what we want isn't to generate electricity through bacteria that digests wastewater -- what we want is to feed electricity to the bacteria that digests wastewater, just to enable that bacteria to do it better, faster, and "wider".
Let's imagine I have a septic tank behind the house. I don't need to find out how I can run a LED light from the bacteria in the tank. What I want is to plug that biosystem to electrical power, so that the septic tank biorection is 100x speeded up, in a 10x smaller tank. Even if it uses up a few Watts of power.
And, once we do that, think of the possibilities that open up: once you feed power into the biosystem, it becomes energetically possible to drive bioreactions in the opposite direction from the energy gradient, basically driving the reaction "up the hill" with electrical power. Now that's something!