Digesting food to create energy/electricity with little to no waste

(Decided to sum up some of my older questions to get one coherent answer)

Robots that use biomass and “food” to power themselves are beginning to be developed more today, usually using a microbial fuel cell. Using this method, a similar one, or something different (specify/explain if it is through a different method), could a robot, perhaps in the near future, use this “digesting” as a source of power without producing any waste—or poop—that needs to be removed or disposed of?

submitted by /u/teenytoons2
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