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Belt filter presses for sludge dewatering

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Sludge activated treatment: belt press

How belt filter presses work

A belt filter press (BFP) provides sludge dewatering by pressing the sludge to force the water through a permeable medium. The process produces a cake (the dewatered product) having a dry solids (DS) content of 30% or more in the case of primary sludge. This is to be distinguished from the lower-energy gravity belt thickener process which achieves a maximum DS content of ~10%.

A BFP combines drainage and mechanical pressure sequentially to remove water. The equipment normally comprises 2−3 recirculating belts, with two belts combining at some point to compress the sludge and squeeze water from it.

The conditioned sludge feed is first conveyed along a gravity drainage section of one of the porous belts, as with a gravity belt thickener. It is then subject to pressure as it is passed between two recirculating belts, forming a wedge zone at the inlet, which travel over a roller. This squeezing action from the two belts in this pressure zone releases more water.

Example arrangement of belts and rollers for a belt filter press, dual-belt system Credit: Judd Water & Wastewater Consultants
Example arrangement of belts and rollers for a belt filter press, dual-belt system
Belt filter press showing a series of rollers used to recirculate two belts which between which sludge is fed and water squeezed out under pressureCredit: Judd Water & Wastewater Consultants

Many belt filter press technologies contain a second pressure zone comprising a series of rollers through which the two belts pass with the sludge solids retained between them. The rollers in this high pressure zone apply tensioning (i.e. stretching) to the belts, exerting both shearing and compressive forces on the sludge which further release more water.

Some BFP technologies are based on a three-belt system, where the gravity belt is independent of the two pressurising belts. This allows the recirculation rate for the thickening operation to be separately adjusted from the dewatering operation.

In common with the gravity belt thickener, the belts are subject to cleaning with water spray to limit the plugging of the filter belt pores. The waste washwater is combined with the filtrate.

The overall performance of the BFP is dependent on the feed sludge source and characteristics and the dosing conditions. As with all other thickening and dewatering technologies, primary sludges – both raw and anaerobically digested – are more readily filtered than waste activated sludge (WAS).

Dewatered sludge generated from a belt filter press
Dewatered sludge generated from a belt filter press
Dewatered sludge from the outlet of a belt filter press

Belt filter press Alfa Laval

Belt filter press Alfa Laval Source: Alfa Laval / YouTube
Table 1. BFP dewatering DS concentration of feed and dewatered sludge (cake), polymer dose, liquid and sludge solids and solids recovered (Metcalf & Eddy, 2014; Andreoli et al, 2007)
Sludge origin % DS content − Feed % DS content − Cake / Typical % DS content − Cake / Range Loading/m belt width − liquid m3/h Loading/m belt width − solids kg/h Polymer dose − g/kg DS
Waste activated sludge (WAS)1−21612-2011−23180−3405−10
Sequencing batch reactor (SBR)1−21612-1911−22250−3605−7.5
Membrane bioreactor (MBR)1−21511-1816−25230−3205.5−10
Anaerobically-digested (AD) primary2−52824-3514−37680−9102−5
Anaerobically-digested WAS2−32013-236.6−20230−4104−10
Mixed, primary + WAS3−52315-259.0−27340−8203−5.5
Mixed, AD primary + WAS2−42415-289.0−27320−5404−8.5


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

'Belt filter presses for sludge dewatering' was written by Simon Judd

This page was last updated on 28 June 2022


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