I would not put salt water into your septic system. First of all, the salt will kill the beneficial bacteria that degrades your waste. Watch what happens to your lawn if you get salt on it. Second, salt tends to corrode the concrete of holding tanks. The surface starts to decay. This process is called spalling. One example would be if concrete sound barriers are built too close to a highway in northern areas. Salt gets on them in the winter and in some circumstances the whole face of the barrier falls off.