Evaluation of membrane crystallization for the recovery of freshwater and mineral salts from high-saline wastewater By Indira Camryn Theresa Chimanlal A dissertation submitted to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Master of Science (Chemistry) Supervisor: Dr. H. Richards Co-supervisor: Dr. L. Nthunya Date: 5 June 2023 ii DECLARATION I declare that this dissertation is my own, unaided work. It is undergoing submission toward the degree of Master of Science at the University of the Witwatersrand, Johannesburg. Furthermore, this work has not been submitted before for any other degree at any other university. Signature of candidate Indira Chimanlal iii DEDICATION This work is dedicated to my loving parents who have always supported and helped me through everything that I do. Thank you for all your sacrifices, it did not go unnoticed. I hope that this makes you proud and I love you immensely. This work is also dedicated to my very special foi, Kirti, you have supported me through everything, guided me, and you have always been there for me. Your wisdom, smile, and laughter remain in my heart forever. You were proud of me. I love you. iv ACKNOWLEDGEMENTS I was able to produce this work with the assistance of many people who have offered support, help, and advice when I needed it. I would like to acknowledge Dr. Heidi Richards for the financial acquisition that funded this project and for the support offered during this project. I would like to thank Prof. Cejna Anne Quist-Jensen (Aalborg University) who provided me with support, guidance, and assistance during my time spent in Denmark. I would also like to acknowledge Dr. Lebea Nthunya for all the assistance and guidance provided in this project. I wish to express my gratitude to the Danish International Development Agency (DANIDA) and the University of the Witwatersrand for their financial support during my studies. I would like to acknowledge the great support offered by Dr. Shaeen Chetty. Even though you were not my supervisor, you were there whenever I needed help no matter what time of day it was. You offered to support me each time I had a problem and you assisted me with finding solutions. I cannot express enough how grateful I am. I would like to acknowledge the contribution of my friends to the Environmental and Analytical Chemistry research group. Thank you for looking out for me, supporting me, and lending a much-needed helping hand. I would like to extend my gratitude to the Chemistry and Bioscience research group at Aalborg University who welcomed me with open arms and created a friendly and inviting atmosphere. Thank you for assisting me to progress my research and for providing guidance whenever I experienced challenges. I thoroughly enjoyed my time spent there. It was truly a memorable experience, something I would not trade for anything. I would like to acknowledge my parents throughout this phase in my life. You have supported me, guided me, and encouraged me. I would like to acknowledge my very special aunt who always kept my best interests at heart and provided me with the best wisdom and strength when I needed it the most. You helped shape me into the person I am today. Whenever I experienced challenging moments throughout this degree and contemplated quitting, when each failure contributed to my frustration, and when difficulties got in the way of my well-being, my family was always around to comfort and encourage me to continue. To my friends and family, I could not have made it this far without you. You have supported me in great ways. You have been there to celebrate my successes and encouraged the rise from my falls. Thank you for allowing me to reach greater heights. Words cannot describe what you mean to me and your impact on my life. Thank you for always assisting me whenever I needed v help, when I did not know whom to turn to, you were there. I am blessed to have such people in my life. To my nieces, Eliana, and Gianna, I hope to be a good role model to you. Thank you for all the joy you bring to my life. To the many pets in my life, you have always put a smile on my face when I needed it most and you have brought me happiness. You may not understand what goes on, but you have left paw prints in my heart. vi PUBLICATIONS This dissertation is largely based on articles that were produced from this study. Both published articles and those in preparation make up this dissertation and the details thereof are provided below for reference: Membrane distillation crystallization for water and mineral recovery : The occurrence of fouling and its control during wastewater treatment Indira Chimanlal, Lebea N. Nthunya, Cejna Anne Quist-Jensen, and Heidi Richards Published in Frontiers in Chemical Engineering, (2022) Vol. 4, ISSN = 2673-2718 DOI=10.3389/fceng.2022.1066027 Nanoparticle-Enhanced PVDF Flat-Sheet Membranes for Seawater Desalination in Direct Contact Membrane Distillation Indira Chimanlal, Lebea N. Nthunya, Oranso T. Mahlangu, Bastian Kirkebæk, Aamer Ali, Cejna A. Quist-Jensen, and Heidi Richards Published in Membranes, MDPI, (2023) Vol. 13(3), 317 https://doi.org/10.3390/membranes13030317 The Evaluation of Modified Synthetic Flat-Sheet PVDF Membranes for Membrane Distillation Crystallization (MDC) Using Simulated Wastewater Indira Chimanlal, Lebea N. Nthunya, Cejna Quist-Jensen, Heidi Richards (Manuscript in preparation) vii CANDIDATE CONTRIBUTION TO ARTICLES Membrane distillation crystallization for water and mineral recovery: The occurrence of fouling and its control during wastewater treatment Indira Chimanlal, Lebea N. Nthunya, Cejna Quist-Jensen, and Heidi Richards Candidate contribution: Principle author responsible for investigation; methodology; validation; writing and editing. Nanoparticle-Enhanced PVDF Flat-Sheet Membranes for Seawater Desalination in Direct Contact Membrane Distillation Indira Chimanlal, Lebea N. Nthunya, Oranso T. Mahlangu, Bastian Kirkebæk, Aamer Ali, Cejna Quist-Jensen, and Heidi Richards Candidate contribution: Principle author who was responsible for investigation; formal analysis; methodology; writing and editing. The Evaluation of Modified Synthetic Flat-Sheet PVDF Membranes for Membrane Distillation Crystallization (MDC) Using Simulated Wastewater Indira Chimanlal, Lebea N. Nthunya, Cejna Quist-Jensen, and Heidi Richards Candidate contribution: Principle author involved in the investigation and methodology; data analysis; writing and editing viii OTHER RESEARCH OUTPUTS Book chapter Hybrid membrane processes equipped with crystallization unit for simultaneous recovery of freshwater and minerals from saline wastewater in Innovative Trends in Removal of Refractory Pollutants from Pharmaceutical Wastewater Tshepiso Mpala, Indira Chimanlal, Heidi Richards, Anita Etale, Lebea N. Nthunya (In press) ix TABLE OF CONTENTS DECLARATION …………………………………………………………………... ii DEDICATION ……………………………………………………………………... iii ACKNOWLEDGEMENTS ………………………………………………………... iv PUBLICATIONS ………………………………………………………………….. vi CANDIDATE CONTRIBUTION TO ARTICLES ………………………………... vii OTHER RESEARCH OUTPUTS …………………………………………………. viii TABLE OF CONTENTS ………………………………………………………….. ix LIST OF FIGURES …………………………………………………………........... xiii LIST OF TABLES ………………………………………………………………… xv NOMENCLATURE ………………………………………………………………. xvi CHAPTER ONE – INTRODUCTION ………………………………………….. 1 1.1 Background …………………………………………………………………….. 1 CHAPTER TWO – LITERATURE REVIEW …………………......................... 4 Membrane distillation crystallization for water and mineral recovery: The occurrence of fouling and its control during wastewater treatment 2.1 Introduction ……………………………………………………………………. 4 2.2 Principles of membrane distillation crystallization ……………………………. 5 2.3 Parameter optimization to enhance membrane distillation crystallization …….. 7 2.3.1 Process temperature ………………………………………………………….. 7 2.3.2 Solution supersaturation ……………………………………………………… 8 2.3.3 Duration of crystallization ……………………………………………………. 9 2.3.4 Recirculation rate …………………………………………………………….. 9 2.4 Fouling of MDC membranes …………………………………………………… 10 2.5 Fouling control …………………………………………………………………. 12 2.5.1 Pre-treatment …………………………………………………………………. 12 2.5.2 Use of anti-scalants …………………………………………………………... 14 2.5.3 Membrane flushing and gas bubbling ………………………………………... 14 2.5.4 Temperature adjustments and backflow ……………………………………… 15 2.5.5 Chemical cleaning ……………………………………………………………. 16 2.5.6 Membrane modification ….…………………………………………………... 21 2.6 Application in wastewater treatment …………………………………………… 24 2.7 Conclusions and future perspectives ..………………………………………….. 29 x References ………………………………………………………………………….. 30 CHAPTER THREE – RESEARCH OBJECTIVES ……………………………. 42 3.1 Research significance and study objective …………………………………….. 42 3.2 Specific research objectives ……………………………………………………. 42 3.3 Hypothesis ……………………………………………………………………… 43 CHAPTER FOUR – METHODOLOGY………………………………………… 44 4.1 Instrumentation …………………………………………………………………. 45 4.1.1 Inductively coupled plasma–optical emission spectroscopy (ICP-OES) ……. 45 4.1.2 Ion chromatography (IC) ……………………………………………………. 46 4.1.3 Fourier transform infrared spectroscopy (FTIR) ……………………………. 46 4.1.4 Scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM/EDX) ………………………………………………………………………… 46 4.1.5 Transmission electron microscopy (TEM) …………………………………… 47 4.1.6 Atomic force microscopy (AFM) ……………………………………………. 47 4.1.7 Tensile strength ………………………………………………………………. 47 4.1.8 Water contact angle ………………………………………………………….. 48 4.1.9 Powder x-ray diffraction (PXRD) and single crystal x-ray diffraction (SXRD ……………………………………………………………………………. 48 4.1.10 Optical microscopy …………………………………………………………. 48 References …………………………………………………………………………. 48 CHAPTER FIVE …………………………………………………………………. 50 Nanoparticle-Enhanced PVDF Flat-Sheet Membranes for Seawater Desalination in Direct Contact Membrane Distillation 5.1 Introduction …………………………………………………………………….. 50 5.2 Materials and methods …………………………………………………………. 52 5.2.1 Chemicals and equipment …………………………………………………… 52 5.2.2 Synthesis and functionalisation of silica nanoparticles (SiO2NPs) …………... 53 5.2.3 Functionalisation of CNTs …………………………………………………… 53 5.2.4 Membrane preparation ………………………………………………………. 54 5.2.5 Characterisation of the NPs and membranes ………………………………… 54 5.2.6 Membrane distillation ………………………………………………………... 55 5.3 Results and discussion …………………………………………………………. 56 5.3.1 TEM analysis of SiO2NPs and CNTs ………………………………………… 56 xi 5.3.2 Membrane porosity, pore size, WCA, and LEP ……………………………… 68 5.3.3 SEM analysis of the membranes ……………………………………………... 60 5.3.4 AFM analysis of the membranes …………………………………………….. 61 5.3.5 Mechanical properties of the membranes …………………………………….. 62 5.3.6 Flux and salt rejection evaluation in DCMD using synthetic saltwater .....…... 64 5.4 Conclusion ……………………………………………………………………… 69 References …………………………………………………………………………. 69 CHAPTER SIX …………………………………………………………………… 76 The Evaluation of Modified Synthetic Flat-Sheet PVDF Membranes for Membrane Distillation Crystallization (MDC) Using Simulated Wastewater 6.1 Introduction …………………………………………………………………….. 76 6.2 Methods and materials …………………………………………………………. 78 6.2.1 Chemicals and equipment ……………………………………………………. 78 6.2.2 Solution preparation …………………………………………………………. 78 6.2.2.1 Working standards …………………………………………………………. 78 6.2.2.2 Feed solutions ……………………………………………………………… 78 6.2.3 Wastewater sampling and analysis …………………………………………… 79 6.2.4 Membrane fabrication and characterization ………………………………….. 80 6.2.5 Membrane distillation crystallization (MDC) tests ………………………….. 80 6.2.6 Crystal characterization ……………………………………………………… 81 6.3 Results and discussions ………………………………………………………… 82 6.3.1 Quantitative determinations of minerals present in wastewater samples …….. 82 6.3.1.1 Physical parameters of all wastewater samples …………………………….. 