A quotation from the EPA about the safety of using water softeners. //

High concentrations of calcium and manganese in the softener backwash water have no deleterious effect on the biological functions occurring in the septic tank and may, in some cases, be helpful.
The additional volume of wastewater generated (typically about 50 gallons per recharge cycle) is added slowly to the wastewater stream and does not cause any hydraulic overload problems.
Soil structure in the soil absorption field is positively affected by the calcium and magnesium ions in water softener effluent