Size the chlorine feed rate for water or wastewater disinfection. The required dose is the sum of the chlorine demand and the desired free residual; this tool converts that dose into a daily chemical mass in both US (lb/day) and SI (kg/day) units using the flow rate.
Chlorination is the most widely used disinfection method in water and wastewater treatment. Getting the dose right is a balance: too little leaves pathogens or no protective residual, while too much wastes chemical, raises cost, and forms regulated disinfection by-products. This calculator sizes the chlorine feed rate from the flow, the chlorine demand, and the free residual you want to carry into the distribution system or discharge.
Three terms drive every chlorination calculation. The chlorine demand is the amount consumed by reactions with ammonia, iron, manganese, organics, and other reducing agents in the water. The residual is the free or combined chlorine you want left over after demand is satisfied — typically 0.2–0.5 mg/L free chlorine entering distribution. The required dose is simply their sum: dose = demand + residual. Apply more than the dose and the surplus shows up as residual; apply less and there is no residual at all because demand is not yet met.
In US customary practice, the feed rate is: lb/day = dose (mg/L) × flow (MGD) × 8.34. The 8.34 is the weight of one US gallon of water in pounds, and it makes the units cancel: (mg/L)·(million gal/day)·(8.34 lb/Mgal per mg/L) = lb/day. It is the single most-used conversion factor in plant operations — for chemical feed, solids loading, and mass-balance work alike. Memorise it.
In SI units the mass-loading form is kg/day = dose (mg/L) × flow (m³/day) / 1000, because 1 mg/L equals 1 g/m³ and there are 1000 g in a kilogram. This calculator converts your flow from MGD to m³/day (× 3785.41) so you can read the chemical demand in both systems. The two results describe the same physical feed rate — 1 lb/day ≈ 0.4536 kg/day.
When ammonia is present, added chlorine first forms chloramines (combined residual); as more is added these are destroyed and oxidised, passing through a "breakpoint" beyond which a free chlorine residual finally appears. Operators dose past the breakpoint to guarantee free residual. Disinfection effectiveness is governed by CT — the product of residual concentration C and contact time T (mg·min/L). Regulators publish required CT values for given pathogen log-removals, temperature, and pH; a longer contact tank lets you hit the same CT with a lower residual.
It is the mass of one US gallon of water, 8.34 pounds. It converts a concentration in mg/L and a flow in million gallons per day (MGD) directly into a chemical mass in pounds per day: lb/day = mg/L × MGD × 8.34. The same factor appears throughout treatment-plant arithmetic for any chemical feed or loading calculation.
Demand is what the water consumes through chemical reactions before any measurable residual forms. The required dose is larger — it equals the demand plus the residual you want to maintain. If you feed only the demand, you finish with zero residual and no ongoing protection; the residual term is what guards the distribution system against regrowth and recontamination.
When ammonia is present, increasing chlorine dose first builds combined chlorine (chloramines), then destroys it through oxidation. The breakpoint is the dose at which combined residual is consumed and additional chlorine appears as free residual. Dosing past the breakpoint ensures a reliable free chlorine residual and removes most ammonia and taste-and-odour compounds.
CT is the product of disinfectant residual concentration C (mg/L) and contact time T (minutes). Pathogen inactivation depends on CT, so the same kill can be achieved with high C and short T or low C and long T. Drinking-water regulations specify minimum CT values for target log-reductions of Giardia and viruses as functions of temperature and pH.
Excess chlorine reacts with natural organic matter to form disinfection by-products (DBPs) such as trihalomethanes and haloacetic acids, which are regulated for health reasons. Over-dosing also causes taste and odour complaints, accelerates pipe corrosion, and wastes chemical. The goal is the minimum dose that meets the CT and residual targets — not the maximum.