Omnipure K5555 BB 580 Grain Ion exchange resin inline water filter cartridge. 2.5" x 10" is used in both Ultra pure 6 stage reverse osmosis configurations. Both the benchtop and under bench system utilise this Ion exchange resin cartridge to reduce water TDS as close as possible to zero.
To determine the expected lifespan of this cartridge in your situation you need to calculate the following formula.
Each grain (580 grains in this particular cartridge) can absorb approximately 17.1 parts per million (PPM) of impurities per litre of water.
So if after you Reverse osmosis membrane you have a TDS reading of 10ppm then you need to multiply 17.1 x 580 = 9,918 then divide that by the TDS after the membrane to get the number of litres of pure water you could expect the K5555 Ion exchange resin cartridge to perform. If the TDS reading was 10ppm then you could expect approximately 991 litres of pure water before needing to replace this cartridge.
Generally speaking, you should plan to replace this cartridge every 6 months with regular use. It is recommended that you do not exceed 6 months usage as the Ion exchange resin has been known to dump it's impurities back into the water if left unreplaced beyond the recommended servicable life period.
It is usually installed along the 1/4" tube line (pure line from the membrane housing) immediately following the reverse osmosis membrane filter. This cartridge can reduce TDS to zero or very close to it, to give you the Ultra pure water you are after. If you plan on drinking this water it is recommended that a remineraliser cartridge and a post carbon filter cartridge are used proceeding the Ion exchange resin filter.