SARS-CoV-2 RNA abundance in wastewater as a function of distinct urban sewershed size

Abstract

During the COVID-19 pandemic, wastewater-based epidemiology has emerged as a promising approach for monitoring SARS-CoV-2 prevalence on a community-level. Despite much being known about the utility of making these measurements in large wastewater treatment plants, little is known about the correlation with finer geographic resolution, such as those obtained through sewershed sub-area catchments. This study aims to identify community wastewater surveillance characteristics between sewershed areas that affect the strength of the association of SARS-CoV-2 RNA detection in a metropolitan area. For this, wastewater from 17 sewershed areas were sampled in Louisville/Jefferson County, Kentucky (USA), from August 2020 to April 2021 (N = 727), which covered approximately 97% of the county's households. Solids were collected from the treatment plants from November 2020 to December 2020 (N = 42). Our results indicate that the sewersheds differ in SARS-CoV-2 trends; however, high pairwise correlation spatial trends were not observed, and the mean SARS-CoV-2 RNA concentrations of smaller upstream community sewershed areas did not differ from their respective treatment centers. Solid samples could only be collected at treatment plants, therefore not allowing us to evaluate SARS-CoV-2 abundance as a function of the sewershed scale. The population size sensitivity of SARS-CoV-2 concentration detection is non-linear: at low population levels the measures are either too sensitive and generate a high level of variability, or at high population levels the estimates are dampened making small changes in community infection levels more difficult to discern. Our results suggest selecting sampling sites that include a wide population range. This study and its findings may inform other system-wide strategies for sampling wastewater for estimating non-SARS-CoV-2 targets.