Belt filter presses for sludge dewatering

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18 June

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

Credit: 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

Belt filter press Alfa Laval

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Table 1. BFP dewatering DS concentration of feed and dewatered sludge (cake), polymer dose, liquid and sludge solids and solids recovered (Metcalf & Eddy, ; Andreoli et al, ) 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 DSPrimary4&#;-&#;&#;.5&#;2.5Waste activated sludge (WAS)1&#;-&#;&#;&#;10Sequencing batch reactor (SBR)1&#;-&#;&#;&#;7.5Membrane bioreactor (MBR)1&#;-&#;&#;.5&#;10Anaerobically-digested (AD) primary2&#;-&#;&#;&#;5Anaerobically-digested WAS2&#;-236.6&#;&#;&#;10Mixed, primary + WAS3&#;-259.0&#;&#;&#;5.5Mixed, AD primary + WAS2&#;-289.0&#;&#;&#;8.5

Belt filter press working principle and operation

The belt filter press working principle and operation can be divided into four important stages: pretreatment, gravity dewatering, pre-pressure dewatering in the wedge zone and press dewatering. It is an environmentally friendly filtration product with reliable performance, stable work, low water content of finished products, low start-up consumption, relatively simple management control and reasonable price. And belt sludge dewatering machine is less affected by the fluctuation of sludge load. At present, many sewage treatment plants mostly use belt filter.

Belt filter press working principle

1. Thick dehydration: chemical pretreatment stage

The belt filter press has higher requirements for flocculation effect. This machine uses a unique dosing mixer to achieve rapid flocculation and the best flocculation effect.

2. Gravity dewatering: gravity concentration dehydration stage

The sludge is evenly fed into the filter belt through cloth bucket and runs forward with filter belt. The free water flows into water tank through the filter belt under the action of its own weight. The main function is to remove the free water in sludge, reduce the fluidity of sludge and prepared for further extrusion.

3. Low/medium pressure dewatering: wedge preloading dewatering stage

After gravity dewatering, it is still difficult to meet the sludge flowability requirements of the press dewatering section. After this stage of slight extrusion dewatering, the sludge is smoothly pressed and dewatered.

4. High pressure dewatering: the pressurization dewatering stage

The filter cake is repeatedly pressed and sheared around the press roll by the upper and lower filter belts, and a large amount of capillary water is removed to gradually reduce the moisture of filter cake.

Belt press machine is composed of two upper and lower tensioned filter belts, which is bent in an s shape from a series of regularly arranged roller cylinders, and is pressed against the sludge layer by the tension of filter belt itself to form the shear force to squeeze the capillary water in the sludge layer to obtain a mud cake with high solid content, thereby achieving sludge dewatering. The belt type filter press dewatering machine is widely used in the field of sludge dewatering because of its low moisture content and stability, low energy consumption, and simple management control.

Belt filter press operation

1. Turned on the air compressor and ensure the Kia be controlled within the specified range.
2. Open the filter cloth cleaning water, and the amount of water should not be too small.
3. Turn on the vacuum pump and vacuum pump circulating water.
4. Turn on the filter, the speed should not be too fast at the beginning, wait for the vacuum adsorption filter cloth, then adjust the speed appropriately.
5. When there is vacuum, open the lowering valve (there is a little vacuum to open the lowering valve).
6. Observe the thickness of material at the lower end and adjust the belt speed. If the material is too thin, slow down the speed; if it is too thick, adjust the speed and try to make the thickness of the material not controlled at 2-3 cm.
7. When shutting down, first close the blanking valve, then wait for the filter to idle for 5 minutes. When the filter cloth is washed, turn off the filter, then stop the water and stop the gas.

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