Investigation on the FO-RO osmotic dilution process for the integration of seawater desalination and wastewater reclamation : fouling quantification, index development and scale-up demonstration

다국어 초록 (Multilingual Abstract)

Forward osmosis (FO) is an osmotically driven membrane process that can be used as an alternative to conventional pressure-driven membrane processes for desalting unconventional saline water resources with enhanced sustainability. Recently, the FO process integrated with reverse osmosis (RO) as an osmotic dilution process was proposed to attain two goals simultaneously: wastewater reclamation and seawater desalination. In the applications of FO, most previous lab-scale studies of organic fouling on the FO membrane surface caused by wastewater have investigated the fouling behavior in terms of fouling reversibility (FR). However, notably, the implementation of this FO-RO dilution process is still considered to be hampered by the potential irreversible membrane fouling over a long period, as the organic pollutants contained in real wastewater are complex in nature.
Well-designed pretreatment stage is particularly important for successful RO applications. Thus, fouling index such as silt density index (SDI) and modified fouling index (MFI) were suggested to predict the fouling rate of RO process in advance. However, efforts on quantifying the FO fouling through a standard fouling index have not been made and realized yet. As an indication parameter of water quality, SDI have some limitations on giving precise real-time prediction and guide the FO system operation. Therefore, during real wastewater or seawater testing in the FO process for large scale operations, an assessment to verify whether the MFI-based fouling index and development of FO specialized fouling simulator are required for in-situ real-time fouling assessment and understanding the fouling mechanism during long term operation and industrial scale.
The goal of this thesis is to quantify FO irreversible fouling in hybrid FO-RO process for wastewater reclamation. First, the fouling behavior in the long-term performance of the FO-based osmotic dilution process was systematically explored by assessing the impact of irreversibility. Then, a novel osmotically driven reversibility index (ORI) and tailored MFI were developed and employed to reflect FO membrane fouling reversibility. At last, the feasibility of developed fouling indexes (ORI and MFI) was verified through plant study.
The dissolved organic matters in secondary wastewater effluent, commonly known as effluent organic matter (EfOM), strongly adhere to the FO membrane surface and raise irreversible membrane fouling concerns regarding the deterioration of the FO performance over the long term. Thus, a quantitative correlation analysis of the importance of organic components was performed by incorporating the concept of FR for simulating the FO irreversible fouling and long-term performance. It was revealed that a strong correlation between irreversible foulant concentrations, especially that of protein-like biopolymer, and physical fouling reversibility was observed, which caused continual irreversible fouling in the FO process.
As FO uses osmotic pressure as a driving force, a loose and dispersed fouling layer is produced, as opposed to the compacted fouling layer in RO, which is operated under high pressure conditions to overcome the osmotic pressure of the feed solution. The behavior of fouling reversibility in FO was first investigated thoroughly with various secondary effluents obtained from real-scale wastewater treatment plants in Korea, to provide a foundation for the ORI development. After conducting a long-term stable FO operation, it is deduced that the ORI is a useful evaluation parameter for continuous FO and its cleaning cycles in which periodic flushing is normally utilized to exploit and enhance the fouling reversibility of FO.
Lastly, practical study was carried out to evaluate the applicability of the MFI-UF fouling potential index and the specific operation ORI index in a FO-RO pilot plant. The correlation method between the fouling indexes and FO membrane performance (i.e., transmembrane inlet pressure (TMIP), differential pressure (DP)) was well validated based on seven months of pilot operation data. MFI-UF was selected for fouling potential measurement according to the pretreatment process (or source water) while ORI was obtained for optimizing the cleaning protocol of FO process in order to support the stable operation of FO plant applications. The results from the current study are expected to provide practical insight and technical guidelines for FO based plant engineers/operators.
Overall, this study investigated to improve the FO performance through the development of novel FO-based indexes at both the lab and pilot scales. It highlighted fouling index would become to the fore in designing and operating practical application of FO plants. We expect that the accumulation of field data in FO using multiple FO fouling indexes (MFI-UF and ORI) could aid in establishing new criteria for a more accurate prediction of fouling potential, and thus, a more effective fouling control strategy can be delicately designed for the sustainable operation of FO in a comprehensive perspective.

