Deformation-induced cleaning of organically fouled membranes: Fundamentals and techno- economic assessment for spiral-wound membranes The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Goon, Grace S.S. et al. "Deformation-induced cleaning of organically fouled membranes: Fundamentals and techno-economic assessment for spiral-wound membranes." Journal of Membrane Science 626 (May 2021): 119169. © 2021 Elsevier B.V. As Published http://dx.doi.org/10.1016/j.memsci.2021.119169 Publisher Elsevier BV Version Author's final manuscript Citable link https://hdl.handle.net/1721.1/130373 Terms of Use Creative Commons Attribution-Noncommercial-Share Alike Detailed Terms http://creativecommons.org/licenses/by-nc-sa/4.0/ Deformation-induced cleaning of organically fouled membranes: Fundamentals and techno-economic assessment for spiral-wound membranes Grace S. S. Goona;y , Omar Labbanb;y , Zi Hao Foob;c , Xuanhe Zhao b;z , John H. Lienhard Vb;z∗ a Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge MA 02139-4307, USA b Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge MA 02139-4307, USA cSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore yJoint first authors. zJoint senior authors. Abstract Membrane fouling is a ubiquitous challenge in water treatment and desalination systems. Current reverse osmosis (RO) membrane cleaning technology relies on chemical processes, incurring considerable costs and generating waste streams. Here, we present a novel chemical-free membrane cleaning method applicable to commercially existing RO spiral-wound membrane modules. The method employs controlled membrane deformation through pressure modulation, which induces shear stresses at the foulant-membrane interface that lead to detachment and removal of the foulants. To investigate the effectiveness of the method, experi- ments on organic fouling by alginate are conducted on a flat-sheet membrane coupon followed by tests ona commercial spiral-wound module with feeds of varying fouling propensities. Cleaning durations are six-fold lower, and the experimental results demonstrate flux recoveries and cleaning efficiencies comparable tothose of chemical cleaning. The experiments on the spiral-wound module indicate that this method will have applicability in industrially-relevant settings. To elucidate the underlying cleaning mechanisms, membrane deformation experiments with no flow are conducted, and in situ visualization techniques are employed for both the flat-sheet and spiral-wound modules. The results show that cleaning is caused by a reduction in shear strength at the foulant-membrane interface after cycles of repeated loading, a behavior typical of fatigue. By enabling more frequent cleanings, deformation-induced cleaning is shown to considerably lower operating costs in an economic case study while offering a more sustainable and environmentally sound solution to membrane cleaning and antifouling in desalination. Keywords: reverse osmosis, membrane fouling, chemical cleaning, chemical-free cleaning, deformation-induced cleaning. ∗Corresponding author: [email protected] Preprint submitted to Journal of Membrane Science February 19, 2021 G. Goon, O. Labban, Z.H. Foo, X. Zhao, and J.H. Lienhard V, “Deformation-induced cleaning of organically fouled membranes: Fundamentals and techno-economic assessment for spiral-wound membranes,” J. Membrane Sci., 626:119169, 15 May 2021. https://doi.org/10.1016/j.memsci.2021.119169 1. Introduction With four billion people already facing water scarcity [1], water demand is only projected to intensify, fueled by a growing world population, climate change, and the rising standards of living in the developing world [2]. Tapping into nature’s most abundant resource, the ocean, desalination has become a key tool for tackling the problem [3, 4, 5] with membrane-based reverse osmosis (RO) supplying the majority of the world’s installed desalination capacity [2]. In spite of their success, however, membranes are plagued by fouling, often dubbed the “Achilles’ heel of membrane processes”, resulting in considerable downtime for cleaning and lost productivity [6]. Given the lack of effective physical cleaning methods, however, desalination plants currently rely upon chemical cleaning despite the numerous drawbacks, which include chemical costs, undesirable plant shut- downs, membrane degradation, and environmental concerns associated with chemical waste. For these reasons, alternative cleaning methods could make desalination cheaper, more widely accessible, and more environmentally sustainable. Interest in chemical-free membrane cleaning has lately been on the rise, due to its potential to negate the drawbacks