This lesson is written in mg/L, million gallons per day and pounds per day, and it is written that way on purpose: it is the convention every North American operator certification exam uses, and the constant is the point of the lesson. The metric twin is at the bottom.

$\text{lb/day} = \text{mg/L} \times \text{MGD} \times 8.34$ — read aloud pounds per day equals milligrams per litre, times M G D, times eight point three four. The letters: mg/L is the dose $C$, the chemical concentration you want in the water. MGD is the flow $Q$, in millions of US gallons per day — not gallons, not gpm: millions per day, and a plant running 694 gpm is running 1 MGD. 8.34 is the weight of one US gallon of water in pounds. The answer is $\dot m$, a mass feed rate in pounds of chemical per day.

The 8.34 is not a fudge factor and it is not a conversion someone invented. mg/L is parts per million by weight. One million gallons of water weighs $10^{6} \times 8.34 = 8{,}340{,}000$ lb, so one part per million of it is 8.34 lb. That is the whole derivation: the formula is a mass balance wearing a costume.

Two places it goes wrong. It assumes the fluid weighs what water weighs, so for a concentrated solution you must bring in specific gravity — that is the tank-inventory form, $\text{lb} = \text{gal} \times \mathrm{SG} \times 8.34 \times \dfrac{\%}{100}$, where SG says how much heavier than water the liquid is and the percentage keeps only the part that is chemical. And it quotes the ACTIVE ingredient, so a dose of chlorine is not a weight of hypochlorite until the next lesson has divided by the strength.

The metric twin is cleaner and worth knowing, because it needs no constant at all: $\text{kg/day} = \text{mg/L} \times \text{ML/day}$. One megalitre weighs a million kilograms, so a part per million of it is exactly one kilogram, and the 8.34 vanishes. Same physics, and the SI units did the bookkeeping for you.