The digester used in this study was a 2.5-L filter bottle with a working volume of 2 L and consisted of a sampling outlet, a gas sampling port, and a feed inlet. It was sealed using a rubber stopper in which there was a pipe to extract biogas. The digester was connected to a gas collection system consisting of a saturated brine displacement bottle and a brine gathering bottle. Prior to operation, the reactors were purged with nitrogen gas for 5 min to ensure anaerobic conditions. Thereafter, the digesters were placed in a water bath at 37 ± 1 C. Each digester was manually mixed twice a day. The experiments were terminated when no significant gas production was observed.All of the reactors were started at an initial substrate concentration of 54 g VS/L. The substrate, inoculum, and water were added according to the desired experimental conditions (Table 2). All batch digesters were run in duplicate.