process, with conversion of the complex organic material to organic acids and other intermediate products as described in Phase Ill. In Phase Il, the pH of the leachate, if any is formed, starts to drop due to the presence of organic acids and the effect of the elevated concentrations of COz within the landfill Phase Ill- acid phase. In Phase Ill, the acid phase, the microbial activity initiated in Phase ll accelerates with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three-step process involves the enzyme-mediated transformation(hydrolysis)of higher-molecular mass compounds (e. g, lipids, polysaccharides, proteins, and nucleic acids) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process(acidogenesis) involves the microbial conversion of the compounds resulting from the first step into lower-molecular mass intermediate compounds as typified by acetic acid(CH3 COOH)and small concentrations of fulvic and other more Typical concentrations of trace compounds found complex organic acids. in landfil in landfill gas at 66 California sw landfills Carbon dioxide(CO2) is the principal gas compound Concentration, ppbve generated during Phase Median IIL Smaller amounts of hydrogen gas(H2) willChiorobenzene also be produced. The 1.1 chloroform 2,801 microorganIsms 1.15 nvolved in this Diethylene ethene conversio collectively 2,3- Dichloropr。pan nonmethanogenic consist of facultative and obligate anaerobic Methyl ethyl ketone These microorganisms Trichloroethylene bacteria ,1,2 14.500 are often identified 8,125 engineering 11. 2.2-Tetrachloroethane 6 literature as acidogens or acid formers inyl acetate The pH of the leachate, Xylenes 38.000 if formed. will often Ac od fron Rof. 5 drop to a value of 5 o pbv- parts per billion by volume. lower because of the presence of the organic acids and the elevated concentrations of COz within the landfill. The biochemical oxygen demand(BODs), the chemical oxygen demand (COD), and th conductivity of the leachate will increase significantly during Phase Ill due to the dissolution of the organic acids in the leachate. Also, because of the low pH values in the leachate, a number of inorganic constituents, principally heavy metals, will be solubilized during Phase Ill. Many essential nutrients are also removed in the leachate in Phase Ill. If leachate is not recycled, the essential nutrients will be lost from the system. It is important to note that if leachate is not formed, the conversion products produced during Phase Ill will remain within the landfill as sorbed constituents and in the water held by the waste as defined by the field capacity Phase IV-methane fermentation phase. In Phase IV, the methane fermentation phase, a second group of microorganisms, which convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to CHa and CO, becomes more predominant. In some cases, these organisms will begin to develop toward the end of Phase Ill. The microorganisms responsible for this conversion arc strict anaerobes and are called methanogenic. Collectively, they are identified in the literature as methanogens or methane formers In Phase Iv, both methane and acid formation proceed simultaneously, although the rate of acid formation is considerably reduced Because the acids and the hydrogen gas produced by the acid formers have been convened to Cha and CO2 in Phase IV, the ph within the landfill will rise to more neutral values in the range of 6.8 to 8. In turn, the ph of the leachate, if formed, will rise, and the concentration of BODs and COD and the conductivity value of the leachate will be reduced. With higher pH values, fewer inorganic constituents
6 process, with conversion of the complex organic material to organic acids and other intermediate products as described in Phase III. In Phase II, the pH of the leachate, if any is formed, starts to drop due to the presence of organic acids and the effect of the elevated concentrations of CO2 within the landfill . Phase Ill－acid phase. In Phase III, the acid phase, the microbial activity initiated in Phase II accelerates with the production of significant amounts of organic acids and lesser amounts of hydrogen gas. The first step in the three-step process involves the enzyme-mediated transformation (hydrolysis) of higher-molecular mass compounds (e.g., lipids, polysaccharides, proteins, and nucleic acids) into compounds suitable for use by microorganisms as a source of energy and cell carbon. The second step in the process (acidogenesis) involves the microbial conversion of the compounds resulting from the first step into lower-molecular mass intermediate compounds as typified by acetic acid (CH3COOH) and small concentrations of fulvic and other more complex organic acids. Carbon dioxide (CO2) is the principal gas generated during Phase III. Smaller amounts of hydrogen gas (H2) will also be produced. The microorganisms involved in this conversion, described collectively as nonmethanogenic, consist of facultative and obligate anaerobic bacteria. These microorganisms are often identified in the engineering literature as acidogens or acid formers. The pH of the leachate, if formed, will often drop to a value of 5 or lower because of the presence of the organic acids and the elevated concentrations of CO2 within the landfill. The biochemical oxygen demand (BOD5), the chemical oxygen demand (COD), and the conductivity of the leachate will increase significantly during Phase III due to the dissolution of the organic acids in the leachate. Also, because of the low pH values in the leachate, a number of inorganic constituents, principally heavy metals, will be solubilized during Phase III. Many essential