Decanter Centrifuge - less is sometimes more

Decanters are one of the most common centrifuges in the industry. They are often used for sludge dewatering in municipal plants. A decanter is characterized by a rotating bowl and scroll while the scroll operates with a small differential speed to the bowl.

Fundamentally three process steps happen in a decanter simultaneously: Sedimentation of the particles, Conveying of the sludge and Dewatering of the sludge on the conical part of the centrifuge. The usually adjustable differential speed controls the duration of the sludge in the machine and consequently the throughput and performance.

If the sludge would be compressed and acting like a block, higher differential speeds would lead to higher throughputs. However, in practice the decanters are often operating with too high differential speeds. Less differential speed is sometimes more. Critical is the conveying part of the sludge. If the differential speed is too high the sludge is not compressed enough to be easily conveyed along the conical part.

In this case the sludge slides back and increases unnecessarily the residual sludge volume in the machine. Unfortunately particles in the filtrate and residual moisture of the sludge go up too. Of course if your differential speed is much too small you loose performance too. If you have a sludge decanter running try to reduce the differential speed - usually there is a peak for throughput and separation performances depending on the differential speed.

[1] Keller K., Centrifugation in the Chemical Industry, AFS Annual Meeting 2002

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