Solid and Hazardous Waste: Systems, Recovery and Containment

Running case. A medium-sized city depends on open dumping and proposes material recovery plus a controlled landfill.

Unit product. Prepare a waste flow diagram and compare source separation, composting, digestion and controlled disposal options.

Unit orientation

This unit asks what can and cannot be concluded from observations in the running case. Begin by listing state variables and the time basis, identifying which quantities have been measured and which are assumptions. The engineering task is to distinguish evidence of transport, transformation, dilution and removal, and to prevent an untested model from becoming a safety claim. Compare at least two approaches with the same functional requirements, and include one sensitivity or uncertainty test in the written submission.

6.1 Waste characterization and collection systems

Waste composition varies with location, season, income, packaging and collection system. A municipal mass estimate that excludes informal recovery or uncollected material is not a complete generation inventory. Bulk density affects truck capacity, storage needs and landfill airspace, while wet organic fraction strongly influences odour and leachate potential.

Carry out a stratified composition study rather than extrapolating one market-day sample to a whole city. Measure wet and dry masses as appropriate, account for sorting losses and contamination, and distinguish generation, collection and disposal. Route design should consider vehicle accessibility, transfer points, worker safety, fuel use and the reliability of collection schedules.

Governing relationship and units

Annual generated mass = population × per-capita generation ×365, with consistent kg/person/day units.

Worked context

A population of 180,000 at 0.55 kg/person/day generates about 99 tonnes/day. A collection rate of 75% leaves roughly 24.75 tonnes/day outside formal collection, not necessarily inside a controlled disposal site.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.2 Recycling quality and materials recovery

The presence of recyclable material in mixed waste does not establish an economically viable recovered product. Capture efficiency, contamination, sorting purity, demand, transport distance and occupational safety determine outcomes. Source separation can improve quality but depends on participation, reliable separate collection and actual processing capacity.

Track input mass, rejected contamination, recovered saleable mass and final destination for each material stream. A high facility diversion claim may be inflated if rejected bales later go to landfill. Compare avoided virgin-material production with sorting, washing and transport impacts using an explicit functional unit and locality-specific emissions data.

Governing relationship and units

Recovered mass = incoming mass × target fraction × capture rate × processing yield.

Worked context

A 100 t/day stream containing 18% recoverable plastic at 70% capture and 85% processing yield produces about 10.71 t/day of recovered plastic, before any later product losses.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.3 Organics treatment and landfill dynamics

Composting uses managed aerobic decomposition to stabilize organics when moisture, oxygen, porosity and carbon-to-nitrogen balance are suitable. Anaerobic digestion produces biogas in the absence of oxygen but requires feedstock control and digestate management. Landfills produce leachate and gas whose composition and timing depend on waste age, moisture, composition and containment.

A first-order landfill methane model is a screening inventory model; landfill-cover oxidation, collection efficiency, site climate and historical disposal alter actual releases. Decisions about composting and digestion should consider contamination by glass, plastics or metals and whether the end product has a viable, safe use.

Governing relationship and units

Illustrative gas generation G(t)=k L₀ M exp(−kt), with unit-consistent mass and gas potential.

Worked context

A legacy deposit can continue producing gas decades after placement. Doubling the assumed first-order decay coefficient increases initial predicted generation but does not double integrated lifetime potential.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.4 Landfill engineering and governance

An engineered landfill needs a defensible site, drainage control, containment, leachate collection, gas management, stable slopes, operational cover and monitoring. A liner does not eliminate long-term risk; leak detection, maintenance, groundwater surveillance and financial provisions matter. Unregulated dumping often transfers environmental cost to neighbours and water resources rather than eliminating it.

Compare disposal sites by geological and hydrogeological context, flood exposure, access, leachate treatment, settlement, gas generation and land-use conflict. Build a closure and post-closure plan early. Avoid design-level conclusions from simple landfill volume or gas estimates; actual capacity and stability require geotechnical studies and local regulations.

