I often receive a phone call with the following request: “I need a price for a DI water system.” Simple enough, right! Well, not nearly so. Sometimes to the frustration of the caller, I come back with a list of questions that the caller is not prepared to answer. Not unlike any other water treatment application or technology, deionization or DI, has its inherent limitations, based on the water supply being treated, the quality of water needed as an end product and the volume of that water required for the application or process. More times than not, what the caller needs is different from what he or she originally considered.
DI water systems typically consist of beds (tanks or cartridges) comprised of cation and anion resins. DI systems produce various endpoint water qualities at various flow rates, depending on the configuration of the system, therefore, each application must be considered independently. Most higher volume DI systems consist of components to pretreat the water like water softeners or anti-scalants, carbon filters and reverse osmosis.
For low volume/low quality commercial and industrial applications requiring 50K-Ohm to 2 Meg Ohm resistivity water, cartridge systems can prove cost effective. DI cartridges have a finite capacity, based on cartridge size, to perform the ion exchange before becoming exhausted and requiring replaced. These are the simplest of systems, as they are flow-through systems requiring little more than a line in and line out for installation, along with a chlorine reduction pre-filter (carbon).
For higher volume and higher purity DI water applications (like laboratory grade water), it is typically recommended to pretreat the water via reverse osmosis (RO/DI). In doing so, the reverse osmosis unit will perform the bulk of the total dissolved solid (TDS) reduction, allowing the DI cartridges to polish the low TDS water. The resulting benefits are higher water purity (2 to 10+ Meg Ohm) and longer DI cartridge runtime, resulting in lower consumable costs.
For ultra-high purity critical applications, which require up to 18.2 Meg Ohm resistivity water, additional system components will be required. These can include recirculation loop/pump, UV sterilization/TOC reduction, along with additional final filtration and a more highly sophisticated monitoring system. A typical high-purity system may look like this Purelab Flex system from Elgo (Veolia). Such a system requires a combination of pretreatment, reverse osmosis and DI to meet the water specification. These systems can be self-contained in a bench-top system or floor-standing, based on the application and volume.
Remember, if high purity water is critical for your process or application, one size does not fit all!
What is the supply water quality?
What is the application specific required water quality?
What is the demand? Gallons/Liters per minute and Gallons/Liters per day?