[Model-based Optimization for the Coordinated Supply of Clear and Reclaimed Water in the Central Districts of Beijing, China].

Coordinated utilization of clear and reclaimed water in urban water supply systems has attracted wide attention from both academic and industrial communities. Although the potential and impacts of water reuse have been explored in previous studies, there is still a lack of model-based research that can optimize the utilization of clear and reclaimed water among multiple sectors and at multiple locations and the associated operations of multiple water treatment plants. In this study, an optimization model was developed for supporting the coordinated supply of clear and reclaimed water in urban water supply systems. This model was formulated based on the conversion relationships among clear water, wastewaters, and reclaimed water resources as well as a number of constraints such as the water reclamation capacities of existing wastewater treatment plants. The developed model provides optimal plans for allocating clear and reclaimed water and for operating wastewater treatment plants in a complex water supply system. The optimization model was applied to a case study in the central districts of Beijing, China. Optimal results were generated under both business-as-usual (BAU) and alternative scenarios in which the utilization scale of reclaimed water was capped and not capped, respectively, according to the existing city plans. Under the BAU scenario, reclaimed water accounted for 32% of the total water used, and contributed 60%, 30%, and 42% of agricultural, industrial, and environmental water consumption, respectively. The supply-demand contradiction of reclaimed water was apparent in Haidian and Shijingshan districts. Compared to the BAU scenario, the alternative scenario achieved 0.621 billion yuan more in economic gain, consumed 36.59% more reclaimed water, and reduced clear water use by 14.02%. The alternative scenario also improved the use of the capacities of existing facilities and promoted water reuse. Moreover, the operation plans of wastewater treatment plants for increasing the utilization amount of reclaimed water was provided. The developed model could be widely applied to other water-scare cities with water reclamation potential without loss of generality.