Chemical Feed Rate from Dose

This is the calculation a water treater does more than any other, and in the field it is written lb/day = ppm × MGD × 8.34. The 8.34 is the weight of a US gallon of water at 60 °F, so a million gallons weighs 8.34 million pounds and one part per million of that is 8.34 lb. Dosing 20 ppm into 100 gpm — which is 144,000 gal/day, or 0.144 MGD — needs 20 × 0.144 × 8.34 = 24.0 lb of product per day, or almost exactly 1 lb/h on the pump.

The near-universal mistake is confusing product with active ingredient. If the drum is a 20% active phosphonate blend, feeding 24 lb/day of product delivers only 4.8 lb/day of active, and your residual test will read a fifth of target. Decide up front whether the ppm you are quoting is as-product or as-active, write it on the log sheet, and stay consistent. Also decide which flow you are dosing against: inhibitor fed proportional to makeup holds a residual against dilution, while feeding proportional to blowdown replaces what is actually leaving the system — the two differ by the cycles of concentration.