Influent, as opposed to activated sludge, is more suitable for SARS-CoV-2 surveillance in wastewater treatment plants

Wastewater surveillance programs based at wastewater treatment plants (WWTPs) have been widely implemented, becoming a crucial measure for public health. Recently, the scope of monitoring has expanded from influent wastewater to include primary settled solids and activated sludge. The effectiveness of monitoring primary settled solids has been widely validated, but the suitability of activated sludge as a monitoring target remains unclear. In this work, we investigated the total amount distribution coefficients of SARS-CoV-2 RNA in both solid and liquid fractions of influent and biological treatment process in WWTPs. Capitalizing on the strategic timing of policy adjustments in China, we conducted a quantitative analysis of the SARS-CoV-2 monitoring results over a three-month span during the first large-scale COVID-19 outbreak from three WWTPs in Hefei city, China. Importantly, in the monitoring of activated sludge, we observed a significant delayed effect, with the viral peak occurring 1 to 2 weeks later than in the influent. In addition, we also reveal a significant correlation between the abundance of SARS-CoV-2 in influent and urban resident behaviors, providing novel insights into the pandemic's dynamics. Collectively, this work demonstrates that influent sample is more appropriate for wastewater surveillance compared to sludge sample.