Evaluation of nanofiltration for the extraction of volatile faffy acids from fermentation broth

Nanofiltration (NF) is a pressure-driven membrane process used for liquid-phase separation, which is promising for biomass separation and purification. In this work, two NF membranes (NF-45 and XLE) are used for the separation and valorization of volatile fatty acids (VFAs), one kind of useful chemical building blocks, from fermentation broth produced using municipal solid waste. The objective of this work is to investigate NF process for the separation and purification of VFAs from fermentation broth. Experiments are carried out with synthetic solutions with increasing complexities, from single, binary and ternary solutions of VFAs, i.e., acetic (Ac), propionic (Pr), and butyric acid (Bu), to mixed solutions containing VFAs and inorganic salts at different compositions. The influence of ionic composition as well as of the solution pH on the transfer of the solutes are discussed. Then, a real fermentation broth containing three VFAs, inorganic ions and organic matters is also investigated. For synthetic solutions containing dissociated VFAs (pH 8), it is observed that for a given VFAs proportion in the feed, regardless of the total concentration, the proportion of VFAs in the permeate remains constant when the filtration flux is higher than a specific value. From the perspective of mass transfer, this observation means that in a mixed solution, the individual charged solute transfer is controlled by the total amount of charge transferred through the membrane. Indeed, it is found that the individual VFA flux increases linearly with the total solute flux and does not depend on the total concentration but only on the VFAs proportions in the feed. More complex solutions containing VFAs with the addition of three inorganic salts (Na2SO4, NaCl, and CaCl2) are further investigated. Again, constant permeate proportions for organic and inorganic solutes (anions as well as cations) are observed for a given feed proportion, when the filtration flux is higher than a certain value. Then, it is concluded that, for any couple of VFAs (Ac/Pr, Pr/Bu, or Ac/Bu), the plateau value of the relative proportion of the individual VFA in the permeate is nearly fixed by its proportion in the feed, for all the conditions investigated. This conclusion can be extended to couples of VFAs/inorganic anions and couples of cations. Solution pH significantly changes the retention of VFAs and the permeate proportion of VFAs. [...]