You do not need to be good at math to pass your operator exam. You need to know about a dozen formulas cold. Here are the ones worth memorizing.
The pounds formula
The single most important formula in water and wastewater:
lbs/day = flow (MGD) × concentration (mg/L) × 8.34
It shows up as chlorine dosage, BOD loading, solids loading — same formula every
time. The 8.34 is the weight of a gallon of water in pounds.
Detention time
detention time = volume ÷ flow
Keep your units consistent. If volume is in gallons and flow is in gallons per minute, your answer is in minutes.
Activated sludge control (wastewater)
- F:M ratio = BOD applied (lbs/day) ÷ MLVSS (lbs)
- MCRT / SRT = solids in system (lbs) ÷ solids leaving (lbs/day)
- SVI = (settled sludge volume in mL/L × 1000) ÷ MLSS (mg/L)
Hydraulics (distribution & collection)
- Pressure — 1 psi = 2.31 feet of head
- Velocity = flow ÷ area
Print these on one page, drill them, and bring them to the exam. Then test yourself with our free practice questions — most of them are the applied-math problems these formulas solve.