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Inclined Plate Clarifier (Parallel Plate Clarifier, Lamella
Clarifier) Basic Theory of Operation
The Inclined Plate Clarifier, or
Lamella, was first described and quantified by Hazen in 1904.
The Lamella consists of a series of closely spaced flat plates
inclined at an angle of from 45 to 60 degrees from horizontal.
Preconditioned water with entrained solids enters the plate pack and flows
between the plates. The path length,
plate spacing, and angle of the plate are the usual engineering
variables. As the water flows between pairs of plates, the heavy
solids with a specific gravity higher than the surrounding water
will settle onto the top surface of the lower plate, and slide down
the inclined surface to be collected in the sludge hopper. Clear,
near solids-free water then exits the top of the plate area and
flows over an adjustable weir.
The
settling pattern of a discrete particle in a rectangular basin is
that, as the particle settles, it is carried forward by the velocity of
the liquid flow through the basin. Thus, if Vf is the
velocity of fluid, Vs the settling velocity of the
particle, L the length of the basin, and d its depth, then a
particle at the influent will settle to the bottom of the basin only
if:
Vf
max = Vs L/d
Therefore,
the flow velocity at which a basin can operate successfully is
directly proportional to the length of the basin, and inversely
proportional to the depth.
This
scenario applies to horizontal plates as well as rectangular basins.
In horizontal plates, a particle must fall a few inches. In a
rectangular basin, a particle must fall 10 to 20 feet. The more even
the flow the more efficient the settling process.
Horizontal
plate settlers are spaced 1 - 2 inches apart; therefore, the
particle only needs to travel vertically 1 - 2 inches. The settled
solids must be removed mechanically from the horizontal plates.
Inclined
plate settlers are positioned on a 55-60 degree angle to allow
the settled solids to slide down the plate and to the bottom of the
basin. In this case the distance the particle must travel vertically
is:
D=
distance the particle travels vertically
d= plate
spacing
Cos µ = Cosine of plate angle (550)
D = d
, D = 2
Cosµ
.574
D = 3.48"
Since the particle sees
the horizontal area, horizontal plates are calculated as the actual
settling area. Inclined plate settlers use the horizontally
projected area and, therefore, the total plate area may be calculated
by:
At
= HP
Cosµ
HP =
Horizontally projected area
Cos µ = Cosine of plate angle (550)
At
= Total ft2 of plate surface required
Because each plate
settler provides an effective settling area equal to that of its
horizontal projection, MRI Plate Settlers will increase a basin's
effective settling area by up to ten times.
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