A septic tank system is a method of wastewater treatment that utilises only anaerobic bacteria (bacteria not requiring oxygen to live) in the treatment or septic tank. The tank has no aeration and treats the wastewater to about 20% of the amount needed to render it harmless to the environment.
Septic tank systems can be vulnerable to poor soil conditions (clay, sand etc) high water table, clogging of the drain lines, damage to the drain lines by vehicles, animals, trees etc and neglect by failure to regularly (at least every 3 years but preferably every year) pumping out the sludge and solids from the tank.
Wastewater enters the septic tank systems from the inlet and flows below the scum layer that develops on the surface. The wastewater (liquid with entrained solids) mixes with the liquid contents of the tank, the more dense particles settle to the bottom and the lighter particles float to the surface to become part of the surface scum seal. This scum on the surface is the most important part of the operation, as it excludes air from the liquid, allowing the anaerobic bacteria to thrive. An effective septic tank system will generally only require pump-out every three to seven years, depending upon vigilance of what goes into the system