Intrinsic Performances of Reverse Osmosis and Nanofiltration Membranes for the Recovery and Concentration of Multicomponent Mixtures of Volatile Fatty Acids: A Semi-Pilot Study

This study presents data from Reverse Osmosis (RO) and Nanofiltration (NF) spi-ral-wound polyamide modules tested in a semi-pilot plant with multicomponent mix-tures of Volatile Fatty Acids (VFAs) comprising acetic, propionic, butyric, valeric, and hexanoic acids. A robust method combining film theory and dissociation equilibria was developed to estimate interfacial concentrations, enabling accurate analysis of concentra-tion polarization, real rejection, and effective transmembrane driving force. Concentration polarization strongly affects NF membranes, resulting in real rejections up to 20% higher than apparent values, while its effect is negligible for RO membranes. NF rejections show marked sensitivity to pH and VFAs feed concentration: at 20 g/L and highest flux, acetic acid real rejection increases from 80% to 91% as pH rises from 6 to 9. At pH 7, rejections decline with feed concentration, with acetic acid dropping from 55% at 20 g/L to 32% at 63 g/L, at the same flux. These changes correlate with the molecular weight of the acids. Conversely, RO rejections are marginally affected by pH and not influenced by concentra-tion due to dominant steric exclusion. Membrane permeabilities remain unaffected by VFAs and align with pure water values. The data analysis framework is effective and ap-plicable across a wide range of conditions and membranes.