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CLARIFICATION 7.23

• Sludge blanket clarification
• Dissolved air flotation
• Contact clarification

Tube Settlers
Tube settlers take advantage of the theory that surface overflow loading, which can also
be defined as particle settling velocity, is the important design parameter. Theoretically,
a shallow basin (i.e., short settling distance) should be effective. By using several shal-
low parallel tubes, surface area can be greatly increased and low flow-through velocity
maintained in each tube to reduce scouring.
The first tube settlers were introduced in the 1960s by Microfloc. Typical tubes are 2
in. (5 cm) square, reducing the settling distance from several feet in a conventional basin
to 2 in. (5 cm) or less inside the tube. The large effective settling surface results in a low
overflow rate compared with the area of the tubes, which allows for a smaller basin, shorter
detention time, and increased flow rates.
When tubes are placed at a 60 ° angle, they provide efficient settling and allow for set-
tled solids removal from the tubes by gravity. As flocculated water rises through the tube,
solids settle to the inclined surface, where they gradually gain mass and weight and even-
tually slide down the incline. In this way, a countercurrent flow pattern is developed. As
the solids fall from the bottom of the tubes, they settle to the floor of the basin, where
they can be removed by conventional sludge collection equipment. Localized velocities
caused by thermal currents are damped by the tubes. Likewise, surface wind currents have
little effect because settling occurs within the tubes.
Overall depth of a tube clarifier is usually the same as that of a conventional basin.
This is necessary to provide room: below the tubes for sludge collection equipment, for
uniform flow approaching tube inlets, for the tubes themselves, and for uniform flow dis-
tribution through the tubes up to the collection launders. Figure 7.13 shows a typical tube
settler installation.
Design Criteria. Tube settler clarifiers are designed on the basis of the total projected
surface area of tubes. A loading rate of about 0.5 gpm/ft 2 (1.2 m/h) is typical for
aluminum- and iron-based coagulants, but the loading rate may range from 0.4 to 0.8
gpm/ft 2 (1.0 to 2.0 m/h). A 0.5 gprn/ft 2 (1.2 m/h) rate is equivalent to a loading rate of
about 2 gpm/ft 2 (4.9 m/h) over the top area of tubes. Rates ranging from 1.0 to 3.0 gpm/ft 2
(2.4 to 7.3 m/h) over the top area may be used, depending on the settling characteristics
of the flocculated solid (Neptune Microfloc, Inc., 1980).

Inlet Conditions. For tube settlers to operate with uniform loading, the hydraulics of the
influent and effluent is very important. Influent turbulence adversely affects settling effi-
ciency in two ways. First, high velocities do not allow even flow distribution into the
tubes, important for ensuring equal loading on the tubes. Second, sludge falling from the
tubes must be able to settle to the bottom of the basins. High velocities below the tubes
break up and shear the falling floc, causing it to be resuspended and carried into the tubes.
This overloads the tubes and affects operating efficiency.
To avoid inlet turbulence effects, tube settlers must be placed to create a stilling zone
between the inlet and the settler modules. This stilling zone is usually at least 25% of the
total basin area.
A minimum depth of 10 ft (3 m) is generally provided below the tubes to create low
velocities approaching the tubes, to allow sludge to settle without breaking. This depth
also allows for access to sludge collection equipment.