Page 408 - Water and wastewater engineering

SEDIMENTATION 10-25

second stage. These flocculate with the preformed turbidity floc. After flocculation, the ballasted
floc is settled in a high-rate settler, and the sludge is pumped to a hydrocyclone where the mic-
rosand is recovered for reuse.
The surface loading rate ranges from 35 to 62 m/h. The floc settling velocities are 20 to
60 times greater than conventional sedimentation. This allows a reduction in detention time to
between 9 and 10 minutes. The sludge contains 10 to 12 percent sand by weight.

10-4 SEDIMENTATION BASIN DESIGN
Rectangular tank and high-rate settlers are the primary focus of this discussion. Because ballasted
sedimentation is a proprietary process, the design is under the control of the manufacturers. The
design of this process will not be covered.

Rectangular Sedimentation Basins
At a minimum, to provide redundancy, two tanks are provided. These are placed together, in
parallel, with a common wall. In general, four tanks are preferred (Willis, 2005). This provides
maximum flexibility in operation over a wide range of flow rates with allowance for one unit be-
ing out of service at the maximum flow rate. Four tanks, however, may result in an unacceptable
capital cost.

Inlet Zone. The preferred arrangement is a direct connection between the flocculation basin
and the settling tank. The diffuser wall between the two tanks is designed using the same proce-
dure that was used for baffle walls in flocculation tanks (Chapter 6).
When the flocculated water must be piped to the settling tank, the flow velocity commonly
used is in the range of 0.15 to 0.6 m/s. This velocity must be reduced and the flow spread evenly
over the cross section of the settling tank. A diffuser wall is the most effective way to accomplish
this. The design process is the same as that used for baffle walls in flocculation tanks (Chapter 6).
The diffuser wall is placed approximately 2 m downstream of the inlet pipe. The headloss
through the holes should be 4 to 5 times the velocity head of the approaching flow. Port velocities
typically must be about 0.20 to 0.30 m/s for sufficient headloss. The holes are about 0.10 to 0.20 m
in diameter spaced about 0.25 to 0.60 m apart. They are evenly distributed on the wall. The lowest
port should be about 0.6 m above the basin floor (Willis, 2005).

Settling Zone. Overflow rate is the primary design parameter for sizing the sedimentation ba-

sin. Typical overflow rates are given in Table 10-2 . These rates are usually conservative enough
that the inlet zone does not have to be added to the length calculated for the settling zone. If the
overflow rate is based on pilot studies, then the length of the inlet zone is added to the length
calculated from the overflow rate.
In theory the sedimentation basin depth [also called side water depth (SWD)] should not
be a design parameter because removal efficiency is based on overflow rate. However, there is
a practical minimum depth required for sludge removal equipment. In addition, depth may be a
controlling parameter to limit flow-through velocities and/or scour of particles from the sludge
blanket. Basins with mechanical sludge removal equipment are usually between 3 and 5 m deep
(MWH, 2005, and Willis, 2005).
To provide plug flow and minimize short circuiting, a minimum length to width ratio (L:W)
of 4:1 is recommended. A preferred L:W is 6:1 (Kawamura, 2000).