Reshaping the urban water management paradigm: towards a new decision-support framework for the sustainable design of hybrid systems

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Large centralized systems, providing universal coverage and taking advantage of economies of scale, have dominated municipal water services worldwide for decades. Most cities in Canada rely on a single water source, and Montreal's 88-m3/s wastewater treatment facility, with probably the highest maximal flowrate capacity in the world, exemplifies this paradigm very well. Yet today, many municipal wastewater systems operate at maximum capacity or beyond, limiting urban development and causing adverse environmental impacts. To avoid large system expansion costs, fully decentralized wastewater systems (like septic tanks) are increasingly proposed to serve new urban developments, causing potentially disastrous long-term environmental and public health impacts due to inadequate operating conditions. Between both extremes, small and medium scale 'satellite' wastewater systems can be incorporated into large urban schemes. In fact, to support strategic planning of urban water systems (UWS), the centralized versus fully decentralized paradigm needs to be revisited with novel alternative, multiscale UWS options, and new analytical approaches and tools are needed to guide decision making in designing future sustainable and resilient cities. This study examines the technical, economic, environmental, and social potential of new 'hybrid' configurations of UWS, i.e., that operate at a mix of scales. To this end, a mixed methodology of future scenario analysis, system modelling and integrated assessment tools will be developed. An analysis of existing UWS across the provinces of Québec and Ontario will be performed to identify urbanisation patterns that affect UWS's efficiency in terms of (i) cost-effectiveness in delivering water services and economies of scale; (ii) environmental impacts from wastewater effluents and combined sewer overflows; and (iii) maximal social and public health benefits from different uses and exposure to improved water quality (O1). In a case study city located in Southern Quebec, stakeholder and community preferences for future UWS will be assessed according to the three dimensions of sustainability, forming the basis for a scenario analysis that informs UWS development (O2). New hybrid configurations to meet future system capacity needs in the case study city will be modelled using wastewater treatment and sewage system modelling tools (O3) and will be compared with traditional options in terms of sustainability using a new spatial multicriteria decision analysis framework that will be developed and integrated into a spatial planning tool (O4). This highly interdisciplinary approach will enhance scientifically robust, innovative spatial planning of UWS that has the potential to overcome technology lock-in and path-dependency. Proposing such a new UWS planning paradigm is innovative, field evaluations of such a management model have not yet been implemented at city scale and stakeholder engagement is challenging; yet, the expected social, environmental and economic benefits are huge.