Water – De-Ionization Process

There are many ways to de-ionize water.  Some are small and simple while others are large and complex.  The method used is determined by the volume of de-ionized water required and the desired purity.  This blog will explore some of the de-ionizing means used in facilities with typical needs for parts cleaning applications.

Removal of ions using resin beads is probably the most commonly used industrial technique.  This is accomplished in much the same way as water “softening” described previously.  The resin typically used to remove Calcium and Magnesium ions in water softening are replaced with resins that are treated to remove a wider wider range of ions.  In order to do this, the resins are treated to selectively exchange “cations” which are  positively charged or “anions” which are negatively charged ions.  The products once the exchange has taken place are the ions captured by the resin and water.

In applications requiring a relatively large quantity of de-ionized water at the relatively low quality level required for applications including “spot-free” rinsing, two resin-containing canisters are connected in series.  The resins contained in the canisters are often called “resin beds.”  The resin in one canister removes cations while that in the other removes anions.  An activated carbon filter is often used (also in series) to remove dissolved gasses and some organics from the processed water.   The contents of the canisters are used until their ability to trap contaminants has been depleted.  Once this has happened (as indicated by a resistivity meter or other suitable instrument) the contents of the canisters must stripped of the ions or other contaminants they have collected in a process called regeneration.  The regeneration process involves acids and other hazardous chemicals (not just salt brine like the resin used for water softening) and is usually not within the capability of most industrial cleaning facilities.  In some cases, the canisters containing the resins are returned either to the supplier or a facility that specializes in their regeneration.  There are also services that supply fresh resins and collect the spent resins for regeneration.  The resins are either transported by an agent of the regeneration facility or are especially packaged for shipping.

In applications requiring only a small amount of de-ionized water it is common to use a single de-ionizing canister containing a “mixed-bed” resin.  This resin is formulated to capture both cations and anions but, typically, can’t supply as much de-ionized water as a system with multiple canisters as described above.  Because of its ability to remove both cations and anions very effectively (although not in quantity) mixed bed exchange canisters are often employed at the point of use to “polish” water previously processed through a multiple bed system.

For reasons discussed in the preceding blog, it is best to locate the de-ionization system as close to the point of use as possible.  If de-ionized water is used in a single location in a facility, this is usually not a problem.  In larger facilities it is not uncommon to have a single pre-treatment facility with final, “polishing” process performed at the point of use as described above.

The process of de-ionization effectively removes ions but is ineffective or minimally effective at removing contaminants that are not negatively or positively charged.  Most organic material including bacteria and pyrogens are NOT removed by de-ionization.  Although their removal in many applications is not required, removal may be critical in applications in the medical industry and others.  Other means to remove these contaminants will be discussed in upcoming blogs.

–  FJF  –