Each tube-shaped mic-robot is a sandwich of three materials. A graphene outer layer, which binds to heavy metals. A middle layer of nickel, which gives the bots magnetic polarity, so they can be pulled through wastewater with magnets. And platinum inside for propulsion. Just add a bit of peroxide to the wastewater, and it'll react with the platinum to form water and oxygen bubbles, which propel the tubes along.
Beginning in 35 seconds.