Municipal Sludge Thickening Technologies

Sludge thickening is the first step in the sludge treatment process and at the same time an important component in sewage sludge management. The volume of sludge after thickening is reduced considerably to less than half of the original amount. Depending upon the physical process employed, sludge thickening can be carried out using gravitational, centrifugal, or buoyancy force. Dissolved air flotation is the only thickening method that utilizes buoyance forces for sludge thickening. Centrifuges use centrifugal forces, whereas gravity thickener, dewatering tables, drum and disk thickeners employ gravitational forces to separate sludge from the water phase. The main factors that influence the selection of the thickener are sludge type, the size of the UWTP plant and the downstream process, the requested sludge concentration, footprint, investment, and operational costs. A gravity thickener is typically used to thicken primary sludge, whereas dissolved air flotation and mechanical thickening are mainly utilized to thicken biological waste sludge. Nonetheless, all of these methods can also be implemented to thicken mixed, chemical and digested sludge. The performance results of each methodology are strongly influenced by the sludge origin and type; however, performance can be improved by applying chemical conditioning. To this end, cationic polymers are used frequently in order to improve the thickening of the waste activated sludge. As a rule, gravity thickeners and centrifuges do not require chemical sludge conditioning, but their performance can be increased by the dosing of flocculants, especially in the case of overloads.