Solid Waste Disposal

Incineration is the controlled burning of solid waste at high temperature (800-1000 °C or more) in a furnace called an incinerator, converting combustible waste to ash, flue gas and heat. It is used for waste that is combustible and has high calorific value (hospital, pathological, hazardous waste) and in cities with little land.

Process

1. Waste is received in a storage pit and fed to the furnace by crane or conveyor.
2. Drying and ignition on the grate, then combustion with excess air at 800-1000 °C; flue gases are held about 1 s at high temperature to destroy odours and toxic compounds.
3. Flue gas passes through a heat recovery boiler (steam or electricity) and then a scrubber, filter or electrostatic precipitator before the chimney.
4. Residual ash and clinker (10-15% of original volume, 25-30% by mass) is removed and landfilled or used in blocks.

 waste -> [pit] -> [furnace 800-1000C] -> [boiler]
                       |                     |
                     ash/clinker     [scrubber/filter] -> chimney

Merits

• Volume reduced by 80-90% and weight by 70%, so little land is needed.
• Kills pathogens; suitable for hospital and hazardous waste.
• Heat can be recovered to generate steam/electricity.
• Can be placed near the city, reducing transport; no odour or fly nuisance when operated properly.

Demerits

• High capital, operating and maintenance cost; needs skilled operators.
• Air pollution (dioxins, fly ash, SO₂, NOx) unless costly gas cleaning is provided.
• Not suitable for wet waste with high organic and low calorific value as in Nepal (needs auxiliary fuel).
• Ash still needs disposal, and noncombustibles (glass, metals) remain.

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 soil conditioner. Nepal's waste is 60-70% organic, so composting is well suited.

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. Aerobic; 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 1 m deep and covered with soil. No turning; matures in 4-6 months.
3. Indore method (aerobic): layers of waste, dung and soil in pits, turned at set intervals (4-5, 10, 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 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, plenty of air (aerobic).

Merits

• Recycles organic matter and nutrients to soil; improves soil structure.
• Reduces waste going to the landfill; cheap and simple.
• Heat 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; compost may contain heavy metals or glass/plastic bits.
• Market for compost is limited; unsold compost accumulates.

A sanitary landfill is an engineered method of disposing solid waste on land in which the waste is spread in thin layers, compacted, and covered daily with soil, so that there is no nuisance or hazard to health or environment. It differs from open dumping because of liner, leachate and gas control and daily cover.

Methods

1. Trench method: used on flat land with deep ground-water; trenches 1-2 m deep, 4-8 m wide are dug, waste is placed, compacted and covered with 15 cm soil daily, with the excavated soil used as cover.
2. Area method: used where the land is flat or the ground-water is high; waste is spread on the ground surface in layers and covered with soil brought from elsewhere.
3. Ramp / depression (valley) method: used in valleys, quarries or ravines (common in Nepal, e.g. Sisdol, Banchare Danda); waste is spread on slopes, compacted and covered.

 final cover 0.6 m
 ____________________________
 | daily soil cover 15 cm    |
 |  compacted waste cells    |
 |___________________________|
 clay/HDPE liner -> leachate collection pipe -> pond
 gas vent pipes

Operation: waste is spread in 0.5 m layers, compacted to reduce volume, covered daily with 15 cm soil, and finally with 60 cm soil and vegetation. Leachate is collected and treated; methane is vented or flared.

Merits

• Cheap compared with incineration or composting; handles all kinds of waste.
• No odour, flies or fire if covered daily; land can later be used as park.
• Landfill gas can be used as fuel; flexible in quantity.

Demerits

• Needs large land, usually far from city so transport cost is high.
• Leachate can pollute ground-water; gas (methane) causes explosion risk.
• Settlement of the site, so building on it is restricted; public opposition to siting.

Open dumping

Open dumping is the uncontrolled disposal of solid waste on open land, river banks or road sides without covering, compaction or any environmental control. It is the cheapest method but causes:

• bad odour, flies, rodents and disease spread;
• burning of waste (air pollution) and fires;
• leachate that pollutes surface and ground water;
• loss of aesthetics and land value; scavenging by animals and people.

Sanitary landfill

A sanitary landfill is an engineered disposal site where waste is spread in thin layers, compacted and covered with soil daily. It has a liner (clay or HDPE) and leachate collection, gas vents, drainage and monitoring wells. The final cover is soil with vegetation. It has no odour or vermin problems and little pollution when managed well.

Landfill (general)

A landfill is any land disposal site where waste is buried. A controlled landfill has some of the controls of a sanitary landfill (daily or periodic cover, fences, supervision) but not always a full liner or gas collection.

Solid waste is finally disposed of by one or more of these methods, chosen by waste type, land availability, cost and climate.

1. Open dumping: waste is thrown on open land without treatment. Cheapest, but causes odour, flies, rats, disease and water pollution; not recommended.
2. Sanitary landfill: waste is spread in layers, compacted and covered daily with soil, with leachate and gas control. Methods: trench, area and ramp (depression). Most common in Nepal and widely accepted.
3. Incineration: controlled burning at 800-1000 °C reduces volume by 80-90% and kills pathogens; heat can be recovered. Costly, causes air pollution, and unsuitable for wet waste; used for hospital waste.
4. Composting: biological decomposition of organic waste into compost (windrow, Bangalore, Indore, vermicomposting). Gives soil conditioner; needs segregation.
5. Biogas / anaerobic digestion: organic waste is digested in the absence of air to give methane-rich biogas for cooking and electricity, and slurry manure.
6. Pulverisation (grinding): waste is ground into small pieces and used as landfill cover or in compost; reduces volume.
7. Recycling and reuse: paper, glass, metal and plastics are recovered and reused, reducing waste (3R: reduce, reuse, recycle).
8. Dumping into sea/river: discharge into water is not allowed in Nepal because it pollutes water and is banned.
9. Feeding to animals (hog feeding): food waste fed to pigs after cooking; used in some places.

 waste -> segregation -> organic -> composting / biogas
                      -> recyclable -> recycling
                      -> residual -> landfill / incineration