Ridex or none of the other septic system bacteria "additives" will help your septic system OR get rid of roots. They're all a waste of money.
A cup or two of copper sulphate crystals flushed down a toilet about three times a year will keep the hairlike rootballs out of your system.
However, if you have a sectional sewer line (cast iron, etc.) - instead of a solid "glued" (actually chemically "welded") PVC white plastic or ABS black plastic sewer line - that will not allow any roots in, you may need to have the line "roto-rootered". If you do that, it will only last a couple of years at most.
The only permanent solution to roots is "solid" plastic line that doesn't allow them to get in.
Good Luck!
Mike