--- title: "Factors affecting self-purification" subject: "Sanitary Engineering" chapter: 6 key: "semester_6/sanitary_engineering:self-purification-factors" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/self-purification-factors site: BCT Engineering --- # Factors affecting self-purification Sanitary Engineering · Chapter 6: Wastewater Disposal - Asked in: 2068 Magh (old course), 6 marks - Video: [Self Purification of River | Stages of Self Purification | Environmental Engg | Simplified Learning](https://www.youtube.com/watch?v=RfMpIWeB_pA) — Simplified Learning Civil Engineering - 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-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-6 ## Question Describe the factors affecting self purification of streams. ## Short answer Key factors affecting self-purification are dilution (higher flow speeds recovery), temperature (increases decomposition but lowers oxygen solubility), sunlight (kills bacteria, aids photosynthesis), velocity and turbulence (enhance reaeration), reaeration rate, initial dissolved oxygen, sedimentation, pollutant nature and quantity, presence of microorganisms and plants, chemical actions, and stream length/travel time. ## Answer **Self-purification** is the natural recovery of a polluted stream. Its rate depends on the following factors. 1. **Dilution:** the greater the flow and volume of the river compared to the sewage, the more the pollutants are diluted and the more oxygen is available. A high dilution factor (above 500) means a fast recovery. 2. **Temperature:** high temperature increases the rate of bacterial decomposition ($K_T=K_{20}\,1.047^{T-20}$) and so the self-purification is quicker, but oxygen solubility falls, which reduces the DO. In cold weather the process is slow. 3. **Sunlight:** kills bacteria by UV rays and allows algae to produce oxygen by photosynthesis. Turbid water lowers this effect. 4. **Velocity and turbulence of the stream:** turbulence mixes the sewage and increases reaeration by exposing the water to the air. Fast, shallow, rocky streams recover faster than slow, deep ones. 5. **Rate of reaeration:** depends on the surface area, depth, wind and temperature; the constant $K_2$ is high in shallow, fast streams. 6. **Dissolved oxygen in the river water:** a higher initial DO gives more capacity for oxidising the pollutants without going anaerobic. 7. **Sedimentation:** the settleable solids settle in a quiet stream, which clarifies the water (but the deposited sludge then decomposes and may use oxygen). 8. **Nature and quantity of the pollutant:** a strong waste with high BOD, toxic industrial wastes, oil, and non-biodegradable matter reduce the rate. The more biodegradable the waste, the faster is the purification. 9. **Presence of bacteria, algae, protozoa and plants:** microorganisms decompose waste; algae give oxygen by photosynthesis; protozoa feed on bacteria. 10. **Chemical actions:** oxidation, reduction, coagulation by dissolved chemicals, and pH. 11. **Time of travel / length of stream:** self-purification needs time; a long stream with no new pollution allows full recovery. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does a high dilution factor (river flow much greater than sewage flow) affect self-purification? A. It slows recovery because pollutants spread out B. It makes recovery fast, since pollutants are diluted and more oxygen is available C. It has no effect on oxygen availability D. It makes the stream turn anaerobic Answer: B. More river water dilutes the pollutants and supplies more oxygen, so recovery is quicker. (Key point: Dilution) 2. What is the two-sided effect of high temperature on self-purification? A. Both decomposition rate and oxygen solubility increase B. Both decomposition rate and oxygen solubility decrease C. Bacterial decomposition rate increases, but oxygen solubility falls D. Decomposition slows but oxygen solubility rises Answer: C. Higher temperature speeds bacterial action (K_T = K20 x 1.047^(T-20)) but lowers saturation DO, reducing available oxygen. (Key point: Temperature effect) 3. Which stream recovers fastest? A. Slow, deep stream B. Stagnant pool C. Deep stream with a smooth bed D. Fast, shallow, rocky stream Answer: D. Turbulence mixes the sewage and exposes water to air, giving a high reaeration constant K2 in fast shallow streams. (Key point: Velocity and turbulence) 4. Which waste reduces the rate of self-purification most? A. Toxic industrial waste and non-biodegradable matter B. Highly biodegradable organic waste C. Settleable solids D. Waste with low BOD Answer: A. Toxic, oily and non-biodegradable wastes cannot be easily decomposed by bacteria, so purification is slowed. (Key point: Nature of pollutant) ## Related in Sanitary Engineering - [Self-purification of rivers; sag curve](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Aself-purification-sag-curve) — 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 - [Factors affecting water demand; variation](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Afactors-affecting-demand) — Water Supply Engineering, chapter 3, asked 5× - [Factors affecting bearing capacity; water table](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Abearing-capacity-factors-water-table) — Foundation Engineering, chapter 6, asked 5× - [Incentives; factors affecting wage/salary](https://www.bctengineer.com/question.md?key=semester_7%2Forganization_and_management%3Aincentives-wage-factors) — Organization and Management, chapter 7, asked 3× - [Forms of chlorination and affecting factors](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aforms-of-chlorination-factors) — Water Supply Engineering, chapter 6, asked 3×