Selective separation of volatile fatty acids and phosphorous recovery from fermented broth using flow-electrode capacitive deionization

Volatile fatty acids (VFA) extracted from fermented liquids are a viable carbon source to improve the removal of biological nutrients due to a carbon shortage of sewage influent in China. However, nutrients released from the fermented liquid increase the influent nutrients loading, resulting in a lower removal efficiency. In this study, a flow-electrode capacitive deionization (FCDI) system was developed to verify the feasibility of VFA separation and simultaneous ammonium and phosphate recovery from synthetic fermented broth. The results showed that 35.87 % of NH4+-N (cations) were removed from the influent under the optimal conditions (carbon = 5 wt%, current density (j) = 8.75 A/m2, hydraulic retention time (HRT) = 7.5 min, and pH 4.5). Meanwhile, only 9 % of acetate and 32.4 % of phosphorous (P) were migrated into anodic chamber. The difference removal efficiencies of anions caused by the different charging characteristics in electric field contributed the selective separation of acetate. The removed P was concentrated in the anode and converted into neutral H3PO4 by controlling the pH of the electrolyte below 2. This facilitated P enrichment in the anode compartment during discharge. The long-term operation results showed that 110.78 mg/L of P in the anode chamber and 92 % of acetate in the effluent were obtained with a lower power consumption of 1.89 kWh/kg P. These findings indicated that FCDI is a feasible approach for separating VFA and recovering P from fermented broth.