Additives/chemicals are not necessary to break down waste material in septic systems.
The biological material needed to break down waste is already in the waste itself. For as long as your septic system is well-maintained and not misused, there is no need for additives or chemicals.
Related topics:
What You Need to Know Before Septic System Installation (A Checklist)
How to Solve The Biggest Problems With Septic Tanks and Systems