82 6.3.1.2 Wastewater analysis – ICP-OES and IC …………………………………… 83 6.3.2 Freshwater and mineral recovery in MDC …………………………………… 86 6.3.3 Crystal analysis ………………………………………………………………. 87 6.3.3.1 Microscopic imagery ……………………………………………………….. 87 6.3.3.2 Crystal structure refinement and identification …………………………….. 90 6.3.3.3 Powder x-ray diffraction (PXRD) ………………………………………….. 91 6.3.4 Membrane deposition ………………………………………………………… 92 6.4 Conclusion ……………………………………………………………………… 94 References ………………………………………………………………………….. 95 CHAPTER SEVEN – CONCLUSIONS AND RECOMMENDATIONS ……... 99 xii 7.1 Conclusions and recommendations …………………………………………….. 99 Appendix xiii LIST OF FIGURES Figure 2.1: Schematic representation of MDC for recovery of freshwater and minerals from industrial wastewater ……………………………………………….. 6 Figure 2.2: A graphical representation of membrane pore wetting experienced in MDC ……………………………………………………………………………….. 11 Figure 2.3: A summary of pre-treatment processes in MDC (a) pre-treatment classifications and (b) process selection for specific foulant ………………………. 14 Figure 2.4: Average cleaning efficiencies as a function of varying NaOCl concentrations, (b) overall cleaning efficiencies (OCE) observed for aged, fouled membranes (Puspitasari et al., 2010) ………………………………………………. 17 Figure 2.5: Normalized water flux (J/Jo) vs. concentration factor for the individual membranes upon evaluation of (a) CaSO4 scaling, (b) casein protein organic fouling (Xiao et al., 2020) ………………………………………………………………….. 22 Figure 5.1: TEM micrographs and corresponding particle size distribution (A1-A2) SiONPs, (B1-B2) fSiONPs, (C1-C2) CNTs, and (D1-D2) fCNTs ………………… 58 Figure 5.2: SEM micrographs of (A1-A2) M1, (B1-B2) M2, (C1-C2) M3, (D1-D2) M4, (E1-E2) PTFE-20, and (F1-F2) PTFE-45: Surface and cross-sectional structures 61 Figure 5.3: Topographical micrographs displaying surface roughness of (A) M1, (B) M2, (C) M3, (D) M4, (E) PTFE-20, and (F) PTFE-45 …………………………. 62 Figure 5.4: Stress-strain plots of (A) the as-synthesised membranes and (B) the commercial membranes …………………………………………………………….. 63 Figure 5.5: Permeate flux, deltaT, permeate conductivity vs time at various temperatures for NaCl separation in DCMD (A1-A3) M1, (B1-B3) M2, (C1-C3) M3, (D1-D3) M4, (E1-E3) PTFE-20, and (F1-F3) PTFE-45 ………………………. 65 xiv Figure 6.1: (A1) Permeate flux plots for all three membranes and (A2) their corresponding conductivity profiles ………………………………………………… 87 Figure 6.2: Micrographs depicting crystal development during MDC using membranes (A) PTFE-20, (B) M3, and (C) M5 ……………………………………. 88 Figure 6.3: SEM micrographs showing crystal morphology following MDC tests using (A) PTFE-20, (B) M3, and (C) M5 …………………………………………… 90 Figure 6.4: Crystal structures of gypsum produced with PTFE-20 and M5, respectively …………………………………………………………………………. 91 Figure 6.5: Experimental PXRD patterns for using (A) PTFE-20, (B) M3, and (C) M5 …………………………………………………………………………………... 92 Figure 6.6: SEM micrographs depicting deposition on the membrane surface of (A) PTFE-20, (B) M3, and, (C) M5 ……………………………………………………... 93 Figure 6.7: EDS spectra showing the elemental composition of membrane fouling deposition on (A) PTFE-20, (B) M3, and, (C) M5 …………………………………. 93 xv LIST OF TABLES Table 2.1: Summary of the advantages and disadvantages associated with MDC ... 6 Table 2.2: Summary of the effect of varying the feed temperature on the permeate flux …………………………………………………………………………………. 8 Table 2.3: Implications of fouling during experimental procedures ………………. 11 Table 2.4: Summary of the cleaning strategies used in various studies …………… 19 Table 2.5: Summary of various membrane modification strategies and their effects on membrane properties ……………………………………………………………. 23 Table 2.6: Various application of MDC towards treatment of wastewater and mineral recovery ……………………………………………………………………. 26 Table 5.1: Composition of the as-synthesised membranes ………………………… 54 Table 5.2: Physical characteristics of the synthesised PVDF and PTFE membranes 60 Table 5.3: Young’s modulus of the as-synthesised PVDF membranes and commercial PTFE membranes ……………………………………………………... 63 Table 5.4: Comparison of nanoparticle modified PVDF membranes for water desalination in DCMD ……………………………………………………………... 67 Table 6.1: Composition of the synthetic AMD solution used for MDC …………... 81 Table 6.2: Physical parameters of the various wastewater samples collected and measured at room temperature ……………………………………………………... 82 Table 6.3: Concentrations of analytes contributing to water salinity using ICP-OES and IC …………………………………………………………………………. 85 Table 6.4: Crystallographic information for gypsum obtained with PTFE-20 and M5 ………………………………………………………………………………….. 90 xvi NOMENCLATURE AFM Atomic force microscopy CNTs Carbon nanotubes DCMD Direct contact membrane distillation DMac Dimethylacetamide DMF EDS Dimethylformamide Energy dispersive x-ray spectroscopy fCNTs Functionalised carbon nanotubes FTIR Fourier transform infrared Spectroscopy IC Ion chromatography ICP-OES Inductively coupled plasma optical emission spectroscopy LEP Liquid entry pressure MCr Membrane crystallisation MD MDC NPs Membrane distillation Membrane distillation crystallisation nanoparticles POTS 1H,1H,2H,2H-Perfluorooctyl triethoxysilane PTFE Polytetrafluoroethylene PVDF Polyvinylidene-fluoride PVP PXRD Polyvinylpyrrolidone Powder x-ray diffraction SEM/EDX Scanning electron microscopy/ energy dispersive x-ray spectroscopy TEM Transmission electron microscopy TEOS Tetraethyl orthosilicate WCA Water contact angle XRD X-ray diffraction 1 CHAPTER ONE – INTRODUCTION 1.1 Background Water scarcity has been a distressing crisis in many countries around the world for years and will continue to pose a threat in the future. Approximately 2.2 billion people around the world are unable to access clean and safe drinking water (UNICEF, 2019). Particularly for developing countries, other factors such as degenerating wastewater treatment infrastructure, reducing freshwater sources, and financial constraints contribute to the enormity of water scarcity (Farmani et al., 2021). Moreover, high salt content in water is yet another contributing factor to water shortages because it requires intensive pre-treatment to ensure its suitability for consumption (Vineis et al., 2011). This is a consequence of wastewater discharged from industrial sources and when left untreated can prove to be detrimental for many species. Some of the disadvantages of high saline water include deteriorating water quality and aquatic health, eutrophication, plant dehydration, and ecological imbalances (Sparenberg et al, 2020; Dow et al., 2022). Approximately 309 million people in developing African countries experience intermittent water supply where drinking water is supplied for less than a full day (Farmani et al., 2021; Loubser et al., 2021). In addition to a water scarcity crisis, mineral shortages present itself as an emerging economic problem. Mineral shortages typically arise due to the growing industrial and economic sector thus further stimulating greater demand for these resources. The increased pressure generated by the renewable energy sector also encourages a greater demand for minerals as they are used in products such as electric generators for wind turbines, electric vehicles, and solar panels (Calvo and Valero, 2022). Therefore, research was directed toward wastewater treatment with an aim to recover valuable components such as mineral salts, nutrients, organic products, and freshwater. The utilization of wastewater for mineral resource reclamation has the potential to drive a circular economy and provide an alternative avenue apart from the conventional methods (Kharraz et al., 2022). Moreover, this is advantageous for the environment as harmful and hazardous wastewater can be adequately treated prior to discharge. Due to the small percentage of drinkable water on earth, clean drinking water is commonly produced from treated wastewater. Examples of membrane separation processes that are used to treat wastewater include nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), and reverse osmosis (RO). Other methods include electrodialysis (ED) and membrane bioreactors (MBR), and evaporation ponds that are used to retrieve valuable salt components in the water. 2 However, these procedures have their associated disadvantages. Processes such as RO are limited by operating pressures (Hickenbottom and Cath, 2014), MBR processes experience poor effluent quality due to the presence of organic contaminants (Kharraz et al., 2022), and evaporation ponds suffer from high water losses to the environment. The gravity of the setbacks discussed herein motivate the need to find sustainable technology that may assist to alleviate the problems experienced. One such solution is membrane crystallisation (MCr), otherwise known as membrane distillation crystallisation (MDC), which was first introduced by Curcio et al. (2001). This technique is an augmented form of membrane distillation (MD) with an added crystallisation component which MD does not include. It is relatively new but is an avenue of membrane application technology that is gaining popularity. This technology has demonstrated its potential in sustainably providing solutions to the water and mineral shortage crises. Principally, this technology utilises a hydrophobic membrane as an interface, which separates a warmer feed side from a cooler permeate region. The feed solution is heated below boiling point, thereby facilitating a water phase change. The water vapour that is generated passes through the hydrophobic membrane, which is condensed by a colder permeate stream, thereby collecting freshwater from a previously polluted feed stream. The temperature difference initiated across the membrane interface introduces a vapour pressure gradient which is the driving force of this technique. The gradual elimination of water from the feed solution drives the solution to supersaturation which allows the salt constituents to be crystallised. A unique advantage of this technology is that it can utilise renewable energy sources, therefore making it a sustainable technology. It can treat brine and has the ability to achieve near zero liquid discharge (ZLD) (Balis et al., 2022). It can also achieve higher total water recoveries compared to MD alone (Balis et al., 2022). Furthermore, it is not hindered by osmotic pressure and can operate independently of the feed concentration, therefore even industrial wastewater can be utilised (Shi et al., 2022). It is not energetically intensive as it can operate at lower temperatures and pressures (Quist-Jensen et al., 2017), and it can simultaneously provide freshwater and mineral salts (Hickenbottom and Cath, 2014). However, the success of this technology is limited due to disadvantages such as membrane fouling and wetting. The former is the result of feed contaminant deposition onto the membrane surface or within its pores thereby leading to pore blocking. (Pramanik et al., 2016). Common types of fouling include inorganic (scaling), organic and biofouling (Drioli et al.,2015). Fouling can lead to structural damage and overall performance deterioration. This problem is alleviated using pre-treatment 3 strategies to eliminate or reduce the foulants in the feed solution, or membrane cleaning to restore its original performance. MDC has the added benefit of reducing the occurrence of scaling as compared to MD, however it does not completely prevent it from happening (Shi et al., 2022). Membrane wetting occurs when water in its liquid state passes through the membrane and provides an avenue for the permeation of any minerals or contaminants in the feed solution. This can be circumvented using hydrophobic membranes. Consequences of this includes a reduction of mass transfer and declining process efficiency. Other disadvantages include polarization effects namely: temperature and concentration polarisation. Temperature polarization results in reduced heat transfer which diminishes the driving force, additionally, concentration polarization stimulates scaling as it facilitates an increase in the solute concentration at the feed-membrane interface. Moreover, this reduces mass transfer across the membrane (Hickenbottom and Cath, 2014). In this work membrane crystallization was evaluated using a selection of synthesised polyvinylidene fluoride (PVDF) membranes containing additives to improve their properties. Moreover, these membranes were comparatively assessed alongside commercial polytetrafluoroethylene (PTFE) membranes. These membranes were evaluated through a series of Direct Contact Membrane Distillation (DCMD) experiments using simulated seawater (3.5 wt% NaCl), after which they were evaluated in MDC application using simulated acid mine drainage (AMD), which has not yet been widely reported in literature thus far. AMD was targeted in this study as it is highly prevalent in South Africa due to the high level of mining activity especially in inland provinces. Its high occurrence provides an opportunity of waste reclamation due to the harmful environmental and health risks AMD poses. 