Chemical Feed Rate (lb/day = mg/L × MGD × 8.34)

Every North American operator learns this one as a chant: pounds per day equals milligrams per litre times million gallons per day times 8.34. The 8.34 is nothing mysterious: it is the weight of a US gallon of water, 8.345 lb at 60 °F, rounded. Dosing 2 mg/L of chlorine into a 1 MGD plant needs 2 × 1 × 8.34 = 16.7 lb/day. Underneath the trade constant it is simply concentration times flow, mass balance in its plainest form, and this page will do it in kilograms per hour and cubic metres per hour just as happily.

Three warnings from the field. First, this gives pounds of active chemical; if you are feeding 12.5% sodium hypochlorite you still have to divide by the strength to get pounds of product, and again by the solution's density to get gallons the pump must deliver. Second, the 8.34 quietly assumes the water is water: feeding into a brine, a slurry or a glycol loop needs the actual density. Third, calibrate the pump against a drawdown cylinder rather than trusting the dial: a diaphragm pump's stroke setting drifts with discharge pressure, and the difference between a calculated feed rate and a measured one is where half of all chemical overspend hides.