Page 138 - ro membanes

FIGURE 6.6 Iron accumulation on SDI test pad due to coagulant overdosing.
6.2.3.2 Coagulation Chamber With Flash Mixers
The key design criteria for such flocculation systems are listed below:
6.3 FLOCCULATION 121
Number of coagulation tanks Tank width  length  depth Tank volume (V)
Contact time (T)
Mixing energy per tank at 2.5 hp/10,000 m3/day (W)
Absolute viscosity of water (ma)
Velocity gradient multiplied by time G  T
Type of mixer
Blade area/tank area Shaft speed
1⁄44
1⁄41.2m1.2m1.4m (3.9ft3.9ft4.6ft)
1⁄4 2.0 m3 (21.5 ft3)
1⁄4 (2.0 m3  4 tanks  24 h  60 min)/127,910 m3/day 1⁄4 0.09 min (5.4 s)
1⁄4 [(2.5 hp  127,910 m3/day)/4]/10,000 1⁄4 8 hp 1⁄4 5966 W 1⁄4 0.0014 N s/m3
1⁄4 W/(ma  V)0.5  T
1⁄4 [5966 W/(0.00114 N s/m3  2.0 m3  4)]0.5  5.4 s 1⁄4 4368
Vertical-shaft with hydrofoil blades 0.15%
40e80 rpm
It is important to point out that the velocity gradient G  T provided by the coagulation chambers (4368) is significantly higher than that provided by the static mixer (1300), which would correspond to a more robust mixing.
6.3 FLOCCULATION
Flocculants (polymers) are sometimes applied in addition to coagulants to improve pre- treatment. However, polymer addition, even if slightly overdosed, may also cause organic fouling of the RO membranes. Often, the potential for RO membrane fouling due to polymer overdosing is of a greater concern than the benefit of polymer use. Therefore, many