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

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

Infiltration and inflow to wastewater sewer systems - A literature review on risk management and decision support

Wastewater sewer systems are one of our largest infrastructural assets. By transporting the sewage from our homes and other facilities to the wastewater treatment plants, the sewer systems protect public health, properties, and the environment. However, in addition to the sanitary sewage, there is an infiltration and inflow (I/I) of other water to the sewer system. This additional load can result in adverse effects such as basement flooding, combined sewer overflows, and larger pumping and treatment costs. I/I can originate from rainfall but also from sources such as groundwater, surface water or leaking drinking water pipes. Expected climate change effects include more intense rain events and periods of higher water levels which will increase the problem of I/I. Hence, it is important to manage I/I in a proper way by implementing efficient measures that provide the largest societal gain from a sustainability point of view.This literature review was performed to form a basis for research on developing risk-based decision support models to evaluate I/I in wastewater sewer systems from a system perspective and with focus on sustainability. It reviews publications on I/I focusing on sources, impacts, quantification and mitigation measures, addresses risk definitions, and the risk management process. Further, common decision support methods are described and literature on decision support models to evaluate I/I are reviewed. Important conclusions are that a vast amount of literature exists on finding and reducing I/I from a technical point of view and in several publications different decision support models are used to evaluate measures aiming at reducing I/I. However, existing models are focused on project internal and financial aspects and a need for future studies is identified, evaluating I/I from a broader societal and sustainability perspective, including project external, environmental, and social criteria

Framework for Risk-Based Decision Support on Infiltration and Inflow to Wastewater Systems

Infiltration and inflow (I/I) to wastewater systems cause e.g., flooding, pollution, and the unnecessary use of the limited resources in society. Due to climate change and an increased need for the renewal of piping systems, making the right decisions on how to handle I/I is more important than ever. This paper presents a novel framework for risk-based decision support on I/I based on established theories on risk assessment and decision-making. The framework is presented on a general level and suggests that uncertainties are included in the decision-making process, together with criteria representing the economic, social, and environmental dimensions of sustainability. Published models on I/I and decision support are evaluated based on criteria from the framework showing that (1) the models rarely include risk-based decision-making or uncertainties in the analyses and that (2) most models only include project-internal financial aspects, excluding social and environmental, as well as project-external aspects, of I/I and I/I measures. A need for further research to develop a more holistic decision support model for I/I is identified, and it is concluded that the application of the proposed framework can contribute to more sustainable decisions on how to handle I/I and provide transparency to the process

Cost to society from infiltration and inflow to wastewater systems

Water from infiltration and inflow to wastewater systems is an extensive problem causing costs to society in various ways. Comprehensive methods for supporting decisions on how to efficiently mitigate the problems in a sustainable manner are, however, missing today. This paper presents a novel risk-based model to assess the cost to society from infiltration and inflow to wastewater systems by monetising effects related to treatment of wastewater, pumping, combined sewer overflows, and basement flooding. The present value is calculated for a specified time horizon and discount rate, using a cost-benefit analysis approach. To acknowledge the various uncertainties, a probabilistic approach is applied where probability distributions represent the input variables. The model is shown to be applicable by illustrating its use in a case study area in Gothenburg, Sweden. Main results from the case study show that most of the costs are related to investments at the wastewater treatment plant and restoration due to basement flooding events. Sensitivity analyses show that the result is highly dependent on factors such as the volume of infiltration and inflow water, the share of basement flooding, and the discount rate. Using expert elicitation to quantify input data is also illustrated and shown to be a valuable method. The presented model fills an important research knowledge gap and will facilitate a more sustainable and comprehensive handling of water from infiltration and inflow