1. Is the salt-brine discharged from a water softener toxic to the bacteria in a septic system?

These tests confirmed that water softener waste effluents actually exert a beneficial influence on septic tank by stimulating biological action and caused no operational problems in the typical anaerobic or aerobic septic tanks.

2. What effect does the flow rate and volume of backwash and regeneration water discharged from a softener have on the settling and floatation process by reducing the digestion time in the septic tank, thus causing carry-over of solids into the drain field?

The volume of softener wastes (about 50 gallons per regeneration) are added to the septic tank slowly and are not of sufficient volume to cause any deleterious hydraulic load problems in septic tank systems.

3. Since sodium is contained in the regeneration solutions of softeners and sodium is known to cause certain soil to swell and thus reduce the percolation (hydraulic conductivity) of water through the soil, how severe is this effect on the soil going to be?

Water softener regeneration wastes not only should not interfere with septic tank system drain field soil percolation, but actually might improve soil percolation.
Reprinted from the Research Reports: Septic Tank/Water Softener, by the University of Wisconsin - Madison and the Geological and Natural History, University of Wisconsin.