nutrients are also removed in the leachate in Phase III. If leachate is not recycled, the essential nutrients will be lost from the system. It is important to note that if leachate is not formed, the conversion products produced during Phase III will remain within the landfill as sorbed constituents and in the water held by the waste as defined by the field capacity. Phase IV—methane fermentation phase. In Phase IV, the methane fermentation phase, a second group of microorganisms, which convert the acetic acid and hydrogen gas formed by the acid formers in the acid phase to CH4 and CO2, becomes more predominant. In some cases, these organisms will begin to develop toward the end of Phase III. The microorganisms responsible for this conversion arc strict anaerobes and are called methanogenic. Collectively, they are identified in the literature as methanogens or methane formers. In Phase IV, both methane and acid formation proceed simultaneously, although the rate of acid formation is considerably reduced. Because the acids and the hydrogen gas produced by the acid formers have been convened to CH4 and CO2 in Phase IV, the pH within the landfill will rise to more neutral values in the range of 6.8 to 8. In turn, the pH of the leachate, if formed, will rise, and the concentration of BOD5 and COD and the conductivity value of the leachate will be reduced. With higher pH values, fewer inorganic constituents 15-1
can remain in solution; as a result, the concentration of heavy metals present in the leachate will also be reduced Phase V-maturation phase. Phase V, the maturation phase, occurs after the readily available biodegradable organic material has been converted to CH4 and CO2 in Phase IV. As moisture continues to migrate through the waste, portions of the biodegradable material that were previously unavailable, will be converted. The rate of landfill gas generation diminishes significantly in Phase V, because most of the available nutrients have been removed with the leachate during the previous phases and the substrates that remain in the landfill are slowly biodegradable. The principal landfill gases evolved in Phase V are CH4 and CO2 Depending on the landfill closure measures, small amounts of nitrogen and oxygen may also be found in the landfill gas. During maturation phase, the leachate will often contain humic and fulvic acids, which are difficult to process further biologically Duration of phases. The duration of the individual phases in the production of landfill gas will var depending on the distribution of the organic components in landfill, the availability of nutrients, the moisture content of waste, moisture routing through the fill, and the degree of initial compaction. For balpanple, if several loads of brush are compacted together the carbon/nitrogen ratio and the nutrient balance may not be favorable for the production of landfill gas. Likewise, the generation of landfill gas will be retarded if sufficient moisture is not available. Increasing the density of the material placed in the landfill will decrease the possibility of moisture reaching all parts of the waste and, thus, reduce the rate of bioconversion and gas production Variation in Gas Production with Time. Under normal conditions, the rate of decomposition, as measured by gas production, reaches a peak within the first two years and then slowly tapers off, continuing in many cases for periods up to 25 years or more. If moisture is not added to the wastes in a well-compacted landfill, it is not uncommon to find materials in their original form years after they were The variation in the rate of gas production from the anaerobic decomposition of the rapidly(five years or less-some highly biodegradable wastes are decomposed within days of being placed in a landfill) and slowly (5 to 50 years) biodegradable organic materials in MSW can be modeled. Gas production model in which the peak rate of gas production occurs one and five years, respectively, after gas production starts. Gas production is assumed to start at the end of the first full year of landfill operation The area under the triangle is equal to one half the base times the altitude, therefore, the total amount of gas produced from the waste placed the first year of operation is equal to Total gas produced, ft/lb 1/2(base, yr)x(altitude, peak rate of gas production, ft/lb. yr) (15-1) Using a triangular gas production model, the total rate of gas production from a landfill in which wastes were placed for a period of five years is obtained graphically by summing the gas produced from the rapidly and slowly biodegradable portions of the MSw deposited each year. The total amount of gas roduced corresponds to the area under the rate curve As noted previously, in many landfills the available moisture is insufficient to allow for the complete conversion of the biodegradable organic constituents in the MS w. The optimum moisture content for the conversion of the biodegradable organic matter in MSw is on the order of 50 to 60 percent. Also in many landfills, the moisture that is present is not uniformly distributed. When the moisture content of the landfill is limited, the gas production curve is more flattened out and is extended over a greater period of Sources of Trace Gases. Trace constituents in landfill gases have two basic sources. They may be brought to the landfill with the incoming waste or they may be produced by biotic and abiotic reactions ccurring within the landfill. Of the trace compounds found in landfill gases, many are mixed into the incoming waste in liquid form, but tend to volatilize. The tendency to volatilize can be shown to be approximately proportional to the vapor pressure of the liquid, and inversely proportional to the surface area of a sphere of the volatile liquid within the landfill. In newer landfills where the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been reduced Complex biochemical pathways can exist for the production or consumption of any of the trace constituents. For example, vinyl chloride is a byproduct of the degradation of di- and trichloroethene Because of the organic nature of these gases they can be sorbed by waste constituents in the landfill. At present, very little can be stated definitively about the rates of biochemical transformation of the trace