of chemical methods. Unlike other membrane processes, however, the structure of RO mem- branes and the module designs implemented in practice prevent direct mechanical backwashing. While the RO market had been dominated until the mid-1990’s by the hollow fiber configuration, the spiral-wound configuration has lately overtaken it in prevalence due to its mechanical robustness for high pressure oper- ation and desirable compromise among competing operating factors, such as fouling control and pressure drop [6, 7, 8]. Nonetheless, commercially existing RO membranes are unable to withstand high pressures on the permeate side during backwashing without incurring damage, nor has the piping on the permeate side in RO spiral-wound modules been designed to tolerate pressures exceeding manufacturer specifications [9]. In an attempt to provide a commercial solution combining the benefits of the spiral-wound configura- tion with backwashing capability, MICRODYN-NADIR recently introduced the SpiraSep 960 ultrafiltration (UF) module. The product features UF membranes that are less tightly wounded than a typical spiral- wound element, allowing for mechanical backwashing [10]. A similar product for RO, nonetheless, remains elusive. Instead, the chemical-free methods employed in RO can largely be categorized into osmotically and mechanically-driven fouling mitigation techniques. In osmotically-induced cleaning (OIC), the net driving pressure (NDP) is controlled such that the osmotic pressure difference across the membrane becomes the dominant driving force. Water is thus transported by osmosis from the purer permeate side to the more concentrated feed side, causing the foulant layer to experience a variety of mechanisms that potentially encourage detachment [11]. Liberman and Liberman [12, 13], first coining the term Direct Osmosis cleaning by the High Salinity Solution (DO-HS), proposedsuch an approach as an environmentally friendly on-line cleaning technique. Further research by Qin et al. [14], Lee and Elimelech [15], and Sagiv and Semiat [16] investigated the viability of this approach for wastewater reclamation and membrane scaling mitigation. 2 Despite its potential, widespread adoption of OIC in practice has been challenged by operational chal- lenges. A study by Farooque et al. [17], for example, found OIC to be ineffective at mitigating fouling in commercial spiral-wound modules used for seawater desalination. Furthermore, recent experiments by Lab- ban et al. [11] using organic foulants demonstrate the mechanism underlying OIC and visually capture the foulant layer detachment and cleaning procedure as they occur in situ. These experiments, however, show the effectiveness of OIC decreases considerably in the presence of spacers. Faced with these limitations for OIC, mechanically-driven vibration of the membrane presents yet another means of chemical-free fouling mitigation. While the method of generating motion can vary widely depend- ing on the membrane configuration and geometry, the mechanism of fouling mitigation had mostly been surrounding methods that would increase the unsteady shear rate and turbulence on the membrane surface [18]. The Vibratory Shear-Enhanced Process (VSEP) is an example that employs a rotational mechanism, especially at the torsional modal frequency, to generate large oscillatory shear stresses at the membrane sur- face [19]. The ability of VSEP to induce large turbulent flows at the surface allows it to mitigate fouling on the membrane effectively, paving the way for treating streams featuring high fouling propensities [20].Due to its limited capacity and high capital and maintenance costs associated with moving parts, nonetheless, VSEP may not be suitable for large-scale deployment in desalination [20, 21]. Despite the ability of rota- tional mechanisms to mitigate fouling, frequent chemical cleaning of the membranes remains inevitable [19]. Furthermore, higher fouling rates at smaller radii may lead to potential blockages, and uneven membrane defects. Another mechanically-driven approach of fouling mitigation involves the introduction of hollow fiber modules, where membrane motion may be classified as longitudinal (parallel to the axis), transverse (per- pendicular to the axis), or rotational. Several studies have explored the introduction of hollow fiber membrane transverse oscillation as an antifouling mechanism [22, 23, 24, 25]. In these studies, membrane vibration was achieved either by the generation of oscillating flow [25, 26] in the liquid or the direct movement ofthe membranes [23, 27]. Li et al. [27] performed vibration experiments with a relatively wide parameter space, concluding that