Managing infiltration and inflow into wastewater systems: Assessing social gains by using probabilistic cost-benefit analysis

The wastewater system is an essential part of our infrastructure as it prevents flooding and protects public health and the environment. Infiltration and inflow into wastewater systems (I/I-water) is a significant problem for water utilities and should be managed so that investments maximise benefits to society. This paper presents a decision-support model based on cost-benefit analysis, with the novelty of including and monetising 14 effects of I/I-water measures which either affect the water utility or other parts of society and considering various uncertainties by assigning probability distributions to the input variables and performing Monte Carlo simulations. Currently, no such model is available to quantify the total value to society from measures to handle I/I-water. The proposed model is illustrated in a case study in Gothenburg, Sweden. In the case study, a reference alternative is compared to three conceptual project alternatives: total separation of the combined system, relining of the entire separate sanitary sewer system, and remediation at private properties. The case study highlights an imbalance in the distribution of costs and benefits for the water utility and society and shows the significance of including externalities to avoid underestimating the social benefits of removing I/I-water. Using the presented novel model facilitates a transparent and structured decision-making process for handling I/I-water in the manner most effective to society.