The introduction of sodium into septic systems is known to clog soils.
Anaerobic conventional septic systems have been historically treated as a dumping ground for any type of waste liquid. This is now fast becoming a problem for the industry. Basically putting anything into a septic tank that you wouldn’t first put into your mouth, or anything that is fundamentally toxic to bacteria should not be put into a septic tanks.
Obviously, as a culture, we won’t follow that exact rule. Reasonable amounts of the normal products we use in our homes can go into a septic tank. The real problem still remains Biomat clogging of soils around drain fields.
The back wash from water softeners is loaded with sodium as rock salt is the medium used for ionic exchange that binds up minerals with the salt. This sodium in the back wash will stratify in the soils around the leach lines and slow down the movement of liquid through it.
Eventually, with biomat and other organic compounds that infiltrate the soil pores, failure is the outcome. As failure is designed into and expected for these anaerobic systems within a couple of decades, excessive amounts of any of the materials described merely decreases the time period to failure.
You should try and discharge the back wash to another place and leave the septic system to handle your biological waste from the home.
Installing a separate small drain field to handle the back wash is easy and shouldn’t be expensive.
