Water Treatment
Water Treatment
The Water Treatment domain tests process control from raw water to finished water: coagulation, flocculation, sedimentation, filtration, disinfection, chemical feed, residuals, and the math that links them. North Dakota systems often treat groundwater for iron, manganese, hardness, nitrate, or arsenic, while surface-water plants add clarification and filtration before disinfection. Expect questions that ask you to diagnose a process change, not just recite a definition.
Core rules
- Coagulation destabilizes particles; dose is set by jar test, and pH and alkalinity affect performance.
- Filtration depends on loading rate, backwash, and filter effluent turbidity; changing flow changes particle capture.
- Disinfection is judged by demand, residual, and contact time (CT), not by dose alone.
- Chemical feed: lb/day = mg/L × 8.34 × MGD.
Worked example
A clearwell holds 60,000 gal and plant flow is 600 gpm. Detention time = 60,000 gal ÷ 600 gpm = 100 min. If chlorine dose is 2.0 mg/L and residual is 0.7 mg/L, demand is 2.0 − 0.7 = 1.3 mg/L. If flow rises to 750 gpm, detention time falls to 80 min, so CT drops unless dose or storage changes. The reasoning: contact time is volume divided by flow; demand is dose minus residual.
Traps
Mixing gpm with MGD; forgetting 8.34 in the pounds formula; using dose as if it were demand; ignoring pH and temperature effects on chlorine; assuming a filter is fine because turbidity is low at the plant but not checking individual filter effluent.