Page 314 - Water and wastewater engineering

LIME–SODA SOFTENING 7-31

Although rapid mixing may be provided ahead of the solids contact unit, generally the soft-
ening chemicals are applied to the mixing zone of the contact unit. The mixing and recirculation
zone is separated from the sedimentation zone by a conical baffle identified as the “hood” in
Figure 7-14 .
Effective solids contact units draw the settled precipitate from near the floor at the cen-
ter of the basin with a large diameter turbine or impeller and recirculate it with the incoming
water. When multiple units are used, the precipitate may be recirculated from one unit to
another.
Because the units are sold as proprietary manufactured items, the designs are established by
the manufacturer. Whenever possible, design requirements should be based on successful plants
using the same or similar source water. The following information from Horsley et al. (2005) is
provided for guidance in evaluating proprietary designs:

• The maximum recirculation rate is typically 10:1 based on the incoming raw water flow
rate. The mixer is provided with a variable-speed drive to allow the operator to adjust the
recirculation rate.
• The side water depth of the tank generally varies from 4.3 to 5.5 m. Contact time in the
mixing zone is typically measured by the volume of water within and directly under the
baffle cone.
• Overflow rate (that is, flow rate divided by the surface area, Q / A s ) is generally measured
0.6 m below the water surface, based on the surface area between the baffle wall and the
basin wall.

GLUMRB (2003) recommends the following design criteria:

1. Flocculation and mixing period should not be less than 30 minutes.
2. Detention time should be two to four hours for solids contact clarifiers and softeners
treating surface water.
3. Detention time should be one to two hours for solids contact clarifiers treating only
groundwater.
4. Upflow rate (overflow rate) shall not exceed 2.4 cubic meters of flow per square meter
3
2
of surface area per hour (m /h · m or m/h) at the slurry separation line for units used for
clarifiers.
2
3
5. Upflow rate (overflow rate) shall not exceed 4.2 m /h · m or m/h at the slurry separa-
tion line for units used for softeners.
3
6. Weir loading shall not exceed 0.120 m /min · m of weir length for units used as clarifiers.
3
7. Weir loading shall not exceed 0.240 m /min · m of weir length for units used as
softeners.

Table 7-4 provides information similar to that provided by manufacturers for selection of an
upflow solids contact unit.