Treatment Processes: Coagulation, Flocculation, Sedimentation, Filtration
The treatment train
Coagulation destabilizes colloidal clay and natural organic matter by charge neutralization; rapid mix disperses the chemical in seconds using intense energy. Flocculation is gentle, tapered stirring for 15–30 minutes that builds settleable floc; over-mixing shears floc apart. Sedimentation removes floc by gravity - the key design number is surface overflow rate (gpd/ft²), computed as flow divided by tank surface area, not by volume. Filtration polishes: rapid gravity filters with sand or anthracite media typically run 2–4 gpm/ft², slow sand filters about 0.05–0.15 gpm/ft².
Chemicals and control
Alum is most effective in roughly the pH 6–7 range and consumes alkalinity (about 0.5 mg/L as CaCO3 per mg/L of alum), which can drag pH down and hurt floc. Iron salts tolerate a wider pH range. Jar tests select dose and pH. Filter behavior to recognize: head loss rise, turbidity breakthrough near the end of a run, air binding from dissolved gas release, mudballs and media loss from poor backwash. Backwash expands media roughly 20–30% at 15–20 gpm/ft²; filter-to-waste protects finished water after backwash.
Traps
Questions swap coagulation (chemical destabilization) with flocculation (physical growth), or overflow rate with detention time. Unit conversions are the usual killer: gpd/ft² versus gpm/ft², and 1 MGD = 694 gpm.
Worked example
A plant treats 1.2 MGD through 500 ft² of filter area. Flow = 1.2 × 694 = 833 gpm. Loading rate = 833 ÷ 500 = 1.67 gpm/ft² - normal for a rapid gravity filter.