Deionisation (DI) is a common form of purifying water. Mixed Bed Resin Filters are the most popular and far exceed the performance of the older type of cartridges. The DI resin process purifies the water by passing water through two resins that have opposite charges to remove ionised minerals and salts from the water. Simply put, DI resins attract non-water ions and replace them with water ions which will result in more pure water.
The mixed bed resin filters include cartridges and need to be replaced when the resins lose their ability to properly purify the water. The best DI resin to use is a colour changing resin that will change from a blue/green colour to a brown colour when the resin is no longer capable of properly purifying the water. This indicates that the cartridge needs to be replaced and is a good visual indicator.
The DI Cartridge Lifespan
DI removes total dissolved solids (TDS) from the water as the water passes through these DI resins. Depending on the TDS of the incoming water, the life of the cartridge will vary dramatically. You will get much greater purified water output from soft water than you would hard water.
If you are experiencing less than desired litre output from your DI cartridge, it is worthwhile checking the TDS levels of the source water. TDS levels of standard tap water will vary greatly depending on your location. For example, Melbourne (or parts of Melbourne) have a TDS level of around 40 mg/L whilst a more rural area may have a TDS level in excess of 100 mg/L. Australian Drinking Water Guidelines (ADWG) state that tap water shall not exceed 600 mg/L so it is safe to assume that tap water across Australia will vary between 0 to 600 mg/L. As such, your location will significantly impact the life of your DI cartridge.
Another factor that will reduce your litreage output is the levels of CO2 present in the source water. CO2 can decrease the life of the cartridge dramatically and can often be the cause of DI resin depleting relatively quickly. If your TDS levels of the source water are at an acceptable level and you are still finding the cartridge not lasting long, high CO2 levels in the water – it is likely the problem.
Increase the Lifespan of a DI Cartridge
Whilst in most instances, you should be able to simply connect a water deioniser to your mains water and get a good lifespan from your cartridge. If you are finding as mentioned above that your TDS or CO2 levels are quite high and dramatically affecting your cartridge life then it may be worth considering adding in an additional step to the purification process. Adding in a reverse osmosis (RO) system prior to the water passing through the DI resins will help to reduce the TDS levels of the water and also gives the water some time to gas off the excess CO2 if set up correctly. For most laboratories, however, this step is not required and only needs to be considered if you require ultrapure water or if your cartridges are depleting too quickly.