4 CHAPTER TWO – LITERATURE REVIEW 2 Membrane distillation crystallization for water and mineral recovery: The occurrence of fouling and its control during wastewater treatment The information presented in this chapter was published as a review article in the journal Frontiers in Chemical Engineering. This work was authored by Indira Chimanlal, Lebea N. Nthunya, Cejna Anne Quist-Jensen, and Heidi Richards and titled “Membrane distillation crystallization for water and mineral recovery: The occurrence of fouling and its control during wastewater treatment”. 2.1 Introduction Presently, about 4 billion people globally are affected by water scarcity (Mekonnen and Hoekstra, 2016). Water scarcity is influenced by an increase in urbanization and industrialization, population growth, and climate change (Ahmed, Hashaikeh and Hilal, 2020). Additionally, mineral resource depletion is emerging as an industrial problem. Thus, the decline in raw materials results in energy and financial challenges in several industries (Quist- Jensen et al., 2016). The shortage of raw materials consequently minimizes industrial production required to meet the market demand. Therefore, recycling mineral resources from waste streams while recovering freshwater is imperative. This avenue circumvents the search for freshwater sources due to their steady depletion. A progressively attractive technique addressing the issues of mineral and freshwater shortages is membrane distillation crystallization (MDC). Interestingly, MDC affords simultaneous recovery of both mineral crystals and freshwater from high-saline wastewater (Quist-Jensen et al., 2016). Technically, MDC is a hybrid process consisting of membrane distillation (MD) and a crystallization reactor wherein the feed solution is concentrated in the MD system to reach supersaturation, followed by crystallization to recover the minerals (Quist-Jensen et al., 2017). Particularly, MDC can overcome challenges associated with common wastewater treatment options such as reverse osmosis (RO) and nanofiltration (NF) (Pramanik, Shu and Jegatheesan, 2017). Additionally, MDC operates at low temperatures and pressures, uses simple configuration, and consumes less energy compared to other thermal processes (Bouchrit et al., 2017; Pramanik, Shu and 5 Jegatheesan, 2017). This review aims to unpack the principles and process characteristics of MDC for mineral and water recovery. Secondly, membrane fouling, and scale control measures are highlighted. Furthermore, process parameter optimization towards permeate flux, and crystal growth and selectivity are discussed. Additionally, membrane fabrication and modification strategies are reviewed to provide further insight into the development of more efficient and competitive membranes. Lastly, the latest developments towards MDC application are reported. 2.2 Principles of membrane distillation crystallization (MDC) Membrane distillation (MD) has been extensively evaluated for the desalination of seawater and the treatment of high saline industrially discharged wastewater. During desalination processes, concentrated brines are generated and discharged to the environment. However, these brines could be treated further to recover mineral resources. Drioli, Ali and Macedonio (2015) regard mineral resources to be more economically valuable compared to fresh water produced from MD processes. In their study, the researchers presented a proof-of-concept to extract mineral resources from MD desalination plants (Drioli, Ali and Macedonio, 2015). In this regard, MDC emerged as a new technology with similar mechanisms to MD. The MDC saturates the feed solution to recover mineral crystals. The feed solution is concentrated through the MD process while recovering fresh water (Pramanik et al., 2016). In this process, the feed solution becomes concentrated towards super-saturation, thus enabling nucleation and mineral crystallization while simultaneously recovering freshwater on the permeate side of the membrane (Figure 2.1) (Quist-Jensen et al., 2016). To facilitate selective passage of water in vapour state while exclusively retaining liquid, this technique requires the use of a hydrophobic membrane (Das, Dutta and Singh, 2021). This process operates in various MD modes namely, direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), sweep gas membrane distillation (SGMD), and vacuum membrane distillation (VMD) (Pramanik et al., 2016; Quist-Jensen et al., 2016). The detailed description of each mode is reported elsewhere (Nthunya et al., 2019). Interestingly, the water recovery in MDC ranges from 50 – 90%, thus emerging as an alternative water desalination technology (Quist-Jensen et al., 2019). According to Quist-Jensen et al. (2019), MDC can increase water production, mineral recovery and advance zero-liquid discharge (Quist-Jensen et al., 2019). The advantages and disadvantages of this technique are summarized in Table 2.1. 6 Figure 2.1: Schematic representation of MDC for recovery of freshwater and minerals from industrial wastewater. Table 2.1: Summary of the advantages and disadvantages associated with MDC. Advantages Disadvantages Reference Independent of the feed concentration. Not affected by osmotic pressures from concentrated brines Membranes are susceptible to fouling due to the contaminant deposition in membrane pores, resulting in clogging among other consequences (Drioli, Di Profio and Curcio, 2012; Pramanik et al., 2016; Ali et al., 2018) Can be utilized for salt separation processes to circumvent salt co- crystallization. Also, crystal growth and nucleation are controlled May suffer from scaling which is due to the collection of inorganic salts on the membrane surface (Drioli, Di Profio and Curcio, 2012; Bouchrit et al., 2017; Ruiz Salmón and Luis, 2018) Lower energy consumption and can make use of alternative energy sources such as solar power Membrane performance may deteriorate due to membrane wetting (Ruiz Salmón and Luis, 2018; Das, Dutta and Singh, 2021) 7 Provides sustainable and simultaneous water and mineral salt recovery - (Pramanik et al., 2016) 2.3 Parameter optimization to enhance membrane distillation crystallization. The development of a viable MDC process requires optimization to prevent undesired crystallization inside the module and tubing. For this reason, the selection of appropriate MD and crystallization operating conditions are imperative. These parameters include process temperature, solution supersaturation, flow rates and duration of crystallization. Moreover, the temperatures and flow rates affect the crystal size distribution. Therefore, analysis of these parameters provides a better understanding of the MDC process and requirements to realize the maximum performance while ensuring zero discharge to the environment. 2.3.1 Process temperature The effect of process temperature on permeate flux is best described by Antoine equation, where α, β, and γ are constants relating to the specific substance and Pi is the vapour pressure (Pa) and T is the temperature (K). 𝑃𝑖(𝑇) = 𝑒 (𝛼− 𝛽 𝛾+𝑇 ) According to the Antoine equation, vapour pressure exponentially increases with temperature (Choudhury et al., 2019). Furthermore, water flux is directly proportional to feed temperature (Banat and Simandl, 1998). However, flux increments are limited by the process temperature and declines once an optimum has been attained (Banat and Simandl, 1998). Moreover, Attia et al. (2017) evaluated the effect of temperature using synthetic electrospun PVDF, superhydrophobic alumina, and commercial PVDF membranes in a comparative AGMD process. A direct relationship between permeate flux and feed temperature was established (Attia et al., 2017). Liu et al. (2022) assessed the effect of temperature and flow velocity to obtain lithium chloride from air-conditioning systems via DCMD. Reportedly, an increase in feed temperature improved solute generation although the membrane's hydrophobicity was altered. However, the increase in solute concentration reduced the water flux due to a decreased partial vapour pressure (Liu et al., 2022). Although high water fluxes are obtained at higher 8 temperatures, the water recovery factor is reduced due to salt precipitation (Zhu et al., 2021). The effect of feed temperature on process operation is summarized in Table 2.2. Table 2.2: Summary of the effect of varying the feed temperature on the permeate flux. Membrane type MD process type Feed temperature variation (℃) Flow rate (L min-1) Permeate flux (L m-2 h-1) Ref. Electrospun PVDF AGMD 30 – 70 1.5 Increased from 9.17 to 26.22, (Attia et al., 2017) Flat sheet PVDF membrane ADMD 25 - 80 5.5 Increased from 0.5 to 9.1 (Banat and Simandl, 1998) Commercial polypropylene (PP) membrane Integrated FO – MD process 40 - 70 0.4 Increased from 8.1 to 35.4 (Husnain et al., 2015) PTFE DCMD 55 -65 0.4 – 1.0 Significant increase in flux. (Liu et al., 2022) PTFE AGMD 50 – 80 0.03 – 0.06 3.06 (Kargari and Yousefi, 2021) Polyimide fibrous membrane (PI FM) DCMD 30 - 50 0.24 Increased from 26.12 to 64.15 (Zhu et al., 2021) Commercial PTFE DCMD 40 - 60 1.0 Increased from 4 to12 (Ramos et al., 2022) 2.3.2 Solution supersaturation The capability of the MD technology to progressively concentrate a feed solution to supersaturation gave rise to MDC (Yadav et al., 2022). The gradual passage of water vapor from the feed stream to the distillate results in the eventual concentration of the feed solution to its critical saturation. Further increase in feed supersaturation enables the recovery of crystal salts from the crystallization reactor (Das et al., 2021). Importantly, this process facilitates the recovery of higher quality mineral crystals in terms of size and purity. Other benefits include controlled rate of supersaturation and nucleation (Yadav et.al., 2022). However, the increase in feed concentration towards solution supersaturation induces temperature and concentration polarization, thus reducing the permeate flux (Martínez, 2004). Moreover, pore blockage 9 occurs due to the formation of crystals on the surface of membrane (Yadav et al., 2022). Martínez (2004) investigated the effect of feed concentration on the permeate flux using a flat sheet PTFE membrane and feed solutions of pure water, sodium chloride, and sucrose. Notably, the pure water flux remained stable towards supersaturation. However, the deposition of sodium chloride and sucrose crystal on the membrane surface resulted in a decrease in the permeate flux (Martínez, 2004). When supersaturation is attained in the bulk feed solution, nucleation is then induced which is succeeded by crystallization (Yadav et.al., 2022). Moreover, a higher feed temperature increases rate of solvent evaporation, thus facilitating an increased rate of supersaturation compared to that experienced at low feed temperatures (Edwie and Chung, 2013). 2.3.3 Duration of crystallization The formation and crystal growth are influenced by the solubility of the salt, rate of water recovery and process temperature. For instance, feed solutions with a low concentration containing extremely soluble solutes requires a lengthy period to form crystals (Rudolph, 2010; Liu et al., 2021). Additionally, slow crystal growth rate facilitates formation of large crystals. Therefore, longer crystallization periods give rise to larger crystals (Alvarez et al., 2020). In their study, Wagstaff et al. (1964) evaluated the impact of crystallization duration to the size of cristobalite. Based on their findings, the size of the crystals increased quadratically upon increase in duration of process crystallization (Wagstaff, Brown and Cutler, 1964). Essentially, the rate of crystal growth is governed by the various factors including flow of latent heat from the growing crystal, diffusion and reactions occurring at the crystal interface (Rudolph, 2010). In MDC processes, the inclusion of the membrane provides a site for heterogeneous nucleation. The Gibbs free energy is lower at the membrane-solution interface, thus favoring heterogeneous nucleation rather than homogenous nucleation (Ruiz Salmón and Luis, 2018). According to Edwie and Chung (2013), a high feed temperature encourages a higher rate of evaporation resulting in a lower average crystal size. Once nucleation has been established, the nuclei begin to grow until the critical cluster size has been achieved. Thereafter, crystals form and grow in saturation zones (i.e. metastable and unstable growth zones) (Yadav et al., 2022). Technically, rate of supersaturation and nucleation affect crystal network growth, consequently the duration of crystallization (Das et al., 2021). 2.3.4 Recirculation rate 10 High recovery rates of MDC processes are realized at higher recirculation rates (Swaminathan and Lienhard, 2018). For an efficient and high performing MDC process, the overall recovery factor should be greater than that of a single pass process (Lokare et al., 2018). To achieve high recovery factors, the retentate is mixed with the new feed solution prior to crystallization (Lokare et al., 2018). In addition to high recoveries, an increase in the recirculation rate enhances the heat transfer coefficient. Consequently, this minimizes the boundary layer thus improving the permeate flux (Srisurichan, Jiraratananon and Fane, 2006). Due to the improvement of water turbulence, a high recirculation rate reduces temperature polarization and membrane fouling, thus ensuring the stable water flux (Lokare et al., 2018). 2.4 Fouling of MDC membranes Occurrence of fouling in MDC is a common problem affecting process performance. To minimize fouling, its developments and successions should be established. Briefly, fouling occurs due to the deposition of microbial, colloidal, organic, or inorganic constituents on the surface or inner pores of the membrane, thus causing blockages (Choudhury et al., 2019; Mpala et al., 2022). Due to changes in the membrane physicochemical properties, fouling reduces permeate water flux, salt rejections and also increases the operating expenditure (OPEX) of the process (Nthunya et al., 2022). Additionally, fouling reduces membrane hydrophobicity leading to membrane wetting (Wang and Lin, 2017). Reduced membrane hydrophobicity encourages the passage of water in liquid state, thus reducing mineral salt rejection (Wang and Lin, 2017; Choudhury et al., 2019). Moreover, fouling is not limited to the membrane surface, but can also occur within the membrane pores. This was evident in a study conducted by Kim, Kim and Hong (2018) reporting deposition of foulants within the membrane pores in conjunction with reduced water recoveries and permeate flux. Usually, permeate flux reduction is caused by partial and complete wetting while the latter is true for water quality deterioration (Figure 2.2) (Yao et al., 2020). Technically, the membrane is partially wetted by process conditions with limited passage of water in both liquid and vapour state. However, during full pore wetting, the water carrying salt ions passes through the membrane in liquid state, thus reducing the quality of the distillate. 11 Figure 2.2: Graphical representation of membrane pore wetting experienced in MDC. Common factors influencing fouling include feed solution properties, hydrodynamic conditions, and membrane characteristics (Yao et al., 2020). The most prevalent form of fouling in MDC is scaling caused by sparingly soluble salts (Pramanik et al., 2016; Charfi et al., 2021). Inorganic scaling occurs via two mechanisms, namely; 1) nucleation and precipitate growth on the surface or pores of the membrane and 2) the build-up of precipitates materializing in the bulk solution (Horseman et al., 2021). Common scalants causing membrane damage include calcium sulphate and calcium carbonate (Alkhatib, Ayari and Hawari, 2021). Fouling can be classified into porous and non-porous where the former causes thermal resistance and the latter results in both thermal and hydraulic resistance (Abdel-Karim et al., 2021; Alkhatib, Ayari and Hawari, 2021). Therefore, to maintain high MDC process performance, operational challenges associated with a high concentration of salts and a complex feed solution should be overcome. Fouling and its implications are presented in Table 2.3 below. Table 2.3: Implications of fouling during experimental procedures. Membrane type Fouling classification Implications Reference Commercial PP hollow-fiber Calcium carbonate and sodium scaling Reduced water recovery and permeate flux (Kim, Kim and Hong, 2018) 12 Synthesised PVDF hollow-fiber Organic fouling (dyes) Decreased flux with long term operation (Shi et al., 2022) Commercial PVDF Scaling Rapid flux decline (Choi et al., 2020) PTFE/PP Calcium sulphate scaling Permeate flux decreased almost to zero (Nghiem and Cath, 2011) PTFE and PE Organic fouling (from petrochemical wastewater) and scaling Decreased permeate flux (Venzke et al., 2021) Commercial PTFE Organic fouling Reduced water recovery rate and permeate flux (Ramos et al., 2022) Synthesised PVDF/PSF hollow fiber Organic (ginseng) and inorganic fouling Reduced overall flux and rejection factor (Zou et al., 2022) Commercial PP Organic and inorganic fouling 40 % flux decline (Gryta, 2020) 2.5 Fouling control Membrane fouling is inevitable and therefore requires strategic measures to minimize its effects on process performance. Fouling control increases the membrane lifespan and maintains the performance of MDC processes (Laqbaqbi et al., 2017). Membrane fouling is controlled through several measures including pre-treatments, backwashing, and chemical cleaning. These processes lengthen membrane longevity. Chemical cleaning and backwashing are employed post membrane fouling to recover flux and salt rejection. To increase water recoveries, fouling control is optimized to minimize cost and damage of membranes. 2.6 Pre-treatment Flux decline caused by membrane fouling requires frequent membrane cleaning and possibly replacement, consequently increasing operating and maintenance costs (OPEX). Therefore, wastewater pre-treatment integrated to MDC improves process performance. Primarily, pre- treatment strategies limit fouling by reducing foulants concentration in the feed water. The choice of pre-treatment depends on the feed water. Typically, a combination of pre-treatment strategies is required to improve efficiency of foulant removal from the feed solution. These combinations involve physical and chemical processes such as low-pressure membrane 13 filtration, coagulation and flocculation, adsorption, pH adjustments, and the addition of anti- scalants. Mechanical pre-treatments consist of membrane processes such as microfiltration (MF), ultrafiltration (UF), and nanofiltration (NF). Particularly, NF is used for water softening and reduction of natural organic matter (NOM). The UF and MF reduces colloidal, suspended and biological matter (Alkhatib, Ayari and Hawari, 2021). These pre-treatment methods have been evaluated in water processing of various complexities (Nthunya, Mbakop and Mhlanga, 2021). El-Abbassi et al. (2013) studied coagulation-flocculation and MF pre-treatment of olive mill wastewater in DCMD. Coagulation-flocculation pre-treatment reduced the concentration of TDS and phenolic compounds by 23% and 18%, respectively. The TDS removal was improved to 30% while that of phenolic compounds was reduced to 4.8% upon the MF treatment (El- Abbassi et al., 2013). In another study, Karakulski & Gryta (2005) investigated NF pre- treatment of tap water for use in MD. Reportedly, untreated feed water caused membrane scaling leading to rapid flux decay. However, NF pre-treatment removed scalants thus ensuring high process performance (Karakulski and Gryta, 2005). Additionally, adsorption has been proven to effectively remove organic matter prior to MD water purification. Nthunya et al. (2019) reported removal of phenolic compounds from feed wastewater using a candle filter (pore size ~100 µm) equipped with polyethyleneimine-functionalized polyacrylonitrile nanofibre membranes. The membranes presented 39.9 mg·g-1 adsorption capacity (Lebea N Nthunya, Gutierrez, Derese and Mamba, 2019). Notably, MD process performance remained relatively stable upon feeding with pre-treated wastewater. Coagulation-flocculation is another process proven to effectively remove foulants prior to MD water processing. In this process, foulant particles are converted into larger flocs, thus reducing their adhesive interaction with the membranes. Moreover, coagulation-flocculation coupled with conventional treatment or membrane filtration processes remove the flocs from the feed water (Alkhatib, Ayari and Hawari, 2021). Li et al. (2016) investigated the purification of biologically treated coking wastewater using MD coupled with coagulation pre-treatment. A poly-aluminium chloride (PACl) flocculant reduced the foulants thus promoting the stable performance in MD (Li et al., 2016). Lastly, pH-adjustments have been extensively used to treat feed solutions in membrane processes. The increase in feed pH promotes formation of metal precipitates which are removed as insoluble metal hydroxides prior to MDC. Similarly, the feed solution is acidified to dissolve the foulant, thus impeding their interaction with the membranes (Karakulski and Gryta, 2005). A summary of MDC pre-treatment processes is presented in Figure 2.3. 14 Figure 2.3: A summary of pre-treatment processes in MDC (a) pre-treatment classifications and (b) process selection for specific foulant. 2.5.1 Use of anti-scalants Anti-scalants are precipitation-inhibiting chemicals impeding nucleation or crystal growth of scalants on membrane surfaces. Anti-scalants adsorb on the nuclei surface to obstruct the rate of crystal growth and agglomeration (Lin and Singer, 2005; Gloede and Melin, 2008; Abdel- Karim et al., 2021). The anti-scaling mechanism of action takes place through ligand exchange or electrostatic interactions (Horseman et al., 2021). Commonly used anti-scalants include organophosphates, polyelectrolytes and polyphosphates (Ketrane et al., 2009). Yin et al. (2021) evaluated gypsum anti-scaling in reverse osmosis (RO) coupled with MD using Poly(acrylic) acid (PAA). A 1300 min test recorded 95% water flux decay in the absence of an antiscalant. However, the decay was reduced by 30% upon addition of anti-scalant, thus corresponding to 40% water recovery (Yin et al., 2021). Lin & Singer (2005) utilized polyphosphates to minimize calcite crystal growth in MD. The process performance remained stable with minimal flux decay recorded. Though anti-scalants improve MDC processes, their addition beyond maximum threshold promote membrane biofouling (Tijing et al., 2015). Therefore, the anti- scalant dosage should be optimized to meet the process requirement upon treatment of a specific feed solution. 