Separation of volatile fatty acids (VFAs) from nutrients in food waste using membrane contactor: Analysis of VFA-membrane interactions, separation efficiency, and nutrient rejection

• Membrane contactor separated 40–73 % VFAs and rejected >95 % nutrients. • Membrane surface energy ≤ 36 mJ/m 2 and WCA ≥ 102° favored VFA/nutrient separation. • VFAs increased membrane dispersive, polar, and Lewis-base components by 2–27-fold. • C 5 to C 8 VFAs increased membrane surface energies up to 9.3-fold than C 2 to C 4 . • C 5 to C 8 VFAs increased membrane surface acidity by three orders of magnitude. Membrane surface properties and interactions with organic species can influence selective separation processes such as hydrophobic membrane contactors (HMC). The selective separation of volatile fatty acids (VFAs) from nutrients in industrial fermented food waste (FFW) was investigated using HMC. The focus was on (1) interactions induced by VFAs on membrane surface, (2) trends in membrane surface free energy (ϒ S tot ), and (3) nutrient rejection (R Nut ) and VFA separation efficiency (S eff ). The ϒ S tot and interactions varied with feed composition and concentration. The presence of VFAs increased the dispersive and polar interactions, and Lewis base components of the membrane surface by up to 2, 27, and 7-fold, respectively, while the Lewis acid components increased by three orders of magnitude. Consequently, the ϒ S tot tripled at maximum compared to the Control membrane, leading to decreased R Nut and VFA S eff . The HMC separated 40–73 % of VFAs to the permeate while rejecting >95 % of nutrients in the feed. Increasing ϒ S tot above 36 mJ/m 2 combined with decreasing water contact angle (WCA) below 90° were most detrimental to VFA S eff and R Nut .