Monitoring, modeling, and controlling membrane fouling and concentration polarization in reverse osmosis systems
Reverse osmosis (RO) has emerged as an important technology in the field of water purification and desalination, offering a highly efficient and sustainable solution for addressing the global water scarcity challenges. The development of thin-film composite (TFC) membranes recently with improved selectivity, permeability, and durability has significantly enhanced the efficiency and reliability of the RO systems. Despite these advantages, membrane fouling and concentration polarization (CP) remain as the key obstacles, diminishing membrane performance and necessitating frequent maintenance and cleaning. Membrane fouling stems from the deposition and accumulation of foulants, including colloids, inorganic particles, organic matter, and microorganisms, on the membrane surface and within its pores. The accumulation of foulant reduces permeate flux, increases transmembrane pressure, and elevates energy consumption. On the other hand, the phenomenon of CP arises from the accumulation of solutes on the membrane surface during filtration, leading to a decline in water flux and salt rejection. This is primarily attributed to the balance of convective and diffusive mass transport within the boundary layer adjacent to the membrane surface. An extensive understanding of the components of fouling and CP layer and their interaction holds potential for improving RO membrane performance. Additionally, modeling the energy consumption related to fouling and CP in full-scale RO facilities provides insights into potential energy savings by applying antifouling and CP reduction methods. Consequently, the main purpose of this dissertation is to enhance the understanding of monitoring, controlling, and modeling membrane fouling and CP in RO processes. Membrane fouling has received intense attention in research because of its clear and visible impact on permeate flux and energy consumption. Regarding fouling control, monitoring of biofilm formation poses a significant challenge because of the insufficient understanding of the ecophysiology and dynamics of microorganisms responsible for the fouling. Development of sustainable biofouling control strategies depends largely on a more detailed understanding of the diversity and dynamics of RO membrane biofilms. This dissertation introduces a comparative analysis of 16S rRNA gene amplicon sequencing data to reveal biofilm communities associated with seawater desalination RO facilities operations and the home-used point-of-use (POU) RO filtration units. While initially targeted towards understanding biofouling, using this approach on POU units offers an advantage in capturing species that may be missed by other instantaneous sampling methods and I propose to use such systems as “sentinels” to monitor and thus ensure drinking water microbiological safety. In addition to fouling, recent developments have integrated detection devices into filtration cells to allow for monitoring CP. The measurements of concentration gradients have enabled the development of active methods to alleviate or even eliminate CP during filtration by “active” membranes. Active membranes aim to eliminate CP by increasing turbulence by external energy supplied such as mechanical, ultrasonic, chemical, electric, magnetic, and light. A comprehensive review provided in this dissertation summarizes the RO CP monitoring methods and “active” membranes. This chapter of literature review revealed that elimination of CP can be a pathway for reducing energy consumption and enhancing the sustainability of RO facilities. Following this revelation, an innovative active membrane approach was employed by infusing hydrogen peroxide into a polydopamine and manganese dioxide-coated RO system at the laboratory scale to mitigate CP and silica induced scaling. In the final part of this dissertation, operational data from three distinct RO facilities—dedicated to water reuse, groundwater desalination, and seawater desalination—was used for modeling fouling and CP related energy consumption. This part of the dissertation aimed to provide insights into the primary pathways for energy conservation in full-scale RO facilities. One of the conclusions is that RO facilities operating with low-conductivity feed should prioritize antifouling strategies, while those dealing with high-conductivity feed, such as seawater desalination facilities, should address CP mitigation to achieve optimal energy efficiency.
Membrane fouling, Reverse osmosis, Concentration polarization
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