Cartridge Logic for Q Series RO Units

In a reverse osmosis unit, a prefilter is a filter that comes before the membrane. Its function is to prepare the water so that the membrane will be protected and to remove chemical contaminants. The postfilter follows the membrane and the storage tank. Its function is to remove any remaining chemical contaminants and to give the water a final taste/odor polish.

Standard Cartridge and Membrane Setup for Q 363 and Q 364 Units

Unit Prefilter 1 Prefilter 2 RO Membrane Postfilter
Q363 None CTO Carbon Block (FC901) 36 gallon per day (MM036) Granular Coconut Shell Carbon Cartridge (FC902)
Q364 5 Micron Sediment Cartridge (FC904) CTO Carbon Block (FC901) 36 gallon per day (MM036) Granular Coconut Shell Carbon Cartridge (FC902)

The setup above is what's in the unit when you purchase it new. The original sediment, if any, and carbon cartridges should be replaced after each year of service. The RO Membrane will normally last several years. The best way to know if the membrane needs replacement is to test it with a TDS Tester — replace the membrane when the TDS of the RO water measures to more than 15% to 20% of the tap water TDS. If you don't have a TDS tester, replace the membrane after no more than 3 years service.

When you change the filter cartridges, you have some options with the carbon cartridges.

Here's the issue:

FC902, the standard postfilter, puts out a heavy dose of "fines," or carbon particles, when water goes through it for the first time. FC901, the carbon block prefilter, is acid pre-washed so that it does not put out fines. For this reason a brand new FC902 coconut shell filter is a terrible prefilter. When it starts up, it can blow a large amount of carbon debris into the membrane, damaging the membrane and stopping up flow restrictors and check valves downstream. The FC901 carbon block puts out nothing that threatens the membrane, so it makes an excellent prefilter. Your unit comes new with a FC901, so it's ready to go.

The coconut shell carbon postfilter, however, after a year of use as a postfilter, is well rinsed and can be safely used as a prefilter. Not only is it rinsed well, but it's almost new in terms of filtration capacity. Since the prefilter has removed virtually all the chlorine and the membrane has stripped the water of contaminants in general, the year-old postfilter has processed only extremely clean water. Moreover, since it doesn't have to process the unit's rinse water as does the prefilter, it has treated only a few hundred gallons of very clean water. The prefilter, by contrast, has treated at least 5 times as much raw tap water and is ready to be replaced.

This is definitely a place where recycling can work to your advantage. Here's how:

To recycle the carbon post filter, remove the year-old prefilter and discard it. Then move the year-old carbon post filter into the prefilter location, and install a new FC902 post filter. The following year, repeat the procedure. You'll always have a well rinsed prefilter and a new postfilter.

On four stage units, the sediment prefilter must always be replaced each year. It does not interchange with the other filters and can't be recycled.

If you prefer to replace all the filters with new filters each year, we also offer Complete Replacement packs. Both packs include complete service instructions and the filters you need.

Cartridge packs for these units, as well as individual cartridges, can be ordered from our Q Series Cartridge page.

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