Environmental Water-Treatment Knowledge: Meaning and Function of Sludge Thickening
sludge thickening A process that uses gravity or dissolved-air flotation to reduce the water content of sludge and thicken it.
A sludge-treatment method that reduces the water content of sludge discharged from water-treatment structures to shrink its volume. Suitable for sludge with a high moisture content. For example, activated sludge has a moisture content of about 99%. When the moisture content of sludge drops from 99% to 96%, its volume shrinks to 1/4 of the original. To treat sludge effectively and economically, thickening must be carried out first. The moisture content of thickened sludge is generally 95-97%. The sludge water discharged during sludge thickening contains a large amount of organic matter and is generally mixed with the raw sewage for treatment together; it must not be discharged directly, so as to avoid polluting the environment.
Gravity thickening method: A sludge-thickening tank is used, of two types, continuous and intermittent. The structure of the thickening tank is similar to that of a sedimentation tank, mostly a circular tank with a diameter of 5-20 m, with a slow-stirring mechanism 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 the nature of the sludge: for primary-sedimentation-tank sludge it is 100-150 kg/(m2·day), and for activated sludge it is 20-40 kg/(m2·day). In the thickening tank, solid particles settle by gravity, water is squeezed out of the sludge, the thickened sludge is discharged from the bottom of the tank, and the sludge water overflows from the weir at the tank surface (continuous type) or flows out from the side outlet (intermittent type). Dissolved-air flotation thickening method: Opposite to the gravity thickening method, it makes sludge particles attach to fine bubbles and float to the water surface, then a scraper scrapes the thickened sludge into a sludge-discharge trough, while the sludge water flows out from the bottom of the tank (see dissolved-air flotation). This method is especially suitable for sludge whose particle specific gravity is only slightly greater than 1, such as activated sludge and sludge from the aerobic digestion method. Dissolved-air flotation thickening commonly uses the dissolved-air flotation method; the equipment includes a flotation tank, a booster pump, a pressure-dissolving tank, and a pressure-reducing gas-release device (valve). The dissolved-air pressure is generally 0.3-0.5 MPa. The amount of solids processed 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 can also be added.
Sludge-thickening methods include the sedimentation method [1], the dissolved-air flotation method, and the centrifugal method. When selecting a thickening method, in addition to the characteristics of each method itself, the nature and source of the sludge, the entire sludge-treatment process, and the final disposal method should also be considered. For example, the sedimentation method works well for thickening a mixture of primary-sedimentation sludge and excess activated sludge. Pure excess activated sludge is generally thickened by the dissolved-air flotation method, and in recent years the centrifugal method has been partially adopted for thickening.
Centrifugal thickening method: Carried out in a specially manufactured centrifugal thickener. It uses the different specific gravities of the solid and liquid in the sludge, which have different centrifugal tendencies, to separate the sludge and water and achieve the purpose of thickening.
