MSW Collection, Transfer, Composting, WTE & Recycling

Quick Answer: MSW management follows the waste hierarchy (reduce, reuse, recycle, compost, WTE, landfill). Know collection/transfer economics, aerobic composting parameters, WTE with APC residues, and recycling contamination limits.

Integrated solid waste systems appear on the FE as process selection, capacity, and environmental tradeoff questions.

Waste Management Hierarchy

EPA hierarchy (most to least preferred):

1. Source reduction — less material used.
2. Reuse/repair
3. Recycling/composting
4. Energy recovery (WTE)
5. Landfill disposal

Engineering decisions weigh cost, diversion rate, environmental impacts, and local markets.

Collection Systems

Haul distance drives cost — transfer stations compact waste into long-haul trailers.

Worked example: Collection trucks average 8 mpg; landfill 60 miles away vs. transfer station 10 miles + rail haul — transfer often saves fuel and labor.

Transfer Stations

• Floor dumping with push walls.
• Compaction into transfer trailers.
• Leachate and odor control — ventilation, misting.
• Traffic and noise — siting concerns.

Recycling and MRFs

Material Recovery Facilities sort commingled recyclables:

• Disk screens — glass, 2D vs. 3D paper/cardboard.
• Eddy current — aluminum.
• Optical sorters — plastics by resin.

Contamination (food residue, plastic bags) lowers commodity value and can reject entire bales.

Recycling rate = diverted / total waste × 100%.

Composting

Aerobic composting of yard waste and food scraps:

• C:N ratio ~25–30:1 ideal.
• Moisture 40–60%.
• Temperature >55°C for pathogen reduction (PFRP — Process to Further Reduce Pathogens).
• Turning or aeration supplies oxygen.

Anaerobic digestion — biogas (CH4) + digestate; overlaps with wastewater sludge digestion.

Compost quality — maturity tests, metals limits for land application.

Waste-to-Energy (WTE)

Mass burn incineration:

• Combustion ~1800°F+ destroys organics, reduces volume ~90%.
• Air pollution control — scrubbers, baghouse, activated carbon for metals/dioxins.
• Bottom ash — often landfilled; fly ash — hazardous characteristics possible (test TCLP).
• Energy recovery — steam → electricity (~550–750 kWh/ton order of magnitude — use stem).

FE tradeoffs: WTE reduces landfill volume but has capital cost and air emissions requiring controls.

Landfill Gas Energy

LFG-to-energy is recovery at landfills — not WTE but uses methane that would otherwise flare.

Special Wastes in MSW Stream

• C&D debris — concrete recycling, wood.
• E-waste — state take-back; hazardous components.
• Bulky waste — furniture, appliances (Freon recovery).

Planning and Capacity

Waste generation per capita ~4–5 lb/day US average (use problem data):

[ \text{Annual tonnage} = \text{population} \times \text{per capita rate} \times 365 ]

Design landfill cell volume with compaction density (typical 1000–1200 lb/yd³ in place).

FE Exam Patterns

• Hierarchy ordering.
• Composting parameters (aerobic vs. anaerobic).
• WTE ash and APC residues.
• Transfer station purpose.
• Diversion rate arithmetic.

Exam trap: Calling open burning or unlined dumps acceptable MSW management — Subtitle D prohibits open dumping.

Solid waste management integrates with air (WTE APC) and water (leachate) — think cross-media on exam scenarios.

Collection Vehicle Productivity and Routing

Haulers optimize routes for one-side versus two-side collection. Packers increase legal load weight before trip to transfer or landfill. Contamination in recycling carts increases residual sent to disposal — public education affects diversion rates as much as engineering design.

Pay-as-you-throw pricing reduces MSW generation per capita — policy lever appearing in sustainability stems.

Anaerobic Digestion of Organics

Anaerobic digestion of source-separated organics or biosolids produces biogas for energy and digestate for land application after pathogen reduction. Compare to aerobic composting — digestion better for wet wastes and energy recovery; composting simpler for yard trimmings.

Landfill Capacity and Siting

Airspace consumption depends on compaction density and daily cover soil volume. Cell development sequences liner, leachate, gas, and final cover in phases. NIMBY and environmental justice considerations affect siting alongside hydrogeologic criteria.

Waste Characterization Formula

[ \text{Generation rate} = \text{population} \times \text{lb/capita/day} \times 365 / 2000 \text{ tpy} ]

City 80,000 people, 4.2 lb/capita/day → 80,000 × 4.2 × 365 / 2000 ≈ 61,320 tpy MSW.

Composting Energy Balance

Thermophilic phase >55°C for 3 days (PFRP) kills pathogens. Turning frequency prevents anaerobic pockets that generate odors — engineering control of O₂ and moisture.

WTE Ash TCLP

Fly ash from municipal waste incinerators may exhibit toxicity characteristic for lead or cadmium — test before disposal; stabilization may be required before Subtitle D landfill.

Subtitle D Solid Waste Management

Municipal solid waste (MSW) under RCRA Subtitle D focuses on nonhazardous solid waste landfills: location restrictions, composite liners, leachate collection, groundwater monitoring, closure/post-closure care, and financial assurance.

Quantity Example

If a city generates 4 lb/capita·day and population is 100,000, MSW ≈ 200 ton/day. Landfill volume depends on compaction density (e.g., 1,200 lb/yd³ in-place) — FE may ask order-of-magnitude airspace.

On the Exam: Subtitle C = hazardous; Subtitle D = nonhazardous solid waste. Do not swap.

Waste Hierarchy Application

Source reduction beats recycling economically when redesign eliminates material — engineer evaluates life-cycle.

Transfer Station Throughput

200 tpd incoming, 8 hr/day → 25 tph average; peak 1.5× for equipment sizing.

Composting C:N

Food waste C:N ~15:1 — add bulking agent (wood chips) to reach ~25:1–30:1.

WTE Ash

Bottom ash ~10% of MSW mass; fly ash may be hazardous — test TCLP. APC residues often managed as hazardous.

Diversion Rate

[ \text{Diversion} = \frac{\text{Recycled + composted}}{\text{Total generated}} \times 100% ]