Solid and Hazardous Waste Management I: Municipal Solid Waste — Types, Sources, Characteristics, Generation and Composition, Proximate and Ultimate Analysis, Collection and Transport by Hauled and Stationary Container Systems, Routes and Transfer Stations, Processing and Recovery, Refuse-Derived Fuel, Composting, Vermicomposting and Biomethanation, and Disposal by Sanitary Landfill and Incineration
2. Collection and transport: container systems, collection routes and transfer stations
Collection, the gathering of waste and its haul to a processing or disposal site, usually takes the largest share of the cost of municipal solid waste management, often well over half. Tchobanoglous distinguished two systems. In the hauled container system, the container itself is taken to the disposal site, emptied and returned (or replaced by an empty one); it suits sources that generate a lot of waste, such as markets, construction sites and industries, using roll-off containers or tilt-frame vehicles, and each trip carries one container. In the stationary container system, the container stays at the point of generation and is emptied into a collection vehicle, usually a compactor truck that serves many containers before going to the disposal site; it suits households and small commercial sources. Door-to-door collection of segregated waste, as required by India's Solid Waste Management Rules 2016, is a form of the stationary system.
Collection routes are laid out by heuristic rules so that a vehicle covers its area with the least unproductive travel: routes should not overlap or double back; they should start near the garage and end near the disposal site or transfer station; heavily trafficked streets should be avoided at rush hours; waste on steep hills should be collected going downhill; turns across oncoming traffic should be minimised by laying out loops in the appropriate direction; and each route should give each crew a balanced day's work. Geographic information systems now optimise routes and track vehicles. A transfer station is a facility where waste from small collection vehicles is transferred to large, long-haul vehicles, by direct discharge from an elevated platform or from a storage floor or pit. It pays when the disposal site is far away, because the crews and small trucks return to collection instead of spending hours driving; the break-even haul distance, beyond which transfer is cheaper, is found by comparing the cost per tonne of direct haul with the cost of transfer plus long haul.
3. Processing and recovery: recycling, refuse-derived fuel and energy, composting, vermicomposting and biomethanation
Recovery begins with segregation at source into wet (biodegradable), dry (recyclable) and domestic hazardous waste, which the Solid Waste Management Rules 2016 make the duty of every waste generator. Dry waste is sorted in material recovery facilities, manually and mechanically by screens, magnets, eddy-current separators and air classifiers, and India's informal sector of waste pickers recovers a large part of the recyclables. Recovered materials go to recycling or to the direct manufacture of products such as compost, bricks and paving blocks from inert waste, and panels or furniture from plastics. The combustible fraction, paper, plastics, textiles and wood, can be shredded and dried into refuse-derived fuel (RDF), fired in cement kilns and power stations, or densified into fuel pellets that are easier to store and transport; the 2016 Rules require cement and thermal power plants within 100 km of an RDF plant to use it to replace part of their fuel. Mass-burn waste-to-energy plants generate electricity directly from mixed waste, but only where it has adequate calorific value.
| Biological process | Conditions | Product and notes |
|---|---|---|
| Aerobic composting | C/N ratio about 25 to 30:1, moisture about 50 to 60%, oxygen by turning windrows or forced aeration, particle size of a few centimetres; the pile passes through mesophilic and thermophilic phases, reaching about 55 to 65 °C, which kills pathogens and weed seeds, before curing | Stable, humus-like compost; methods include windrows, aerated static piles and in-vessel systems; odour and leachate if too wet or anaerobic |
| Vermicomposting | Earthworms such as Eisenia fetida (the red wiggler), Eudrilus eugeniae and Perionyx excavatus feed on partly decomposed organic waste at moderate temperatures (below about 35 °C) and moisture | Vermicast, rich in available nutrients and microbial activity; suited to decentralised, household and institutional scale |
| Biomethanation (anaerobic digestion) | Wet, segregated food and market waste digested without oxygen in closed reactors | Biogas of about 55 to 65% methane for cooking, power or compressed biogas, and a digestate used as manure; the BARC-designed Nisargruna plants are an Indian example |
4. Disposal: sanitary landfilling and its management, and incineration
A sanitary landfill is an engineered facility in which waste is placed in thin layers, compacted, covered daily with soil and isolated from the environment, unlike an open dump. Sites are chosen away from water bodies, flood plains, wetlands, airports (because of bird strikes), habitation and shallow groundwater. The base carries a composite liner, under the Indian rules a high-density polyethylene geomembrane over a thick layer of compacted clay of very low permeability, with a drainage layer and pipes above it to collect leachate for treatment. Landfill gas, roughly half methane and half carbon dioxide once the methanogenic stage is reached, is collected through wells and flared or used for energy. When full, the landfill is closed with a multi-layer cap that sheds rain and limits leachate, and gas, leachate and groundwater are monitored for years afterwards. Only waste that cannot be reused, recycled or processed, the rejects and inerts, should be landfilled; the old open dumps of Indian cities are being remediated by biomining, which excavates and screens the waste to recover soil-like material and combustibles and reclaim the land.
Leachate is the liquid that percolates through the waste, carrying dissolved and suspended matter. Its composition changes with the age of the landfill: in the early acid phase it is strongly polluting, with high BOD and COD, a high BOD/COD ratio, low pH and dissolved metals; as the landfill matures into the methanogenic phase, the readily degradable organics are consumed, pH rises and the BOD/COD ratio falls, though ammonia remains high. The capacity needed is found from the waste volume: 200 tonnes a day compacted to 800 kg/m³ occupies 250 m³, and with daily and intermediate cover taking about 20% more, about 300 m³ of airspace a day. Incineration burns waste at high temperature, in modern plants at 850 °C or above with a residence time of at least two seconds, reducing its volume by about 90% and recovering heat; its residues are bottom ash and fly ash, the latter often hazardous. Dioxins and furans can form when flue gas cools slowly through about 200 to 450 °C, so gases are quenched rapidly and cleaned with activated-carbon injection and fabric filters. Because Indian mixed waste is wet, inert-rich and low in calorific value, several incineration plants have failed, starting with Delhi's Timarpur plant in the 1980s, and incineration is suited to the high-calorific, segregated combustible fraction rather than to mixed waste.
