Coagulation, Flocculation, and Sedimentation
What you must understand
This is the particle-removal train: coagulation destabilizes particles, flocculation builds them into settleable flocs, and sedimentation removes them by gravity. Know the chemistry: most raw-water particles carry negative surface charge, so repulsive forces keep them in suspension (zeta potential). Coagulants-alum, ferric chloride, ferric sulfate, PACl-hydrolyze to positive species that neutralize charge and sweep particles into flocs (sweep floc).
Understand pH control: alum consumes alkalinity and lowers pH; if alkalinity is insufficient, add lime or caustic. Know coagulant dose optimization via jar tests, streaming current, or turbidity. In flocculation, velocity gradient (G) and detention time govern floc growth-too much shear breaks flocs. In sedimentation, know surface overflow rate (flow ÷ surface area), weir loading rate, and detention time, and why short-circuiting and density currents degrade performance.
Traps
A classic trap: confusing coagulation (rapid mix, seconds, charge neutralization) with flocculation (slow mix, minutes, floc growth). Another: assuming more coagulant is always better-overdosing can restabilize particles or drop pH too far. Watch for questions mixing surface overflow rate with hydraulic loading on filters, and for alum vs. polymer roles (polymer often as flocculant aid).
Worked example
A clarifier handles 2.0 MGD with a 40-ft-diameter surface. Area = π(20²) ≈ 1,257 ft². Convert flow: 2.0 MGD × 1.547 cfs/MGD ≈ 3.09 cfs. Overflow rate ≈ 3.09 ÷ 1,257 ≈ 0.00246 cfs/ft², or in gpd/ft²: 2,000,000 ÷ 1,257 ≈ 1,591 gpd/ft². Compare to design criteria to judge whether settling capacity is adequate.