Waste water treatment and recovery of nutrients from waste water is a strategy that will be used for long term sustainability of both water and nutrients. Waste water generally contains large quantities of useful nutrients such as Nitrogen and Phosphorus (P) that has detrimental impact on environment such as eutrophication of water bodies when lost to environment, further reducing the quality of water resources. Therefore, containing and treating waste water is an important activity to maintain the supply of water. Further, P is a critical element for agricultural production and reserves are limited, hence it is crucial to recover P from waste for long term availability for food production. Currently, there are several industrial scale existing technologies for P recovery from waste water such as AirPrex, Phosnix and, Crystalactor with recovery efficiencies 98%, 90%, and 75% respectively. All these established technologies recover P as struvite and/or calcium phosphate and depend extensively on large quantities of chemicals for processes like acid leaching and precipitation. These upstream chemical requirement imply high energy usage, thus resulting in an energy-water nexus problem for sustainable supply of both energy and water. Hence, it is necessary to study the thermodynamic performance of these recovery technologies at large scale that can inform the future expansion of these waste water treatment and recovery technologies. In this work, we focus on using thermodynamic metrics for performance of a novel Ion-Exchange based P recovery technology. The proposed Ion-Exchange based technology is independent of large quantities of additional chemicals; potentially being more sustainable relative to current technologies. However, the energy requirements for resin regeneration and maintaining the ideal Temperature and Pressure may result into higher energy requirements. Thus, before these technologies can be scaled up for adoption in expanding waste water treatment facility the energy requirement along with water and P fertilizer productions needs to be analyzed. To address this necessity, we use Entropy generation or S
gen as a metric that can provide information on the theoretical minimum energy requirement for the successful operation of this ion-exchange process.
Our approach involves rigorous systems modeling to quantify the total Sgen where process modeling tools are used for each of the sub-processes in the ion exchange based P recovery technology. The proposed model relies on two unit operations: Ion-exchange and anaerobic-digestion. ASPEN Plus process modeling software package for process modeling of all the sub-processes has been utilized. Results show that both these unit operations involve an entropy change of 34.5 kCal/mole-K and -478 kCal/mole-K respectively. We will also perform an energy analysis of the system to demonstrate the difference between insights obtained from entropy vs energy analysis along with performing the reaction level entropy change in both cases. Availability of theoretical minimum energy value from Sgen will potentially inform the energy implications of treating waste water for clean water generation on large scale.
