To maximize the purity of product water, EDI feedwater needs pre-treatment, usually done via reverse osmosis. When fed with feedwater that is low in total dissolved solids (e.g., purified by RO), the product can reach very high purity levels. The contents of the feedwater must be kept within certain parameters to prevent damage to the EDI instrument.
Common feedwater quality concerns are:
* Hardness, which is often limited to 1 part per million (ppm) of CaCO or corresponding molecule, with limited exceptions up to 2 ppm.
* Silica content (SiO), which generally must be no more than 1 ppm in most EDI cells or 2 ppm in thin-cell modules.
* CO, which must be monitored to prevent excessive loading of anion exchange resin.
* TOC, which can foul resins and membranes, must be minimized.
* Chlorine, ozone, and other oxidizers can oxidize resins and membranes and create permanent damage, and must be minimized.