When to replace RO Filters?

[cdraughon]

I have had my RODI system for about 9 months now. How often should I replace the sediment, carbon and TFC filters? I just tested the output from the system (prior to DI) and am geting 84 ppm. Tap is 308. This is only about 70% efficiency, right?

Thanks, Chris

[bertoni]

Hmm, do you have a 100gpd membrane?

I wouldn't think the membrane would be bad after only 9 months. I'd just keep testing the TDS and replace the membranes when that starts to go downhill. My first membrane lasted about 2-3 years.

[cdraughon]

The membrane is 50 gpd. When should you replace the carbon and sediment filters?

[bertoni]

Every six months or when they plug up and the RO output drops. That's the guidelines I use.

What brand of RO-DI unit?

[AZDesertRat]

That sounds awful high for 9 months usage. How long did you let it run before testing and what did you use to collect the sample in. Always make sure you triple rinse your TDS meter and glass jar or sample glass in DI water when you are done with them and between samples. I always start with the raw water first, triple rinse with DI, RO output next and again triple rinse and finally the DI out put so it is as clean as possibe when I am done.
Let your water run 5 minutes or so before you test it. A 50 GPD membrane should be 98% when new and should still be somewhere better than 90% after 2 to 3 years of usage given your raw water TDS. My raw water is 550 to 800 and my membranes last 18 months or so and I replace them when they get to about 92% or 93%.
Ditto on the 6 month replacements on carbon and prefilters.

[Unresistible Blue]

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 filers is beginning to clog.

Also be cognizant of the chlorine capacity of the carbon block. The Matrikx+1 for example will remove >90% 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 ppm. Common readings are 100 to 400. So for sake of discussion, let's say your tap water reads 400. 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 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 its 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 permeate, but have a higher rejection rate (98%).

If you measure the TDS in your system after the RO membrane, and before the DI housing, you'll be able to measure the rejection rate of your membrane. A declining rejection rate is a good way to tell when your membrane needs to be replaced.

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. Usually the culprit is a malfunctioning RO membrane sending the DI resin "dirty" water. This will exhaust the resin quicker then would otherwise have been the case.

[cdraughon]

The RODI system is similar to the Captive Purity brand, 4 stage model. The carbon and sediment filters have never been replaced, and only today learned the DI needs to be replaced now. I just got a TDS meter. Maybe this "not so pure" water is the root cause of my red/brown turf algae.

I will try re-testing following your method of sterile glass. I just used a "clean" glass from the kitchen.

Thanks, Chris