Utilizing reclaimed water to enhance the sustainability of the water environment in valley urbanization in Chongqing

Abstract

The process of urbanization would result in a series of environmental issues, posing significant challenges to the urban water environment and consequently impact the overall sustainability of urban development. The evaluation of the sustainability of urban water environment has attracted more and more attention from scholars and urban planners in both governments and civil societies. Climate change will exacerbate water scarcity and, along with population growth, land use and energy choices, make the sustainability of water environment a moving target. The cities in valley areas face greater uncertainty in environmental integrity due to their unique spatial patterns and hydrological and water resources conditions.

This thesis evaluates the sustainability of the water environment sustainability and the role of reclaimed water use on sustainability in valley cities in Chongqing, Southwestern China, which, like most Chinese cities, are still in the process of urbanization. The study classified valley cities and identified their water environment characteristics by building models of “conceptual valley city” and “conceptual water system”. An evaluation indicator system and methodology for assessing water environment sustainability in valley cities are developed through a comprehensive challenging analysis. The indicator framework encompasses four elements: water resource diversity, water environment quality, water ecological health, and low carbon emissions.

The thesis investigates the driving factors and purposes of reclaimed water use as an alternative urban water source. The multiplier effect of reclaimed water utilization is formally defined for the first time and patterns of urban reclaimed water utilization identified, including single-pipe and multi-pipe systems, which influence the multiplier. Several valley cities in Chongqing are examined as case studies to evaluate specific aspects of sustainability and the role of reclaimed water utilization in enhancing sustainability, utilizing respective watershed data and the methods of environmental capacity, environmental flow, cumulative accounting of carbon intensity, as well as DHI’s Mike II model.

The results show that, reclaimed water can be adopted to reduce the pollution loads discharged into rivers and thus reduce the environmental risk of rivers; Reclaimed water replenishment is beneficial to enhance the river environmental flow in the valley city; Using reclaimed water to substitute inter-basin water transfer in valley cities can significantly reduce carbon emissions, and the carbon reduction effect is affected by the use and pattern of reclaimed water utilization; As an important aspect of urban sustainability, the indicator of water resources diversity is denoted by the inverse HHI (Herfindahl-Hirschman Index) and determined by quantifying and assessing the quantity and proportion of urban water resources potentially available to each valley city, which ultimately depending on the natural endowments of valley cities (including land resources, water resources, and spatial patterns) as well as the scale and intensity of urban development.

The water system of Valley City is characterized as a semi-closed, non-self-sustaining framework. Human activities must be conducted within certain limits, controlling the scale of urban development appropriately and establishing suitable environmental quality targets and discharge standards. These measures will contribute to enhancing the sustainability of the urban water environment in the valley. As a mitigation strategy to address these challenges, reclaimed water can be used to promote water environmental sustainability.

The research methodologies employed, including the conceptual city, conceptual water system, sustainability indicator systems, and quantitative assessment techniques, can serve as valuable references for studies on water environment sustainability in other relatively isolated urban water environments, particularly in water-scarce regions where diversification of water resources is essential. The concepts of the reclaimed water utilization multiplier and the identified patterns of reclaimed water use introduced in this study would help to develop practical tools for planning reclaimed water utilization and provide a solid foundation for policy-making decisions to realize sustainable cities.