--- title: "Composting: methods, merits, demerits" subject: "Sanitary Engineering" chapter: 10 key: "semester_6/sanitary_engineering:solid-waste-composting" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/solid-waste-composting site: BCT Engineering --- # Composting: methods, merits, demerits Sanitary Engineering · Chapter 10: Solid Waste Disposal - Asked in: 2075 Bhadra, 4 marks; 2074 Bhadra, 4 marks; 2073 Bhadra, 4 marks; 2069 Bhadra, 4 marks; 2068 Magh (old course), 4 marks (asked 5 times) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/solid-waste-composting - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-10 ## Question Describe the methods of composting for solid waste management/disposal and its merits and demerits. ## Short answer Composting biologically decomposes organic waste into humus. Methods include open windrows (aerobic turning), Bangalore anaerobic pits, Indore aerobic pits, mechanical drums, and vermicomposting. It recycles nutrients, reduces landfilling, but needs segregation and time; market may be limited. ## Answer **Composting** is the biological decomposition of the organic fraction of solid waste by micro-organisms under controlled conditions into a stable, humus-like material called compost, which is used as a soil conditioner. Nepal's waste is 60-70% organic, so composting is well suited to it. #### Methods 1. **Open windrow / heap method:** segregated waste is piled in long heaps (windrows) about 1.5 m high and 2-3 m wide, and turned every 5-7 days for aeration. It is aerobic; the compost is ready in 2-3 months. 2. **Bangalore method (anaerobic):** alternate layers of waste (15 cm) and night soil or cow dung (5 cm) are laid in trenches about 1 m deep and covered with soil. There is no turning; the compost matures in 4-6 months. 3. **Indore method (aerobic):** layers of waste, dung and soil are placed in pits and turned at set intervals (e.g. after 4-5, 10 and 30 days); ready in about 3 months. 4. **Mechanical composting:** waste is fed into enclosed rotating drums or digesters with forced air; ready in 1-2 weeks, but expensive. 5. **Vermicomposting:** earthworms feed on the organic waste and produce rich castings; common in household and community composting. ```text raw waste -> segregation -> shredding -> pile / pit -> turning (air, moisture) -> maturing -> screening -> compost ``` Conditions: C:N ratio about 30:1, moisture 50-60%, temperature 55-65 °C in the active stage, and plenty of air (aerobic methods). #### Merits - Recycles organic matter and nutrients to the soil; improves soil structure. - Reduces the waste going to landfill; cheap and simple. - The heat produced during composting kills pathogens and weed seeds; creates jobs. #### Demerits - Needs segregation of waste and land for pits; odour and flies if poorly managed. - Takes weeks to months; the compost may contain heavy metals or bits of glass and plastic. - The market for compost is limited, so unsold compost accumulates. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is composting? A. Burning waste B. Burying waste in a lined cell C. Chemical treatment D. Biological decomposition of organic waste to humus-like compost Answer: D. Micro-organisms stabilise the organic fraction into compost. (Key point: Definition) 2. What distinguishes Bangalore method? A. Anaerobic with alternate layers of waste and night soil in trenches, no turning B. Aerobic with frequent turning C. Enclosed rotating drums D. Earthworms Answer: A. It is anaerobic and takes 4-6 months. (Key point: Bangalore method) 3. Which conditions suit aerobic composting? A. C:N 5:1, dry B. C:N about 30:1, moisture 50-60%, 55-65 °C C. Moisture 10%, 20 °C D. No air Answer: B. These give active aerobic decomposition. (Key point: Conditions) 4. A key demerit? A. Produces no useful product B. Destroys all organics C. Needs segregation and limited market D. Needs no land Answer: C. Segregation, land and a limited compost market are drawbacks. 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