Drinking Water Treatment and Distribution
Drinking Water Treatment and Distribution
This domain tests how Nevada public water systems protect source water, treat it to primary and secondary standards, and move it through the distribution system without losing quality. Expect process-control math, disinfection and CT, sampling rules, cross-connection control, and Nevada-specific contaminants such as naturally occurring arsenic.
Core rules
- Public water systems in Nevada are regulated by NDEP's Bureau of Safe Drinking Water under NAC 445A and the federal Safe Drinking Water Act. Know MCLs: arsenic 0.010 mg/L (10 µg/L), nitrate 10 mg/L as N, and the total coliform and E. coli rules.
- Treatment train: coagulation, flocculation, sedimentation, filtration, disinfection. Filtration must reduce turbidity; disinfection must meet CT (mg/L × minutes) and maintain a residual.
- Distribution: maintain positive pressure, prevent backflow with an approved cross-connection program, control storage tank turnover, flush dead ends, and sample at approved sites.
- Lead and copper: first-draw samples at high-risk sites; corrosion control may require pH or alkalinity adjustment.
Worked example
A well pumps 500 gpm. Target free chlorine dose is 2.0 mg/L. Using 12.5% sodium hypochlorite (about 1.19 lb available Cl2 per gallon), what feed rate is needed?
- Convert flow: 500 gpm × 1,440 min/day = 720,000 gpd = 0.72 MGD.
- Chlorine demand: lb/day = MGD × mg/L × 8.34 = 0.72 × 2.0 × 8.34 = 12.0 lb/day.
- Feed: 12.0 lb/day ÷ 1.19 lb/gal = 10.1 gal/day. Check: if the residual after contact is only 0.5 mg/L, the dose is not the same as the residual; increase feed based on actual demand and verify CT.
Traps
- Mixing µg/L and mg/L (10 µg/L = 0.010 mg/L).
- Using gpm in the pounds formula instead of MGD.
- Treating a chlorine residual reading as the applied dose.
- Sampling lead at a kitchen tap that has been run, or outside the approved site.
- Ignoring backflow hazards at irrigation, boiler, and chemical-feed connections.