Subtitle D Landfill Design, Liners & Leachate
Key Takeaways
- Subtitle D municipal landfills require composite liners, leachate collection, and groundwater monitoring.
- Composite liner = compacted clay (low hydraulic conductivity) plus synthetic geomembrane.
- Leachate collection systems use granular drainage layers and pipes to limit head on the liner.
- Landfill gas (methane and CO2) must be monitored and controlled — flaring or energy recovery.
- Final cover includes low-permeability cap, drainage, and vegetative soil to shed precipitation.
Quick Answer: Modern Subtitle D landfills use composite liners (clay + geomembrane), leachate collection to keep head off the liner, groundwater monitoring wells, landfill gas control, and a low-permeability final cover to minimize infiltration after closure.
Municipal solid waste landfills are engineered structures — not open dumps. Design separates waste from groundwater and controls gas migration.
Subtitle D vs. Subtitle C Disposal
| Feature | Subtitle D (MSW) | Subtitle C (HW) |
|---|---|---|
| Waste type | Non-hazardous MSW | Hazardous waste |
| Liner | Composite required | Double composite, stricter |
| Leachate | Collect and treat | Collect and treat to standards |
| Location restrictions | Hydrogeologic siting | Additional security |
FE focus is Subtitle D MSW landfills unless the stem specifies hazardous waste landfill (Subtitle C).
Liner System Components
Typical composite bottom liner from bottom upward:
- Compacted subgrade — prepared foundation.
- Recompacted clay layer — low hydraulic conductivity (often K ≤ 1×10⁻⁷ cm/s order of magnitude — use problem values).
- Geomembrane (HDPE) — synthetic barrier.
- Geonet/geocomposite drainage — protects membrane, conveys leachate.
- Granular drainage layer (sand/gravel).
- Filter fabric — prevents fines clogging drainage.
- Waste.
Composite design — if geomembrane leaks, clay delays contaminant breakthrough.
Leachate Collection
Leachate forms when precipitation percolates through waste and extracts dissolved and suspended constituents.
| Component | Function |
|---|---|
| Drainage blanket | High-permeability layer |
| Collection pipes | Sloped to sumps |
| Sumps/pumps | Remove leachate to storage/treatment |
| Head criteria | Regulatory max leachate head on liner (e.g., 12 inches — use stem) |
Worked concept: If collection pipes clog, head on liner rises → increased leakage risk. Slope toward pipes must be maintained during operations.
Leachate Management
Leachate is often recirculated (bioreactor landfills accelerate decomposition) or sent to WWTP with pretreatment. Toxicity and BOD/ammonia drive treatment needs.
Landfill Gas (LFG)
Anaerobic decomposition produces ~50% CH4, ~50% CO2, plus trace compounds:
- Active gas collection — vertical wells, headers, blower/vacuum.
- Control — flare or LFG-to-energy.
- Migration monitoring — probes at perimeter for methane > 5% LEL in soil.
NMOC (non-methane organic compounds) triggers Title V / NSPS in large landfills.
Daily and Final Cover
Daily cover — soil or alternative at end of each working day controls vectors, odors, fires.
Final cover system:
- Low-permeability barrier (compacted clay or geomembrane).
- Drainage layer — shed infiltration.
- Protective soil — erosion control.
- Vegetation — long-term stability.
Goal: reduce post-closure leachate and gas generation.
Siting and Hydrogeology
Landfills require site suitability:
- Depth to groundwater.
- Bedrock fracturing.
- Floodplains and wetlands restrictions.
- Buffer to property lines.
Hydrogeologic report documents background groundwater quality.
Stability and Operations
Waste placement in lifts; compaction reduces void space and settlement. Slope stability analysis prevents waste mass slides. Cover integrity during operations limits stormwater infiltration.
Monitoring
Detection monitoring — upgradient and downgradient wells; statistical comparison to background. Assessment monitoring if detection limits exceeded.
FE Exam Patterns
- Identify liner layer purposes.
- Know composite = clay + geomembrane.
- Leachate head management.
- Methane control requirements.
- Difference Subtitle C vs. D stringency.
Exam trap: Confusing leachate (liquid percolating through waste) with landfill gas (vapor phase decomposition products).
Landfill design questions test whether you understand how each layer protects groundwater — not memorizing state-specific acreage rules.
Leachate Treatment and Recirculation
Bioreactor landfills recirculate leachate to accelerate waste decomposition and gas generation — increases moisture within waste mass but requires liner integrity and leachate head management. Leachate pretreatment before sewer discharge may include equalization, ammonia stripping, metals precipitation, and activated carbon for organics — cross-reference water treatment knowledge.
Leachate strength varies with waste age — young waste produces high BOD and ammonia; methanogenic phase leachate is less biodegradable but may contain higher metals concentrations from reducing conditions.
Post-Closure Care and Financial Assurance
Post-closure care typically 30 years includes cap maintenance, gas system operation, and groundwater monitoring. Financial assurance (surety bond, trust fund) ensures closure and post-closure funding if operator defaults — engineering cost estimates support assurance amounts.
Slope Stability and Waste Placement
Lift heights and benching on high landfills affect stability. Settlement after closure impacts cap integrity — design includes sacrificial soil thickness. Daily cover reduces odor, vectors, and fire risk while shedding precipitation from active face.
Slope Stability Quick Check
Waste unit weight ~75–85 lb/ft³ in place. Steep side slopes (>3H:1V) risk waste sliding on weak liner interfaces — geosynthetic interfaces have low friction; bench design and toe berms add stability.
Daily Cover Alternatives
Alternative daily cover (ADC) — foam, tarps — reduces soil use and airspace consumption. Must meet state approval for disease vector, fire, and odor control equivalent to 6 inches soil.
Groundwater Monitoring Well Placement
Background wells upgradient; detection wells downgradient at compliance point before property boundary. Statistical tests (e.g., Mann-Kendall) demonstrate no significant trend after closure.
Landfill Design, Leachate, and Liners
| Component | Function |
|---|---|
| Composite liner | Geomembrane + clay/GCL barrier |
| Leachate collection | Pipes/geocomposite above liner |
| Cover system | Infiltration control, gas management |
| Gas collection | LFG energy or flare |
| QA/QC | Weld testing, CQA |
Water Balance Idea
Leachate generation rises with precipitation and poor cover performance. FE items link liner failure modes (punctures, poor seams) to groundwater risk and the need for leak detection in some designs.
On the Exam: Primary barrier is the geomembrane; clay/GCL is the low-permeability backup in composite systems.
Leachate Head
Max 12 inches on liner typical regulatory limit — collection pipes in gravel blanket at 2% minimum slope.
Landfill Gas Collection
Vertical wells every 50–200 ft; vacuum -2 to -5 in H₂O. Flare or engine for energy.
Final Cover Layers
6 in low-permeability + 18 in drainage + 6 in topsoil + vegetation — shed precipitation post-closure.
Settlement
Waste settles 15–25% — final grade design accounts for differential settlement over waste cells.
Worked Example
Leachate generation 15 gal/ton-waste/yr × 500,000 tpy → 7.5 Mgal/yr — size storage and treatment accordingly.
A composite landfill liner typically consists of:
The primary purpose of a leachate collection system is to:
Landfill gas from MSW decomposition is predominantly:
Subtitle D landfills primarily accept: