SILVER NANOPARTICLE-MODIFIED CELLULOSE NANOCRYSTALS FOR FOULING CONTROL IN MEMBRANE DISTILLATION Josephine Tshepiso Mpala Student number: 731264 MASTER OF SCINCE in School of Chemistry, Faculty of Science, University of the Witwatersrand Supervisors: Dr. Lebea Nthunya; Dr. Heidi Richards and Dr. Anita Etale ii DECLARATION I declare that this thesis is my own, unaided work. It is being submitted for the Degree of Master of Science at the University of the Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination at any other University. J.T. Mpala (Signature) _______day of __________________ 20_______at_________________ 08 June 23 Wits University iii ABSTRACT A global reduction in water resources and the growing demand for fresh water has motivated the quest for the development of sustainable water-augmenting technologies. Membrane distillation (MD) is envisaged as an attractive desalination technology, surpassing cost challenges faced by conventional desalination technologies. Yet, its industrial commercialization faces multiple limitations, including the production of low water fluxes, membrane wetting and membrane fouling. This study sought to investigate the performance of silver nanoparticles (AgNPs) embedded on cellulose nanocrystals (CNCs) (CNC-capped AgNPs) to lessen the impact of biofouling in MD. This was conducted through coating the polyvinylidene fluoride (PVDF) membrane with CNCcapped AgNPs. Prior to coating with CNC-capped AgNPs, PVDF membrane properties were improved (for MD suitability) through blending with polyvinylpyrrolidone (PVP) and functionalized carbon nanotubes (f-CNTs). The resulting membrane had an improved overall porosity, and a respective increase in surface roughness (75%) and mechanical strength (45%). Pristine CNC-capped AgNPs’ characterization presented stable AgNPs with minimal leaching. Transmission electron microscopy (TEM) micrographs revealed a uniform dispersion of spherically shaped AgNPs exhibiting 13.3 ± 3.4 nm average diameter. The presence of AgNPs on the surface of CNCs afforded excellent thermal stability and good anti-microbial activity, mainly against E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus. Following membrane modification, preliminary anti-bacterial tests conducted on the CNC/AgNP-modified PVDF membrane revealed a 98.7%, 52.3%, 78.0%, 53.9% and 93.3% reduction of E. coli, P. aeruginosa, S. aureus, S. epidermis, and S. saprophyticus cells, respectively, demonstrating its ability to control biofouling. Although the CNC/AgNP-modified PVDF membrane exhibited improved membrane properties, such as high surface roughness, high liquid entry pressure (LEP), and good hydrophobicity, its performance in MD (with artificial seawater as the feed stream) was poor, producing the lowest average water flux (0.179 ± 0.0303 kg/m2 /hr) compared to the unmodified PVDF membrane (0.528 ± 0.0838 iv kg/m2 /hr), mainly due to pore blockage. However, upon spiking the artificial seawater with a monoculture of G. Stearothermophilus, the CNC/AgNP-modified PVDF membrane displayed the most stable water flux while the unmodified PVDF membrane’s water flux decreased by 79.3% over the 24-hour (h) period. This was attributed to the formation of a biofouling layer on the PVDF membrane which was absent on the CNC/AgNP-modified PVDF membrane. The AgNPs on the surface of the membrane afforded minimal bacterial deposition during operation. These results ascertain the possibility of biofouling minimization in MD using CNC-capped AgNPs, contributing to MD’s body of work for its ultimate realization for up-scaling. v DEDICATION I dedicate this dissertation to my late aunt, Rakgadi Tiny Mabel Ntlatleng, for upholding our family with love and humility. “Forever in our hearts.” vi ACKNOWLEDGEMENTS Immense appreciation firstly to God for His tremendous mercy and love. He carried me through and provided health, strength, and wisdom. Thank you to my supervisors; Dr Lebea, Dr Heidi, and Dr Anita – for affording me the opportunity to become a part of the analytical and environmental chemistry group, for providing technical assistance and for offering support. I will always be thankful for the time I spent and the overall impact it had. A special thank you to Dr Lebea for assisting step by step and showing the way. This work would not have been half its quality without his input. Massive gratitude to Dr Heidi and Dr Anita as well, for offering well-thoughtful suggestions. Immense thanks to Prof Hope from UJ for opening the doors of her lab for us to do our microbial work. A special thanks to Indira for the insightful and enlightening chats around our work in the lab. Massive thank you as well to the postgraduates within our research group for assistance and motivation. A special thanks to Ms. Mokgaetji for assistance with safety assurance in the lab and equipment training and to Dr. Oranso Mahlangu from UNISA for assistance with membrane characterization. Thank you to the MMU department for assistance during characterization. Lastly, thank you to the School of Chemistry and Wits University for affording me the opportunity to complete my studies. I remain eternally grateful to my parents for their support, the patience, and the love that they have shown me throughout my life. Embarking on an MSc journey would have remained a dream without them. To my sister, Dineo, who’s words of affirmation became my strength, words are not enough. I want to thank her for offering an ear to listen when I needed it. To the Mpala and Ntlatleng clan – my maternal and paternal – it takes a village, and I am immensely grateful for everything. A titanic thank you to my best friends, Lerato and Uni, for being my biggest cheerleaders. I want to thank them for encouraging me to do this degree and for being my inspiration. Massive thank you to the National Research Foundation (NRF) for providing financial assistance. vii PUBLICATIONS AND PRESENTATIONS List of presentations: July 2022 – Gradflash presentation: Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation, Cross Faculty Symposium, Wits University. September 2022 – Poster presentation: Silver nanoparticle-modified cellulose nanocrystals for fouling control in membrane distillation, MSc poster presentations, Wits University. List of publications: Published: Tshepiso J. Mpala, Heidi Richards, Anita Etale and Lebea N. Nthunya. Biofouling phenomena in membrane distillation: mechanisms and mitigation strategies. Environmental Science Advances. 2022; DOI: 10.1039/d2va00161f. A review article forming part of Chapter 2. Tshepiso J. Mpala, Heidi Richards, Anita Etale, Hope Serepa-Dlamini and Lebea N. Nthunya. Cellulose nanocrystal-mediated synthesis of silver nanoparticles via microwave assisted method for biofouling control in membrane distillation. Materials today communications. 2022; 34, 105028. A research article forming part of Chapter 3 and 4. Submitted: Tshepiso J. Mpala, Heidi Richards, Anita Etale, Oranso T. Mahlangu, and Lebea N. Nthunya. Preparation of nanoparticle enhanced PVDF membrane for improved performance in membrane distillation. Frontiers in membrane science and technology. A research article forming part of Chapter 5. viii In press: Tshepiso J. Mpala, Heidi Richards, Indira Chimanlal, Anita Etale, and Lebea N. Nthunya. Hybrid membrane processes equipped with a crystallization unit for the simultaneous recovery of freshwater and minerals from saline wastewater. A book chapter forming part of a book titled: “Innovative trends in removal of Refractory pollutants from Pharmaceutical wastewater.” Elsevier In preparation: Tshepiso J. Mpala, Heidi Richards, Anita Etale, Oranso T. Mahlangu, and Lebea N. Nthunya. Biofouling control in membrane distillation using CNC/AgNP- modified PVDF membranes. In preparation. A research article forming part of Chapter 6. ix TABLE OF CONTENTS DECLARATION ........................................................................................................... ii ABSTRACT ................................................................................................................ iii DEDICATION ............................................................................................................. v ACKNOWLEDGEMENTS .......................................................................................... vi PUBLICATIONS AND PRESENTATIONS ................................................................ vii LIST OF FIGURES .................................................................................................... xii LIST OF TABLES ...................................................................................................... xii LIST OF ABBREVIATIONS ..................................................................................... xvii Chapter 1: Introduction ............................................................................................ 1 1.1 Background and motivation .................................................................................... 1 1.2 Problem statement ................................................................................................. 3 1.3 Aims and Objectives .............................................................................................. 4 Chapter 2: Literature Review ................................................................................... 8 2.1 Membrane distillation ............................................................................................. 8 2.2 Biofilm development pathway .............................................................................. 11 2.2.1 Conditioning film formation and surface attachment ...................................... 11 2.2.2 Biofilm stability and EPS secretion ................................................................ 14 2.3 Enumeration and identification of microbial cells on the membrane surface ........ 