As on-site waste treatment alternatives to municipal sewage systems, residential septic systems don't come in one-size-fits-all packages. Because the lay of the land and the type of soil varies from site to site, septic systems must be designed to include these variables. When you add a homeowner's preference of additional options to the mix, there's even more factors to consider.
A Basic Setup
The simplest and most common residential septic system includes a single-stage tank that holds solid and liquid waste. Buried in the ground, this tank receives piped-in household sewage waste. The solid waste settles at the bottom of the tank where anaerobic bacteria -- bacteria that live in the absence of oxygen -- digest it. At this point, the wastewater, called effluent, flows out of the septic tank by one of two methods: gravity or pumping.
A gravity system relies on the natural fall of the land to move water from a septic tank into pipes that carry it away. A distribution box, commonly called a d-box, disperses the effluent through multiple pipes in the leach field.
A pressure distribution system uses a pump to create the pressure that moves water out of a septic tank. This is a suitable choice if the septic system is installed in sandy soil, or if the effluent removal area is uphill from the septic tank or a long distance away.
A two-stage septic system expands the process of a simple septic system by including an additional component -- an aerobic treatment unit. An aerobic septic treatment system uses a settling tank to separate the liquids from the solids, similar to a conventional single-stage unit.
But instead of moving the wastewater directly into the distribution box, which disperses the effluent into the drain field, a two-stage system first moves the wastewater into an aerobic treatment tank. Air might be pumped into this tank, which helps aerobic bacteria -- bacteria that need oxygen to live -- to digest the waste.
Two-stage systems are suitable for sites that have a greater need for effluent treatment before water is released safely into the environment.
Septic media systems use varied media, such as peat moss, sand or synthetics to filter the effluent before discharging it into the environment. This step comes after the wastewater has already traveled through the septic tank and, in some cases, an aerobic unit as well. This additional filtration process is needed in areas with high groundwater levels, poor soil or shallow bedrock.
Gray-water separation systems use treated effluent to irrigate plants by employing seepage lines that direct the water toward plant roots. These systems are optional and installed at a homeowner's request. Gray water consists of all the wastewater in the house that doesn't come from the toilet, such as showers, laundry, dishwashers and sinks.
The Final Treatment
An essential component of a septic system is the area beyond the septic tank -- the septic field, also called the drain field or leach field. This final step in the wastewater treatment process moves treated water into the landscape where the septic field removes residual impurities.
A sand or gravel filtration leach field, typically part of a conventional septic system, moves effluent through perforated pipes buried in parallel trenches filled with gravel or sand. As the wastewater flows out through the holes in the pipe, it slowly leaches into the gravel.
A gravel-less system works the same way as a gravel system, except it uses other media in the trenches, such as clay, shale, foam or plastic.
A mound system is a septic drain field raised above the soil's surface instead of being buried underground. This system is installed when it's impossible to dig three feet into the soil to lay a conventional drain field. The mound system must use a pump to move the water above-grade, and they're needed in areas of shallow soil, high water tables or bedrock.