Balancing Human Health Protection with Sustainable Design in Water Reuse: How Do Log Reduction Targets Affect System Cost and Environmental Impacts?

On-site nonpotable water systems (ONWS) must be designed to reduce pathogen risks to protect human health. However, variation among quantitative microbial risk assessment (QMRA) models used in this process has led to a range of treatment goals or pathogen log reduction targets (LRTs). When evaluating a range of potential treatment levels, designers and regulators should be aware of the cost and environmental implications of alternative system designs and how these metrics balance against public health objectives. In this study, we quantified and compared the life cycle costs and environmental impacts of several on-site nonpotable reuse systems designed to meet different LRTs within the range of available QMRA model variability. Though treatment system disinfection processes vary considerably in their dosages, the net effect on overall system cost and environmental impacts is generally on the order of ±5% or less. We find that other factors such as geographic location, dual-pipe plumbing requirements, and energy needed for the removal of organics are considerably more important and discuss ways in which total system cost and environmental impacts could be reduced while maintaining high levels of human health protection.