Missouri Department of Natural Resources fact sheet
Missouri Department of Natural Resources Director: Dru Buntin

Natural disasters can damage an individual on-site wastewater treatment (septic) system. Many systems may be adversely affected by flooding. Some of these systems may be so damaged that they require major repairs or replacement. Most of these systems will remain inoperable until the flood water recedes and the soil dries.

One problem is the release of untreated sewage from the septic tanks onto the ground. The ponding sewage left behind on the soil's surface from the septic tanks can be a significant health hazard. Another issue is sewage backup into the building, caused by a blockage that results from the damaged system or piping.


  • Avoid contact with sewage. Raw sewage is a public health concern and can contribute to the spread of disease. Good personal hygiene, including hand washing, is necessary.
  • Treat surfacing sewage with hydrated lime or diluted household bleach. Wear gloves. Handle chemicals carefully. Follow all listed precautions.
  • Check for missing septic tank or treatment system access covers. Secure openings with temporary cover.
  • Check for electrical system damage on systems with electric pumps and motors.
  • Take care during cleanup operations not to drive over lateral lines including low-pressure pipe fields or drip irrigation fields. This could damage your system.
  • Carefully remove debris from wastewater lagoons. Repair damaged berms and fences.
  • Avoid using the house plumbing system if the septic tank or the lateral field is still underwater.
  • Do not use the plumbing system if sewage is backing up into the house.
  • Carefully investigate the cause of sewage backups. Check the septic tank to see if it has shifted, or if the grease layer in the top of the tank has blocked the inlet or outlet pipes. Any of the sewage pipes either leading to the septic tank or to the lateral field may have become broken or filled with silt. Look for broken and blocked plumbing and vents. Have the wastewater system serviced by a professional.
  • Try to minimize the amount of mud and silt entering the plumbing system during cleanup operations. Mud can fill the septic tank and clog lateral lines.
  • The greatest threat to flood-damaged septic systems is the clogging of drainfield lines with sewage sludge and silt left by receding floodwaters.
  • Installation of a septic tank effluent filter can help prevent drain-field clogging.
  • Flooded systems will remain inoperable until floodwater recedes and the ground where the treated wastewater is absorbed becomes somewhat dry.
  • Do not pump flood-damaged septic tanks dry until the groundwater level has dropped sufficiently. Doing so may cause the tank to float upward or to collapse due to groundwater pressure outside the tank.
  • Do not enter the septic tank. Septic tanks can contain hazardous fumes and gases that are potentially fatal. After the groundwater has dropped sufficiently, have a professional check the tank to determine the solids level prior to using the system. If solids are near the outlet pipe or baffle, the tank should be pumped before use.
  • Individual lagoons that have silted in or have been physically damaged should be repaired as soon as possible before long-term use is resumed.
  • Contact your local county health department or the Missouri Department of Health and Senior Services, Onsite Wastewater Treatment Program, Bureau of Environmental Health Services, at 573-751-6095, with questions about on-site wastewater system repair or replacement.

Wastewater System Abandonment

Some on-site wastewater treatment systems might be abandoned after a natural disaster. The following are recommendations that will help ensure that the site formerly used for wastewater treatment and disposal is safe:

  • Disconnect power at the source to all electrical controls and remove controls and panels.
  • Have a septage hauler pump the wastewater and sludge from all the tanks in the system.
  • Fill tanks with sand or gravel to prevent future collapse, remove the tanks or crush in place. Backfill the excavation to a natural grade and establish a vegetative cover.
  • Remove all parts of the system on the ground surface such as valves, valve boxes and risers. Backfill the area to a natural grade and establish a vegetative cover.
  • Coat all surface areas exposed to wastewater with hydrated lime and establish a vegetative cover.
  • Wait at least 18 months before using the wastewater disposal area for gardening or construction.

Produced in cooperation with the Missouri Department of Health and Senior Services.

Nothing in this document may be used to implement any enforcement action or levy any penalty unless promulgated by rule under chapter 536 or authorized by statute.

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