Operator Math
Three Names, One Formula
Three different names show up on your formula sheet. They are the same three numbers multiplied together. What changes is what someone hands you — and that tells you which name you are looking at.
The middle slot decides
| On your sheet | What you have | What you get |
|---|---|---|
| Feed RateYou are adding a chemical | a dose and a flow in MGD | pounds per day |
| Loading RateSomething is coming at you | a concentration and a flow in MGD | pounds per day |
| MassSomething is sitting in a tank | a concentration and a volume in MG | pounds, total |
Look at the middle number. A flow moving past you gives a rate — pounds per day. A tank volume sitting still gives a total — just pounds.
On shift
Feed rate — you are adding it
You are carrying 2.0 mg/L of chlorine into 0.5 MGD. How much chlorine leaves the day tank?
MG/L
GALLON
MGD
That is the number you order against and the number your day tank has to keep up with.
Loading rate — it is coming at you
Influent BOD came back at 180 mg/L and the plant is taking 2.0 MGD. How much BOD is the plant actually being asked to treat?
MG/L
GALLON
MGD
Same arithmetic as the feed rate. Nobody dosed anything — this is what showed up at the headworks.
Mass — it is sitting in the tank
Your basin holds 0.75 MG and the mixed liquor is running 2,400 mg/L. How many pounds of solids are under you right now?
MG/L
GALLON
MG
No per day on the end. This is inventory — the pounds you are holding. It is the starting point for wasting decisions and for sludge age.
Why 8.34? It is the weight of one gallon of water in pounds. It is what lets you turn a concentration in mg/L into pounds, which is the only unit a pump, a scale or a purchase order understands.