Lamella Design Program

Lamella Design Program Inputs and Outputs

Lamella Program Inputs
Lamella Program Outputs

Lamella Design Program Algorithm

The Lamella Design Program uses three constraints to determine design values. The critical velocity of 10 m/day, the upward velocity at the bottom of the tank of 70 m/day and the predetermined length of the sedimentation tank are used to calculate the space needed between the lamella. The length of the sedimentation tank is set by the Sedimentation Program. The 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 remain in the water sent through the distribution system for drinking. However, a small critical velocity comes at the expense of a large area (so it is not practical to have an unnecessarily small velocity). The upward velocity at the bottom of the tank is important for sludge blanket formation, too high and the blanket will form to 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. Either of these issues would result in the sludge blanket being detrimental to the sedimentation process.

The program starts by determining the height available for the lamella. After the height available is determined the length of the lamella can be found given an assumed angle of 60deg. In order to find the vertical height available for the lamella, first the height of the water needed above the lamella is found.

The distance below the lamella, H.SedBelowSlope, was found in the sedimentation program and based off of the tank fraction given by the user. The vertical height available for the lamella is simply the remainder of the water depth in the sedimentation tank after the bottom slopes and the space needed above the lamella has been accounted for.

The lenght of the lamella:

The next step is to determine the space between the lamella needed to satisfy the critical velocity given the tank length found previously. In order to determine the space between the lamella the inactive length of the tank and the upward velocity under the lamella must be found first. The inactive length of the tank consists of the space occupied by the inlet and exit channels.

The upward velocity under just the active lamella area is flow through the sedimentation tank divided by the cross sectional area of the active portion of the sedimentation tank.
Upward Velocity under the Active Area below the Lamella:

The perpendicular distance between the lamella is found from the equation below. Typically the lamella spacing should be around 5cm apart. Closer distances are thought to possibly lead to improper floc settling and possible clogging.
Distance from Center to Center between Lamella:

Horizontal Distance Center to Center between Lamella:

The open space between the lamella is the area for water to flow up through the lamella. This spacing is the center to center spacing between the lamella minus the thickness of the lamella material.
Open Space between Lamella:

The number of lamella placed in the tank is determined by the maximum number of lamella that can fit in the tank. This number is function of the geometry of the space available and the following equation is used.
The Number of Lamella:

Without knowing the exact number of lamella that will be placed in the tank it is not possible to calculate the exact parameters of the tank. Now that the exact number of lamella has been calculated, more accurate values of active tank length, upward velocity, and critical velocity up through the lamella can be found. Calculations for these values are shown below.

Active Length of the Tank:

The Actual Upward Velocity at the Bottom of the Tank:

The Critical Velocity Up through the Lamella: