Evaluating the impact of membrane properties and feed pH on concentration and fractionation of volatile fatty acid using nanofiltration

Waste-derived volatile fatty acids (VFAs) are emerging as a promising sustainable alternative to petroleumderived VFAs. However, the post-treatment of waste-derived VFAs becomes imperative for the purpose of concentration, purification, and fractionation. This study delves into the application of the nanofiltration process for post-processing of solubilized VFAs, focusing on a comprehensive exploration of the influence of membrane properties and feed pH on process efficiency. Four commercial nanofiltration membranes, with molecular weight cut-off ranging from 150 to 500 Da and substantial differences in zeta potential, were tested under four different feed pH values (4, 5.5, 7, and 9), constant pressure (20 bar) and controlled temperature (20 +/- 1 degrees C). The VFAs' rejection mechanisms were investigated by analyzing membrane behavior at different pH levels. At feed pH 4, all membranes achieved low VFAs rejection (concentration ratios ranging from 1.38 to 1.62) associated with size exclusion. Transitioning from feed pH 4 to 9, electrostatic repulsion became predominant, leading to increased VFAs rejection (from a minimum of 213 % to a maximum of 311 %, with a sharp increase up to 272 % when transitioning from pH 4 to 7, followed by a more gradual increase of up to 114 % from pH 7 to 9) and decreased permeability (with an average reduction of from about 25 % to about 56 %). Notably, the highest VFAs concentration obtained was 40.1 g/L, representing a 4.4-times increase over the VFAs concentration in the feed. These findings underscore the potential of implementing nanofiltration as an efficient process for the VFAs postprocessing, emphasizing the importance of membrane selection and operating conditions for optimized performance.