Validation and Applications of a Chemical Equilibrium Model for Struvite Precipitation

Abstract

Removal of nutrients like nitrogen and phosphorus from wastewater can be accomplished by precipitating these as the mineral struvite (NH 4 MgPO 4 . 6H 2 O). Predicting struvite precipitation potential, yield, and purity is important for designers and operators of reactors for struvite precipitation. In this paper, a mathematical model of this precipitation process is developed using physicochemical equilibrium expressions, mass balance equations for nitrogen, phosphorous and magnesium, and charge balance. The model was simulated to explicitly solve for equilibrium concentrations of eighteen species that included dissolved (three), ionic (ten), and solid (five) species for a given set of initial concentrations of ammonium-nitrogen, magnesium and phosphate-phosphorus, and pH. The model simulations were validated against literature experimental data, which used synthetic as well as actual wastewater, and data from our experiments. The model satisfactorily predicted most data. Struvite fraction in the precipitate ranged from 27% to 100%. The purity of struvite in the precipitate and the pH that maximizes struvite fraction was dependent on the initial concentrations of ammonium, magnesium, and phosphate. Optimum pH and struvite fraction was, respectively, 8.5 and 29.3% for an equimolar mixture of ammonia, magnesium, and phosphate and 9.8 and 98.3% for 10:1.7:3.4 mM ratio. Struvite fraction in the precipitate increased as magnesium became limiting or as ammonia to phosphate ratio increased and magnesium to phosphate ratio decreased. Since the struvite component is only a fraction of the total solids, it is erroneous to report the total precipitate produced as being struvite as is conventionally done.

Phosphate recovery from animal manure wastewater by struvite crystallization and CO2 degasification reactor

“…Iterative jar experiments are expensive and manpower-consuming. Several researchers have presented mathematical theoretical models of struvite crystallization to evaluate the reaction condition [13]. However, mathematical theoretical calculation, while effective, may be very complicated and time-consuming.…”

Integrated, multi-process approach to total nutrient recovery from stored urine

“…However, due to the elemental composition of K-struvite additional chemical input of Mg 2+ and P is required for effective K recovery. 28,30 TAN recovery by ammonia stripping-acid absorption 32,34,35 and TP recovery by struvite precipitation 21,25,27,[36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] have been studied extensively to recover a significant portion of N and P in urine. For example, major conclusions from previous research include >95% TP recovery using MgCl 2 and >90% TAN recovery by increasing the pH and temperature of the solution.…”

Modelling the Thermodynamic Equilibrium of Struvite Precipitation using a Hybrid Optimization Technique

“…Several thermodynamic models based on chemical equilibrium have been developed to predict with reasonable accuracy the potential P removal as struvite by precipitation [9][10][11][12][13][14][15][16][17]. In particular, the use of computer models, such as PHREEQC [17][18][19], MINTEQA2/ Visual Minteq [10], [16], [19][20][21][22], and MINEQL+ [11], [23] have been used to determine the equilibrium speciation of struvite species constituents and therefore to predict struvite formation potential and PR.…”

“…The model developed by [13] uses an iterative residual procedure in MAPLE, the total phosphate concentration is obtained within a given tolerance, as well as the concentration of different precipitates; [9] uses Newton's method to find the approximate values of species concentration in urine; [15] express the struvite species concentration in terms of the known values and solves it numerically with iterative methods using MATLAB; [14] combine a genetic algorithm for initialization purpose with a standard Newton-Raphson method implemented in MATLAB environment to describe the chemical equilibria at different temperatures. This paper presents the development of a simple thermochemical model, representative of struvite precipitation when the concentrations of ammonium is low enough to neglect the presence of NH 3 (g) in the system and the pH is less than 10.0 to avoid the presence of Mg(OH) 2 in the solid phase [32], [33], since it reduces the P removal (PR) which is defined at Eq.…”