Primary Clarifier Tanks - Wastewater flows from the preliminary treatment building into four primary clarifiers, three rectangular and one circular. The velocity slows down in these tanks and allows solids to fall to the bottom of the clarifier. This settled material, called sludge, is scraped into a hopper and pumped to the anaerobic digester. The scrapers also skim the surface to remove scum, grease and other floating material, which is also pumped to the digesters. The liquid from the primary clarifiers passes over weirs to the secondary treatment process. Manitowoc uses a trickling filter process consisting of two stack filters and two rock filters in series.