Three energy self-sufficient networks of wastewater treatment plants developed by nonlinear bi-level optimization models in Jeju Island

Abstract

Classical consumer communities are gradually turning to energy prosumers who employ innovative and self-sufficient systems. Despite the importance of wastewater treatment plants (WWTPs) in environmental protection and public health, they still consume considerable energy. This study employs bi-level nonlinear optimization models and a systematic platform of four novel techniques to design an energy self-sufficient network of WWTPs on Jeju Island, South Korea. A systematic platform consisting of four novel techniques is employed for designing energy self-sufficient WWTPs. The four techniques are dehydration of generated biogas from an anaerobic digestion, a nonlinear thermo-mathematical program for maximizing the generated electricity per input sludge, a deficit-surplus trade to avoid storage usage, and a bi-level multi-integer nonlinear program for optimal sizing of photovoltaic (PV) arrays. Three self-sufficient designs (SSDs) are proposed, including a network of biogas-fed combined heat and power plants (CHPPs) coupled with WWTPs (SSD-1), solar-powered WWTPs (SSD-2), and hybrid CHPP WWTPs retrofitted with PV systems (SSD-3). The biogas obtained from anaerobic digestion of waste-activated sludge in the WWTPs is dehydrated to fuel the CHPPs that use the available grid as a storage unit to establish a self-sufficient network in SSD-1. The PV units substitute for the CHPPs in SSD-2, while the hybrid systems minimize the local power trade in SSD-3. Economic, social, and environmental models of the optimal configurations are developed to compare the proposed SSDs. The resulting life-cycle impacts (LCI) of the proposed designs vary between 2 and 4531 pts/day depending on the energy demand and SSDs of the WWTPs. When the energy demand is 155.1 kWh/day in SSD-2, the LCI is 2 pts/day, whereas it reaches 150 pts/day for the energy demand of 10,786.6 kWh/day. A maximum of 26,278 new jobs could be created at an annual cost of −1,345,458 USD in SSD-2. The solar-powered scenarios had lower environmental impacts and greater social benefits, but were more expensive, compared with anaerobic digestion-assisted designs. Gradually adding bio-energy to an energy mix using hybrid renewable networks can therefore facilitate a smooth transition to a carbon-free economy on Jeju Island by 2030.