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Digging Deep

Digging Deep

Florida landfill turns to innovative mining project to increase its capacity without expanding its footprint.

Facing a need to increase capacity at its Perdido Municipal Solid Waste Landfill without expanding the site's footprint, Escambia County, Fla., has launched an innovative on-site mining project. As a result of the project, the Perdido Landfill will be able to operate for an additional 26 years without having to expand significantly beyond its current boundaries.

When the landfill opened in 1980, it was an unlined MSW landfill. Thirty years later, the site includes both unlined landfill areas and active lined Class I landfill areas, along with other facility operations such as yard waste processing areas, recyclables management areas and employee offices. For the landfill's first decade, municipal and industrial solid waste was disposed of in unlined cells using a trench-and-fill method. Later, the lined cells that today make up the active landfill were constructed adjacent to the 50 acres of the original unlined cells. Wanting to increase capacity at the site as well as address the long-term impact of unlined areas on their modern facility, Escambia County decided to explore mining the unlined areas to create new lined cells.

With relatively few examples of similar projects at other landfills, the Escambia County Division of Solid Waste Management proceeded cautiously, conducting a full-scale search of existing literature on the method as well as economic and engineering analyses of landfill mining. A feasibility study revealed many potential benefits, including:

  • Recovery of landfill airspace.

  • Recovery of soils from the unlined cells.

  • Recovery of materials of value, such as metals, from the unlined cell.

  • Prevention of groundwater contamination by removing waste from unlined area.

  • Optimization of available land at the site.

Based on its research, the county decided to proceed with the mining project. "The landfill mining provides a great opportunity for reuse of materials," says Pat Johnson, chief of landfill operations for Escambia County. "The recovered soils and composted organics are reused on-site as cover materials and recovered metals are sent for recycling."

To further increase the feasibility of the expansion, the project incorporates a mechanically stabilized earthen (MSE) wall to build up the outer edge of the landfill using soils mined from under the closed, unlined area of the landfill. "The decision to design an MSE wall allowed a substantial increase in landfill airspace without any significant expansion of the existing landfill footprint and avoided impacts to nearby wetlands and other sensitive environmental features," says Ron Hixson, Escambia County's environmental manager for the Neighborhood and Community Services Bureau.

In this instance, the MSE wall offers a double benefit. Not only does it contribute to increasing the landfill's airspace by providing a vertical expansion, it also provides a beneficial use of the soils excavated from the mined areas. By mining, the county can design modern landfill cells that are significantly deeper than the old unlined ones, which increases airspace.

Digging In

The first step in the landfill mining project was to develop reliable estimates of both the volume of waste that could be excavated from the unlined cells and the capacity of new lined cells that could be built on the land reclaimed by mining. The landfill did have the original topographic design plan from the 1980s, when the unlined cells opened, but no as-built drawings existed, making reliable volume calculations challenging.

To fill this data gap, the Perdido Landfill drilled a series of 39 boreholes to determine the accuracy of the topographic design plan. The boreholes were drilled to within about five feet of the naturally-occurring clay soil. The waste depth was estimated by measuring the length of the auger below the landfill surface grade and the penetration depth into the soil below the deposited waste. Air emissions were monitored with a portable multi-gas meter near each borehole. When complete, each borehole was backfilled with bentonite pellets.

Determining the composition of buried waste materials was another major challenge. To get a better sampling than that provided by the tiny amounts retrieved from the boreholes, the landfill excavated eight test pits, each about 10 by 20 feet and up to 15 feet deep. The landfill selected areas that would not disturb the integrity of existing landfill recovery gas wells, stormwater control channels and other site features. Waste screening separated the recovered waste into four categories, based on size.

The mining project is executed in stages, with waste excavated and moved from the unlined landfill area for sorting, reclamation and placement in the lined landfill. In the meantime, excavated areas of the closed, unlined landfill are turned into modern, permitted and lined landfill cells. The project will continue section by section through the unlined area, progressing until the entire unlined landfill is transformed into a double-lined, environmentally safe landfill.

As part of the mining, processed soil and composted organics from the waste screening process are removed and stockpiled for use at the active landfill as daily cover. Excavated waste is moved to a special processing area where leachate can be collected. A series of berms and channels are used to prevent stormwater from entering the processing and excavation areas.

Excavated waste is placed on screens, where it is separated by size. Any very large objects are removed prior to placement on a screen. The waste is processed to reclaim recyclable materials, which are placed in covered containers for pick up. The waste that remains from the screening process is placed in an open area of the Class I landfill. Any waste that is unacceptable for Class I disposal is segregated for suitable disposal or treatment.

Odors are always a concern in landfill operations but the waste screening process at the Perdido Landfill has not resulted in any serious odor or dust problems. Occasional gusty winds require tight control over lightweight items, especially plastic recovered from the mining operations. "Litter, stormwater, and leachate control continues to be a key component of our landfill mining project," Johnson says.

The Perdido Landfill has multiple demonstrated benefits, including:

  • Upgrading the closed, unlined waste disposal area into a fully lined facility that complies with all Florida Department of Environmental Protection and U.S. Environmental Protection Agency solid waste management regulations.

  • Adding landfill airspace without impacting undeveloped property and avoiding the cost of siting new areas for landfill development.

  • Removing a source of leachate and landfill gas migration with opportunities for additional groundwater and soil remediation.

  • Reclaiming soils from excavated areas for onsite reuse.

  • Recycling materials previously discarded as solid waste and generating revenue from the sale of reclaimed metals and other items of value.

  • Removing hazardous waste materials during the excavation process.

  • Prolonging the life cycle of the site as a waste disposal option for Escambia County.

"The Perdido Landfill mining project offers the county the opportunity to reclaim up to 70 percent of excavated material from the waste stream while also addressing environmental concerns associated with the aging, closed and unlined portions of the landfill," Hixson says.

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Mark Roberts is a vice president with Jacksonville, Fla.-based HDR Engineering. He can be reached at mark.roberts@hdrinc.com. Dr. Kanishka Perera is a senior engineer with HDR. He can be reached at kanishka.perera@hdrinc.com. Sandy Prince Jennings is the bureau chief for Escambia County, Fla.'s Neighborhood and Community Services. She can be reached at sandra_jennings@co.escambia.fl.us.