Sludge Dewatering: Decanter Centrifuge vs. Filter Press & Belt Press

Sludge dewatering is the step of removing water from sludge after biological or physico-chemical treatment, to reduce volume and lower transportation and disposal costs. Three different working principles:

The decanter centrifuge uses centrifugal force (G-force): a conical-cylindrical bowl rotates at high speed while the scroll (conveyor screw) rotates at a differential speed to push solids toward the conical end, with the liquid phase overflowing from the other end. The process runs continuously, enclosed within the machine body.

The filter press pumps sludge into chambers between filter cloth-covered plates, where pressure forces water through the cloth, retaining solids to form a sludge cake. This is a batch process: feed – press – open plates – discharge cake – clean.

The belt press spreads sludge onto a continuously moving filter belt and squeezes it between two belts through a roller system to extract water. The system is typically open, easy to observe but also prone to odor dispersal.

Sludge cake dryness (solids content) is a key criterion as it determines the volume of sludge requiring further handling. For the decanter centrifuge, dryness depends on G-force, the differential speed between the bowl and scroll, and flocculant polymer dosage. Increasing G-force and optimizing polymer dosage yields drier cake from the same machine.

The filter press typically produces very dry sludge cake due to high pressing pressure and extended batch hold time, making it suitable when the goal is maximum volume reduction and batch handling is not a concern. The belt press delivers moderate dryness in exchange for low power consumption and straightforward operation.

One important shared requirement: all three technologies require flocculant polymer to flocculate sludge particles before separation. The wrong polymer type or incorrect dosage results in wet cake, turbid filtrate, and excessive chemical use — applicable to decanter, filter press, and belt press alike.

Equipment purchase price is only one component. Annual operating costs truly reflect the total cost of ownership:

• Decanter: runs continuously and automatically via PLC, requires minimal operator attendance, and the enclosed system limits odor within the facility. The trade-off is high-speed rotating components that require periodic maintenance.
• Filter press: each batch requires an operator to open the plates, remove the sludge cake, and clean the filter cloth — labor-intensive with downtime between batches.
• Belt press: low power consumption but the open system readily disperses odors; the filter belt requires continuous washing and periodic replacement.

For wastewater treatment stations located near residential areas or within food processing plants, odor containment is typically prioritized, tipping the balance toward enclosed systems such as the decanter.

No single technology suits every situation — the right choice must be based on sludge flow rate, sludge characteristics, available space, and odor requirements. General guidance:

For stations requiring continuous and automated sludge dewatering, the decanter centrifuge is a commonly selected option thanks to its combination of enclosed system, compact footprint, and high automation level. Refer to the wastewater treatment solutions page to place the dewatering equipment in its correct position within the process line.