15 2.4 Microorganisms responsible for membrane biofouling in MD ............................... 16 2.5 Effects of biofouling on process performance ...................................................... 20 2.5.1 Distillate flux decay ........................................................................................ 22 2.5.2 Salt rejection decline ...................................................................................... 25 x 2.5.3 Frequency of membrane replacements ......................................................... 25 2.6 Mitigation of biofouling in MD ............................................................................... 26 2.6.1 Pre-treatment of the feed solution .................................................................. 27 2.6.2 Membrane cleaning ....................................................................................... 28 2.6.3 Membrane modification ................................................................................. 30 Chapter 3: Equipment and techniques overview ................................................ 45 3.1 Techniques used in study .................................................................................... 45 3.1.1 Microwave irradiation ..................................................................................... 45 3.1.2 Phase inversion ............................................................................................. 45 3.1.3 Minimum inhibitory concentration (MIC) ........................................................ 46 3.1.4 Cell viability .................................................................................................... 47 3.2 Equipment used for characterization .................................................................... 47 Chapter 4: Cellulose nanocrystal-mediated synthesis of silver nanoparticles via microwave assisted methods for preliminary anti-microbial evaluation against Gram-negative and Gram-positive bacteria ......................................................... 59 4.1 Introduction .......................................................................................................... 59 4.2 Materials and methods ......................................................................................... 60 4.2.1 Materials ........................................................................................................ 60 4.2.2 Methods ......................................................................................................... 60 4.3 Results and discussion ........................................................................................ 64 4.3.1 Characterization of CNC-capped AgNPs ....................................................... 64 4.3.2 Preliminary antimicrobial tests ....................................................................... 70 4.4 Conclusion ........................................................................................................... 74 Chapter 5: Preparation of nanoparticle enhanced PVDF for improved performance in membrane distillation ........................................................................................ 78 xi 5.1 Introduction .......................................................................................................... 78 5.2 Materials and methods ......................................................................................... 79 5.2.1 Materials ........................................................................................................ 79 5.2.2 Methods ......................................................................................................... 79 5.3 Results and discussion ........................................................................................ 83 5.3.1 Characterization of CNTs and f-CNTs ........................................................... 83 5.3.2 Physiochemical properties of PVDF membranes ........................................... 85 5.3.3 Leaching studies of AgNPs ............................................................................ 92 5.3.4 Membrane performance evaluation using DCMD .......................................... 93 5.4 Conclusion ........................................................................................................... 95 Chapter 6: Evaluation of biofouling control in membrane distillation against G. Stearothermophilus ............................................................................................. 102 6.1 Introduction ........................................................................................................ 102 6.2 Materials and methods ....................................................................................... 103 6.2.1 Materials ...................................................................................................... 103 6.2.2 Methods ....................................................................................................... 103 6.3 Results and discussion ...................................................................................... 105 6.3.1 Membrane anti-microbial performance evaluation using DCMD .................. 105 6.3.2 Fouled membrane surface morphology analysis ......................................... 108 6.4 Conclusion ......................................................................................................... 111 Chapter 7: Overall conclusion and future work ................................................. 115 7.1 Overall Conclusion ............................................................................................. 115 7.2 Future work ........................................................................................................ 116 xii LIST OF FIGURES Figure 2.1: Mechanism of operation of MD: (a) process mechanism and (b) heat and mass transfer, where Tf, Tmf, Tmp and Tp represent temperatures of the feed solution in the tank, near the membrane surface, the distillate solution in the tank and near the membrane surface, respectively. Pf, Pp represent the partial pressures on the feed side and the distillate side and cf, cmf represent feed solution concentrations in the tank and near the membrane surface, respectively. ...................................................................... 9 Figure 2.2: Mechanistic relationship of the salinity, heat, nutrient, and flow stresses on the distribution of bacteria on membrane surfaces during MD operation ...................... 13 Figure 2.3: Biofilm progression in MD ........................................................................... 15 Figure 2.4: Temperature polarization phenomenon in MD. Tf,in, Tfm, Tpm and Tpb represent temperatures in the bulk feed solution, near the membrane surface (feed side), near the membrane surface (distillate side) and in the bulk distillate, respectively. ..... 24 Figure 2.5: Latest developments of biofouling control strategies in MD........................ 26 Figure 2.6: Membrane modification using MNPs .......................................................... 31 Figure 3.1: Synthesis of polymer membranes through the phase inversion technique . 46 Figure 3.2: UV/vis spectroscopy principle of operation ................................................. 48 Figure 3.3: Interaction of material with X-rays .............................................................. 49 Figure 3.4: Dead-end filtration cell set up ..................................................................... 52 Figure 3.5: Typical goniometer used to assess WCA ................................................... 53 Figure 3.6: ICP-OES principle of operation .................................................................. 54 Figure 4.1: UV−vis absorbance spectra of CNC-capped AgNPs .................................. 64 xiii Figure 4.2: Schematic illustration of the microwave-assisted reduction of AgNPs followed by their support on CNCs ............................................................................................. 64 Figure 4.3: Transmission electron micrograph of CNC-capped AgNPs (a), coupled with corresponding histogram plot (b). .................................................................................. 66 Figure 4.4: XRD spectra of CNCs and CNC-capped AgNPs ........................................ 67 Figure 4.5: FTIR spectra of CNCs and CNC-capped AgNPs ........................................ 68 Figure 4.6: TGA (a) and DTG (b) curves for CNCs and CNC-capped AgNPs .............. 69 Figure 4.7: EDS spectra of CNCs (a) and CNC-capped AgNPs (b).............................. 70 Figure 4.8: MIC of CNCs .............................................................................................. 71 Figure 4.9: MIC of CNC-capped AgNPs ....................................................................... 72 Figure 5.1: Interaction between CNTs and POTS to produce f-CNTs. .......................... 80 Figure 5.2: MD lab-set up: (1) Feed tank, (2) and (5) Circulation pumps, (3) Membrane module, (4) Distillate tank, and (6) Weighing balance ................................................... 83 Figure 5.3: TEM images of (a) CNTs and (b) f-CNTs .................................................... 84 Figure 5.4: FTIR plot (a) and XRD plot (b) of CNTs and f-CNTs ................................... 84 Figure 5.5: EDS Spectra of CNTs (a) and f-CNTs (b) ................................................... 85 Figure 5.6: FTIR spectra of the as-synthesised membranes ........................................ 86 Figure 5.7: SEM micrographs of PVDF membranes: Top surface and corresponding cross-sectional morphology ........................................................................................... 87 Figure 5.8: Contact angle measurements of the as-synthesized membranes .............. 89 Figure 5.9: Surface roughness characteristics of the as-synthesised membranes ....... 