• Bonnie Palmer

How a Septic System Works

If you are buying a home in the Austin area you may very well run into houses with septic tanks. As the city of Austin developed over the years, there were several areas that were not included in the city's sewage infrastructure and are therefore run on septic tanks.


A typical septic system consists of a septic tank and a drainfield (or soil absorption field).


Household wastewater is flushed from the house through the exit pipe into the septic tank. The septic tank is a buried, water-tight container usually made of concrete, fiberglass, or polyethylene. Its job is to hold the wastewater long enough to allow solids to settle down to the bottom forming sludge, while the oil and grease floats to the top as scum. The septic tank digests organic matter and separates floatable matter (e.g., oils and grease) and solids from the wastewater. Compartments and a T-shaped outlet prevent the sludge and scum from leaving the tank and traveling into the drainfield area. In conventional, or soil-based systems, the liquid (known as effluent) is discharged from the septic tank into a series of perforated pipes buried in a leach field, chambers, or other special units designed to slowly release the effluent into the soil. This area is known as the drainfield. The drainfield is a shallow, covered, excavation made in unsaturated soil. Pretreated wastewater is discharged through piping onto porous surfaces that allow wastewater to filter through the soil.


Alternative systems use pumps or gravity to help septic tank effluent trickle through sand, organic matter (e.g., peat and sawdust), constructed wetlands, or other media to remove or neutralize pollutants like disease-causing pathogens, nitrogen, phosphorus and other contaminants. Some alternative systems are designed to evaporate wastewater or disinfect it before it is discharged to the soil.


The liquid drains through the soil layers, where bacteria filter and digest contaminants. By the time the wastewater reaches groundwater, it is fully treated.


View an animated, interactive model of how a household septic system works created by the Guadalupe-Blanco River Authority.





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