11 research outputs found
A modeling approach to estimate the solar disinfection of viral indicator organisms in waste stabilization ponds and surface waters
Sunlight is known to be a pertinent factor governing the infectivity of waterborne viruses in the environment. Sunlight inactivates viruses via endogenous inactivation (promoted by absorption of UVB sunlight by the virus) and exogenous processes (promoted by adsorption of sunlight by external chromophores, which subsequently generate inactivating reactive species). The extent of inactivation is still difficult to predict, as it depends on multiple parameters including virus characteristics, solution composition, season and geographical location. In this work, we adapted a model typically used to estimate the photodegradation of organic pollutants, APEX, to explore the fate of two commonly used surrogates of human viruses (coliphages MS2 and X174) in waste stabilization pond and natural surface water. Based on experimental data obtained in previous work, we modeled virus inactivation as a function of water depth and composition, as well as season and latitude, and we apportioned the contributions of the different inactivation processes to total inactivation. Model results showed that X174 is inactivated more readily than MS2, except at latitudes >60°. X174 inactivation varies greatly with both season (20-fold) and latitude (10-fold between 0 and 60°), and is dominated by endogenous inactivation under all solution conditions considered. In contrast, exogenous processes contribute significantly to MS2 inactivation. Because exogenous inactivation can be promoted by longer wavelengths, which are less affected by changes in season and latitude, MS2 exhibits smaller fluctuations in inactivation throughout the year (10-fold) and across the globe (3-fold between 0 and 60°) compared to X174. While a full model validation is currently not possible due to the lack of sufficient field data, our estimated inactivation rates corresponded well to those reported in field studies. Overall, this study constitutes a step toward estimating microbial water quality as a function of spatio-temporal information and easy-to-determine parameters
Solar disinfection (SODIS) of viruses in PET bottles
Solar disinfection of drinking water in PET bottles (SODIS) is a simple point-of-use technique efficient for the inactivation of many bacterial pathogens. In contrast, the efficiency of SODIS toward viruses is not well known. In this work, we studied the inactivation of bacteriophages (MS2 and ɸX174) and human viruses (echovirus 11 and adenovirus type 2) by SODIS. We conducted experiments in PET bottles exposed to (simulated) sunlight at different temperatures (15, 22, 26 and 40°C) and in water sources of diverse composition and origin (India and Switzerland). Good inactivation of MS2 (more than 6-log inactivation after exposure to a total fluence of 1.34 kJ/cm2) was achieved in Swiss tap water at 22°C, while less efficient inactivation was observed in Indian waters and for echovirus (1.5-log at the same fluence). The DNA viruses studied, ɸX174 and adenovirus, were resistant to SODIS and the observed inactivation was equivalent to that occurring in the dark. Temperature enhanced MS2 inactivation substantially; at 40°C, a 3-log inactivation as achieved in Swiss tap water after exposure to a fluence of only 0.18 kJ/cm2. Overall, our findings demonstrate that SODIS may reduce the load of ssRNA viruses such as echoviruses, particularly at high temperatures and in photo-reactive matrices. In contrast, further complementary measures may be needed to ensure an efficient inactivation during SODIS of viruses resistant to oxidation such as ɸX174, or viruses undergoing rapid inactivation in the dark
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 ÎĽm 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 dispersed viruses. However, aggregated viruses started to tail more readily than dispersed ones. Neither light shielding, nor the presence of resistant sub-populations could account for the tailing. Instead, tailing was consistent with genome recombination arising as a result of the simultaneous infection of the host by several impaired viruses. We argue that UV254 treatment of aggregates permanently fuses a fraction of viruses, which increased the likelihood of multiple infection of a host cell and ultimately enables the production of infective viruses via recombination. Our results suggest that UV disinfection followed by the addition of a mild disinfectant should yield efficient disinfection for waters containing viral aggregates. Outside Europe and North America wastewater is rarely efficiently treated as treatment costs are often too high and additionally require highly-skilled personnel. To mitigate this issue, waste stabilisation ponds (WPSs) are a viable option because both construction and maintenance are inexpensive and easy to be executed. Pathogen inactivation in these pond systems is mainly attributed to sunlight. Although viruses are among the most resistant pathogens, little is known about sunlight-mediated inactivation mechanisms for these pathogens. Viruses can either be inactivated directly by UV light absorption by the viral genomes, or indirectly via light absorption by sensitizers. The excited sensitizers lead to the formation of reactive species, typically oxidants, which can further react with the viruses and lead to inactivation. Two bacteriophages were chosen as model viruses to investigate these inactivation mechanisms; phiX174, which is resistant to oxidants, and MS2, which is relatively resistant to direct UV inactivation. The efficiency of direct inactivation by solar UVB light was determined, and the rate constants associated with the inactivation by four potentially important reactive species present in sunlit surface waters (singlet oxygen, triplet state organic matter, hydroxyl and carbonate radicals) were quantified. A model was developed that computes the contribution of each of these inactivation mechanisms for ponds with different depths and solution parameters. Direct inactivation was found to be the major inactivation mechanism for phiX174 and also X contributed importantly to inactivation of MS2. Singlet oxygen was the most important reactive species for both virus below 0.5 m. Nevertheless, it did not contribute significantly to the overall inactivation of phiX174. These results suggest that maturation ponds should either be shallow or continuously-mixed to achieve good disinfection results. They furthermore demonstrate that virus inactivation can be reasonably approximated based on easy to determine solution parameters, along with information regarding the sensitivity to viruses toward direct inactivation and few selected reactive species
Inactivation and tailing during UV254 disinfection of viruses: contributions of viral aggregation, light shielding within viral aggregates and recombination
UV disinfection of viruses frequently leads to tailing after an initial exponential decay. Aggregation, light shielding, recombination or resistant virus sub-populations were proposed as explanations; however, none of these options has conclusively been demonstrated. This study investigates how aggregation affects virus inactivation by UV254 in general, and the tailing phenomena in particular. Bacteriophage MS2 was aggregated by lowering the solution pH before UV254 disinfection. Aggregates were redispersed prior to enumeration to obtain the remaining fraction of individual infectious viruses. Results showed that initial inactivation kinetics were similar for viruses incorporated in aggregates (up to 1000 nm in radius) and dispersed viruses; however, aggregated viruses started to tail more readily than dispersed ones. Neither light shielding, nor the presence of resistant sub-populations could account for the tailing. Instead, tailing was consistent with recombination arising as a result of the simultaneous infection of the host by several impaired viruses. We argue that UV254 treatment of aggregates permanently fused a fraction of viruses, which increased the likelihood of multiple infection of a host cell and ultimately enabled the production of infective viruses via recombination
Impact of virus aggregation on inactivation by peracetic acid and implications for other disinfectants
Viruses in wastewater and natural environments are often present as aggregates. The disinfectant dose required for their inactivation, however, is typically determined with dispersed viruses. This study investigates how aggregation affects virus inactivation by chemical disinfectants. Bacteriophage MS2 was aggregated 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. Inactivation kinetics were determined as a function of aggregate size (dispersed, 0.55 and 0.90 µm) and PAA concentration (5-100 mg/L). Aggregation reduced the apparent inactivation rate constants 2-6 fold. The larger the aggregate and the higher the PAA concentration, the more pronounced the inhibitory effect of aggregation on disinfection. A reaction-diffusion based model was developed to interpret the experimental results, and to predict inactivation rates for additional aggregate sizes and disinfectants. The model 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. Our results suggest that mild disinfectants should be used for the treatment of water containing viral aggregates