Photodynamic inactivation of phage Phi6 as SARS-CoV-2 model in wastewater disinfection: effectivity and safety

The past two years have been marked by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. This virus is found in the intestinal tract and reaches the wastewater system, and, consequently, the natural receiving water bodies. As such, inefficiently treated wastewater (WW) can be a means of contamination. The currently used disinfection methods of WW can be expensive, lead to the formation of toxic compounds, or be inefficient. So, new alternative approaches must be considered for this matter, namely photodynamic inactivation (PDI). In this work, the bacteriophage Phi6 was used as a surrogate model for the SARS-CoV-2. First, to understand the virus survival in the environment, phage Phi6 was subjected to different laboratory-simulated environmental conditions of temperature, pH, salinity, and solar and UV-B irradiation, and its persistence over time was monitored. Thereafter, the PDI efficiency towards virus was assessed, both in phosphate-buffered saline (PBS), as control aqueous matrix, and in real secondarily treated WW. Then, the safety of WW PDI-treated was assessed through the determination of the viability of native marine water microorganisms after their contact with the PDI-treated effluent, since generally WW is discharged into the marine environment after treatment. The results revealed that phage Phi6 (SARS-CoV-2 surrogate) remained viable in different environmental conditions for a considerable period. Also, PDI was confirmed to be an effective approach in the virus inactivation, and the PDI-treated effluent showed no toxicity to native marine microorganisms under realistic conditions of treated WW dilutions, supporting PDI as an efficient and safe WW tertiary disinfection method. Though the studies were performed with phage Phi6, considered a suitable model of SARS-CoV-2, further studies using the SARS-CoV-2 are needed, nevertheless, the observed results envisage the potential of the PDI to control SARS-CoV-2 in WW.

Acknowledgments: Thanks are due to the University of Aveiro and FCT/ MEC for the financial support to LAQV-REQUIMTE (UIDB/50006/2020) and CESAM (UID/AMB/50017/2019 and UIDB/50017/ 2020 + UIDP/50017/2020) research units, and to the FCT projects (PREVINE-FCT-PTDC/ASP-PES/29576/2017) through national founds (OE) and, where applicable, co-financed by the FEDER-Operational Thematic Program for Competitiveness and Internationalization−COMPETE 2020, within the PT2020 Partnership Agreement, and to the Portuguese NMR Network. The doctoral grants to M.B. and C.V. are funded by FCT (SFRH/BD/121645/2016, SFRH/BD/150358/2019, respectively).