Cation concentration represents an appropriate inhibition indicator for anaerobic acetate and propionate oxidation under mesophilic conditions.

Volatile fatty acid accumulation is a common issue encountered in anaerobic digestion, indicating potential process failure through the inhibition of microorganisms consuming those acids. However, the related inhibition mechanisms and thresholds remain poorly characterized for digesters treating concentrated feedstocks such as food waste. Considering two inocula adapted to high ammonia concentration, this study assessed the inhibition of acetate or propionate consumers exposed to various salts (NHHCO, NaHCO or KHCO) under mesophilic conditions in a range of total cation concentration between 0.23 and 0.86 mol.L. For each inocula, a single inhibition model could explain most variance (>92 %) of the methanogenic activity reduction without considering the substrate (i.e., acetate or propionate) nor the cation nature (i.e., K, Na or NH). This result highlights that the total salt concentration had a predominant impact compared to the specific nature of the ions in solution, supporting osmotic pressure as principal cause of the inhibition.