Methane Releases from Septic Tanks and the Frequency of Emptying.

[NNadir]

9. On reflection, I think I can hazard an answer, albeit with limitations.

In scientific papers on water pollution, including this one, one often sees reference to BOD "biological oxygen demand." The organisms that are methanogens do not utilize oxygen for respiration, hence the produce a reduced molecule, methane, as a function of their metabolism.

Oxygen is less available in water, so I'd guess that wet sewage does produce more methane than dry feces.

Again, though, it's speculation on my part.