The precipitated sludge settles to the tank bottom. A mechanical scraper at the bottom of the unit moves the sludge toward the sludge hopper and prevents the formation of shoals on the tank bottom. Surplus sludge is removed by opening the sludge blow-down valve, located just outside the clarifier tank. The surplus sludge should be removed periodically while the mechanical scraper blades are in operation. Sludge removal reduces the opportunity of the sludge going septic in that there is some small amount of BOD in the raw water that is removed as turbidity. A concentrated sludge bed depth of approximately two feet above the cone bottom is recommended for operating as a solids contact unit. Regular intervals of sludge blow-down should be established based on the influent flow rate and raw water turbidity.