Process performance and precipitate quality of phosphorus recovery by struvite precipitation in a fluidized bed reactor using a MgO industrial by-product

Phosphorus recovery through struvite precipitation has gained interest due to the potential use of struvite as a fertiliser, with fluidised bed reactors being a popular technology for carrying out the process. Struvite precipitation requires a magnesium source and an alkaline reagent. This research uses a low-grade magnesium oxide (LG-MgO) industrial by-product with a 56 wt% of MgO as magnesium source and an alkaline reagent to lower operating costs and value-add an industrial by-product. LG-MgO is poorly soluble in water, but its solubility increases significantly when dissolved in anaerobic digestion supernatants due to its circumneutral pH and high buffer capacity. Phosphorus precipitation was carried out in a laboratory-scale fluidised bed reactor where three operating variables (i.e. P:Mg molar ratio, feed inlet position, and recirculation flow rate) were studied to determine the LG-MgO impact on precipitate struvite content. Experimental results showed a high struvite content in all precipitates, close to the values reported for pure magnesium sources. The P:Mg molar ratio influenced precipitate composition. The percentage of struvite in the precipitate were 75–82 wt%, 85–88 wt%, and 75–76 wt% for the P:Mg ratio of 1:0.5, 1:1 and 1:3, respectively. The feed inlet position (side or bottom) also had an impact on precipitate struvite content when the P:Mg molar ratio was 1:3, but not for the other molar ratios. The recirculation flow rate did not have a significant impact on precipitate struvite content

Struvite precipitation in wastewater treatment plants anaerobic digestion supernatants using a magnesium oxide by-product

Struvite precipitation is a well-known technology to recover and upcycle phosphorus from municipal wastewater as a slow-release fertiliser. However, the economic and environmental costs of struvite precipitation are constrained by using technical-grade reagents as a magnesium source. This research evaluates the feasibility of using a low-grade magnesium oxide (LG-MgO) by-product from the calcination of magnesite as a magnesium source to precipitate struvite from anaerobic digestion supernatants in wastewater treatment plants. Three distinct LG-MgOs were used in this research to capture the inherent variability of this by-product. The MgO content of the LG-MgOs varied from 42 % to 56 %, which governed the reactivity of the by-product. Experimental results showed that dosing LG-MgO at P:Mg molar ratio close to stoichiometry (i.e. 1:1 and 1:2) favoured struvite precipitation, whereas higher molar ratios (i.e. 1:4, 1:6 and 1:8) favoured calcium phosphate precipitation due to the higher calcium concentration and pH. At a P:Mg molar ratio of 1:1 and 1:2, the percentage of phosphate precipitated was 53-72 % and 89-97 %, respectively, depending on the LG-MgO reactivity. A final experiment was performed to examine the composition and morphology of the precipitate obtained under the most favourable conditions, which showed that (i) struvite was the mineral phase with the highest peaks intensity and (ii) struvite was present in two different shapes: hopper and polyhedral. Overall, this research has demonstrated that LG-MgO is an efficient source of magnesium for struvite precipitation, which fits the circular economy principles by valorising an industrial by-product, reducing the pressure on natural resources, and developing a more sustainable technology for phosphorus recovery

Effect of the shear rate and supersaturation on the nucleation and growth of struvite in batch stirred tank reactors

Controlled struvite precipitation is a promising solution for phosphorus recovery in wastewater treatment plants. Particle size distribution of recovered struvite affects its efficacy as a fertilizer, so should be considered in the design and operation of struvite recovery reactors. This contribution analyzes the effect of varying the average shear rate (between 150 s−1 and 876 s−1) and saturation index (between 0.76 and 2.96) in two different experimental set-ups. Solution pH and particle number and size measurements using an electric zone sensing method are used to monitor the process. In addition, photomicrographs are used to observe the shape of the precipitated particles. Interestingly, for identical thermochemical conditions, a higher mixing intensity, associated with the shear rate, leads to shorter induction times, faster precipitation and a greater particle density. On the other hand, for similar mixing conditions, a higher saturation index is also linked with shorter induction times, faster precipitation and a greater particle density. From the experimental data it is concluded that the effect of the fluid shear rate cannot be ignored and should be further studied in the precipitation process

New mass-based population balance model including shear rate effects: Application to struvite recovery.

