Some solids float to the top and others settle to the bottom. Natural bacteria in wastewater break down the organic solids. Solids not broken down by bacteria are stored in the septic tank until the tank is pumped. They should be pumped out of the tank through the manhole every one to two years. Septic tanks do not destroy disease-causing pathogens.
Septic tanks prepare the liquid for final treatment by the soil.
Wastewater from septic tanks is distributed into the soil by a drainfield or mound system. A biomat forms where sewage meets the soil. Once wastewater is through the biomat, harmful pathogens are destroyed. When owners don't clean septic tanks often enough, too many solids clog the biomat. Liquids can't pass through.