Upcycling fermentation liquors by integrating membrane contactors for nitrogen recovery and forward osmosis for volatile fatty acids up-concentration

Fermentation liquors from organic wastes are characterised by relatively low concentrations of volatile fatty acids (VFA) and high concentrations of total ammoniacal nitrogen (TAN), which reduce their commercial value and potential application. Therefore, downstream processes are necessary to concentrate VFA and increase the carbon-to-nitrogen ratio of fermentation liquors. In this study, a biomimetic forward osmosis (FO) membrane was studied to concentrate acetic acid and TAN at different pH levels (i.e. 5, 7, and 9). Acetic acid rejection exceeded 97 % at pH levels at or above 7, but it dropped to 65 % at pH 5. TAN rejection remained around 58 % regardless of the pH level. These results suggest that TAN should be recovered from fermentation liquors before up-concentrating VFA by FO membranes. Accordingly, a gas-permeable membrane (GPM) contactor was used to recover nitrogen from fermentation liquors, achieving nitrogen recoveries up to 96 %. This process produced a highly pure ammonium sulphate solution with 36 g N/L, with potential interest as liquid fertiliser. Applying the FO to a nitrogen-spent fermentation liquor resulted in a VFA rejection of 98 % and increased the VFA concentration from 32 to 77 g COD/L. The combination of GPM and FO membrane technologies has proven to be an efficient approach for concentrating VFA and recovering TAN from fermentation liquors, offering enhanced separation performance and efficient reagent utilisation for pH control.