Population normalization with ammonium in wastewater-based epidemiology: Application to illicit drug monitoring

Fluctuations in ammonium (NH₄⁺), measured as NH ₄-N loads using an ion-selective electrode installed at the inlet of a sewage treatment plant, showed a distinctive pattern which was associated to weekly (i.e., commuters) and seasonal (i.e., holidays) fluctuations of the population. Moreover, population size estimates based on NH₄-N loads were lower compared to census data. Diurnal profiles of benzoylecgonine (BE) and 11-nor-9-carboxy-δ9-tetrahydrocannabinol (THC-COOH) were shown to be strongly correlated to NH₄-N. Characteristic patterns, which reflect the prolonged nocturnal activity of people during the weekend, could be observed for BE, cocaine, and a major metabolite of MDMA (i.e., 4-hydroxy-3- methoxymethamphetamine). Additional 24 h composite samples were collected between February and September 2013. Per-capita loads (i.e., grams per day per 1000 inhabitants) were computed using census data and NH₄-N measurements. Normalization with NH₄-N did not modify the overall pattern, suggesting that the magnitude of fluctuations in the size of the population is negligible compared to those of illicit drug loads. Results show that fluctuations in the size of the population over longer periods of time or during major events can be monitored using NH₄-N loads: either using raw NH₄-N loads or population size estimates based on NH ₄-N loads, if information about site-specific NH₄-N population equivalents is available.