Thickening Zone and Sludge Removal in Circular Final Settling Tanks

Abstract

In horizontal flow final settling tanks mechanical sludge removal methods are necessary to remove the thickened and stored sludge. The frequently designed circular tanks are commonly fitted with travelling bridge scrapers and their performance is given by their velocity, their blades and the number of arms per clarifier. In five final settling tanks investigations were made at different operation parameters, modified scraper velocities and deepening the scraper blade to measure the influence of scraper performance and operation parameters on the thickening zone and the sludge retention time. The results show the relation between sludge return rate and short circuiting flow from the clarifier inlet to the sludge drawoff. The thickening zone is mainly influenced by the sludge volume flow and the sludge volume loading. The positive effect of higher scraper velocities and deepening the scraper blade on the thickening depth was found.