This is the 12th and final lesson in the independent learning correspondence course on municipal solid waste (MSW) landfills. Each lesson in this series was published in Waste Age magazine each month in 2002.
If you are interested in taking the course for two continuing education credits (CEUs), send a check (payable to the University of Wisconsin) for $149 to Phil O'Leary, Department of Engineering Professional Development, University of Wisconsin, 432 N. Lake Street, Madison, Wisc. 53706. Phone (608) 262-0493. E-mail: [email protected]. Website: www.wasteage.com. You can register for the course until December 2006. Previous lessons will be sent to you.
To understand the important considerations in closing a landfill.
To know the activities of a landfill's long-term care.
To understand the financial resources necessary to sustain closure and long-term care.
Efficiently using a landfill site following closure requires long-range planning. The best strategy is to plan for the eventual site use before the landfill is constructed and operated. Determining the final use of a closed facility during site selection also may reduce possible opposition to a new landfill. Potential uses for a closed sanitary landfill can include a nature or recreation park; animal refuge; tennis court; golf course; parking lot; or commercial or industrial building site.
Planning is particularly important when constructing a building on or near the landfill. Design features such as locations of structures requiring special support, recreational facilities featuring specific topography and gas control systems to protect future buildings should be anticipated during landfill operation. Areas also should be set aside for groundwater monitoring. Setting aside space to install remediation equipment or structures to handle potential groundwater contamination also is a good strategy.
Completed Site Features
When planning final site use, the critical factors that must be considered are: settlement, foundation characteristics, leachate and gas control, vegetation, and final grade. Depending on the end-use, efficient site access may be necessary.
Waste settlement and consolidation is a major issue when constructing facilities on a closed landfill. Older sites with poor initial compaction and large amounts of precipitation percolated into the waste, settle quicker and more extensively. About 90 percent of the settlement will occur in the first five years, although it may continue for 25 or more years at a slower rate. A site with in-place densities of about 800 pounds per cubic yard may settle quicker and more extensively compared to a well-run site with in-place densities of 1,400 pounds per cubic yard.
The amount of water in the landfill can significantly affect decomposition rates. For example, a site with limited available water for biochemical decomposition will settle more slowly. Efforts to promote decomposition by recycling leachate should speed up settlement. Water, which leaks through the landfill cover, can cause uneven settlement that is difficult to manage.
Settlement also is affected by the amount of cover material used. A landfill with a cover-to-waste ratio of 1-to-4 should settle more than a site with a ratio of 1-to-3. The composition of solid waste influences the subsidence rate. A landfill with a high percentage of construction and demolition (C&D) waste will likely settle less than a landfill with high organic content.
A final closure plan should include periodic landfill inspections and, when necessary, provide for regrading to prevent ponding.
Monitoring the vertical movement of the individual levels of intermediate covered waste can provide an initial indication of the eventual settlement rate. Conditions on the landfill's finished surface will depend on the conditions of each lift within the landfill.
Surcharge loading where heavy materials, usually soil, are piled onto the site will accelerate consolidation. Wetting solid waste can cause biological decomposition and, subsequently, volume reduction. Settlement due to volume reduction creates cracks in the cover material. Settlement can occur within a few days of filling or over many years.
Closing procedures will differ depending on whether the entire landfill or a phase of it, is filled to capacity. Phased closure is recommended [See “Site Closure Checklist” on right].
A landfill's design should limit water from entering the site to reduce leachate generation. Correct cover placement is an important control technique. Regulations somewhat dictate the type of final cover. Covers may include an initial two-foot layer of relatively impermeable soil followed by topsoil, or a geomembrane with topsoil. Construction techniques, which ensure quality installation, will enhance the cover's ability to resist water percolation into the landfill.
In humid regions, the closed landfill should be immediately vegetated after cover placement to prevent erosion. [See “Steps for Vegetating Landfills” on page 46]. A timetable should be prepared to address:
Settlement, cover soil integrity and a need for grading;
Buffers and vegetation;
Sediment and erosion control facilities;
Leachate and gas controls;
Integrity of the final site facility;
Regular intervals of monitoring.
It also is important to consider the high cost of removing leachate for treatment, monitoring groundwater quality and maintaining the gas management system when closing a site. The costs of leachate treatment will be directly influenced by the landfill cover's integrity. Properly installing and maintaining the cover will reduce the cost of leachate treatment. Reducing leachate generation also reduces the potential for groundwater contamination.
