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7.30 CHAPTER SEVEN

ing on water quality and temperature. Selection should be based on an understanding of
flocculation principles. The designer should refer to Chapter 6 on flocculation for addi-
tional information.

Basin Dimensions. Slurry recirculation units are center-fed units with uniform collec-
tion of clarified water across the surface. To provide good flow distribution, units are gen-
erally circular with a diameter up to about 150 ft (46 m). Square tanks can take advan-
tage of common wall construction, but they generally have problems associated with sludge
removal from comers.
Side water depth, especially center depth, is an important design parameter to ensure
adequate space for the mixing zone, uniform distribution of slurry to the basin clarifica-
tion zone, and transition flow vertically to effluent collection troughs. Basin depth is usu-
ally established by proprietary equipment manufacturers' requirements.
InfluentDesign. Influent design is based on requirements for mixing chemically treated
source water with previously formed flocculated and settled solids. To achieve this mix,
flow is introduced into a recirculation drum or zone at the center of the unit.
Flow is then introduced to a flocculation reaction zone in a center well separated from
the clarification zone by a hood. The design of this area varies depending on the manu-
facturer. The hood is designed to control flow uniformly to the clarification zone. The de-
signer should work with the manufacturer to select equipment most suitable to the appli-
cation and to obtain necessary dimensions for basin structural design.

Effluent Design. Clarified water is collected in radial launders to maintain a uniform
vertical upflow within the settling area to help reduce short-circuiting. Launders are de-
signed with low loading rates, typically about 20 gpm per linear foot [248 (L/min)/m],
which sets the spacing. Spacing is usually about 15 to 20 ft (4.6 to 6.1 m).
Launders are designed with submerged orifices to overcome the problems of trying to
level many weirs to the same elevation. Small changes in orifice elevation do not greatly
affect flow distribution, but small changes in weir elevations have a major impact on flow.

Solids Removal. Most slurry recirculation units use a rake rotating around a center col-
umn to plow settled solids toward the center, where they are deposited in an annular hop-
per for discharge to waste. Some solids are picked up by the recirculating flow to be mixed
with influent. The rake consists of a steel truss with squeegees and is supported from the
rotating recirculation drum. Rake arms must be designed to withstand the torque caused
by the weight of the solids being moved and by friction forces. The rake is driven by a
center drive assembly similar to equipment for conventional circular clarifiers.

Equipment
Basin equipment consists of a conical reaction well, recirculation drum, influent pipe, and
influent baffle. The equipment is generally mounted on a bridge extending across the basin
and designed to act as a walkway for access to the center drive unit. Dual drives are pro-
vided, one to drive the sludge collection rakes, the other to drive the turbine mixer used
to recirculate slurry and provide flocculation.
In large-diameter tanks, equipment is supported from a center column, and a bridge is
provided only to access drive units. Some large-diameter tanks use peripheral drive units
with rakes supported from the walls and a center column.