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Operator Math

Demand, Strength and Purity

The pie chart always answers in pure chemical. Real water and real jugs are not pure. Two things get adjusted: what the water consumes before you measure it, and what the product actually contains when you pump it.

Choose an adjustment

Demand = dose − residual

The chlorine that got used up — eaten by iron, ammonia and organics — before anything was left over to measure. Adjust this before the chart, on the way in.

END OF SHIFT

Your feed is set for 3.0 mg/L into 1.0 MGD. Post-contact residual reads 0.5 mg/L. The water ate the difference.

3.0DOSE
FED
0.5RESIDUAL
LEFT
2.5MG/L
DEMAND

Now run that through the chart:

2.5DEMAND
MG/L
8.34LB PER
GALLON
1.0FLOW
MGD
21LB/DAY CONSUMED
WATCH THIS NUMBER

Demand is a water quality signal, not just bookkeeping. Track it shift to shift. If demand climbs while your flow holds steady, something upstream changed — a slug of ammonia, iron breaking through, a line that got stirred up.

Feeding 3.0 and consuming 2.5 is different from feeding 3.0 and consuming 1.0. Same feed rate on the invoice; very different plant.

Your chemical is not pure

The chart tells you the pounds of chlorine you need. It does not tell you how much product to pump. Adjust this after the chart, on the way out.

SIZING THE DAY TANK

You need 25 lb/day of chlorine. What is in the tote is 12.5% sodium hypochlorite. How much liquid actually has to move?

Hypochlorite is only 12.5% chlorine — the rest is water and caustic. Divide by the decimal to get pounds of product:

25 ÷ 0.125 = 200 lb/day of solution

DOES IT MAKE SENSE?

Weaker product means you feed more of it. Eight times the weight here. If your answer came out smaller than the pounds of chlorine, you multiplied when you should have divided.

ON THE FLOOR

Size pumps, day tanks and delivery schedules against the 200, never the 25. This is also why hypo plants run out faster than the chlorine numbers suggest — and why hypo that has been sitting all summer no longer feeds at label strength.

Two multipliers stacked

Same rule as strength — it just runs twice. Some products are rated on purity and again on how much of the useful ion they carry.

FLUORIDE FEED

You are holding 0.8 mg/L fluoride in 3.0 MGD using sodium fluorosilicate — 98.5% pure, 60.6% available fluoride.

STEP 1 — POUNDS OF FLUORIDE YOU ACTUALLY NEED

0.8DOSE
MG/L
8.34LB PER
GALLON
3.0FLOW
MGD
20.0LB/DAY FLUORIDE

STEP 2 — DIVIDE BY WHAT FRACTION IS REALLY FLUORIDE

20.0LB/DAY
NEEDED
0.985PURITY
0.606AVAILABLE
FLUORIDE
33.5LB/DAY PRODUCT
THE RULE NEVER CHANGES

One divisor or two, you always feed more product than pure chemical. The answer must come out larger than the pounds you needed. Here: 20.0 needed, 33.5 fed.