|Better Farming Series 32. Biogas 2: Building a Better Biogas Unit (FAO Better Farming Series, 1986)|
110. Sometimes a hard layer of scum may form on top of the waste mixture in your biogas unit. If this happens, less gas will be made and gas will not collect in the inner tube.
111. If the waste is well mixed before it is put into unit, there will be less chance for scum to form and your biogas unit will make gas well.
112. Scum is more likely to form if you use plant materials than if you use only animal waste.
113. To keep scum from forming, shake your biogas unit from time to time. The drawing below shows you how.