Volatile fatty acids (VFAs) are widely used across various industries, representing a significant global market. However, conventional petroleum-based VFAs production raises environmental concerns. Waste-derived VFAs offer a sustainable alternative, but the complexity of feed compositions poses significant challenges for efficient post-processing. This study analyzes the effects of operating conditions and coexisting compounds – ammonium/phosphorus-containing compounds and inorganic salts – on the performance of nanofiltration (NF) membranes for VFA recovery. It addresses a gap in the literature by providing a comprehensive analysis and theoretical insights into the combined effects of feed composition, membrane properties, and operating parameters on the semi-selective separation and concentration of VFAs from complex aqueous matrices. Three NF membranes with different molecular weight cut-offs and zeta potentials were tested under varying feed pH (5.5 and 9), applied pressure (10 bar, 20 bar, and 30 bar), and operating temperature (20 °C and 40 °C). Synthetic solutions simulating pretreated anaerobic digestion effluents were used in partial recirculation cross-flow NF experiments. Results demonstrate that ammonium/phosphorus-containing compounds reduced VFAs rejection by up to 10 %, while inorganic salts increased it by up to 15 %, both contributing to reduced permeate flux. The combined effects of membrane properties, feed composition, feed pH, and operating conditions impact the VFAs rejection mechanisms. At lower pH, size exclusion is the dominant mechanism, whereas at higher pH, electrostatic repulsion becomes more important, enhancing VFAs rejection but reducing permeate flux. Temperature and pressure had strong effects: higher pressure improved both solute rejection and flux, while higher temperature increased flux but reduced solute rejection. Feed pH 9, pressure 30 bar, and temperature 40 °C were identified as the most suitable operating parameters for maximizing VFAs rejection, minimizing the rejection of coexisting compounds, and maximizing permeate flux. Under these conditions, a retentate containing about 29 g/L of VFAs was recovered. This study offers insights for optimizing NF processes to improve waste-derived VFAs valorization, advancing sustainable resource recovery and supporting the circular bioeconomy.

Substituting waste-derived Volatile Fatty Acids (VFAs) with their conventionally applied fossil-derived counterparts in a spectrum of industrial applications necessitates its proper fractionation into individual acids. This study explored a multi-stage batch adsorption approach for fractionating acidogenic fermentation VFAs effluents from food waste (FW) and chicken manure (CKM) using Diaion HP-20 and activated charcoal. Initial screening at different washing conditions and pH (3.5 and 6.5) revealed the unwashed granular-activated charcoal (GAC-Unwashed) and milli-Q water-washed Diaion (DI-MQ Washed) as the most promising candidates for VFA fractionation of a synthetic VFA mixture at 4 gL−1. At pH 3.5 (<𝑝⁢𝐾𝑎), GAC-Unwashed adsorbed 2–6 carbon atom VFAs completely, while DI-MQ Washed exhibited minimal adsorption of acetic acid (AA) (8%), favoring caproic (CA) and valeric acids (VA) (>97%). While at pH 6.5 (>𝑝⁢𝐾𝑎), GAC-Unwashed selectively targeted VA (79%) and CA (100%). Fractionating VFAs from FW and CKM were conducted in a two-stage adsorption process with optimal results being achieved using GAC-Unwashed at FW initial pH (5.3) and DI-MQ Washed at pH below CKM 𝑝⁢𝐾𝑎 (3.5), respectively. The first adsorption stage primarily adsorbed higher molecular weight (MW) VFAs (FW:99.1% CA, CKM:72.9% butyric acid (BA)) with a minor quantity of lower ones (FW:56.5% BA, CKM:29.3% propionic acid (PA)), leaving AA intact. Subsequent stages aimed to isolate AA by adsorbing the remaining low MW VFA (FW:58.9% BA, CKM:27.8% PA, 70% BA) other than AA, indicating effluent fractionation while preserving and purifying AA. Applied selective multi-stage adsorption approach offers a promising method to broaden waste-derived VFA applications.