16 Laboratory Analysis Practice Questions & Answers

1. 1. A BOD batch is set up correctly, but the incubator's temperature log shows the unit drifted outside the method's specified range overnight. The analyst notices this before reporting. What is the correct action?

   • A.Apply a temperature correction factor to the depletion before reporting
   • B.Report the values with a footnote describing the temperature drift
   • C.Report the values because the duplicate bottles agreed closely
   • D.Invalidate the batch and rerun the test on freshly collected samples✓ Answer

   Incubation temperature is a defining condition of the test, not an adjustable variable: outside it the rate of microbial oxygen uptake no longer represents the standardized measurement, so the numbers are not a result at all. A result produced outside the written procedure cannot be rescued by a footnote, a correction factor, or good precision between duplicates — precision only shows the two bottles were wrong the same way. The batch is invalidated and the analysis repeated on new samples.

   Source: WPI 2025 Need-to-Know Criteria, Wastewater Treatment Class II, Laboratory Analysis — 'follow laboratory SOPs'; Standard Methods 5210 B incubation conditions; 40 CFR 136 approved-method requirementReport a problem with this question

2. 2. An automatic sampler collects a 24-hour flow-proportional composite of final effluent. Which parameter CANNOT be validly reported from that composite jug and must be run on a grab sample?

   • A.Total residual chlorine✓ Answer
   • B.Chemical oxygen demand
   • C.Total suspended solids
   • D.Ammonia nitrogen

   Chlorine residual is chemically unstable — it reacts with organics and off-gasses within minutes — so any value measured hours after collection reflects decay in the jug, not the effluent leaving the plant. Parameters that change this fast or cannot be chemically preserved (chlorine residual, pH, temperature, dissolved oxygen, coliform) are grab-only. COD, TSS and ammonia are stable enough with cooling and acid preservation for a composite, which is the better sample for loading because it averages the day's variation.

   Source: 40 CFR 136 Table II sample-type requirements; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.), laboratory chapter — grab vs. compositeReport a problem with this question

3. 3. An operator collects the daily influent sample by dipping a beaker into the quiet surface water of a wide channel just behind a baffle. Why is this sample unlikely to be representative?

   • A.It cools too rapidly at the surface and loses its dissolved oxygen
   • B.It picks up extra grit scoured from the bottom of the channel
   • C.It over-represents floating material and misses suspended and settled solids✓ Answer
   • D.It is diluted by plant effluent recirculating past the baffle

   A quiescent surface behind a baffle is where grease and floatables accumulate while heavier solids settle out below, so a surface dip samples a stratified layer rather than the flow. Solids and BOD results from it will not match the load the plant actually receives, and every downstream calculation built on that number — loading, percent removal, F/M — is wrong by the same amount. Samples are taken at a turbulent, well-mixed point where the stream is homogeneous.

   Source: CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.), laboratory chapter — representative sampling and sample locationReport a problem with this question

4. 4. What does a chain-of-custody record establish about a sample submitted for permit reporting?

   • A.That the analysis met the method's precision and accuracy criteria
   • B.That the analyst holds current certification for the method that was run
   • C.Who possessed the sample and how it was handled from collection to analysis✓ Answer
   • D.Instrument tolerance held between its calibration checks

   Chain of custody is a possession and handling history: location and time of collection, sample type, collector, preservation, transfer signatures and storage conditions. It answers the question 'is this sample still the water we say it is?' — the traceable link between the point sampled and the number reported. Analyst qualification, instrument drift and method performance are documented separately, in training files, calibration logs and QC records.

   Source: 40 CFR 122.41(j) monitoring records content; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — sample records and chain of custodyReport a problem with this question

5. 5. A metals sample is acidified in the field and placed on ice. What does preservation accomplish, as distinct from holding time?

   • A.It removes the interferences that would otherwise bias the analysis
   • B.It extends the sample past the maximum time allowed before analysis
   • C.It replaces refrigeration when a chilled cooler is not available
   • D.It slows the physical, chemical and biological changes that alter the sample✓ Answer

   Preservation is chemical and thermal retardation: acid keeps metals in solution off the container walls and cold suppresses biological activity, so the sample stays closer to what it was at the moment of collection. Holding time is a separate, regulatory ceiling on how long the sample remains acceptable even when properly preserved — preservation buys quality within that window, never an extension of it. Acid and cold work together; neither substitutes for the other.

