Kelle2, methane gas is a product of digestion of specific anaerobic bacteria in a septic tank. Other gasses produced are Carbon dioxide, Hydrogen sulfide, and Mercaptans. There are other gasses produced depending upon the constituents of the organic matter being digested, and the various species of microbes present in the tank.
Typically, environmental conditions which exist in a septic tank are not conducive to efficient methane production, mostly due to the relatively low temperatures of the sewage in the zone where the methane producing bacteria reside in the tank. The optimum temperature for efficient methane production is in a range of 90 to 95 degrees Fahrenheit. As a result, Carbon dioxide is the predominant gas present in sewer gas produced by microbial digestion in a septic tank.