On the Control Strategy to Improve the Salt Rejection of a Thin-Film Composite Reverse Osmosis Membrane

Abstract

Since the specific energy consumption (SEC) required for reverse osmosis (RO) desalination has been steeply reduced over the past few decades, there is an increasing demand for high-selectivity membranes. However, it is still hard to find research papers empirically dealing with increasing the salt rejection of RO membranes and addressing the SEC change possibly occurring while increasing salt rejection. Herein, we examined the feasibility of the process and material approaches to increase the salt rejection of RO membranes from the perspective of the SEC and weighed up a better approach to increase salt rejection between the two approaches. A process approach was confirmed to have some inherent limitations in terms of the trade-off between water permeability and salt rejection. Furthermore, a process approach is inappropriate to alter the intrinsic salt permeability of RO membranes, such that it should be far from a fundamental improvement in the selectivity of RO membranes. Thus, we could conclude that a material approach is necessary to make a fundamental improvement in the selectivity of RO membranes. This paper also provides discussion on the specific demands for RO membranes featuring superior mechanical properties and excellent water/salt permselectivity to minimize membrane compaction while maximizing the selectivity.

The Ionic Composition and Chemistry of Nanopore-Confined Solutions

“…From an engineering perspective this channel is salt rejecting and ideally cation permselective, ,,− meaning it transports only cations and not anions. In addition, it has chemically based selectivity that allows it to transport essentially only Ca 2+ .…”

A Novel Method to Quantify Real-Time Compaction of Water Treatment Membranes

“…8 In pressure-driven applications (e.g., ultrafiltration, nanofiltration, and reverse osmosis), compaction changes the membrane morphology and internal structure 10−12 and can cause decreased water flux, higher energy consumption, 7,8,13,14 and lower salt rejection. 11,12 In thermally driven applications (e.g., membrane distillation), membrane compaction results in increased heat loss 15−18 and reduced water permeability. 9,15−21 Compaction can occur instantly or over longer time periods and, depending on the recovery properties of the membrane, can cause irreversible, partially reversible, or reversible changes in membrane morphology and mechanical properties.…”

“…When pressure is applied to thin-film materials during use or during manufacturing, the thin-film material often undergoes compaction. − For water treatment membranes, a hydraulic pressure differential results in membrane compaction that can decrease performance and limit practical use of the membrane . In pressure-driven applications (e.g., ultrafiltration, nanofiltration, and reverse osmosis), compaction changes the membrane morphology and internal structure − and can cause decreased water flux, higher energy consumption, ,,, and lower salt rejection. , In thermally driven applications (e.g., membrane distillation), membrane compaction results in increased heat loss − and reduced water permeability. ,− …”

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“…When organic agro-industrial residues are used in bioconversion processes, not only is the amount of polluting organic matter in the environment reduced, but an added value is also given to these residues by using them as inputs for the production of electricity. On the other hand, the transition to a low-carbon economy is being facilitated by reducing the amount of carbon burned to produce energy, which contributes to the reduction of the carbon footprint [10,11].…”