Page 416 - Water and wastewater engineering

SEDIMENTATION 10-33

TABLE 10-5
Typical design criteria for small to medium horizontal-flow rectangular
sedimentation basins

Parameter Typical range of values Comment
3
Number of tanks 1 1 spare < 10,000 m /d
3
2 20,000 m /d
Inlet zone
Distance to diffuser wall 4% of length up to 2 m
Diffuser hole diameter 0.10–0.20 m
Settling zone
3
3
Overflow rate 20 m /d · m 2 < 10,000 m /d
3
3
40 m /d · m 2 >10,000 m /d
Side water depth (SWD) 3–5 m
Length 30 m Wind constraint
60 m Chain-and-flight
Width 0.3 m increments Chain-and-flight
6 m maximum per train Chain-and-flight
L:W minimum of 4:1 6:1 preferred
L:D 15:1 Minimum
Velocity 0.005–0.018 m/s Horizontal, mean
Reynolds number < 20,000

Outlet zone
Launder length 1/3–1/2 length of basin Evenly spaced
3
Launder weir loading 250 m /d · m of launder
Sludge zone
Depth 0.6–1 m Equipment dependent
Slope 1:600 Mechanical cleaning
Sludge collector speed 0.3–0.9 m/min
Sources: AWWA, 1990; GLUMRB, 2003; Kawamura, 2000; MWH, 2005; Walker, 1978; Willis, 2005.
High-Rate Settler Modules
The guidance provided for the number of tanks for rectangular horizontal flow sedimentation
also applies to high-rate settler modules.

Inlet Zone. A diffuser is designed and placed in the tank inlet zone in the same fashion as it is
for a plain rectangular horizontal flow clarifier.

Flow Pattern. The three flow patterns, countercurrent, cocurrent, and crosscurrent, in theory,

have little difference in performance. In practice the countercurrent pattern is the one most com-
monly employed because cocurrent designs often have trouble with resuspended sludge and
crosscurrent designs have trouble with flow distribution (MWH, 2005).

Plates versus Tubes. Although little difference has been reported for various tube shapes, the
hexagon and v-shapes may have some advantage because the sludge can collect in the notch.