If you could build something cheap enough that works water might do something. Maybe not alot.
Just for fun I thought about building a water wheel with a small dc motor and installing it under the bath tub facet. Maybe try to modify one of those water pumps you attach to a drill, stick it on the shower pipe before the shower head and have it turn a small dc motor as water runs through it. Maybe do a drill type pump for the main water line into the house. Anytime water runs the motor spins and hopefully make a little power.
Around here people can still use a lagoon instead of a septic system. So there is no problem with running washing machine grey water into a small pond as a lagoon. Thought about doing that and put a small water wheel at the end of the pipe.
Perhaps make myself a 55gal barrel of charcoal (great filter) run washing machine water into top of barrel, filters though charcoal, drains out bottom, runs through buried pipe comes out side of hill like a spring, turns water wheel and dumps into fake pond, then evaporates in hot sun. Would be more for landscaping, decoration and looks, but should make some small power if done right. Maybe light Leds at night around the pond or driveway.
Like I say mostly just for fun, no great power, but it might be a good way to recharge Ni-cads, batteries dead, take a bath or shower and recharge them 
