As BillyZ points out the water from the dehumidifier is definatly NOT RO
filters run water thru a Reverse Osmosis membrane filter that in effect blocks all particles larger than a water molicule (over simplification). As its been pointed out above a dehumidifer is basicly a reverse AC
unit in that it pulls the mosture out of the air and it collects on condensing coils. As it collects it drops into the collection tub to be dumped. Dust in the air will wind up in this collection tub as well as other things again like mentioned above airborn bactera, mold, pollen, etc.
NOW with that said I know someone who did just what your thinking. He mounted a dehumifiyer unit above his sump (his sump was in the basement and his display tank above him on the main floor). The unit was setup to drain into his sump directly. he used this as his "topoff" device with the aid of a float switch and RO
as backup for many months with little signs of negitive results. But there is the potental expecially if you have someone in the hosue who likes to go spraying things like hair spray, air freshner, etc into the air that it will find its way into the dehumidifer.