Though harvesting energy from waste water has been discovered there is definitely room for research and improvement. Engineers at Oregon State University have discovered that putting a nanoparticle coating on the anodes in the fuels cells that transform the waste into currents increases production nearly 20-fold. The anode captures the electrons produced by the bacteria as they break down the sewage. Unfortunately this coating is made out of gold but they say iron may work as well with different bacteria in the tanks.
UF - Env Sci and Sustainability