Chemical Feed, Storage, and Handling
What you must understand
Chemical feed must be accurate, reliable, and safe. Know the common chemicals-alum, ferric salts, lime, caustic soda, sodium hypochlorite, fluoride, polymers, and potassium permanganate-and their storage and handling needs. Understand feed systems: dry feeders, solution feeders, metering pumps, and gas feeders (chlorine, ammonia, sulfur dioxide). Know how to compute dose (mg/L) and feed rate (lb/day), and how to convert between them.
Key concepts: calibration of feeders, dilution of stock solutions, day tanks and secondary containment, and chemical compatibility (never store acids with hypochlorite-releases chlorine gas). Understand sodium hypochlorite degradation (strength declines over time, faster when warm), so dosing must be adjusted for measured strength. Know fluoride feed accuracy is critical because of the narrow therapeutic range.
Traps
Frequent errors: mixing percent solution with mg/L dose, forgetting specific gravity and pounds-per-gallon conversions, and assuming hypochlorite strength is constant. Another trap: assuming all chemicals can share a room-oxidizers and acids must be separated. Also watch gas chlorine safety: never use ammonia to detect leaks; use proper detectors.
Worked example
Feed rate (lb/day) = dose (mg/L) × flow (MGD) × 8.34. For 2.5 mg/L alum at 3.0 MGD: 2.5 × 3.0 × 8.34 ≈ 62.6 lb/day. To convert to gallons of solution, divide by (solution lb/gal) = 8.34 × specific gravity × fraction strength.