Conventional rock and Pipe System

Additional gravel is poured around and over the pipe, then the gravel is covered by a semi permeable barrier such as geotextile fabric so that backfill soil doesn't filter between the rocks and reduce the field's ability to absorb water. While some treatment of waste occurs in the septic tanks as bacteria within the tank operate on the waste, most of the treatment occurs as wastewater discharged from the tank enters the drain field and is filtered through the gravel and the soil below. Over time, bacteria and other organisms in the soil consume any organic material in the wastewater. These organisms multiply and form a layer called a biomat that sits on the soil layer. When sufficient oxygen is available, worms and other parasites feed on the bacteria as well as the material in the wastewater. When the drain field is in balance, these organisms keep the biomat from becoming so thick that it won't allow passage of wastewater to the soil below.

Generally there are two types of conventional septic systems: those that use gravel in the drain field and those that use some form of chamber system. As its name suggests the older style graveled system contains a layer of gravel in the drain field. During construction, a drain field ditch 1 to 3 feet below ground level is constructed. Its length is determined by the anticipated flow of effluent into the system from the home or rural office, as well as the soil's ability to absorb water. Washed gravel is poured into the bottom of the ditch, then over and around a perforated plastic pipe.