Virus Inactivation during Water Treatment : the Role of Virus Aggregation and the Disinfection Potential of Sunlight

Virus removal and inactivation is still a major challenge for water treatment facilities in both industrialised nations and developing countries. This may seem surprising as chlorine disinfection started to spread broadly over a century ago. However, many viruses are more resistant to disinfection by chlorine and other oxidants than other pathogens. Additionally, some viruses are known to be particularly difficult to disinfect by UV. Finally, viruses are extremely small (18-120 nm diameter), which makes sedimentation impossible and filtration difficult. As no real-time methods exist to enumerate viruses, disinfectant doses are based on lab experiments typically conducted with dispersed viruses. However, viruses in wastewater and natural environments can be present as aggregates. Previous studies have shown that aggregates protect viruses from disinfection, but it remains unclear what renders these aggregates more resistant compared to dispersed viruses. In order to elucidate this observation, aggregates of bacteriophage MS2 of well-defined sizes up to 1 μm diameter were produced by lowering the solution pH, and aggregates were inactivated by peracetic acid (PAA). Aggregates were re-dispersed before enumeration to obtain the residual number of individual infectious viruses. In contrast to enumerating whole aggregates, this approach allowed an assessment of disinfection efficiency which remains applicable even if the aggregates disperse in post-treatment environments. Aggregation reduced the apparent inactivation rate constants 2-6 fold, depending on the aggregate size. The larger the aggregate and the higher the PAA concentration, the more pronounced was the inhibitory effect of aggregation on disinfection. A reaction diffusion model, developed to simulate aggregate disinfection, showed that the inhibitory effect of aggregation arises from consumption of the disinfectant within the aggregate, but that diffusion of the disinfectant into the aggregates is not a rate-limiting factor. Aggregation therefore has a large inhibitory effect if highly reactive disinfectants are used, whereas inactivation by mild disinfectants is less affected. This finding leads to the counterintuitive notion that mild disinfectants, rather than aggressive ones, should be used when virus aggregates are present. During UV disinfection, viruses disinfection curves frequently exhibit a tailing after an initial exponential decay. Aggregation, light shielding, genome recombination or resistant virus sub-populations were proposed as explanations. However, none of these options has conclusively been demonstrated. We investigated how aggregation affects virus inactivation by UV254 in general, and the tailing phenomena in particular. A similar experimental set-up was used as described above with the difference that UV254 disinfection was applied instead of PAA addition. Results showed that initial inactivation kinetics were similar for viruses incorporated in aggregates and disperse