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CLARIFICATION 7,39

formly spaced pipes. Recycle flow must also be uniformly distributed. When pipes are
used for flocculated water, recycle flow is sometimes introduced into pipes just before the
outlet.

Effluent Design
Clarified water is collected from the tank bottom by being forced under an end baffle wall
or by pipe laterals or cast-in-place concrete flumes on the basin floor. In either case, flow
must then be discharged over a control weir to maintain a stable water surface in the clar-
ifier. The weir may be adjustable to fine-tune the water surface level. This is especially
important when scrapers are used to remove the float.

Floated Solids Removal

Floating solids (called float) are removed from the surface by mechanical skimming or
by hydraulic flooding. Mechanical skimming equipment must be maintained, but it allows
the float to be removed at a dry solids content of 3% to 6%. At this concentration it may
be possible to eliminate or at least reduce thickening equipment required for mechanical
dewatering.
Flooding eliminates mechanical equipment in the flotation tank but results in a dilute
solids stream of around 0.5% or less. This float removal method could create a need for
larger thickening equipment.

Mechanical Skimming. Three common types of mechanical skimming equipment are
the chain-and-flight skimmer, oscillating skimmer, and rotating skimmer.
Chain-and-Flight Skimmer. A typical mechanical skimming device is the chain-and-
flight type, similar to those used to scrape settled solids in gravity sedimentation basins.
In this case, however, flights are equipped with rubber squeegees or nylon brushes that
drag the float along the water surface and up onto a dewatering beach. As solids are
dragged up the beach, water is squeezed out and drains away, further thickening the float.
The float then drops off the beach into a hopper from which it flows by gravity or is
pumped to waste or thickening.
Solids content of the float is controlled by how often it is removed and by how much
excess water is dragged along with the solids. Adjusting the flight depth into the water is
critical to the amount of excess water and to treatment efficiency. If the moving flights
are set too deep, they can create flow pattems in the basin that will cause poor particle
removal and floc carry-through to the effluent. If the flight is set too shallow, it can dis-
rupt the adhesive forces in the float blanket. This disruption causes solids to settle back
into the tank, resulting in floc carry-through.
The skimmer is usually operated on a timer, with the frequency of operation based on
solids production and the desired level of thickening. All mechanical components are
above water, making maintenance easier than that for gravity sedimentation basins.
Oscillating Skimmer. This skimming device consists of a carriage-and-blade assem-
bly that is pushed and pulled across the surface by a gear motor. Blade spacing is typi-
cally 5 to 7 ft (1.5 to 2.1 m), and the mechanism covers two-thirds of the flotation area.
The lead carriage, closest to the sludge beach, is equipped with a profile-duplicating arm
that allows the skimmer blade to exactly duplicate the shape of the beach. The beach can
be either flat or curved.
All moving components are located above water, which allows easy inspection and
maintenance. The gear motor oscillates a drive rod back and forth in 15-in. (38-cm) in-