90 Figure 5.10: Stress-strain plot of the as-synthesised membranes ................................ 92 xiv Figure 5.11: Concentration of silver of CNC/AgNP-modified PVDF membrane (M4) following a 72-hour silver leaching test .......................................................................... 93 Figure 5.12: Conductivity and water flux for M1, M2, M3 and M4 using synthetic seawater in DCMD ........................................................................................................................ 95 Figure 6.1: Factors affecting microbial deposition ...................................................... 103 Figure 6.2: Water flux and distillate salinity results as obtained from DCMD operations where artificial seawater spiked with thermophilic bacteria was used as the feed stream .................................................................................................................................... 108 Figure 6.3: SEM images (at 10 µm) for M1, M4 and M2, M3 following DCMD operation .................................................................................................................................... 109 Figure 6.4: SEM images (at 5 µm) for M1 and M4 following DCMD operation ........... 111 xv LIST OF TABLES Table 2.1: Biofouling-causing microbial organisms in MD ............................................. 18 Table 2.2: A summarized impact of biofouling on MD efficiency ................................... 20 Table 2.3: Mitigation strategies of biofouling in MD ...................................................... 33 Table 4.1: Casting solution composition for each membrane ........................................ 61 Table 4.2: MIC values of CNC-capped AgNPs against different bacterial strains ......... 72 Table 4.3: Antibacterial performance of PVDF and CNC/AgNP-modified PVDF against bacterial strains. ............................................................................................................ 73 Table 5.2: Physical properties of the membranes ......................................................... 88 Table 5.3: Mechanical properties of the membranes .................................................... 91 xvi LIST OF ABBREVIATIONS PVDF – Polyvinylidene fluoride PVP – Polyvinyl pyrrolidone CNCs – Cellulose nanocrystals AgNPs – Silver nanoparticles MD – Membrane distillation DCMD – Direct contact membrane distillation RO – Reverse Osmosis LEP – Liquid entry pressure TP – Temperature polarization CP – Concentration polarization PTFE – Polytetrafluoroethylene PP – Polypropylene MWCNTs – Multi-walled carbon nanotubes POTS – 1H, 1H, 2H, 2H-perfluoro-octyltriethoxysilane f-CNTs – Functionalized carbon nanotubes NIPS – Non induced phase inversion EPS – Extracellular polymeric substance SLM – Soluble microbial products MNPs – Metal nanoparticles FTIR – Fourier transform infrared spectroscopy xvii TEM – Transmission electron microscopy SEM – Scanning electron microscopy AFM – Atomic force microscopy XRD – X-ray diffraction ICP-MS – Inductively coupled plasma – mass spectroscopy Chapter 1: Introduction 1.1 Background and motivation The demand for potable water is predicted to exceed supply by 2030 (Prins et al., 2022), a projection necessitating the development of well-thoughtful water-augmenting strategies. The importance of water accessibility stems from its ability to enhance economic growth and advance agricultural activities, which have a direct impact on food security (Liu, 2022). Moreover, the basic survival of a human being requires freshwater (White et al., 2010). However, freshwater accessibility is deteriorating at an alarming rate (Wang et al., 2019). It is estimated that about 2.7 billion people are affected by this deterioration annually (Barrios et al., 2020). According to a study by Giri et al., (2022), diarrhea caused by inadequate clean water and sanitation is a major cause of death among children. Worse, predictions by the world bank project a water deficit (in 2050) where approximately 5 billion people in developing countries will be affected (Dhakal et al., 2022). Therefore, global urgent responses are required to tackle water related challenges. Currently, strategies presented to lessen the impact of water shortage include saving water, increasing water reuse, transporting water to semi-arid areas and desalination (recovery of fresh water from seawater). Although desalination is considered the last resort owing to its high cost of operation; its independence on climate change, river flows and reservoir levels has enhanced its advancement in research and development (Dhakal et al., 2022). As of 2022, the total desalination capacity is estimated to be 115 Mm3/d (Dhakal et al., 2022). Desalinations’ consideration for application is further enhanced by the abundance of seawater (Castro-Muñoz, 2020). In particular, seawater constitutes 97% of the total water on earth (Biswas, 2016). The remaining 3% constitutes freshwater, with 1.74% accounting for water locked up in glaciers/ice-caps and 1.75% accounting for water in lakes, rivers and groundwater (Balasubramanian, 2015). Through desalination, independent remote areas near the coast can be supplied with fresh water (Schwantes et al., 2013). Several techniques aimed at seawater desalination exist, 2 including reverse osmosis (RO), multi effect distillation (MED), multi stage flash (MSF), and membrane distillation (MD), amongst others (Nthunya et al., 2022). The main drawback of RO includes overcoming the hurdle of disposing brine concentrates in the environment, which pollute existing water systems and impact negatively on fauna and flora, especially in regions on the coast (Kiefer, Präbst and Sattelmayer, 2019). Given the need to meet global environmental integrity and zero liquid discharge (ZLD), thermal desalination technologies have emerged, including MD, MED and MSF (Ruiz-Aguirre, Andrés-Mañas and Zaragoza, 2019). While MED and MSF are able to treat highly concentrated brine, MD is reported to exhibit better energy efficiency compared to the two (Choudhury et al., 2019). In addition, MD exhibits less sensitivity to the concentration of solutes in the feed solution (due to its driving force), achieves exceptional separation efficiencies (˃99%) and requires moderate conditions (i.e., pressure) for operation (Alkhudhiri et al., 2012; Lalia et al., 2014). Indeed, MD displays superior features. The implications of a MD system operating under moderate conditions include the possibility of commercializing a decentralized system due to a decrease in system complexity (Bogler and Bar-Zeev, 2018). In addition, less rigid membrane mechanical properties are required (Liu, Zhu and Chen, 2020b). With MD, alternative energy sources such as solar energy and geothermal energy can be considered for use during operation (Sarbatly and Chiam, 2013; Guillen-Burrieza et al., 2014). Factors affecting seawater desalination in MD include: (1) hydrodynamic conditions of the system, (2) feed solution properties, and (3) membrane properties. MD achieves freshwater recovery through the use of a hydrophobic/microporous membrane, where volatile substances are transferred from the feed compartment to the distillate compartment across the membrane (Teoh et al., 2021). Membranes often require modification to operate optimally, with substantial work being done on certain nanoparticles (NPs) to exploit them for use during membrane modification. CNTs are NPs consisting of rolled-up sheets of graphene and are largely used during membrane synthesis due to their high mechanical strength and excellent aspect ratio (Gao et al., 2020). CNTs are further able to lower membrane surface energy when surface 3 functionalized with fluorosilanes. This is because these have low surface tension and low polarity (Zheng et al., 2016). This work involved the use of CNTs during PVDF synthesis. The main drawback of a hydrophobic membrane during MD operation is the tendency for membrane fouling, which refers to the adherence of unwanted contaminants on the membrane surface and within its pores (Tripathi, 2020). Amongst other fouling types, microbial accumulation (i.e., biofouling) has proven to be the most intricate and the severest, inducing biofilm development through elaborate pathways (Bogler, Lin and Bar- Zeev, 2017). Little has been known about biofouling in MD through the years as minimal microbial deposition was anticipated, given the high temperatures and the high saline conditions under which the system operates (Liu, Zhu and Chen, 2020a). However, reports of biofouling in MD have been on the rise and have attracted significant research attention (Costa et al., 2021). 1.2 Problem statement Challenges associated with biofouling include considerable microbial deposition on the surface of the membrane, leading to the formation of a biofilm. Biofilms cause a severe irreversible reduction in water flux through impeding the maximum transfer of the vapor produced, largely as a result of an increase in the restriction of flow (Bogler, Lin and Bar- Zeev, 2017). Moreover, these microbial contaminants pollute the distillate stream. The production of extracellular polymers (EPS) by bacteria following microbial adsorption cause a decrease in membrane hydrophobicity, inducing membrane wetting and a subsequent decrease in the salt rejection efficiency (Bogler and Bar-Zeev, 2018). According to previous reports, operational costs are increasingly affected by biofouling (Wang et al., 2019; Jafari et al., 2021). A rise in energy consumption has been witnessed when the driving force (i.e., vapor pressure) in MD plummets due to biofouling. Therefore, the large scale application of MD has been limited by biofouling, among other factors (Krivorot et al., 2011). With an increase in efficiency losses and a reduction in membrane lifespan, biofouling eradication has become a key research concern. This project focused on the exploitation of an environmentally benign and renewable resource, cellulose, to 4 immobilize AgNPs on PVDF to prevent microbial deposition and accumulation. Limiting biofouling in MD will ensure that higher fluxes are realized, and pure freshwater is recovered. 