poultry dung a difficult substrate. In case of serious doubt, the building of a biogas plant
should be re-assessed.
Scum can be avoided by stirring, but...
Scum is not brittle but very filthy and tough. Scum can become so solid after only a
short time, that it needs heavy equipment to break it. It remains at the surface after
being broken up. To destroy it by fermentation, it must be kept wet. Either the scum
must be watered from the top or pushed down into the liquid. Both operations demand
costly apparatus. For simple biogas plants, stirring is not a viable solution for breaking
the scum.
The only solution for simple biogas plants to avoid scum is by selecting suitable feed material
and by sufficient mixing of the dung with liquid before entering the plant.
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