Optimization of energy-water-waste nexus at district level: a techno-economic approach

Abstract

The United Nations (UN) Sustainable Development Goals (SDG) includes sustainable clean water and clean energy supply. Contrary to the typical dogma, wastewater is now a valuable source to recover certain resources as the effluent stream is also applicable to be reused and/or reclaimed, minimizing freshwater consumption. Hence, clean water and clean energy can be obtained from the integration works. However, the attempt to establish a tripartite energy-water-waste nexus at the district level is yet to be made, hence requiring the use of the mathematical approach to perform simultaneous optimization of the nexus. This study develops a mathematical model to optimize the nexus for a centralized facility, which is applicable at the district level. The sources are from domestic and industrial wastewaters and multiple contaminants are considered for optimization. The recovery processes are intended to recover biogas, phosphorus, nitrogen, metal, digestate, solid sludge, and microalgae oil from the wastewater streams, while supplying reused and/or reclaimed water for various application options. Connection items and segregation tanks are also included in the model, as well as the techno-economics equations. The model is formulated as mixed-integer non-linear programming (MINLP). As multiple resources can be recovered in the facility, a case study is conducted. A total of 3.2 MW of renewable energy can be recovered, and freshwater consumption is reduced by 66%. Supply water selling price, industrial processing fee value, and maximum length of the main transmission pipeline are the important factors affecting the project's economics. As there are no subsidies set, the study provides insight on how the tripartite nexus can be symbiotically integrated at the district level.