   Source: 40 CFR 136.3(e) Table II preservation requirements; Standard Methods 1060 B sample preservation principlesReport a problem with this question

6. 6. A BOD test requires a dilution with at least 2.0 mg/L (2.0 ppm) of DO depletion AND at least 1.0 mg/L (1.0 ppm) of DO remaining at day 5. All bottles started at 8.8 mg/L. Final DO was 7.5 mg/L in the 5 mL bottle, 4.9 mg/L in the 15 mL bottle, and 0.4 mg/L in the 50 mL bottle. Which dilution is usable?

   • A.The 5 mL and 50 mL dilutions
   • B.The 50 mL dilution only
   • C.The 5 mL dilution only
   • D.The 15 mL dilution only✓ Answer

   The 5 mL bottle depleted only 1.3 mg/L, too little for the measurement to be distinguishable from meter and handling error. The 50 mL bottle depleted 8.4 mg/L but ended at 0.4 mg/L, meaning oxygen ran short before day 5 and the true demand is unknown and understated. The 15 mL bottle depleted 3.9 mg/L and still held 4.9 mg/L, satisfying both criteria: enough change to measure, and proof that oxygen never limited the organisms.

   Source: Standard Methods 5210 B dilution validity criteria; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.), BOD procedureReport a problem with this question

7. 7. On the same secondary effluent sample, BOD is reported as 145 mg/L (145 ppm) and CBOD as 82 mg/L (82 ppm). What does the difference between the two results indicate?

   • A.The dilution water blank depleted more oxygen than the method allows
   • B.Nitrifying organisms are exerting an oxygen demand in the uninhibited bottle✓ Answer
   • C.The CBOD bottle was seeded while the BOD bottle was left unseeded
   • D.The inhibitor suppressed the carbonaceous organisms in the CBOD bottle

   CBOD is the same test with a nitrification inhibitor added, which blocks the ammonia-oxidizing bacteria without touching the heterotrophs that consume carbonaceous material. The oxygen that disappears in the uninhibited bottle but not the inhibited one is therefore nitrogenous demand, so a wide BOD-minus-CBOD gap is the lab's signature that nitrification is well established in the plant. CBOD can never exceed BOD on the same sample.

   Source: Standard Methods 5210 B nitrogenous demand and inhibition; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — CBODReport a problem with this question

8. 8. An operator suspects an industrial slug hit the plant this morning and wants a laboratory number today to decide whether to divert flow to the equalization basin. Which test fits that decision, and why?

   • A.BOD, because it responds only to biodegradable organic material
   • B.BOD, because the result is available within hours of setup
   • C.COD, because the result is available within hours of setup✓ Answer
   • D.COD, because it responds only to biodegradable organic material

   This is the central trade-off in loading tests: BOD measures the demand the biology will actually see, but it arrives five days late and cannot steer today's process, while COD chemically oxidizes the sample in a couple of hours. COD is the broader measure — it also counts material the bugs cannot use — so it is not interchangeable with BOD for permit reporting, but for a same-day operating decision a fast, broader number beats an exact number that arrives after the slug has passed through.

   Source: CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — COD vs. BOD, use of COD for process controlReport a problem with this question

9. 9. An unseeded BOD is set up with 10 mL of sample in a 300 mL bottle. Initial DO is 8.6 mg/L (8.6 ppm) and final DO after 5 days is 3.4 mg/L (3.4 ppm). What is the BOD?

   • A.258 mg/L (258 ppm)
   • B.5.2 mg/L (5.2 ppm)
   • C.156 mg/L (156 ppm)✓ Answer
   • D.15.6 mg/L (15.6 ppm)

   Unseeded BOD = (initial DO - final DO) x 300 / mL of sample, so (8.6 - 3.4) x 300 / 10 = 5.2 x 30 = 156 mg/L. The bottle factor scales the depletion measured in a mostly-dilution-water bottle up to the concentration of the undiluted sample; leaving it out reports the raw depletion of 5.2 mg/L, and using the initial DO instead of the difference gives 258 mg/L. The multiplier is the whole point of the dilution technique and is supplied on the exam formula table.

