Agricultural engineer: Septic tanks vulnerable to flooding

WEST LAFAYETTE, Ind. — Every toilet flush and running faucet can place an additional burden on septic systems in homes ravaged by Indiana’s floods, said a Purdue University Extension agricultural engineer.

Older septic tanks or those in poor condition might be vulnerable to flooding, which can cause wastewater to back up into the house and possibly even lead to permanent septic system damage, said Don Jones.

Jones advised homeowners with flooded septic systems to limit their water use until floodwaters recede and soils have had an opportunity to dry.

About 800,000 homes in Indiana have private sewage systems.

“A high water table – the kind you would see in a flood – can become a problem if there are cracks in the septic tank or the seals around the tank aren’t very good,” Jones said. “This is probably a bigger concern for septic systems that are at least 10 years old.”

Septic systems in Indiana use soil absorption fields to treat and disperse the wastewater that leaves the septic tank. In normal conditions, the systems remove nitrogen and pathogens from the wastewater before the pathogens reach groundwater. In flood conditions, however, saturated soils can prevent proper wastewater treatment and dispersion from taking place, resulting in sewage backups and potential groundwater contamination.

“You could even get water running back into a newer, watertight septic tank if there is not much slope in the soil field,” Jones said.

Septic tanks not equipped with effluent filters could face long-term damage to the filter field, Jones said. Many older tanks are not fitted with the filters.

“The filter is used to prevent solids from being washed out of the tank, even when the liquid level in the pump is high,” he said. “This keeps solids and scum in the tank from reaching the filter field, even when the filter cartridge is removed for cleaning.

“If no filter is present when the wastewater level in the tank is above the outlet baffle, some of the floating solids and scum can overflow into the soil absorption field as it dries out. If water from the soil field is able to back up into the tank, is stirred up and then runs out again, even more solids could end up in the soil field. This can affect the system in the near term and even shorten the life of the soil field.”

Homeowners with submerged septic tanks should have the systems inspected once floodwaters abate, Jones said. He also recommended homeowners:

  • Avoid removing the septic tank lid while the tank is still under water. “In addition to flooding the system, someone could even fall into the tank,” Jones said. “It’s good to just leave it alone until the water goes away.”
  • Minimize water use until soils are no longer soggy. “The soil doesn’t need any more water,” Jones said. “If you need to do laundry, go to a coin-operated laundry. Keep water use in the home to a minimum, even if the water is still flowing out of the house into the septic system.”
  • Shut off power to septic system sewage pumps until floodwater goes down.
  • Remove all silt and other material from the septic tank, distribution boxes and sewage pump chambers after water recedes. This requires a professional.

Above all, be patient and allow the soil a chance to dry, Jones said.

“With the water table as high as it is in some parts of southern Indiana right now, that’s going to take a few days, or even weeks,” he said.

For additional information and tips on septic systems and flooding, read Purdue Extension publication HENV-10-W, “Septic Systems in Flooded and Wet Soil Conditions,” by Jones and Purdue agronomist Brad Lee. The publication is available online at or by calling Purdue’s toll-free Extension hotline at 1-888-EXT-INFO (398-4636).

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 July 2012