High-Rate Vacuum-Enhanced anaerobic Digestion: Performance, ammonia recovery and process kinetics

• Operation at 4x OLR of a conventional AD with comparable COD and VSS destruction. • Ex-situ vacuum enriched Methanosarcinaceae population and weakened Methanosaeta. • Recovery of 44%-47% of ammonia mitigated potential ammonia inhibition. • Vacuum-resistant syntrophs were detected. • Smithella was weakened under ex-situ vacuum and high OLR. This study investigated the applicability of side-stream vacuum for intensification of anaerobic digestion (AD) at organic loading rates (OLRs) of 3.8–4.5 kgVS/m 3 /d (6.1–7.3 kgCOD/m 3 /d), and ammonia recovery at four times the OLR of a conventional AD. Initial operation showed a performance decline due to Methanosaeta dominance and its inability to resist vacuum, resulting in low methane yield (0.06 L/gCOD feed , 15 % COD destruction) and high SCOD accumulation (12 g/L). Methanosarcinaceae enrichment under vacuum restored stability, achieving 49 %–53 % VSS destruction. Ammonia inhibition was mitigated by 44 %–47 % TKN recovery through ex-situ vacuum stripping, maintaining ammonia concentrations below 1 gN/L, compared to a potential inhibitory level of > 4 gN/L without ex-situ vacuum. The dominance of Methanosarcinaceae and successful ammonia recovery corresponded with stable operation at four times the OLR of conventional digesters. Biokinetic studies revealed a 155 %–232 % improvement in biomass-specific acetoclastic methanogenic activity through ex-situ vacuum. Fluorescence excitation-emission matrix-parallel factor analysis showed accumulation of the slowly biodegradable fluorescing organic. High fermentative bacterial counts were maintained, with increased populations of DMER64 , Mesotoga , Synergistaceae , Clostridium_sensu_stricto_1 , Pelotomaculum, and Syntrophomonas , suggesting resistance to vacuum stress, while Smithella decreased.