LAMELLA CLARIFICATION: DESIGN-APPLICATION
Charles B. Grimes, Industry Manager
Evan K. Nyer, Manager/Testing and Development
Ecodyne Corporation
Industrial Waste Treatment Division
Union, New Jersey 07083
INTRODUCTION
The theories behind lamella separation have been around for a long time. Most
people refer to a paper by Hazen in 1904 as the original theoretical evaluation of
lamella separation. In recent years, many research groups have refined the original
theories, and several manufacturers now produce commercially available sedimentation
units based on lamella separation.
As shown in Figure 1, the settling efficiency of each lamella is equivalent to the
horizontal projection of the lamella. Since the lamella are packed closely together, their
projections will, in effect, overlap. The projections are, of course, merely conjectural and
not physically present in this system. To build a conventional sedimentation system with
the same efficiency, you would have to spread out the theoretical settling surfaces side
by side. The result would be a settling tank whose total length would be up to 10
times the length of an equivalent lamella system. Figure 2 represents the volume saving
a lamella sedimentation unit can have over a conventional sedimentation unit.
While this technology provides tremendous volume savings, it also adds many variables
to the sedimentation process. Plate angle, plate size, length-to-width ratio, spacing, plate
support, flow direction, flow control, scale up, overflow rate, solids loading, and many
others have to be considered in the design of a lamella unit. Years of experience have
been able to eliminate most of these variables from most applications. Pilot plant and
full scale data have provided a superior design for the application of the lamella concept.
Simple laboratory tests to determine solids settling rate and sludge volume index are all
that is needed to design a lamella unit. These well-known tests can be related to the
three main variables needed in the design of a lamella unit; surface load, plate spacing,
and sludge draw off.
LAMELLA DESIGN
Flow Direction
The lamella sedimentation is a method of separating solid particles from a suspension
flowing between inclined plate-lamella (Figure 3). By the inclination, the separated
particles settling on the lamella will slide down into the sludge hopper below the
lamella plate. When analyzing the method, theoretically, Stoke's law and Hazen's theory
of surface load can be applied. The lamella sedimentation insures laminar and stable
flow conditions throughout the sedimentation equipment, which leads to a very high
degree of separation. Reynold's numbers have calculated at less than 100, which is considerably less than the 500 mentioned as a limit of laminar conditions. Thus, there is no reason
to adjust sedimentation efficiency because of turbulence, except at the inlet zone.
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