Your web browser is out of date.

Update your browser for better security, speed and to get the best experience on this website.

Update your browser

Centrifugal sludge dewatering

Posted on
Whispy swirls of blue on a black background

How centrifugal sludge dewatering works

Centrifugation is used for both thickening and dewatering of sewage sludge, where dewatered sludge has a higher dry solids (DS) concentration. The centrifuge technologies used for each is almost identical. The key operational differences between the two functions are:

  • the rotation speed employed
  • the throughput, and
  • the nature of the concentrated solids product generated.

Dewatering demands more energy than thickening since more water must be removed to achieve the higher solids concentrations. The dewatered product, whose dry solids (DS) content may be as high as 50%, takes the form of a cake: a deformable semi-solid which forms lumps rather than a free-flowing fluid. It can therefore only be conveyed using a conveyor belt, whereas a thickened product retains the fluid properties of the feed and can be pumped.

As with thickening, the most common type of centrifuge used for dewatering applications is the solid bowl centrifuge, usually referred to as a decanter or a decanting centrifuge. Its dewatering performance and solids recovery depends on the feed sludge quality and dosing conditions.

Solid bowl dewatering centrifuge, counter-current operation Credit: Judd Water & Wastewater Consultants
Solid bowl dewatering centrifuge, counter-current operation
Centrifugal sludge dewatering technology, solid bowl: sludge enters the bowl at one end and is fed along the inside of the centrifuge by a rotary screw while the outer bowl rotates, extracting water from the sludge through the action of a centrifugal force.Credit: Judd Water & Wastewater Consultants

Solid bowl decanting centrifuge, Alfa Laval

Solid bowl decanting centrifuge, Alfa Laval Source: Alfa Laval / YouTube
Table 1. Solid bowl centrifugal dewatering DS concentration of feed and dewatered sludge (cake), polymer dose, and solids recovered (Metcalf & Eddy, 2014; Andreoli et al, 2007)
Sludge origin % DS content − Feed % DS content − Cake Polymer dose g/kg DS % solids recovered

this is a test footnote

Waste activated sludge (WAS)1−216−257.5−15≥95
Anaerobically-digested (AD) primary2−525−404−6≥95
Aerobically-digested WAS1−318−2510−15≥95
Mixed, primary + WAS3−525−352.5−8≥95
Mixed, AD primary + WAS2−422−357.5−15≥95

The solid bowl centrifuge design can be modified to provide greater dewatering capacity, i.e. a drier solids product, by:

  • extending the length of the solids outlet section (referred to as the beach)
  • reducing the differential velocity – the difference in velocity between the screw conveyor and the bowl, and
  • modifying the screw to provide pressing of the solid cake in the beach area.

The first two of these modifications extend the residence time in the centrifuge. The third provides additional dewatering of the cake.

Solid bowl centrifuges also form the basis of a hybrid dewatering−drying process, where the bowl is adapted to allow it to be heated with a sweep gas. DS concentrations of up to 90% are apparently possible from this process from an unthickened sludge starting material, implying that dewatering and drying of raw sludges can be carried out in a single stage.


Andreoli, C.V., von Sperling, M., & Fernandes. F. (2007). Sludge treatment and disposal, IWA Publishing (Lon, NY)

Metcalf & Eddy (2014). Wastewater engineering, treatment and resource recovery, 5th ed. Eds. Tchobanoglous, G., Stensel, D., Tsuchihashi, R., and Burton, F., McGraw-Hill (NY)

YouTube channel, Alfa Laval,

About this page

'Centrifugal sludge dewatering' was written by Simon Judd

This page was last updated on 05 May 2022


Information on this page may have been supplied by third parties. You are reminded to contact any third parties to confirm information is accurate, up to date and complete before acting upon it. accepts no liability for information provided by third parties, actions taken on the basis of this information or information held on third-party websites.

© Text copyright Judd & Judd Ltd unless otherwise indicated on this page