Buy a water softener that only regenerates on demand (if you don't have iron in the water). WaterBoss (like ours) only uses about 15 gallons to regenerate. Use a "dry well" or a "French Drain" in your yard for disposing of the salt water. You can find info online how to construct either one.
Some of the large tank type use 60 gallons to regenerate and some use up to 120 gallons, so be sure to select one with low water useage.
Cities prefer that users put the salt water into a dry well or French Drain rather than into the sewer system.