Conventional septic systems have traditionally been the most commonly used technology for treating wastewater. These systems use gravity to distribute waste water in the soil. These systems are generally cheaper to install and require the least amount of maintenance.

A conventional septic system consists of a series of tanks and a distribution field which uses soil to filter wastewater . The septic tanks are used to settle out solids and partially treat the waste water before it is distributed into the soil. There are several standard subsurface distribution options. They generally consist of plastic chambers, graven filled trenches, or plastic pipe installed underground to hold waste water that has left the tanks until it can seep into the surrounding soil.

The soil does most of the treatment of waste water when using a conventional gravity flow septic system. Once waste water is absorbed into the soil, microbes in the soil break down solids and kill pathogens and bacteria in the waste water.