Closed Landfill Prepares For Takeoff

Ron Brookshire

With today's technology, building on and around landfills is becoming a reality. For example, the McClellan-Palomar Airport in San Diego's North County has been expanded onto a closed landfill site.

The 36-year-old airport and its 4,000-foot runway sit on an east-west oriented mesa, approximately 320 feet above mean sea level. The county built the nearby municipal solid waste (MSW) landfill in 1962. In 13 years of operation, it accumulated approximately 830,000 tons of MSW. The waste was predominantly municipal and agricultural. No gas extraction system was installed during operation. When the site was closed, it was believed the land could no longer be used.

As the airport grew and began encroaching on the closed landfill, the county assessed the site and discovered it was plagued by the typical problems: settlement and landfill gas (LFG) control. Because landfills can produce gas for approximately 30 years after closure, the county had to consider residents' safety as well as the structures' and the site's integrity.

During the airport expansion, permanent structures such as the runways, tower and boarding areas were built on solid ground outside the landfill boundaries. Numerous hangers, tie-down areas and other temporary structures were built on the settling landfill. Since these buildings were intended to be mo-bile, the settlement reportedly has not compromised their integrity. For maintenance, owners periodically re-level the hangers with blocks. The tie-down areas, parking and roadways are maintained by adding fill material and re-paving when necessary.

In 1994 the county installed a LFG control system. During design and installation, the project's na-ture presented unique concerns involving cost, functionality and safety. The primary issue was to install a system that would not interfere with the airport's operations and would also take into ac-count long-term maintenance and repair costs.

The gas collection system was permanently placed below grade. A blower and flare facility was sited at the airport's lowest point to prevent interference with air traffic or visibility.

The LFG collection system consists of 38 vertical collection wells and nine horizontal wells. A high-density polyethylene (HDPE) header system ties the wells together; eight sumps collect the landfill gas condensate. Once at the flare facility, the gas is destroyed and the condensate is pumped and treated. The whole system has been de-signed for minimal maintenance and is automated where possible.

The LFG wellheads, which control gas flow and gather data, are one-piece units that slip down into the well. Through a sliding compression fitting, the wellheads can be adjusted during landfill settlement. The wellhead features a built-in orifice plate flow meter and gas sampling ports with disconnect fittings; type II high-impact PVC gate valves control flow.

Wellheads are protected by fiber glass reinforced plastic (FRP) vaults with built-in meter reading lids for data gathering and wellhead control. The vault system can be extended just under the lid to accommodate settlement and to maintain the integrity of the vault and the surrounding roads or parking areas.

The header system is placed underground throughout the airport. The system's HDPE construction aims to provide maximum strength and flexibility during landfill settlement. To minimize future pipe adjustment, the system also features grade breaks or condensate collection points throughout.

The condensate sumps are fully automated, pneumatic systems that use the air system at the blower and flare station. The entire unit comprises a HDPE sump tank, pneumatic level control, a double-diaphragm pneumatic pump with variable discharge head and pressure, built-in air conditioners and the FRP vault with RPM lid.

The sump tank provides temporary storage for the liquid. When the liquid reaches a pre-set level, the pneumatic control automatically switches on the pump. The pump removes the liquid from the tank and transfers it to the condensate holding tanks at the blower and flare facility.

The condensate sumps reportedly can be used at any location or elevation on the site; the discharge is regulated by the air pressure and volume. The pumps are protected by the vault, which also helps contain spills. Like the wellhead vault, the condensate pump vaults can be extended.

The landfill gas control system was completed in 1995. Today, the McClellan-Palomar Airport reportedly can handle up to 65,000 commuters a year. It has more than 400 private planes.

Preparing landfills after closure for future use is a modern-day necessity. With proper safety measures, we may be able to look at closed landfills as land ready for development.