The weight and trouble associated with anaerobic digestion on board a 100 person ship is not worth the effort for methane propellant.

http://www.dept.aoe.vt.edu/~mason/Ma...toryGalley.ppt

With four toilets for 100 persons at 450 kg per toilet that's 18 kg per passenger. Watering down each passengers manure for efficient bacterial growth and holding it for 28 days masses 26 kg for the water alone per passenger.

A toilet processing the waste back into oxygen and fresh water in a closed cycle system operating alongside the air reprocessing system is far less massive then holding watered down waste in a huge airtight tank for 28 days while it rots and produces a few grams of methane that then must be purified and mixed without mishap in a zero boil off tank.

Far less odour and disease risk too processing it back to water and oxygen and carbon Black and phosphate.