Faced with a Flooded Septic System?

Floods can wreak havoc on septic systems, but they can often be rescued if the septic professional knows how to minimize the damage

Faced with a Flooded Septic System?

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Prior to the flood
If a septic system is located in a flood-prone area, a plumber should install a backflow preventer on the building sewer so sewage cannot back up into the home during a flood. A backflow preventer is recommended as a simple check valve may not close properly and sewage may back up into the home. Also, make sure all inspection caps are in place. Threaded caps can be installed and the pipes can be cut flush with the ground.

Immediately prior
If you know a flood is coming and the building sewer has a backflow preventer, nothing further needs to be done. If the backflow preventer is a manual valve, ensure it is shut. If the tank does not have a backflow preventer, it may be desirable to pump the tank to remove the sewage, but the tank must be properly anchored to prevent flotation. If pumped, some sewage solids will remain in the tank and could mix with any floodwaters that enter the tank. It may be advantageous to block any lower level drains in the dwelling to prevent backup.

During the flood
If a soil treatment system has not been flooded but is soggy due to heavy rain, recommend that the owners minimize water use within the home. The additional water from regular household use can cause poorly treated sewage to surface in the yard or raw sewage to back up into the house.

When floodwaters cover the septic system it should never be used. If the ground above the septic tank, advanced treatment system or soil treatment area is covered with water, the septic system is not adequately treating wastewater. Turn off water softeners to prevent them from regeneration. Turn off all the system’s electric devices (pumps, alarms, blowers, etc.).

After the flood
The system should not be used until the soil has adequately dried to allow sewage to be absorbed and not back up. This may take several weeks. The system’s user should try to conserve water until the system is completely dry.  

There is potential for damage to the system; however, action can be taken after the flooding to minimize the damage. Here are some things to consider when helping the system recover:

  • Pump the tank(s) and treatment units as soon as possible after the flood recedes and prior to resuming use of the system. Be sure to pump both the septic tank and the pump/lift station (if one is present). Silt and other debris may have collected in the septic tank while it was under water, which could ultimately find its way to and damage the soil treatment systems. Additionally, a variety of substances such as pesticides, petroleum products and other contaminants may have entered the tank. These contaminants could be detrimental to the beneficial bacteria in both the tank and the soil treatment systems and therefore need to be removed. However, it is not advisable to leave the septic tank empty after pumping if the soil around the area of the tank(s) is saturated; this can cause the tank to "float" toward the ground's surface if the soil's water pressure remains high. In this case you may want to fill the tank with clean water. Effluent screens (if present) should be cleaned at this time as well.
  • Protect the soil treatment system from compaction by keeping all traffic off the area. Often there will be a considerable traffic increase around a flooded home as flood cleanup and home restoration occur. This traffic could include but is not limited to foot traffic, debris piles, dumpsters and heavy equipment. It may be recommended to fence off the system to protect it.
  • Check electrical connections for damage or wear before turning electricity back on.
  • Check that the septic tank manhole cover is secure and that inspection ports have not been blocked or damaged. Check for animal damage or intrusion in the soil treatment area.
  • Check the vegetation over the septic tank and soil treatment area. Repair erosion damage; sod or reseed as necessary to provide a good plant cover. You may need to mulch the area to provide insulation if the grass has not become well established before winter.
  • Inside the home, be sure the property owner disinfects thoroughly if sewage backed up into the house or garage. Pathogens in wastewater can cause serious illness, such as dysentery, hepatitis and other waterborne illnesses. However, avoid flushing these disinfectants into drains that empty into the septic system. The disinfectants could be detrimental to the beneficial bacteria in both the tank and the soil treatment systems. If they are discharged into the septic system it is best to pump the tanks (a second time if floodwaters were previously pumped) to avoid discharging of these chemicals into the soil portion of the septic system.
  • If, after the floodwater has receded from the soil treatment area and the surrounding soil has had a chance to dry, the soil treatment systems still will not accept effluent from the septic tank, the pipes or soil might be "plugged." The systems will then need a thorough investigation to determine if a repair or new system is needed.

If the system was destroyed
Often floodwaters can cause components of a septic system to be partially or completely washed away. The owner of such a system should not assume that soil or other "fill" can be added and new system components constructed. Heavy rains can cause slides to partially or completely cover septic system components with rock, mud or silt. These slides can affect the operational integrity of the system, especially the soil treatment systems. Care needs to be taken during debris removal from the area on or around a septic system in order to protect system components, taking special care to keep vehicle and equipment traffic off the soil treatment system to avoid compaction. If the soil treatment system is saturated or has standing water long after other areas have dried out there may be a long-term problem indirectly related to the flood.

About the Author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association (MOWA) and the National Onsite Wastewater Recycling Association (NOWRA), and serves on the NSF International Committee on Wastewater Treatment Systems. Send her questions about septic system maintenance and operation by email to kim.peterson@colepublishing.com.


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