There are many types of “pure” water: chemically pure, biologically pure, or a combination of both. RO water, while biologically pure, it is not very chemically pure. The purity (and therefore corrosivity) of RO water is a function of the total dissolved solids (TDS) of the feed water: the lower the TDS, the more pure the RO water. In hospitals, RO is often combined with deionization (DI), and many hospitals follow AAMI TIR standards. RO/DI water is starved for minerals and tends to want to “steal” them from the piping.

All of these factors need to be fully understood and identified before selecting the piping material.

So What Is the Best Material?
Our experts overwhelmingly agree that copper should not be used in this application. Copper can easily leech into the RO water, diminishing its purity. Brass isn’t as easily corroded as copper by RO water, but it still should be avoided for this application.

For any system that includes deionization, Schedule 80 PVC is not recommended. The aggressive nature of the DI water will extract additives from the PVC, weakening the piping structure and contaminating the high-purity water. However, for single-stage RO systems, PVC is acceptable.

For this application, our experts suggest the following materials.

PEX
Many have found that cross-linked polyethylene (PEX) piping/tubing work best in this application. PEX is resistant to the corrosion that is caused by minerals and other contaminants in the water. These contaminants do not build up in the piping, and they do not break down from the constant exposure of the contaminants, which means the RO system can do its job while minimizing the possibility of damage to the plumbing system.

PP
For Types II and III water, polypropylene (PP) piping is suggested. PP is commonly used in these applications and will support not only the resistivity/conductivity levels, but also the other components of the water spec, primarily total organic carbon (TOC).

Keep in mind that PP does not meet ASTM E84 25/50 flame/smoke Ratings, so take care if the piping is within a plenum.

PVDF
For Type I water, PVDF is suggested. PVDF will support the high resistivity levels desired, as well as the tight TOC and other constituent restrictions. Also, to meet AAMI/TIR 34 standards, PVDF with BCF (bead and crevice free) joints to prevent bacteria buildup in the joint crevices should be specified.

Due to its cost, PVDF would usually be reserved for ultra-pure water in applications where water approaching 18 µohm is required.