   Source: WPI Wastewater Formula/Conversion Table — BOD, unseeded; Standard Methods 5210 B calculationReport a problem with this question

10. 10. A 100 mL portion of mixed sample is filtered for TSS. The pre-weighed filter is 1.2140 g and the dried filter with residue is 1.2352 g. What is the TSS concentration?

    • A.2.12 mg/L (2.12 ppm)
    • B.212 mg/L (212 ppm)✓ Answer
    • C.21.2 mg/L (21.2 ppm)
    • D.2,120 mg/L (2,120 ppm)

    The residue weight is 1.2352 - 1.2140 = 0.0212 g, and mg/L = (grams of dry solids)(1,000,000) / (mL of sample), so 0.0212 x 1,000,000 / 100 = 212 mg/L. The million in the numerator is doing two unit conversions at once, grams to milligrams and millilitres to litres; dividing by 1,000 mL instead of 100 mL gives 21.2 and treating the volume as 10 mL gives 2,120. Unit handling, not the arithmetic, is where this item is normally lost.

    Source: WPI Wastewater Formula/Conversion Table — solids, mg/L; Standard Methods 2540 D total suspended solidsReport a problem with this question

11. 11. A settleometer test on mixed liquor gives 320 mL/L of settled sludge after 30 minutes, and the MLSS on the same mixed liquor is 2,400 mg/L (2,400 ppm). What is the sludge volume index?

    • A.1,333 mL/g
    • B.133 mL/g✓ Answer
    • C.750 mL/g
    • D.13.3 mL/g

    SVI = (settled volume, mL/L)(1,000 mg/g) / (MLSS, mg/L) = 320 x 1,000 / 2,400 = 133 mL/g. The 1,000 converts milligrams to grams so the index reports the volume occupied by one gram of solids; dropping it gives 13.3 and inverting the ratio gives 750. A value near 133 is normal settling, but the operating value is in the trend — an index climbing day over day alongside rising effluent solids points to filamentous bulking rather than hydraulic overload.

    Source: WPI Wastewater Formula/Conversion Table — sludge volume index; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — SVI interpretationReport a problem with this question

12. 12. A permit expresses the fecal coliform limit as a geometric mean. Four results for the period are 40, 100, 250 and 1,000 colonies/100 mL. What value is reported?

    • A.175 colonies/100 mL
    • B.178 colonies/100 mL✓ Answer
    • C.348 colonies/100 mL
    • D.1,000 colonies/100 mL

    The geometric mean is the nth root of the product of the n values: 40 x 100 x 250 x 1,000 = 1,000,000,000, and the fourth root (the square root taken twice) is about 178 colonies/100 mL. Bacterial counts span orders of magnitude, so an arithmetic mean lets one high value dominate the period — here it would report 348 and could fail a limit the geometric mean meets. Reporting the median (175) or the maximum are the other two ways this is commonly missed.

    Source: WPI Wastewater Formula/Conversion Table — geometric mean; 40 CFR 122.45(d) bacterial limit expressionReport a problem with this question

13. 13. Which laboratory test provides the settled-volume number an operator uses with MLSS to track activated sludge settling day to day?

    • A.One litre of mixed liquor settled 30 minutes in a graduated cylinder✓ Answer
    • B.One litre of final effluent settled 30 minutes in a cone
    • C.One litre of final effluent settled 60 minutes in a graduated cylinder
    • D.One litre of mixed liquor settled 60 minutes in a cone

    The settleometer or settled-sludge-volume test uses mixed liquor in a graduated cylinder or settleometer read at 30 minutes, and that volume divided into MLSS produces the sludge volume index. The cone test read at 60 minutes measures settleable solids in influent or effluent in mL/L and never feeds SVI — mixing up these two is the classic error. Either test is read as a trend, since one day's number says little without the days around it.

