Somatic coliphages are conservative indicators of SARS-CoV-2 inactivation during heat and alkaline pH treatments

Varbanov, M.; Bertrand, I.; Philippot, S.; Retourney, C.; Gardette, M.; Hartard, C.; Jeulin, H.; Duval, R. E.; Loret, J.-F.; Schvoerer, E.; Gantzer, C.; Bertrand, Isabelle; Gantzer, Christophe; Le Guyader, Soizick; Maday, Yvon; Maréchal, Vincent; Mouchel, Jean-Marie; Moulin, Laurent; Boni, Mickael; Wurtzer, Sébastien

High concentrations of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genome have been described in wastewater and sewage sludge. It raises the question of the security of land sludge disposal practices during a pandemic. This study aimed to compare SARS-CoV-2's resistance to the main inactivating factors in sludge treatments, pH and heat, to that of native wastewater somatic coliphages. The latest can be easily used as an indicator of treatment efficiency in the field. The effects of heat treatment and pH on the survival of SARS-CoV-2 and somatic coliphages were investigated in simple media. The T90 value (time required for a 90% reduction in the virus or a 1 × log10 decline) at 50 °C was about 4 min for infectious SARS-CoV-2, and around 133 min for infectious somatic coliphages, with no decrease in SARS-CoV-2 genome. For infectious SARS-CoV-2, a slight decrease (<1 log10 unit) was observed at pH 9 or 10 for 10 min; the decrease was over 5 log10 units at pH 11. However, both SARS-CoV-2 genome and infectious somatic coliphages decreased by less than 1 log10 unit at pH 12. All thermal or pH-based treatments that can remove or significantly reduce infectious somatic coliphages (>4 log10) can be considered efficient treatments for infectious SARS-CoV-2. We concluded that somatic coliphages can be considered highly conservative and easy to use indicators of the inactivation of SARS-CoV-2 during treatments based on heat and alkaline pH.