Bacteria that do not use oxygen from the air grow in the tank. These bacteria are called anaerobic and the by-products of their activity are methane and hydrogen sulfide gas, plus other substances having odor. But the septic tank might also be described as a settling tank where the sewage solids are stored while the bacteria decompose them and reduce their volume. The volume is never reduced to zero, however, so a residue remains. It is this residue that must be cleaned out of the tank when the volume becomes too great.
Sewage flows to the septic tank through the house sewer.
A grade of from one to two inches in eight feet is used. This is a slope of one to two percent. A one percent slope is a one-foot drop in a hundred feet of pipe. The house sewer should not have any low spots where liquid can remain. In freezing climates these low spots are the places where sewer pipe freezing happens.