CHAPTER VII - LANDFILLS AND MUNICIPAL
WASTEWATER TREATMENT PLANTS
Landfill gas is produced by microorganisms in the landfill under anaerobic (in the absence of oxygen) conditions. Anaerobic decomposition of materials such as cellulose, fats, and proteins is performed by a type of bacteria called acid formers that convert these materials into fatty acids. Fatty acids are consumed by methanogenic bacteria, which convert fatty acid to methane and carbon dioxide. Most of the gas produced within landfills consists of methane and carbon dioxide. In addition to these, trace quantities of other potentially toxic gases are formed. Manmade materials such as chlorinated solvents, household hazardous wastes, cleaning products, oil and solvent based paints, pesticides, and adhesives are converted into gases by bacteria. For example, bacterial decomposition of chlorinated organic solvents produces vinyl chloride.
Varying quantities and types of landfill gases are generated depending on the age of the landfill, how wastes are buried and covered, and the specific components of the solid waste stream. Landfills that receive less commercial and industrial wastes produce less toxic gas. Despite these differences, landfill gas generally has the following composition: 50 percent methane, 40 percent carbon dioxide, 9 percent nitrogen, and 1 percent nonmethane organic compounds (NMOCs). Although they are present in very low concentrations, NMOCs are of concern since they are considered potentially toxic. NMOC concentrations in landfill gas have been estimated to range from 300 to 1,000 parts per million (ppm). NMOCs are primarily composed of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). VOC levels may range from 2 to 85 percent of the total NMOC concentration depending on waste characteristics. The remaining compounds within the NMOC portion of the gas are HAPs. HAPs identified in highest concentrations in landfill gas are vinyl chloride, ethyl benzene, zylene, and toluene.
Collection of landfill gas is by active and passive collection methods. Active systems use vertical wells or trenches to collect the gas. Blowers or compressors are used to move gas from the landfill to treatment devices.
Passive landfill gas collection relies on natural pressure to capture landfill gas and move it to treatment devices.
Combustion of landfill gas is done with flares and afterburners. For example, in California landfills are required to install enclosed flares. A fire-resistant wall extends above the top of the flame. Air is admitted in a controlled manner to the bottom of the flare. Monitoring and sampling can be performed above the flame. Sampling results can determine if 98 percent of NMOCs are removed from gas in enclosed flare systems. In California, sampling results showed that 98 percent of NMOCs were destroyed in the enclosed flare systems.
Ninety-eight percent of the NMOC present in the landfill gas can be destroyed by adsorption techniques. Adsorption involves intermolecular forces that attract and hold gas molecules to a solid surface. A common adsorption system is activated carbon, which acts as the adsorbent. For example, when landfill gas is filtered through activated carbon, the carbon dioxide and NMOCs are adsorbed into the carbon medium.
Gas turbines, internal combustion engines, and steam turbines are use to produce energy from landfill gases. In order to economically utilize landfill gas for energy purposes, a landfill generally needs two million tons of solid waste buried 35 feet deep over an area of 35 acres. Gas turbines are used at about 20 landfills in the U.S. to control landfill gases and produce electricity.
The new source performance standards (NSPS) and emission guidelines (EG) were originally proposed as part of Section 111 of the CAA and were published in draft form in the Federal Register on May 30, 1991. They represent the first comprehensive federal effort to regulate landfill gas emissions. When the EPA released the rule in 1991, industry opposition caused the agency to reconsider the original proposal. The U.S. EPA considered public and private comments throughout the rule-making process. The final rule, promulgated in 1996, achieves the purpose of safely managing landfill gas and industry's need for an enforceable regulation.
The NSPS are applicable to new landfill construction, modification, or acceptance of waste on or after May 30, 1991. The EG are applicable to existing landfills (i.e., those that accepted waste since November 8, 1987). This is considered the date after which landfill operations became documented. Existing landfills have three to five years to comply with the regulations.
New or existing landfills with a design capacity equal to or greater than 2. million cubic meters must file a design capacity report and are required to calculate their annual emission of NMOCs. If NMOC emissions are equal to or greater than 55 tons per year, owners are required to install gas collection and control equipment at the facility that is capable of reducing NMOCs by 98 percent (by weight).