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Tube settler design basics - Solid settling with lamella clarifiers

Lamella clarifiers - Increase settling performance

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Karl-Uwe Schmitz
Karl Schmitz is a consultant and sales agent for FBBR, diffused air and lamella system solutions. We are working together with the lead manufacturers for drinking water and waste water components in the United States.
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Idrica

Already in 1904 Allen Hazen defined the laws of sedimentation. He noticed that a particle in water can only settle down when its sedimentation velocity Vs is higher than the result of Flow Q divided by tank area A. Q divided by A is called surface loading qa.

Except with flocculation the sedimentation velocity Vs of particles and Flow Q cannot be changed. By increasing the only variable - tank area – we decrease the surface loading qa and therefore increase the settling performance. To increase tank area, more horizontal surfaces have to be installed. Obviously this is not practical so that horizontal plates are replaced by inclined plates. The additional surface area of the inclined plates is created by their projection in horizontal direction.

Nowadays settling plates are replaced by plastic modules because they have a lower cost, are easier to maintain and to install. Lamella clarifiers increase the settling area by 5 to 10 times! Also advantageous is that incoming water will be slowed down and redirected to flow between lamella plates which improves the flow pattern.

Another advantage is that inclined plates result in a countercurrent flow – one upward flow of water and one downward stream of sludge. As in a water pipe frictional forces get higher as closer water gets to the pipe surface. Therefore particle settling and sludge accumulation is improved because drag forces are reduced by frictional forces.

In total lamella clarifiers are a great solution to upgrade existing sedimentation tanks. Compared to conventinal sedimentation tanks the installation is significantly faster and the total cost is about 1/3.

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