Water in a septic system aids the bacteria in digesting the solids. If there is "excess" water going into a septic system it simply flows out to the leach field. The water flowing out does no carry solids into the leach field, solids only go into the leach field if the system is undersized or the bacterial action fails for some reason (rare).
The amount of reject water added to a septic system be an under counter RO filter system is minuscule under any normal usage, comparable to an extra shower or load of laundry. It will not in any way harm a septic system that is in normal operational condition.