Operator Math
Cost and Days of Supply
Once you have pounds per day, you are one step from the two questions anyone actually asks you: what is it costing and when do I reorder. Pounds first — always — then the last step.
Dollars per day
You are running alum at 25 mg/L through 3.0 MGD. Alum lands at $0.40/lb. What is coagulation costing per day?
POUNDS FIRST, THEN DOLLARS
25 × 8.34 × 3.0 = 626 lb/day
626 × $0.40 = $250/day
About $7,500 a month. That is the number that makes a jar test worth running — drop the dose 2 mg/L and you have found real money.
Days of supply
You are feeding fluoride at 0.8 mg/L into 2.0 MGD and there is 600 lb on the pad. How long does that last?
POUNDS FIRST, THEN DIVIDE THE SHELF BY IT
0.8 × 8.34 × 2.0 = 13.3 lb/day
600 ÷ 13.3 = 45 days
Forty-five days is when you run dry, not when you order. Back off your lead time and a safety margin — on a two-week delivery, you are placing that order around day 25.
Both of these are a pie chart plus one more step, and the pounds in the middle look like a finished answer. They are not. If the question was dollars, keep going to dollars. If it was days, keep going to days.
Also watch the difference between running out and reordering. If you keep a minimum on the pad, subtract it before you divide — 600 lb with a 150 lb floor is 450 usable pounds, which is 34 days, not 45.
Rising cost, steady dose? Same logic runs backwards. If your dollars per day climb while the dose and the price stay put, then flow went up — or someone changed the setpoint and did not write it down.