|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.