Will Membranes Break Barriers on Volatile Fatty Acid Recovery from Anaerobic Digestion?

The conversion of organic waste into value-added chemicals provides a sustainable pathway toward a circular economy, but anaerobic digestion (AD) as an established technology has faced great challenges primarily due to the low value of its primary product—biogas. Recent developments on arrested methanogenesis allow the AD process to be rewired to suppress methanogenesis and promote the production of higher valued volatile fatty acids (VFAs). However, the separation and recovery of VFAs from the mixed reactor constituents (microbes, salt, and organic solids), remains a challenge. Membrane-based separation processes are showing increased promise compared to other methods, as recent studies reported high yield, high selectivity VFA recovery with low energy consumption and reactor footprint. However, competing membrane processes have not been quantitatively reviewed or compared, making it difficult to understand the current state-of-the-art. This study provided the first such analysis, comparing nanofiltration (NF), reverse osmosis (RO), pervaporation (PV), membrane contactors (MC), and membrane distillation (MD) for VFA separation and recovery. The study offers a comprehensive characterization of the different membrane’s selectivity and permeability under different conditions and compares them side-by-side using normalized measures. In addition, the energy demand and fouling potential were also analyzed. New opportunities such as mixed matrix pervaporation membrane, antifouling electroactive membrane, solar-heating membrane distillation, and integrated processes were also explored to provide insights on technology advancement and synergistic benefits.