Drinking Water Treatment
Drinking Water Treatment
This domain tests how a Montana surface-water or groundwater system turns raw water into safe finished water, and how the operator proves it with process control and compliance data. Expect questions on coagulation, filtration, disinfection, CT, turbidity, DBP precursors, and dose/feed calculations.
Core rules and concepts
- Conventional treatment: coagulation, flocculation, sedimentation, filtration, disinfection. Each barrier must be optimized; filtration alone is not a pathogen barrier for viruses.
- Combined filter effluent turbidity: ≤0.3 NTU in at least 95% of monthly samples, never >1 NTU (40 CFR 141.73).
- Disinfection must achieve required CT (residual concentration × contact time), adjusted for temperature, pH, and baffling.
- Maintain a detectable disinfectant residual in distribution; monitor TTHM/HAA5 against 80/60 µg/L LRAA limits.
- Nitrate MCL is 10 mg/L as N; arsenic 0.010 mg/L; lead action level 0.015 mg/L.
Worked example
A 2.0 MGD plant doses 2.5 mg/L chlorine using 12.5% sodium hypochlorite (specific gravity 1.2). Pounds per day = 2.0 MGD × 8.34 lb/gal × 2.5 mg/L = 41.7 lb/day. One gallon of solution weighs 8.34 × 1.2 = 10.0 lb and contains 0.125 × 10.0 = 1.25 lb available chlorine. Feed = 41.7 ÷ 1.25 = 33.4 gal/day. Check: 33.4 gal × 1.25 lb/gal = 41.8 lb, matching the demand.
Traps
Using 8.34 lb/gal for the hypochlorite solution instead of the solution density; treating mg/L as pounds; forgetting that CT tables are based on free chlorine residual, not dose; and reporting a single turbidity value instead of the 95th-percentile calculation.