bentov:
You state you have 7,200 ft of PVC conduit on hand; you fail to state the lineage you need. Is it also 7,200 ft? I doubt it. More importantly, I consider good, potable-rated PVC piping one of the lowest material costs involved in a drinking water pipe installation. I don't see a practical reason for not doing the installation the "right" way: buy the appropriate PVC for the installation. Your supposed "savings" are only = (cost of new, correct PVC - cost of existing PVC conduit). So you don't really save the 100% PVC quantity needed.
Additionally, you don't mention your site or location. I foresee rough local water piping code violations are are a strong posibility. Getting a clean bill of goods on what you have previously purchased as PVC conduit may be next to impossible and could easily prevent you from using it for potable water. You may be willing to drink water out of it; however, I and others wouldn't do it without knowledge that it is according to local drinking water codes and has been approved.
If you can't get local code approval, I urge you to abandon the idea.