Coordinated allocation of water resources and wastewater emission permits based on multi-objective optimization model: From the perspective of conflict between equity and economic benefits

Increasing demand for water resources, shortage of fresh water resources and imperfect management of wastewater emission permits compose the main conflicts of water resource in various basins. Therefore, to alleviate these supply and demand conflicts, watershed managers should search for optimal allocation strategies. Overall, there are two challenges in water resource allocation: the first one is the contradictions among various water sectors exacerbated by existed unfair water resource allocation; the second is the unification of wastewater emission permits and water resource allocation while considering economic, social, and environmental benefits. Based on this, a multi-objective optimization model for united allocation of water resources and wastewater emission licenses was proposed in this paper, which takes into account the maximizing of economic benefits per unit of wastewater emission. Applying this model, the balanced development of economic, social, and environmental benefits can be assured, and the equity of water resource allocation to alleviate the supply-demand contradiction can be maximized. An improved particle swarm optimization algorithm was designed to solve the nonlinear programming model and simulate the cooperative allocation scheme of water resources and wastewater emission permits under different preferences. The case study showed that the proposed model is realistic and sensible, and that watershed managers' preferences has an influence on the coordinated allocation strategy of water resources and wastewater emission permits. For example, the economic benefit differential between the basic trade-off scheme (BTS) and the economic benefit maximization scheme (EBM) is 95.39717 billion yuan, and the Gini coefficient between the BTS and the EMS falls from 0.3874 to 0.3627. The proposed model can successfully balance equity and economic benefits, and provide a reference for the construction of wastewater emission and water resource co-allocation systems.