Coagulation, Flocculation, and Sedimentation
Coagulation destabilizes particles, flocculation builds them into settlable flocs, and sedimentation removes them. Keep those three verbs straight - questions love to swap them.
Coagulants include alum, ferric sulfate or chloride, polyaluminum chloride, and cationic polymers. Mechanisms are double-layer compression, charge neutralization (positive coagulant meeting a negative particle), adsorption bridging, and sweep floc, in which a metal-hydroxide precipitate enmeshes particles. Key side effect: alum consumes alkalinity, roughly 0.45–0.5 mg/L of alkalinity per mg/L of alum, and depresses pH. If alkalinity is insufficient, add lime or change coagulant. Optimum pH windows are roughly 5.5–7.5 for alum and wider for ferric salts.
Mixing: rapid mix at a high velocity gradient (G about 300–1,000 s^-1) for seconds to a minute; flocculation at a gentle, often tapered G (about 10–75 s^-1) for 15–45 minutes. Sedimentation design parameters are detention time (typically 1.5–4 h), surface overflow rate (about 0.4–1.0 gpm/ft2), and weir loading. The jar test is the standard bench tool: identical rapid mix, varied coagulant dose and pH, slow mix, settle, then measure turbidity.
Traps: confusing G with rpm; assuming more coagulant is better (overdosing restabilizes floc and wastes alkalinity); forgetting the pH/alkalinity consequence; and using surface overflow rate when the question supplies weir length.
Worked example: 2.0 MGD over a 1,000 ft2 basin = 2,000,000 gal/day ÷ 1,440 min/day = 1,389 gpm ÷ 1,000 ft2 = 1.39 gpm/ft2, high for a conventional clarifier.