Level 4 · Expert sample questions
5 free sample questions on math and calculations for the Level 4 · Expert wastewater treatment operator exam. Try each one, then open the answer to check your work.
A chlorine basin has 1.2 MG of effective volume and a 1.5 mg/L outlet residual. The plant requires a minimum CT of 600 mg-min/L. Current flow is 4.0 MGD, but a forecast peak is 4.5 MGD. Which conclusion is correct?
Answer: D. Current passes; peak flow fails
The current condition exceeds the plant requirement, but the forecast hydraulic peak reduces detention time enough to fall below it. Calculation: At 4.0 MGD, time = 432 min and CT = 648. At 4.5 MGD, time = 384 min and CT = 576.
A plant treats 3.0 MGD and needs an active sodium-bisulfite dose of 2.2 mg/L. The solution is 38% active by weight, density correction is not required, and the feed system can deliver 130 lb/day of solution. What is the operating conclusion?
Answer: C. Solution capacity is short by 15 lb/day
Feed capacity must be compared with total solution demand, not active chemical alone. Calculation: Active demand = 3.0 x 2.2 x 8.34 = 55.0 lb/day. Solution demand = 55.0 / 0.38 = 145 lb/day, about 15 lb/day above capacity.
A land-application site will be closed for 10 days. The plant produces 24 wet tons/day, covered storage starts empty and holds 100 tons, and an authorized landfill can accept 16 tons/day beginning on Day 2. Which plan stays within storage capacity?
Answer: C. Use storage, then the authorized landfill
Covered storage and the authorized landfill keep the maximum inventory at 96 tons, below the 100-ton capacity. Calculation: Day 1 inventory is 24 tons. From Day 2 through Day 10, storage gains 8 tons/day for nine days: 24 + (8 x 9) = 96 tons.
Four BOD results are 180, 186, 183, and 242 mg/L. The 242 mg/L bottle leaked during incubation. The other bottles and blank pass. A plant action level is 190 mg/L, and the same-day COD trend is unchanged. What should the supervisor use for the action-level decision?
Answer: B. Use the 183 mg/L mean and document the bottle exclusion
The passing blank and three intact bottles support the run. The damaged bottle is excluded and documented, so the supported mean remains below the stated plant action level. Calculation: (180 + 186 + 183) / 3 = 183 mg/L.
A verified monthly totalizer shows 88.0 MG. Daily records show 2.4 MGD for 10 days, 3.1 MGD for 19 days, and one missing day. The reconciled missing day exceeds the plant's 4.5-MGD review trigger. What should the supervisor report and review?
Answer: D. Report 2.93 MGD and investigate the 5.1-MG day
The verified monthly total provides the weighted average and reconciles the missing day to 5.1 MG, which exceeds the stated review trigger and warrants follow-up. Calculation: Missing day = 88.0 - [(2.4 x 10) + (3.1 x 19)] = 5.1 MG. Monthly average = 88.0 / 30 = 2.93 MGD.
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