NC Water/Wastewater Operator common mistakes
NC Water/Wastewater Operator · Understand the traps and how to avoid them.
Plugging numbers straight into the pounds formula without converting the flow to MGD first, e.g., using 150 gpm in 'mg/L x MGD x 8.34' and reporting a chlorine feed in lb/day that is roughly 1,000 times too small.
The 8.34 constant is tied to million gallons. Candidates memorize 'mg/L x MGD x 8.34' as a string but do not track the units of the flow they were handed, and gpm feels like 'a flow' so it goes in unchanged.
What to do instead
Before any calculation, write a unit-cancellation chain on scratch paper: gpm x 1440 min/day x 1 MG/1,000,000 gal = MGD. Do three practice conversions (gpm, cfs, gpd) to MGD until the conversion factors 0.00144, 0.6463, and 0.000001 are automatic, and always write the unit on the final answer.
Treating the chlorine dose as the residual, or subtracting the residual from the demand instead of from the dose when a problem gives dose and residual and asks for demand.
In the field a residual is what you measure, so the residual feels like 'the' chlorine number. The dose/demand/residual relationship (dose = demand + residual) is a bookkeeping identity that is never seen as a single equation on the plant floor.
What to do instead
Draw a one-line bar model every time: total applied dose, a chunk consumed by demand, the remainder is residual. Practice by being asked for each of the three unknowns from the other two, and check that the residual is always the smallest of the three numbers.
Setting a chemical feed pump by computing gallons per day of neat chemical and ignoring solution strength and specific gravity, or confusing 'percent solution' with 'mg/L of active ingredient'.
Feed calculations chain three separate ideas: dose in mg/L, active strength of the solution, and the weight of a gallon of that solution. Candidates collapse them into one step and lose the strength conversion entirely.
What to do instead
Use a fixed three-box worksheet: (1) lb/day of active chemical from mg/L x MGD x 8.34; (2) lb of active per gallon of solution = percent strength x 8.34 x specific gravity; (3) gpd = box 1 / box 2. Then verify with a drawdown test (mL/min out of a graduated cylinder) before trusting the number.
Computing pipeline or storage volume in cubic feet and then entering that number as gallons, or using 7.48 in the wrong direction when converting gallons back to cubic feet.
The volume formula produces cubic feet, which is an intermediate, not an answer. Under time pressure the intermediate gets treated as the destination, and 1 ft3 = 7.48 gal is recalled as a bare fact with no direction attached.
What to do instead
Annotate the working: 'ft3 (not an answer)' then multiply by 7.48 gal/ft3 and label the result 'gal'. For pipes, memorize gal = 0.785 x D2 (ft) x length (ft) x 7.48 and sanity-check that a mile of 12-inch main holds roughly 0.5 MG.
Using the wrong constant or omitting specific gravity and pump efficiency in horsepower problems, e.g., dividing gpm x head by 3,960 to get brake horsepower and then reporting that as the motor size.
Water horsepower and brake horsepower look like the same question, and 3,960 (water) versus dividing by efficiency (brake) is a one-word difference in the prompt that is easy to skim past.
What to do instead
Underline the words 'water' or 'brake' or 'motor' in the stem before solving. Memorize whp = gpm x head (ft) x SG / 3,960 and bhp = whp / efficiency, and always state the efficiency as a decimal (0.75, not 75) in the equation.
Choosing an RPZ, double check valve, or pressure vacuum breaker as the universal backflow answer, including for a situation that calls for a physical air gap.
All the devices appear together in a backflow chapter and the distinctions (backsiphonage only vs. backpressure, high vs. low hazard, continuous pressure) are memorized as a list rather than as decision rules.
What to do instead
Build a two-question decision tree and rehearse it: (1) Is the hazard high or low? (2) Is backpressure possible (pump, boiler, elevation) or only backsiphonage? Air gap is the only answer that protects against everything; RPZ for high hazard with backpressure; DCVA for low hazard, backsiphonage only; PVB never under continuous pressure.
Calculating percent removal as (in - out) / out x 100, which produces removal efficiencies over 100 percent on high-removal problems.
The 'out' value is the number the candidate just computed or focused on, so it ends up in the denominator by default. There is no physical intuition check because the answer 'sounds like' a big removal.
What to do instead
Always divide by the INFLUENT value, since removal is a fraction of what came in. Cross-check by asking what the formula gives for 200 in / 20 out: 90 percent is possible, 900 percent is not. Write 'in' under the denominator before substituting numbers.
Confusing MCRT, F/M ratio, and SVI, or computing solids inventory in pounds while leaving MLSS in mg/L instead of converting through aeration tank volume in MG.
All three are biomass-control numbers built from the same four inputs (MLSS, aeration volume, waste rate, influent BOD), so the formulas blur together, and the mg/L to lb conversion is an invisible step in every one of them.
What to do instead
Keep a single card listing what each number answers: MCRT = how long solids stay (lb solids in aeration / lb solids leaving per day); F/M = food per pound of bugs per day; SVI = how well sludge settles (settled volume mL/L x 1,000 / MLSS mg/L). For each, note that the numerator and denominator must be in the same units, usually pounds via mg/L x MG x 8.34.
Treating sewer slope as a percent rise over a run measured along the pipe, or adding the fall to the upstream invert instead of subtracting it, when computing invert elevations.
Percent slope and feet of fall per 100 feet of pipe are numerically the same number, which hides the unit, and gravity flow 'downstream' is obvious in the field but easy to invert on paper under time pressure.
What to do instead
Compute fall = slope (ft per 100 ft) / 100 x pipe length in feet, then subtract fall from the upstream invert to get the downstream invert. Sketch a small profile with two boxes and an arrow pointing downhill before substituting any numbers, and always check that the downstream invert is the lower number.
Answering confined space and trench questions from memory of 'test the air' without the required order (oxygen, then combustible gas, then toxics) or without the depth thresholds that trigger a protective system and a ladder.
The safety items are learned as a checklist of things to do rather than a sequence with numbers attached, so order and depth triggers feel interchangeable when they are not.
What to do instead
Memorize two short chains: test order O2 -> LEL -> H2S/CO, and depth rules (protective system required at 5 ft unless a competent person documents stable rock; ladder or other egress within 25 ft of lateral travel, required once depth reaches 4 ft). Rehearse them by writing the chain from blank paper, not by rereading notes.