Water – DI vs. RO

Preceeding blogs have described two ways of reducing the ionic content of water for use in washing and rinsing applications.  Although both reduce ionic content, the process commonly called “De-Ionization” uses a treated resin to capture ions while the “Reverse Osmosis” process uses a semi-permeable membrane.  Choosing between the two in the design of a cleaning process is not always an easy task.  This blog will discuss some of the considerations that may help in the selection of the most appropriate of the two.

Allowable Ionic Contamination –

To make water with extremely high resistivity (generally over 1 megohm) requires a resin exchange or DI water system.  If this level of purity is not required, the reverse osmosis process may be adequate.  Depending on the ionic content of the feed water and the efficiency or “rejection factor” of the membrane, several RO units may need to be used in series to achieve output water with 1 megohm resistivity.  Each progressive unit reduces the ionic content but creates its own reject stream.  The result is that it may take a LOT of water going in to produce a relatively small amount of low ionic content water using this technique.

Cost of Source Water and Waste Disposal –

A significant percentage of water used to produce water with reduced ionic content using the reverse osmosis process ends up in the reject stream.  Therefore a process using resin exchange may be preferred if source water or the disposal of waste water is expensive.  This balance may be swayed if the reject stream can be used productively in applications where ionic content is not an issue.

Closed Loop Systems –

In rinsing applications, it may be possible to re-use water indefinitely with a setup that recirculates it through a resin exchange de-ionizing system.  This scheme is particularly attractive if the incoming water supply is high in ionic content.  Re-using the water means that only the ions rinsed off the parts need to be removed in the re-circulating loop once the initial ionic content of the incoming water has been removed.  This option, of course, may not “fly” if the parts being processed add more ions than would be present in the incoming water.

Caution – Since resin exchange systems have temperature limits (some as low as 120F), a re-circulating system must either operate at a temperature below the recommended temperature for the resin or be fitted with heat exchangers to reduce the temperature of the water prior to processing through the resin bed(s).  This fact is often the deciding factor against re-circulation of rinse water.

Maintenance and Upkeep –

Ion exchange resins used in de-ionization systems require replacement with a frequency dependent on the number of ions they remove from the water being processed.  There are several ways in which this can be accomplished but all require considerable logistics.  These resins can not be rejuvenated by a simple backflush with salt water like those used for water “softening.”  In most cases, the resin canisters or collected resins are returned to a facility that specializes in rejuvenating them (a process that requires some “heavy duty” chemicals).

The membranes used in reverse osmosis systems usually have a relatively long life when used properly.  Improper use may result in their premature failure.  Membrane replacement is usually accomplished by replacing a cartridge and is similar to replacing the media in a filter.

Let there be no doubt that as important as it is in many applications, the cost of de-ionized water adds significantly to the overall cost of many cleaning operations.  Doing your homework before committing to a particular DI water system may result in considerable savings both in the initial plant and in its upkeep over the years.

–  FJF  –