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7.40 CHAPTER SEVEN

crements while engaging the skimmer carriages. When the carriage reaches the effluent
end and the profile blade deposits the float into the hopper, the blades retract above the
water surface. The carriage is then mechanically reversed to return to the influent end in
an indexing manner.
The skimmer unit is constructed of stainless steel and plastic. High-hardness plastics
provide low operating friction, allowing the mechanism to operate efficiently. The pro-
file duplicating blade is typically a nylon brush that aids in sludge dewatering. The brush
allows the sludge to thicken to a slightly higher solids percentage while the blade pauses
inherently during operation on the sludge beach. Maintenance is minimal, consisting of
checking the plastic wear blocks and regular servicing of the gear motor.
Rotating Skimmer. Another type of mechanical skimming device consists of a rotat-
ing shaft with curved or straight blades attached. The shaft is mounted at the effluent end
of the basin above a sludge beach. As the blades rotate, they pull a portion of the float
blanket onto the beach and into a hopper. The general flow of water toward the effluent
causes the blanket to flow up to the beach, where the rotating blades continually remove
a portion.

Hydraulic Removal. Float can be hydraulically removed from the surface of a DAF unit
by flooding. This is accomplished by partially closing the basin effluent gate or in some
cases raising a mechanical effluent weir, which causes increased head loss and a small
rise in the water surface level. This increase in level brings the surface up above a weir
that discharges to a sludge drain. Part of the flow through the basin is now diverted over
this weir, dragging the float blanket along with it.
Adhesion of the sludge particles helps keep the blanket intact as it moves across the
surface to the weir. Water is trickled down the basin walls to break adhesion to the wall
so that the blanket can move freely. Float removal is relatively frequent to prevent the
blanket from becoming too thick because a heavy blanket does not flow well. A solids
content of about 0.5% is best.
When most of the blanket is removed, usually in about 10 min or less, the effluent
gate reopens and operations return to normal. This mode of float removal eliminates all
mechanical equipment in the basins. The disadvantage is that the removed solids are di-
luted and may require more extensive thickening for subsequent mechanical dewatering.

CONTACT CLARIFICATION

Contact clarification resembles filtration more than clarification. Coagulated water floc-
culates within the contact medium, may stick to the medium grains, and builds in size,
eventually clogging the medium, at which point it is backwashed. Because this process is
more filtration than gravity settling, it is capable of operating at rates of up to 20 times
that for conventional settling on low-solids source waters requiring low coagulant dosages.
High-solids waters and high-color waters that require high coagulant dosages quickly clog
the medium, resulting in excessive backwashing. The process is generally marketed as a
two-stage process, along with filters in package plants, but may be used separately to up-
grade existing plants.
Roughing filters have been used for as long as filtration has been around. The contact
clarification process, however, gained popularity with the development in the mid-1980s
of the Adsorption Clarifier by Microfloc, now owned by USFilter. Similar processes are
also marketed by Infilco Degremont, Inc., and Roberts Filter Co. The Adsorption Clari-
tier is shown in Figure 7.22.