How often should I change RO/DI filters?

[sam11909]

I am getting a RO/DI system soon and wanted to know how often would need to change the filters and resin. Would I need to replace all of them or just a few? Would I have to replace the carbon more often than the other filters? Also, I heard that GE made the best RO chamber, but the one I'm looking at comes with a dow filmtec RO. How do these two compare? is there a difference? Here's the one I'm looking at

http://www.bulkreefsupply.com/RO/DI-Filters-&-Systems-RO/DI-Systems/c9_10/p603/Economy-4-stage-RO/DI-unit/product_info.html

Thanks as always!

[Sisterlimonpot]

IMO an RODI unit is an RODI unit. Some have more bells and whistles than others. But to answer the filter question it all depends on how often you use it. I would (at a minimum) replace the filters once a year. And replace your di resin when your TDS go above 0. And the ro membrane when the tds after the ro gets above normal readings (depending on your local water) TDS of 10-20 (maybe less maybe more)

[sam11909]

I would only use about ten gallons a week.

[Zestay]

well your prefilters need to be changed every 6 months or so. regardless of use. ( bacteria growth )
your RO membrane needs to be replaced every 2 - 4 years.
your DI resin everytime it changes color or when your TDS reads above 0

[Agu]

Replace my prefilters every six months with filters from the big box home stores. Just bought two 2 micron carbon filters for about $12, put them both in for a filter and prefilter. Usually the DI is a color changing medium, replace it at about a 90% depletion. The RO membrane can last up to ten years. Replaced mine at the 9 year mark only because I got a deal from a person getting out of the hobby. At your usage the RO membrane should last at least ten years.

[Buckeye Hydro]

A good rule of thumb is to replace your sediment filter and carbon block after six months. A more precise way to maximize the useable life of these two filters is to use a pressure gauge to identify when pressure reaching the membrane starts to decline. This is your indication one or both of the filters is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 (“Chlorine Guzzler”) for example will remove 99% of chlorine from 20,000 gallons of tap water presented at 1 gpm. Original equipment suppliers commonly provide carbon cartridges rated at 2,000 to 6,000 gallons.

Regarding your RO membrane and DI resin, use your TDS meter to measure, record, and track the TDS (expressed in parts per million) in three places:
1. Tap water
2. After the RO but before the DI
3. After the DI.

The TDS in your tap water will likely range from about 50 ppm to upwards of 1000 parts per million (ppm). Common readings are 100 to 400 ppm. So for sake of discussion, let's say your tap water reads 400 ppm. That means that for every million parts of water, you have 400 parts of dissolved solids. How do we go about getting that TDS reading down to somewhere near zero?

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon block filter (collectively called “prefilters”) do very little to remove dissolved solids. So with your tap water at 400 ppm, you can measure the water at the “in” port on your RO housing and you'll see it is still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 100 gpd Filmtec membranes have a rejection rate of 90% (i.e., they reject 90% of the dissolved solids in feed water). So the purified water coming from your 100 gpd membrane would be about 40 ppm (a 90% reduction). Filmtec 75 gpd (and below) membranes produce less purified water (aka “permeate”), but have a higher rejection rate (96 to 98%). The life span of a RO membrane is dependant upon how much water you run through it, and how dirty the water is. Membranes can function well for a year, two years, or more. To test the membrane, measure the total dissolved solids (TDS) in the water coming in to the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce less water as their function declines.

After the RO membrane, water will flow to your DI housing. DI resin in good condition will reduce the 40 ppm water down to 0 or 1 ppm. When the DI output starts creeping up from 0 or 1 ppm to 3 ppm, 5 ppm, and higher, you know that your resin needs to be replaced. Sometimes people complain that their DI resin didn't last very long. Often the culprit is a malfunctioning RO membrane sending the DI resin “dirty” water. This will exhaust the resin quicker than would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ @
Buckeye Field Supply

[sam11909]

Thanks everyone! Buckeye, how often is average time for resin to last? my RO membrane will be a dow filtec 98% rejection rate.

[Buckeye Hydro]

It really depends upon the TDS in your RO water (which depends upon the TDS in your feedwater), and the amount of DI water you make. Something between 4 and 8 months wouldn't be unusual.

Russ @
Buckeye Field Supply

[sam11909]

Thanks!