Water Treatment Processes
The treatment train
Conventional surface water treatment runs coagulation → flocculation → sedimentation → filtration → disinfection, and each step has a distinct job that questions like to blur. Coagulation (rapid mix, seconds, high energy) destabilizes the negative surface charge on particles so they can stick together. Flocculation (minutes, gentle tapered mixing) lets collisions build visible floc. Sedimentation removes settleable floc by gravity. Filtration polishes the rest, and disinfection inactivates what remains.
Concepts to know cold
- Alum, ferric chloride, and polymer doses are set by jar test; overdose can reverse the charge and restabilize turbidity.
- Coagulants consume alkalinity and depress pH; confirm enough alkalinity remains.
- Sedimentation design is governed by overflow rate (gpm/ft²), not depth alone; two basins with the same surface area settle the same.
- Filters are rated by loading rate; watch head loss, breakthrough, air binding, and mudballs. Backwash uses air scour plus water at a rate expanding the bed about 20–30%.
- CT (residual × contact time) links disinfection practice to required log credit.
Traps
More coagulant is not better. Filtered turbidity is a treatment technique, not an MCL, and individual filter effluent is judged on a 95th percentile plus consecutive exceedances.
Worked example
Overflow rate = flow ÷ surface area. A 1.5 MGD plant with a 40 ft × 80 ft clarifier: 1,500,000 ÷ 1440 = 1,042 gpm; 1,042 ÷ 3,200 ft² = 0.33 gpm/ft². Typical design is 0.5–1.0 gpm/ft², so this basin is conservatively loaded.