1.3 Aims and Objectives The main aim of this study was to investigate the performance of CNC-capped AgNPs embedded on the PVDF membrane for biofouling resistance in MD. This was achieved through small work packages, each related to a specific objective: 1. Synthesis and characterization of CNC-capped AgNPs 2. Synthesis and characterization of flat sheet pristine and modified PVDF membranes 3. Preliminary assessment of the antimicrobial properties of CNC-capped AgNPs and CNC/AgNP-modified PVDF membrane 4. Assessment of the performance of flat sheet pristine and modified PVDF membranes towards biofouling minimization in a laboratory-scale MD system. 1.4 References 1. Balasubramanian, A. (2015) ‘the World ’ S Water Crisis’, Research Gate, 44(0). 2. Alkhudhiri, A., Darwish, N. and Hilal, N. (2012) ‘Membrane distillation: A comprehensive review’, Desalination, 287, pp. 2–18. doi: 10.1016/j.desal.2011.08.027. 3. Barrios, A. C. et al. (2020) ‘Prolonging the antibacterial activity of nanosilver- coated membranes through partial sulfidation’, Environmental Science: Nano, 7(9), pp. 2607–2617. doi: 10.1039/d0en00300j. 4. Biswas, A. K. (2016) ‘Water Availability and Use’, Water Resources of North America, (April 2018), pp. 163–174. doi: 10.1007/978-3-662-10868-0_19. 5. Bogler, A. and Bar-Zeev, E. (2018) ‘Membrane Distillation Biofouling: Impact of 5 Feedwater Temperature on Biofilm Characteristics and Membrane Performance’, Environmental Science and Technology, 52(17), pp. 10019–10029. doi: 10.1021/acs.est.8b02744. 6. Bogler, A., Lin, S. and Bar-Zeev, E. (2017) ‘Biofouling of membrane distillation, forward osmosis and pressure retarded osmosis: Principles, impacts and future directions’, Journal of Membrane Science, 542, pp. 378–398. doi: 10.1016/j.memsci.2017.08.001. 7. Castro-Muñoz, R. (2020) ‘Breakthroughs on tailoring pervaporation membranes for water desalination: A review’, Water Research, 187, p. 116428. doi: 10.1016/j.watres.2020.116428. 8. 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. 9. Costa, F. C. R. et al. (2021) ‘Biofouling in membrane distillation applications - a review’, Desalination, 516(July). doi: 10.1016/j.desal.2021.115241. 10. Dhakal, N. et al. (2022) ‘Is Desalination a Solution to Freshwater Scarcity in Developing Countries ?’, (Figure 1), pp. 1–15. 11. Gao, C. et al. (2020) ‘Superhydrophobic electrospun PVDF membranes with silanization and fluorosilanization co-functionalized CNTs for improved direct contact membrane distillation’, Engineered Science, 9, pp. 35–43. doi: 10.30919/es8d905. 12. Giri, M. et al. (2022). 'Water, Sanitation, and Hygiene Practices and Their Association With Childhood Diarrhoea in Rural Households of Mayurbhanj District, Odisha, India', Cureus, 14(10). https://doi.org/10.7759/cureus.29888. 13. 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. 14. Jafari, M. et al. (2021) ‘Cost of fouling in full-scale reverse osmosis and nanofiltration installations in the Netherlands’, Desalination, 500, p. 114865. doi: 10.1016/j.desal.2020.114865. 6 15. Kiefer, F., Präbst, A. and Sattelmayer, T. (2019) ‘Membrane scaling in Vacuum Membrane Distillation - Part 2: Crystallization kinetics and process performance’, Journal of Membrane Science, 590, pp. 1–10. doi: 10.1016/j.memsci.2019.117293. 16. Krivorot, M. et al. (2011) ‘Factors affecting biofilm formation and biofouling in membrane distillation of seawater’, Journal of Membrane Science, 376(1–2), pp. 15–24. doi: 10.1016/j.memsci.2011.01.061. 17. Lalia, B. S. et al. (2014) ‘Nanocrystalline cellulose reinforced PVDF-HFP membranes for membrane distillation application’, Desalination, 332(1), pp. 134– 141. doi: 10.1016/j.desal.2013.10.030. 18. Liu, C., Zhu, L. and Chen, L. (2020a) ‘Biofouling phenomenon of direct contact membrane distillation (DCMD) under two typical operating modes: Open-loop mode and closed-loop mode’, Journal of Membrane Science, 601. doi: 10.1016/j.memsci.2020.117952. 19. Liu, C., Zhu, L. and Chen, L. (2020b) ‘Effect of salt and metal accumulation on performance of membrane distillation system and microbial community succession in membrane biofilms’, Water Research, 177, p. 115805. doi: 10.1016/j.watres.2020.115805. 20. Liu, X. (2022) ‘Global Agricultural Water Scarcity Assessment Incorporating Blue and Green Water Availability Under Future Climate Change Earth ’ s Future’. doi: 10.1029/2021EF002567. 21. 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, p. 113922. doi: 10.1016/j.jenvman.2021.113922. 22. Prins, F. X. et al. (2022) ‘Water scarcity and alternative water sources in South Africa: can information provision shift perceptions?’, Urban Water Journal, 00(00), pp. 1–12. doi: 10.1080/1573062X.2022.2026984. 23. Ruiz-Aguirre, A., Andrés-Mañas, J. A. and Zaragoza, G. (2019) ‘Evaluation of distillate quality in pilot scale membrane distillation systems’, Membranes, 9(6), pp. 1–14. doi: 10.3390/membranes9060069. 7 24. Sarbatly, R. and Chiam, C. K. (2013) ‘Evaluation of geothermal energy in desalination by vacuum membrane distillation’, Applied Energy, 112, pp. 737–746. doi: 10.1016/j.apenergy.2012.12.028. 25. Schwantes, R. et al. (2013) ‘Membrane distillation: Solar and waste heat driven demonstration plants for desalination’, Desalination, 323, pp. 93–106. doi: 10.1016/j.desal.2013.04.011. 26. Teoh, G. H. et al. (2021) ‘Impacts of PVDF polymorphism and surface printing micro-roughness on superhydrophobic membrane to desalinate high saline water’, Journal of Environmental Chemical Engineering, 9(4), p. 105418. doi: 10.1016/j.jece.2021.105418. 27. Tripathi, B. P. (2020) ‘Encyclopedia of Membranes’, Encyclopedia of Membranes, pp. 1–3. doi: 10.1007/978-3-642-40872-4. 28. Wang, Wenyi et al. (2019) ‘Palygorskite/silver nanoparticles incorporated polyamide thin film nanocomposite membranes with enhanced water permeating, antifouling and antimicrobial performance’, Chemosphere, 236, p. 124396. doi: 10.1016/j.chemosphere.2019.124396. 29. White, M. et al. (2010) ‘Blue space: The importance of water for preference, affect, and restorativeness ratings of natural and built scenes’, Journal of Environmental Psychology, 30(4), pp. 482–493. doi: 10.1016/j.jenvp.2010.04.004. 30. Zheng, L. et al. (2016) ‘Preparation of PVDF-CTFE hydrophobic membranes for MD application: Effect of LiCl-based mixed additives’, Journal of Membrane Science, 506, pp. 71–85. doi: 10.1016/j.memsci.2016.01.044. 8 Chapter 2: Literature Review In this chapter, an in-depth review of MD is given as well as its application in water purification. First, the history of MD and its mechanism of operation is presented, followed by conditions under which separation occurs and the pressing concerns around membrane biofouling. Later, literature relating to solutions regarding biofouling prevention are offered. 2.1 Membrane distillation The process of MD was originally described and patented in 1963 by Bodell, with no presentation of quantitative data (Bodell, 1968). Having developed an apparatus that could circulate brine water and produce distillate vapor, Bodell’s idea of the “structure” that was liquid impermeable and vapor permeable was unclear (Lawson & Lloyd, 1997). The discovery of a porous hydrophobic membrane was reported by Weyl, (1967), proposing that the minimization of energy consumption was possible by means of directly contacting the feed solution and distillate solution with a hydrophobic membrane. Reportedly, water fluxes reaching 1 kg m-2 h-1 were attained. However, MD water fluxes remained critically low compared to the widely explored RO desalination processes (70 kg m-2 h-1), causing the technique to lose traction (Lawson & Lloyd, 1997). It was only in the late 1980s that the availability of a hydrophobic membrane that exhibited better properties was attained (Nakoa, Date and Akbarzadeh, 2015). Subsequently, MD research grew exponentially, displaying major advances in its configurations (El-Bourawi et al., 2006). Currently, four different MD configurations have been reported, depending on how water vapor is treated on the distillate side following production. These include air gap membrane distillation (AGMD), sweep gas membrane distillation (SGMD), vacuum membrane distillation (VMD) and direct contact membrane distillation (DCMD) (Eykens et al., 2016). DCMD remains the most widely used configuration (Choi et al., 2019), 9 involving a direct contact of the hydrophobic membrane with the heated feed solution and the cold distillate (Jiang et al., 2017). With DCMD, the water fluxes generated are more stable, especially compared to VMD (Drioli, Ali and Macedonio, 2015). However, DCMD suffers from massive conductive heat loss, hampering its industrial application. To reduce energy loss, an air gap was introduced into the system, establishing high energy savings with, however, significantly lower fluxes (Eykens et al., 2016). In terms of the mechanism of operation, vapor generated from the vapor pressure difference existing across the membrane gets transported to the distillate side where it condenses to form pure water. As illustrated in Figure 2.1, both mass (vapor) and heat are transferred though the hydrophobic membrane. The mechanism of mass transport occurs through Poisseuille flow, molecular diffusion, and Knudsen diffusion, conditional to the existence of trapped air in the pores and the membrane pore size (Khayet, 2011). Mass transport depends on the concentration of the feed solution, the mass transfer coefficient, the type of membrane used and its properties. Figure 2.1: Mechanism of operation of MD: (a) process mechanism and (b) heat and mass transfer, where Tf, Tmf, Tmp