Recovery of bio-based volatile fatty acids from anaerobically treated winery wastewater using a closed-loop liquid-liquid hydrophobic membrane contactor system

Abstract

This research evaluated a closed-loop hydrophobic liquid–liquid membrane contactor (MC) for the recovery of volatile fatty acids (VFAs) from the effluent of an up-flow anaerobic sludge blanket (UASB) bioreactor treating winery wastewater. Sodium bicarbonate was used as a stripping solution for its operational flexibility and compatibility with downstream bioprocesses. An electrochemical acidification cell (EAC) was tested to continuously adjust of pH of the feed solution to avoid the use of inorganic acids. VFA recovery was optimized under different flow rates and feed pH conditions using a synthetic VFA solution and validated with actual UASB effluent. Lower feed pH resulted in higher VFA recovery due to increased protonation of the acids. This effect was more pronounced when using synthetic VFA solutions compared to real UASB effluent. Specifically, the MC system recovered 16 ± 5 % of acetic acid at 0.3 ± 0.1 gCOD/m2h flux and 34 ± 4 % of butyric acid at 0.5 ± 0.1 gCOD/m2h flux at an unadjusted UASB effluent feed pH of 4.9, compared to 22 ± 3 % at 0.5 ± 0.1 gCOD/m2h flux and 46 ± 5 % at 0.7 ± 0.1 gCOD/m2h flux, respectively, at pH 2. The EAC effectively lowered the feed pH of UASB effluent to 3.9, achieving acetic acid recovery of 22 ± 3 % at a mass flux of 0.2 ± 0.1 gCOD/m2h, and butyric acid recovery of 45 ± 1 % at a mass flux of 0.6 ± 0.1 gCOD/m2h. These moderate recovery efficiencies are likely influenced by the lower pH gradient resulting from the use of sodium bicarbonate. However, the compatibility of sodium bicarbonate with bioprocesses offers the advantage that the proposed UASB-MC system can be seamlessly integrated with downstream bioprocesses to produce valuable end products for other markets, such as PHA for plastics and microbial proteins for animal feed.