Key takeaways
- Solid waste is municipal, industrial, agricultural, construction and demolition, bio-medical, hazardous or electronic; Indian municipal waste is wet, dense, inert-rich and low in calorific value, generated at a few hundred grams per person per day.
- Proximate analysis gives moisture, volatile matter, fixed carbon and ash; ultimate analysis gives C, H, O, N, S and ash; moisture on a wet basis is (wet − dry) ÷ wet.
- In hauled container systems the container travels to the disposal site; in stationary systems a compactor empties many containers; routes avoid overlap and doubling back; transfer stations pay beyond a break-even haul distance.
- Segregation at source feeds recycling, RDF and fuel pellets, composting (C/N 25 to 30, 55 to 65 °C), vermicomposting with Eisenia fetida and biomethanation to biogas.
- Sanitary landfills need composite liners, leachate and gas collection, daily cover and capping; young leachate has a high BOD/COD ratio; incineration at 850 °C or above needs rapid quenching against dioxins and suits dry, high-calorific waste rather than India's mixed waste.
Practice questions (10)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
A 100 kg sample of municipal solid waste weighs 60 kg after drying at 105 °C. What is its moisture content in per cent on a wet basis?
Numerical answer — type the value.
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Answer: 40
Moisture (wet basis) = (wet − dry) ÷ wet × 100 = (100 − 60) ÷ 100 × 100 = 40%. On a dry basis the same water would be 40 ÷ 60 = 67%, which is why the basis matters; solid waste figures are conventionally on a wet basis.A town sends 200 tonnes of waste a day to a landfill, where it is compacted to 800 kg/m³. If daily and intermediate soil cover take up an additional 20% of the compacted waste volume, how many cubic metres of landfill space are used each day?
Numerical answer — type the value.
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Answer: 300
Compacted waste volume = 200,000 kg ÷ 800 kg/m³ = 250 m³, and cover adds 20%, 50 m³, for a total of 300 m³ a day. Forgetting to convert tonnes to kilograms, or to add the cover, are the usual slips.A hauled container system of solid waste collection is best suited to
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Answer: B — sources that generate large quantities of waste, such as markets and construction sites
In a hauled system each trip carries one large container, so it is economic only where a container fills quickly, as at markets, industries and construction sites. Households and small shops are served by stationary containers emptied into compactor trucks.Which of the following are favourable conditions for aerobic composting? Select all that apply.
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Answer: A — A thermophilic phase of about 55 to 65 °C; C — A C/N ratio of about 25 to 30:1; D — A moisture content of about 50 to 60%
Balanced carbon and nitrogen, moist but not waterlogged material and a hot thermophilic phase give fast, hygienic composting. Sealing the pile makes it anaerobic, slowing decomposition and producing odours; air is supplied by turning or forced aeration.Which earthworm species is most widely used for vermicomposting?
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Answer: C — Eisenia fetida
Eisenia fetida, the red wiggler, is a surface-dwelling (epigeic) worm that lives in decaying organic matter, eats rapidly and breeds fast, which makes it ideal for composting; Eudrilus eugeniae and Perionyx excavatus are also used. Soil-dwelling burrowers such as Pheretima and Lumbricus terrestris do not thrive in compost beds, and Tubifex is an aquatic worm.Assertion (A): Mass-burn incineration of mixed municipal waste has often failed in India. Reason (R): Indian mixed waste has high moisture and inert content and hence a low calorific value. Choose the correct option.
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Answer: A — Both A and R are true, and R is the correct explanation of A
Both are true and R explains A. Wet food waste and inert street sweepings cannot sustain combustion without support fuel, so plants such as Delhi's Timarpur incinerator could not run as designed; segregation and processing of the wet fraction by composting or biomethanation suit Indian waste better.A transfer station in solid waste management becomes economical when
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Answer: A — the disposal or processing site is far from the collection area
Beyond a break-even haul distance, it is cheaper to transfer waste from small collection vehicles to large long-haul vehicles than to have every collection crew drive to a distant site. Near a landfill, direct haul is cheaper.Which of the following are determined in the proximate analysis of solid waste? Select all that apply.
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Answer: C — Ash; D — Moisture
Proximate analysis measures moisture, volatile matter, fixed carbon and ash by heating under set conditions. The elemental composition, carbon, hydrogen, oxygen, nitrogen and sulphur, belongs to ultimate analysis.In a waste incinerator, dioxins and furans form mainly when
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Answer: A — the flue gas cools slowly through about 200 to 450 °C
Dioxins re-form (de novo synthesis) on fly-ash particles as gases cool through that temperature window, so modern plants quench gases rapidly and inject activated carbon. High combustion temperature with adequate residence time destroys them, and chlorine in the waste is a prerequisite for their formation.Compared with leachate from an old landfill, leachate from a young landfill typically has
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Answer: B — a higher BOD/COD ratio and a lower pH
In the early acid phase, readily degradable organics and volatile fatty acids make leachate strong, biodegradable and acidic. As methanogenesis consumes them, pH rises and the remaining organics are resistant, so the BOD/COD ratio falls, which changes how the leachate must be treated.