Gas Production

Generation of digester gas is a direct result of the destruction of volatile solids. Specific gas production for wastewater sludge generally ranges from 0.8 to 1.1 m3/kg (13 to 18 ft3/lb) of volatile solids destroyed. Specific gas production values will be closer to the high end of this range if the sludge contains a higher percentage of fats and grease as long as adequate SRT is provided for these slow-metabolizing materials. A healthy digestion process produces a gas with about 65 to 75% methane, 30 to 35% carbon dioxide, and very low levels of nitrogen, hydrogen, and hydrogen sulfide. The heat value of digester gas is approximately 24 MJ/m3 (640 Btu/ft3), compared to 38 MJ/m3 (1010 Btu/ ft3) for methane.

Figure 5.9 Volatile solids reduction as function of solids retention time.

Solids Retention Time, days

Figure 5.9 Volatile solids reduction as function of solids retention time.

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