Landfill mining uses conventional soil excavating and processing techniques. Using soil excavators, the process produces material that is screened by a trommel or finger screen to remove large items, such as appliances, wood, tires, metals, plastics and fabric. The oversize materials are sold or disposed. If off-site disposal is too costly or not available, the oversize can be shredded and placed in a lined section of the excavated site.
The process includes six phases: excavate and separate landfill material into soil, recyclables, combustible and residual materials; remediate the soil; remove and dispose of any hazwastes; refill to bring the site to grade or reline and upgrade to Subtitle D standards; and monitor after the project is completed.
Landfill mining may reduce or eliminate closure costs and future environmental problems; recover commodities and land; enhance the adjoining land's value; help achieve state recycling goals; and provide low-cost fuel for waste-to-energy plants.
Planning and implementing a landfill mining project is not simple. Much of the complexity is in managing its environmental and economic aspects. In order to determine its economic viability, landfill mining should be considered in the context of overall solid waste management costs.
Every landfill has its own unique set of circumstances that affect the economics of a potential project. Landfills come in different shapes and sizes. The amount of decomposition varies with climate and age; rural landfills have different compositions then urban landfills. Some landfills may have accepted more construction and demolition wastes, while others contain more industrial wastes. Some landfills have serious environmental problems while others are benign. A few states have well-defined regulations governing landfill mining, while many others have little experience with the process.
Major factors influencing costs include landfill volume and topography; equipment parameters; soil conditions; climate; labor rates; the regulatory approval process; excavation and screening costs; sampling and remediation; development costs; the contractor's fees; material disposal and logistics; hazardous wastes disposal; and revenue from the sale of commodities.
The Costs Are In The Soil Since landfills are mostly soil, mining economics are dominated by the costs of soil excavation, screening, testing and deposition, which can account for 80 percent of the project's cost. Any cost that relates to soil can rapidly escalate the cost of the project. For example, a 30 acre by 30 foot deep site contains about 1 million cubic yards of soil. For every dollar added to the cost of soil processing, approximately 12 percent is added to the cost of the project.
Overlooking an issue related to soil can mean the difference between profit or loss on a project. For example, if soil will be used as daily cover at another site, state regulatory authorities might require a Toxicity Characteristic Leaching Procedure (TCLP) for every 1,000 cubic yards shipped. A TCLP can cost $1,500 per analysis or $1.50 per cubic yard shipped.
However, the state may have no testing requirement if the soil is shipped to another state and the importing state may have a more lenient requirement. In this case, there is a trade-off between shipping costs, testing requirements and the market for the soil.
Since not all costs can be anticipated, a contingency must be included in the costs. For example, unusually wet weather can result in muddy soil conditions, which slows excavation rates, clogs screens and decreases productivity. These conditions can increase screening and excavation costs by several dollars per cubic yard.
To control the project's cost, contractors must get the most out of their soil processing equipment. Limiting the contractor to one shift per day will minimize any inconvenience to residents but will be costly. Reducing shifts from two to one shift per day can increase the cost by $1.50 per cubic yard and the project's completion time from two years instead of one. Operating at three shifts per day reduces costs by about 50 cents per cubic yard and could cut the project time to as few as seven months.
Excavator bucket capacities and cycle rates also influence landfill mining costs. For example, changing from a 3- to a 5-yard bucket can decrease costs by $1.20 per cubic yard. A drop in excavator cycle rate from four to two cycles per minute increases costs by $3 per cubic yard.
Landfill contractors report that trommels and finger screens are the major cause of downtime. Trommels and finger screens take a tremendous pounding, with heavy, bulky items being dumped directly into the screen hopper.
Screens are blinded by flat items such as sheet plastics and cardboard. Wet soil clogs screen openings. Unless a contractor has a back-up screen, downtime can drive up costs. For example, an increase from 10 to 30 percent downtime can increase costs by $1 per cubic yard. By adding an additional screen the downtime can be controlled at an added cost of only about 20 cents per cubic yard.
