Content area
Full Text
INTRODUCTION
Wastewater reclamation is one of the practical options to mitigate water stress, in which reclaimed wastewater is used for multiple purposes, including irrigation (Lubello et al. 2004), ground water recharge (Asano & Cotruvo 2004), recreational impoundment (Levine & Asano 2004), and drinking water source (Rodriguez et al. 2009). However, chemical and microbial constituents impose health risks on users of reclaimed wastewater and individuals who work in wastewater treatment (Toze 2006). Enteric viruses, such as human noroviruses, are major microbial constituents causing infection risks in wastewater reclamation, because these viruses are released to sewage with feces from symptomatic/asymptomatic individuals (Ozawa et al. 2007), and the reduction efficiency of these viruses from sewage is relatively lower than those of indicator microorganisms such as Escherichia coli (Ottoson et al. 2006).
To reduce the risks of waterborne disease outbreaks through reclaimed wastewater, it is critical to significantly reduce the virus quantity in reclaimed wastewater. The World Health Organization (WHO) guidelines stipulate that virus infection risks in wastewater reclamation should be managed by the concept of multiple-barrier system, in which a wastewater reclamation process is designed to achieve a target log reduction (LR) value by combining treatment unit processes with predetermined virus reduction efficiency (WHO 2006a). The target reduction efficiency is the sum of virus LR values in each unit process, which is determined not to exceed the additional tolerable burden of disease (10−6 disability adjusted life year per person per year (DALYpppy)) in wastewater reclamation (Sano et al. 2016).
Under the multiple-barrier system concept, the virus reduction efficiency of wastewater treatment unit processes, such as secondary treatment and disinfection, has to be determined prior to the operation of the wastewater reclamation system. Commonly, the ratio of virus concentration in influent to that in effluent is regarded as the virus reduction efficiency, and this ratio is repeatedly analyzed to obtain the average efficiency of virus reduction (Ottoson et al. 2006; Sima et al. 2011; Frohnert et al. 2015; Schmitz et al. 2016). However, this practice of evaluating virus reduction efficiency is not always successful because the virus concentrations in influent and effluent often fall below the analytical quantification limit, which makes it impossible to calculate the virus concentration ratio at some sampling events. It is necessary to estimate the...