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Any convenient width

FIGURE 3.32 Typical passive gas control synthetic membrane.

Any convenient width

FIGURE 3.32 Typical passive gas control synthetic membrane.

in Figure 3.31 may be reached in six months to five years depending on the landfill. Acidic conditions inhibit growth of methane-producing bacteria; alkaline conditions have the opposite effect. Methane production is quite variable depending on the amount and type of decomposable material in the landfill, moisture content, temperature, and resulting rate of microbial decomposition under anaerobic conditions.

Methane is odorless, has a heat value of about 500 Btu/ft3 compared to 1000 Btu for commercial gas, has a specific gravity less than air, and is nearly insoluble in water. The gases from landfills, after anaerobic conditions have been established, are quite variable, ranging from 50 to 60 percent methane and 40

FIGURE 3.33 Passive gas control using permeable trench. (Source: SCS Engineers, Inc., Procedural Guidance Manual for Sanitary Landfills: Volume I Landfill Leachate Monitoring and Control Systems and Volume II Landfill Gas Monitoring and Control Systems, California Waste Management Board, Sacramento, CA, April, 1989.)

FIGURE 3.33 Passive gas control using permeable trench. (Source: SCS Engineers, Inc., Procedural Guidance Manual for Sanitary Landfills: Volume I Landfill Leachate Monitoring and Control Systems and Volume II Landfill Gas Monitoring and Control Systems, California Waste Management Board, Sacramento, CA, April, 1989.)

to 50 percent carbon dioxide. Included are small amounts of nitrogen, oxygen, water, mercaptans (very odorous), and hydrocarbons. Hydrogen sulfide may also be released if large amounts of sulfates are in the landfill. Vinyl chloride, benzene, and other toxics in trace amounts may also be produced by the action of bacteria on chlorinated solvents deposited in the fill. The presence of oxygen and nitrogen with methane gas would indicate the entrance of air into the landfill due to methane being withdrawn too rapidly. If methane extraction is not controlled to reduce or eliminate the entrance of oxygen and nitrogen, the production of methane will slow down or stop.

In the early stages the landfill gases are primarily carbon dioxide, with some methane. The carbon dioxide is heavier than air and can dissolve in water to form carbonic acid, which is corrosive to minerals with which it comes into contact. Mercaptans, carbon dioxide, and water are usually extracted to upgrade the methane to pipeline quality. Removal of carbon dioxide may improve Btu content to 900 or 1,000 Btu/ft3. Methane as it comes from a landfill is often very corrosive. Deep landfills, 30 feet or deeper, and 30 acres or more in area with a good cover are better methane producers. Actually, gas will be generated as long as biodegradable material remains and is primarily dependent on precipitation, infiltration, and moisture content. Gas can be extracted using plastic tube wells in each cell with perforations or well screens toward the bottom connected to a controlled vacuum pump (see Figure 3.34) or a series of covered horizontal gravel trenches connected to a pipe collection system (see Figure 3.35). The extracted gas may be used for heating and generating electricity.

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