Joking aside, the power of any water driven electricity generation is dependent on the difference in water levels on either side of the generator. Dams don't ge

Joking aside, the power of any water driven electricity generation is dependent on the difference in water levels on either side of the generator. Dams don't get built in deep canyons for convenience: the sites are chosen because they maximize the height of the water column. The potential energy of a municipal sewage system, while not zero, since there is flow, is so small as to be useless, unless your city is bisected by a 400 foot cliff.