Originally Posted by jessl_99
Im mulling over the idea of adding a gray tank to a 1967
tradewind. Just following this to get some of the plus and minus of storing gray water.
The type of plastic used for the gray tank determines whether it will absorb odors from the contents.
"Food-grade" plastics are designated as such because they do not absorb stains or odors from the organic matter it touches (whether that organic matter is actually food or not) and does not impart stains, odors, or tastes to the organic matter it touches.
In the RV industry, no one expects waste tanks to be food-grade plastic, but everyone expects fresh tanks to be food-grade plastic. So the best gray tank may actually be a fresh tank modified for the purpose.
High-density polyethylene (HDPE) is generally food-grade, and makes an excellent holding tank for an RV, whether it's a fresh tank, gray tank, or black tank.
As an aside— even in one's home, not all plastics used for preparing or storing food are necessarily food-grade. Ever have a Tupperware container get ruined by a food stain that soaks into the plastic and won't come out? Older Tupperware wasn't food-grade plastic, because Tupperware predates the FDA regulations for food-grade plastics. Many newer sports bottles aren't food-grade either, as I learned when two that I bought picked up permanent tea stains. When buying plastics for contact with food or beverages, look for a "wine glass and fork" symbol on the bottom; that's the international symbol for food-grade plastics.