The order have been cleaning polluted water since they first arrived on this planet. Even ocean water is easily cleaned up and turned into pure water that the order consume every day. They use an alien liquid that destroys all bacteria and all saline contents from the water and this basically turns the water into the purest form of liquid in the world. This kind of cleansing has not been made available to humans yet. The alien liquid is manufactured on another planet and brought to Earth for the order to use.
There might be a time when the order will decide to finally allow humans to get a chance to enjoy this purifying liquid, but it will not be allowed until the order determines that humans have evolved enough to be allowed to have such a powerful way to purify water. The order considers that this is still not the case.