Ammonia-induced constraints on butyrate degradation in anaerobic digestion: Impact of ammonia levels and pH conditions, and recovery behaviour

The accumulation of butyrate is a recurrent phenomenon in anaerobic digesters utilizing animal manure as the primary feedstock. Despite its prevalence, the factors governing the anaerobic degradation of butyrate remain inadequately explored. In this study, a series of experiments were carried out under different total ammonia concentrations (TAN, 0.18–20 g N/L) and pH conditions (7.0–8.0) to investigate the inhibition of butyrate anaerobic degradation by different ammonia species (NH₄⁺ and NH₃) and to assess the recoverability following severe ammonia inhibition. The findings indicate that at pH 7.5, butyrate degradation experienced remarkable inhibition when TAN exceeded 8.0 g N/L, while no discernible impact was observed at pH 7.0–8.0 and 4.0 g TAN/L. Additionally, the lag phase for butyrate degradation extended with increasing TAN concentration. Notably, the activity of butyrate-degrading bacteria exhibited full recovery from severe ammonia inhibition (TAN 20 g N/L or NH₃ 779.2 mg N/L), provided prolonged adaption time was allowed. The analysis using a modified Monod inhibition model highlighted that NH₄⁺ contributed more to inhibition than NH₃ at TAN concentrations of 2.0–20.0 g N/L. Therefore, simply reducing pH levels would not adequately counteract ammonia inhibition. Implementing an extended hydraulic retention time emerges as an effective measure to reduce butyrate accumulation in anaerobic digestion systems, particularly the feedstock being nitrogen-rich materials (e.g., animal manure).