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How Does a Septic System Work?

a typical residential septic system (septic tank and drainfield)

A septic system, properly installed and maintained, is a good way to treat wastewater and to protect groundwater quality when municipal sewer service is not available.  A typical septic system consists of two major parts, the septic tank and drainfield.

Waste from toilets, sinks, washing machines and showers enters the septic tank, which is a holding tank generally made of pre-cast concrete or fiberglass and is sized according to the estimated wastewater flow from a given-sized residence or business.

The septic tank separates the wastewater into three general components -- solids or "sludge", floatables or the "scum layer", and a zone of relatively clear water.  Anaerobic bacteria (able to live in an oxygen-free environment) perform the first treatment of the wastewater, generating gas that is vented through the vent stack of the building's plumbing, and breaking the solids into a liquid form.  The oxygen-free conditions inside the septic tank also deactivate some of the disease germs that are found in sewage.

a typical septic tank, showing the three major components of wastewater

From the septic tank, the liquid portion of the wastewater flows into the drainfield, which is generally a series of perforated pipes or slotted panels that are usually surrounded by a layer of gravel, tire chips, or other lightweight materials such as styrofoam pieces.  The drainfield provides secondary treatment of the sewage by allowing aerobic (oxygen-using) bacteria to continue deactivating the disease germs that remain in the wastewater.  The drainfield also provides filtration of the wastewater as gravity draws the water downwards through the soil layers.  In addition, evaporation of water occurs through the layer of soil covering the drainfield.

In some areas where soil types such as clay layers or bedrock exist, or in areas where there is a shallow seasonal high water table, septic systems must be elevated above the ground surface ("mounded" systems).  This ensures the wastewater has sufficient permeable or unsaturated soil in order to provide adequate filtration before the remaining wastewater reaches the groundwater table and the underlying aquifer.

In other areas, such as flood zones near rivers or other bodies of water, traditional septic systems may not be sufficient to treat the wastewater.  In these cases, advanced wastewater treatment systems that "aerate" or add oxygen to the wastewater may be required.  Other advanced wastewater treatment systems may have chlorinating chambers or peat moss-based filtration chambers which neutralize the disease germs before they can reach groundwater levels.

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