? First high-salinity bioanode-biocathode system operated >290 days on food waste
Coupling bioanodes and biocathodes in three-compartment bioelectrochemical systems (BES) enables waste-to-chemicals conversion with simultaneous carboxylate production and extraction, yet long-term performance under high-salinity conditions remains unexplored. Here, we report 290 days of operation comparing high-salinity (HS, 626 mM NaCl) and low-salinity (LS, 26.7 mM NaCl) coupled BES, with food waste hydrolysate oxidized at the bioanode and CO2 reduced to carboxylates at the biocathode. Despite an initial eight-fold lower internal resistance in HS systems (20.9 vs. 158.3 O), both conditions achieved comparable anodic current densities (8.8 vs. 8.1 A.m-2) and maximal carboxylate production (5.8 vs. 5.9 g). Critically, salinity did not compromise performance but acted as an ecological filter, shaping distinct microbial communities: Geoalkalibacter vs. Geobacter anodireducens at anodes, and Alkalibaculum bacchi vs. Acetobacterium at cathodes. HS conditions markedly reduced methanogenic competition while yielding diversified products (formate + acetate), whereas LS cathodes produced predominantly acetate with up to 94.9% electron recovery. Ion accumulation driven by anodic pH control progressively constrained system performance, ultimately leading to resistance convergence and limiting carboxylate extraction by electrodialysis. Notably, these operational constraints were observed under both high-salinity and low-salinity conditions, indicating that high ionic loads are not specific drawbacks of high-salinity BES. Instead, ion and pH management emerge as critical design challenges common to coupled BES, irrespective of salinity level.