--- title: "BOD and COD with significance" subject: "Sanitary Engineering" chapter: 5 key: "semester_6/sanitary_engineering:bod-cod-significance" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bod-cod-significance site: BCT Engineering --- # BOD and COD with significance Sanitary Engineering · Chapter 5: Characteristics and Examination of Wastewater - Asked in: 2071 Magh, 4 marks - Video: ["Difference Between BOD and COD | Water Pollution Indicators"](https://www.youtube.com/watch?v=57sV4wX8j3I) — TotalBlend - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bod-cod-significance - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-5 ## Question Describe in detail about BOD and COD with their significances. ## Short answer BOD measures biodegradable organic matter under aerobic conditions; it indicates sewage strength, guides treatment design, and determines stream oxygen demand. COD uses chemical oxidation for all organic matter, providing rapid results(2-3 hours). It is essential for industrial wastes and the BOD/COD ratio indicates biodegradability (above0.5 easily treatable). ## Answer **BOD (Biochemical Oxygen Demand)** is the amount of oxygen required by bacteria to oxidise (stabilise) the biodegradable organic matter in a water or wastewater sample under aerobic conditions at a specified temperature and time (standard: 5 days at 20°C). It is given in mg/L. **COD (Chemical Oxygen Demand)** is the amount of oxygen needed to oxidise all organic matter (biodegradable and non-biodegradable) by a strong chemical oxidant, potassium dichromate ($K_2Cr_2O_7$) in boiling acid medium, expressed in mg/L. The test takes about 2-3 hours. #### Significance of BOD 1. It measures the **strength (organic pollution load)** of sewage; raw domestic sewage has BOD₅ of 200-400 mg/L. 2. It is used to **design treatment plants** (trickling filter, activated sludge, ponds) and to find their efficiency. 3. It is used to find the **oxygen demand on a receiving stream**, and so the degree of treatment needed before discharge (self-purification calculations). 4. It is the main parameter in effluent standards (for example, effluent standards in Nepal and India limit BOD₅ to about 30-50 mg/L for discharge to inland surface water). 5. It measures only the biodegradable organic matter and gives an idea of how much will be decomposed in nature. #### Significance of COD 1. It is a **rapid test** (hours against 5 days) and can be used for plant control. 2. It also includes non-biodegradable matter, so it is used for industrial wastes that are toxic to bacteria, where BOD cannot be measured. 3. COD is always greater than BOD. The ratio **BOD/COD** shows biodegradability: above about 0.5 the waste is easily treated biologically; below 0.3 it is difficult and contains toxic or refractory matter. 4. It is used with BOD to estimate the non-biodegradable part: $COD-BOD_u$. #### Difference in brief | Point | BOD | COD | |---|---|---| | Oxidising agent | Bacteria (biological) | Potassium dichromate (chemical) | | Measures | Biodegradable organic matter only | Biodegradable and non-biodegradable organic matter | | Time | 5 days (standard) | 2-3 hours | | Value | Lower | Higher: COD > BOD | | Use | Domestic sewage, plant design, stream study | Industrial wastes, toxic wastes, quick plant control | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which statement correctly defines BOD? A. Oxygen used by bacteria to stabilise biodegradable organic matter under aerobic conditions, standard 5 days at 20°C B. Oxygen needed by potassium dichromate to oxidise all organic matter in boiling acid C. Oxygen present in the water sample after 5 days at 20°C D. Oxygen used by bacteria in 20 days at 5°C Answer: A. BOD is the oxygen consumed by bacteria in oxidising biodegradable organic matter, measured at 20°C for 5 days. (Key point: Definition of BOD) 2. Which is a significance of BOD in sewage practice? A. It gives the pH of the sewage directly B. It gives the strength of sewage and is used to design treatment plants and find their efficiency C. It measures non-biodegradable matter in industrial waste D. It is a rapid test usable within 2-3 hours Answer: B. BOD measures organic pollution load and is used for design, efficiency and stream oxygen demand studies. (Key point: Use of BOD) 3. Why is COD preferred for toxic industrial wastes? A. COD takes 5 days, which suits industrial waste B. COD measures only biodegradable matter C. Toxic wastes kill the bacteria, so BOD cannot be found, but a chemical oxidant still works D. Industrial wastes contain no organic matter Answer: C. Bacteria are inhibited by toxic wastes, so the BOD test fails, while COD relies on chemical oxidation. (Key point: Why COD for industrial waste) 4. Which pair correctly describes the COD test? A. Bacteria at 20°C, 5 days B. Potassium permanganate at 37°C, 24 hours C. Bacteria in acid, 2-3 hours D. Potassium dichromate in boiling acid, about 2-3 hours Answer: D. COD uses K2Cr2O7 in boiling acid and is a rapid test compared with BOD. (Key point: COD test details) ## Related in Sanitary Engineering - [BOD/COD significance; BOD at 30°C](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-cod-significance-temp-conversion) — Sanitary Engineering, chapter 5 - [BOD, COD; ultimate BOD of 480 mg/l](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-cod-ultimate-bod-480) — Sanitary Engineering, chapter 5 - [Define BOD, COD; derive BOD equation](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Abod-cod-derive-equation) — Sanitary Engineering, chapter 5 ## Related in other subjects - [BOD versus COD and a BOD and COD calculation](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Abod-cod-difference-numerical) — Environment and Pollution Control, chapter 4