Centrifugal thickening of wastewater sludge is accomplished by subjecting the sludge to high centrifugal forces. Sludge is fed into a rotating bowl. Solids separation occurs due to centrifugal force impelling the solids to the bowl wall where they are compacted. The liquid and fine solids (centrate) exit the unit through the effluent line. Thickened sludge is discharged as a liquid or cake. There are several configurations of centrifuges on the market. The most common type of centrifuge is the scroll type centrifuge. Scroll centrifuges rotate a tapered bowl along a horizontal axis. An inner scroll is used to evenly distribute the feed sludge. An adjustable weir controls the discharge of the centrate.
The performance of centrifuges is affected by the following factors:
1. Type of Sludge. The type of sludge being thickened is important. Generally, centrifuges are not used to thicken primary sludge because the centrifuge sludge inlets are susceptible to clogging. Secondary sludges are well suited to thickening by centrifugation because they usually lack material that will plug the centrifuge inlets. Centrifuges are less affected than other thickening processes by adverse sludge characteristics such as bulking sludge, rising sludge and old sludge. It should be noted that unlike other sludge thickening processes, off gassing of sludges will occur due to the high separation forces applied. Adequate ventilation is required and consideration must be given to monitoring air quality.
2. Solids and Hydraulic Loading. Unlike gravity thickeners and DAF units, the hydraulic and solids loading of centrifuges is not related to units of area (gpm/sq ft or gpd/sq ft). The accepted loading terminology for centrifuges is gal/hr/unit and lbs./ hr/unit. The size of the centrifuge sets the upper limit for solids and hydraulic loadings.
3. Bowl Speed. Increasing the bowl speed will increase the thickness of the sludge cake. However, unless the centrifuge is equipped with a hydraulic back drive, the speed cannot be changed except by changing the drive belt sheaves. Generally, once the ideal speed has been determined for a particular sludge there is no reason to change it.
4. Differential Scroll Speed. The scroll speed will affect the sludge cake thickness and the centrate quality. In general, as cake concentration increases, solids removal efficiencies decrease. Operator observation and experience is the best way to determine the scroll speed setting.
5. Liquid Depth (pool depth). The liquid level in the centrifuge can be varied by adjusting the effluent weirs. A deeper liquid depth results in greater solids capture but a lower thickness sludge cake. A shallower liquid depth results in less solids capture but a thicker sludge cake. Most wastewater sludge thickening operations run will a high liquid depth because solids capture is important to preventing disruption of the wastewater treatment system due to poor quality centrate.
6. Sludge Conditioning. Most centrifuges are operated with polymer addition to improve the thickness of the sludge cake and the solids recovery, which improves the quality of the centrate. Proper polymer dosages, steady sludge feed rates, and internal cleaning of centrifuge for accumulated greases/solids, are important to maintaining centrifuge performance.