Depends on what all is in the tap water wether it is safe to drink. Normal filtering of drinking water will remove some contaminants but it does leave a large amount behind. Distillation is the best treatment for drinking water , as it leaves it over 99% pure. I dont personaly think that tap water will kill a dialysis patient, as dialysis is just an artificial way of doing one of your bodies normal fuctions, and it is fairly effective at doing it. But the purer the water the better for you it is going to be, dialysis or not.