Coagulation, Flocculation, and Sedimentation
Destabilize, then build
Raw water particles are tiny colloids whose negative surface charge keeps them apart. Coagulation neutralizes that charge and sweeps particles into precipitates so they can collide; flocculation is the gentle, tapered stirring that grows the floc without shearing it; sedimentation removes the floc by gravity. Candidates must keep the three processes straight - exams love to swap the terms.
Know the coagulants: alum, ferric sulfate, ferric chloride, polyaluminum chloride (PACl), and cationic polymers, plus coagulant aids. Alum and iron salts work over a narrow pH band (alum roughly 5.5–7.0), consume alkalinity, and depress pH - the classic trap is a correct dose with insufficient alkalinity, causing a pH crash and worse turbidity carryover. Overdosing causes charge reversal and restabilization. Cold water slows hydrolysis, so winter operation needs longer reaction time or adjusted dose. Rapid mix needs G ≈ 300–1,000 s⁻¹ for 10–30 s; flocculation runs G ≈ 10–75 s⁻¹ for 15–45 min. Jar tests, run at plant temperature and pH, set the dose.
Sedimentation is classed by particle behavior: Type I discrete, Type II flocculant (nearly all alum floc), Type III hindered/zone settling, and Type IV compression. The controlling number is surface overflow rate = flow ÷ surface area, not detention time; baffling, weir loading, and tube settlers matter too.
Worked example: 30 mg/L dry alum consumes ≈0.5 mg/L alkalinity per mg/L of alum = 15 mg/L as CaCO₃. Against raw alkalinity of 25 mg/L, only 10 mg/L remains - feed lime before or with the coagulant.
Worked example: A 100 ft × 40 ft clarifier at 2.0 MGD: area 4,000 ft²; overflow rate = 2,000,000 ÷ 4,000 = 500 gpd/ft² ≈ 0.35 gpm/ft².
Traps: more coagulant does not always mean clearer water; overflow rate uses surface area, not volume; over-mixed floc cannot be recovered downstream.