Many landfills have gas recovery systems, which generate income from electricity sales. These sales can help to offset long-term closure costs, but the revenue will decline as less gas is emitted over time. Some landfills must operate gas control systems to prevent migration into residential areas. However, the gas quality may be low and have no economic value. In this situation, the cost of operating the gas management system must be in the long-term care fund.
Building on a Landfill
Engineering techniques are available to construct many types of facilities over a closed landfill. For example, it may be necessary to extend pilings through the landfill and into the underlying base material. In certain instances, it may be possible to use special foundations or footings to support the load.
When building on a landfill, it is imperative to consider consolidation-time-settlement relationships and loadbearing capacities developed from field and laboratory data. Specialized techniques also can help designers identify appropriate structural techniques.
Bearing capacity is the ability to support foundations and heavy equipment. Although the bearing capacity of the landfill will depend on the operation, values ranging from 500 to 800 pounds per square foot have been reported. Low bearing capacity can be overcome by increasing the soil thickness used in the final cover. This helps the soil to resist both punching and rotational shear. The minimum recommended soil thickness is 1.5 times the width of structural footings.
Placing and compacting special waste under planned buildings also is an option. For example, fly ash and bottom ash may be compacted in 1-foot lifts in assigned areas while other solid waste is distributed in adjacent areas. Incorporating stabilizers into the soil during cover placement can improve the landfill's bearing capacity. Common chemicals mixed with soil include lime, portland cement and various organic chemicals. Soils also can be strengthened by cementation, waterproofing or dispersion to provide greater density and lower permeability.
Where other means of supporting a landfill's foundation are unsuccessful, a system of piles or piers can strengthen important structures. But even these extreme measures do not eliminate problems. A landfill may continue to settle around and away from the stable structure and incidentally cause the piling to take additional, unanticipated loads through negative skin friction mobilization.
When constructing facilities on a landfill, consider the placement and building service entrances of the sewer, water, electric, and gas utilities. If trees will be located on the landfill cover, appropriate locations with deeper soil cover should be constructed [See “Tree Planter with Hydraulic Barrier” on page 47].
When designing structures on a landfill, operators also must be cognizant of gas-venting techniques to prevent methane gas accumulation within structures. These controls generally will involve installing extraction systems, which prevent landfill gas from entering buildings. Natural ventilation systems to prevent gas from entering buildings are not recommended.
Often, drainage problems can cause accelerated erosion in particular landfill areas. Differential settling can limit the usefulness of drainage control structures and may result in failing to properly direct storm water off the site. Failure to maintain the physical integrity of the landfill cover promotes additional water infiltration and eventually results in increased leachate production.
Gas and Leachate Control
Quantities of gas and/or leachate may be found migrating away from the landfill if control mechanisms fail or were not installed. Gas and leachate management control systems require continuous attention after closure. Groundwater monitoring wells and gas probes should be incorporated into the design to check control system performance. Remedial steps to control gas and leachate involve implementing an engineering control system along the perimeter of the site and, in some cases, within the landfill. Often, remedial actions are more expensive than if the gas or leachate had been properly managed and facilities installed when the landfill was being developed.
Groundwater that has been contaminated by leachate can be managed in many ways. Impermeable barriers of bentonite slurry, cement or chemical grouts, or sheet piling can be installed vertically to prevent groundwater migration, or to divert groundwater and prevent contact with leachate. These methods are considered passive groundwater control. Pumping groundwater with subsequent surface treatment is an active remedial measure. Placing a more effective cover over the landfill also can reduce leachate generation. Implementing a leachate control program requires an extensive engineering study to determine the most effective and economical approach.
Landfill closure and long-term care is expensive. While some income may be expected through the sale or lease of the site, and possibly through the sale of methane gas, this is not enough to pay for all closure and long-term care costs.
Expenses incurred at closure and during a 30-year, long-term care period, for example, can be a significant portion of the overall cost of operating a site. The federal government requires that closure and long-term care costs be identified, and a plan established for their financing when operating a solid waste disposal facility. Most states have similar requirements.
Landfills can set up separate accounts to manage closure costs. [See “Long-Term Care Fund Deposits and Payments” on page 46]. When a site closes, funds are withdrawn from this account to pay for closure, and then a sum is withdrawn annually to pay for long-term care. As you create this account, remember to consider inflation when calculating the yearly payments.
