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A modeling approach to estimate the solar disinfection of viral indicator organisms in waste stabilization ponds and surface waters

Tamar Kohn, Michael Jon Mattle

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.

Elsevier2016

Solar disinfection (SODIS) of viruses in PET bottles

Anna Carratala Ripolles, Alex Dionisio Calado, Tamar Kohn, Michael Jon Mattle

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.

American Society for Microbiology2016

A conceptual model and experimental framework to determine the contributions of direct and indirect photoreactions to the solar disinfection of MS2, phiX174 and adenovirus

Tamar Kohn, Michael Jon Mattle

Sunlight inactivates waterborne viruses via direct (absorption of sunlight by the virus) and indirect processes (adsorption of sunlight by external chromophores, which subsequently generate reactive species) processes. While the mechanisms underlying these processes are understood, their relative importance remains unclear. This study establishes an experimental framework to determine the kinetic parameters associated with a virus’ susceptibility to solar disinfection, and proposes a model to estimate disinfection rates and to apportion the contributions of different inactivation processes. Quantum yields of direct inactivation were determined for three viruses (MS2, phiX174 and adenovirus), and second-order rate constants associated with indirect inactivation by four reactive species (1O2, OH., CO3.-, and triplet states) were established. PhiX174 exhibited the greatest quantum yield (1.42•10-2), indicating that it is more susceptible to direct inactivation than MS2 (2.9•10-3) or adenovirus (2.5•10-4). Second-order rate constants ranged from 1.7•107 to 7.0•109 M-1s-1 and followed the sequence MS2>adenovirus>phiX174. A predictive model based on these parameters accurately estimated solar disinfection of MS2 and phiX174 in a natural water sample, and approximated that of adenovirus within a factor of three. Inactivation mostly occurred by direct processes, though indirect inactivation by 1O2 also contributed to the disinfection of MS2 and adenovirus.

Amer Chemical Soc2015