Flocculation and Sedimentation
Flocculation builds small, dense floc into larger, settleable floc using gentle, tapered mixing-fast at first, slower later-so floc grows without being sheared apart. Know velocity gradient (G), detention time, and the product G×t as measures of mixing energy; excessive G breaks floc, insufficient G leaves fine particles that carry to the filters. Understand type I, II, and III settling, and that most treatment floc is type II (hindered) settling, described by zone settling rather than discrete Stokes settling.
Sedimentation basin concepts: surface overflow rate (flow ÷ surface area), weir loading, hydraulic detention time, short-circuiting, density currents, and sludge collection. Traps: computing overflow rate with the wrong units (gpm/ft² vs m³/m²·d) or using total basin volume instead of surface area; assuming longer detention always improves settling when short-circuiting or thermal density currents dominate; confusing flocculation (gentle, for growth) with rapid mix (intense, for dispersion). Also watch questions on tube/plate settlers, which increase effective settling area and improve performance.
Rule of thumb: settling performance scales with surface area, not depth-doubling depth at constant flow does not halve overflow rate.
Worked example: A basin 40 ft × 100 ft treats 2.0 MGD. Surface area = 4,000 ft². Overflow rate = 2,000,000 gal/day ÷ 4,000 ft² = 500 gpd/ft² ≈ 0.35 gpm/ft². That is a reasonable conventional loading; if flow doubled, overflow rate doubles and settling would suffer.