Methods of Solid Waste Disposal and Management

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Methods of Solid Waste Disposal and Management

Methods of solid waste disposal and management are as below:

• Open burning
• Dumping into the sea
• Sanitary Landfills
• Incineration
• Composting
• Ploughing in fields
• Hog feeding
• Grinding and discharging into sewers
• Salvaging
• Fermentation and biological digestion

Open burning of Solid Wastes

Not an ideal method in the present day context

Dumping into Sea

• Possible only in coastal cities
• Refuse shall be taken in barges sufficiently far away from the coast (15-30 km) and dumped there
• Very costly
• Not environment friendly

Sanitary Landfilling of Solid Wastes

• Simple, cheap, and effective
• A deep trench (3 to 5 m) is excavated
• Refuse is laid in layers
• Layers are compacted with some mechanical equipment and covered with earth, leveled, and compacted
• With time, the fill would settle
• Microorganisms act on the organic matter and degrade them
• Decomposition is similar to that in composting
• Facultative bacteria hydrolyze complex organic matter into simpler water soluble organics
• These diffuse through the soil where fungi and other bacteria convert them to carbon dioxide and water under aerobic conditions
• Aerobic methanogenic bacteria utilize the methane generated and the rest diffuses into the atmosphere
• Too much refuse shall not be buried – fire hazard
• Moisture content – not less than 60% for good biodegradation
• Refuse depth more than 3m – danger of combustion due to compression of bottom layers – hence should be avoided
• Refuse depth is generally limited to 2m
• Temperature in the initial stages of decomposition – as high as 70 degree C – then drops
• Reclaimed areas may be used for other uses

Engineered Landfills of Solid Wastes

• Bottom of the trench is lined with impervious material to prevent the leachate from contaminating groundwater
• A well designed and laid out leachate collection mechanism is to be provided
• Leachate so collected is treated and then disposed off

Figure: A Typical Sanitary Landfill for Solid Waste

Figure: A Typical Sanitary Landfill

Components of a Typical Landfill

Figure: Examples of systems in municipal solid waste landfills

Where LCS = Leachate collection system GCS = Geosynthetic clay liner LDS = Leachate detection system

MSW Landfill Gas

Incineration of Solid Waste

• A method suited for combustible refuse
• Refuse is burnt
• Suited in crowded cities where sites for land filling are not available
• High construction and operation costs
• Sometimes used to reduce the volume of solid wastes for land filling
• Primary chamber – designed to facilitate rapid desiccation of moist refuse and complete combustion of refuse and volatile gases
• A ledge or drying hearth is provided for this purpose
• Secondary chamber – between the primary chamber and the stack – temperatures above 700 degree C
• All unburnt and semi burnt material are completely burnt here

Waste to Energy Combustors

• Incinerators – Refuse was burned without recovering energy – exhaust gas is very hot – exceeds the acceptable inlet temperature for electrostatic precipitators used for particulate emission control
• Modern combustors – combine solid waste combustion with energy recovery

Combustors for Solid Waste

• Storage pit – for storing and sorting incoming refuse
• Crane – for charging the combustion box
• Combustion chamber consisting of bottom grates on which combustion occurs
• Grates on which refuse moves
• Heat recovery system of pipes in which water is turned to steam
• Ash handling systems
• Air pollution control systems
• Grates – Provide turbulence so that the MSW can be thoroughly burned, moves the refuse down, provides under fire air to the refuse through openings in it (to assist in combustion as well as to cool the grates)
• Operating temperature of combustors ~ 980 to 1090 degree C

Figure: A typical MSW Combustor

Grates of MSW Combustor

Figure: Grates of MSW combustor. The under fire air is blown through the holes in the drawings show three types grates. (b) reciprocating (c) rocking (d)travelling

Composting

• Similar to sanitary landfilling
• Yields a stable end product – good soil conditioner and may be used as a base for fertilizers
• Popular in developing countries
• Decomposable organic matter is separated and composted

Methods

1. Open window composting
2. Mechanical composting

Open window composting

• Refuse is placed in piles, about 1.5m high and 2.5m wide at about 60% moisture content
• Heat build up in the refuse piles due to biological activity – temperature rises to about 70 degree C
• Pile is turned up for cooling and aeration to avoid anaerobic conditions
• Moisture content is adjusted to about 60%
• Piled again – temperature rises to about 70 degree C
• The above operations are repeated
• After a few days (~ 7 to 10 weeks) temperature drops to atmospheric temperature – indication of stabilization of compost

Mechanical composting

• Process of stabilization is expedited by mechanical devices of turning the compost
• Compost is stabilized in about 1 to 2 weeks
• To enrich compost – night soil, cow dung etc. are added to the refuse
• Usually done in compost pits
• Arrangements for draining of excess moisture are provided at the base of the pit
• At the bottom of the pit, a layer of ash, ground limestone, or loamy soil is placed – to neutralize acidity in the compost material and providing an alkaline medium for microorganisms
• The pit is filled by alternate layers of refuse (laid in layers of depth 30 – 40 cm) and night soil or cow dung (laid over it in a thin layer)
• Material is turned every 5 days or so
• After ~ 30 days – it is ready for use

Methods used in India Indore method – aerobic – brick pits 3 x 3 x 1 m – up to 8-12 weeks materials are turned regularly in the pits and then kept on ground for about 4-6 weeks – 6 to 8 turnings in total Bangalore method – anaerobic – earthen trenches 10 x 1.5 x 1.5 m – left for decomposition – takes 4 to 5 months

Vermicomposting

• Ideal for biodegradable wastes from kitchens, hotels etc.
• At household level, a vessel or tray more than 45 cm deep, and 1 x 0.60m may be sufficient
• A hole shall be provided at one end in the bottom for draining the leachate out into a tray or vessel
• Lay a 1” thick layer of baby metal or gravel at the bottom of the tray
• Above that lay an old gunny bag or a piece of thick cloth, a layer of coconut husk upside down over it and above that a 2” thick layer of dry leaves and dry cow dung (powdered)
• Lay the biodegradable waste over it
• Introduce good quality earthworms into it (~ 10 g for 0.6 x 0.45 x 0.45 m box)
• If the waste is dry, sprinkle water over it daily
• Rainwater should not fall into the tray or vessel or box
• Keep it closed
• If the box is kept under bright sun earthworms will go down and compost can be taken from the top
• Compost can be dried and stored
• Continue putting waste into the box
• Add little cow dung at intervals
• Do not use vermiwash directly. Dilute in the ratio 1:10 before use

Disposal by Ploughing into fields

• Not very commonly used
• Not environment friendly in general

Disposal by hog feeding

• Not common in India
• Refuse is ground well in grinders and then fed into sewers
• Disposal of garbage into sewers – BOD and TSS increases by 20-30%
• Disposal of residual refuse – still a problem

Salvaging

• Materials like paper, metal, glass, rags, certain types of plastic etc. can be salvaged, recycled, and reused

Fermentation or Biological Digestion

• Biodegradable Waste – convert to compost
• Recycle whatever is possible
• Hazardous wastes – dispose it by suitable methods
• Landfill or incinerate the rest