Sludge Dewatering and Separation Processes

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When separating solids and liquids from a suspension, various methods can be employed, such as a belt filter press, a dewatering table (or thickening table, belt thickener), a chamber filter press (or plate filter press), or a decanter. The result is a filtered liquid and a solid mass. Examples of suspensions include wastewater sludge, manure, digestate, or dredged material. Below, we will explain the operation of a belt filter press, a dewatering table, and a chamber filter press.

Example of an belt filter press

(Sludge) dewatering with a belt filter press

A belt filter press is a machine that presses a suspension between filter cloths (filter belts), using rollers to build up pressure. This process separates the solid from the liquid. A belt filter press is part of a larger installation, including auxiliary equipment such as a polymer system, pumps (sludge feed, belt wash water, and filtrate discharge), and a cake discharge system.

Applications for belt filter presses can be found in dewatering sludge and slurry in wastewater treatment, soil remediation, mining, the chemical industry, agriculture, and horticulture. Belt filter presses are also used in fruit processing (preparation of fruit juices and wine production). Unlike a (chamber) filter press, a belt filter press operates continuously.

Sludge disposal is a significant operational cost in water treatment. The lower the solids content of the sludge, the more you pay for water disposal. The solids content in organic wastewater sludge can reach approximately 25%; for some inorganic sludges, 60% is possible.

Advantages of a Belt Filter Press

Robust and continuous process
Autonomous machine: operates without operator intervention (system self-cleans)
Large pre-dewatering and pressing zone
Maximum dewatering through crack formation in the sludge cake as it passes through the rollers
Maximum number of rollers in a relatively compact installation (7 to 15 for extra high solids content)
Maintenance-friendly

Operation Filter Belt Press

Separation

A belt filter press separates solid and liquid from a suspension (a mixture of liquid with a powdery substance). This separation process occurs under pressure using a filter cloth or filter belts.

Flocculants

In many applications, flocculants (polymers/poly-electrolytes) are used to clump smaller suspended particles into larger ones, known as flocs. These flocs are easier to separate from the water and are better captured between the relatively coarse filter belts in a press. Before dewatering, the suspension (sludge) is mixed with the flocculant, creating sludge flocs that are easier to separate from the water fraction and pressable. Flocculant is added using a dosing system that is part of the installation.

The flocculated suspension is poured onto the horizontal filter belt. Adjustable guide plates on either side of the belts ensure that the sludge does not slide off. Here, the first separation of sludge and (free) water occurs. So-called “chicanes” distribute the suspension over the filter belt, mixing and turning it, creating a pressable product.

The released water from the sludge, the so-called filtrate, flows through the filter belt due to gravity and is collected in a tray. This filtrate contains a small amount of solids, resulting in a relatively high yield.

Pressing Process

The sludge stream then comes between two filter belts and is compressed under increasing pressure. The actual pressing process occurs next. The filter belts (with the sludge between them) pass through several rollers, further dewatering the sludge. Two scrapers remove the remaining sludge cake from the filter belt (spray nozzles clean both filter belts).

Solid Mass

A belt filter press dewaters until a solid mass remains. A well-designed installation achieves high solids content with low polymer and energy consumption. The belt speed is relatively low. There is also a speed difference between the two filter belts, so besides pressure, shear forces help in dewatering, resulting in a higher solids content.

Example of a (pre)dewatering table or belt thickener

(Pre)dewatering with a dewatering table

Advantages of a dewatering table

Relatively inexpensive
Energy-efficient
Separation efficiency of over 99% for biological sludge
Fully autonomous operation
Maintenance-friendly

Operation Dewatering Table

Pre-dewatering

A dewatering table or belt thickener is used as the first dewatering step, separating solid and liquid from a suspension. This separation process occurs based on gravity (gravity drying step) using filter belts. A suspension is transported on the filter belt where chicanes guide and turn the sludge. The liquid falls through the filter belt, and the more solid particles are scraped and captured at the end of the belt.

After this step, the sludge is mixed with a flocculant, creating conditioned sludge. A dewatering table forms a simplified version of a belt filter press.

schets_ontwateringstafel_principe_engels

Example of an belt filter press

Voorbeeld van een kamerfilterpers

(Sludge)dewatering with a chamber filter press

A chamber filter press or filter press is technically a relatively simple and highly flexible pressing installation suitable for many applications, especially in the food industry and mining. The medium to be filtered is batch-wise pressed through vertically hanging plates with filter chambers. Each plate has a filter cloth. This type of press achieves the highest separation efficiency of all mechanical separation techniques because it can press directly on the product in closed “chambers.” Depending on the suspension and dewatering properties, a chamber filter press achieves solids content in the filter cake of 30% to about 70%.

Advantages of a chamber filter press:

Maximum separation efficiency: dry cake, pure liquid
Robust technology with a long lifespan
A mechanical separation technique where both cake filtration and cake drying can occur together.
Everything in contact with the filtered product can be made of plastic.

Operation Chamber Filter Press

Separation with plates with filter chambers

In a chamber filter press, the sludge is pumped into different plates, which are provided with a permeable woven filter medium on both sides, such as cloth or pin seam. The design of the plate, combined with the filter cloth, results in so-called filter chambers. The filter cloths trap solid particles, and the liquid can pass through the mesh openings, forming a filter cake. As the filter chambers fill with solid material, filling them becomes more challenging, resulting in increasing pressure. To control this, the feed pump is switched off at a preset maximum pressure. This completes the filling phase.

The solid cakes in each plate must then be removed from the filter chambers. After this, a new cycle can begin. The emptying of the filter chambers and repositioning of the plates is a secondary process that, depending on the application, can be automated, allowing the separation process to be continuous

Woven Filter Cloth

Using very strong but finely woven cloth materials with small mesh openings, optimal filtration or fine separation is achieved. This process is ideal for applications requiring maximum purity of the filtrate or the lowest residual moisture content in the solid fraction. The focus can be either the recovery of the solid (filter cake) or the purification of the initial product. The liquid content in the filter cake depends on the physical and chemical properties of the suspension and the pressure with which the suspension is pumped into the (chamber) filter press.

Membrane Filter Press

A membrane filter press can remove more liquid from a suspension than a chamber filter press and generally has shorter cycle times compared to a classic chamber filter press. Membrane filter presses are used for more challenging products to separate or applications where separation efficiency is crucial. The operation of a membrane filter press starts like that of a chamber filter press, with the feed pump first filling the filter chambers to a predetermined maximum pressure. In the next phase, a membrane in each filter chamber is inflated with air or a liquid. This reduces the volume in each chamber, resulting in more liquid being removed from the cake. This significantly increases the solids content of the filter cakes and makes them more homogenous in composition.

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