Fixed – Liquid level is controlled by pipes in drawoff boxes on tanks with steel or concrete covers.
Floating – Hollow steel covers move up and down with the liquid level.
Pressure and Vacuum Reliefs – Weights resting on a platten let gas out or air in to prevent excess pressure or vacuum from damaging the cover.
Flame Traps – A stack of metal plates in the gas piping will extinguish fire as it moves toward the tank.
Thermal Valves – Fire in the gas piping will melt a fusible plug, causing the valve plate to drop closed.
Drip Traps – Condensation in the gas piping is collected in drainage pots.
Earthquake Valves – A jiggle will cause the valve plate to drop closed, shutting off gas flow.
Steam Injection – Steam is diffused into digester.
Mechanical Mixers – Large propellors provide mixing.
Draft Tubes – Propellers of gas diffusers are surrounded by a large tube to pull sludge from the bottom.
Gas Recirculation – Compressors recirculate gas to shear boxes in the tank bottom or pipes coming down from the cover.
Steam injection – Steam is diffused into the digester.
Spiral Exchangers – Sludge and hot water flow through several separate spiral passages.
Concentric Pipe – Sludge flows through a pipe within a hot water pipe.
Self – Contained – Digester gas is burned in a unit which also contains sludge piping in a hot water chamber.