Solvent-free membrane extraction of volatile fatty acids from acidogenic fermentation

Diversification of anaerobic digestion into higher value products, namely volatile fatty acids (VFAs), is receiving interest. One of the biggest challenges with this is recovery of the VFAs. Membrane extraction can be used, and a novel process configuration using a non-porous silicone membrane and water for an extractant is proposed here. This process would enable the reduction in the number of downstream unit operations compared to other membrane extraction processes. Selective recovery in favour of longer chain VFAs was demonstrated. Testing with a synthetic solution resulted in an overall mass transfer coefficient of 0.088 μm s^-1 for butyric acid, and 0.157 μm s^-1 when fermentation broth was used. This indicates this process is not hindered by fouling, but improved somehow. Although the preliminary economic analysis showed this process to require a larger membrane area compared to porous membrane alternatives, it also has a significantly reduced cost associated with the extractant.