7 can remain in solution; as a result, the concentration of heavy metals present in the leachate will also be reduced. Phase V—maturation phase. Phase V, the maturation phase, occurs after the readily available biodegradable organic material has been converted to CH4 and CO2 in Phase IV. As moisture continues to migrate through the waste, portions of the biodegradable material that were previously unavailable, will be converted. The rate of landfill gas generation diminishes significantly in Phase V, because most of the available nutrients have been removed with the leachate during the previous phases and the substrates that remain in the landfill are slowly biodegradable. The principal landfill gases evolved in Phase V are CH4 and CO2 Depending on the landfill closure measures, small amounts of nitrogen and oxygen may also be found in the landfill gas. During maturation phase, the leachate will often contain humic and fulvic acids, which are difficult to process further biologically. Duration of phases. The duration of the individual phases in the production of landfill gas will vary depending on the distribution of the organic components in landfill, the availability of nutrients, the moisture content of waste, moisture routing through the fill, and the degree of initial compaction. For example, if several loads of brush are compacted together the carbon/nitrogen ratio and the nutrient balance may not be favorable for the production of landfill gas. Likewise, the generation of landfill gas will be retarded if sufficient moisture is not available. Increasing the density of the material placed in the landfill will decrease the possibility of moisture reaching all parts of the waste and, thus, reduce the rate of bioconversion and gas production. Variation in Gas Production with Time. Under normal conditions, the rate of decomposition, as measured by gas production, reaches a peak within the first two years and then slowly tapers off, continuing in many cases for periods up to 25 years or more. If moisture is not added to the wastes in a well-compacted landfill, it is not uncommon to find materials in their original form years after they were buried. The variation in the rate of gas production from the anaerobic decomposition of the rapidly (five years or less－some highly biodegradable wastes are decomposed within days of being placed in a landfill) and slowly (5 to 50 years) biodegradable organic materials in MSW can be modeled. Gas production model in which the peak rate of gas production occurs one and five years, respectively, after gas production starts. Gas production is assumed to start at the end of the first full year of landfill operation. The area under the triangle is equal to one half the base times the altitude, therefore, the total amount of gas produced from the waste placed the first year of operation is equal to Total gas produced, ft3 /lb = 1/2 (base, yr) × (altitude, peak rate of gas production, ft3 /1b • yr) (15-1) Using a triangular gas production model, the total rate of gas production from a landfill in which wastes were placed for a period of five years is obtained graphically by summing the gas produced from the rapidly and slowly biodegradable portions of the MSW deposited each year. The total amount of gas produced corresponds to the area under the rate curve. As noted previously, in many landfills the available moisture is insufficient to allow for the complete conversion of the biodegradable organic constituents in the MSW. The optimum moisture content for the conversion of the biodegradable organic matter in MSW is on the order of 50 to 60 percent. Also in many landfills, the moisture that is present is not uniformly distributed. When the moisture content of the landfill is limited, the gas production curve is more flattened out and is extended over a greater period of time. Sources of Trace Gases. Trace constituents in landfill gases have two basic sources. They may be brought to the landfill with the incoming waste or they may be produced by biotic and abiotic reactions occurring within the landfill. Of the trace compounds found in landfill gases, many are mixed into the incoming waste in liquid form, but tend to volatilize. The tendency to volatilize can be shown to be approximately proportional to the vapor pressure of the liquid, and inversely proportional to the surface area of a sphere of the volatile liquid within the landfill. In newer landfills where the disposal of hazardous waste has been banned, the concentrations of VOCs in the landfill gas have been reduced significantly. Complex biochemical pathways can exist for the production or consumption of any of the trace constituents. For example, vinyl chloride is a byproduct of the degradation of di- and trichloroethene. Because of the organic nature of these gases they can be sorbed by waste constituents in the landfill. At present, very little can be stated definitively about the rates of biochemical transformation of the trace