Composting: methods, merits, demerits

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.

 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.