Lamella clarifier guideline – tube settler design and CFD simulation (Video animation)

Apart from the selection of the right lamella clarifier module the design of lamella clarifier tanks requires several design considerations. More details on the selection of the right lamella clarifier module can be found in the linked video.

Other parts of a lamella clarifier tank are the water inlet, sludge settling and water outlet zone. The most important design guideline is to achieve an equal flow inside the lamella clarifier tank and to reduce turbulences so that the maximum number of solids can be settled down in the lamella clarifier modules.

Therefore, the water inlet should be as big as possible to allow the incoming water to slow down and to spread out. Ideally is an overflow edge as shown in this picture. Water enters the tank over a wide opening and is then directed into the open tank area. Water molecules close to the overflow plate slow down due to frictional forces so that water molecules with a longer distance can catch up.

Sharp overflow edges in comparison can create a high velocity water stream. To reduce the flow velocity further, deflector plates can be installed in the inlet zone.

The design goal of the sludge zone is to collect all settled particles and to prevent that settled sludge is carried out by the incoming water flow. This can be achieved by using sludge hoppers and other geometries but more important is an adequate distance between sludge settling zone and bottom of lamella clarifiers.

If the distance between lamella clarifiers and sludge settling zone is too short the water flow accelerates and carries out sludge particles.

Another trick for a more equal flow pattern in the sludge settling zone is to install the lamella clarifiers against the direction of the flow. In this way the water stream must change its direction to flow trough the lamellas which reduces the water velocity.

Finally, effluent launders in the outlet zone have to be big enough to carry out clean water at a moderate flow velocity and their overflow weirs have to ensure an equal water outlet. There is a fine line for the correct sizing of effluent launders. If they are too big, they increase the overall cost for the lamella clarifier tank. If they are too small the flow velocity in the channels will be very high which can create an unequal water level in the launders or even worse the effluent launders will not be capable of handling the incoming water volume and spill over.

An unequal overflow over the launder weirs causes water streams and turbulences within the lamella clarifiers because more water has to flow trough the lamella clarifier channels in a certain area. Consequently, particles in these channels will not be settled down but carried out. To fix the issue effluent launders should be equipped with adjustable overflow weirs. Then overflow weirs can be adjusted according to the water level in the lamella clarifier tank and ensure an equal water outlet.