Urban sludge to value-added products for promoting the development of circular economy: Supply network design and optimization

In order to realize effective sludge treatment as well as energy recovery simultaneously for sustainable urban sludge management, a three-layer optimization model based on mixed-integer programming was proposed to solve the sustainable supply chain design problem. The three layers include wastewater treatment plants, sludge treatment facilities, and final disposal sites. Aiming at minimizing the total costs of the sludge management system, mass constraints were considered in the model as well as the regulations on environmental emissions. A case study based on the conditions in Hong Kong was conducted to verify the feasibility of the proposed model. Four sludge-to-energy technical routes were studied as the alternatives, covering incineration with landfill, incineration with cement production, gasification, and hydrothermal pyrolysis technology (HPT). Eleven sewage treatment services, four alternative sludge treatment facilities and three landfill sites were considered. Results indicated that three sludge treatment facilities would be constructed to satisfy the daily sludge treatment demand. Incineration followed by cement production, gasification and HPT were recommended in the context. Sensitivity analysis revealed that centralized management in one sludge treatment facility with a suitable capacity could be more conducive to the cost reduction. Permitting the same competitive technology adapting by different sludge treatment facilities could also help with the cost reduction. All the results showed that the proposed model possesses the feasibility, reliability, and flexibility for solving the supply chain design problem of sludge management with the consideration of economic benefits and emissions requirements and providing valuable reference to the decision-makers for better sustainable management.