Chemical Feed, Storage, and Handling
Chemical feed must be accurate, verifiable, and safe. Know dry feeders (volumetric and gravimetric) and liquid feeders (positive-displacement pumps, rotameters), and understand dose vs feed rate vs stock concentration. Master the calculation chain: desired dose (mg/L) × flow (MGD) × 8.34 = pounds per day of chemical; then convert to gallons of solution using specific gravity and percent strength.
Storage and handling: know secondary containment, compatibility (never store acids with hypochlorite-releases chlorine gas), ventilation, and sodium hypochlorite degradation (strength and pH drop over time; store cool, dark, and short-term). Understand day tanks, dilution, and calibration by weight or drawdown. Safety: know the hazards of chlorine gas (heavier than air; use SCBA, never a cartridge respirator), ammonia, caustic (thermal and eye burns), alum (corrosive, slippery), and fluoride (acute toxicity at high dose). Traps: forgetting the 8.34 lb/gal factor or the specific-gravity correction; using percent strength as a decimal incorrectly (12.5% = 0.125); and assuming hypochlorite strength stays constant in storage.
Rule of thumb: lb/day = dose (mg/L) × flow (MGD) × 8.34, then divide by the fraction of active chemical in the product.
Worked example: Dose 2.0 mg/L at 1.5 MGD using 12.5% hypochlorite (SG ≈ 1.17). Active lb/day = 2.0 × 1.5 × 8.34 = 25 lb/day. Product lb/day = 25 ÷ 0.125 = 200 lb/day. Gallons/day = 200 ÷ (8.34 × 1.17) ≈ 20.5 gal/day.