Selective separation of volatile fatty acids, nitrogen and phosphorus from anaerobic acidogenic fermentation via forward osmosis membrane process

Selective separation and recovery of volatile fatty acids (VFA) and other valuable resources promotes a sustainable wastewater treatment system, but its cost-effective separation approaches are poorly understood. This study investigated the feasibility of applying a forward osmosis (FO) dewatering process for the selective separation of VFA, ammonium and phosphate from anaerobic acidogenic fermentation. Limited VFA rejection of fermentation broth, i.e., less than 20 %, was observed under acidic conditions, while the rejection rate was significantly increased to ∼90 % at neutral pH. The transfer of ammonium across the membranes was remarkably impacted by the feed solution pH. Lower pH (∼4) values resulted in higher rejection rates of ammonium (∼96 %), and the rate gradually declined as pH increased. The rejection of phosphate, which remained higher than 98 %, was less sensitive to pH variations. The increase in water flux promoted the rejection of VFA, ammonium and phosphate, and the membrane orientation significantly impacted the transfer of nutrients in the FO process. The active layer facing the feed solution (AL-FS) mode exhibited higher rejection rates of VFA, ammonium and phosphate than the active layer facing the draw solution (AL-DS) mode, which was ascribed to the internal concentration polarization effect. Furthermore, approximately 80 % of water could be effectively reduced from a real acidogenic fermentation liquor during the FO process; ammonium and phosphate were 3.72-fold and 4.10-fold concentrated in feed solution, respectively, and VFA was selectively separated in draw solution. The work confirmed the feasibility of utilizing FO processes in separating and enriching VFA, ammonium and phosphate from anaerobic fermentation liquors, providing meaningful information on promoting FO-based processes in treating wastewater.