I thought they were not supposed to freeze...
In the early days, people were able to bury septic systems deeper in the ground. With today's rules they are installed very shallow. This is to help limit the amount of bad stuff getting into the water supply. During cold winter months, frost penetrates several feet into the ground. This makes septic systems very vulnerable to freezing. Systems that are not used daily are especially susceptible to freezing. High-efficiency furnaces that release small amounts of water into the septic system also add to the problem.
Freezing can occur anywhere within a septic system. If there is a sag in the pipe, it is often the culprit because the standing water freezes and then builds as water trickles past until the pipe is totally blocked. Then raw sewage begins to back up toward the house.
Where freeze-up occurs.
Photo courtesy of the University of Minnesota Extension Services.
Another common area of freeze-up is where the sewage enters the drainfield area, at which point the pipe connects to a drop box. The drop box is usually less than 3 feet underground and always has water in it. This will freeze when surrounded by frozen ground. When the drop box water freezes, ice builds up as water trickles from the septic tank, eventually blocking the pipe.
In mound systems or systems with lift stations, freezing can occur between the house and the tank, within the tank or lift station itself, after the lift station, and/or in the mound. The most common area is the lift station and the pipe that goes to the mound. If the pipe that goes to the mound freezes, it is often because the weep hole has frozen shut, not allowing the residual water drain back into the lift station.
If your septic system freezes...
There were two ways to deal with a frozen septic system, both of which, costly, time consuming, and inconvenient:
Call your plumber and have them jet or steam the pipes, usually a few times during the cold spell. This is an effective way to unclog the pipes, but there are risks of overstressing the pipe and joints. The amount of pressure and heat used can exceed 2000 psi and 500º F. Sewer pipe is not made to handle such extremes. This solution usually requires multiple service calls because the pipe refreezes because nothing is done to prevent future freezing. Several people have reported that their pipes actually collapsed from repeat steaming, causing several thousands of dollars of damage.
Have your tank pumped regularly while your pipes are clogged and allow mother nature to thaw at her own pace. Use your water sparingly!
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