What happens to your septic system during heavy rain?

Category: Septic system
16 May 2022

If you have a conventional septic system, heavy rainfall that comes with floods can make the system to stop working. As rainwater floods over your drain field, the effluent from the septic tank will have no place to drain because the ground under the drain field is already saturated with water. This will make the septic waste to start backing up in the house and to overflow on the lawn.

In a conventional system, the septic tank holds wastewater for 2-3 days as the anaerobic bacteria treat it. When the effluent leaves the tank and drains into the leach field, the partially treated water percolates through the gravel and into the sand via perforated pipes in the drain field. As the water passes through the gravel in the leach field, pathogens are removed by aerobic bacteria before the water goes back into the groundwater. If the leach field gets saturated, the partially treated water from the septic tank doesn’t go through the final treatment process in the drain field. As a consequence, the wastewater will back up in the tank and overflow in the leachfield.

Signs of a flooded drain field

If you are experiencing heavy rainfall in your area, it is best to be on the lookout for any tale-tell signs of a flooding drain field. Here are some of the signs;

  • Sluggish drains in the house
  • Toilets draining slowly when flushed
  • Gurgling sounds in toilet and drains
  • Backing up of water into the floor drains and basement

Septic systems are designed to only handle wastewater from the house. In fact, the size of the septic tank installed on a property is determined by the occupancy of the house (number of bedrooms). If runoff water from the storm gets into the septic tank, it will get full and since the soil in the leachfield will be already too saturated, the water will start backing up into the house or from the manhole.


Maintaining the septic system BEFORE the heavy rains

It is possible for your septic system to withstand heavy rainfall but it needs to be well maintained. For this reason, you should always pump your septic tank on schedule and you should ensure it is working efficiently throughout the year. The septic tank relies on anaerobic bacteria in order to liquefy the waste so it is in your best interest to ensure the bacteria in the tank are as healthy as possible. First and foremost, you must desist from using any toxic substances that might kill the helpful bacteria like scented soaps, antibacterial soaps, paint, etc. But since it is not possible to totally avoid getting harmful substances in the septic tank, adding biological additives can help. The additives introduce billions of enzymes and bacteria into the septic tank which helps the septic tank to regain its efficiency.

What to do if the weather forecast warns of a looming storm

If the weather forecast has warned of an impending flood, take these precautionary measures in advance to help safeguard your system;

  • Seal any possible points of entry into the septic system.
  • Seal of all inspection points to ensure the excess rainwater will not find its way into the tank.
  • Switch the pump off at the circuit box before the area gets flooded in water.
  • Cut off the power supply to the pump in the lift station of your mound system if you have one. You can also remove the pump from the system in order to protect it from damage.
  • Waterproof any electrical connection in the system in order to safeguard electrical wiring from getting damaged and also to avoid getting shocked.

Maintaining the septic system DURING the heavy rains

Once the heavy rains start, it is advisable to avoid any non-essential usage of water. The idea is to avoid straining the system even more than it already is. For instance, flush the toilet only when necessary and reduce the number or duration of showers. If the drain field gets flooded, you should stop using the toilet and faucets altogether. A flooded drain field means the system is already blocked so you do not want to make the bad situation even worse. Additionally, avoid contact with any flooded water because there is a good chance the water is contaminated.

Maintaining the septic system AFTER the heavy rains

Do not attempt to have the septic tank pumped before the floods rescind. Pumping the tank in the midst of floods could make the tank float out of the ground and cause serious damage to the entire system. The thing to remember is, the problem is not actually the septic tank but the saturated soil in the drain field. the best cause of action is to quit using the system altogether until the flooded water recedes and the soil around the drain field area dries out. Here are some tips to help you limit the water that is going into the septic tank.

  • Do not send the basement sump pump water into the septic tank
  • Reroute any rainwater from your roof gutters away from the drain field
  • Stop using the garbage disposal and dishwasher
  • Reduce the number and duration of showers and if possible, take sponge baths
  • Do not run water while brushing your teeth
  • Wash your clothes at a laundromat

Sometimes, the backup is a bigger problem than stormwater – it could be caused by a clogged up drainfield. If organic waste leaves the septic tank prematurely, it could end up clogging the drainfield thereby causing backups.  In such a scenario, pumping will not fix the problem because after pumping it, the tank will fill up again in no time.  The best approach would be to clear the clog through shock treatment. Shock treatment is the introduction of biological additives that are made from bacteria and enzymes. These biological additives introduce billions of bacteria into the septic system which liquefying the organic waste thereby unclogging the system.


Safety precautions after a heavy downpour

If the floods were really serious, you might have to temporarily evacuate the house. If evacuation becomes necessary, do not move back in before you check with the authorities to ensure all advisories have been lifted. Here are other important safety tips to observe;

  • Do not attempt to dig around the drain field area when the soil is still wet. Avoid working with heavy machinery over the drainfield too because it can cause compaction of soil which will then make it hard for aerobic bacteria in the drainfield to get enough oxygen.
  • The scum layer in the septic tank could have lifted up and blocked the outlet. You should, therefore, check the outlet tee when the flooding stops to ensure it is not blocked.
  • Do not handle any of the electrical devices that are part of the system until they have completely dried.
  • Upflow filters, media filters, aerobic plant, and other such components of advanced systems could get blocked by mud and debris from the floods. You should, therefore, wash these systems thoroughly before putting them back to use.


As long as you take good care of the system before the flood arrives, it should weather the storm just fine.  That said, there are some storms that are just too rough for any system to handle especially if you continue using the water like you usually do. If this is the case, then you might want to call a professional to inspect the system and help you in repairing any damage that might have been done. Otherwise, just follow the tips shared above and you will be fine.

About the author


CEO at Bio-Sol

Our goal is not only to offer you the best products on the market but also to educate and empower you with the insights you need for successful wastewater treatment.


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