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What Are the Causes of an Overflowing Septic Tank?

author image Emily Beach
Emily Beach works in the commercial construction industry in Maryland. She received her LEED accreditation from the U.S. Green Building Council in 2008 and is in the process of working towards an Architectural Hardware Consultant certification from the Door and Hardware Institute. She received a bachelor's degree in economics and management from Goucher College in Towson, Maryland.
What Are the Causes of an Overflowing Septic Tank?
A septic tank is being lowered into the ground. Photo Credit travellinglight/iStock/Getty Images


Septic systems process household waste in areas without municipal sewer systems. They consist of a large holding tank and a nearby drainage field made from underground trenches. As waste exits the home, it passes through a series of pipes until it reaches the septic tank. Solid waste settles to the bottom of the tank, while liquid waste and effluent pass into the drainage fields, where they are slowly absorbed into the earth. An overflowing tank can be caused by a variety of different factors. By pinpointing the cause of an overflow, you can determine the best way to fix your system and restore its operation.

Inadequate Maintenance

As waste passes through the septic tank, materials that are nonbiodegradable immediately settle to the bottom of the tank. Some solid waste also ends up in the bottom of the tank, and over the years the level of sludge in the tank rises. According to the University of Georgia, septic tanks must be pumped out every three to five years to remove this sludge and prevent overflow. The period of time between septic tank cleanings depends on the size of the tank and the amount of waste. For example, a 1,000 gallon tank in a four-person household should be pumped out about every two and a half years. A 1,500 gallon tank in a four-person household only needs to be pumped out about every four years.

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Bacterial Deficiency

Naturally occurring bacteria in a septic tank help to break down waste and allow it to pass through to the drainage field. If bacteria levels are low, solids won't break down, and will build up much more quickly than usual. This can cause the tank to overflow, or lead to clogs in drainage pipes or trenches. According to Thomas Refuse, any cleaning product that is toxic to humans can also kill helpful bacteria in a septic system. Keep bleach, disinfectants and toilet cleaners out of your waste piping to avoid killing bacteria needed to maintain your system.

Clogs and Design Flaws

As liquids and broken-down solids exit the tank, they pass through a series of pipes into an underground drainage field. If the tank overflows, you'll notice that the ground is very wet above this drainage area. This type of overflow is usually caused by either poor design or damaged drain pipes. If tree roots grow through pipes, the walls of the pipe could collapse and prevent proper drainage. Clogged or broken pipes can also cause overflow.

Some septic system overflow happens because of improper design. According to Purdue University, drainage pipes need a slope of 1 percent to 2 percent for waste to drain effectively. If the slope is too shallow, materials will not flow as intended, and the pipe will need to be replaced.

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