Proof of concept: Recovery of high-purity volatile fatty acids by membrane system from source-separated waste streams

Abstract

Volatile fatty acids (VFAs) are platform chemicals with a higher value with wide applications. The feasibility of producing high-purity VFAs from source-separated wastewater using mixed-culture fermentation and membrane techniques was investigated. Batch studies were conducted for VFA production from blackwater and food waste under acidic and alkaline conditions. The VFA production from blackwater was higher at initial pH 9 with yield of up to 0.82 ± 0.03 gCOD/gCOD_fed due to higher buffer capacity, homogeneity, and biodegradability. The highest VFA yield from food waste was 0.36 ± 0.02 gCOD/gCOD_fed at initial pH 5. VFAs from the blackwater were dominated by acetic acid (up to 93 %), regardless of pH VFAs from the food waste were dominated by butyric acid (up to 76 %) and propionic acid (up to 52 %) at pH 5 and 9, respectively. Both the substrate types and pH influenced the microbial communities of the fermentation reactors. Bacteroides (up to 40 %) and Atopostipes (up to 51 %) were dominant genii for blackwater at initial pH 5 and 9, while Clostridium_sensu_stricto clusters (up to 58 %) and Romboutsia (up to 37 %) dominated food waste fermentation microbial communities at pH 5 and 9, respectively. VFAs were separated and purified with nanofiltration (NF) and reverse osmosis (RO). NF produced high-purity, but low-concentration permeate (900 −1500 mgCOD/L) at 50 % permeate recovery (with up to 42 % of acetic acid permeating the NF membrane). A higher concentration of pure VFAs (3x higher) was achieved with a subsequent step of RO. The study highlights the feasibility of the recovery of functional chemicals from waste.