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Tube settler vs. plate settler - Best option for clarifier capacity improvement

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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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Tube settler vs. Plate settler - Clarifier capacity improvement options

One of the first technology for drinking water and wastewater treatment is sedimentation.

Conventional sedimentation often utilizes large basins called clarifiers to settle out suspended solids by gravity. However, depending on solid density and size, settling by gravity might take a long time and requires therefore large clarifiers.

In 1904 it was discovered by Allan Hazen that the sedimentation process in clarifiers can be significantly improved if the settling path of the solids is reduced. Please click on the pop up banner for more information on the Hazen theory.

To decrease the settling path of solids multiple surface layers need to be added to the clarifier basin. To prevent solid build up and clogging the surface layers need to be inclined so that settled solids can slide down to the bottom of the basin and carried out. If the layer slope is too small solids wont slide down however if the layer slope is too large the solid settling path is increased and less solids will settle out. The ideal angle of the surface layers is between 55 and 60 degree.

The surface layers are provided by tube settler or plate settler equipment. Tube settlers are usually made of PVC plastic and consist of multiple corrugated sheets which are glued together to form rectangular blocks with multiple channels. Plate settlers in comparison usually consist of stainless steel plates which are assembled into large modules. Therefore tube settlers have the advantage of being lightweight compared to plate settlers which makes their handling much easier especially during transportation and installation.

Additionally, tube settler block dimensions can be configured to any basin size and geometry which makes them also suitable for existing covered, underground, or round tanks. Instead, plate settler modules are usually too large to fit into existing covered or underground tanks and they cannot be used in most round tanks.

Equal flow distribution is very important for both tube settles and plate settlers to ensure that 100% of the added clarifier capacity is used and bot bypassed. Larger gaps in tube settlers or plate settlers can cause short circuiting of flow so that solids bypass the settling areas and are carried out with the effluent.

Tube settlers have in general an advantage over plate settlers regarding equal flow distribution. Because tube settlers consist of several smaller channels the water flow is split between each channel resulting in similar flow velocities throughout each channel of the tube settler. However, plate settlers consist of large layers which favor zones of higher flow velocities (short circuiting) and zones of little or no upward flow velocity at all (Deadzones).

If this happens the settling surface area of plate settlers is not fully utilized so that the clarifier capacity can decrease significantly.

Some plate settler manufacturers are aware of this problem and try to overcome the short circuiting by reducing the channel sizing (flow control angles) at the top of the plates to limit water flow in certain areas.

Advantages of plate settler are in general a better product robustness and less risk of clogging because of larger settling planes.

Also, plate settler modules can be made higher compared to tube settler blocks which can reduce the required footprint of new clarifier basins.

Finally, tube settlers have a much lower capital cost compared to plate settlers, however depending on the application plate settlers can have a longer lifetime.

AET provides both tube settler and plate settlers so depending on the customer situation and requirements the matching product can be provided.

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