Recycled water safety: Current status of traditional and emerging viral indicators

The use of recycled or reclaimed water for direct or indirect potable uses is increasing because of water scarcity. Wastewater contains pathogens that are capable of causing infections in humans. Among the microbial pathogens, viruses pose significant human health risks because of their high environmental persistence and low infectious doses. Routine monitoring of pathogenic viruses is challenging, time-consuming, and expensive. Moreover, the concentrations of most pathogenic viruses are not stable in wastewater throughout the year. As an alternative, viral indicators can be used to predict the presence, behavior, and removal of viral pathogens through wastewater treatment processes. This article highlights the characteristics of various viral indicators, and their usefulness as a proxy for the log reduction of pathogenic viruses for recycled water scheme. Concentrations of indicator viruses, such as pepper mild mottle virus and emerging crAssphage, are 2-3 orders of magnitude greater in untreated sewage than those of bacteriophages and other viral indicators. However, more research is needed for their comparative log reduction with pathogenic viruses. Developing effective methods to determine virus concentrations, the ability to culture indicator viruses, and new detection tools will enhance the application of indicator viruses in water recycling.