Waste material deposited in landfills contains organic material. If sufficient moisture is available (more than 20%), indigenous microbes degrade this material. While sufficient external electron acceptors are available, degradation is achieved through respiratory processes that produce primarily carbon dioxide and water. Invariably, oxygen is available at first, entrained in the waste during collection, transport, and unloading. Usually, microbial activity consumes the available oxygen within a short period of time, i.e., days or weeks. If alternative electron acceptors are available to substitute for oxygen, respiratory processes continue (Suflita et al. 1992). The most common alternative electron acceptor in landfills is sulfate, found in gypsum dry wall debris.
Alternative electron acceptors are not available in most of the volume of a typical landfill; subsequently, fermentative processes predominate, ultimately producing landfill gas that is primarily carbon dioxide and methane (see Table 11.13.4). Observed gas yields are less than the the oretical maximum based on stoichiometry and are generally in the range of 4000 cu ft per tn of waste (O'Leary and Walsh 1991b).
Landfill gas must be removed from landfills. The final cover, used to keep out water and support vegetation for erosion control, can trap landfill gases. A build-up of gas in a landfill can rupture the final cover. In addition, the vegetative cover can be killed if the pore space in the final cover topsoil becomes saturated with landfill gases. Finally, methane is explosive if present in sufficient concentration, above 5%. Methane traveling through the landfill or surrounding soils can collect at explosive concentrations in nearby buildings. Migration distances greater than 1500 ft have been observed (O'Leary and Walsh 1991a). For these reasons, landfill gases must be vented or collected.
Gas control can be accomplished in a passive or active manner. Passive landfill gas control relies on natural pressure and convection to vent gas to the atmosphere or flares. Passive systems consist of gas venting trenches or wells, either in the landfill or around it. However, passive systems are not always successful because the pressure generated by gas production in the landfill may not be enough to push landfill gas out.
Active gas control removes landfill gases by applying a vacuum to the landfill. In other words, the landfill gases are pumped out. However, overpumping draws air into the landfill, slowing the production of more methane. After expensive landfill gas extraction equipment is installed, slowing methane production is not desirable. If migration control is the primary purpose of active gas control, recovery wells can be placed near the perimeter of the landfill. However, landfill gas can be an energy source, in which case vertical or horizontal recovery wells are typically placed in the landfill. Landfill gas with 50% methane has a heating value of 505 Btu/standard cu ft, about half that of natural gas (O'Leary and Walsh 1991a).
Collected landfill gas can be vented, burned without energy recovery, or directed to an energy recovery system.
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