and Tp represent temperatures of the feed solution in 10 the tank, near the membrane surface, the distillate solution in the tank and near the membrane surface, respectively. Pf, Pp represent the partial pressures on the feed side and the distillate side and cf, cmf represent feed solution concentrations in the tank and near the membrane surface, respectively (Tomaszewska, 2020). Membranes used in MD include PVDF, polytetrafluoroethylene (PTFE) and polypropylene (PP) (Hou et al., 2014). The widespread application of PVDF in MD stems from its advantages including mechanical, thermal and chemical stability, as well as its beneficial dissolution in a wide variety of solvents (Lai et al., 2014). PVDF consists of 59.4 wt% fluorine and 3 wt% hydrogen (Dohany, 2000), with 35% - 70% crystallinity. Fluorinated polymers generally have high thermal stability over their hydrocarbon counterparts, largely due to the highly electronegative fluorine atom (Liu et al., 2011). Although MD displays attractive features, it is its ability to use residual heat that continues to heighten its advancement in research (Choi et al., 2019). A study by Schwantes et al., (2013) coupled MD operation (pilot-scale) with a diesel power station, where residual heat from cooling circuits of the diesel power station was utilized to heat the MD feed stream. During MD operation, temperatures of up to 70⁰C-80⁰C could be obtained throughout a 24-hour day, with MD producing about 4 m3 of water per day. The utilization of residual heat was found to be energy-saving and cost-effective. One significant limitation of MD application is the tendency for membrane fouling (Drioli, Ali and Macedonio, 2015). Membrane fouling is reported to increase membrane wetting, causing a reduction in the quality of the distillate stream (Chew, Krantz and Fane, 2014). Moreover, membrane fouling introduces regular membrane cleaning and frequent membrane replacements, further increasing costs of water production (Shirazi, Lin and Chen, 2010). Current studies reported in literature mostly focus on inorganic and organic fouling, with limited information available pertaining to the dynamics and succession of biofouling (Liu, Zhu and Chen, 2020a). By definition, biofouling refers to the deposition and accumulation of microorganisms on the surface of the membrane and within its pores, leading to the formation of a biofilm (Zahid et al., 2021). The biofilm consists of microbial cells and 11 extracellular polymeric substances (EPS), covering the surface of the membrane. A study by Zodrow et al.,(2014) investigated the initial stages of biofilm formation in MD and RO. Waters collected during the autumn season had greater amounts of EPS, dead bacteria, and nutrients than waters collected during winter. Consequently, a 50% decline in water flux was observed in the first 12 h of MD operation. This was attributed to the high nutrients present in the feed solution which were not removed though pre-treatment via microfiltration (10 µm pores). Elucidating the mechanism and effects of biofouling and biofilm development in MD is highly imperative as it will ensure a sustained operation of MD over long periods of time. The latter part of this Chapter will cover biofilm development, its implications on performance and its mitigation. 2.2 Biofilm development pathway Biofilm formation follows a pathway of successive steps including (1) conditioning film formation with the migration and adhesion of bacterial cells to the membrane surface, (2) EPS secretion with the growth/maturation of bacterial cells, and (3) proliferation and cell detachment for the colonization of new areas (Aslam, Ahmad and Kim, 2018). Enumeration of bacterial cells on the membrane surface will be further highlighted as well as predominant microbial cells responsible for initiating biofouling in MD. 2.2.1 Conditioning film formation and surface attachment A conditioning film composed of macromolecules, organic matter, proteins, amino acids, and nucleic acids initially cover the surface of the membrane. Concomitantly, dead bacteria and soluble microbial products (SMP’s) capable of surface adsorption adhere on the surface. The presence of the conditioning film alters the membrane surface, promoting better adhesion of cells (Petrova and Sauer, 2012). Planktonic (free-floating) cells migrate from the bulk solution to the membrane surface though the Brownian motion, where movement is induced by the collision of particles (Figure 2.2) (Guo et al., 2022). Attachment is not permanent, as planktonic cells preferentially select sites for adhesion 12 though cell locomotion using flagella and pili (Uneputty et al., 2022). Cells exhibiting better motility and better microbial activity attach first on membrane surfaces (Zheng et al., 2022). Following attachment, cells undergo proliferation, where constant multiplication and division takes place. The interaction between cells and the membrane (cell-to-surface) occur largely though electrostatic and hydrophobic-hydrophobic interactions (Zhang et al., 2018). Although positively charged bacterial cells adhere readily on membrane surfaces, negatively charged bacterial cells tend to adhere less, especially in cases where the membrane surface is predominantly negatively charged and the ionic strength of the feed solution is low (Hori and Matsumoto, 2010). Solutions of low ionic strength contain less ions to counter-act the electronegativity of bacterial cells, resulting in increased repulsion between the membrane and the cell. However, at high ionic strength, cells adhere readily and irreversibly on membrane surfaces. Abu-Lail & Camesano, (2003) evaluated the bio-adhesion of Pseudomonas putida KT2442 and noted low adhesion rates at low ionic strength. This was attributed to the high energy barrier that was required to be overcome for adhesion to take place, a theory best described by the Derjaguin– Landau–Verwey–Overbeek (DLVO) theory (Adair, Suvaci and Sindel, 2001). From a hydrophobic-hydrophobic interaction perspective, hydrophobic surfaces tend to adsorb hydrophobic microorganisms. In particular, bacteria contain hydrophobins (also known as adhesins), which promote adhesion and the formation of the conditioning layer, responsible for initiating biofilm development (Bogler, Lin and Bar-Zeev, 2017). In the same space, bacteria that are predominantly hydrophilic tend to adsorb on hydrophilic surfaces. However, bacteria generally adsorb more on hydrophobic than hydrophilic surfaces (Bogler et al., 2017). Maikranz et al., (2020) studied the binding mechanism of Staphylococcus aureus on hydrophilic and hydrophobic surfaces. It was noted that many macromolecules tethered on hydrophobic surfaces while a few macromolecules tethered on hydrophilic surfaces. This was attributed to the cell-surface contact time and adhesion force. It was further stated by Fabre et al., (2018) that the adhesion of protein (precursor of bacterial adhesion) occurs in low amounts and less tightly on hydrophilic surfaces than 13 hydrophobic surfaces. It is important to note that full assurance of absolute adherence/non-adherence of cells is complicated as bacteria exhibit endless mechanisms that enable their adaptation under different conditions (Luan et al., 2018). Figure 2.2: Mechanistic relationship of the heat, nutrient, and flow stress on the distribution of bacteria on membrane surfaces during MD operation (Aslam, Ahmad and Kim, 2018; Zheng et al., 2022) 14 2.2.2 Biofilm stability and EPS secretion Following attachment and proliferation, bacterial cells secrete EPS and together, essentially constitute the biofilm (Figure 2.3). The biofilm forms between the solid and the liquid phase, where ultimate separation takes place. EPS tend to immobilize and encapsulate bacterial cells, enhancing the firmness of the biofilm. The biofilm structure contains interstitial water channels, facilitating the movement of oxygen, nutrients, and genetic material (Bogler, Lin and Bar-Zeev, 2017). Bacteria are usually larger than other biofilm microorganisms and typically range from 0.5 – 2 µm (Hori and Matsumoto, 2010). Movement of bacteria though the biofilm results in the colonization of new areas. EPS vary in terms of composition, but mainly contain polysaccharides, lipoproteins, proteins, glycoproteins, and carbohydrates (Manzoor et al., 2022). Contributing towards bacteria resistance against inhibitors, EPS further bind the cells to the surface. EPS are capable of enhancing communication within cells and act as a source of nutrients for the bacteria (Costa et al., 2021). Bacteria are known to release significant EPS under thermal stress, making EPS production during MD operation highly favorable (Zheng et al., 2022). Notably, different biofilm layers contain different pore-size distributions. Goh et al., (2013) used evapoporometry to study the particle-size distribution of the biofilm layer caused by two sludge solutions of different hydrophobicity’s. Reportedly, the hydrophilic sludge containing smaller pores was responsible for vapor pressure reduction. The depression of vapor pressure was attributed to the Kelvin effect rather than the effect of an increase in hydraulic resistance. During biofilm development, bacteria can sense each other within the vicinity, a process known as quorum sensing (QS). Bacteria secrete autoinducers during QS development, enhancing bacteria communication (Tabraiz et al., 2021). QS further promotes microbial social activities and enhance community behavior though the expression of certain genes (Pagliero et al., 2021). The dispersion of the biofilm is augmented by QS (Ruhal and Kataria, 2021). Thus, QS further encourages biofilm development. In a study by Zheng et al., (2022), the development and succession of the microbial community was exerted by 15 QS, amongst other factors. The obstinacy of biofilm development and EPS secretion has deemed membrane