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With the rapid onset of the COVID-19 pandemic, wastewater-based epidemiology sampling methodologies for SARS-CoV-2 were often implemented quickly and may not have considered the unique drainage catchment characteristics. This study assessed the impact of grab versus composite sampling on the detection and quantification of SARS-CoV-2 in four different catchment scales with flow rates ranging from high flow (wastewater treatment plant influent) to medium flow (neighborhood scale) to low-flow (city block scale) to ultralow flow (building scale). At the high-flow site, grab samples were comparable to 24 h composite samples with SARS-CoV-2 detected in all samples and differed in concentration from the composite by <1 log 10 unit. However, as the size of the catchment decreased, the percentage of negative grab samples increased despite all respective composites being positive, and the SARS-CoV-2 concentrations of grab samples varied from those of the composites by up to almost 2 log 10 units. At the ultra-low-flow site, increased sampling frequencies generated composite samples with higher fidelity to the 5 min composite, which is the closest estimate of the true SARS-CoV-2 composite concentration that could be measured. Thus, composite sampling is more likely to compensate for temporal signal variability while grab samples do not, especially as the catchment basin size decreases.

With the rapid onset of the COVID-19 pandemic, wastewater-based epidemiology (WBE) sampling methodologies for SARS-CoV-2 were often implemented quickly and may not have taken the unique drainage catchment characteristics into account. One question of debate is the relevance of grab versus composite samples when surveying for SARS-CoV-2 at various catchment scales. This study assessed the impact of grab versus composite sampling on the detection and quantification of SARS-CoV-2 in catchment basins with flow rates ranging from high-flow (wastewater treatment plant influent), to medium-flow (neighborhood-scale micro-sewershed), to low-flow (city block-scale micro-sewershed) and down to ultra-low flow (building scale). At the high-flow site, grab samples were reasonably comparable to 24-h composite samples with the same non-detect rate (0%) and SARS-CoV-2 concentrations that differed by 32% on the Log10 scale. However, as the flow rates decreased, the percentage of false-negative grab samples increased up to 44% and the SARS-CoV-2 concentrations of grab samples varied by up to 1-2 orders of magnitude compared to their respective composite sample concentrations. At the ultra-low-flow site, increased sampling frequencies down to every 5 min led to composite samples with higher fidelity to the SARS-CoV-2 load. Thus, composite sampling is superior to grab sampling, especially as flow decreases.SynopsisThe need for composite sampling to generate reliable SARS-CoV-2 wastewater based epidemiology results increases as the collection basin scale decreases.Table of Content Art