A septic tank is the central component of a self-contained disposal system for household wastewater, from toilets, sinks, showers, tubs, dishwashers and clothes washers. Septic tanks serve homes at locations without municipal sewer service; locations with sewer service don’t need septic tanks. According to the U.S. Environmental Protection Agency, about 25 percent of the 115 million U.S. households, mostly in rural and far suburban areas, are served by septic tank systems.
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How the Tank Works
A septic system has four parts: the wastewater drain pipe from the home, the septic tank, a drain field and the drain field’s soil. Household wastewater flows down the drain pipe and enters the septic tank, which is a buried watertight box made of concrete, fiberglass or polyethylene that holds a couple of hundred gallons of liquid. The tank holds the wastewater long enough for solids to settle out as sludge and for cooking grease and oils to float to the top as scum. Bacteria in the tank partially decomposes sludge and scum.
A tee-shaped outlet allows liquid waste to leave the septic tank for the drain field. The outlet tee prevents solids and scum from leaving the tank. The liquid waste travels by gravity to the drain field, which consists of parallel rows of perforated pipe buried in porous soil. The wastewater percolates into the soil where natural chemical and microbial actions neutralize harmful waste contents that could pollute groundwater, such as nitrates, phosphates and disease-causing bacteria and viruses.
A typical septic tank system works quietly and constantly but does require maintenance. You should have your system inspected and the tank pumped out every three to five years. Your tank needs to be pumped if the top of the sludge is within 12 inches of the outlet tee. Signs of a septic system failure include wastewater backups into the house and pooling water or mucky soil around your septic drain field. You can “kill” your septic tank by dumping down your drains' toxic solvents, gasoline, antifreeze, pesticides or harsh household cleaners. These substances can poison the bacteria that make the tank work. Look for household products that say they are safe for septic tanks.
If your soil isn’t porous, a conventional septic drain field won’t work. An alternative method involves lining deep drain field trenches with coarse gravel before laying the perforated pipe. Leaching chambers are open-bottom vessels 6 feet high and 3 feet wide installed singly or in multiples. Septic effluent floods into the chamber and gradually seeps into the soil for purification. Pressurized drain fields use a pump to force septic tank effluent into the ground. Alternatives for areas with little or no soil drainage include shallow lagoons where effluent is purified by sunlight, air and bacterial action, artificial wetlands where aquatic plants and bacteria purify effluent, and pumping the effluent into a drain field installed in an artificial raised bed of soil.