Ammonia inhibition on thermophilic anaerobic digestion

This study evaluated both chronic and acute toxicity of ammonia in thermophilic anaerobic digestion of synthetic wastewater over a range of acclimation concentrations. The inhibition effects of ammonia, in terms of total ammonia nitrogen (TAN), under various pH values and acclimation conditions on thermophilic aceticlastic methanogens were investigated. Completely mixed thermophilic anaerobic reactors operated at a chemical oxygen demand (COD) loading rate of 4 g/lday and a solid retention time (SRT) of 7 days were subjected to TAN concentrations of 0.40, 1.20, 3.05, 4.92, and 5.77 g/l. The reactor operations presented a case of chronic inhibition and it was observed that TAN concentrations of 4.92 and 5.77 g/l caused a drop in methane production by as much as 39% and 64%, respectively with respect to control. Batch anaerobic toxicity assays (ATA) were also performed to evaluate the acute toxicity effects of TAN and pH on methanogenesis at thermophilic condition. Modeling based on the results of ATA indicated that aceticlastic methanogens acclimated to high concentrations of TAN were less sensitive to increase in TAN and could tolerate wider pH ranges. TAN concentration causing 100% inhibition occurred in the range of 8-13 g/l, depending on acclimation condition and system pH.