Septic systems are designed to handle household wastes, but improper maintenance of a septic tank can often make waste a big problem. Most causes of septic overflow are overuse and incorrect equipment.
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A septic tank stores wastewater underneath the house. In the tank, solids sink to the bottom and oils float to the top. Bacteria then break down the top layer of scum and partially treated water flows into nearby soil, according to the Environmental Protection Agency's Septic Tank Guide.
Septic tank overflow can occur due to problems with maintenance and the capacity of the septic system, or the condition and capacity of the drainage field.
Too much wastewater flowing into the septic tank can clog the system and flow back into the plumbing system. Drainage fields sometimes cannot handle the amount of water flowing out of the septic system; pavement and large roots can prevent soil from absorbing wastewater.
An overflowing septic system can reach groundwater sources and spread waterborne illness. Septic overflow can also leach into lakes and streams, spreading infections and hepatitis to swimmers.
Preventing overflow requires pumping the septic tank and inspecting it every three to five years. Reducing the use of water by installing low-flow toilets, as well as not flushing solid products down drains, eases the septic system load.