I have the exact same problem. All my white synthetics are gray. I wear them as around-the-house-lounge wear. I have complained to my town but they insist the water is tested daily and is healthy to drink.

There are a lot of experts who post on GardenWeb and someone probably can explain what is happening. Try posting this on the Laundry Room forum and the Plumbing forum. GardenWeb discourages people from cross posting, but in this case, I think it makes sense.

Have you ever had your water tested? Maybe when you had the water purification system installed?

I had my whites start doing the same thing after I started using different laundry soaps. I went back to my tried and true and the white came back. I have also had great success with adding Simple Green to the wash water.
My Mom swears by soaking the dingy whites in baking soda to restore the brightness.

Good point kellied! I just was about to say that baking soda is what I use because I don't like to use too much chemicals on my clothes. It works wonderful - a good household remedy.

I want to try it because my white towels are graying, too! How much baking soda? Do you use it to soak first or mix with laundry soap?
Many thanks!