Wet Anaerobic Digestion
When dry matter (DM) concentrations are below 15% the process is described as wet. A wet digester will typically process a slurry of around 10% DM which has the consistency of thin soup and must be constantly stirred to prevent suspended solids from precipitating. Stirring is usually either mechanical in which case a rotating paddle is the preferred method, or gas, in which case the biogas given off is redirected through the substrate to ensure stirring. Wet digesters are nearly always constant flow digesters.
Wet digesters are ideally suited to the processing of low DM feedstock such as farm slurries and source separated food wastes.
Dry Anaerobic Digesters
The material processed during dry AD is normally around 25 – 30% DM and has the consistency of thick porridge. In constant flow digesters this must always be mechanically stirred, not only to prevent settling and unequal pocketing, but also to assist the flow of the material through the reactor.
Dry digesters are ideally suited to high DM feedstock such as energy crops, garden wastes and mechanically recovered municipal wastes in MBT plants.