GFS Biogas Digestion Tanks Provider: What the Vessel Has to Do Beyond Holding Slurry A digester is the only vessel in a biogas plant that is simultaneously a chemical reactor, a pressure vessel, a heated tank and a gas holder. It has to maintain a temperature band within a degree or two, keep a biological population stable for years, mix a material that separates if left alone, contain a flammable gas at low pressure, and do all of that while resisting an internal environment