Sunlight Inactivation of Viruses in Open-Water Unit Process Treatment Wetlands: Modeling Endogenous and Exogenous Inactivation Rates

Abstract

Sunlight inactivation is an important mode of disinfection for viruses in surface waters. In constructed wetlands, for example, open-water cells can be used to promote sunlight disinfection and remove pathogenic viruses from wastewater. To aid in the design of these systems, we developed predictive models of virus attenuation that account for endogenous and exogenous sunlight-mediated inactivation mechanisms. Inactivation rate models were developed for two viruses, MS2 and poliovirus type 3; laboratory- and field-scale experiments were conducted to evaluate the models' ability to estimate inactivation rates in a pilot-scale, open-water, unit-process wetland cell. Endogenous inactivation rates were modeled using either photoaction spectra or total, incident UVB irradiance. Exogenous inactivation rates were modeled on the basis of virus susceptibilities to singlet oxygen. Results from both laboratory- and field-scale experiments showed good agreement between measured and modeled inactivation rates. The modeling approach presented here can be applied to any sunlit surface water and utilizes easily measured inputs such as depth, solar irradiance, water matrix absorbance, singlet oxygen concentration, and the virus-specific apparent second-order rate constant with singlet oxygen (k2). Interestingly, the MS2 k2 in the open-water wetland was found to be significantly larger than k2 observed in other waters in previous studies. Examples of how the model can be used to design and optimize natural treatment systems for virus inactivation are provided.

Constructed Wetlands

“…Figure 9b shows free water surface flow wetlands are expected to achieve a wide range of removal of bacteria and viruses that can range from less than 1 log 10 reduction to up to 2 or 3 log 10 reduction. This supports the conclusions of others (e.g., Nguyen et al, 2015;Silverman et al, 2015) that sunlight inactivation is very important for virus and bacteria reductions and this mechanism may be limited in a free water surface flow constructed wetland that contains emergent plants because of shading of the water by the vegetation. The observations in Figure 9a and 9b are also supported by who showed that horizontal subsurface flow constructed wetlands typically have a better capacity to remove pathogens than a planted free water surface flow constructed wetlands.…”

“…Sunlight is also rapidly attenuated within the first few centimeters of a pond-like system that contains wastewater or natural organic matter (Davies-Colley et al, 2005b). The issue of sunlight exposure in a planted system is especially important because there is a direct relationship between the amount of sunlight that reaches a pathogen and the inactivation rate Silverman et al 2015). The inactivation of viruses by sunlight and the influence that different parts of the solar spectrum have on different viral species is discussed by Silverman et al (2013).…”

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

“…In contrast, the inactivation of human adenovirus was less accurate, and was underestimated two-fold. Silverman et al (2015) pursued a similar approach. This group also considered total solar inactivation as the sum of endogenous and exogenous processes.…”

Sulfide-Induced Dissimilatory Nitrate Reduction to Ammonium Supports Anaerobic Ammonium Oxidation (Anammox) in an Open-Water Unit Process Wetland

“…As a subclass, shallow open-water unit process wetlands were developed as a specialized component of larger engineered wetland systems that receive municipal wastewater effluent and effluent-dominated surface waters. These units were initially designed to enhance the removal of recalcitrant trace organic contaminants such as pharmaceuticals and personal care products (24-26) but were found to have ancillary benefits for nitrate (2) and pathogen attenuation (27,28) through an interplay of photolytic and biological processes that rivaled or even surpassed their vegetated cousins. The open-water unit process wetland is lined with a geotextile fabric in order to prevent the growth of macrophytes that would shade the water column and contains a benthic photosynthetic biomat that when mature can be up to 10 cm thick.…”