Phenomenological Interpretations of Membrane Properties Following Repeated Chemical Cleaning of an End-of-Life Potable Reuse Reverse Osmosis Element Dominated by Inorganic Fouling

Abstract

A tail end-of-life reverse osmosis (RO) element from the third stage of a three-stage train extensively fouled by silicon was investigated for the effects of repeated alkaline cleaning and their consequences on foulant reversal and membrane integrity. Detailed surface characterization revealed that after four years of operation in the world’s largest potable reuse facility, it was severely fouled by inorganic substances with lesser contributions from bioorganic materials that together had reduced its water permeance and salt rejection to only ∼20 and ∼80% of their initial values, respectively. Swatches of the heavily fouled membrane were exposed repeatedly (but separately) to two high-pH cleaning agents (NaOH or TPP/DBS, a mixture of sodium tripolyphosphate and sodium dodecylbenzesulfonate) simulating repetitive cleaning-in-place (CIP) protocols typical of real-world operations. Although five-to-ten cleaning cycles fully recovered the fouled membrane’s water permeance, salt rejection always remained below 90% confirming its end-of-life. X-ray photoelectron (XPS), energy-dispersive X-ray (EDS), and Fourier transform infrared (FTIR) spectroscopy of fouled membranes implicated silicon as the dominant foulant, which was only partially removed even after ten cleaning cycles, although water permeance was completely restored. Importantly, exposing a virgin membrane to identical “cleaning” regimens as the end-of-life membrane artificially increased water permeance without changing its salt rejection. FTIR and XPS scans of the virgin membrane following repetitive exposure to NaOH or TPP/DBS revealed no damage/degradation of its polyamide layer as demonstrated by the relatively constant amide I/amide II absorbance ratios and consistent oxygen/nitrogen atomic ratios, both symptomatic of maintaining membrane integrity. Hence, we phenomenologically invoked swelling and/or surface property modifications to mechanistically explain the quantitative increase of water permeance after repeatedly exposing the virgin membrane to CIP agents (while maintaining the active polyamide layer’s integrity). Similarly, we attributed a portion of the restored permeance of the fouled membrane upon progressive chemical cleaning to swelling and/or surface property modification that could be indirectly inferred. Therefore, it is paramount to comparatively characterize virgin and fouled membranes prior to and after exposure to CIP chemicals to distinguish foulant removal from other mechanisms potentially contributing to recovering water permeance.