The mechanism of ammonia inhibition in the thermophilic digestion of livestock wastes

This paper proposes a scheme for the inhibition of thermophilic methane digestion processes by high ammonia concentrations. Ammonia acts as a strong inhibitor of the formation of methane from H₂ and CO₂. It has only a minor effect on the formation of methane from acetate, as indicated by the independence of the specific growth rate of acetate-consuming methanogens from the ammonia concentration (up to 4500 mg NH₄⁺-N per litre). On the basis of thermodynamic considerations it is shown that the inhibition of the hydrogen consumption leads to an inhibition of propionate breakdown. Accumulated propionate, in turn, acts as an inhibitor of the acetate-consuming methanogens. This scheme explains the discrepancy between the observed acetate accumulation in thermophilic methane digestion systems with high ammonia levels and the independence of acetate mediated methanogenesis from high ammonia concentrations under laboratory conditions.