APPENDIX A BASICS OF LANDFILL GAS Basics of Landfill Gas (Methane, Carbon Dioxide, Hydrogen Sulfide and Sulfides) Landfill gas is produced through bacterial decomposition, volatilization and chemical reactions. Most landfill gas is produced by bacterial decomposition that occurs when organic waste solids, food (i.e. meats, vegetables), garden waste (i.e. leaf and yardwaste), wood and paper products, are broken down by bacteria naturally present in the waste and in soils. Volatilization generates landfill gas when certain wastes change from a liquid or solid into a vapor. Chemical reactions occur when different waste materials are mixed together during disposal operations. Additionally, moisture plays a large roll in the speed of decomposition. Generally, the more moisture, the more landfill gas is generated, both during the aerobic and anaerobic conditions. Landfill Gas Production and Composition: In general, during anaerobic conditions, the composition of landfill gas is approximately 50 percent methane and 50 percent carbon dioxide with trace amounts (<1 percent) of nitrogen, oxygen, hydrogen sulfide, hydrogen, and nonmethane organic compounds (NMOCs). The more organic waste and moisture present in a landfill, the more landfill gas is produced by the bacteria during decomposition. The more chemicals disposed in a landfill, the more likely volatile organic compounds and other gasses will be produced. The Four Phases of Bacterial Decomposition: “Bacteria decompose landfill waste in four phases. The composition of the gas produced changes with each of the four phases of decomposition. Landfills often accept waste over a 20-to 30-year period, so waste in a landfill may be undergoing several phases of decomposition at once. This means that waste in one area might be in a different phase of decomposition than more recently buried waste in another area. Phase I: During the first phase of decomposition, aerobic bacteria-bacteria that live only in the presence of oxygen-consume oxygen while breaking down the long molecular chains of complex carbohydrates, proteins, and lipids that comprise organic waste. The primary byproduct of this process is carbon dioxide. Nitrogen content is high at the beginning of this phase, but declines as the landfill moves through the four phases. Phase I continues until available oxygen is depleted. Phase I decomposition could last for days or months, depending on how much oxygen is present when the waste is disposed of in the landfill. Oxygen levels will vary according to factors such as how loose or compressed the waste was when it was buried. Phase II: Phase II decomposition starts after the oxygen in the landfill has been used up. Using an anaerobic process (a process that does not require oxygen), bacteria convert compounds created by aerobic bacteria into acetic, lactic, and formic acids and alcohols such as methanol and ethanol. The landfill becomes highly acidic. As the acids mix with the moisture present in the landfill, they cause certain nutrients to dissolve, making nitrogen and phosphorus available to the increasingly diverse species of bacteria in the landfill. The gaseous byproducts of these processes are carbon dioxide and hydrogen. If the landfill is disturbed or if oxygen is somehow introduced into the landfill, microbial processes will return to Phase I. 1 Phase III: Phase III decomposition starts when certain kinds of anaerobic bacteria consume organic acids produced in the Phase II and form acetate, an organic acid. This process causes the landfill to become a more neutral environment in which methane-producing bacteria begin to establish themselves. Methane-and acid-producing bacteria have a symbiotic, or mutual beneficial, relationship. Acid-producing bacteria create compounds for the methanogenic bacteria to consume. Methanogenic bacteria consume the carbon dioxide and acetate, too much of which would be toxic to the acid-producing bacteria. Phase IV: Phase IV decomposition begins when both the composition and production rates of landfill gas remain relatively constant. Phase IV landfill gas usually contains approximately 45 percent to 60 percent methane by volume, 40 percent to 60 percent carbon dioxide and 2 percent to 9 percent other gasses, such as sulfide. Gas is produced at a stable rate in Phase IV, typically for about 20 years; however, gas will continue to be emitted for 50 or more years after the waste is placed in the landfill (Crawford and Smith 1985). Gas production might last longer, for example, if greater amounts organics are present in the waste, such as at a landfill receiving higher than average amounts of domestic animal waste" (ATSDR, 2001). Due to the fact that refuse is placed in a landfill at different times and consists of different types of