Bacteria are what enable wastewater to be treated and safely disposed off. This applies to home septic systems as well as municipal sewage treatment plants.
The treatment difference between the two systems is the amounts of bacteria involved. A home septic system uses the same three types of bacteria aerobic, facultative, and obligate anaerobic. Aerobic bacteria require oxygen induced compressed air and are more aggressive than anaerobic bacteria, which live off non-oxygen containing food. Typically there are very few aerobic bacteria in your septic tank. A septic tank itself does not have a pure oxygen supply. Facultative bacteria also need oxygen. It can use either molecular (dissolved) oxygen or oxygen obtained from food material or sulfates or nitrates ions from wastewater and other sources. This is what produces the “rotten egg odor, ” also known as hydrogen sulfide gas (H2S).
The problem with anaerobic bacteria is that they leave the septic tank and go into the leach field bed of the septic system. The anaerobic bacteria continue to feed on the non-oxygen containing unfiltered foods in the wastewater. In the process, they eventually build up a thick layer of smelly, slimy bacteria that clogs the leach field beds. Wastewater then accumulates above the bacteria layer. This causes odorous ponds and septic tank backups.