Sedimentation and lamella clarifiers

Particles in water and waste water accelerate towards the tank bottom as long as the gravity force is bigger than the sum of all drag forces. The settling velocity of the particle is Vs = H/t. In reality the settling velocity is never constant as other particles, water turbulences and sludge accumulation hinder the settling process. Additionally particles differ a lot in size and densities which results in different settling velocities. A practical experiment to define the settling velocities and to calculate an overflow rate for a plant is the Imhoff cone test. The Imhoff cone is filled with a water sample and the settled sludge is analyzed at different levels and time intervals. AET can provide equipment and support for a Imhoff cone test (Here). Adding flocculants will increase the particle size from submicroscopic microfloc to visible suspended particles. When microflocs collide they bond together and form larger, visible pinflocs. More collisions and interaction with added coagulants (inorganic or organic polymers) will form Macroflocs with an increased settling rate.

To determine the size of a sedimentation tank two criteria based on findings by Hazen have to be considered:

1) Sedimentation velocity of the particles Vs

2) Surface loading of tank (qa) defined as Flow Q / Tank Area

To achieve sedimentation the velocity of the particle must be higher than the surface loading of the tank. (Vs > qa)

Consequently, for a given water flow and settling time of the particles settling surface is the only variable. Therefore, the only possibility (except coagulation, flocculation or sludge recirculation) for improved settling performance is to increase the settling surface. This is solved by installing lamella clarifiers into the tank. Lamella clarifiers consist of rows of 60degree inclined plates with a distance normally between 40 and 80mm. To simplify the installation of inclined plates in a settling basin plates are mostly replaced by a tube design nowadays. The tubes are assembled to blocks that can easily be installed in a basin.

As shown in the picture the settling pathway of the particles is now significantly reduced from a few meters to less than 200mm. In other words the settling area of the basin was increased from the tank base to the projected surfaces of all lamella clarifiers. Particles will now be settled as long as the particle sedimentation velocity Vs is higher than the water flow / projected surface (qp). Other than increased surface area lamella clarifiers have more advantages that result in an improved settling performance. The tube structure helps to create a controlled flow pattern in the entire basin which reduces turbulences and therefore drag forces. The 60degree inclination of the lamella clarifiers causes two streams in the tubes – One upward stream of water and one downward stream of sludge.

Also, due to frictional forces, the water velocity gets slower the closer it gets to the tube surface. That means the influence of the water stream will have a decreasing effect on the particle settling in this areas. Directly above the surface is a zone of almost stagnant water. Particles in the stagnant water zone can freely slip downwards only controlled by gravity and its own friction. There is also a friction between the particles and the tube surface which has a very important influence of the lamella clarifier design. Accumulated bigger particles can push smaller particles downwards and increase their sliding velocity.

Overall lamella clarifiers are a very effective way to increase settling performance when they are designed properly. AET provides consulting services and product solutions for lamella clarifiers. Feel free to contact us here for more information.