What Is Sludge Thickening? A Detailed Explanation of a Wastewater Treatment Term
Sludge thickening is the process of reducing the water content of sludge by gravity or flotation to thicken it.
Centrifugal thickening method: carried out in a specially made centrifugal thickener. It exploits the different specific gravities of solid and liquid in the sludge and their different centrifugal tendencies to separate sludge and water, achieving the purpose of thickening.
Sludge thickening methods include sedimentation [1], flotation, and centrifugation. When selecting a thickening method, besides the characteristics of each method itself, the nature, source, the entire sludge treatment process, and the final disposal method of the sludge should also be considered. For example, the sedimentation method works well for thickening the mixed sludge of primary sedimentation sludge and excess activated sludge. Pure excess activated sludge is generally thickened by flotation, and in recent years partial centrifugal thickening has been developed.
Gravity thickening method: uses a sludge thickening tank, of which there are continuous and intermittent types. The thickening tank is constructed similarly to a sedimentation tank, mostly a circular tank with a diameter of 5-20 m, equipped with a slow-stirring mechanical agitator inside. The retention time of sludge in the thickening tank is generally about 12 hours. The surface sludge solid loading rate of the thickening tank varies with sludge properties: 100-150 kg/(m2.day) for primary sedimentation sludge, and 20-40 kg/(m2.day) for activated sludge. In the thickening tank, solid particles descend by gravity, water is squeezed out of the sludge, the thickened sludge is discharged from the tank bottom, and the sludge water overflows from the weir on the tank surface (continuous type) or flows out from the side outlet. Flotation thickening method: opposite to the gravity thickening method, it makes sludge particles attach to fine bubbles and float to the water surface, then uses a scraper to scrape the thickened sludge into a sludge discharge trough, while the sludge water flows out from the tank bottom (see flotation). This method is especially suitable for sludges whose particle specific gravity is only slightly greater than 1, such as activated sludge and aerobic digestion sludge. Flotation thickening commonly uses dissolved-air flotation, with equipment including a flotation tank, a pressure pump, a dissolved-air tank, and a pressure-release valve. The dissolved-air pressure is generally 0.3-0.5 MPa. The solid amount treated per square meter of flotation tank per day is 100-200 kg for general sewage sludge and 25-100 kg for activated sludge. To improve the flotation thickening effect, a coagulant may also be added.
A sludge treatment method that reduces the water content of sludge discharged from water treatment structures to shrink its volume. It is suitable for sludge with a high moisture content. For example, activated sludge has a moisture content of about 99%. When the sludge moisture content drops from 99% to 96%, its volume can shrink to 1/4 of the original. To treat the sludge effectively and economically further, thickening must be done first. The moisture content of thickened sludge is generally 95-97%. The sludge water discharged during sludge thickening contains large amounts of organic matter and is generally mixed into the raw sewage for treatment together; it cannot be discharged directly so as to avoid polluting the environment.