2.5.2 Membrane flushing and gas bubbling 15 Membrane flushing and gas bubbling are classified as physical fouling mitigation strategies. Flushing is often carried out to remove adsorbed solutes from the membrane surface using deionized water. Nonetheless, flushing fails to remove solutes within the membrane pores (Alkhatib, Ayari and Hawari, 2021). Flushing is often operated in two modes namely, forward and backwashing. Technically, deionized water is pumped in a forward direction during forward flushing while the reverse is true for backflushing (Alkhatib, Ayari and Hawari, 2021). Gas bubbling enhances shear rate and fluid dynamics thus reducing temperature and concentration polarization (Alkhatib, Ayari and Hawari, 2021). Reportedly, finely dispersed bubbles are more efficient compared to course bubbles (Lu et al., 2008). Choi et al. (2020) assessed the recovery of sodium sulfate from seawater brine using a hollow fiber PVDF membrane in fractionally submerged MD crystallization. Two cleaning procedures were used, namely air backwashing and deionized water flushing in the presence of ammonium sulfate. Air backwashing enabled 90% flux recovery. Similarly, flushing recovered 82% water flux from the original level. However, multiple air backwashing caused progressive permeate flux decline (Choi et al., 2020). To reduce scaling of a commercial PTFE membrane supported on polypropylene (PP), Nghiem & Cath (2011) used MilliQ water. Five cycles of membrane flushing recovered 30% of the original flux (Nghiem and Cath, 2011). Though flushing is more efficient for removal of inorganic foulants, it can also be used for removal of organic foulants upon treatment of an oil-contaminated feed (Gryta, 2020). 2.5.3 Temperature adjustments and backflow Temperature and flow reversal (backflow) techniques are novel methods used to mitigate fouling in MD/MDC. This experimental procedure was evaluated by Hickenbottom & Cath (2014) to minimize scaling while ensuring stable process performance. The temperature swap between the feed and distillate effectively reversed the driving force across the membrane, thus reducing the surface interactions between the membrane and scalants. Water flux and rejection efficiencies were recovered to 95%. Remarkably, both methods minimized scaling, thus ensuring stable fluxes and maintaining high salt rejection (Hickenbottom and Cath, 2014). Notably, these mitigation strategies avoid the use of expensive and toxic chemicals. Therefore, temperature and flow reversal are attractive alternative measures to control membrane fouling. However, extensive research is required to ascertain their sustainability at an industrial scale. 16 2.5.4 Chemical cleaning Chemical cleaning is the most evaluated reactive measure used to control membrane fouling. The mechanism of action involves breaking foulant-membrane interactions (Alkhatib, Ayari and Hawari, 2021). Chemical reagents include acids, bases, surfactants, chelating agents, enzymes, and oxidants (Al-Amoudi and Lovitt, 2007; Porcelli and Judd, 2010). Typically, bases and surfactants are used to address organic and biofouling (Alkhatib, Ayari and Hawari, 2021; Charfi et al., 2021) while acids and chelating agents are true for inorganic fouling (Alkhatib, Ayari and Hawari, 2021; Gryta, 2021). To ensure a synergistic cleaning process, a combination of chemicals is generally used during the treatment of complex feed solutions characterized by various foulants (Alkhatib, Ayari and Hawari, 2021). Charfi et al. (2021) optimized cleaning procedures of the MD process during treatment of anaerobic digestate. Reportedly, deionized water flushing was followed by 0.2% NaOCl and 3% citric acid for 60 min. NaOCl and citric acid were effective for organic and inorganic foulant removal, respectively thus ensuring 75.5% flux recovery. Furthermore, the cleaning process recovered 87% of membrane hydrophobicity with minimal membrane wetting (Charfi et al., 2021). In another study Guillen-Burrieza et al. (2014) evaluated a variety of cleaning agents in long-term scaling control in MD processes. As per reported findings, a combination of 0.1 wt% oxalic acid and 0.8 wt% citric acid recovered 97% of the membrane WCA. Furthermore, formic, and sulfuric acid recovered 96.7% and 94.6% of the membrane WCA respectively. Although these processes restored WCA, the integrity and mechanical strength of the membranes were affected (Guillen-Burrieza et al., 2014). The destruction of membrane integrity depends on cleaning conditions including the concentration of reagents, duration, and cleaning frequency. To understand the impact of chemical cleaning pertaining to physicochemical properties, various characterization techniques should be employed. These include chemical, morphological, topological, hydrophobic/hydrophilic, and mechanical analysis of the membrane. Puspitasari et al. (2010) investigated the cleaning and ageing of PVDF membranes using oxidative sodium hypochlorite (NaOCl). The effect of chemical concentration on the cleaning and ageing of the membrane is presented in Figure 2.4a. The cleaning efficiency improved with an increase in NaOCl. The same trend was observed for cyclical membrane cleaning. However, following the cleaning protocol, SEM micrographs showed presence of foulants on the membrane surface. Moreover, FTIR results presented changes in the chemical functional groups of membrane, thus alluding to an ageing effect. Furthermore, higher concentrations NaOCl damaged the integrity of the membrane (Figure 2.4b) (Puspitasari et al., 2010). To minimize the damage, a 17 combination of cleaning reagents and anti-scalants is commonly used. This was evaluated by Peng et al. (2015) during the MD treatment of RO concentrated brine. A series of chemicals namely, NaCl, NaOH, KCOOH, citric acid, and EDTA-4Na were used. While operating at elevated temperatures, EDTA-4Na enabled highest flux recovery. Improved recovery was associated to chelation of calcium ions, thus reducing their interactions with the membranes (Peng et al., 2015). In another study, Zhang et al. (2021) used a combination of organic phosphoric acid and hexamethylene diamine tetra(methylene phosphonic acid) (HDTMPA) during treatment of landfill leachate in FO/MD system. A combination of these chemicals reduced the foulant-membrane interactions, thus improving the process performance. Although 90% of flux was recovered in the first cycle, continuous cleaning did not show significant increase in performance recovery (Zhang et al., 2021). Figure 2.4: (a) Average cleaning efficiencies as a function of varying NaOCl concentrations, (b) overall cleaning efficiencies (OCE) observed for aged, fouled membranes (Puspitasari et al., 2010). Some foulants bind strongly on membrane surfaces, thus causing irreversible fouling. This phenomenon was reported by Naidu et al. (2015) upon NaOH cleaning MD membranes fouled by humic substances. Partial regeneration of the membrane with 19% hydrophobicity recovery was reported (Naidu, Jeong and Vigneswaran, 2015). Further improvements in chemical cleaning involves the use of 3D spacers. Spacers amplify flow turbulence, thus reducing foulant-membrane interaction. In their study, Castillo et al. (2019) investigated a step-wise cleaning of MD membrane using citric acid and water in the presence of spacers. Upon 18 cleaning, 87% of membrane WCA was recovered (Castillo et al., 2019). The effect of various cleaning strategies is presented in Table 2.4. 19 Table 2.4: Summary of the cleaning strategies used in various studies. Cleaning strategy Cleaning duration Frequency Effect on Flux Effect on WCA Comments Ref. 60 min rinsing with deionized water followed by 0.2 % NaOCl and 3 % citric acid Every two days 87% water flux was recovered 75.5 % WCA was restored Membrane was resistant to wetting (Charfi et al., 2021) 30 min rinsing with deionized water followed by 0.1 wt% oxalic acid and 0.8 wt% citric acid - - 126.4° compared to 129° for the unused membrane Mechanical integrity of the membrane was reduced (Guillen- Burrieza et al., 2014) 1% NaOCl followed by 10 min rinsing with deionized - - - 95% cleaning efficiency was recorded (Puspitasari et al., 2010) 60 min washing with NaOH, absolute ethanol, and pure water - - - Combined cleaning strategy was effective (Shi et al., 2022) EDTA-4Na - - - - Higher flux recoveries achieved at higher temperatures (Peng et al. 2015) 2.5 wt% HCl 30 – 70 hrs - - - 100% flux recovery was recorded (Gryta, 2007) Deionized water rinsing and NaOH - - - Average hydrophobicity was reduced by 19 % Fouling was irreversible (Naidu et al., 2015) 20 Membrane modification using SiO2-PNIPAM particles, and thermal actuation 5 and 10 min - - Membrane surface free energy was reduced, thus restoring hydrophobicity (Lyly et al., 2021) Hydraulic rinsing - Flux recovery of > 90 % - HDTMPA facilitated antiscaling (Zhang et al., 2021) 0.1 wt% citric acid and deionized wate. Also, 3D Gyroid spacer was used 24 hrs, - - WCA of membranes was reduced by 13% Acid improved cleaning process (Castillo et al., 2019) 21 2.5.5 Membrane modification Membrane modification improves resistance to fouling and wetting. Typically, modification is achieved through systematic manipulation. Currently, superhydrophobic membranes characterized by self-cleaning properties are explored with low success rate. To improve membrane resistance to fouling while retaining high salt rejection, omniphobic and Janus membranes are also reported (Wang and Lin, 2017; Yao et al., 2020; Tjale et al., 2022). These membranes are characterized by asymmetric wettability to minimize fouling while retaining process stability (Afsari, Shon and Tijing, 2021). Xiao et al. (2020) prepared omniphobic membranes through incorporation of silica nanoparticles (SiNPs)-coated micropillars (MP) to PVDF. Reportedly, SiNPs-MP-PVDF membrane reduced scaling and fouling, thus maintaining the process performance over a longer period. Figures 2.5 (a) and (b) present the role of membrane modification towards preventing flux decay. In another study, Toh et al. (2019) modified PVDF-co-hexafluropropylene membranes using silica nanoparticles to improve their resistance to wetting and fouling. The modified membranes were characterized by high WCA and low surface energy (Toh et al., 2019). Zhang et al. (2021) reported hydrophilic surface modification of PVDF hollow fibre membrane through co-deposition of polydopamine (PDA) and poly(MPC-co-2-aminoethyl methacrylate hydrochloride) (MPC-co-AEMA). The smooth hydrophilic thin layer reduced the foulant- membrane interaction (Zhang et al., 2021). In addition to hydrophilic coating, antimicrobial nanoparticles are embedded on hydrophobic membranes to combat organic, inorganic and biofouling. These additives include silver nanoparticles, cellulose nanocrystals and carbon nanotubes (Nthunya et al., 2019; Nthunya et al., 2020). Membrane modifications processes addressing fouling are presented in Table 2.5. 22 Figure 2.5: Normalized water flux (J/Jo) vs concentration factor for the individual membranes upon evaluation of (a) CaSO4 scaling, (b) casein protein organic fouling (Xiao et al., 2020). 