Economic Justification Justifying landfill mining goes beyond a simple comparison of landfill mining verses conventional closure. Landfill mining alternatives should be evaluated in the context of the overall management of solid waste. Taking this systems approach can reveal unexpected benefits including:
* Consolidation - Reducing the Footprint. The area subject to closure can be decreased significantly by reducing the landfill volume. For a 10-acre landfill, the site would be excavated and the oversize placed in a one- or two-acre lined area, which would be closed. This is referred to as consolidation or footprint reduction.
The cost of closing a landfill depends on its conditions, the clean-up required and state regulations. Closure costs have been estimated at anywhere from $170,000 to $250,000 per acre. Remember that the cost of landfill mining increases by the landfill volume, while closure costs increase by the area.
If conventional closure costs are high, consolidation might be cheaper for landfills less than 25 feet deep. Since large landfills are usually deep, consolidation probably cannot be justified for landfills much larger then 30 acres (see Table I on page 50).
* Re-siting. Most landfill mining projects are justified on the basis of re-siting, since tipping fees range from $16 to more than $70 per cubic yard, while re-siting costs about $6 to $8 per cubic yard of new space.
Re-siting replaces wastes with space. Excavation and screening can recover about 25 percent of the space by removing the oversize, as long as it can be sold or disposed off-site at little or no cost. Soil is still 75 percent of the volume of the material excavated. The markets for this type of soil varies between regions. Some use it for constructing levees or in flood control. Soil can be sold as landfill cover or stockpiled for daily cover for the new landfill.
Often, the contractor will finance the project and mine and operate the landfill, paying a fee for each ton of waste accepted. In addition, the contractor will establish a sinking fund to finance the landfill's closing.
Table II (on page 50) shows a fee structure for a 30 acre by 30 foot site and the financial projections for the project. In this case, the landfill operator finances $10,500,000 over five years based on a contractual arrangement for waste. The operator refills the landfill over a period of five years and then the landfill is closed.
Potential Benefits Landfill mining produces other tangible benefits. As sites are enlarged, more money can be borrowed through bonds based on firm waste contracts. Equipment can be more fully utilized and back-up equipment can be brought to the site. It is easier to justify investing in equipment that will recover and upgrade steel, aluminum and other commodities. Market specialists can be hired to negotiate the sale of commodities and soil and more money can be spent on equipment to shred and grind residual wastes.
Even with a relatively small landfill, a town can form a consortium with nearby towns facing landfill closure. For example, if one landfill is selected as the hub site, it can host excavation and screening as well as all the oversize processing equipment. A section of this site accepts residuals from the mining operation. It also could be prepared as a Sub-title D landfill for the consortium's use. As the mining progresses to the other landfills, the oversize is brought to the hub site for processing, sale or disposal.
Large landfills are a storehouse of valuable steel and aluminum scrap. For example, a 50-acre site might contain as much as 240,000 tons of steel and 20,000 tons of aluminum depending on the site's depth. These amounts of scrap represent mill quantities of material.
Steel scrap recovered from landfills is finding its way through scrap dealers into the steel industry. There are estimates of more than 400 million tons of steel in these closed (or soon to be closed) sites. Considering that the U.S. steel industry consumes about 100 million tons of scrap every year, the scrap recovered from landfills represents a significant supply source.
Today's landfill mining uses conventional soil excavation and screening equipment. As the industry gains experience, equipment is being modified to fit the needs of the mining contractor. In addition, these contractors are learning how to use and modify equipment to improve productivity. Landfill contractors are becoming familiar with field separation, sorting and grinding equipment such as scalping magnets, eddy current separators and tub grinders.
The industry still needs inexpensive methods of reducing soil volume or improving the soil's marketability.
The cost of landfill mining will continually decline as the industry gains experience and new equipment and techniques become available. Over time, landfill mining will be-come more attractive to towns facing landfill closure or operators wanting to extend the life of their landfills.