--- title: "BOD test by dilution method" subject: "Sanitary Engineering" chapter: 5 key: "semester_6/sanitary_engineering:bod-determination-dilution-method" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bod-determination-dilution-method site: BCT Engineering --- # BOD test by dilution method Sanitary Engineering · Chapter 5: Characteristics and Examination of Wastewater - Asked in: 2073 Magh, 8 marks; 2072 Magh, 4 marks (asked 2 times) - Video: [Experiment No. 8: Determination of Biochemical Oxygen Demand (BOD)](https://www.youtube.com/watch?v=aMyhmUB4eb8) — Dr. Harish Kumar .S - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bod-determination-dilution-method - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-5 ## Question Why is BOD value important for waste water? Describe the procedure for determining BOD of waste water sample in laboratory by dilution method. ## Short answer BOD measures biodegradable organic strength for treatment design and effluent standards. Dilution method: incubate diluted sample (1-5% for sewage) in 300 mL BOD bottles at 20°C for 5 days in dark. Measure initial and final DO; BOD₅ = (D₁-D₂)/P where P = sample volume/bottle volume. Ensure 2 mg/L DO depletion and 1 mg/L residual DO. Seed correction applied if needed. ## Answer #### Importance of BOD value BOD tells the amount of biodegradable organic matter in wastewater, i.e. its **strength and polluting power**. It is needed to - design the treatment units and assess their performance (percentage BOD removal), - decide the degree of treatment before disposal into a river, from the oxygen demand on the stream, - check effluent against standards, and - know the oxygen the sample would take from receiving water (cause of fish kill and odours). A high BOD shows heavily polluted wastewater (raw sewage 200-400 mg/L; treated effluent should be below 30-50 mg/L). #### Principle A diluted sample is incubated for 5 days at 20°C in the dark in a standard bottle, and the oxygen consumed by the microorganisms in oxidising the organic matter is measured by the **dissolved oxygen (DO) before and after** incubation. #### Apparatus and reagents BOD bottles (300 mL, glass-stoppered), incubator at 20 ± 1°C, pipettes, DO meter or Winkler titration set. Dilution water is prepared from distilled water, with 1 mL each per litre of phosphate buffer, $MgSO_4$, $CaCl_2$ and $FeCl_3$ solutions (nutrients for bacteria), and aerated to saturate with oxygen. Seed (settled domestic sewage) is added if the sample has no bacteria. #### Procedure 1. **Select dilutions.** Sewage needs far more oxygen than water can hold, so it is diluted in such a way that, after 5 days, at least **2 mg/L of DO is used up and at least 1 mg/L of DO remains**. Take 1-5% for strong wastes (raw sewage). 2. **Prepare** the diluted samples: pour a measured volume of sample (e.g. 5 mL) into each of the BOD bottles and fill to the neck with dilution water without trapping air bubbles. Make a **blank** with only dilution water. 3. **Initial DO ($D_1$):** measure the DO of one set of bottles immediately (Winkler azide method or probe). 4. **Incubate** the second set, with stoppers sealed by a water seal, for **5 days at 20°C in the dark** (to stop algae making oxygen). 5. **Final DO ($D_2$):** after 5 days measure the DO of the incubated bottles. 6. **Calculate** $$ BOD_5=\frac{D_1-D_2}{P}\ \text{mg/L},\qquad P=\frac{\text{volume of sample}}{\text{volume of diluted mixture}} $$ For example, 5 mL of sample in a 300 mL bottle gives $P=5/300=0.0167$. With $D_1=9$ and $D_2=5$ mg/L, $BOD_5=4/0.0167=240$ mg/L. For seeded samples, a seed correction is applied, and the blank is used to check the dilution water (its depletion should be below 0.2 mg/L). The ultimate BOD can be found by $BOD_t = L(1-10^{-kt})$ if $k$ is known. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does BOD indicate about wastewater? A. Hardness B. Total dissolved solids only C. Amount of biodegradable organic matter, i.e. its strength D. Colour Answer: C. BOD measures biodegradable organic strength. (Key point: Importance) 2. What are the standard BOD test conditions? A. 1 day at 37°C in light B. 3 days at 25°C in light C. 10 days at 4°C D. 5 days at 20°C in the dark Answer: D. Dark prevents algae producing oxygen. (Key point: Standard test) 3. What criteria guide dilution selection? A. At least 2 mg/L DO used and at least 1 mg/L remaining after 5 days B. Zero DO used C. All DO consumed D. DO increases Answer: A. Ensures measurable depletion while some oxygen remains. (Key point: Dilution) 4. BOD5 = ? A. (D1 + D2) x P B. (D1 - D2)/P where P is sample volume/diluted volume C. D1/D2 D. (D1 - D2) x P Answer: B. Depletion divided by the dilution fraction. (Key point: Calculation) ## Related in Sanitary Engineering - [Sewage sampling and BOD dilution method](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asampling-bod-dilution) — Sanitary Engineering, chapter 5 - [BOD of Bagmati sample at 30°C](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-thapathali-4day-30c) — Sanitary Engineering, chapter 5 - [BOD5 and ultimate BOD, 5% dilution](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-dilution-5pct-blank) — Sanitary Engineering, chapter 5 - [Temperature effect on BOD; 5-day BOD](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atemperature-effect-decomposition) — Sanitary Engineering, chapter 5 - [Ultimate BOD and rate constant](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-ultimate-rate-constant-100-210) — Sanitary Engineering, chapter 5 - [Why examine wastewater; solids procedure](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aexamination-necessity-solids-procedure) — Sanitary Engineering, chapter 5 ## Related in other subjects - [Procedure: difference of two proportions, large sample](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Adifference-two-proportions-large-procedure) — Probability and Statistics, chapter 7, asked 2× - [Aggregate crushing value test](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Acrushing-value) — Transportation Engineering I, chapter 6, asked 7× - [Aggregate impact value test](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aimpact-value) — Transportation Engineering I, chapter 6, asked 3× - [t-test assumptions; two means small sample](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3At-test-assumptions-small-sample-procedure) — Probability and Statistics, chapter 6