solid waste at different moisture content, all four phases of degradation may be occurring simultaneously within the landfill. Factors Affecting Landfill Gas Production: Gas production rates depend on the rate of decomposition, which in turn is affected by moisture content of the waste, temperature, soil cover permeability, amount of precipitation, composition of the waste, refuse particle size, compaction, and landfilling practices. Moisture is essential for bacterial survival. Ideal moisture content for decomposition is one that approaches saturation. Biological reactions can be retarded if moisture content drops below 40 percent and essentially stops when moisture content is below 20 percent. Thus, in very dry climates, methanogenic (methane producing) decomposition will have a very small possibility of occurring. Methane will be produced only when oxygen (O2) is no longer present in the landfill. Capping of a landfill with a low permeability layer reduces the available moisture, thus retarding landfill gas production. Smaller particles decompose more quickly due to the high surface area to mass ratio. Thus, shredded waste produces gas more quickly than non-shredded waste. For additional information on landfill gas production and composition refer to Chapter 7 of MassDEP’s “Landfill Technical Guidance Manual” (http://www.mass.gov/dep/recycle/laws/policies.htm). Figure 1: Production Phases of Typical Landfill Gas 2 Hydrogen Sulfide and Sulfides: Sulfides are naturally occurring gasses that often give a landfill gas mixture its rotten egg smell. Sulfides can cause unpleasant odors even at very low concentrations. Hydrogen sulfide is a colorless, flammable gas and is one of the most common sulfides responsible for landfill odors. Some people can smell hydrogen sulfide (individual’s odor threshold) at concentrations as low as 0.5 parts per billion (ppb). However, the odor threshold can vary significantly among individuals based on the olfactory sensitivity of 3 the person. For many compounds, including hydrogen sulfide, there is a wide variability in published odor thresholds (refer to Table 1). Odors alone cannot be relied upon as providing an early warning for elevated concentrations of hydrogen sulfide. "At concentrations around 100 ppm,” (parts per million) “no odor is detected due to a loss of olfactory sensation, resulting in loss of warning properties at lethal levels." (Integrated Risk Information System (IRIS)). Hydrogen sulfide is more dense than air, and therefore, more likely to pool at lower elevations under still conditions, depending upon topography. Hydrogen sulfide is found naturally in the environment and is also produced from man-made processes. Hydrogen sulfide occurs naturally in crude petroleum, natural gas, volcanic gasses, hot springs, and tidal flats and can also be produced from bacterial breakdown of organic matter. Hydrogen sulfide is produced by human and animal waste, and landfilling of solid waste, and can result from industrial activities such as food processing, coke ovens, paper mills, tanneries and petroleum refineries, to name a few. Dimethyl sulfide and mercaptans are sulfides that can also cause odors but typically are not emitted from a landfill at the high concentrations or at production rates associated with hydrogen sulfide. The concentration of hydrogen sulfide detected in landfill gas samples at solid waste landfills that receive construction and demolition (C&D) waste is usually much higher than at landfills that do not accept C&D. The higher concentrations of hydrogen sulfide are believed to be associated with the gypsum board component (e.g. wallboard) present in C&D material. The combination of gypsum, organic material, moisture and anaerobic conditions present in C&D landfills is believed to provide a favorable mixture and environment for bacteria to produce hydrogen sulfide gas. Concentrations of hydrogen sulfide detected in raw landfill gas samples, collected from within a landfill waste mass, have ranged from 50,000 ppb to 15,000,000 ppb (50 ppm - 15,000 ppm) for those landfills that accepted C&D solid waste. Landfills which do not accept C&D typically have much lower concentrations of hydrogen sulfide in the raw gas, usually less than 100,000 ppb (100 ppm). Hydrogen sulfide is generated as a result of a series of reactions that biologically reduce sulfate leached from gypsum contained in the C&D. In general, wallboard consists of the gypsum core (CaSO4 x 2H2O) with facing and backing consisting of paper. The microorganisms responsible for generating hydrogen sulfide include sulfate-reducing bacteria and sulfur-reducing bacteria (Florida, 2002). Sulfate reducing bacteria require the following to produce hydrogen sulfide: a sulfate source (gypsum), a carbon source