Sedimentation and Other Clarification Processes - Public Water Systems - page 320

plicable primarily to small plants (1 mgd [3.8 ML / d] or less in capacity) and is often
used in package plant systems (described in Chapter 25).
Steeply Inclined. Sediment in tubes or on plates inclined at angles in excess of 45
(normally 45 to 60 ) does not accumulate; rather, it moves down the tubes and even-
tually exits into the plenum below (see Fig. 11-11). A flow pattern is established in
which the settling solids are trapped in a downward-flowing stream of concentrated
solids, as shown in Figure 11-12. The continuous sludge removal achieved in the
steeply inclined tubes or plates eliminates the need for drainage or backflushing of the
tubes for sludge removal. The advantage of shallow settling depth coupled with that
of continuous sludge removal extends the range of application of this principle to
installations with capacities of many millions of gallons per day.
Various manufacturers have developed alternative approaches for incorporating
steeply inclined tubes into a modular form that can be built economically and can
easily be supported and installed in a sedimentation basin. One type of modular con-
struction is shown in Figures 11-13 and 11-14, in which the material of construction
may be polyvinyl chloride (PVC) or alkyl benzene sulfonate (ABS) plastic. Extruded
PVC or ABS channels are installed at a 60 inclination between thin sheets of PVC
or ABS. Inclining the tube passageways rather than the entire module enables the
rectangular module to be readily installed in either rectangular or circular basins. If
the direction of inclination of each row of the channels forming the tube passageways
alternates, the module becomes a self-supporting beam that needs support only at its
ends. The tubular passageways provide approximately 2 to 4 in. 2 (0.0013 to 0.0026 m 2 )
of cross section and are available with inclined passageway lengths of 24 in. (0.61 m),
36 in. (0.91 m), and 48 in. (1.2 m).
Other manufacturers use rectangular channels similar to the construction shown in
Figure 11-13 but with all of the channels inclined in the same direction.
All of the systems just described are used in configurations in which the influent
is introduced beneath the tubes and the flow passes up through the tubes. In the
Lamella-type separators, which were initially developed at the Chalmers University of
Fig. 11-12. Liquid versus sludge flow pattern in steeply inclined settler tubes (From Culp, Gor-
don, and Williams, Robert, Handbook of Public Water Systems. Copyright 1986 by John Wiley
& Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)