12.2 BOD, CBOD & Whole-Effluent Toxicity Analysis
Key Takeaways
- Select BOD or CBOD according to the required parameter.
- Use multiple dilutions and evaluate blank/seed/DO validity before reporting.
- WET depends on organism controls and prescribed endpoints.
- Connect results to process evidence without overstating what the method proves.
12.2 BOD, CBOD & Whole-Effluent Toxicity Analysis
2025 WPI alignment: This section teaches conducting and interpreting biological analyses such as BOD, CBOD, and whole-effluent toxicity in Laboratory Analysis, the 15-question area containing 3 recall, 12 application, and 4 calculation items.
Why this laboratory task matters
BOD/CBOD estimates oxygen consumed during a specified test, while WET exposes test organisms to effluent to assess aggregate toxicity. Both require method-controlled samples, valid controls, and careful distinction between what the result measures and what it does not.
Analytical foundation
| Element | What makes the result defensible |
|---|---|
| BOD | Dissolved oxygen depletion in a diluted sample reflects biodegradable demand plus any nitrogenous demand allowed by the method. |
| CBOD | A nitrification inhibitor suppresses nitrogenous oxygen demand so the test emphasizes carbonaceous demand. |
| Seed and blank | Seed supplies organisms when needed; blanks and seed controls identify oxygen demand not attributable to the sample. |
| Valid depletion | Initial/final DO and dilution must meet the approved method’s validity criteria; a depleted bottle cannot be rescued by arithmetic. |
| WET | Acute or chronic organism response evaluates combined effluent toxicity rather than naming one chemical cause. |
| Controls/dilution series | Control survival, reference toxicant or other required QC, test conditions, and concentration series determine WET validity. |
Laboratory workflow
- Review sample type, preservation, collection time, residual neutralization, expected demand/toxicity, and method deadline.
- Prepare dilution water, seed if required, inhibitor for CBOD, blanks, controls, and a range of sample dilutions.
- Measure initial conditions accurately and incubate/test under controlled method conditions.
- Measure final DO or organism endpoints at specified times without substituting observations.
- Evaluate blank, seed, depletion/residual, control, and other QC criteria before calculation.
- Report the correct BOD/CBOD basis or WET endpoint with qualifiers and compare with related process data.
Quality and diagnostic evidence
| Finding | Meaning | Correct response |
|---|---|---|
| Bottle DO is exhausted | Dilution was too strong or demand too high for a valid result | Use an acceptable lower sample volume if method/time permits. |
| Blank depletion excessive | Dilution water, reagents, seed, or handling introduced demand | Investigate and reject affected calculations as required. |
| BOD high but CBOD much lower | Nitrogenous oxygen demand contributed to BOD | Review nitrification/ammonia context and permit parameter. |
| WET control survival fails | Test conditions cannot support a valid toxicity conclusion | Follow invalidation/retest rules. |
Calculation and interpretation
WPI gives unseeded BOD = (initial DO − final DO) × 300 mL / sample volume for the standard 300 mL bottle arrangement, and a seeded relationship that subtracts seed correction. Use the version matching the stem. Do not apply the CBOD inhibitor to a BOD test unless the specified method requires it. WET endpoints use the method’s statistical or pass/fail procedure, not the BOD formula.
Worked laboratory scenario
An unseeded 15 mL sample in a 300 mL bottle drops from 8.5 to 3.5 mg/L DO and otherwise meets method criteria. BOD is (8.5 − 3.5) × 300 / 15 = 100 mg/L. If final DO had been exhausted or the blank failed, that arithmetic would not make the bottle valid.
Common exam traps
- BOD and CBOD differ because CBOD suppresses nitrification under its method.
- COD cannot be substituted for BOD using a universal conversion factor.
- A WET failure shows aggregate biological effect, not a uniquely identified toxicant.
- Check validity criteria before calculating a bottle result.
Field-to-exam checklist
- Select BOD or CBOD according to the required parameter.
- Use multiple dilutions and evaluate blank/seed/DO validity before reporting.
- WET depends on organism controls and prescribed endpoints.
- Connect results to process evidence without overstating what the method proves.
Using a dilution series
The best BOD dilution is not known precisely before testing, so a series protects against both complete DO depletion and too little measurable depletion. Review all valid bottles and apply the method’s selection or averaging rule; do not choose only the bottle that produces a favorable permit result. Historical wastewater strength helps plan dilutions, but storm flow, industrial loading, nitrification, and treatment changes can shift demand. Seeded and inhibited bottles must remain clearly identified throughout preparation and calculation.
Dilution planning, seeding, and validity criteria
The five-day BOD test has explicit validity criteria, and they are the most commonly tested facts in this area. The test incubates in the dark at 20 ± 1 °C for 5 days ± 6 hours. A bottle counts toward the result only if it shows at least 2.0 mg/L of dissolved-oxygen depletion and retains at least 1.0 mg/L of residual dissolved oxygen at the end. Dilution-water blank depletion should not exceed 0.20 mg/L. Arithmetic cannot rescue a bottle that went anaerobic, and a bottle that barely moved is not precise enough to report.
Plan the dilution series from expected strength. Raw domestic wastewater near 200 mg/L needs a small sample volume; a well-nitrified secondary effluent near 5 mg/L may use a large one. Preparing three volumes across the expected range is the practical protection against being wrong about the strength, especially after a storm or an industrial discharge.
Seeding is required where organisms are scarce or damaged — chlorinated effluent, high-temperature waste, or a stream with an unusual industrial character. The seed correction subtracts the oxygen the seed alone consumed, which is why the seed control bottles must be prepared and read in the same batch.
Worked seeded calculation. A 30 mL sample in a 300 mL bottle shows a 4.2 mg/L dissolved-oxygen drop; the seed correction for that bottle is 0.4 mg/L. BOD = (4.2 − 0.4) x 300 / 30 = 3.8 x 10 = 38 mg/L. Using the unseeded formula would have reported 42 mg/L and would have credited the seed's oxygen demand to the sample.
CBOD is a different reported parameter. The nitrification inhibitor suppresses nitrogenous demand, so a strongly nitrifying effluent can show BOD well above CBOD. The two cannot be reported interchangeably regardless of which is more favourable.
An unseeded 15 mL sample in a 300 mL bottle loses 5.0 mg/L DO. Using the WPI formula, what is BOD?
What does a valid whole-effluent toxicity result evaluate?