목차 (Table of Contents)

• 1. General Introduction 1
• 1.1 Motivations 2
• 1.2. Objectives and scope of thesis 4
• 2. Literature Review 7
• 2.1 Introduction 8
• 2.2 Where does FO membrane stand? 14
• 2.2.1 Historical Development 14
• 2.2.2 Current Limitation 18
• 2.3. Commercial Application of Forward Osmosis Membrane 28
• 2.3.1. Osmotic Dilution-system 28
• 2.3.1.1 Forward osmosis bag (FOB) 28
• 2.3.1.2 Osmotic pump 29
• 2.3.1.3 Fertilizer-drawn forward osmosis (FDFO) 33
• 2.3.2. Osmotic Concentration 41
• 2.3.3. Simultaneous Osmotic Dilution and Concentration 44
• 2.4 Concluding Remark: Future Prospect 55
• 2.5. References 70
• 3. Quantitative Analysis of the Irreversible Membrane Fouling of Forward Osmosis during Wastewater Reclamation: Correlation with the Modified Fouling Index 89
• 3.1. Introduction 90
• 3.2. Methods and materials 94
• 3.2.1. Source water 94
• 3.2.2. Lab-scale FO experiments 95
• 3.2.3. Analytical methods 96
• 3.2.4. Quantitative analysis of the irreversible membrane fouling of FO 97
• 3.2.5. Modified fouling index (MFI) analysis 99
• 3.2.6. Statistical analysis 100
• 3.3. Results and discussion 100
• 3.3.1. Fouling behavior on the FO membrane during wastewater reclamation 100
• 3.3.2. Investigation of irreversible membrane fouling: quantitative analysis 104
• 3.3.3. Applicability of the MFI 112
• 3.4. Conclusions 117
• 3.5. References 119
• 4. Indexing Fouling Reversibility in Forward Osmosis and its Implications for Sustainable Operation of Wastewater Reclamation 131
• 4.1. Introduction 132
• 4.2. Material and method 135
• 4.2.1. Feed and draw solutions 135
• 4.2.2. FO experiments 136
• 4.2.3. Foulant analysis on the FO membrane 137
• 4.3. Results and discussion 138
• 4.3.1. Quantification of FO fouling by its reversibility 138
• 4.3.1.1. Investigation on fouling propensity and reversibility 138
• 4.3.1.2. Correlation analysis between water flux and fouling reversibility 142
• 4.3.2. Osmotically-driven reversibility index: protocol development 146
• 4.3.3. ORI measurement and verification 154
• 4.4. Conclusions 159
• 4.5. References 160
• 5. Application of fouling index for forward osmosis (FO) hybrid system: demonstration in pilot 165
• 5.1. Introduction 166
• 5.2. Material and method 169
• 5.2.1. Overview of experimental pilot-scale system description 169
• 5.2.2. Pilot data collection 171
• 5.2.3. Osmotically-driven reversibility index (ORI) measurement 173
• 5.2.4. Modified fouling index (MFI) analysis 174
• 5.2.5. Membrane fouling characterization 175
• 5.3. Results and discussion 175
• 5.3.1 Predicting FO performance through ORI: Lab-scale continuous FO fouling test 175
• 5.3.2. Pilot–scale FO operating performance: pretreatment strategy 180
• 5.3.3. Application of MFI–UF and ORI to monitor FO membrane performance 182
• 5.3.4. Sensitivity analysis of seasonal effect 185
• 5.3.5. Implications of FO fouling indexes protocol based on field operations 187
• 5.4. Conclusion 189
• 5.5. References 190
• 6. Concluding Remarks 195
• 7. Appendix I: Pilot–scale Evaluation of FO–RO Osmotic Dilution Process for Treating Wastewater from Coal–fired Power Plant Integrated with Seawater Desalination 198
• 7.1. Introduction 199
• 7.2. Material and method 203
• 7.2.1. Overview of experimental approaches 203
• 7.2.2. Pilot–scale system design 203
• 7.2.3. Water quality analysis 205
• 7.2.4. Assessment of fouling potential in RO 206
• 7.2.5. Liquid chromatography – organic carbon detection 207
• 7.2.6. Energy consumption evaluation 207
• 7.3. Results and discussion 209
• 7.3.1 Pilot–Scale evaluation of FO–RO dilution process 209
• 7.3.2. Analysis of foulants on the pilot–scale RO membrane 213
• 7.3.3. Water quality requirements for power plant cooling water 214
• 7.3.4. Energy consumption evaluation of FO–RO pilot plant 215
• 7.4. Conclusion 222
• 7.5. References 224