Water Treatment Processes: Coagulation, Flocculation, Sedimentation, Filtration, Softening
The unit-process chain
Coagulation destabilizes particles by charge neutralization and, at higher doses and pH, by sweep floc enmeshment in metal hydroxide precipitate. Rapid mix needs intense energy - velocity gradient G roughly 700 to 1000 per second for 10 to 30 seconds. Flocculation is gentle: G around 10 to 75 per second for 15 to 45 minutes, tapering so floc grows without shearing. Alum performs best near pH 6 to 7; ferric salts tolerate a wider range. Always confirm dose and pH with a jar test before changing full-scale feed.
Sedimentation is governed by surface loading (overflow) rate, not depth. Particles with settling velocity greater than the overflow rate are removed in theory. Short-circuiting, density currents, and wind reduce the real detention time below the calculated value. Weir loading and sludge collection matter as much as the basin volume.
Filtration removes residual particles by straining, sedimentation onto media, interception, and attachment. Rapid gravity filters run at roughly 2 to 6 gpm/ft2, with backwash near 15 to 20 gpm/ft2 plus air scour. Watch for breakthrough (turbidity rising at the end of a run), air binding, mudballs, and media loss. Slow sand and membrane filters have their own loading and integrity rules.
Softening with lime and soda ash raises pH to precipitate calcium carbonate; magnesium removal requires excess lime to pH above about 10.5 to 11, followed by recarbonation to stabilize the water.
Worked example
A 40 ft by 100 ft clarifier treats 2.0 MGD. Area = 4000 ft2, so overflow rate = 2,000,000 / 4000 = 500 gpd/ft2. At 10 ft depth, volume = 40,000 ft3 x 7.48 = 299,200 gal, giving detention time = 299,200 / 2,000,000 = 0.15 day, about 3.6 hours.