Key parameters for high volatile fatty acids concentration and ethanol separation through membrane technologies

Abstract

Volatile fatty acids (VFA) are organic compounds of high importance due to their usage as building blocks for different industrial processes. Novel biotechnological routes using organic waste as feedstock are under development to replace current VFA production from petrochemical sources. In this scenario, VFA concentration and purification is the main bottleneck for further implementation. Membrane-assisted processes could be a promising solution, from a technological and economical point of view. In this study, forward osmosis (FO), reverses osmosis (RO) and nanofiltration (NF) membranes were used for high VFA concentration and ethanol removal. Overall VFA rejections reached more than 90% and with final concentration up to 50 g·L-1 when pH was adjusted above VFA pKa (i.e. neutral pH), due to the electrostatic effects generated between dissociated acid forms and the polymeric membrane. Additionally, and interestingly, high ethanol extraction from the VFA solution was achieved taking advantage of the electrostatic effect, as its separation from the VFA was enhanced at higher pH conditions. However, increased pH led to higher conductivity values which increased osmotic pressure of the VFA solution, limiting the potential concentration of the VFA by FO, RO, and NF.

Postprint (published version)