Biogas slurry often contains sand and grit and organic matter that eventually sediments. Clearing thetank from sedimentation is expensive (plant downtime, additional equipment) and dangerous. So it would be interesting to know how much sediment there is.
Typical tank heights are around 8m, so I think opening a hatch in the roof and poking with a long stick is not an option. What are other ways to do this?
Liquid level gauging is done with a pressure sensor mounted in the lower part of the wall, or a radar sensor on the roof.