Ammonia Inhibition of Methanogenesis and Identification of Process Indicators during Anaerobic Digestion

In continuously stirred tank reactor experiments using pig manure codigested with solid fractions separated from pig manure (a mass mixture ratio of 40: 60) with an organic loading rate of 9.4 gVS/l(digester)/day at a thermophilic temperature of 51 C, we investigated acute inhibitory effects of ammonia on methanogenesis by using pulses of ammonium chloride, and assessed whether volatile fatty acids (VFAs) could be used as process indicators. Codigestion with the solid fractions seemed to result in a slight ammonia inhibition effect, though the digestion process was stable and the methanogens were adapted to a high ammonia concentration. We found a strong negative correlation (R(2) = 0.91) between total ammonia concentration and methane gas yield. However, free ammonia concentration did not reflect the acute ammonia inhibition significantly. Compared to a control reactor which was at an ammonia concentration of 4.6 gNH(4)-N/l, without pulsing ammonium chloride, a 50% decrease in methane gas yield in terms of loaded volatile solids was observed at an ammonia concentration of 11.0 gNH(4)-N/l (1.45 gNH(3)-N/l). Acetic acid and propionic acid, the two VFAs produced most abundantly during digestion of animal manure, did not accumulate with increased ammonia concentration; thus total VFA concentration was not a clear indicator of ammonia inhibition. However, we found that using isobutyric, butyric, and isovaleric acids individually as process indicators was useful. Isobutyric acid, in particular, accumulated initially by ammonia inhibition even before the yield of methane gas was affected.