Selective Separation and Concentration of Volatile Fatty Acids, Nitrogen, and Phosphorus from Alkali-Fermented Liquid Using Flow-Electrode Capacitive Deionization System

The volatile fatty acids (VFAs) produced through alkaline fermentation of waste-activated sludge (WAS) can be utilized as carbon sources for biological nutrient removal (BNR) in mainstream wastewater treatment. However, the presence of refractory organics, ammonia, and phosphate in the fermented liquid often limits its effectiveness as a carbon source. This study introduces a novel liquid membrane chamber flow-electrode capacitive deionization (LMC-FCDI) technology for selectively and simultaneously separating and recovering VFAs, nitrogen, and phosphorus from actual fermented liquid. Under the influence of an electric field, cations (ammonium ions) and anions (including VFAs, non-VFAs dissolved organic matter (DOM), and phosphate ions) were effectively separated in the fermentation liquid. Consequently, ammonium ions accumulated in the cathode chamber, reaching a concentration of 361.43 mg/L, while anions moved toward the LMC. Notably, the non-VFAs DOM molecules were larger and migrated more slowly than the smaller VFAs (<1 kDa) under the electric field. Meanwhile, negatively charged VFAs moved to the acidic extraction solution in the LMC, becoming uncharged and thus concentrating there. This process successfully enhanced VFAs purity from 41.80 to 57.21%. Simultaneously, negatively charged phosphate ions (HPO42– or H2PO4–) migrated toward the anode chamber, where they accumulated to a concentration of 84.33 mg/L. The method proved efficient, with a normalized energy consumption of 1.63 kWh/kg of COD VFAs, lower than that reported in previous studies. This approach enabled the simultaneous recovery of 65.47% VFAs, 80.54% ammonium, and 79.43% phosphorus from alkaline fermentation liquids, facilitating selective separation and purification of these key products.