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 con- nected to a gas collection system consisting of a saturated brine. displacement bottle and a brine gathering bottle. Prior to opera- tion, the reactors were purged with nitrogen gas for 5 min to en-. sure 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 concentra-. tion 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