Viral load-dependent masking effect by activated sludge during the aeration process

Abstract

Emission of bioaerosols from wastewater treatment processes is a significant risk for the workers involved and the residents around, but the driving factors of viral aerosol emission and are not clear. In this study, four model viruses (Phi6, MS2, PhiX174, and T7) were used to investigate the effects of activated sludge on the emission of viral aerosols. It was found that more than 95% of the viruses could be adsorbed by the activated sludge within three hours. During aeration, when the activated sludge was present, the virus-containing aerosols became larger compared to no sludge situation. This implies that some viruses were adsorbed by the sludge particles during the aerosolization. The viral aerosol concentrations in air were proportion to the virus concentrations in the sludge-free wastewater, but much lower when aerosolized from the sludge suspension. This result showed a masking effect by the sludge. However, the masking effect was gradually weakened or even disappeared, when the spiked virus concentrations were less than 10⁷ copies/mL, which are much closer to the presence of pathogenic viruses in the real wastewater. One possible explanation could be that small sludge particles might adsorb viruses preferentially compared to large ones and could help the aerosolization of viruses. This was demonstrated by the increased emission of viral aerosols after reducing the particle size of the activated sludge by sonication. This work reveals evidence that the sludge will actually not mask the aerosolization of pathogenic viruses in the wastewater treatment plant as the concentrations of most viruses are very low, and health risks always exist.