biofouling a key research concern. Figure 2.3: Biofilm progression in MD (Bogler, Lin and Bar-Zeev, 2017) 2.3 Enumeration and identification of microbial cells on the membrane surface Biofilm identification and characterization on the membrane surface is conducted using a variety of techniques including scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), optical coherence tomography (OCT) and confocal laser scanning microscopy (CLSM) (Yoon et al., 2013; Valladares Linares et al., 2014). Zodrow et al., (2014) evaluated membrane biofilm formation using CLSM, assessing the architecture, heterogeneity and biovolume of the microbial community. The biofilms were heterogeneous and contained several colonies, with a plethora of Burkholderiales, Rhodobacterales, and Flavobacteriales. Bogler & Bar-Zeev, (2018) identified the presence of polysaccharides and detected dead and live bacterial cells using CLSM, with a further evaluation of the average biofilm thickness and total biovolume. Krivorot et al., (2011) evaluated the progression of biofilm development (over-time) on the membrane surface using SEM. The experiment was conducted over a period of 19 days and the 16 deposition of microbial cells increased qualitatively as a function of time. Initial bacterial attachment was detected after 28 h, with the conditioning film seen only after 20 h of operation. In 48 h, the biofilm had formed and over the next 19 days, biofilm coverage on the membrane surface increased. This later caused a reduction in process performance. The use of highly specialized sequencing techniques for the identification of bacterial species within the biofilm structure has gained significant attraction. Goh et al., (2013) for example, identified bacterial species on the fouled membrane through DNA amplification using polymerase chain reaction (PCR). The bacterial organisms were predominantly thermophilic and halotolerant, including Rubrobacter taiwanensis, Caldalkalibacillus uzonensis, Caldalkalibacillus uzonensis, Tepidimonas sp, and Meiothermus hypogaeus. In addition to identifying bacterial species; growth patterns and behavioural changes of specific microorganisms can be identified, particularly using 16S rDNA and 16S rRNA gene sequencing techniques C. Liu et al., (2020a) studied the bacterial composition of the biofilm on the membrane surface using 16S rRNA and 16S rDNA gene sequencing. Analysis was done following DNA extraction. The abundance of live bacteria was higher during the initial biofilm development stage which sharply reduced as a function of time. This decrease was associated with an increase in salt crystal deposition on the membrane. 2.4 Microorganisms responsible for membrane biofouling in MD Since feed solutions sourced from different environmental locations contain microorganisms of different identity, studying feed solution composition is imperative. Feed solutions sourced from marine environments are discovered to cause biofouling through enhancing the deposition of phyla Firmicutes, Bacteroidetes and Proteobacteria, especially at high temperatures and high water salinity (Belila et al., 2016). From a general perspective, bacteria enhancing biofilm development in MD include genera Mycobacterium, Bacillus, Lactobacillus, Cytophaga, and Flavobacterium (Matin et al., 2011). It is important to note that not all microorganisms present in the feed solution cause biofouling in MD, as microorganisms evolve and adapt to changing conditions. 17 Gryta, (2002) evaluated biofouling occurrence in MD. Prior to MD operation, the feed solution was characterized by genera Pseudomonas, Penicillium bacteria, Aspergillus fungi and species S. faecalis. Evaluation of the biofilm on the membrane surface following MD operation displayed the presence of S. faecalis and Aspergillus fungi only. Genera Pseudomonas growth was hindered by oxygen deficiency and high-water salinity. In another study, Zheng et al., (2022) reported a change of plump sphere or short rod to lankier rhabditiform with a microbial community transformation from Algoriphagus, Marinobater, and Sulfurihydrogenibium to Chelativorans, Acinetobater, and Idiomarina. The change in microbial community was attributed to the elevated temperatures and high saline conditions under which MD processes operate. Moreover, the latter strains notably survived due to having high motility, good quorum sensing and high secretory ability. C. Liu et al., (2020a) used lake water (Xuanwu Lake) characterized by Proteobacteria, Actinobacteria, Bacteroidetes, and Cyanobacteria to assess biofilm development under closed and open loop operations of MD. Early biofilm development consisted of genera Acidovorax and Acetobacteraceae, which were replaced by thermophilic Methyloversatilis at stage 2 of flux decay. Only Gammaproteobacteria and Deinococcus- Thermus were detected from the biofilm under closed loop operation. The viability of these strains was explained by their halotolerant mechanism induced by a change in morphology and identity to withstand heat (Zheng et al., 2022). Under open loop operation, the membrane biofilm was dominated by Anoxybacillus, Meiothermus, Schlegelella, Tepidimonas, and Vulcaniibacterium. These examples show that the viability of microorganisms and the succession of microbial communities strongly depend on process parameters. Microbial succession is detrimental to MD as it leads to the production of more EPS, which decreases membrane life span and further lowers process performance. Sometimes, however, microbial communities present in the feed solution do not evolve, but deposit on the membrane surface as they are, just in varying degrees. Zodrow et al., (2014) evaluated the biofouling impact of seawater, predominantly characterized by Octadecabacter, Sediminicola, Loktanella, and Pelagibacteraceae. All aforementioned 18 bacteria were detected on the membrane surface as well, although in varying degrees. The most abundant strain was Octadecabacter, due to its capability to survive high temperatures. Other strains detected included thermophilic Bacillales and spore self- protective Ralstonia. Additional microbes identified from the biofilm are listed in Table 2.1. Table 2.1: Biofouling-causing microbial organisms in MD Feed source Operating conditions Microorganisms Reference Saline wastewater (From animal intestines) Temp: F – 80℃ P – 25℃ CV: 0.367 m/s S. faecalis (S) (Gryta, 2002) Seawater (Long Island Sound) Temp: F – 50.4℃ P – 18.1℃ CV: 4.3 cm/s Ralstonia (G) Octadecabacter (G) Pelagibacteraceae (F) Loktanella (G) Sediminicola (G) Vibrionaceae (F) Rhodobacteraceae (F) Cryomorphaceae (F) Flavobacteriaceae (F) Bacillales (O) (Zodrow et al., 2014) Xuanwu Lake (China) Temp: F – 60℃ P – 10℃ CV: Anoxybacillus (G) Meiothermus (G) Schlegelella (G) Tepidimona (G) Vulcaniibacterium (G) Proteobacteria (P) Deinococcus-Thermus (P) (C. Liu et al., 2020a) Xuanwu Lake, Nan Lake and Qinhuai Temp: F – 60℃ P – 15℃ CV: 10.5mm/s Tepidimonas (G) Meiothermus (G) OLB14_norank (G) (Chen et al., 2021) 19 River (Nanjing, China) Schlegelella (G) Hydrogenophilaceae (F) Env.OPS 17_norank (G) Armatimonadetes_norank (G) Wastewater from power plant Temp: F – 55℃ P – 25℃ CV: Idiomarina (G) Chelativorans (G) Phenylobacterium (G) Methyloversatilis (G) Schlegelella (G) Aeribacillus (G) Bacillus (G) Actinobacteria (P) Chloroflexi (P) Microgenomates (P) (Zheng et al., 2022) Xuanwu Lake (China) Temp: F – 60℃ P – 10℃ CV: 10.5 mm/s Tepidimonas (G) Meiothermus (G) Sphingobium (G) env.OPS 17_norank (G) Curvibacter (G) OLB14_norank (G) Pelomonas (G) Novosphingobium (G) Sphingomonas (G) Bradyrhizobium (G) Chelatococcus (G) Geobacillus (G) (Liu, Zhu and Chen, 2020b) Artificial sterile wastewater Temp: F – 55℃ P – CV: Anoxybacillus sp (G) (Bogler and Bar-Zeev, 2018) F and P are the feed and distillate temperatures, CV is the crossflow velocity, and the letters G, F, O, P and S represent the taxonomic classification of the microorganisms namely: G=genus, F=family, O=order, P=phylum, and S=species. 20 2.5 Effects of biofouling on process performance Microbial accumulation on the membrane surface produces undesirable consequences, including water flux decline, salt rejection decline and frequent membrane replacements. These effects are largely caused by vapor pressure depression/pore blockage, membrane wetting, and the accumulation of EPS and protein. Microbial accumulates on feed spacers (inserted inside the membrane module during operation to increase flow turbidity) can further exacerbate fouling though providing more surface area where these foulants can adhere. Table 2.2 summarizes the impact of biofouling on MD efficiency. The conclusion that can be drawn from this table is that the feed solution composition is the most important factor to consider (for any operation) if biofouling is to be minimized. For example, while the treatment of coastal seawater presented minimal effects on the salt rejection efficiency of the system, the treatment of sludge resulted in a 71.9% decrease in the salt rejection efficiency, and therefore lowering process performance. Overall, it is important to note that the decline of MD efficiency is complex and may not be attributed to biofouling alone. Table 2.2: A summarized impact of biofouling on MD efficiency 21 Configurat- ion of MD Type of membrane Feed type Temperatur e (ºC) Flux decay (L/m2/h) Salt rejection decay Reference DCMD PP Coastal seawater Fe – 40 P – 20 I – 3.85 F – 2.55 No effect on salt rejection efficiency (Krivorot et al., 2011) MDBR PVDF Sludge suspension Fe – 55 P – 19.5 I – 8.42 F – 3.36 I – 217 µS/cm F – ˂600 µS/cm (Goh et al., 2013) DCMD - Artificial sterile wastewater Fe – 65 P – I – 23.0 F – 15.6 I – 1000 µS/cm F – 90000 µS/cm (Bogler and Bar-Zeev, 2018) MDBR PVDF/PTFE Sludge Fe – 56 P – 25 I – 12.7 F – 1.90 I – 1.6 g/L F – 5.7 g/L (Phattaran awik et al., 2009) DCMD PTFE Estuarine water Fe – 50.4 P – 18.1 I – 20.0 F – 10.0 No effect on salt rejection efficiency (Zodrow et al., 2014) DCMD PTFE Lake water Fe – 60 P – 10 I – 9.67 F – 4.28 I – 2.33 µS/cm F – ˂12.7 µS/cm (Liu, Zhu and Chen, 2020a) DCMD PTFE Qinhuai river Fe – 60 P – 15 I – 8.10 F – 4.30 I – 324.7 µS/cm F – ˂23.0 µS/cm (Chen et al., 2021) DCMD PVDF Effluent water Fe – 60 P – 20 I – 40 F – 21 I – 100% F – 97% (Nthunya, Gutierrez, Khumalo, et al., 2019a) 22 Where Fe, P, I and F, represent the feed temperature, distillate temperature, initial and final water flux/salt rejection efficiency, respectively. 