Solid Waste and Resource Recovery Systems Exercises

These exercises practise first-pass calculations used in solid waste and resource recovery engineering. They connect waste characterization, material balance, diversion, contamination, collection capacity, leachate storage, landfill gas, energy recovery, equipment reliability, risk ranking, and validation evidence.

Assume simplified nominal values unless an exercise states otherwise. Real waste-system decisions require representative sampling, regulatory classification, facility operating data, product-quality specifications, environmental controls, health and safety review, market feedback, and responsible engineering approval.

How to Use These Exercises

For each problem:

1. define the waste boundary, time basis, and material streams;
2. state whether the result is mass, volume, recovery, residual, load, or reliability;
3. keep density, moisture, energy, and flow units consistent;
4. distinguish reported diversion from verified downstream acceptance;
5. identify which record proves that the waste stream was actually controlled.

The most common mistake is reporting recovery without tracking rejects. A waste system is credible when recovered products, residuals, leachate, gas, emissions, and downstream acceptance all reconcile.

For each result, state whether it supports route planning, facility capacity, product-quality release, buyer acceptance, leachate or gas control, fire-risk response, or recovery-claim closeout. Diversion is not proven until material quality, residuals, and downstream records reconcile.

Exercise 1: Waste Characterization by Mass

A facility receives 42\ \text{t/day} of mixed waste. A characterization study estimates:

• organics: 38\%;
• paper and cardboard: 22\%;
• plastics: 14\%;
• metals and glass: 11\%;
• residuals and fines: 15\%.

Find the daily mass of each category.

Solution

Organics:

Paper and cardboard:

Plastics:

Metals and glass:

Residuals and fines:

Engineering Comment

The percentages should come from representative sampling, not a single visual audit. Seasonal change, moisture, commercial activity, construction debris, and sampling bias can change recovery design.

Exercise 2: Diversion Rate with Rejects

A materials recovery facility receives 120\ \text{t/day}. It ships 72\ \text{t/day} of recovered material to buyers. Buyers reject 9\ \text{t/day} because of contamination.

Find the reported shipment rate and the accepted recovery rate.

Solution

Reported shipment rate:

Accepted recovered mass:

Accepted recovery rate:

Engineering Comment

The accepted rate is the stronger performance metric. Shipping material that is later rejected transfers the problem downstream and can overstate environmental performance.

Exercise 3: Contamination in a Recovered Bale

A recovered plastic bale has mass 820\ \text{kg}. A quality sample estimates contaminant mass of 54\ \text{kg}. The buyer specification allows at most 5.0\% contamination.

Check whether the bale meets the specification.

Solution

Contamination rate:

Since:

the bale does not meet the specification.

Engineering Comment

The response should include source review, sorting adjustment, sampling method check, and buyer feedback. Recovered material is a product stream, not only a diverted waste stream.

Exercise 4: Collection Vehicle Capacity

A collection route serves 680 households. Each household sets out an average of 34\ \text{L} of organics per pickup. The compacted bulk density in the vehicle is 0.42\ \text{t/m}^3. The vehicle payload limit is 9.0\ \text{t}.

Estimate collected volume and mass.

Solution

Total volume:

Mass:

Engineering Comment

The route exceeds the payload limit under the assumed density. Options include route split, larger vehicle, lower pickup count, transfer point, or density verification. Volume capacity and payload capacity should both be checked.

Exercise 5: Leachate Storage During Wet Weather

A landfill cell generates leachate at 68\ \text{m}^3/\text{day} during a wet-weather period. The treatment or hauling capacity is 45\ \text{m}^3/\text{day}. The period lasts 5 days.

Estimate required temporary storage for the excess leachate.

Solution

Daily excess:

Five-day storage:

Engineering Comment

The storage estimate should be checked against available tank volume, freeboard, pump reliability, pipe fouling, treatment acceptance, emergency hauling, and stormwater run-on controls.

Exercise 6: Landfill Gas Energy Potential

A landfill gas system collects 520\ \text{m}^3/\text{h} of gas. Methane content is 48\% by volume. Use methane lower heating value 35.8\ \text{MJ/m}^3. The generator electrical efficiency is 34\%.

Estimate electrical power output.

Solution

Methane flow:

Thermal energy rate:

Convert to kW:

Electrical power:

Engineering Comment

This estimate assumes gas quality, flow, and generator operation are stable. Siloxanes, moisture, hydrogen sulfide, oxygen intrusion, condensate, flare downtime, and wellfield balance can reduce usable energy.

Exercise 7: Waste-to-Energy Net Efficiency

A waste-to-energy unit receives waste with useful thermal input of 28\ \text{MW}_{th}. Gross electrical output is 7.4\ \text{MW}. Auxiliary loads for fans, pumps, feed systems, and controls are 1.1\ \text{MW}.

Find net electrical efficiency.

Solution

Net electrical output:

Net efficiency:

Engineering Comment

Net efficiency is more useful than gross output. The review should also include feed moisture, heating value variability, air emissions, ash quality, uptime, heat recovery, and whether recyclables should be removed before thermal treatment.

Exercise 8: Equipment Availability in a Sorting Line

A sorting line requires a screen, magnet, optical sorter, and baler to operate. Their estimated availabilities during the production window are:

Estimate simplified series availability.

Solution

Series availability:

Engineering Comment

The line availability is lower than each individual item. Reliability planning should include bypass rules, spare parts, cleaning access, fire response, jam clearing, and product-quality checks after restarts.

Exercise 9: Hot-Load Failure Mode RPN

A transfer station reviews the failure mode “hot load reaches tipping floor without isolation.” Initial scores are:

After adding thermal screening, driver questioning, and an isolation bay, occurrence is estimated as O=2 and detection as D=2.

Find the initial and revised risk priority numbers.

Solution

Initial:

Revised:

Reduction:

Engineering Comment

The controls are credible only if staff can use them during peak traffic. Isolation area access, fire water, communication, and emergency procedures should be validated under real operating conditions.

Exercise 10: Residual Accountability Closeout

A monthly resource-recovery report tracks 18 required records. Fifteen are accepted, one buyer rejection report is missing, one residual-disposal ticket is pending, and one scale calibration record is rejected.

Find the accepted-record percentage and unresolved count.

Solution

Accepted-record percentage:

Unresolved records:

Engineering Comment

The report should remain open. Buyer rejection, residual disposal, and scale calibration directly affect whether the material balance and recovery claims are defensible.

Review Checklist

Before accepting a solid waste or recovery calculation, check:

• whether the system boundary and time basis are explicit;
• whether mass, volume, moisture, density, and energy bases are compatible;
• whether diversion includes buyer rejection and residual disposal;
• whether product quality is sampled against downstream specifications;
• whether leachate, gas, emissions, and fire risk are part of the same operating model;
• whether reliability calculations include bypass and residual consequences;
• whether RPN reductions correspond to usable controls on the floor;
• whether scale calibration, sampling method, buyer rejection, disposal tickets, and inventory changes support the reported material balance;
• whether hot loads, contamination, equipment downtime, leachate storage, gas quality, and fire response can be handled during peak operations;
• whether validation records reconcile scales, products, rejects, residuals, and market feedback.

Good resource recovery engineering closes the loop between incoming waste, recovered products, residual streams, environmental controls, and evidence that downstream users accepted the material.