MO Water/Wastewater Operator common mistakes
MO Water/Wastewater Operator · Understand the traps and how to avoid them.
Computing chlorine dose or demand by plugging numbers into the formula without first converting all concentrations to the same units (mg/L vs. percent vs. lb/day), producing an answer off by a factor of 10,000.
The 'pounds formula' (lb/day = mg/L × MGD × 8.34) is memorized, but the concentration is often given as a percent or as a feed rate, and candidates skip the unit conversion under time pressure.
What to do instead
Before substituting, write every quantity with its unit and convert to mg/L and MGD first (percent × 10,000 = mg/L). Do a magnitude sanity check: a plant feeding a few hundred lb/day of chlorine is plausible; millions is not.
Confusing chlorine demand with chlorine residual and reporting demand as the target residual, or subtracting in the wrong direction.
The terms are related but the arithmetic (dose − demand = residual) is easy to invert when the question is worded as 'how much must be fed to maintain X residual.'
What to do instead
Always restate the relationship in words first: dose applied minus what the water consumes leaves the residual. Solve for the unknown explicitly before computing.
Miscalculating filter loading rate or backwash rate by using the wrong area (one filter cell vs. total filter area) or forgetting to convert gpm to gpm/sq ft.
Questions often give total plant flow across multiple filters, and candidates divide by the wrong number of cells or omit the area entirely.
What to do instead
Write the rate as flow ÷ area with units shown (gpm/ft²). Confirm whether the flow is per filter or plant-total and how many cells are in service. Typical rapid-sand loading is around 2–5 gpm/ft²; use that as a plausibility check.
Misapplying the CT concept - treating CT as a fixed number rather than contact time × residual, or ignoring that required CT changes with pH and temperature.
CT tables are complex, and candidates memorize a single value instead of the relationship, so they cannot adapt when the question changes pH or water temperature.
What to do instead
Remember CT = disinfectant residual (mg/L) × contact time (minutes) and that required CT rises as temperature drops and (for free chlorine) as pH rises. Compute actual CT, then compare to the required value for the stated conditions.
On distribution questions, calculating head loss or pressure without accounting for elevation change, or mixing up psi and feet of head.
The 2.31 ft per psi conversion is known but applied in the wrong direction, and elevation head is forgotten when a question mentions a hill or elevated tank.
What to do instead
Convert all elevations and pressures to the same unit (feet of head) before adding: total head = elevation head + pressure head + friction. 1 psi = 2.31 ft of head; divide feet by 2.31 to get psi.
Confusing the purpose and direction of flushing versus directional flushing, or stating that flushing raises disinfectant residual when it actually removes water and can lower it in the system.
Flushing is taught as a general 'water quality fix,' so candidates give a generic answer instead of the specific mechanism (removing sediment and stale water, restoring turnover).
What to do instead
Frame each distribution practice by mechanism: flushing removes sediment/stagnant water and restores flow; it does not add disinfectant. Booster chlorination adds residual. Match the practice to the stated problem.
Treating a cross-connection/backflow question as a pressure question and choosing 'backpressure' when the scenario describes backsiphonage (or vice versa).
Both are backflow, and the distinguishing cue - a negative pressure event vs. a downstream pressure higher than supply - is easy to miss in a short stem.
What to do instead
Identify the driving force: backsiphonage is caused by negative or reduced supply pressure pulling water backward; backpressure is a downstream pressure exceeding supply. Read the stem for which condition is described.
Computing F/M ratio, MCRT (sludge age), or SVI with the wrong numerator or denominator - e.g., using MLSS where MLVSS belongs, or omitting the aeration tank volume.
The formulas look similar and are often recalled as 'bugs over bugs,' so the specific mass balance is blurred.
What to do instead
Write each formula with its units before plugging in: F/M = lb BOD applied per day ÷ lb MLSS (or MLVSS) in aeration; MCRT = lb solids in system ÷ lb solids wasted per day; SVI = settled volume (mL/L) × 1000 ÷ MLSS (mg/L). Check that the units cancel to the expected dimensionless or day result.
Misdiagnosing filamentous bulking vs. pin floc and choosing the wrong corrective action (e.g., increasing wasting when the real problem is low DO or low F/M).
Both show poor settling, but the causes and fixes are opposite; candidates pick the most familiar fix (waste more sludge) regardless of the actual cause.
What to do instead
Use SVI plus observation: high SVI with filaments points to low DO, low F/M, or septicity - correct the cause (raise DO, adjust wasting). Pin floc with low SVI and turbid effluent points to young sludge/overwasting. Match the fix to the indicated cause.
On collection questions, sizing or interpreting pipe slope and velocity by treating slope as a percent when the formula needs feet per foot, or accepting velocities outside the self-cleaning range.
Slope is often expressed as a percentage in the field, and candidates plug percent directly into a feet-per-foot calculation, or they ignore the 2 ft/s self-cleaning threshold.
What to do instead
Convert percent slope to ft/ft by dividing by 100 before using it. Check that the resulting velocity is at least about 2 ft/s for gravity sewers to keep solids moving, and flag any answer below that as suspect.