Effects of acclimation and pH on ammonia inhibition for mesophilic methanogenic microflora

This work evaluated the toxicity of ammonia toward mesophilic methanogenic microflora, with respect to the acclimation and pH. Microflora acclimated to total ammonia nitrogen (TAN) concentrations of 1.0–2.5 g N/L and neutral to alkaline pH, 7.5–8.0, were used for anaerobic toxicity assays (ATA) with TAN up to 6.1 and pH ranged from 7 to 8.2. The effect of ammonia on the specific methanogenic activity (SMA) and methane yield (MY) was described using a noncompetitive model. Acclimation increased the tolerance of the methanogenic microflora to ammonia over the acclimation range. There was no significant difference in the values of SMA and MY at each TAN concentration in the examined pH range, although free ammonia nitrogen (FAN) concentrations increased along with pH. It implied that TAN rather than FAN would be the useful criterion for ammonia inhibition. Propionate degradation was more inhibited compared to acetoclastic methanogenesis by ammonia. The half-maximal inhibitory concentrations (IC₅₀) of TAN for the SMA from acetate, SMA from propionate, MY from acetate, and MY from propionate were 3.42–4.26, 3.14–3.91, 3.67–4.07, and 3.34–3.88 g N/L, respectively, at pH 7.4. 454-pyrosequencing analysis of the inoculum showed that the archaeal community was dominant of Methanosarcinaceae and Methanobacteriaceae, which were known as tolerant to ammonia inhibition.