Ammonia inhibition in thermophilic anaerobic digestion of dairy cattle manure

Abstract

This study investigated the effect of five different concentrations of total ammonia nitrogen (TAN) and free ammonia (FA) on the methane yield and digester performance under thermophilic conditions (50°C). Five identical continuously fed digesters were used. The experiment was run for four times hydraulic retention time (HRT). Data were statitically analysed using the data Analysis Tool Pack available with the Microsoft Excel program. Different ammonia levels were obtained by pulsing urea to obtain the target level of TAN and FA, and to subsequently maintaining the concentration of ammonia levels by daily urea additions. The result showed a strong negative correlation between both TAN and FA concentrations and methane yield. The methane yield was reduced by 24, 30, 52 and 66% in digesters that had TAN levels of 2.9, 3.6, 4.4 and 5.1 g/L, respectively, corresponding to 0.7, 1.1,1.5 and 1.8 g/L FA. Total volatile fatty acid and especially isovaleric and isobutyric acid concentrations were elevated during ammonia inhibition. Concentration of TAN in the biogas digester exceeding 3 g/L indicating that very often the full biogas potential is not achieved due to ammonia inhibition and that longer HRT should be used to compensate or a lower process temperature should be chosen.