Much of the impurities in raw wastewater are readily removed by sedimentation. Sedimentation is simply the separation of materials from water by gravity settling. Referring to the illustration, raw wastewater is pumped through a center shaft and enters the tank via a stilling baffle. The large size of the tank, 490,000 gallons, slows the flow enough to allow heavier organic matter to settle to the bottom and light materials like plastic and grease to float to the top. Settled organic matter is called sludge. The sludge is moved to the center of the tank by two rotating arms fitted with squeegees. The sludge is pumped directly from the tank to the anaerobic digesters. Material that floats to the top is called scum. The scum is collected into a holding tank by a rotating arm. The scum is then periodically treated with enzymes and pumped to the digesters.