Entropy generation analysis of sequential Anaerobic Digester Ion-Exchange technology for Phosphorus extraction from waste

Phosphorus (P) recovery from waste is a crucial sustainability challenge both for meeting the growing food demand and maintaining water quality effected by P run-off. Several new technologies are under development to address this issue, however each technology presents a trade-off in terms of cost vs benefits. Energy consumption forms one such critical trade-off as energy intensive technologies can lead to unintended consequences. However, the energy assessment of waste treatment and nutrient recovery systems are limited in literature and mostly focused on empirical analysis. This limits the design optimization of these systems for improving energy utilization. In this work, this critical gap of rigorous energy assessment of waste recovery technologies has been addressed by proposing the method of entropy generation analysis (EGA) as a tool for evaluating the design of waste recovery systems in order to improve energy utilization. To demonstrate the method of EGA for waste recovery technologies, the method was applied on a sequential Anaerobic Digester (AD)-Ion Exchange (IE) based side stream process used for extraction of P from waste water system which is the first step in recovery of nutrient from waste. EGA was performed by assuming a black box model for the technology and thermodynamic parameters were calculated using mechanism based approach by considering biochemical reactions in AD, stream flow analysis in ASPEN PLUS and batch IE experiments. The study successfully shows how to account for entropy generation in processes like AD and IE using mechanisms that result in entropy generation leading to energy losses. Results from the study indicate that the entropy generated from the heat exchange system to maintain the operating temperature for AD and IE was leading cause of entropy generation which provides guidance for targeting design improvements to design cleaner technologies in future. Such insights about mechanism behind energy losses is not feasible using widely used energy analysis methods and the method of total exergy destruction analysis which combines the total losses for a subsystem but does not elucidate exact causes of losses, thus EGA provides a distinct advantage for guiding the design of technologies to minimize energy losses. (C) 2019 Elsevier Ltd. All rights reserved.