Thermodynamic Modeling of Phosphorus Recovery from Wastewater for Process Optimization

Abstract

Although the estimated global phosphorus (P) resources could last 230 years, economically extractable phosphate rock reserves may only last approximately 60 years [1]. Alternative P sources are increasingly valuable as the demand for P fertilizer continues to rise along with global population and food scarcity. P recovery from wastewater (WW) is often achieved via solid precipitation, which can be promoted using chemical and/or electrochemical systems to create conditions that favor the precipitation of solids containing P. Struvite (NH4MgPO4:6H2O), also known as magnesium ammonium phosphate (MAP), is one such solid that is favored under conditions observed in WW from concentrated animal feeding operations (CAFOs). Precipitation of struvite is favored at high pH values, at low concentrations of calcium, and when the Mg:NH4:PO4 molar ratio is near 1:1:1 [2]. Temperature effects on struvite precipitation have not been explored extensively, but precipitation is theoretically favored at lower temperatures [3].The purpose of this study is to more accurately predict solid precipitates in CAFO WW P recovery systems using the chemical equilibrium modeling computer program, PHREEQC, and a novel thermodynamic database. The novel thermodynamic database includes empirically-derived relationships between temperature and equilibrium constants for reactions that are of interest to CAFO WW P recovery systems (i.e. reactions involving species that affect the likelihood of precipitation for P-containing solids). Therefore, the novel database improves the predictive accuracy of solution simulations because PHREEQC employs the Van’t Hoff equation by default, which produces a linear relationship and consequently inaccurate equilibrium constants at temperatures not near 25ºC. In this study, results produced using the novel thermodynamic database are also compared to results produced using a database automatically generated by SupPHREEQC, a recently developed software utility that prints thermodynamic databases in PHREEQC-friendly formats by extracting data from the high-quality database, SUPCRTBL [4].This study also involves an in-depth literature review of kinetic limitations implicit in CAFO WW treatment systems, which affect the likelihood of precipitation of certain solid species and overall P recovery rates. Based on the review, solids deemed unlikely to precipitate are excluded from the modelling simulations. Using the batch reaction capabilities of PHREEQC, P recovery is predicted based on four separate variables: temperature, pH, Ca:Mg ratio, and N:P ratio. The batch reaction functionality provides more meaningful data compared to the saturation index, which is often cited as a proxy for precipitation likelihood but does not consider the competition for dissolved ions posed by other solids. Based on existing water quality data from different WW sources, the thermodynamic model will be used to identify the most promising sources and conditions for P recovery.