Struvite (MgNH4PO4·6H2O) precipitation is a promising solution for phosphorus recovery in wastewater treatment plants. Controlled struvite precipitation can help to reduce eutrophication in the receiving waterways, fight global phosphorus scarcity and reduce operational problems generated by the uncontrolled precipitation of the mineral in the pipes. Due to the generated interest, the description of the precipitation process has been already included in existing wastewater treatment modelling libraries. However, following the classic wastewater treatment modelling approach, the process has been generally included as a one-step kinetic model. This one-step model type is limited for technological design and optimization purposes, as it does not include information about the mechanisms by which the precipitation occurs, nor the particle size distribution, a key variable for the performance of struvite as an effective fertilizer. Therefore, the aim of this thesis has been to upgrade existing one-step kinetic models by developing a mathematical model that could describe in detail the mechanisms occurring in struvite precipitation in order to be able to predict the resulting particle size distribution. This model is a population balance model in which hydrodynamic effects have been considered. The population balance model has been constructed according to Ceit’s plant wide model methodology, guaranteeing mass and charge balance. Therefore, it can be combined with the simulation of other unit processes used to describe wastewater treatment plants in a systematic and straightforward way. A sensitivity and collinearity analysis performed in the thesis, demonstrated that the model is coherent in its structure and valid to represent struvite precipitation processes. In order to incorporate the hydrodynamic effects to the model, results obtained in an experimental campaign where struvite precipitation was analysed under different mixing and saturation conditions in two different experimental set-ups, were used. Obtained results showed that a higher mixing intensity could be linked with a faster pH decay, an increasing particle density and lower particle size. These effects were included in the population balance model using a calibration procedure based on Bayesian Monte Carlo techniques. From the calibration procedure, new kinetic laws were proposed for struvite nucleation and growth, where the effect of the hydrodynamics had been decoupled by explicitly including the shear rate as a process variable.La precipitación de estruvita (MgNH4PO4·6H2O) es una solución prometedora para la recuperación de fósforo en estaciones depuradoras de aguas residuales. La precipitación controlada de estruvita puede ayudar a reducir la eutrofización provocada por los vertidos de la depuradora, combatir la escasez global de fósforo y reducir los problemas operacionales generados por la precipitación incontrolada del mineral en las tuberías. Debido al interés generado, la descripción del proceso de precipitación ya se ha incluido en las librerías de modelado de tratamiento de aguas residuales existentes. Sin embargo, siguiendo el enfoque clásico de modelado de tratamiento de aguas residuales, el proceso se ha incluido generalmente con una cinética de un solo paso. Este hecho limita el uso del modelo matemático para fines de diseño y optimización de tecnologías, ya que no se consideran ni los mecanismos de precipitación ni la distribución del tamaño de partícula, siendo ésta una variable clave en el desempeño de la estruvita como fertilizante. Por lo tanto, el objetivo de esta tesis ha sido mejorar los modelos cinéticos de un paso existentes mediante el desarrollo de un modelo matemático que describe en detalle los mecanismos que ocurren en la precipitación de estruvita para poder predecir la distribución del tamaño de partícula resultante. Este modelo es un modelo de balance poblacional en el que se han considerado efectos hidrodinámicos. El modelo de balance poblacional se ha construido siguiendo la metodología de modelado integral de planta (‘Plant Wide Model’) de Ceit, garantizando el balance de masa y carga. Por lo tanto, se puede combinar con la simulación de otros procesos que ocurren en las estaciones depuradoras de aguas residuales de una manera sistemática y sencilla. Un análisis de sensibilidad y colinealidad realizado en la tesis, demostró que el modelo es coherente en su estructura y válido para representar procesos de precipitación de estruvita. Para incorporar los efectos hidrodinámicos al modelo, se utilizaron los resultados obtenidos en una campaña experimental donde se analizó la precipitación de estruvita bajo diferentes intensidades de agitación y grado de saturación. Los resultados obtenidos mostraron que una mayor intensidad de agitación podría estar relacionada con una caída más rápida del pH, un aumento del número de partículas y un menor tamaño medio de las partículas obtenidas. Estos efectos se incluyeron en el modelo de balance de población utilizando una metodología de calibración basada en inferencia bayesiana. A partir de la metodología de calibración, se propusieron nuevas leyes cinéticas para la nucleación y el crecimiento de la estruvita, donde el efecto de la hidrodinámica está desacoplado al incluir explícitamente la cizalladura como una variable de proceso

Inclusion of shear rate effects in the kinetics of a discretized population balance model: Application to struvite precipitation

The effect of mixing in the modelling of processes based on mass transfer phenomena is commonly ignored in wastewater treatment industry. In this contribution, the effect of the average shear rate in the nucleation and growth rates of struvite is analyzed by combining experimental data with simulation results obtained with a previously presented mass-based discretized population balance model. According to the obtained results, the effect of the average shear rate is identifiable for the selected data and mechanisms. Therefore, it should be considered when a detailed modelling of the process is needed. Consequently, in this contribution, the average shear rate has been decoupled from the kinetic constants. In addition, kinetic rates where it is explicitly included as a power law function have been proposed. The exponents in these power law functions for the primary homogeneous nucleation and growth are 1.3 and 0.3, respectively. Considering shear rate effects allowed to see in the simulation outputs experimentally observed effects: a faster pH decay and smaller particle distribution for increasing mixing intensities

A CFD-based compartmental modelling approach for long-term dynamic simulation of water resource recovery facilities

This article presents a methodology for compartmental model (CM) creation for long-term simulation of water resource recovery facilities (WRRFs). CMs are often focused on reproducing with a lower computational cost, previously simulated scenarios. In contrast, the methodology presented here can represent variable hydraulic conditions, based on the interpolation of data gathered from a set of computational fluid dynamics (CFD) simulations that reproduce representative hydraulic scenarios. This is achieved by modelling with bidirectional flows the exchange flows between fixed compartments, which are defined based on the geometry of the reactors. The resultant hydraulic surrogate model can be implemented in commercial water treatment software to solve biochemical kinetics. The methodology was applied to simulate in WEST®-DHI a WRRF in Vila-Real, Spain. In this contribution, the CM was validated with real plant data. The developed CM provided a quick response simulation with a high level of hydraulic and biochemical detail. This allowed observation of a spatial distribution of component concentration, which could help with sensor location or plant optimisation. The methodology presented here could also be a useful enabler of digital twins to be implemented in WRRF. HIGHLIGHTS A computational fluid dynamics (CFD)-based compartmental modelling methodology is presented to simulate the dynamic long-term operation of water resource recovery facilities (WRRFs).; A hydraulic surrogate model was developed for the compartmental model (CM) based on multiple CFD simulations to replicate variations in hydraulic conditions.; The CM was validated with real WRRF data.; The developed CM provided quick simulation with high detail and allowed observation of a spatial distribution of component concentration.