Modeling in-sewer transformations at catchment scale – implications on drug consumption estimates in wastewater-based epidemiology

Abstract

To which extent illicit drugs are transformed during in-sewer transport, depends on a number of factors: i) substance-specific transformation rates, ii) environmental conditions, iii) point of discharge (location of drug user) and iv) sewer network properties, primarily hydraulic residence time (HRT) and the ratio of biofilm contact area to wastewater volume (A/V). Assessing associated uncertainties typically requires numerous simulations. Therefore, we propose a new two-step modeling framework: 1) Quantify hydrodynamic conditions. This computationally demanding step was performed once in SWMM to derive HRT and A/V for each potential point of discharge (node) in three catchments of different size. 2) Estimate biomarker loss. In this step, Monte Carlo simulations were performed for defined scenarios. Depending on assumptions about drug user distribution and prevalence, a number of nodes was sampled. For each node an empirical first-order transformation model was applied with flow-path-corresponding HRT and A/V from step 1. Biotic and abiotic transformation rates were sampled from distributions combining variability of different biofilms. In our modeling study, median losses were >30% for amphetamine, 6-monoacetylmorphine and 6-acetylcodeine in all three catchments with high uncertainty (5%-100% loss), which would imply a systematic underestimation of consumption when neglecting in-sewer processes. Median losses for 21 other investigated biomarkers were <10% with different uncertainty ranges - "no substantial transformation" was confirmed for nine substances in a real sewer segment with a 2-h residence time. Transferability of these results must be tested for other catchments. To further reduce uncertainty, mainly additional knowledge on transformation rates, particularly in biofilm, and their distribution across a sewer network is needed to update model input objectively. Our approach allows efficient testing and, furthermore, can be expanded for many other human biomarkers. Accounting for biomarker stability during in-sewer transport will avoid biased estimates and further improve wastewater-based epidemiology.

Transformation and Sorption of Illicit Drug Biomarkers in Sewer Biofilms

“…Although this study compared the biotransformation rate in raw wastewater and in biofilms, the contribution of each of these biotransformation processes to the overall drug biomarkers removal should be assessed. Moreover, the assessment of drug abuse rates at catchment level should account for, importantly, the layout of the sewer system, hydraulic conditions 49 and drug release patterns. Additionally, model-based back-calculation tools should account for abiotic processes and biotransformation induced by suspended biomass and sewer biofilms.…”

Experimental Investigation and Modeling of the Transformation of Illicit Drugs in a Pilot-Scale Sewer System

“…[2][3][4][5] In order to improve the accuracy of back-estimation, researches have been widely conducted to address the uncertainties associated with sampling method and chemical analysis, [5][6][7][8][9] while a comprehensive understanding of biomarkers stability in real sewers is still ongoing. [10][11][12][13][14][15][16][17] Biomarkers are subjected to physiochemical and biological processes during their transport in rising main and gravity sewer pipes, where the hydraulic retention time (HRT) may last for hours. 18 Neglecting the biomarkers transformation (e.g., the degradation or formation in sewers) will lead to an under-or over-estimation of drug consumption in a catchment.…”

Biotransformation of micropollutants in sewers

“…Furthermore, the rapid conversion of VFAs acetate, propionate, butyrate and valerate indicate the activity of heterotrophic microbial communities in the GS pipes, such as sulfide-oxidising bacteria.The GS biofilm activity in full-scale differed significantly from the laboratory-scale (see chapter 5.1.1), likely due the continuous flow and reduced contact time of the sewage with the biofilm in full-scale. The impact of contact time has been previously highlighted by McCall et al86 Although the substrate conversion rates in the laboratory-scale reactor were in accordance with reported values in literature (see chapter 5.1.1), the significantly different substrate conversion rates for sulfide to those in the full-scale GS suggest the population of different microbial communities in both setups.…”

“…Since then, more elaborate sampling campaigns and modelling approaches were published to account for the in-sewer biotransformation of human biomarkers. 15,86,87 Commonly, these studies include selected benchmarking compounds, such as the stable carbamazepine, diclofenac and tramadol and the easily degradable caffeine and acetaminophen, to allow the comparison of biofilm activity between studies. 88 However, the focus of these studies was to assess the suitability of substances as human biomarkers for WBE.…”