    Source: CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — settleability test and SVI; Standard Methods 2540 F settleable solidsReport a problem with this question

14. 14. A basin dissolved oxygen probe has read a comfortable, steady 2.2 mg/L (2.2 ppm) for weeks, yet the mixed liquor has turned septic and effluent quality is slipping. A freshly calibrated portable meter reads 0.3 mg/L (0.3 ppm) at the same point. What is the correct conclusion?

    • A.The basin is adequately aerated because two meters seldom agree closely
    • B.The installed probe has drifted low and the blowers should be throttled back
    • C.The portable meter is in error because its membrane was recently replaced
    • D.The installed probe has drifted high and must be cleaned and recalibrated✓ Answer

    The process evidence and the independent check agree with each other and against the installed probe: septic odour and deteriorating effluent are what a near-zero DO looks like, so the fixed probe is reading high. This is the danger of an uncalibrated or fouled instrument — it does not go obviously dead, it reports a reassuring number while the basin starves and the operator cuts air that is already short. The rule is to verify each instrument against an independent standard or reference on a schedule, and to trust the verified reading.

    Source: WPI 2025 Need-to-Know Criteria, Class II — 'operate and maintain laboratory instrumentation (DO, pH, H2S, ORP)'; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.) — DO probe calibration and maintenanceReport a problem with this question

15. 15. Bench pH readings on aeration basin samples have become erratic. Which practice best restores confidence in the meter?

    • A.Calibrate with two buffers that bracket the expected sample pH✓ Answer
    • B.Calibrate with one buffer chosen near the middle of the scale
    • C.Store the electrode in distilled water to keep the bulb clean
    • D.Store the electrode dry between uses to protect the glass bulb

    A pH meter converts electrode millivolts to pH units using a slope, and a single buffer fixes only one point, leaving the slope assumed rather than measured; two buffers that straddle the sample define the response across the range actually used. Storage matters too, but in the opposite direction from the distractors: the bulb must stay wet in storage or filling solution, because drying it out or soaking it in distilled water leaches ions from the gel layer and produces exactly this drift.

    Source: Standard Methods 4500-H+ B pH electrometric method — multi-point buffer calibration and electrode storage; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.), instrumentationReport a problem with this question

16. 16. The method's countable range for a membrane filter is 20 to 60 colonies per filter. The largest volume filtered in the dilution series produced only 4 colonies. How should the analyst handle this?

    • A.Report the count as an estimate and filter larger volumes next time✓ Answer
    • B.Multiply the count by the dilution factor to bring it into range
    • C.Report the count as a valid result since colonies were clearly countable
    • D.Discard the filter and report the sample as free of coliforms

    The countable range exists because the statistical uncertainty of a plate count explodes at low colony numbers — a handful of colonies could as easily have been half or double that by chance, so the calculated density carries far more error than the arithmetic suggests. A count below the range is reported as an estimate, and the fix is a larger filtered volume in the next dilution series. Multiplying by a dilution factor does not create precision that was never in the plate, and few colonies is not the same finding as none.

    Source: Standard Methods 9222 B/D membrane filter countable range and reporting of estimated counts; CSUS Operation of Wastewater Treatment Plants, Vol. 2 (8th ed.), bacteriological testingReport a problem with this question

Practice questions written against the 2025 standardized Wastewater Treatment Operator Need-to-Know Criteria published by Water Professionals International (formerly the Association of Boards of Certification) and standard references from the CSUS Office of Water Programs and the Water Environment Federation. This site is not affiliated with or endorsed by WPI/ABC, WEF, or the US EPA. Operator certification is issued by your state's certifying authority, which sets plant classification tiers, operator grades, eligibility, and the passing standard — confirm those with your state before testing, and confirm which edition of the exam your program has adopted. Effluent limits, monitoring frequencies and reporting requirements come from an individual discharge permit and are revised over time, so no answer here should be relied on as a current regulatory or permit value; consult the permit in force for your facility. This bank covers the wastewater treatment exam only — drinking-water treatment, water distribution, and wastewater collection are separate certifications. About the Need-to-Know Criteria →