Alternatives such as trust funds, escrow accounts or bonds also are available to fund closure and long-term care. Specific regulatory procedures are more extensive and should be used to create a landfill's long-term financing plan.
When choosing closure funding, consider the political sentiments within the community, tax consequences to operators of private landfill facilities and the desirability of tying up sums of money where the operating authority already appears to have sufficient financial resources.
Designing and funding a landfill for a 10- or 20-year life, followed by 30 years of long-term care, also requires that the landfill authority be in existence for as long as 50 years. Many things can change during this time, and the need for long-term viability must be considered.
Phil O'Leary and Patrick Walsh are solid waste specialists with the University of Wisconsin-Madison. The authors also acknowledge the contributions of authors of previous editions of this Solid Waste Landfills course, including John Reindl, Berrin Tansel and Rick Fero.
SITE CLOSURE CHECKLIST
□ Identify final site topographic plan;□ Prepare site drainage plan;□ Specify source of cover material;□ Prepare vegetative cover and landscaping plan;□ Identify closing sequence for phased operations of on-site structures;□ Specify engineering procedures for the development of on-site structures.
Three Months Before Closure
□ Review closure plan for completeness;□ Schedule closing date;□ Prepare final time-table for closure procedures;□ Notify appropriate regulatory agencies;□ Notify site users by letter if they are municipalities or contract haulers, or by published announcement if private dumping is allowed.
□ Erect fences or appropriate structures to limit access;□ Post signs indicating site closure and alternative disposal sites;□ Collect any litter or debris and place in final cell for covering;□ Place cover over any exposed waste.
Three Months After Closure
□ Complete needed drainage control features or structures;□ Complete as required gas collection or venting system, leachate containment facilities, and gas or groundwater monitoring devices;□ Install settlement plates or other devices for detecting subsidence;□ Place required thickness of earth cover over landfill;□ Establish vegetative cover;□ Record the closure action with the official land records agency.
STEPS FOR VEGETATING LANDFILLS
- Select an end-use.
- Determine cover depth: Cover soil must be at least 60 centimeters deep for grass establishment and 90 centimeters for trees.
- Establish an erosion control program: The soil on recently covered landfills must be stabilized soon after spreading to prevent erosion.
- Determine the soil nutrient status: Before or during the grass and ground cover experiments, soil tests should be made for pH, major nutrients (nitrogen, potassium, and phosphorus), conductivity, bulk density and organic matter.
- Determine soil bulk density: Cover soil is frequently compacted by landfill equipment during spreading operations to bulk densities that will severely restrict plant root growth.
- Amend soil cover: The soil over the entire planting area should be amended with lime, fertilizer and/or organic matter according to soil tests before planting. These materials should be incorporated into the top 15 centimeters of soil.
- Select landfill-tolerant species: Grasses and other ground covers that will be planted in the soil cover can be selected by evaluating the results of the experimental plots established earlier to determine landfill-tolerant species.
- Plant grass and ground covers: It is generally desirable to embed seed in the soil. Mulches can be used as an alternate to embedding the seed but are not as effective.
- Develop tree and shrub growth: Trees and shrubs should not be planted until one or two years after grass has been planted. If grass does not grow because of landfill gases, other deeper-rooted species are not likely to thrive either.
LONG-TERM CARE FUND DEPOSITS AND PAYMENTS
FUND DEPOSITS Site Life Year Annual Payment To Fund Fund Balance January 1 Interest Earned 6% per yr Fund Balance December 31 2002 1 $231,106 $231,106 $13,866 $244,973 2003 2 238,040 483,012 $28,981 $511,993 2004 3 245,181 757,174 $45,430 $802,604 ~ 2016 15 349,569 6,460,123 $387,607 $6,847,731 FUND PAYOUTS Site Closure - 2017 Fund Payout $1,557,169 (Equivalent to $1,000,000 in current $) Fund Balance 5,289,763 Year LTC Year LTC Estimate (current price) Annual Payout (assumes 3% inflation) Fund Balance January 1 Interest Earned 6% per yr Fund Balance December 31 2018 1 $200,000 $320,941 $4,968,822 $298,129 $5,266,951 2019 2 200,000 $330,570 4,936,382 $296,183 5,232,565 ~ 2045 28 150,000 $534,678 1,024,386 $61,463 1,085,849 2046 29 150,000 $550,718 535,132 $32,108 567,239 2047 30 150,000 $567,239 0 $0 0