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6.16 CHAPTER SIX

TABLE 6.5 Typical Mixing and Flocculation Design Criteria for System Shown in Figure 6.8

Plant capacity 150 ft3/s
Design flow 97 mgd
Plant inlet pipe diameter 84 in.
Initial mixer
Pump blender 2 units
Pump rating, each 10 hp
Energy input, G at 20 ° C 1,000 s-1
Mixing zone 538 ft 2
Detention time 3.6 s
Distribution channel
Depth 10 ft
Width 6.5 to 1.5 ft
Maximum velocity 1.2 ft/s
Flocculation
Number of basins 2
Compartments, each basin 4
Depth, average 16 ft
Compartment, width × length 15 × 80 ft
Volume, per basin 76,800 ft 3
Detention time, total 18 min
Horizontal shaft paddles (each basin) 4
Maximum G, per compartment 50 s- t
Maximum power, per compartment 2.0 hp
Chemical dosages
Chlorine, refilter 5 mg/L
Alum, maximum, rapid mixer 20 mg/L
Cationic polymer, rapid mixer 2 mg/L
Nonionic polymer, second-stage flocculator 0.5 mg/L
Potassium permanganate, rapid mixer 2 mg/L

and it appears that earlier texts based detention times on mean, or nominal, flows rather
than on maximum plant capacity.
In addition, temperature adjustments outlined in Table 6.2 should be considered. The
design water temperature is the temperature most likely to be encountered under maxi-
mum flows.
For direct filtration plants, high-energy flocculation is typically in the range of 15- to
20-min detention. When clarification is required, lower energy input and detention times
of 18 to 25 min are a guide. If compartmentalization is not provided, increased detention
times should be provided in addition to adjustments for water temperature. In all cases,
pilot plant or full-scale tests and economic evaluation offer the most reliable indicators.
For low-pressure membranes, the flocculation duration is typically in the range of 5
to 15 min, depending on the type of membrane and required organics removal.

Energy Requirements
Energy input (G values) for flocculation in plants using metal-ion and organic coagulant
chemicals ranges from 20 to 75 s-1. Typical G values and detention times for floccula-