Mixed liquor is transferred by gravity from the aeration basins to the final clarifiers. Two clarifiers are provided. In normal operation one clarifier will be dedicated to each aeration basin. Clarifiers are circular concrete basins of the center feed type with perimeter launders. Each unit is provided with a rapid return suction arm to collect settled sludge from the entire clarifier floor and return it to the aeration basins quickly. Clarifiers also have a scraper arm to collect any settled material not picked up by the suction arm as well as a skimmer arm to remove floating material from the clarifier surface.