23 Table 2.5: Summary of various membrane modification strategies and their effects on membrane properties. Membrane type Membrane modification Physical properties Findings Ref. WCA (°) LEP (Bar) Flux (kg m-2 hr-1) Omniphobic PVDF membrane Incorporation of silica nanoparticles Improved from 130.1 to 175.6 - - Fouling resistant (Xiao et al., 2020) Superhydroph -obic PVDF- HFP) Incorporation of silica nanoparticles Improved from 135 to 151 - - (Toh et al., 2019) PVDF hollow fiber membrane Modified with PDA and AEMA-HCl - Improved from 1.13 to 1.15 - Improved fouling resistance (Zhang et al., 2021) Two-layer superhydroph -obic PVDF membrane Surface fluorination coating Increased from 123.1 to 154.5 - 18% flux increase Fouling resistant (Kharraz & An, 2020) Superhydroph -obic electrospun PVDF membrane Electrosprayed with PDMS and silica fumes 170 - - Anti- abrasive and fouling resistant (Liao et al., 2020) PFPE/PVDF Prepared by UV-curing 162.6 - 34.2 No flux and salt rejection decay (Pan et al., 2022) PVDF-PDMS Janus membrane AgNP deposition on membrane surface Top surface: 85.62, and bottom surface: 119.7 - Flux increased from 11.5 to 20 Improved vaporiz- ation and flux (Yue et al., 2021) PVDF membrane Modified with TNTs - - Water flux increased by 38.7% Improved porosity, thermal and mechanical properties (Rahmaniya n et al., 2021) PVDF Blended with Hyflon and PFPE Hyflon/PV: 138.4 and PFPE/PV: 157.7 - Hyflon/P VDF: > 28 L PFPE/PV DF: 21 Improved permeate quality and resistant to flux decay (Pan et al., 2022) PVDF/PSF hollow fiber Fluorinated 132 - ~ 6.0 Improved mechanical strength, anti- wettability, (Zou et al., 2022) 24 and water permeability 2.6 Application in wastewater treatment Membrane distillation crystallization (MDC) emerged as a promising innovation in response to the global shortage of fresh water and mineral resources. Owing to the challenges associated with industrial application, MDC is extensively tested at laboratory-scale. Various applications of MDC are summarized in Table 2.6. Nonetheless, process optimization with sound findings has motivated its industrial use for treatment of wastewater. For instance, Hamzah et al. (2019) reported a flux of 11.0 kg·m-2·hr-1 during the treatment of a phenolic-rich feed solution using a PVDF/TiO2/SiO2 composite membrane. Remarkably, TiO2-modification improved process resistance to organic fouling (Hamzah et al. 2019). Although fouling is minimized to some extent, it remains critically challenging (Kim et al., 2017). Notably, fouled membranes attract scaling and wetting (Kim et al., 2017). Despite all these challenges, MDC is relatively versatile towards treatment of complex feed solutions. In their study, Lu et al. (2017), reported 99% water purity recovered from oil-processing wastewater. Moreover, MDC is used as a finishing process to recover minerals and freshwater (90% water recovery and 99% salt rejection) from the RO concentrate (Venzke et al., 2021). Nonetheless, treatment of the RO concentrate causes concentration polarization and scaling (Venzke et al., 2021). Interestingly, MDC does not only treat industrial wastewater but also biological waste including human urine (Zhao et al., 2013). During this treatment, 31.9 – 48.6 % water recovery was reported along with ammonia-nitrogen recovery and COD reduction (Zhao et al., 2013). Among other factors related to the economics of the process, MDC is driven by renewable energy sources. The use of solar energy was evaluated by Li et al. (2020) in a pilot-scale where photothermal membrane was used. Although high flux (21.99 kg·m-2·hr-1) was reported, water recovery factors were low and production of photothermal membrane was costly (Li et al., 2020). The successes achieved at lab scale supports the implementation of this technology toward pilot and industrial scale. Memstill ® reported first pilot testing of MD technology implemented at an incineration plant in Singapore in 2006 (Dotremont et al., 2010). Other pilot studies were established at BASF in Antwerp, Belgium in 2011 (Camacho et al., 2013). 25 Table 2.6: Various application of MDC towards treatment of wastewater and mineral recovery. Membrane type Mode of application Feed solution used Process temperature (Feed/ permeate/ crystallizer) (℃) Process flow rates Permeate flux Products Ref. Commercial hollow fiber PVDF Fractional submerged MDC RO brine 50.0/ 20.1/ - 0.8 L min-1 - 72% water recovery, 223.73 g Na2SO4 (Choi et al., 2020) Commercial hollow fiber PP MDC Shale gas produced water 60/ 20/ - - - 84% water recovery, salt production 2.72 kg·m- 2·day-1 (Kim, Kim and Hong, 2018) Hollow fiber PP VMD- crystallization Wastewater from oil extraction. 55 - 75/ 10/ - - - 99% water purity, NaCl and ethylene glycol (Lu et al., 2017) Commercial PVDF DCMD - MDC 2 M concentrated Na2SO4 40 – 70/ 25/ variable 2 L hr-1 - 80% water recovery, 100 kg m-3 Na2SO4 (Bouchrit et al., 2017) PTFE MD/MDC Synthetic shale gas produced water 60/ 20/ 40 25 cm s-1 - 62.5% water recovery and NaCl, BaCl2, and CaCO3 (Kim et al., 2017) 26 Microporous PTFE plate membrane VMD Human urine 50 – 70/ -/ - 30 L hr-1 - 31.9 – 48.6% water recovery (Zhao et al., 2013) Fe3O4 – PVDF – co- hexafluoropropylene nanofibers Solar driven MD Synthetic NaCl solution -/ 20/ - 20 ml min-1 0.97 kg m-2 h-1 Salt rejection of 99.99 % (Li et al., 2020) Cloisite 15 - modified PVDF hollow fiber membrane MDC 26.4 wt% NaCl solution 70/ -/ 25 - - 34 kg NaCl was produced per m3 of feed (Edwie and Chung, 2013) PTFE, standard PE (PE – S) and oleophobic PE (PE – O) DCMD RO concentrates from petrochemical wastewater 60/ 20 - PE – O = 5.0 kg·m- 2·h-1, PE – S = 2.1 kg·m-2·h-1 and PTFE reduced by 30% of initial flux after 250 hrs Recoveries close to 90% and rejection rates > 99.5% (Venzke et al., 2021) PVDF blended with multi-walled carbon nanotubes DCMD 35 g·L-1 synthetic NaCl solution 82/ 20/ - 48 ml min-1 9.5 x 10-3 kg m-2 s-1 Salt rejection of 100 % after 60 min (Silva et al., 2015) PVDF/TiO2/SiO2 DCMD A phenolic- rich solution containing surfactant 40/20/- 300 ml min-1 11.0 kg·m- 2·h-1 after 8 hrs 99.9% gallic acid rejection with a flux decay resistance (Hamzah, Leo and Ooi, 2019) 27 Commercial PP and hollow fiber PVDF DCMD Wastewater produced from oil and gas production 35,45,55/10/- Feed: 150 ml·min-1 and permeate: 70 ml·min-1 Increased flux as a function of temperature NaCl purity: > 99.9%, water and NaCl recovery: 37% and 16 kg·m-3 respectively (Ali et al., 2015) 28 2.7 Conclusion and future perspectives MDC addresses financial challenges affecting developing countries. Various literature reports have documented the successes of this technique in effectively recovering freshwater and mineral salts from a myriad of wastewater feed sources (Quist-Jensen et al., 2016, 2017; Kim et al., 2017; Choi et al., 2020). In conjunction to emerging laboratory-scale studies, implementation of pilot studies at an industrial platform provides a promising trajectory for the future of this technology. Nevertheless, membrane fouling, and wetting requires special attention. Membrane fouling can be classified into organic, inorganic (i.e., scaling), biofouling, and/or colloidal fouling. In some circumstances, a combination of foulants may exist in the feed solutions thus resulting in more complex membrane fouling scenarios. To circumvent these issues, various fouling control measures have been registered including mechanical pre- treatment options such as microfiltration (MF) and nanofiltration (NF). Other pre-treatment strategies include anti-scalants, temperature adjustments, and membrane flushing. Moreover, chemical cleaning has been extensively evaluated to restore MDC performance. While commercial membranes have be used for MDC processes, further research has been directed towards synthesis and modification of various fouling resistant membranes. This includes incorporation of nanoparticles to induce self-cleaning through superhydrophicity enhancement (i.e. improving the lotus effect of the membrane). Similarly, Janus membranes characterized by asymmetric wettability have been evaluated to mitigate membrane fouling. The steady development in this technique and its accompanying components has probed further interest into its applicative potential. Promising feedback established with the use of this technique pave the way towards further implementation in an industrial setting for mineral and water recycling. Future perspectives include, though not limited to: • Production of membranes using environmentally friendly reagents in addressing membrane fouling and wetting. • Optimization of membrane cleaning strategies towards a feasible industrial application. • Further research and implementation of pilot-scale studies to provide a realistic MDC suitability in industrial application. • Further research establishing fouling mechanism is required to understand membrane longevity and process performance. 29 References Abdel-Karim, A. et al. (2021) ‘Membrane cleaning and pretreatments in membrane distillation – a review’, Chemical Engineering Journal, 422(April), p. 129696. doi: 10.1016/j.cej.2021.129696. Afsari, M., Shon, H. K. and Tijing, L. D. (2021) ‘Janus membranes for membrane distillation : Recent advances and challenges’, Advances in Colloid and Interface Science, 289, p. 102362. doi: 10.1016/j.cis.2021.102362. Ahmed, F. E., Hashaikeh, R. and Hilal, N. (2020) ‘Hybrid technologies: The future of energy efficient desalination – A review’, Desalination, 495(August), p. 114659. doi: 10.1016/j.desal.2020.114659. Al-Amoudi, A. and Lovitt, R. W. (2007) ‘Fouling strategies and the cleaning system of NF membranes and factors affecting cleaning efficiency’, Journal of Membrane Science, 303(1– 2), pp. 4–28. doi: 10.1016/j.memsci.2007.06.002. Alanezi, A. A. et al. (2021) ‘Theoretical Investigation of Vapor Transport Mechanism Using Tubular Membrane Distillation Module’. Ali, A. et al. (2015) ‘Application of membrane crystallization for minerals’ recovery from produced water’, Membranes, 5(4), pp. 772–792. doi: 10.3390/membranes5040772. Ali, A. et al. (2018) ‘Evaluation of integrated microfiltration and membrane distillation/crystallization processes for produced water treatment’, Desalination, 434(May 2017), pp. 161–168. doi: 10.1016/j.desal.2017.11.035. Ali, A. et al. (2019) ‘Treatment of Wastewater Solutions from Anodizing Industry by Membrane Distillation and Membrane Crystallization’, Applied Sciences, 9(2), pp. 1–15. doi: 10.3390/app9020287. Alibakhshi, S. et al. (2019) ‘Tuning morphology and transport in ultrafiltration membranes derived from polyethersulfone through exploration of dope formulation and characteristics’, Materials Research Express, 6(12), pp. 1–14. doi: 10.1088/2053-1591/ab56c3. Alkhatib, A., Ayari, M. A. and Hawari, A. H. (2021) ‘Fouling mitigation strategies for different foulants in membrane distillation’, Chemical Engineering and Processing - Process Intensification, 167(June), p. 108517. doi: 10.1016/j.cep.2021.108517. Alvarez, R. et al. (2020) ‘Single crystal growth of water-soluble metal complexes with the help of the nano-crystallization method’, Dalton Transactions, 49(28), pp. 9632–9640. doi: 10.1039/d0dt01236j. Ametek, S. (2023) SPECTRO - The performance benchmark for ICP-OES/ICP-AES spectrometers. Available at: https://www.spectro.com/products/icp-oes-aes- spectrometers?sf_campaign_id=7013V000000AcgW&google_ad=icp+resource-library(subs) (Accessed: 9 January 2023). Ardeshiri, F. et al. (2018) ‘PVDF membrane assisted by modified hydrophobic ZnO 30 nanoparticle for membrane distillation’, Asia-Pacific Journal of Chemical Engineering, 13(3), pp. 1–12. doi: 10.1002/apj.2196. Ashoor, B. B. et al. (2016) ‘Principles and applications of direct contact membrane distillation (DCMD): A comprehensive review’, Desalination, 398, pp. 222–246. doi: 10.1016/j.desal.2016.07.043. Attia, H. et al. (2017) ‘Modelling of air gap membrane distillation and its application in heavy metals removal’, Desalination, 424(September), pp. 27–36. doi: 10.1016/j.desal.2017.09.027. Balis, E., Griffin, J. C. and Hiibel, S. R. (2022) ‘Membrane Distillation-Crystallization for inland desalination brine treatment’, Separation