring data: A case study from Latvia

ABSTRACT

Diagram showing how population, weather and urban activities influence wastewater composition, where parameters BOD5, COD, N, NH4-N, P and PO4-P are analysed to support wastewater-based epidemiology normalization.

Wastewater-based epidemiology uses data from wastewater treatment plants to monitor the health of a population, but requires accurate normalization to provide per-capita, or individual-based, health indicators. This study assessed the long-term stability and suitability of key wastewater parameters for population normalization using data from 45 treatment plants across Latvia over a ten-year period, representing two-thirds of the national population. Specific pollutant loads were calculated for biochemical oxygen demand, chemical oxygen demand, total nitrogen, ammonium nitrogen, total phosphorus, and orthophosphate. Overall, pollutant loads remained stable during the study period, though an increase was observed in recent years, with ammonium rising by 27%, total nitrogen by 15%, and phosphorus by 11%. Industrial plants consistently showed higher pollutant loads than municipal facilities. Nitrogen exhibited the lowest variability among the evaluated hydrochemical parameters, suggesting its suitability as a robust normalization indicator within the studied dataset, while chemical oxygen demand can serve as a supporting indicator. In contrast, biochemical oxygen demand and phosphorus were less suitable due to operational sensitivity and high variability. The study recommends using rolling, plant-specific median values over two to three years to enhance the precision and comparability of population-normalized wastewater-based health assessments.