Chlorine Dosing Calculator

This calculator sizes a chlorine feed system and checks CT (concentration × time) disinfection compliance in one pass. Total dose is simply target residual plus chlorine demand, and the feed rate uses the same lbs/day = flow (MGD) × dose (mg/L) × 8.34 conversion used throughout water treatment chemical dosing. The default Target Chlorine Residual of 0.5 mg/L sits comfortably above the federal floor: under the Surface Water Treatment Rule (40 CFR 141.72(a)(4), promulgated under the Safe Drinking Water Act), the residual disinfectant concentration entering the distribution system cannot be less than 0.2 mg/L for more than 4 hours, so any target you enter should clear that regulatory minimum with margin for decay across the distribution system. CT achieved multiplies the target residual by contact time; CT required comes from a built-in lookup table for four pathogen classes — bacteria, virus, Giardia, and Cryptosporidium — at pH 7, adjusted by a temperature factor that roughly doubles the requirement below 5°C and adds 20% between 5-15°C. Cryptosporidium's required CT value is dramatically higher than the others because free chlorine is a genuinely weak disinfectant against it — if your plant targets Crypto inactivation, chlorine alone is rarely the actual control strategy, and this tool will show a large CT deficit reflecting that reality.

Log inactivation is a simplified proportional estimate, not the full Chick-Watson kinetics regulators use. The calculator sizes daily feed for three chemical forms — 12.5% sodium hypochlorite (gallons/day), gas chlorine (lbs/day), and 65% calcium hypochlorite (lbs/day) — but only prices the first two out to an annual cost, using a $/gallon rate for hypochlorite solution and a $/lb rate for gas chlorine; cal-hypo's daily feed weight is reported without a matching annual-cost figure, so treat that line as a sizing number rather than a cost comparison. DBP (disinfection byproduct) risk is a coarse three-tier flag based on total dose and demand thresholds, meant to prompt further THM/HAA5 monitoring rather than to replace it. Breakpoint dose is a simplified multiple of demand, useful only as a rough planning number when ammonia is present.