INVESTIGATION INTO STRUVITE SOLUBILITY, GROWTH AND DISSOLUTION KINETICS IN THE CONTEXT OF PHOSPHORUS RECOVERY FROM WASTEWATER by MD. IQBAL HOSSAIN BHUIYAN M.Sc, UENSCO-IHE Institute for Water Education, 2002 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Civil Engineering) THE UNIVERSITY OF BRITISH COLUMBIA October 2007 Md. Iqbal Hossain Bhuiyan, 2007 Abstract The present research was conducted to investigate the mechanisms controlling formation, dissolution and decomposition of the mineral, struvite (MgNPLiPO^HiO) in the context of phosphorus recovery from wastewater. Solubility, thermodynamics, kinetics and thermal decomposition of struvite were studied in laboratory and wastewater treatment environments to gain knowledge to optimize the phosphorus recovery process from wastewater through struvite crystallization. The thermodynamic solubility products (Ksp) of struvite were determined by extrapolating measured solubility product values to zero ionic strength, with -log Ksp of 13.36(+0.07) at 25°C, using an appropriate activity coefficient model. A representative temperature compensation factor (a = 0.0198 °CI) has been derived for electrical conductivity (EC) correction, and a relationship between ionic strength (I) and EC has been developed for anaerobic digester supernatant/centrate samples from five different wastewater treatment plants in western Canada. The metastable region, where nucleation is negligible, for struvite precipitation was explored in this study. This region was used in a kinetics study to suppress nucleation of struvite during growth experiments in a bench-scale fluidized bed reactor (FBR). A linear growth rate model has been tested and proposed, which was found to be effective for struvite growth determination in FBRs. The dissolution processes of struvite were investigated in a batch reactor system using two different theoretical models. The experimental values of struvite dissolution were found to fit well with both models. In a mixed flow-through reactor system, the dissolution rates for struvite pellets were found to increase with the hydrogen ion concentration in the acidic pH, while the rate of dissolution in the alkaline pH was found to increase due to hydroxyl-promoted dissolution. The thermal decomposition study of struvite ii showed that the simultaneous loss of both ammonia and water molecules from the struvite structure occurred gradually as a function of temperature, rather than as a distinct step. A pilot-scale struvite recovery FBR developed at The University of British Columbia (UBC) was operated, using the knowledge gained from the thermodynamics and kinetics experiments. The pilot-scale FBR was found to be effective in recovering phosphate from anaerobic digester centrate in the form of a nearly pure struvite. in Table of Contents Abstract ii Table of Contents iv List of Tables viii List of Figures x List of Abbreviations xiv Preface xv Acknowledgements xvii Contribution of others xix Chapter 1 Introduction 1 1.1 Preface 1 1.2 Literature review 3 1.2.1 Why recover phosphorus? 3 1.2.2 Struvite Solubility and Thermodynamics 8 1.2.3 Estimation of Ionic Strength from Electrical Conductivity 10 1.2.4 Precipitation kinetics of struvite 11 1.2.5 Pilot-scale fluidized reactor operation 13 1.2.6 Dissolution kinetics and slow release property of struvite 14 1.2.7 Thermal decomposition of struvite and its phase transition 14 1.3 Research objectives 15 1.4 Thesis outline 17 Chapter 2 A solubility and thermodynamic study of struvite 25 2.1 Introduction 25 2.2 Materials and methods 28 2.2.1 Formation of struvite 28 2.2.2 Equilibration 28 2.2.3 Thermodynamic solubility product, Ksp 30 2.2.4 Speciation and ionic strength calculation 31 2.3 Results and discussion 33 2.3.1 Thermodynamic solubility product 33 2.3.2 Solubility product of struvite at various temperatures 34 2.3.3 Effect of pHon struvite solubility 35 2.3.4 Solubility product (Ksp) value and solubility of struvite 37 iv 2.3.5 Temperature effect on struvite solubility 39 2.3.6 Enthalpy 40 2.4 Conclusions 42 Chapter 3 Determination of temperature dependence of electrical conductivity and its relationship with ionic strength of anaerobic digester supernatant, for struvite formation 55 3.1 Introduction 55 3.2 Materials and methods 58 3.2.1 Temperature dependence ofEC 58 3.2.2 EC-I relationship 59 3.2.3 Analyses 59 3.3 Results and discussion 60 3.3.1 Temperature dependence of EC 60 3.3.2 EC-1 relationship 62 3.4 Conclusions 65 Chapter 4 Nucleation and growth kinetics of struvite in a fluidized bed reactor 77 4.1 Introduction 77 4.2 Materials and methods 81 4.2.1 Determination of induction time 81 4.2.2 Determination of relative supersaturation 83 4.2.3 Identification of metastable region 83 4.2.4 Crystallization system 84 4.2.5 Determination of crystal growth rate 85 4.2.6 Analysis 86 4.3 Results and discussion 87 4.3.1 Nucleation and induction time 87 4.3.2 Metastable region 89 4.3.3 Growth 89 4.3.4 Growth rate expressions 91 4.4 Conclusions 93 Chapter 5 Assessing struvite precipitation in a pilot-scale fluidized bed crystallizer 109 5.1 Introduction and background 109 5.2 Material and methods 112 v 5.2.1 Reactor design and operation.. 