Chlorine Dose from a Weight of Product

This is the shock-chlorination and batch-dosing formula: you have a bucket of product, a tank or a well of known volume, and you need the resulting parts per million. One pound of 65% calcium hypochlorite in 1,000 gallons gives 0.65 lb of chlorine in 8,340 lb of water, which is 0.65/8,340 = 78 parts per million. Well disinfection specifications typically call for 50–200 mg/L held for 12–24 hours; a new water main is usually chlorinated to 25 mg/L for 24 hours or 50 mg/L for 3 hours before flushing and bacteriological clearance.

Getting the volume right is more than half the job. A drilled well's water column is π/4 × d² × depth-of-water, which for a 6-inch casing is about 1.5 gallons per foot of standing water — and you must add the volume of the pressure tank, the plumbing and any storage, because the chlorine has to reach everywhere the bacteria are. The other trap is where you pour it. Dumping cal-hypo granules down a well casing without pre-dissolving drops undissolved pellets onto the pump and can pit stainless screens; make up a slurry in a clean pail with soft water first, and circulate by running each fixture until it smells of chlorine, then shut everything and let it stand.