Fig. 3

Plug flow digesters for biogas production. a “Kompogas” reactor. Horizontal plug flow reactor. Additional mixing by axial mixer. Increased process condition stability by partial effluent recycling. Gas outlet on top of the outlet side. 23–28% total solids. b Valorga reactor. Substrate entry at the bottom; plug flow over a vertical barrier to the outlet. Additional mixing by biogas injection at the bottom. 25–35% total solid content. c Dranco reactor. Substrate entry wit partial effluent recycling at the bottom, upward flow through substrate pipes. Downward plug flow to outlet. 30–40% total solids (Li et al. 2011; Nizami and Murphy 2010)