P.A. Schneider

Model-driven experimental evaluation of struvite nucleation, growth and aggregation kinetics

Nutrient stewardship is emerging as an issue of global importance, which will drive the development of nutrient recovery in the near to medium future. This will impact wastewater treatment practices, environmental protection, sustainable agriculture and global food security. A modelling framework for precipitation-based nutrient recovery systems has been developed, incorporating non-ideal solution thermodynamics, a dynamic mass balance and a dynamic population balance to track the development of the precipitating particles. The mechanisms of crystal nucleation and growth and, importantly, aggregation are considered. A novel approach to the population balance embeds the nucleation rate into the model, enabling direct regression of its kinetic parameters. The case study chosen for the modelling framework is that of struvite precipitation, given its wide interest and commercial promise as one possible nutrient recovery pathway. Power law kinetic parameters for nucleation, crystal growth and particle aggregation rates were regressed from an ensemble data set generated from 14 laboratory seeded batch experiments using synthetic solutions. These experiments were highly repeatable, giving confidence to the regressed parameter values. The model successfully describes the dynamic responses of solution pH, the evolving particle size distribution subject to nucleation, growth and aggregation effects and the aqueous magnesium concentration in the liquid phase. The proposed modelling framework could well be extended to other, more complex systems, leading to an improved understanding and commensurately greater confidence in the design, operation and optimisation of large-scale nutrient recovery processes from complex effluents.

Assessments of critical flux in a pilot-scale membrane bioreactor

In this study, the influence of various parameters such as determination methods and step height and length of incremental flux on critical flux values were investigated. Experiments were carried out on a pilot-scale membrane bioreactor (MBR) treated municipal wastewater. Three of the five critical flux determination methods, such as flux linearity, 90% permeability and flux cycling conducted in this study, indicated a decline in critical flux values as the step height of incremental flux increased. However, the hysteresis method and the two-third (2/3) flux limitation method showed an increase and independence of critical flux to the step height of incremental flux, respectively. On the other hand, the step length of incremental flux had no obvious effect on critical flux values evaluated by all critical flux determination methods. Like critical flux, sustainable flux has negative relationship with the increase of step height but no influence of step length was found in this study.

Crystallization of struvite from metastable region with different types of seed crystal

Abstract The main feature of this paper was to recognize struvite crystallization in the metastable region of supersaturation. Thermodynamic equilibria of struvite were simulated to identify the minimum struvite solubility limit, thereafter validated by existing thermodynamic modelling packages such as PHREEQC and the derived data from existing struvite solubility curve. Using laser light scattering detection, spontaneous nucleation was identified by the slow increase of pH in a supersaturated solution of struvite. The crystallization experiment, conducted close to the saturation region in metastable zone, initiated struvite growth. The conducted experiment showed that mother crystal (struvite) was more effective as seeds for struvite crystallization.

Modelling and dynamic simulation of struvite precipitation from source-separated urine

A model of a mixed-mode nutrient recovery reactor is developed for a urine feed, incorporating complex solution thermodynamics, dynamic conservation relations and a power-law kinetic expression for crystal growth from seed crystals. Simulations at nominal operating conditions predict phosphorus recoveries greater than 99%, based on existing process kinetic parameters and operating conditions employed in previously published studies. The overall rate of nutrient recovery depends on the saturation index of the precipitating solid, the available surface area for mass transfer and the feed rate of the limiting constituent ion. Under the conditions considered, the nutrient feed rate appears to be the limiting factor for precipitation. Simulations demonstrate that diurnal feed flow variations of ±50% have a small effect on the rate of nutrient recovery. Overall, the study shows that valuable insights are gained in relation to process performance predictions, which should lead to more confident process design, operation and control.

Dielectric properties of sewage biosolids: Measurement and modeling

Abstract The measurement of dielectric properties of materials is critical to understanding the electromagnetic field distribution in the materials. Reliable knowledge of the dielectric constant is also important when designing a microwave pyrolysis unit for effective pyrolysis of materials of interest. The dielectric probe technique was used to evaluate the dielectric properties of biosolids. Biosolid samples were collected from various sources and the dielectric properties were measured across the microwave frequency range of 1.0 GHz to 10.0 GHz. The dielectric constant of the materials varied in the range from 80 to 2 and depended mainly on the origin of the sample, the electrical conductivity and the moisture content of the material. A mathematical model was developed to accurately predict the values of complex permittivity. This model was verified using the measured data.

Silver removal from aqueous solution by biochar produced from biosolids via microwave pyrolysis

The contamination of water with silver has increased due to the widespread applications of products with silver employed as antimicrobial agent. Adsorption is a cost-effective method for silver removal from aqueous solution. In this study biochar, produced from the microwave assisted pyrolysis of biosolids, was used for silver removal from an aqueous solution. The adsorption kinetics, isotherms and thermodynamics were investigated to better understand the silver removal process by biochar. X-ray diffraction results demonstrated that silver removal was a combination two consecutive mechanisms, reduction and physical adsorption. The Langmuir model fitted the experimental data well, showing that silver removal was predominantly a surface mechanism. The thermodynamic investigation demonstrated that silver removal by biochar was an exothermic process. The final nanocomposite Ag-biochar (biochar plus silver) was used for methylene blue adsorption and photodegradation. This study showed the potential of using biochar produced from biosolids for silver removal as a promising solution to mitigate water pollution and an environmentally sustainable approach for biosolids management and re-use.

Modelling the dynamics of solids transport in flighted rotary dryers

This paper proposes a simple dynamic solids transport model for flighted rotary dryers, which results by discretising the dryer in the axial direction into a series of equivolume elements. Each resultant element is partitioned into two zones; one active and the other passive. Solids interchange between the active and passive zones is included, leading to a tanks-in-series/parallel approach, traditionally used by reaction engineers. Modelling solids transport in this manner allows the residence time distribution (RTD) characteristics of the rotary dryer to be elucidated. In this work gPROMS is used to simulate the proposed rotary dryer model. Data from a 100 tonne per hour raw sugar dryer is reconciled against the dynamic solids transport model, by estimating overall solids transport coefficients.

Improving the Control of an Industrial Sugar Crystalliser: a Dynamic Optimisation Approach

The controllability of an industrial sugar crystalliser is investigated. Using a simultaneous integration and optimisation approach, the optimal control problem is formulated in GAMS, based on a dynamic model of the vacuum pan validated against plant data. MINOS 2.50 solves this open-loop model, yielding: the minimum batch time, set-point trajectories and the optimal switching from high to low purity feed. These results are implemented within double-loop PI controllers. Alternative control variables are proposed, replacing traditional process outputs, which are not fundamentally process-relevant. Dynamic simulation results show that the proposed control schemes are satisfactory in the face of errors in growth rate correlations and variations in feed properties and initial batch conditions. Preliminary full-scale investigations have shown promise and will be extended into next years sugar cane crushing season.