Governing relationship and units

Nominal airspace volume = waste mass / achieved in-place density; add cover and operational allowances separately.

Worked context

If 50,000 tonnes of compacted waste average 0.8 t/m³, nominal waste airspace is 62,500 m³ before cover, settlement and buffer volumes. A design that ignores cover could exhaust usable space early.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

Synthesis and field decision

In the case scenario, prepare a waste flow diagram and compare source separation, composting, digestion and controlled disposal options. For each quantity used, identify its spatial representativeness, time basis, uncertainty and the responsible person who can confirm the value. Explicitly document how the plan changes during equipment failure or extreme conditions.

Unit assessment

1. For solid waste: systems, recovery, treatment and disposal, define a control volume and list all stocks, transports and reaction terms with units.
2. Explain how different sampling times or locations could reverse the conclusion in this scenario: A medium-sized city depends on open dumping and proposes material recovery plus a controlled landfill.
3. Choose two uncertainties and quantify which has the larger effect on the recommendation. Show a reproducible parameter table.
4. Use one of the four governing relationships to calculate a low, central and high case, then explain why a numerical result is not a permit limit.
5. Design an independent field or bench experiment that discriminates between a transport limitation and a reaction limitation.
6. Write a corrective-action plan for equipment failure or a surprising monitoring result; identify response time and independent verification.
7. Construct a single-page public explanation of the issue without concealing unresolved uncertainty.
8. Critique a proposal that reports percent removal but omits residual streams, peak-flow capacity and monitoring quality.

Assessment policy: A numerical answer without units, documented assumptions, and interpretation is incomplete. Explain what additional observation or regulation would be needed to authorize real-world action.

Unit 6: Solid waste: systems, recovery, treatment and disposal

Running case. A medium-sized city depends on open dumping and proposes material recovery plus a controlled landfill.

Unit product. Prepare a waste flow diagram and compare source separation, composting, digestion and controlled disposal options.

Unit orientation

This unit asks what can and cannot be concluded from observations in the running case. Begin by listing state variables and the time basis, identifying which quantities have been measured and which are assumptions. The engineering task is to distinguish evidence of transport, transformation, dilution and removal, and to prevent an untested model from becoming a safety claim. Compare at least two approaches with the same functional requirements, and include one sensitivity or uncertainty test in the written submission.

6.1 Waste characterization and collection systems

Waste composition varies with location, season, income, packaging and collection system. A municipal mass estimate that excludes informal recovery or uncollected material is not a complete generation inventory. Bulk density affects truck capacity, storage needs and landfill airspace, while wet organic fraction strongly influences odour and leachate potential.

Carry out a stratified composition study rather than extrapolating one market-day sample to a whole city. Measure wet and dry masses as appropriate, account for sorting losses and contamination, and distinguish generation, collection and disposal. Route design should consider vehicle accessibility, transfer points, worker safety, fuel use and the reliability of collection schedules.

Governing relationship and units

Annual generated mass = population × per-capita generation ×365, with consistent kg/person/day units.

Worked context

A population of 180,000 at 0.55 kg/person/day generates about 99 tonnes/day. A collection rate of 75% leaves roughly 24.75 tonnes/day outside formal collection, not necessarily inside a controlled disposal site.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.2 Recycling quality and materials recovery

The presence of recyclable material in mixed waste does not establish an economically viable recovered product. Capture efficiency, contamination, sorting purity, demand, transport distance and occupational safety determine outcomes. Source separation can improve quality but depends on participation, reliable separate collection and actual processing capacity.

Track input mass, rejected contamination, recovered saleable mass and final destination for each material stream. A high facility diversion claim may be inflated if rejected bales later go to landfill. Compare avoided virgin-material production with sorting, washing and transport impacts using an explicit functional unit and locality-specific emissions data.

Governing relationship and units

Recovered mass = incoming mass × target fraction × capture rate × processing yield.

