Flow-electrode capacitive deionization for enhanced selective separation of ammonia, phosphorus, and caproate from sewage sludge fermentation: Performance and mechanistic insights

Caproic acid has broad applications and can be produced from activated sludge via fermentation, but its quality is hindered by ammonia (NH₄⁺-N) and reactive phosphorus (RP) in the fermentation broth. However, flow-electrode capacitive deionization (FCDI), a novel ion separation technology that operates continuously without secondary pollution seems to be an efficient process that separates the ions. The results showed that at pH 5.0, the majority of N and P presented as NH₄⁺ and H₂PO₄^-, with removal efficiencies of 59.5 % for NH₄⁺-N, 49.5 % for RP, and 17.4 % for caproate. Higher caproate concentrations increased boundary layer thickness, thereby promoting caproate transport to compensate for the ions consumed. The anion exchange membrane exhibited stronger rejection of divalent phosphate than acetate and caproate, resulting in lower HPO₄²⁻ selectivity. The FCDI holds potential as a viable technology for resource recovery from fermentation broth, offering an alternative method in bioprocessing applications.