Factors influencing recovery of SARS-CoV-2 RNA in raw sewage and wastewater sludge using polyethylene glycol-based concentration method.

Wastewater surveillance for monitoring severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is an important epidemiologic tool for the assessment of population-wide coronavirus disease 2019 (COVID-19). This tool can be successfully implemented only if SARS-CoV-2 RNA in wastewater can be accurately recovered and quantified. The lack of standardized procedure for wastewater virus analysis has resulted in varying SARS-CoV-2 concentrations for the same sample. This study reports the effect of 4 key factors-sample volume, percentage polyethylene glycol (PEG)-NaCl, incubation period, and storage duration at 4°C-on the recovery of spiked noninfectious SARS-CoV-2 RNA in raw sewage and sludge samples. N1 and N2 genes of SARS-CoV-2 were quantified using the reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and digital droplet PCR (RT-ddPCR) techniques. Results indicate that 1) for raw sewage, 50-ml sample volume, 30% PEG-NaCl addition, 6-h incubation, and sample analysis within 24 h of collection can result in much better RNA recovery (RT-qPCR: 72% for N1 and 82% for N2; RT-ddPCR: 55% for N1 and 85% for N2) when compared with commonly used PEG-based method; 2) for sludge, the sample analysis using raw sewage protocol and all other variations of each factor mostly resulted in false negatives for both N1 and N2. The absence of N1 and N2 suggests that sludge samples probably need a pretreatment step that releases RNA entrapped in sludge solids back into bulk solution. In conclusion, our modified PEG-based concentration method can cut down the analysis time at least by half, which in turn helps to implement early detection system for SARS-CoV-2 in wastewater.