--- title: "Organic pollution and aerobic/anaerobic decomposition" subject: "Technology, Environment and Society" chapter: 5 key: "semester_7/technology_environment_and_society:organic-pollution-decomposition" source: https://www.bctengineer.com/q/semester_7/technology_environment_and_society/organic-pollution-decomposition site: BCT Engineering --- # Organic pollution and aerobic/anaerobic decomposition Technology, Environment and Society · Chapter 5: Water and Air Pollution - Asked in: 2073 Chaitra, 8 marks - Video: [Aerobic Decomposition & Anaerobic Decomposition/Facultative Bacteria/Biogas/Biological Decomposition](https://www.youtube.com/watch?v=4nCY5NxQBD8) — ISHA MISHRA - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/technology_environment_and_society/organic-pollution-decomposition - Whole chapter: https://www.bctengineer.com/md/semester_7/technology_environment_and_society/chapter-5 ## Question What is organic pollution in drinking water? Discuss in detail aerobic and anaerobic decomposition process. ## Short answer Organic pollution is contamination by biodegradable matter (sewage, food waste) measured by BOD/COD. Aerobic decomposition by bacteria with oxygen yields CO₂, H₂O, NO₃⁻, SO₄²⁻, is fast and odorless. Anaerobic decomposition without oxygen yields CH₄, CO₂, NH₃, H₂S, is slow and foul-smelling. ## Answer **Organic pollution** of drinking water is contamination by biodegradable organic matter – human and animal excreta, sewage, food waste, dead plants and animals, and effluent from food, dairy, tannery and distillery industries. These substances are decomposed by micro-organisms, which use up the dissolved oxygen (DO) in water. The strength of organic pollution is measured by **BOD (Biochemical Oxygen Demand)** and **COD**. Organic pollution also usually brings pathogens, so it is closely linked with water-borne disease. **Effects:** fall in DO and death of fish; bad smell, colour and taste; growth of pathogens; formation of harmful by-products (e.g. trihalomethanes when organic-rich water is chlorinated); eutrophication when nutrients are released. Rivers like the Bagmati in Kathmandu show these effects. #### Aerobic decomposition Breakdown of organic matter by **aerobic bacteria in the presence of dissolved oxygen**. - Carbon → CO₂ - Nitrogen → NH₃ → NO₂⁻ → NO₃⁻ (nitrification) - Sulphur → SO₄²⁻ - Phosphorus → PO₄³⁻ General reaction: Organic matter + O₂ → (aerobic bacteria) → CO₂ + H₂O + NO₃⁻ + SO₄²⁻ + PO₄³⁻ + new cells + energy **Features** - Fast and complete; final products are stable and harmless. - **No foul smell.** - Consumes DO – if loading is high, DO falls and conditions turn anaerobic. - Used in engineered systems: activated sludge, trickling filter, oxidation pond, aerobic composting. - Nitrate and phosphate released can cause algal growth (eutrophication). #### Anaerobic decomposition Breakdown of organic matter by **anaerobic bacteria in the absence of oxygen** (after DO is exhausted, e.g. in the bottom of polluted lakes, septic tanks, biogas digesters). - Carbon → CH₄ (methane) + CO₂ - Nitrogen → NH₃ (ammonia), amines - Sulphur → H₂S (rotten egg smell) - Phosphorus → PH₃ (phosphine) General reaction: Organic matter → (anaerobic bacteria) → CH₄ + CO₂ + NH₃ + H₂S + new cells + small energy **Stages:** hydrolysis → acidogenesis (fatty acids) → acetogenesis → methanogenesis (methane). **Features** - Slow, and products are often unstable. - **Foul smell** (H₂S, mercaptans), black colour of water and sludge. - Methane is a greenhouse gas but can be used as fuel (biogas). - Used in septic tanks, UASB reactors, sludge digesters and biogas plants. #### Comparison | Basis | Aerobic | Anaerobic | |---|---|---| | Oxygen | Required | Absent | | Bacteria | Aerobic | Anaerobic (methanogens) | | Speed | Fast | Slow | | End products | CO₂, H₂O, NO₃⁻, SO₄²⁻ | CH₄, CO₂, NH₃, H₂S | | Smell | None | Foul | | Energy released | High | Low | | Use | Activated sludge, oxidation pond | Septic tank, biogas digester | ```text Organic waste in water | DO present? --yes--> Aerobic: CO2, NO3, SO4 | no +-----------> Anaerobic: CH4, NH3, H2S ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does organic pollution affect dissolved oxygen (DO) in water? A. It increases DO B. Micro-organisms decomposing the matter use up DO, measured as BOD and COD C. It has no effect on DO D. It converts DO into methane Answer: B. Decomposers consume dissolved oxygen, so DO falls, fish die and the strength of pollution is measured by BOD and COD. (Key point: Organic pollution and DO) 2. What are the end products of aerobic decomposition? A. CH4, NH3, H2S and PH3 B. H2S and methane with foul smell C. Only methane D. CO2, H2O, nitrate, sulphate and phosphate, with no foul smell Answer: D. Aerobic bacteria convert carbon to CO2, nitrogen to nitrate, sulphur to sulphate and phosphorus to phosphate; the process is fast and odourless. (Key point: Aerobic products) 3. Which is true of anaerobic decomposition? A. It occurs with plenty of oxygen and produces nitrate B. It is fast and releases high energy C. It produces CH4, CO2, NH3 and H2S, is slow and gives foul smell, as in septic tanks and biogas digesters D. It has no use in engineering Answer: C. Without oxygen, bacteria produce methane, ammonia and H2S slowly with low energy; used in septic tanks, UASB reactors and biogas plants. (Key point: Anaerobic decomposition) 4. What happens when organic loading is very high in a water body? A. DO is exhausted and conditions turn anaerobic, with foul smell and black colour B. DO increases further C. The water stays clean D. Only aerobic bacteria multiply Answer: A. Aerobic decomposition consumes DO; if loading is high DO falls to zero and anaerobic decomposition takes over. 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