A conventional septic system is comprised of three parts. The first is the conventional anaerobic (without oxygen) septic tank that receives the solid and liquid waste (called influent) from the house or business that's connected to it. The second is the disposal field that receives liquid waste (called effluent) from the septic tank. The effluent is distributed throughout the disposal field so that it can be absorbed into the surrounding soils. The third is the soil that purifies the effluent.
The way the three parts operate is simple and easy to understand. Within the residence or business, solid waste is put into water so gravity can move it away and into the septic tank. The conventional septic tank is designed to remove the majority of these solids from the water used to carry it to the septic tank. This is accomplished by retaining the influent for several days in the septic tank to allow separation of floatable solids (scum) and sinking solids (sludge) from one another. Intestinal bacteria from human bodily wastes survive within the conventional septic tank and there secrete enzymes that assist in breaking down the solid wastes into these two forms. By this method, 70% to 90% of the solids that were originally put into water are removed and contained within the septic tank. Over time, these solids build up within the septic tank and have to be removed by pumping and disposed of off site. Typically this means taking the contents or the septic tank (septage) to the local municipal wastewater plant for treatment (digestion) and disposal.
The clarified liquid (effluent) between the two forms of solids, then over flows out of the septic tank as new influent comes in, and is piped to a disposal field. The disposal field can be of many designs and constructions. Most typical is the leach line, though all disposal fields have the same function. This function is to expose the liquid effluent to aerobic soils (with oxygen) that can absorb it. Within the aerobic portions of the soil, there is an abundant community of aerobic microbes, including many species of bacteria that digest the remaining organic matter, purify and recycle the liquid back into the environment.
The part of a conventional septic system that fails is the disposal field.