Probably the safest place is back in the pool. You would be adding CH back to the pool but it is probably small compared to what is already in the pool. Just keep track of how much acid is in the solution so you don't add back too much at once. Basically your normal dosing. I clean my cell in a dosing bucket so I just use it for dosing at a later time.

Otherwise, you would have to dilute it further and perhaps neutralize it before disposing in a drain.