2.5.1 Distillate flux decay Membrane biofilm development notably causes a reduction in water flux. The water flux decreases due to: (1) membrane pore-blockage, (2) vapor pressure reduction, (3) temperature polarization (Figure 2.4) and (4) concentration polarization effects. Commonly, the major cause of MD distillate flux decay is the blockage of membrane pores, where the passage of vapor is obstructed by the biofouling layer (Chew, Krantz and Fane, 2014). The degree of pore blockage determines the extent of flux decay. Goh et al., (2013) noted a 60% decrease in water flux caused by the accumulation of microbial contaminants on the surface of the membrane. These contaminants essentially caused a decrease in the vapor pressure of the system by 20% to 32%. Another plausible explanation given for the reduction in flux was the resistance to heat and mass transfer across the boundary layer. Resistance to heat transfer occurs when heat from the feed solution is prevented from reaching the membrane surface where vaporization takes place. This decreases performance efficiency and invariably increases costs of operation (Costa et al., 2021). Future modelling studies of MD looking into vapor pressure depression around biofouling should be considered to gain insights into its mitigation. Notably, the degree of water flux decline depends on the temperature of the feed solution as well. Bogler & Bar-Zeev, (2018) evaluated the impact of biofouling in MD. The feed stream temperatures were controlled at 47℃, 55℃ and 65℃. Correspondingly, the percentages of the water flux declines reported were 30%, 78% and 32%, respectively. The flux declines were attributed to a variety of reasons. At 47℃, the 30% flux drop was attributed to the enhanced TP and CP phenomena caused by the presence of biofoulants. At 55℃, the 78% flux decline was attributed to the increased growth of the bacteria under study (Anoxybacillus sp) on the membrane surface as those were its optimal growth conditions. Here, it was suggested that biofouling in MD can be greatly minimized if the feed temperatures are kept below or above the optimal growth conditions of the bacteria 23 expected to cause biofouling. However, as was seen from previous sections, the adaptability of bacteria induces substantial challenges as new strains may thrive under new conditions. At 65⁰C, substantial pore wetting caused the 32% flux decline. Overall, membrane wetting significantly affected the entire system of the MD operation. In a recent study by Elcik et al., (2022), similar tests were conducted, where feed stream temperatures of 45⁰C, 55⁰C and 65⁰C were evaluated for biofilm progression in MD. Notably, flux drops of 71.8%, 71.5% and 79% were experienced at 45⁰C, 55⁰C and 65⁰C, respectively. Biofilm thickness was reported to induce the flux decline, as a biofilm thickness of 250 ± 10 µm was obtained for 65⁰C, 95 ± 10 µm for 45⁰C and 130 ± 20 µm for 55⁰C. 24 Figure 2.4: Temperature polarization phenomenon in MD. Tf,in, Tfm, Tpm and Tpb represent temperatures in the bulk feed solution, near the membrane surface (feed side), near the membrane surface (distillate side) and in the bulk distillate, respectively (Olatunji and Camacho, 2018). 25 2.5.2 Salt rejection decline Membrane wetting is a fundamental aspect requiring significant attention in MD systems. Wetting promotes the passage of the feed stream though the pores of the membrane, reducing the solute (e.g., salt) rejection efficiency (Nthunya et al., 2022). Membrane wetting occurs as a result of an increase in hydrophilicity of the membrane pores following biofilm development. Bogler & Bar-Zeev, (2018) reported a reduced salt rejection efficiency caused by EPS-induced membrane wetting. A significant difference in salt rejection efficiency was obtained for different feed operating temperatures. At 55℃ and 65℃, a respective 30-fold and 90-fold increase in distillate salinity was obtained. The 90- fold increase was attributed to the improved bacterial growth conditions causing rapid biofilm formation. It is evident from the study that both salt rejection and water flux declines occurred only after inoculation of the feed water with bacteria (Anoxybacillus sp.) Zodrow et al., (2014) evaluated biofilm development in RO and MD systems. For MD systems, it was found that although the hydrophobicity of the membrane decreased from 134 ± 4⁰ to 32 ± 6⁰ due to the presence of a foulant layer, no membrane wetting occurred. This was attributed to the hydrophilic layer on the membrane surface preventing and blocking pores from encountering the feed solution and getting wet. It was thereby concluded that it is not enough for the surface of the membrane to be hydrophilic, the pores also need to be wet for salt rejection decay to be realized. 2.5.3 Frequency of membrane replacements A decrease in membrane lifespan of hydrophobic membranes caused by the accumulation and persistence of biofilm development leads to frequent membrane replacements. This causes significant increases in operational costs (Nthunya et al., 2022). The frequency of membrane replacements largely depend on the type of feed stream treated and the hydrodynamic conditions (Avlonitis, Kouroumbas and Vlachakis, 2003). In desalination plants, membrane replacement occurs within 4 – 5 years whereas industries treating dairy products replace membranes within 3 years (Avlonitis, Kouroumbas and Vlachakis, 2003). In general, membrane replacements are not 26 recommended due to interruptions in the process and significant amount of labor requirements (Agrawal et al., 2021). Hence, mitigation of biofouling in MD is highly encouraged. 2.6 Mitigation of biofouling in MD Major strategies used to mitigate biofouling in MD have been proposed and include pre- treatment of the feed solution, membrane modification and membrane cleaning (Figure 2.5). The detailed impact of these strategies towards water flux and salt rejection stabilities is presented in Table 5.3. Figure 2.5: Latest developments of biofouling control strategies in MD (H. Zhang et al., 2022) 27 2.6.1 Pre-treatment of the feed solution Membranes subjected to contaminated saline wastewaters are prone to biofouling. Therefore, pre-treatment of the feed solution is a useful tool for the minimization of biofouling. Pre-treatment involves the removal of potential foulants from the feed solution before MD processing, reducing the concentration of biofoulants entering the module. This may cause a delay in water flux reduction and reduce the rate of membrane cleaning (B. Zhang et al., 2022). Pre-treatment methods are largely dependent on the composition of the feed solution (Choudhury et al., 2019). There are two types of pre-treatment methods widely applied in membrane-based technologies: (1) physical pre-treatment, and (2) chemical pre-treatment processes. a) Physical pre-treatment processes A variety of physical pre-treatment processes exist including filtration (ultra/micro), sonication and UV radiation, amongst others. Ultra- and Micro-filtration utilize the mechanism of size-exclusion to selectively prevent certain contaminants from becoming a part of the feed stream (Zodrow et al., 2014). This minimizes nutrient availability in the feed stream and minimizes microbial growth during biofilm development. It is important to note here that bacteria are self-replicating and any planktonic cell that survives filtration may enter the module and reproduce, enhancing biofilm development (Mansouri, Harrisson and Chen, 2010). Hence, pre-treatment of the feed solution for biofouling management is not encouraged (Mansouri, Harrisson and Chen, 2010). If sterilization is not 100% effective, some bacteria may survive and reproduce, and exacerbate fouling. Sonication is another alternative strategy to the management of bacterial growth, where ultrasound frequencies of ≥ 18 KHz are used to effectively mitigate fouling (Gizer et al., 2023). Here, fouling is mitigated though cavitation, where the production of strong convective currents is triggered (Aghapour Aktij et al., 2020). Studies related to ultrasound treatment in minimizing bacterial growth are few. Mathieu et al., (2019) studied the ability of ultrasound to inhibit bacterial growth in bulk drinking water “N” spiked with 100 mg/L of 28 Ca(OH)2. Evidently, there was a 7-fold reduction in bacterial growth (from 1.4 × 105 mL-1 to 2.0 × 104 mL-1) caused by the injection of ultrasonic waves. This was achieved though using an ultrasound frequency of 46 KHz. b) Chemical pre-treatment processes Coagulation, chlorine injection and ozone injection are some of the chemical processes used during chemical pre-treatment. Coagulation is the cheapest and most convenient process, involving the stabilization of particulates though addition of a coagulant promoting agglomeration within the feed stream (Alkhatib, Ayari and Hawari, 2021). Inorganic coagulants include ferric chloride (FeCl3), aluminium chloride (AlCl3) and polymeric aluminium chloride (PAC) whereas organic coagulants include polyacrylamide (PAM), poly dimethyl diallyl ammonium chloride (PDMDAAC) and microbial flocculants (Ren et al., 2019; Zhao et al., 2021). Since higher doses of inorganic flocculants are known to cause secondary pollutants due to the presence of residual metal ions, a hybrid system of organic and inorganic coagulants is usually used to enhance the performance of coagulation. B. Zhang et al., (2022) used a hybrid system to evaluate the performance of microbial flocculants modified with PAC (MMF/PAC) to minimize fouling. Cake layer formation on the membrane surface was effectively mitigated, especially in comparison to the individually applied inorganic flocculant, PAC. The optimum concentration of MMF in MMF/PAC (for optimum performance) was 10 mg/L. Management of biofilm development has further been seen though chlorine dosages, largely at concentrations of 1 – 50 ppm (Costa et al., 2021). Although chlorination of the feed stream effectively prevents biofilm development, it produces mutagenic and carcinogenic by-products that are harmful (Zemite et al., 2022). 