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 constituents6 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