Worked context

A 100 t/day stream containing 18% recoverable plastic at 70% capture and 85% processing yield produces about 10.71 t/day of recovered plastic, before any later product losses.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.3 Organics treatment and landfill dynamics

Composting uses managed aerobic decomposition to stabilize organics when moisture, oxygen, porosity and carbon-to-nitrogen balance are suitable. Anaerobic digestion produces biogas in the absence of oxygen but requires feedstock control and digestate management. Landfills produce leachate and gas whose composition and timing depend on waste age, moisture, composition and containment.

A first-order landfill methane model is a screening inventory model; landfill-cover oxidation, collection efficiency, site climate and historical disposal alter actual releases. Decisions about composting and digestion should consider contamination by glass, plastics or metals and whether the end product has a viable, safe use.

Governing relationship and units

Illustrative gas generation G(t)=k L₀ M exp(−kt), with unit-consistent mass and gas potential.

Worked context

A legacy deposit can continue producing gas decades after placement. Doubling the assumed first-order decay coefficient increases initial predicted generation but does not double integrated lifetime potential.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

6.4 Landfill engineering and governance

An engineered landfill needs a defensible site, drainage control, containment, leachate collection, gas management, stable slopes, operational cover and monitoring. A liner does not eliminate long-term risk; leak detection, maintenance, groundwater surveillance and financial provisions matter. Unregulated dumping often transfers environmental cost to neighbours and water resources rather than eliminating it.

Compare disposal sites by geological and hydrogeological context, flood exposure, access, leachate treatment, settlement, gas generation and land-use conflict. Build a closure and post-closure plan early. Avoid design-level conclusions from simple landfill volume or gas estimates; actual capacity and stability require geotechnical studies and local regulations.

Governing relationship and units

Nominal airspace volume = waste mass / achieved in-place density; add cover and operational allowances separately.

Worked context

If 50,000 tonnes of compacted waste average 0.8 t/m³, nominal waste airspace is 62,500 m³ before cover, settlement and buffer volumes. A design that ignores cover could exhaust usable space early.

Reasoning exercise. Using the running case, make a two-column table: available observations and missing evidence. State a plausible parameter range with units, identify one observation capable of disproving the principal assumption, and explain how the engineering action would change if a measured input were doubled. Do not treat the worked illustrative numbers as calibration data.

Model critique. Check dimensional consistency before calculation. If data come from different dates, locations, temperature bases or chemical definitions, reconciliation is required before drawing a conclusion. Report the sensitivity of the decision to uncertain inputs and distinguish the physical result from any legal or public-health judgment.

Synthesis and field decision

In the case scenario, prepare a waste flow diagram and compare source separation, composting, digestion and controlled disposal options. For each quantity used, identify its spatial representativeness, time basis, uncertainty and the responsible person who can confirm the value. Explicitly document how the plan changes during equipment failure or extreme conditions.

Unit assessment

1. For solid waste: systems, recovery, treatment and disposal, define a control volume and list all stocks, transports and reaction terms with units.
2. Explain how different sampling times or locations could reverse the conclusion in this scenario: A medium-sized city depends on open dumping and proposes material recovery plus a controlled landfill.
3. Choose two uncertainties and quantify which has the larger effect on the recommendation. Show a reproducible parameter table.
4. Use one of the four governing relationships to calculate a low, central and high case, then explain why a numerical result is not a permit limit.
5. Design an independent field or bench experiment that discriminates between a transport limitation and a reaction limitation.
6. Write a corrective-action plan for equipment failure or a surprising monitoring result; identify response time and independent verification.
7. Construct a single-page public explanation of the issue without concealing unresolved uncertainty.
8. Critique a proposal that reports percent removal but omits residual streams, peak-flow capacity and monitoring quality.

Assessment policy: A numerical answer without units, documented assumptions, and interpretation is incomplete. Explain what additional observation or regulation would be needed to authorize real-world action.