Water Treatment
Water Treatment
The Water Treatment domain tests whether you can run a conventional surface-water plant end to end - coagulation, flocculation, sedimentation, filtration and disinfection - and prove it with dose calculations, turbidity records, and CT documentation. Expect arithmetic (chemical feed, filter loading, contact time) plus the operational judgment questions NC surveyors care about.
Core rules to hold onto
- Coagulation: a jar test is the only reliable way to set alum or polymer dose. Each mg/L of alum consumes roughly 0.5 mg/L of alkalinity as CaCO3; if raw alkalinity drops much below 30 mg/L, add lime or switch coagulants or you will lose pH and the floc will not form.
- Sedimentation: surface overflow rate = flow ÷ surface area; check it before you blame the filters for solids carryover.
- Filtration: conventional loading is about 2–6 gpm/ft2. Combined filter effluent turbidity must be ≤ 0.30 NTU in at least 95% of monthly measurements and may never exceed 1 NTU; individual filters are monitored to catch a single break-through.
- Disinfection: CT = measured residual (mg/L) × T10 (minutes). T10 comes from basin volume ÷ flow, multiplied by a baffling factor; required CT is read from a table based on pH, temperature and log credit.
Worked example
A 1.0 MGD plant (694 gpm) has a 50,000-gallon clearwell, a peak-exit free chlorine residual of 1.2 mg/L, and a baffling factor of 0.6. Volume ÷ flow = 50,000 ÷ 694 = 72 min theoretical. T10 = 72 × 0.6 = 43 min. CT = 1.2 × 43 = 51.6 mg/L·min. The table for the plant's pH and water temperature calls for 45, so the plant complies with roughly a 15% cushion - and the cushion shrinks fast if flow rises or residual drifts down.
Traps that cost marks
Using gross theoretical time instead of T10; plugging in the feed dose rather than the measured residual; forgetting that pH shifts the hypochlorous/hypochlorite balance and changes required CT; reporting the 95th-percentile turbidity rule as a single grab sample; and starting a jar test at the wrong rapid-mix intensity, which flocculates poorly and misleads the whole dose decision.