Probabilistic life cycle approach for novel systems addressing user-behavior uncertainty: application to toilets

For product systems that heavily rely on user profiles, such as buildings, vehicles, HVAC (heating, ventilation, and air conditioning) systems, or toilet systems, estimating environmental impacts during the use phase can be challenging, which can be a major contributor to life cycle impacts. User-behavior uncertainty has not been sufficiently addressed in life cycle assessment studies. The goal of this article is twofold: i) to propose a probabilistic life cycle approach that combines user-behavior scenario analysis, pairwise comparative analysis, and global sensitivity analysis, and ii) to apply this approach to a novel toilet system (WashOne) for comparison with a conventional system (toilet and bidet). A combination of deterministic usage patterns and stochastic scenarios (as a result of combining uncertain parameters) was employed. A pairwise comparison indicator presents uncertain results, indicating that the WashOne has significantly lower environmental impacts than the conventional system in all categories. Results from the global sensitivity analysis (using Spearman Rank Correlation Coefficient) show that toilet paper (in the conventional system) and washlet use (in the novel system) have the most significant effect on the impacts. This article concludes that (i) novel systems that greatly depend on user preferences can reduce impacts compared to conventional systems, and (ii) it highlights that user behavior becomes less influential as product systems are designed to be more efficient.