It may seem paradoxical, but processes used to clean water actually also create wastewater. Reverse Osmosis creates clean water by forcing the water through a semipermeable membrane that filters out many moleclues. The problem is with the water that's left, that does not pass through the membrane, which has an extremely high concentration of salts and minerals. This water is called RO reject or reverse osmosis concentrate. Reverse Osmosis reject can sometimes equal as much as half the volume of the original feed water.
Draygon's wastewater evaporators can distill RO reject, leaving behind only solids that can be easily and safely disposed of.