Gas Collection
The biogas in an anaerobic digester is collected in an
inverted drum. The walls of the drum extend down
into the slurry to provide a seal. The drum is free to
move to accommodate more or less gas as needed.
The weight of the drum provides the pressure on the
gas system to create flow. The biogas flows through a
small hole in the roof of the drum. A non-return valve
here is a valuable investment to prevent air being
drawn into the digester, which would destroy the
activity of the bacteria and provide a potentially
explosive mixture inside the drum. Larger plants may
need counterweights of some sort to ensure that the
pressure in the system is correct.
The composition of biogas depends on a number of
factors such as the process design and the nature of
the substrate that is digested. The table below lists the
typical properties of biogas from digesters and a
comparison with average values of natural gas.