Abstract
Sustainability and resilience are both key considerations in the design and operation of wastewater systems. However, there is currently a lack of understanding of the relationship between these two goals and of the effects of increasing resilience on sustainability. This paper, therefore, presents a framework for analysis of the effects of resilience-enhancing interventions on sustainability, and applies this to an urban wastewater system. Given that sustainability addresses the long term, the framework includes a novel sustainability assessment approach which captures a continuum of potential future conditions and enables identification of tipping points where applicable. This method allows a wide range of potential futures to be captured whilst removing the need to develop scenarios or future projections. While it may be possible to develop interventions that are beneficial in terms of their effects on both resilience and sustainability, the results obtained from the case study demonstrate that implementing measures designed to increase resilience of an integrated urban wastewater system does not guarantee a universal improvement in sustainability. Therefore, when proposing measures to increase resilience, the potential effects on sustainability should be considered also. It is also shown that the extent of any negative effects on system sustainability can vary significantly depending on future conditions, with the case study intervention (increasing pump capacity) achieving the highest degree of sustainability if rainfall depths or imperviousness in the catchments reduce. However, trade-offs between sustainability indicators are present irrespective of future conditions. Furthermore, while an intervention that enhances resilience may be considered sustainable with respect to specific indicators under current conditions, tipping points exist and it will cease to be sustainable if future threat magnitudes exceed these.
Highlights
Sustainability and resilience are both key considerations in the design and operation of wastewater systems
When evaluating sustainability under future threats, plots of the form shown in Figs. 6 and 7 are useful because they capture the effects of a wide range of possible future conditions, and enable identification of conditions under which an intervention will or will not be sufficiently sustainable
A similar approach has previously been used for analysis of the effects of drought in a water resources system (Borgomeo et al, 2015), where performance was calculated under a continuum of drought duration and deficit values, irrespective of their probability, and specific deficit and duration combinations of interest were highlighted. To illustrate how these plots can be used to explore the effects of future scenarios that are considered likely, Fig. 8 provides an example in which population change and impervious surface area change projections provided by the Office for National Statistics (2015) and Environment Agency respectively (2013a, b) are mapped onto the corresponding results from Fig. 7
Summary
Sustainability and resilience are both key considerations in the design and operation of wastewater systems. In addition to meeting these goals, sustainable water management strategies should be ‘future-proof’ (Haasnoot et al, 2011; Walker et al, 2013) e i.e. effective and in the future or have the flexibility to adapt to future conditions. This includes not just the foreseeable future, and the unforeseen (Walker et al, 2013), which poses a challenge as natural, social and technological uncertainties may all affect long term water management (Haasnoot et al, 2011). Assessments of sustainability, are typically based on current conditions, projections or a limited number of predefined scenarios containing potential future states (based on driving pressures such as climate change), and do not capture all possibilities
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