Integrated Management of Urban Wastewater Systems: Exploring Reliable, Resilient and Sustainable Strategies for an Uncertain Future

Abstract

The integration of the different parts of the urban wastewater system (i.e. catchment, sewer network, wastewater treatment plant and receiving waters) permits the representation of their intrinsic interactions and complexities, allowing for a more sophisticated management of stormwater and wastewater interventions. Emerging threats (e.g. climate change, population growth) and their associated future uncertainties pose an unprecedented challenge to the performance of the integrated urban wastewater system (IUWWS), which is expected to be reliable, resilient and sustainable regardless of future conditions.

The aim of this thesis is to understand the performance and planning implications of catchment-scale infrastructure strategies for the improved management of the IUWWS in the face of future uncertainty. To this end, green and grey infrastructure strategies are proposed and assessed in the context of two different IUWWS models. Future uncertainties are represented by means of four future scenarios that account for a rich and ample variety of internal and external threats in the horizon 2050. A novel regret-based method is employed in order to: 1) assess the performance of the proposed strategies for multiple objectives (environmental, economic and social) and identify their main trade-offs; 2) evaluate the robustness of the proposed strategies for reliability, resilience and sustainability across future scenarios; 3) explore the dynamic compliance and adaptability of the strategies along pathways of transient scenarios.

The obtained results demonstrate that end-of-pipe grey infrastructure strategies are subject to significant trade-offs that compromise their performance downstream, in spite of addressing localised issues. These operational trade-offs, along with the cost of grey schemes, importantly constrain their robustness to promote sustainability in the future, even in situations where these can become robust for reliability and resilience. In contrast, green infrastructure retrofits prove to be more consistent in their performance, delivering a wide range of performance benefits at a moderate cost. This translates into higher levels of robustness for reliability, resilience and sustainability across future scenarios when compared to their grey counterparts. Hybrid strategies combining grey and green interventions offer additional potential for robustness as they can ameliorate the unintended impacts and consequences of end-of-pipe solutions. Finally, it is also demonstrated that the robust performance of green retrofit strategies reconciles (and can help grey infrastructure alternatives to reconcile) compliance requirements in the short-term with those associated with the need to adapt to uncertain challenges in the long-term.