and Purification Technology, 290(January), p. 120788. doi: 10.1016/j.seppur.2022.120788. Banat, F. A. and Simandl, J. (1998) ‘Desalination by Membrane Distillation: A Parametric Study’, Separation Science and Technology, 33(2), pp. 201–226. doi: 10.1080/01496399808544764. Bouchrit, R. et al. (2017) ‘Membrane crystallization for mineral recovery from saline solution: Study case Na2SO4 crystals’, Desalination, 412, pp. 1–12. doi: 10.1016/j.desal.2017.02.021. Calvo, G. and Valero, A. (2022) ‘Strategic mineral resources: Availability and future estimations for the renewable energy sector’, Environmental Development, 41(April 2021), p. 100640. doi: 10.1016/j.envdev.2021.100640. Camacho, L. M. et al. (2013) ‘Advances in membrane distillation for water desalination and purification applications’, Water (Switzerland), 5(1), pp. 94–196. doi: 10.3390/w5010094. Castillo, E. H. C. et al. (2019) ‘3D printed spacers for organic fouling mitigation in membrane distillation’, Journal of Membrane Science, 581(March), pp. 331–343. doi: 10.1016/j.memsci.2019.03.040. Charfi, A. et al. (2021) ‘Optimal cleaning strategy to alleviate fouling in membrane distillation process to treat anaerobic digestate’, 279. doi: 10.1016/j.chemosphere.2021.130524. Chimanlal, I. et al. (2022) ‘Membrane distillation crystallization for water and mineral recovery : The occurrence of fouling and its control during wastewater treatment’, Frontiers in Chemical Engineering, (November), pp. 1–14. doi: 10.3389/fceng.2022.1066027. Choi, Y. et al. (2020) ‘Recovery of sodium sulfate from seawater brine using fractional submerged membrane distillation crystallizer’, Chemosphere, 238, p. 124641. doi: 10.1016/j.chemosphere.2019.124641. Choudhury, M. R. et al. (2019) ‘Fouling and wetting in the membrane distillation driven wastewater reclamation process – A review’, Advances in Colloid and Interface Science, 269, pp. 370–399. doi: 10.1016/j.cis.2019.04.008. Curcio, E., Criscuoli, A. and Drioli, E. (2001) ‘Membrane crystallizers’, Industrial and Engineering Chemistry Research, 40(12), pp. 2679–2684. doi: 10.1021/ie000906d. Das, P., Dutta, S. and Singh, K. K. (2021) ‘Insights into membrane crystallization: A 31 sustainable tool for value added product recovery from effluent streams’, Separation and Purification Technology, 257(August 2020), p. 117666. doi: 10.1016/j.seppur.2020.117666. Ding, Z., Liu, Z. and Xiao, C. (2021) ‘Excellent performance of novel superhydrophobic composite hollow membrane in the vacuum membrane distillation’, Separation and Purification Technology, 268(October 2020), p. 118603. doi: 10.1016/j.seppur.2021.118603. Dong, Z. Q. et al. (2014) ‘Superhydrophobic PVDF-PTFE electrospun nanofibrous membranes for desalination by vacuum membrane distillation’, Desalination, 347, pp. 175–183. doi: 10.1016/j.desal.2014.05.015. Dotremont, C. et al. (2010) ‘Seawater desalination with memstill technology - a sustainable solution for the industry’, Water Practice and Technology, 5(2), p. wpt2010026. doi: 10.2166/wpt.2010.026. Dow, N. et al. (2022) ‘Pilot demonstration of nitrogen removal from municipal wastewater by vacuum membrane distillation’, Journal of Water Process Engineering, 47(January), p. 102726. doi: 10.1016/j.jwpe.2022.102726. Drioli, E., Ali, A. and Macedonio, F. (2015) ‘Membrane distillation: Recent developments and perspectives’, Desalination, 356, pp. 56–84. doi: 10.1016/j.desal.2014.10.028. Drioli, E., Di Profio, G. and Curcio, E. (2012) ‘Progress in membrane crystallization’, Current Opinion in Chemical Engineering, 1(2), pp. 178–182. doi: 10.1016/j.coche.2012.03.005. Edwie, F. and Chung, T.-S. (2013) ‘Development of simultaneous membrane distillation– crystallization (SMDC) technology for treatment of saturated brine’, Chemical Engineering Science, 98, pp. 160–172. doi: 10.1016/j.ces.2013.05.008. El-Abbassi, A. et al. (2013) ‘Integrated direct contact membrane distillation for olive mill wastewater treatment’, Desalination, 323, pp. 31–38. doi: 10.1016/j.desal.2012.06.014. El-Bourawi, M. S. et al. (2006) ‘A framework for better understanding membrane distillation separation process’, Journal of Membrane Science, 285(1–2), pp. 4–29. doi: 10.1016/j.memsci.2006.08.002. Elhenawy, Y. et al. (2022) ‘Performance enhancement of a hybrid multi effect evaporation / membrane distillation system driven by solar energy for desalination’, Journal of Environmental Chemical Engineering, 10(6), p. 108855. doi: 10.1016/j.jece.2022.108855. Eykens, L. et al. (2016) ‘How to optimize the membrane properties for membrane distillation: A review’, Industrial and Engineering Chemistry Research, 55(35), pp. 9333–9343. doi: 10.1021/acs.iecr.6b02226. Eykens, L. et al. (2017) ‘Membrane synthesis for membrane distillation: A review’, Separation and Purification Technology, 182, pp. 36–51. doi: 10.1016/j.seppur.2017.03.035. Farmani, R. et al. (2021) ‘Intermittent water supply systems and their resilience to COVID-19: IWA IWS SG survey’, Aqua Water Infrastructure, Ecosystems and Society, 70(4), pp. 507– 520. doi: 10.2166/aqua.2021.009. 32 Fernandes, C. S. et al. (2021) ‘Explication of hydrophobic silica as effective pore former for membrane fabrication’, Applied Surface Science Advances, 3, p. 100051. doi: 10.1016/j.apsadv.2020.100051. Fontananova, E. (2020) ‘Encyclopedia of Membranes’, Encyclopedia of Membranes, pp. 1–3. doi: 10.1007/978-3-642-40872-4. Gao, C. et al. (2020) ‘Superhydrophobic Electrospun PVDF Membranes with Silanization and Fluorosilanization Co-Functionalized CNTs for Improved Direct Contact Membrane Distillation’, pp. 35–43. doi: 10.30919/es8d905. Gloede, M. and Melin, T. (2008) ‘Physical aspects of membrane scaling’, Desalination, 224(1– 3), pp. 71–75. doi: 10.1016/j.desal.2007.02.081. Goh, S. et al. (2015) ‘Membrane Distillation Bioreactor (MDBR) - A lower Green-House-Gas (GHG) option for industrial wastewater reclamation’, Chemosphere. doi: 10.1016/j.chemosphere.2014.09.003. Gryta, M. (2007) ‘Influence of polypropylene membrane surface porosity on the performance of membrane distillation process’, Journal of Membrane Science, 287(1), pp. 67–78. doi: 10.1016/j.memsci.2006.10.011. Gryta, M. (2020) ‘Separation of saline oily wastewater by membrane distillation’, Chemical Papers, 74(7), pp. 2277–2286. doi: 10.1007/s11696-020-01071-y. Gryta, M. (2021) ‘Surface modification of polypropylene membrane by helium plasma treatment for membrane distillation’, Journal of Membrane Science, 628(February). doi: 10.1016/j.memsci.2021.119265. Guillen-Burrieza, E. et al. (2014) ‘Membrane fouling and cleaning in long term plant-scale membrane distillation operations’, Journal of Membrane Science, 468, pp. 360–372. doi: 10.1016/j.memsci.2014.05.064. Guo, J. L. et al. (2014) ‘Investigation of polyvinylidene fluoride membranes prepared by using surfactant OP-10 alone or with a second component, as additives, via the Non-Solvent-Induced Phase Separation (NIPS) process’, Journal of Macromolecular Science, Part B: Physics, 53(8), pp. 1319–1334. doi: 10.1080/00222348.2014.928163. El Haj Assad, M. et al. (2020) ‘Applications of nanotechnology in membrane distillation: A review study’, Desalination and Water Treatment, 192(March 2019), pp. 61–77. doi: 10.5004/dwt.2020.25821. Hamzah, N., Leo, C. P. and Ooi, B. S. (2019) ‘Superhydrophobic PVDF/TiO 2 -SiO 2 Membrane with Hierarchical Roughness in Membrane Distillation for Water Recovery from Phenolic Rich Solution Containing Surfactant’, Chinese Journal of Polymer Science (English Edition). doi: 10.1007/s10118-019-2235-y. Hernández-Aguirre, O. A. et al. (2016) ‘Surface Modification of Polypropylene Membrane Using Biopolymers with Potential Applications for Metal Ion Removal’, Journal of Chemistry, 2016. doi: 10.1155/2016/2742013. 33 Hickenbottom, K. L. and Cath, T. Y. (2014) ‘Sustainable operation of membrane distillation for enhancement of mineral recovery from hypersaline solutions’, Journal of Membrane Science, 454, pp. 426–435. doi: 10.1016/j.memsci.2013.12.043. Horseman, T. et al. (2021) ‘Wetting, Scaling, and Fouling in Membrane Distillation: State-of- the-Art Insights on Fundamental Mechanisms and Mitigation Strategies’, ACS ES&T Engineering, 1(1), pp. 117–140. doi: 10.1021/acsestengg.0c00025. Husnain, T. et al. (2015) ‘Integration of forward osmosis and membrane distillation for sustainable wastewater reuse’, Separation and Purification Technology, 156, pp. 424–431. doi: 10.1016/j.seppur.2015.10.031. Jung, J. T. et al. (2016) ‘Understanding the non-solvent induced phase separation (NIPS) effect during the fabrication of microporous PVDF membranes via thermally induced phase separation (TIPS)’, Journal of Membrane Science, 514, pp. 250–263. doi: 10.1016/j.memsci.2016.04.069. Kang, G. dong and Cao, Y. ming (2014) ‘Application and modification of poly(vinylidene fluoride) (PVDF) membranes - A review’, Journal of Membrane Science, 463, pp. 145–165. doi: 10.1016/j.memsci.2014.03.055. Karakulski, K. and Gryta, M. (2005) ‘Water demineralisation by NF/MD integrated processes’, Desalination, 177(1–3), pp. 109–119. doi: 10.1016/j.desal.2004.11.018. Kargari, A. and Yousefi, A. (2021) ‘Process intensification through magnetic treatment of seawater for production of drinking water by membrane distillation process: A novel approach for commercialization membrane distillation process’, Chemical Engineering and Processing - Process Intensification, 167(April), p. 108543. doi: 10.1016/j.cep.2021.108543. Ketrane, R. et al. (2009) ‘Efficiency of five scale inhibitors on calcium carbonate precipitation from hard water: Effect of temperature and concentration’, Desalination, 249(3), pp. 1397– 1404. doi: 10.1016/j.desal.2009.06.013. Khalid, A. et al. (2017) ‘Fabrication of polysulfone nanocomposite membranes with silver- doped carbon nanotubes and their antifouling performance’, Journal of Applied Polymer Science, 134(15), pp. 1–12. doi: 10.1002/app.44688. Kharraz, J. A. et al. (2022) ‘Membrane distillation bioreactor (MDBR) for wastewater treatment, water reuse, and resource recovery: A review’, Journal of Water Process Engineering, 47(February), p. 102687. doi: 10.1016/j.jwpe.2022.102687. Kharraz, J. A. and An, A. K. (2020) ‘Patterned superhydrophobic polyvinylidene fluoride (PVDF) membranes for membrane distillation: Enhanced flux with improved fouling and wetting resistance’, Journal of Membrane Science, 595(July 2019), p. 117596. doi: 10.1016/j.memsci.2019.117596. Kim, J. et al. (2017) ‘Membrane distillation (MD) integrated with crystallization (MDC) for shale gas produced water (SGPW) treatment’, Desalination, 403, pp. 172–178. doi: 10.1016/j.desal.2016.07.045. 