Yes, all Morton water softening products are safe for septic systems. In recent years the United States Environmental Protection Agency (EPA) issued a special facts sheet summarizing studies by NSF and the University of Wisconsin on home water softeners and onsite treatment systems. What they found was softener effluent has no deleterious effect on bacterial functions, increased volume of wastewater does not cause hydraulic overloading and that the calcium and magnesium actually improve soil stability and percolation in the leach field. A link to the EPA web page is given below:
https://www.epa.gov/ordntrnt/ORD/NRMRL/Pubs/625R00008/html/fs3.htm