A channel in which algae is bred and gently stirred with a paddlewheel.
Growing algae on sewage could generate affordable biofuels. To make this work you begin with a series of small artificial rivers where waste water flows and algae is cultured. In those ponds is a paddle wheel, which pushes the water about and mixes algae into the waste water. The shallowness of the rivers encourages sunlight and helps the algae grow. And that algae is pretty important, because it’s what gets turned into biofuel.