Gravity Thickener

Gravity thickeners are heavy-duty circular collectors that are custom designed to handle concentrated municipal and industrial sludges. In order to meet the load of increased sludge, these thickeners are designed with high service factors. In addition to this, the thickeners can also be provided with pickets for dewatering, lifting devices for heavy solid uses and applications. As the gravity thickeners are engineered for sludge handling rather than liquid handling, they are designed with steeper floor slopes than traditional clarifiers.

The gravity thickeners are designed and engineered to attain the continuous dewatering of different types of sludges and pulp by gravity drainage through a porous synthetic membrane (belt). Either the static devices or the free turning rollers throughout the unit are used to support this belt. The usual design and construction is to the same high standards and quality as other press technology equipment that is engineered for continuous industrial usage.

The gravity thickeners have proved to be an effective tool for primary thickening of the light consistency slurries either as a pre-thickening device or as an end result prior to introduction into other dewatering equipment. Pre-thickening is an economical method of significantly reducing the hydraulic loading on twin wire and screw presses, thereby improving the performance and throughput.

Gravity Thickening - Process
Gravity thickening is a physical process wherein, the sludge is stirred for comparatively longer period, for around 8-12 hours in a thickener unit. This is done for the basis objective of forming passageways or channels in the sludge mass to permit escape of occluded water and to produce a more cohesive mass of sludge flocks with decreased. Thickening can be carried out in a tank, facilitated with a special mechanism, similar to a clarifier. The thickening units that are equipped with such mechanisms with a series of upright chambers (called pickets) are connected to the raking arms.

The successful operation of gravity thickening depends on the consistent addition and mixing of dissolved oxygen bearing water with the dilute sludge from the secondary clarifier. The over flow of excess sludge from the digester, the continuous flocculation of this anaerobic mixture by interfacial contact, followed by a settling of the sludge particles and the retention of the settled material, also affect the successful completion of process. All these successive stages take place in the same tank, or with the addition of a pre-mix basin. The compacting action takes place in the sludge blanket or mass in the bottom levels of the tank by two means –

• By compression due to weight of the solids themselves
• By the release of occluded water due to the uninterrupted passage of the arms of the mechanism through the sludge mass.

How it works
The material, which is required to be dewatered enters the gravity thickener via a stainless steel head box that distributes the suspension on the belt. As the material passes through the length of the thickener, chicanes can be used to turn the material, thereby preventing “pooling” and aiding free drainage. Chicanes have found to be especially beneficial in sludge thickening applications. A scraper blade mounted on the drive / discharge roll is used to remove the thickened material from the belt, thereby allowing it to drop onto a conveyor for transport to the next phase in the method. The belt is then travels through a cleaning shower and over tracking and tensioning rolls prior to entering the head box again.