1. Will a softener harm a septic system?

Research was done at the University of Wisconsin and the National Sanitation Foundation (NSF) has shown several findings:

The normal salt content found in unsoftened water is less than ideal for bacterial growth. The addition of sodium to the system was found to bring the bacterial environment closer to the optimal range.

The volume of softener wastes are added to the septic tank slowly and are not of sufficient volume to cause any deleterious hydraulic load problems in septic tank systems. Water softener regeneration wastes not only should not interfere with septic tank system drain field soil percolation, but may actually improve soil percolation.

2. Is it a problem if I install a water softener that has a larger capacity than I really need?

No, In fact, there may be savings in salt and the total number of regeneration cycles in a larger unit.