Most septic tanks are hundreds of gallons, so dumping 10 to 30 gallons of saltwater in it is not much. Similar to adding a quarter cup of salt water to a gallon of fresh water. Plus, you only do it once or twice a month, so it really is not a big deal. If you were dumping it daily, I bet there would be a problem. Don't you think urine has tons of salt in it? I know it's very acidic.
Sonny