Sludge thickening is the process of increasing the solids content of sludges from primary and/or secondary settling tanks. The process serves to reduce the size of the subsequent sludge dewatering steps. The influent sludge is fed to the center of the tank and the thickened sludge on the tank bottom is pushed into and central hopper by the scraper blades, from where it is drawn off either hydrostatically or by a pump and sent to further sludge processing steps. Generally, a picket fence structure is installed on the scraper arms for gentle mixing of the settled sludge for better consolidation. In continuously fed sludge thickeners, the supernatant is collected by peripheral weirs while in intermittent feed system alternate are used for decanting the supernatant.