Gases that contribute to air pollution and human health risks - including methane gas, carbon dioxide and other volatile compounds - can be emitted from myriad sources, including municipal solid waste (MSW) landfills.
Landfill gas collection and management systems have become critical to landfill management. To protect air quality and public health, active gas collection systems with flares are replacing passive systems.
MSW landfills that have received wastes since November, 1987 must comply with stringent regulations as part of the Environmental Protection Agency's (EPA) proposed Clean Air Act standards. The regulations apply to landfill emissions that contribute to ambient ozone problems, including smog and high levels of toxics and carcinogens in the air.
A landfill naturally releases gas through its cover system, but the required impermeable cover system traps the gases inside. A traditional, passive landfill gas venting system - the type in place at approximately 10 percent of U.S. landfills - is installed to control gas migration. It relies on a grid of vertical pipes to collect and channel the gases to vents, which release landfill gases into the atmosphere. Since these gases are comparable to the density of air and are released at low velocities and elevations, the landfill gases do not always disperse effectively, causing the odors to linger.
The mixture of methane and carbon dioxide is not odorous itself. But when these gases migrate through the decaying garbage, the gases pick up traces of complex, highly odorous compounds.
Without proper ventilation from a landfill gas collection system, methane, a combustible gas, can migrate from unlined landfills and infiltrate nearby buildings and subsurface structures. A methane accumulation can cause a potential explosion or asphyxiation.
Less than 5 percent of the country's landfills currently have active gas withdrawal systems with flaring. The active aspect of these systems involves drawing gases from the landfill using blowers, extracting the gases into the collection pipes and channeling the gases to flares. Gases are ignited and mixed with oxygen-rich gas at the flares. This results in a nearly complete combustion of landfill gases and destruction of odors.
Due to advanced technology and strict monitoring schedules, active gas collection systems with flares require higher capital investments and incur greater operating costs than passive systems. To help reconcile the needs of the public and the financial constraints of landfill owners, engineers are incorporating energy recovery into the design of active flaring systems. By installing turbines, landfill gas can be converted to steam or electric power to help communities recover costs and, in some cases, actually make a profit. Gas can become a commodity for owners of larger landfills when this method is used.