Solid waste

Municipal solid waste (MSW) is the solid waste generated from households, markets, offices, institutions and streets. Its characteristics decide the method of collection, treatment and disposal.

Physical characteristics

Chemical characteristics

• Proximate analysis: moisture, volatile matter, fixed carbon and ash. Gives combustibility and ash disposal.
• Ultimate analysis: percentage of C, H, O, N, S and ash; used to compute the energy value, air requirement, and gases formed in incineration.
• Calorific (heating) value: energy per kg, about 3000-4500 kJ/kg for MSW in developing countries (higher in developed countries). Incineration needs at least about 6000-7000 kJ/kg or an auxiliary fuel.
• C/N ratio: best 25-30:1 for composting.
• pH, lipids, carbohydrates, proteins, fusing point of ash (clinkering): relate to biodegradability and furnace operation.
• Hazardous constituents: heavy metals and toxic chemicals affecting compost quality and leachate toxicity.

Sampling is carried out by quartering, and the sorting gives composition by weight.

Functional elements

1. Generation and source reduction (houses, markets, industries).
2. On-site handling and storage (bins, segregation at source).
3. Collection (door to door, community bins).
4. Transfer and transport (compactor trucks, transfer stations).
5. Processing and treatment (recycling, composting, incineration).
6. Final disposal (sanitary landfill).

Waste hierarchy (3R)

  Reduce  (most preferred)
    |
  Reuse
    |
  Recycle / recover
    |
  Treatment (compost, energy)
    |
  Landfill (least preferred)

Composting

Aerobic biological decomposition of organic waste by microorganisms into stable humus-like compost. Methods: windrow, pit, and aerated static pile, with vermicomposting using earthworms. Conditions: C/N 25-30, moisture 50-60%, temperature 55-65 $^\circ$C, enough air. Time: 4-8 weeks. Product is a soil conditioner. Suited to the high organic fraction of waste in Nepal.

Incineration

Controlled burning of waste at 850-1100 $^\circ$C to reduce volume by about 90% and mass by 70%. Heat can generate steam or electricity. Needs waste of high calorific value and low moisture, and air pollution control (scrubbers, bag filters) to control dioxins, fly ash and acid gases. Costly, so used mostly for medical and hazardous waste.

Sanitary landfill

Engineered disposal of waste in layers, compacted and covered each day with soil (15 cm). Features: site selection (away from water sources), liner (clay/HDPE), leachate collection and treatment, gas vents (methane collection), final cap and monitoring. Leachate and methane are the main problems; the land can be reused after closure.

Other methods: open dumping (bad), pyrolysis, biogas digestion of organic waste.

Data: population $= 150\,000$, per capita rate $= 0.4$ kg/day, compacted density $= 600$ kg/m$^3$, cover $= 25\%$, depth $= 8$ m.

(a) Annual waste

$$\text{Waste/day} = 150\,000\times0.4 = 60\,000\ \text{kg} = 60\ \text{t/day}$$ $$\text{Waste/year} = 60\times365 = 21\,900\ \text{t/year}$$

(b) Volume per year

Compacted waste volume:

$$V_w = \frac{21\,900\times1000}{600} = 36\,500\ \text{m}^3/\text{year}$$

Including cover soil:

$$V = 1.25\times36\,500 = 45\,625\ \text{m}^3/\text{year}$$

(c) Area for 20 years

Total volume $= 45\,625\times20 = 912\,500\ \text{m}^3$.

$$A = \frac{912\,500}{8} = 114\,063\ \text{m}^2 \approx 11.4\ \text{ha}$$

Extra land (about 20-30%) would be needed for roads, buffer zone, leachate treatment and offices.

Answer: 21 900 t/year; 45 625 m$^3$/year; area $\approx$ 11.4 ha (114 000 m$^2$) for 20 years.

Hazardous waste is any solid, liquid or gaseous waste that, because of its quantity or its chemical, physical or biological nature, can cause or contribute to illness or death, or can pose a danger to human health or the environment if improperly stored, treated, transported or disposed of.

Classification by characteristics (as used by the US EPA and in textbooks)

By form: solid, liquid, sludge, gaseous.

Sources

• Industries: chemical, pesticide, paint, tannery, electroplating, battery manufacture, metal finishing.
• Hospitals and laboratories: biomedical waste, chemicals, radioactive isotopes.
• Agriculture: pesticides, containers.
• Households: batteries, paints, e-waste, fluorescent lamps (small amounts).
• Mining and energy: tailings, fly ash, used oil.

The approach is the same hierarchy: minimise first, treat second, dispose safely last. The "cradle to grave" principle means the generator remains responsible from production to final disposal.

1. Waste minimisation

• Source reduction (process change, material substitution, good housekeeping).
• Recycling, reuse and recovery (solvent recovery, metal recovery, waste exchange).

2. Physical and chemical treatment

• Neutralisation of acids and alkalis.
• Precipitation of heavy metals as hydroxides or sulphides and filtration.
• Oxidation and reduction, e.g. cyanide oxidised by chlorine; chromium(VI) reduced to chromium(III).
• Adsorption on activated carbon, air stripping and ion exchange.

3. Thermal treatment

• Incineration at above 1000 $^\circ$C with 2 s residence time, destroying organics (about 99.99% destruction) with flue gas cleaning; rotary kiln is most common. Standard for pesticides, solvents, and biomedical waste.

4. Biological treatment

• Activated sludge, land farming, and bioremediation for biodegradable organics.

5. Solidification and stabilisation

• Mixing with cement, lime, fly ash or bitumen to make a solid block that reduces leaching of metals before landfilling.

6. Secure (hazardous) landfill

Double liner (clay + HDPE), leachate collection, leak detection, cap, groundwater monitoring, and far from water sources; sometimes deep-well injection is used for liquids.

Transport must follow manifest systems, and storage must use labelled, compatible containers. Basel Convention controls cross-border movement of hazardous wastes.