Investigation of membrane processes for the removal of volatile fatty acids

The accumulation of volatile fatty acids such as acetic acid can cause reactor pH problems and the inhibition of microorganisms utilised in anaerobic digestion processes. A cross-flow membrane process using Teflon and ion-exchange membranes was investigated as a means of separating acetic acid from pure acetic acid solution and rumen fluid. Acetic acid transfer across the Teflon membrane was dependent on the free acid concentration (CH3COOH) in the acid solution. Concurrent transfer of water was minimal due to the hydrophobic nature of the membrane. The strong base anionic exchange membrane facilitated the separation of acetic acid from both pure solutions and rumen fluid with flux again being dependent on the free acid gradient across the membrane. Flux rates were lower than other studies of diffusion dialysis, however, this may be partly attributed to improper preparation of the membrane. The currently achieved rates of transfer using these membranes are very low and are therefore not yet suitable for full-scale use in anaerobic digestion. Additional research is needed to achieve higher trans-membrane transport rates at reasonable costs.