Coagulation, Flocculation, and Sedimentation
Coagulation, Flocculation, and Sedimentation tests whether you can convert raw-water turbidity and color into settleable floc and remove it reliably. Kentucky surface water plants often use alum or ferric salts, so expect questions on charge neutralization, pH and alkalinity effects, jar testing, and basin hydraulics.
Core rules and concepts
- Coagulation destabilizes particles by charge neutralization and enmeshment in a metal hydroxide precipitate. Rapid mix disperses coagulant with high velocity gradient and short detention.
- Flocculation is gentle mixing that builds floc size and density; too much shear breaks floc.
- Sedimentation removes floc by gravity. Overflow rate (gpd/ft²) and detention time are key design and operating checks; short-circuiting reduces performance.
- Alum and ferric coagulants consume alkalinity and lower pH. If alkalinity is too low, add an alkaline chemical or change coagulant dose.
- Jar tests simulate rapid mix, flocculation, and settling to select coagulant, dose, pH, and polymer.
- Sludge production increases with coagulant dose and raw-water turbidity.
Worked example
A sedimentation basin is 100 ft long, 40 ft wide, and 12 ft deep. Flow is 2.0 MGD. Volume = 100 × 40 × 12 = 48,000 ft³. In gallons: 48,000 × 7.48 = 359,040 gal. Detention time = 359,040 ÷ 2,000,000 gal/day = 0.18 day = 4.3 hours. Overflow rate = 2,000,000 gpd ÷ (100 × 40 ft²) = 500 gpd/ft². Both values help judge whether settling has enough time and surface area.
Traps
Confusing rapid mix with flocculation; ignoring alkalinity consumption; carrying a jar-test dose directly to full scale without checking feed strength; assuming a deep basin always settles better; overlooking short-circuiting from baffles or sludge blankets.