Page 313 - Water and wastewater engineering

7-30 WATER AND WASTEWATER ENGINEERING

of the lime slurry. Flocculation mixing is provided by axial flow impellers or paddle wheels. The
1
flocculation mixing velocity gradient is between 130 and 300 s with a mixing time of 30 to
45 minutes, to achieve Gt values ranging from 200,000 to 400,000. The velocity through ports in
the flocculation basin is in the range 0.15 to 0.36 m/s (Davis and Cornwell, 2008; Horsley et al.,
2005; Kawamura, 2000).
The design process follows that shown in the Chapter 6 examples for vertical turbine and
paddle mixers.

Upflow Solids Contact Basins
Conventional softening basins are mostly found at older facilities. They provide a high degree
of process stability, but the size and number of basins result in a high capital cost. Since the
late 1960s most new plants have been built with upflow solids contact basins as shown in
Figure 7-14 .

Launders

Discharge channel

Discharge
channel
and launder
Lime slurry feed

Mass effluent
Launder launder
Chennal feed
Mixer
“Hood” or
“skirt” Separation zones
Sludge
scraper Sludge
blanket
Flocculation

Sludge hopper
Sludge
Sludge
circulation port
withdrawal
Raw water
FIGURE 7-14
Upflow solids contact basin.