34 Kim, Junghyun, Kim, Jungwon and Hong, S. (2018) ‘Recovery of water and minerals from shale gas produced water by membrane distillation crystallization’, Water Research, 129, pp. 447–459. doi: 10.1016/j.watres.2017.11.017. Kiss, A. A. and Kattan Readi, O. M. (2018) ‘An industrial perspective on membrane distillation processes’, Journal of Chemical Technology and Biotechnology, 93(8), pp. 2047–2055. doi: 10.1002/jctb.5674. Kong, X. et al. (2020) ‘Manipulating membrane surface porosity via deep insight into surfactants during nonsolvent induced phase separation’, Journal of Membrane Science, 611(May), p. 118358. doi: 10.1016/j.memsci.2020.118358. Laqbaqbi, M. et al. (2017) ‘Fouling in membrane distillation, osmotic distillation and osmotic membrane distillation’, Applied Sciences (Switzerland), 7(4). doi: 10.3390/app7040334. Lee, M. (2017) X-Ray diffraction for materials research: from fundamentals to applications. CRC Press. Li, J. et al. (2016) ‘Advanced treatment of biologically treated coking wastewater by membrane distillation coupled with pre-coagulation’, Desalination, 380, pp. 43–51. doi: 10.1016/j.desal.2015.11.020. Li, W. et al. (2020) ‘Fe3O4/PVDF-HFP photothermal membrane with in-situ heating for sustainable, stable and efficient pilot-scale solar-driven membrane distillation’, Desalination, 478(September 2019), p. 114288. doi: 10.1016/j.desal.2019.114288. Liao, Y. et al. (2020) ‘Development of robust and superhydrophobic membranes to mitigate membrane scaling and fouling in membrane distillation’, Journal of Membrane Science, 601(December 2019), p. 117962. doi: 10.1016/j.memsci.2020.117962. Lin, Y. P. and Singer, P. C. (2005) ‘Inhibition of calcite crystal growth by polyphosphates’, Water Research, 39(19), pp. 4835–4843. doi: 10.1016/j.watres.2005.10.003. Liu, G. et al. (2021) ‘Directional Crystallization from the Melt of an Organic p-Type and n- Type Semiconductor Blend’, Crystal Growth and Design, 21(9), pp. 5231–5239. doi: 10.1021/acs.cgd.1c00570. Liu, J. et al. (2022) ‘Membrane fouling in direct contact membrane distillation for liquid desiccant regeneration: Effects of feed temperature and flow velocity’, Journal of Membrane Science, 642(September 2021), p. 119936. doi: 10.1016/j.memsci.2021.119936. Loh, C. H. et al. (2011) ‘Fabrication of high performance polyethersulfone UF hollow fiber membranes using amphiphilic Pluronic block copolymers as pore-forming additives’, Journal of Membrane Science, 380(1–2), pp. 114–123. doi: 10.1016/j.memsci.2011.06.041. Lokare, O. R. et al. (2018) ‘Importance of feed recirculation for the overall energy consumption in membrane distillation systems’, Desalination, 428(September 2017), pp. 250–254. doi: 10.1016/j.desal.2017.11.037. Loubser, C., Chimbanga, B. M. and Jacobs, H. (2021) ‘Intermittent water supply: A South 35 African perspective’, Water SA, 47(1), pp. 1–9. doi: 10.17159/wsa/2021.v47.i1.9440. Lu, D. et al. (2017) ‘Simultaneous Recovery and Crystallization Control of Saline Organic Wastewater by Membrane Distillation Crystallization’, AlChE Journal, 63(6). doi: 10.1002/aic. Lu, Y. et al. (2008) ‘The influence of bubble characteristics on the performance of submerged hollow fiber membrane module used in microfiltration’, 61, pp. 89–95. doi: 10.1016/j.seppur.2007.09.019. Lyly, L. H. T. et al. (2021) ‘Development of membrane distillation by dosing SiO2-PNIPAM with thermal cleaning properties via surface energy actuation’, Journal of Membrane Science, 636(February). doi: 10.1016/j.memsci.2021.119193. Macedonio, F. et al. (2014) ‘Direct contact membrane distillation for treatment of oilfield produced water’, Separation and Purification Technology, 126, pp. 69–81. doi: 10.1016/j.seppur.2014.02.004. Makgabutlane, B. et al. (2020) ‘Microwave Irradiation-Assisted Synthesis of Zeolites from Coal Fly Ash: An Optimization Study for a Sustainable and Efficient Production Process’, ACS Omega, 5(39), pp. 25000–25008. doi: 10.1021/acsomega.0c00931. Makgabutlane, B. et al. (2021) ‘Green synthesis of carbon nanotubes to address the water- energy-food nexus: A critical review’, Journal of Environmental Chemical Engineering, 9(1), p. 104736. doi: 10.1016/j.jece.2020.104736. Manawi, Y. M. et al. (2018) ‘Engineering the surface and mechanical properties of water desalination membranes using ultralong carbon nanotubes’, Membranes, 8(4). doi: 10.3390/membranes8040106. Martínez, L. (2004) ‘Comparison of membrane distillation performance using different feeds’, Desalination, 168(1–3), pp. 359–365. doi: 10.1016/j.desal.2004.07.022. Mekonnen, M. M. and Hoekstra, A. Y. (2016) ‘Four billion people facing severe water scarcity’, (February), pp. 1–7. Mericq, J.-P., Laborie, S. and Cabassud, C. (2010) ‘Vacuum membrane distillation of seawater reverse osmosis brines’, Water Research, 44(18), pp. 5260–5273. doi: 10.1016/j.watres.2010.06.052. Mpala, T. J. et al. (2022) ‘Biofouling phenomena in membrane distillation : mechanisms and mitigation strategies’, Environmental Science: Advances. doi: 10.1039/d2va00161f. Naidu, G., Jeong, S. and Vigneswaran, S. (2015) ‘Interaction of humic substances on fouling in membrane distillation for seawater desalination’, Chemical Engineering Journal, 262, pp. 946–957. doi: 10.1016/j.cej.2014.10.060. Nghiem, L. D. and Cath, T. (2011) ‘A scaling mitigation approach during direct contact membrane distillation’, Separation and Purification Technology, 80(2), pp. 315–322. doi: 10.1016/j.seppur.2011.05.013. 36 Nitodas, S. F., Das, M. and Shah, R. (2022) ‘Applications of Polymeric Membranes with Carbon Nanotubes: A Review’, Membranes, 12(454), pp. 1–17. doi: 10.3390/membranes12050454. Nthunya, Lebea N, Gutierrez, L., Derese, S., Edward, N., et al. (2019) ‘A review of nanoparticle-enhanced membrane distillation membranes : membrane synthesis and applications in water treatment’, Chemical Technology and Biotechnology, 94(9), pp. 2757– 2771. doi: 10.1002/jctb.5977. Nthunya, Lebea N, Gutierrez, L., Derese, S. and Mamba, B. B. (2019) ‘Adsorption of phenolic compounds by polyacrylonitrile nano fibre membranes : A pretreatment for the removal of hydrophobic bearing compounds from water’, Journal of Environmental Chemical Engineering, 7, p. 103254. doi: 10.1016/j.jece.2019.103254. Nthunya, Lebea N, Gutierrez, L., Lapeire, L., et al. (2019) ‘Fouling resistant PVDF nanofibre membranes for the desalination of brackish water in membrane distillation’, Separation and Purification Technology, 228, p. 115793. doi: 10.1016/j.seppur.2019.115793. Nthunya, Lebea N. et al. (2019) ‘Removal of Fe and Mn from polluted water sources in Lesotho using modified clays’, Journal of Water Chemistry and Technology, 41(2), pp. 81–86. doi: 10.3103/S1063455X19020036. Nthunya, L. N. et al. (2020) ‘f-MWCNTs / AgNPs-coated superhydrophobic PVDF nanofibre membrane for organic, colloidal, and biofouling mitigation in direct contact membrane distillation’, Journal of Environmental Chemical Engineering, 8(2), p. 103654. doi: 10.1016/j.jece.2020.103654. Nthunya, L. N. et al. (2022) ‘Fouling, performance and cost analysis of membrane-based water desalination technologies: A critical review’, Journal of Environmental Management, 301(July 2021), p. 113922. doi: 10.1016/j.jenvman.2021.113922. Nthunya, L. N., Mbakop, S. and Mhlanga, S. D. (2021) ‘Emerging nanoenhanced membrane- based hybrid processes for complex industrial wastewater treatment’, in Membrane-based hybrid processes for wastewater treatment. Netherlands: Elsevier B.V, pp. 633–656. doi: 10.1016/B978-0-12-823804-2.00024-0 633. Pan, J., Chen, M., et al. (2022) ‘Enhanced anti-wetted PVDF membrane for pulping RO brine treatment by vacuum membrane distillation’, Desalination, 526(December 2021), p. 115533. doi: 10.1016/j.desal.2021.115533. Pan, J., Zhang, F., et al. (2022) ‘Enhanced anti-wetting and anti-fouling properties of composite PFPE/PVDF membrane in vacuum membrane distillation’, Separation and Purification Technology, 282(PB), p. 120084. doi: 10.1016/j.seppur.2021.120084. Peng, Y. et al. (2015) ‘Effects of anti-scaling and cleaning chemicals on membrane scale in direct contact membrane distillation process for RO brine concentrate’, Separation and Purification Technology, 154, pp. 22–26. doi: 10.1016/j.seppur.2015.09.007. Phattaranawik, J. et al. (2008) ‘A novel membrane bioreactor based on membrane distillation’, 37 Desalination, 223, pp. 386–395. doi: 10.1016/j.desal.2007.02.075. Porcelli, N. and Judd, S. (2010) ‘Chemical cleaning of potable water membranes: A review’, Separation and Purification Technology, 71(2), pp. 137–143. doi: 10.1016/j.seppur.2009.12.007. Pramanik, B. K. et al. (2016) ‘A critical review of membrane crystallization for the purification of water and recovery of minerals’, Reviews in Environmental Science and Biotechnology, 15(3), pp. 411–439. doi: 10.1007/s11157-016-9403-0. Pramanik, B. K., Shu, L. and Jegatheesan, V. (2017) ‘A review of the management and treatment of brine solutions’, Environmental Science: Water Research and Technology, 3(4), pp. 625–658. doi: 10.1039/c6ew00339g. Pramono, E. et al. (2017) ‘Effects of PVDF concentration on the properties of PVDF membranes’, IOP Conference Series: Earth and Environmental Science, 75(1). doi: 10.1088/1755-1315/75/1/012027. Puspitasari, V. et al. (2010) ‘Cleaning and ageing effect of sodium hypochlorite on polyvinylidene fluoride (PVDF) membrane’, Separation and Purification Technology, 72(3), pp. 301–308. doi: 10.1016/j.seppur.2010.03.001. Quist-Jensen, C. A. et al. (2016) ‘A study of membrane distillation and crystallization for lithium recovery from high-concentrated aqueous solutions’, Journal of Membrane Science, 505, pp. 167–173. doi: 10.1016/j.memsci.2016.01.033. Quist-Jensen, C. A. et al. (2017) ‘Reclamation of sodium sulfate from industrial wastewater by using membrane distillation and membrane crystallization’, Desalination, 401, pp. 112–119. doi: 10.1016/j.desal.2016.05.007. Quist-Jensen, C. A. et al. (2019) ‘Perspectives on mining from sea and other alternative strategies for minerals and water recovery – The development of novel membrane operations’, Journal of the Taiwan Institute of Chemical Engineers, 94, pp. 129–134. doi: 10.1016/j.jtice.2018.02.002. Rahimpour, M. R. and Esmaeilbeig, M. A. (2019) ‘Chapter 6 - Membrane Wetting in Membrane Distillation’, in Angelo Basile, Efrem Curcio, I. (ed.) Current Trends and Future Developments on (Bio-) Membranes. Elsevier, pp. 143–174. doi: https://doi.org/10.1016/B978- 0-12-813551-8.00006-1. Rahmaniyan, B., Mohammadi, T. and Tofighy, M. A. (2021) ‘Development of high flux PVDF/modified TNTs membrane with improved properties for desalination by vacuum membrane distillation’, Journal of Environmental Chemical Engineering, 9(6), p. 106730. doi: 10.1016/j.jece.2021.106730. Ramos, R. L. et al. (2022) ‘Direct contact membrane distillation as an approach for water treatment with phenolic compounds’, Journal of Environmental Management, 303(June 2021), p. 114117. doi: 10.1016/j.jenvman.2021.114117. Reifenberger, R. G. (2015) Fundamentals of Atomic Force Microscopy-Part I: Foundations. 38 World Scientific. Rudolph, P. (2010) ‘Defect Formation During Crystal Growth from the Melt’, in Springer Handbook of Crystal Growth. 1st edn. Berlin: Springer, pp. 159–201. doi: 10.1007/978-3-540- 74761-1_6. Ruiz Salmón, I. and Luis, P. (2018) ‘Membrane crystallization via membrane distillation’, Chemical Engineering and Processing: Process Intensification, 123(June 2017), pp. 258–271. doi: 10.1016/j.cep.2017.11.017. Salehi, M. (2022) ‘Global water shortage and potable water safety; Today’s concern and tomorrow’s crisis’, Environment International, 158, p. 106936. doi: 10.1016/j.envint.2021.106936. Schmidt, M. (2019) ‘Scarcity and environmental impact of mineral resources-an old and never- ending discussion’, Resources, 8(1), pp. 1–12. doi: 10.3390/resources8010002. Selvarajan, V., Obuobi, S. and Ee, P. L. R. (2020) ‘Silica Nanoparticles—A Versatile Tool for the Treatment of Bacterial Infections’, Frontiers in Chemistry, 8(July), pp. 1–16. doi: 10.3389/fchem.2020.00602. Sershen et al. (2016) ‘Water security in South Africa: Perceptions on public expectations and municipal obligations, governance and water re-use’, Water SA, 42(3), pp. 456–465. doi: 10.4314/wsa.v42i3.11. Shi, W. et al. (2022) ‘An innovative hollow fiber vacuum membrane distillation-crystallization (VMDC) coupling process for dye house effluent separation to reclaim fresh water and salts’, Journal of Cleaner Production, 337(January), p. 130586. doi: 10.1016/j.jclepro.2022.130586. Silva, T. L. S. et al. (2015) ‘Multi-walled carbon nanotube/PVDF blended membranes with sponge- and finger-like pores for direct contact membrane distillation’, Desalination, 357, pp. 233–245. doi: 10.1016/j.desal.2014.11.025. Smith, B. . (2011) Fundamentals of Fourier transform infrared spectroscopy. Second edi. CRC press. Sparenberg, M. C., Ruiz Salmón, I. and Luis, P. (2020) ‘Economic evaluation of salt recovery from wastewater via membrane distillation-crystallization’, Separation and Purification Technology, 235(April 2019), p. 116075. doi: 10.1016/j.seppur.2019.116075. Sri