Risk-based management of the cost to society from infiltration and inflow to wastewater systems

Licentiate thesis, 2022

The wastewater system is an important part of our infrastructure as it protects public health and the environment. Well-functioning maintenance is essential to sustain the functionality of the system. Insufficient maintenance in combination with the complexity of the system often results in problems related to water from infiltration and inflow (I/I-water). I/I-water is all water in the wastewater sewer system that is not sanitary sewage and can originate from, e.g. groundwater, stormwater, and surface water. Common effects of I/I-water are the need of additional treatment at the wastewater treatment plants and pumping of water. After large rains, basement flooding events and discharge of untreated wastewater into streams and rivers are also common. To enable a more efficient management of I/I-water, the problem is comprehensively mapped in this thesis. Furthermore, a risk-based framework for decision support is presented that emphasises the need of considering economic, social, and environmental effects of I/I-water, as well as uncertainty throughout the decision-making process. Based on the framework a model is set up to calculate the cost to society from I/I-water. Important effects of I/I-water are monetised and the present value for a longer time horizon is calculated. Further, a case study in central Gothenburg, Sweden, is used to illustrate the applicability of the model. It is concluded that a comprehensive model for decision support is needed to handle I/I-water in a more sustainable way and that the result of this thesis can work as a basis for this.