Exploring the combined effect of total ammonia nitrogen, pH and temperature on anaerobic digestion of chicken manure using response surface methodology and two kinetic models

Ammonia inhibition is the most challenging issue in the anaerobic digestion (AD) of nitrogen-rich substrates. Total ammonia nitrogen (TAN) concentration, temperature and pH are the main operational parameters affecting the chemical equilibrium between free ammonium nitrogen (NH₃; FAN) and ammonium ions (NH₄⁺). However, it is still unclear how these parameters together affect digestion performance by influencing this equilibrium. To determine the effect and linkages of these parameters, a Box-Behnken design–response surface methodology (RSM), correlation analysis and two kinetic models were carried out. The results revealed that the linear effect of TAN, the interaction effect between TAN and temperature, and temperature and pH were significant, however, the quadratic effect of TAN, temperature and pH were also significant. Furthermore, TAN and temperature were positively correlated with Mmₐₓ, k and Rmₐₓ. Findings from this study could provide a theoretical basis to develop the way of relieving ammonia inhibition.