Simple solution: don't use the words 'sewer' or 'reused' anywhere near the promotional materials for this product. Call it 'filtered water' or some other safe euphemism.
For example, the idea of passing it through an underground aquifer lets people think "yeah, I know it came from a sewer treatment plant, but now it's different". You're not trying to hide anything, you're just trying to get them to view it differently.