Potential of acetic acid to restore methane production in anaerobic reactors critically intoxicated by ammonia as evidenced by metabolic and microbial monitoring

BACKGROUND Biogas and biomethane production from the on-farm anaerobic digestion (AD) of animal manure and agri-food wastes could play a key role in transforming Europe’s energy system by mitigating its dependence on fossil fuels and tackling the climate crisis. Although ammonia is essential for microbial growth, it inhibits the AD process if present in high concentrations, especially under its free form, thus leading to economic losses. In this study, which includes both metabolic and microbial monitoring, we tested a strategy to restore substrate conversion to methane in AD reactors facing free ammonia critical intoxication. RESULTS The AD process of three mesophilic semi-continuous 100L reactors critically intoxicated by free ammonia (> 3.5 g_N L_slurry − 1 ; interrupted hydrolysis and methanogenesis) was restored by applying a strategy that included pH correction using acetic acid, washing out total ammonia with water, re-inoculation with active microbial flora and the progressive re-introduction of sugar beet pulp, a complex organic substrate. After five weeks, two intoxicated reactors restarted the production of CH 4 from the pulp hydrolysis and via hydrogenotrophic methanogenesis. The acetoclastic methanogenesis remained inhibited, probably due to the transient dominance in the slurry of a strictly methylothrophic methanogen of the archaeal genus Candidatus Methanoplasma, to the detriment of a versatile methanogen belonging to the Methanosarcina genus. Concomitantly, the third intoxicated reactor, in which the Methanosarcina members remained permanently dominant, faced transient hydrolysis inhibition but produced CH 4 from restored acetoclastic methanogenesis. After eight weeks, the AD process was functional in all formerly intoxicated reactors with a pulp to CH 4 conversion ratio of 0.41 ± 0.10 L N _CH 4 g_VS − 1 , analogous to the pulp biochemical methane potential (0.38 ± 0.03 L N _CH 4 g_VS − 1 ). CONCLUSIONS Despite a particularly extreme free ammonia intoxication, the proposed process recovery strategy allowed the hydrolysis and methane production to be restored in three intoxicated reactors within eight weeks, a period during which reinitiating the feedstock supply appeared to be crucial to sustain the process. The initial introduction of acetic acid allowed substantial CH 4 production during the recovery period and could enable the agronomic valorization of the effluents produced by full-scale reactors recovering from ammonia intoxication.