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Tube settler comparison - best lamella clarifier for high particle settling velocity

Lamella clarifier types - Advantages of different channel designs

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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

Allen Hazen defined in 1904 the law for sedimentation: Vs – settling velocity of a particle divided by settling height – has to be bigger as – Water flow Q divided by tank volume.

Applied to a lamella clarifier this means that a particle is only settled down when the settling velocity of the particle divided by channel height, is bigger than the water velocity divided by the channel volume. Vs of the particle must be bigger than the upward stream Q. Case 1: Vs divided by H is bigger as Flow Q divided by channel volume. The particle can be settled down.
Case 2: Vs divided by H is smaller as Flow Q divided by channel volume. The particle cannot be settled down.

When the channel distance of the plates is equal, H can be crossed out of the equation. Then L multiplied W projects the surface area of the lamella clarifier module in horizontal direction. Lamella clarifiers are available in many different designs such as parallel plates, chevron shaped, diamond, octagon or triangle shape, and circular shape.

Parallel plates as well as chevron shape designs have an equidistant channel distance in common. Instead the channel distance of octagon, diamond and circular shape designs is not equidistant. Some manufacturers of octagon, diamond or circular shaped Lamella clarifier types claim more projected surface area compared to parallel plate or chevron shaped designs. However the octagon, diamond or circular shape design violate the law for sedimentation. As we found earlier, the projected surface area can only be calculated if we can cross out H. Therefore, the actual projected surface area of octagon, diamond or circular shape designs results from the average plate distance. That means the projected surface area of every lamella clarifier design is pretty much the same. The projected surface area depends on the plate distance… and plate inclination.

Each channel design shows different patterns of sludge accumulation. Whereas parallel plates and chevron shaped channels show an equal sludge accumulation…other shapes show an unequal sludge accumulation. The unequal sludge accumulation is a consequence of not equidistant channel design. Particles at the edges of the channel have a shorter settling path compared to particles in the middle of the channel. The result is that more sludge accumulates in the narrow edges of these channels which increases the risk of clogging.

Another important factor for the design of lamella clarifier tanks are turbulences of the incoming water. Lamella clarifiers with a chevron shape or similar design consist of several channels within one module. But parallel plates usually consist of only 2 plates per one module row.

The large number of channels help to reduce turbulences in the water and support an equal flow through the lamella modules with less short-circuits. In comparison the design of parallel plate settlers does allow short-circuits through the lamella clarifier without reducing water turbulences. The consequence is that particles are carried out because of higher upward forces.

All in all, parallel plates and chevron shaped lamella designs have advantages over other lamella shapes regarding sludge accumulation. The projected surface area does not depend on the channel designs but only on the plate distances and inclination.

The channel structure gives all modular lamella clarifiers an advantage in regard to water turbulences and compared to plate settlers. The highest stability have parallel plates and modules that consist of extruded profiles such as chevron shaped Lamella clarifier types.  

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