Sunlight inactivation of fecal indicator bacteria in open-water unit process treatment wetlands: Modeling endogenous and exogenous inactivation rates

Abstract

A pilot-scale open-water unit process wetland was monitored for one year and found to be effective in enhancing sunlight inactivation of fecal indicator bacteria (FIB). The removal of Escherichia coli and enterococci in the open-water wetland receiving non-disinfected secondary municipal wastewater reached 3 logs and 2 logs in summer time, respectively. Pigmented enterococci were shown to be significantly more resistant to sunlight inactivation than non-pigmented enterococci. A model was developed to predict the inactivation of E. coli, and pigmented and non-pigmented enterococci that accounts for endogenous and exogenous sunlight inactivation mechanisms and dark processes. Endogenous inactivation rates were modeled using the sum of UVA and UVB irradiance. Exogenous inactivation was only significant for enterococci, and was modeled as a function of steady-state singlet oxygen concentration. The rate constants were determined from lab experiments and an empirical correction factor was used to account for differences between lab and field conditions. The model was used to predict removal rate constants for FIB in the pilot-scale wetland; considering the variability of the monitoring data, there was general agreement between the modeled values and those determined from measurements. Using the model, we estimate that open-water wetlands at 40° latitude with practical sizes can achieve 3-log (99.9%) removal of E. coli and non-pigmented enterococci throughout the year [5.5 ha and 7.0 ha per million gallons of wastewater effluent per day (MGD), respectively]. Differences in sunlight inactivation rates observed between pigmented and non-pigmented enterococci, as well as between lab-cultured and indigenous wastewater bacteria highlight the challenges of using FIB as model organisms for actual pathogens in natural sunlit environments.

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.…”

“…In addition, reactions mediated by exogenous photosensitizers can induce lesions in the transport chains in water that contains dissolved organic matter (Muela et al, 2002). However, it is reported that though exogenous photosensitizers played a role in inactivation rates of E. faecalis, they did not play an important role in the inactivation of E. coli in similar experiments (Kadir and Nelson, 2014;Nguyen et al 2015).…”

Staphylococcus aureus Strain Newman Photoinactivation and Cellular Response to Sunlight Exposure

“…A number of studies have therefore investigated bacterial photoinactivation in the context of environmental water quality (1)(2)(3)(4)(5)(6)(7), with further studies also focusing on photoinactivation in drinking water treatment (8)(9)(10)(11), photodynamic therapy (12)(13)(14)(15), and wastewater treatment (16)(17)(18)(19)(20). Bacteria can be inactivated by sunlight by direct and indirect mechanisms.…”

“…Previous studies have identified factors and characteristics associated with increased photoinduced damage of FIB. For example, higher concentrations of dissolved oxygen lead to increased photoinactivation of fecal coliforms and Enterococcus faecalis (6,17), and the presence of carotenoid pigments in certain enterococci tends to decrease their susceptibility to photoinactivation (4,18). However, most previous research has focused on loss of culturability as the sole metric of photoinactivation, with a limited number of studies also addressing the effects of sunlight exposure on membrane integrity, cell metabolism, protein content, and gene expression in FIB (6)(7)(8)29).…”

“…Previous field and laboratory studies on photoinactivation of Enterococcus spp. have found that pigmented enterococci are more resistant to photoinactivation than nonpigmented enterococci (4,18), suggesting that the presence of pigmentation is an important factor for bacterial persistence in sunlight-exposed environments.…”