At the very top is the biogas zone where the gas is collected. Bacteria digest waste in the lowest portion of the upflow reactor; the bioreactor zone. In between these two stages is the clarifier zone where which exports the stabilised waste.
The biogas zone at the top collects the gas produced by bacteria digesting waste in the lower bioreactor zone. The clarifier zone, located between these two areas, separates and exports the stabilized waste.