Is Soda Ash used in Tie Dying septic safe?
Most of the resources that I reviewed agree that the amount of soda ash used for home dyeing is safe to pour down the drain. However, I agree that it is important to take extra precautions with a septic system. So I checked the Dharma Trading Co. website (a great resource with lots of detailed info about dyeing fabric and fiber), 


  One environmental consideration is that if you have added Soda Ash to your dyebath, then it's alkaline (high pH) and can be neutralized by the addition of a little white vinegar (low pH). If you used vinegar or acidic acid in your dyebath, you can add some Soda Ash to neutralize the acid. 


They go on, 


  Highly acidic or alkaline waste water maybe could upset the balance in a septic tank. We have pH test paper you can use to test the pH before flushing it down the drain.


So, I would recommend testing the pH of the leftover liquid, and, if it is overly alkaline, treating it with some white vinegar before you pour it down your drain.