112 5.2.2 Chemicals, storage tanks and pumps 113 5.2.3 Sampling and analysis 113 5.2.4 Product Identification 114 5.3 Results and discussion 114 5.3.1 Centrate characteristics during the study 115 5.3.2 Reactor operation 115 5.3.3 Performance of the crystallization process 116 5.3.4 Supersaturation level 117 5.3.5 Induction time and mixing 118 5.3.6 Apparent upflow velocity 119 5.3.7 Mg:P and N:P molar ratio 119 5.3.8 Effect of Organic ligands 120 5.3.9 Influence of calcium and carbonate.ions 121 5.3.10 Crystal morphology 122 5.4 Conclusions 123 Chapter 6 Dissolution kinetics of struvite grown in a pilot-scale crystallizer 148 6.1 Introduction 148 6.2 Materials and Methods 150 6.2.1 Identification 150 6.2.2 Batch reactor system 151 6.2.3 Treatment of data 151 6.2.4 Mixed flow-through reactor system 153 6.2.5 Treatment of data 154 6.2.6 Analysis 155 6.3 Results and discussion 155 6.3.1 Batch reactor system 155 6.3.2 Mixed flow-through reactor system 158 6.4 Conclusions 160 Chapter 7 Thermal decomposition of struvite and its phase transition 176 7.1 Introduction 176 7.2 Materials and methods 178 vi 7.2.1 Formation of struvite 178 7.2.2 Identification of struvite and transformation compounds 179 7.2.3 Phase transition of struvite in excess water 180 7.2.4 Analytical and thermogravimetric methods 180 7.3 Results and discussion 181 7.3.1 Identification of struvite 181 7.3.2 Thermogravimetric Analysis 181 7.3.3 Evaluation of activation energy 183 7.3.4 Phase transition with heating in excess water 184 7.3.5 Phase transition with boiling in excess water 186 7.4 Conclusions 187 Chapter 8 General conclusions and direction for future research 203 8.1 Introduction 203 8.2 Overall conclusions 204 8.3 Engineering significance 207 8.4 Recommendations for future research 209 vii List of Tables Table 2.1 Published ksp values for Struvite at 25 C from Literature 44 Table 2.2 Major equilibria involved in the computation of the solution species at 25 °C. 45 Table 2.3 Solubility products of struvite determined at various temperatures. (Values in parenthesis are the 95% confidence intervals) 46 Table 3.1 Expressions for activity coefficients 66 Table 3.2 Temperature compensation factor (a) and RMS percentage (e) 67 Table 3.3 Chemical composition of the anaerobic digester supernatant/centrate samples. All values in mg 1"' 68 Table 3.4 pH, temperature, electrical conductivity and calculated ionic strength 69 Table 3.5 Correlation matrix for anaerobic digester centrate supernatant/centarte. All values shown are Pearson's correlation coefficients of struvite constituting ion concentrations [ ] , activities { }, EC (pS cm"1) and I (mol 1"') 70 Table 4.1 Concentrations and conditions used in different run during induction time study 95 Table 4.2 Determination of metastable region for struvite at 25°C 96 Table 4.3 Mass transfer coefficient and surface reaction coefficient of struvite at pH=8.07 97 Table 5.1 Centrate characteristics of the Lulu Island Wastewater Treatment Plant during the pilot-scale operation of the struvite crystallizer (n = 23) 125 Table 5.2 Operational conditions during the pilot-scale operation at LIWWTP (n=23). 126 Table 5.3 Calculated Reynolds Number at three sections of the reactor 127 Table 5.4 Major equilibria involved among acetate and struvite constituents (Ball and Nordstorm, 1991) and corresponding solubility product constants at 25 °C... 128 3 Table 5.5 pKsp values and solubility equilibria of possible precipitates in Ca-Mg-P04 "- 2 C03 "system at 25°C 129 Table 6.1 Solubility of struvite (Cs) and equilibrium pH calculated by PHREEQC for different initial pH values 162 viii Table 6.2 Dissolution rate constants with the R2 values of the regressions for different amount of struvite pellets and pH values 163 Table 6.3 Experimental conditions and corresponding rate of dissolutions rates of struvite pellets at 25°C 164 Table 7.1 Solubility product values (Ksp) available in the literature for the precipitates 2+ 3 + + in Mg - P04 " - NH4 - H system 189 Table 7.2 Activation energy of the struvite decomposition reactions at different heating rates 190 ix List of Figures Figure 1.1 Metastable widths for different possible mechanisms of nucleation 20 Figure 2.1 Identification of crystalline solid as struvite using (a) powder X-ray diffraction.