A Model to Estimate the Population Contributing to the Wastewater Using Samples Collected on Census Day

Abstract

An important uncertainty when estimating per capita consumption of, for example, illicit drugs by means of wastewater analysis (sometimes referred to as "sewage epidemiology") relates to the size and variability of the de facto population in the catchment of interest. In the absence of a day-specific direct population count any indirect surrogate model to estimate population size lacks a standard to assess associated uncertainties. Therefore, the objective of this study was to collect wastewater samples at a unique opportunity, that is, on a census day, as a basis for a model to estimate the number of people contributing to a given wastewater sample. Mass loads for a wide range of pharmaceuticals and personal care products were quantified in influents of ten sewage treatment plants (STP) serving populations ranging from approximately 3500 to 500 000 people. Separate linear models for population size were estimated with the mass loads of the different chemical as the explanatory variable: 14 chemicals showed good, linear relationships, with highest correlations for acesulfame and gabapentin. De facto population was then estimated through Bayesian inference, by updating the population size provided by STP staff (prior knowledge) with measured chemical mass loads. Cross validation showed that large populations can be estimated fairly accurately with a few chemical mass loads quantified from 24-h composite samples. In contrast, the prior knowledge for small population sizes cannot be improved substantially despite the information of multiple chemical mass loads. In the future, observations other than chemical mass loads may improve this deficit, since Bayesian inference allows including any kind of information relating to population size.

Assessing the population equivalent and performance of wastewater treatment through the ratios of pharmaceuticals and personal care products present in a river basin: Application to the River Thames basin, UK

“…The de facto population connected to an STP may not coincide with the determined de jure population 11,13 . It is because the former is population equivalent and accounts for tourist and the difference between daytime and nighttime populations, while the later is based on the latest census.…”

“…More recently, O'Brien et al 13 found the artificial sweetener acesulfame and the anti-epileptic gabapentin effective in estimating the population (the "de facto population") connected to sewage treatment plants (STPs) in Australia on a census day (the "de jure population"). The results revealed that such stable substances can be used as indicators of populations in river basins, as well as of the proportion of treated or untreated wastewater (i.e.…”

Critical review on the stability of illicit drugs in sewers and wastewater samples

“…Numerous efforts have focused on improving the accuracy (trueness and 130 precision) of different analytical methods, and inter-laboratory studies were conducted in order to evaluate and harmonize the different analytical procedures being used (Castiglioni et al, 2013;Thomas et al, 2012). Uncertainties associated with the population estimates have recently been conceptually reduced (Lai et al, 2015;O'Brien et al, 2014), as well as the uncertainty related to wastewater sampling (Ort et al, 2010). Furthermore, sample collection, storage and preparation 135 methods have been evaluated, deducing critical, substance-specific parameters: i) solvent and temperature used during the evaporation of solid-phase extraction (SPE) extracts, and ii) biomarker stability in the wastewater matrix and silanisation of glassware (Baker and Kasprzyk-Hordern 2011a).…”

Sewage-based Epidemiology Requires a Truly Transdisciplinary Approach

“…Ideally, a ubiquitous biomarker could be found that has no other source than human excretion and is stable during transport in sewers (Chen et al 2014). So far, pharmaceutical residues have been successfully combined with Bayesian inference (O'Brien et al 2014). Although transformation of certain drugs during transport in sewers may affect SBE, this is just starting to be investigated (Thai et al 2014).…”