Dear Aruna, A  lamella ...

Dear Aruna,

A  lamella clarifier  or  inclined plate settler  ( IPS ) is a type of settler designed to remove particulates from liquids. They are often employed in primary water treatment in place of conventional settling tanks They are used in industrial water treatment. Unlike conventional clarifiers they use a series of inclined plates. These inclined plates provide a large effective settling area for a small footprint. The inlet stream is stilled upon entry into the clarifier. Solid particles begin to settle on the plates and begin to accumulate in collection hoppers at the bottom of the clarifier unit. The sludge is drawn off at the bottom of the hoppers and the clarified liquid exits the unit at the top over a weir.

There are a number of proprietary lamella clarifier designs. Inclined plates may be based on circular, hexagonal or rectangular tubes. Some possible design characteristics include:

1. Tube or plate spacing of 50 mm.
2. Tube or plate length 1–2 m.
3. Plate pitches between 45° and 70° allow for self-cleaning, lower pitches require backwash.
4. Minimum plate pitch 7°
5. Typical loading rates are 5 to 10 m/h

Lamella clarifiers can handle a maximum feed water concentration of 10000 mg/L of grease and 3000 mg/L of solids. Expected separation efficiencies for a typical unit are:

90-99% removal of free oils and greases under standard operation conditions.

20-40% removal of emulsified oils and greases with no chemical amendment.

50-99% removal with the addition of chemical agent(s).

Treated water has a turbidity of around 1-2 NTU.

The Lamella Design Program uses four constraints to determine design values, the critical velocity of 10 m/day , the upward velocity at the bottom of the tank, the minimum space between the lamella and the predetermined length of the sedimentation tank. All of these constraints come together to determine the length of the lamella. The minimum spacing between the lamella was determine via laboratory experiments, at spacing closer than 2 cm failure occurred. The length of the sedimentation tank is set by the Sadimentation ProgrammeThe critical velocity is the rate at which a particle must fall to ensure that it settles out within the plate settlers. If the critical velocity is too large, flocs will not settle out, and will remain in the water sent through the distribution system. However, a small critical velocity comes at the expense of a large cross sectional tank area (so it is not practical to have an unnecessarily small critical velocity). The upward velocity at the bottom of the tank is important for sludge blanket formation, too high and the blanket will form too thin and will not capture particles, too slow and the blanket will either settle out instead of remaining suspended or the shear value in the blanket will be so high that flocs will get broken up in the blanket

The program starts by determining the height needed for the launders above the lamella. This height needed for the launders is same as the depth of water needed above the lamella. This value is simply a function of leaving enough available head loss through the exit launder about the lamella to keep it properly submerged.

1. Flow entry:

The flow enters from both sides of the plate. Distribution and entry velocities are minimum to optimise the hydraulic flow regime, resulting in full plate utilisation, maximum efficiency and better effluent quality.

2. Weir take-off: A weir launder provides an effective weir length. The weir has orifices on either side of each plate.

3. Removable plates: Individual plates are easily removable even during operation, making the unit very simple to maintain without shutdown. The design offers flexibility to handle changes in influent characteristics.

4. Hopper arrangement: Several options are available for sludge. The standard arrangement is a hopper bottom with a structural support frame. A second option is to mount the lamella clarifier on top of a thickener in order to achieve a higher solids concentration, while providing a large sludge volume.

Regards,

Prem Baboo