--- title: "Self-purification of rivers; sag curve" subject: "Sanitary Engineering" chapter: 6 key: "semester_6/sanitary_engineering:self-purification-sag-curve" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/self-purification-sag-curve site: BCT Engineering --- # Self-purification of rivers; sag curve Sanitary Engineering · Chapter 6: Wastewater Disposal - Asked in: 2070 Bhadra, 8 marks - Video: [oxygen sag curve | Deoxygenation | Reoxygenation | BOD | oxygen deficit | zone of self purification](https://www.youtube.com/watch?v=quBzazdeysw) — Civil Engineering Exam - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/self-purification-sag-curve - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-6 ## Question Discuss the process of self-purification of river and factors affecting the process. Draw the oxygen sag curve showing the zones of pollution along river. ## Short answer Self-purification is the natural recovery of a polluted stream through dilution, sedimentation, aerobic oxidation, reaeration, sunlight, and biological actions. Factors include dilution, temperature, sunlight, velocity, reaeration rate, initial DO, sedimentation, pollutant nature, and aquatic life. The oxygen sag curve shows four zones: degradation (DO drops to 40% saturation), active decomposition (minimum DO at critical point), recovery (DO rises), and clear water (DO near saturation). ## Answer #### Self-purification of a river **Self-purification** is the natural process by which a polluted stream recovers its original quality by physical, chemical and biological actions, without human help. When sewage enters a river, the oxygen in the water is used for decomposition, but atmospheric oxygen and photosynthesis restore it as the stream flows downstream. **Process** 1. **Dilution** by river water reduces concentration of pollutants. 2. **Sedimentation** of the suspended matter. 3. **Oxidation** of organic matter by aerobic bacteria (deoxygenation): $\text{organic matter}+O_2\to CO_2+H_2O+\dots$ 4. **Reaeration** (reoxygenation) from the atmosphere through the surface, aided by turbulence, and oxygen from aquatic plants (photosynthesis). 5. **Sunlight** kills bacteria; **reduction** by anaerobic decomposition of the deposited sludge; and **predation** by protozoa. #### Oxygen sag curve and zones of pollution The DO is used by bacteria (deoxygenation) at a rate $K_1L_t$ and replaced from air (reaeration) at a rate $K_2D_t$. The net result is the **oxygen sag** curve: $$ D_t=\frac{K_1L_0}{K_2-K_1}\left(10^{-K_1t}-10^{-K_2t}\right)+D_010^{-K_2t} $$ ```text DO sat |----. ______ recovery | '. .' | '. .' | '. Dc .' min | '.___v_____.-' | Degradation | Active |Recovery| Clean | zone |decomp. | zone | water +---------------------------------------> distance/time outfall critical point ``` **Zones** 1. **Zone of degradation (just downstream of the outfall):** water turns turbid and grey, DO falls to about 40% of saturation, aerobic bacteria and fungi grow, and fish may disappear. 2. **Zone of active decomposition:** DO falls to a minimum (zero in a heavily polluted stream, then anaerobic with black colour, bubbles of gas and bad smell). The **critical point** (maximum deficit $D_c$) is at the lowest DO. 3. **Zone of recovery:** reaeration exceeds the deoxygenation; DO rises, water clears, fish return, and nitrates form. 4. **Zone of clear water:** the river is restored to its original state with DO near saturation, and with the BOD satisfied. The critical time is $t_c=\dfrac{1}{K_2-K_1}\log_{10}\left[\dfrac{K_2}{K_1}\left(1-\dfrac{(f-1)D_0}{L_0}\right)\right]$, where $f=K_2/K_1$. #### Factors affecting self-purification - **Dilution (river flow):** more flow dilutes the waste and supplies more oxygen. - **Temperature:** speeds up decomposition but lowers the oxygen solubility. - **Sunlight:** kills bacteria and helps algae release oxygen. - **Velocity and turbulence:** increase reaeration. - **Nature and amount of pollutant, and the initial DO of the river.** - **Sedimentation** and the **presence of algae and other aquatic life.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is self-purification of a river? A. Removal of pollutants by a treatment plant built on the bank B. Natural recovery of a polluted stream to its original quality by physical, chemical and biological actions C. Addition of chlorine to the river to kill bacteria D. Dredging of the river bed to remove sludge Answer: B. Self-purification is the natural recovery of a polluted stream without human help, through dilution, sedimentation, oxidation, reaeration and similar actions. (Key point: Definition) 2. In the oxygen sag, what causes the DO to fall and what restores it? A. Reaeration uses up DO; bacterial oxidation restores it B. Sedimentation uses up DO; sunlight restores it C. Bacterial oxidation of organic matter uses up DO; reaeration and photosynthesis restore it D. Dilution uses up DO; turbulence restores it Answer: C. Aerobic bacteria consume oxygen while decomposing waste (deoxygenation, rate K1Lt), while atmospheric reaeration (K2Dt) and photosynthesis add oxygen. (Key point: Deoxygenation vs reaeration) 3. Which is the correct order of zones going downstream from the outfall? A. Active decomposition, degradation, recovery, clear water B. Recovery, degradation, active decomposition, clear water C. Clear water, degradation, active decomposition, recovery D. Degradation, active decomposition, recovery, clear water Answer: D. Just below the outfall is the zone of degradation, then active decomposition (minimum DO), then recovery, and finally clear water. (Key point: Zones of pollution) 4. Where is the critical point on the oxygen sag curve? A. Where the DO deficit is maximum, i.e. DO is lowest B. At the outfall where the sewage enters C. Where the DO is at saturation D. Where reaeration rate becomes zero Answer: A. The critical point lies in the zone of active decomposition where the deficit Dc is maximum and DO is minimum; after it reaeration exceeds deoxygenation. (Key point: Critical point) ## Related in Sanitary Engineering - [Factors affecting self-purification](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aself-purification-factors) — Sanitary Engineering, chapter 6 - [Critical DO deficit, location, sag curve](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asag-curve-effluent-1p5-stream-7p5) — Sanitary Engineering, chapter 6 - [Factors affecting sewage; calculating discharge](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Afactors-sewage-calculate-discharge) — Sanitary Engineering, chapter 2 ## Related in other subjects - [Oxygen sag curve in a stream](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Aoxygen-sag-curve-explain) — Environment and Pollution Control, chapter 4 - [Rating curve and factors affecting runoff](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arating-curve-runoff-factors) — Engineering Hydrology, chapter 4 - [Bridge site factors; river bank protection structures](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Abridge-site-factors-river-bank-protection) — Transportation Engineering II, chapter 5 - [Case study: critical factors in process control](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-critical-factors) — Instrumentation II, chapter 10, asked 2×