2.6.2 Membrane cleaning Membrane cleaning involves the removal of microbial accumulates from the membrane surface following deposition. The concept of membrane cleaning was introduced to 29 restore the original performance of the membrane following fouling interruptions (Costa et al., 2021). Membrane cleaning further reduces the rate of membrane replacements (Alzahrani et al., 2013). There are two types of cleaning methods widely applied in membrane-based technologies: (1) physical cleaning and (2) chemical cleaning. a) Physical cleaning Physical cleaning involves the use of deionized water during MD operation to facilitate foulant detachment (Guo et al., 2022). Using water guarantees minimal damage to the membrane structure. During cleaning, pumping of the feed stream into the module stops, followed by deionised water running through the system, usually at higher flowrates. These flowrates ensure increased shear forces that induce optimal detachment of foulants from the membrane surface and spacers (Bogler, Lin and Bar-Zeev, 2017). In other instances, high pressure water is utilized (Gizer et al., 2023). Lyly et al., (2021) evaluated deionised water’s ability (as a cleaning agent) to restore the performance of PTFE membranes. The feed solution used was a 35 000 ppm NaCl solution spiked with BSA (1000 ppm) and microalgae (Cylindrotheca fusiformis). After running the operation for 35 h, the following steps were implemented for physical cleaning: (1) running of pure cold-water (at 25⁰C) for 10 mins, (2) running of pure hotwater (at 60⁰C) for 10 mins and (3) running of cold-water again for 5 mins. Reportedly, the water flux was restored by 62.69%, displaying promising results for future applications. In the same study, modification of PTFE with PVDF as top later and poly(N-Isopropylacrylamide) as modifying agent presented a water flux restoration of 88.26%. b) Chemical cleaning Chemical cleaning involves the exclusive use of chemicals to continuously remove foulants from the membrane surface. To avoid unalterable damage to the membrane, the concentration of the cleaning agent and frequency of membrane cleaning is optimized (Choudhury et al., 2019). Current literature is dominated by the chemical cleaning of inorganic fouled (using acidic solutions) and organic fouled (using basic solutions) 30 membranes (Guillen-Burrieza et al., 2014). Reported chemical cleaning of biofouled membranes is limited. Zheng et al., (2022) sought to remove microbial accumulates on the membrane surface by applying different concentrations of HCl and water. The process ensured complete removal of Euryarchaeota. However, full restoration of the flux was not achieved, largely due to Idiomarina cells’ affinity for the membrane surface. Although process performance efficiency is notably restored by membrane cleaning, modification of the membrane is highly encouraged to render biofouling-resistance to the hydrophobic membranes (Bogler, Lin and Bar-Zeev, 2017). This approach ensures minimal waste disposal, subsequently posing less risk to the environment (Ben-Sasson et al., 2014). 2.6.3 Membrane modification Membrane modification involves the alteration of membrane properties to render the membrane resistant to biofouling. Membranes are modified though various approaches; including the incorporation of metal nanoparticles (MNPs) (Figure 2.6) and the alteration of membrane surface properties such as surface roughness, membrane hydrophobicity and surface charge (Choudhury et al., 2018). Herein, we will only deal with the incorporation of MNPs. a) Incorporation of MNPs Attachment and proliferation of microbial cells on membrane surfaces is largely minimized through the incorporation of biocidal MNPs. Upon oxidation, ionic counterparts of the biocidal MNPs get released and react with microbial cells, inactivating them. Biocidal MNPs capable of inactivating bacterial cells include Ag, Ti, Zn and Fe (Slavin et al., 2017; Guo et al., 2022). Inactivation occurs due to the electrostatic interaction between the positively charged metal ions and the negatively charged thiol groups on the DNA of the bacteria (Politano et al., 2017). Amongst others, AgNPs are the most commonly used to inhibit biofouling (Somayajula et al., 2019; Xuan et al., 2020). In addition to membrane 31 systems, many different industries apply AgNPs for their anti-microbial properties, including textile, medicinal, wood dressing, and food packaging industries (Fu et al., 2016). Nthunya et al., (2020) modified the PVDF membrane using AgNPs and functionalized carbon nanotubes (f-MWCNTs), achieving successful biofouling, colloidal and organic fouling control. Although biofouling mitigation is evident when applying membrane modification, biocidal MNPs tend to leach from the membrane surface, an event that reduces their antibacterial activity over-time (Bogler, Lin and Bar-Zeev, 2017). In addition, the quality of water produced from the operation may be reduced following leaching. Hence, ensuring the stability of these MNPs on membrane surfaces is crucial. Stability is assured through a variety of ways, including the use of biopolymers such as cellulose, containing abundant negatively charged groups on the surface capable of stabilizing these MNPs. Figure 2.6: Membrane modification using MNPs CNCs are nano-structural forms of cellulose, the most important structural constituent of the cell walls of plants and algae. Paper-producing industries isolate cellulose from wood for use during the production of cardboard and paper (Trache et al., 2017). Other 32 applications of cellulose include its addition in textile and technical products. In addition to exhibiting various advantages such as biocompatibility, bioavailability, chemical functionality and high surface to volume ratio, CNCs have shown to exhibit tremendous tensile strength (Chen et al., 2011). Moreover, the presence of hydroxyl groups on the surface has made the grafting of various chemical functional groups and MNPs on the backbone possible. Isolation methods of CNC’s from wood includes acid hydrolysis (Karim et al., 2016), mechanical refining (Qiao et al., 2015), water hydrolysis (Novo et al., 2016), oxidation methods (Sun et al., 2015) and enzymatic hydrolysis (Anderson et al., 2014). However, the main experimental method for CNC preparation is through acid hydrolysis (Habibi, Lucia and Rojas, 2010). The reason behind acid hydrolysis is for enabling the disintegration of amorphous regions for the removal of hemicellulose and lignin. In addition, the subjection of cellulose to acid is generally known to decrease the degree of Polymerization (DP) between structural linkages of the polymer (Håkansson and Ahlgren, 2005). Following acid hydrolysis, a white crystal-like solid remains that is primarily composed of crystalline regions, which requires dialysis with distilled water (several times) before further application. The two prominent acid solutions used during hydrolysis are hydrochloric acid (HCl) and sulphuric acid (H2SO4). CNC’s produced through HCl subjection tend to flocculate in solution and have been reported to display limited dispersion (Araki et al., 1998). The use of H2SO4, however, promotes the non-dispersion of crystal structures in solution. However, H2SO4 produced CNC’s display little thermostability due to the presence of charged sulphate groups (Roman and Winter, 2004). Consequently, HCl and H2SO4 are used simultaneously during reactions for the production thermally stable and disperse CNC’s. 33 Table 2.3: Mitigation strategies of biofouling in MD Membrane configuration Mitigation strategy Mitigation process Feed solution Duration of operation Impact on water flux and salt rejection efficiency Ref. DCMD Pre-treatment Chlorination (Addition of HCl in feed stream) Tap-water 58 days Stable water flux and salt rejection. (Gryta, 2002) DCMD Pre-treatment and membrane cleaning Magnetic coagulation and HCl cleaning Wastewater 65 days Original flux of membrane restored (at high HCl concentration) (Zheng et al., 2022) DCMD Membrane modification Membrane modification (f- MWCNTs and AgNPs) Effluent water 2 days Stable flux and salt rejection efficiencies obtained as far as biofouling was concerned. (Nthunya et al., 2020) DCMD Pre-treatment Microfiltration Estuarin water 4 days 50% decline in flux (Zodrow et al., 2014) DCMD Membrane cleaning NaOH, distilled water, 70% ethanol cleaning Coastal seawater 14 days Original flux of membrane restored. (Krivorot et al., 2011) AGMD Backwash Reversion of direction of flow Pond water 91 days Original flux of membrane restored (Meinders ma, Guijt and de Haan, 2006) DCMD Membrane modification Coating membrane with hydrophilic active layer Effluent water 2.5 days 47% flux decline and 99.8% salt rejection (Nthunya, Gutierrez, Khumalo, et al., 2019b) 34 DCMD Membrane modification Membrane modification (f- MWCNTs and AgNPs) Scheldt estuary water 2 days 20.8% flux decline and 99.8% salt rejection (Nthunya, Gutierrez, Lapeire, et al., 2019) In this chapter, a summary of the mechanism of operation of MD as well the configurations used during separation have been discussed. 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