activity of mesophilic methanogens. The concentrations of propionic
acid in effluent also gradually increased, with slight variation
compared with acetic acid. Both butyric acid and valeric acid in the
effluent were comparatively low, and the concentrations were
nearly unchanged. The same acetic acid-accumulating phenomenon
was also pointed in other anaerobic reactors (Gao et al.,
2011a; McHugh et al., 2004).