|Better Farming Series 31. Biogas: What it is; How it is Made; How to Use it (FAO Better Farming Series, 1984)|
91. Sometimes a layer of scum may form on top of the waste mixture in your biogas unit. If this happens, less gas will be made and the small drum may not rise.
92. If the waste is well mixed before it is put into the unit, there will be less chance for scum to form and your biogas unit will make gas well.
93. If you use plant materials, scum is more likely to form than if you use only animal manure. You will need to stir the waste mixture in your biogas unit from time to time.
94. You can break up a scum layer by stirring or shaking the waste mixture after it is in the unit. You must do this without opening the small drum and letting out the gas or letting in the air.
95. During warm weather, the waste mixture in your biogas unit may become too thick and little gas will be made.
96. If this happens, add a bucket of water to the unit and stir the waste to thin it. If after a few days no gas is being made and the waste is still too thick, add another half- bucket of water and try again.
97. If a lot of very hard scum forms on top of the waste mixture and no gas is being made, take out all the waste mixture, clean the unit and start again. Do not throw the waste mixture away, use it for fertilizer.