When do you change your RO/DI filters?

[tony13]

I always use to change mine every 6 months, but recently I put a TDS meter on it and after 6 months the input and output are still reading zero. Should I change them anyways or should I wait till I get some sort of reading? What do you guys/gals do?

[ReefDent]

I change the prefilter when I see the pressure guage drop 10 psi or so. I change the carbon block every third prefilter or so. I change my first DI cartridge when the TDS is 2 ppm, and the final DI cartridge when the meter reads 1 ppm TDS.

James

[atvdave]

Originally posted by tony13
but recently I put a TDS meter on it and after 6 months the input and output are still reading zero.

Please explain this. Where is it reading 0? Your tap water input or after your membrane filter go's into your DI filter?

Need a little more info on this.

[tony13]

The TDS meter reads "in" which is before the DI filter (and after the RO unit) and "out" after the DI filter.

[APISTOBORELLI]

I change mine when the TDS out reads 3-5.

[atvdave]

Originally posted by tony13
The TDS meter reads "in" which is before the DI filter (and after the RO unit) and "out" after the DI filter.

First off. I'd change my sediment filter & my 2 Carbon block filters every 6 to 9 months depending on how much you use it. The get the iron and Chlorine out.

My RO membrane filter is dependent on my TDS readings. I have a 98% rejection rate membrane so I just do the math with my Tap TDS and my membrane TDS. My tap is around 350 TDS so if I get a TDS out of my membrane of over 8 TDS I look into changing it.

I change my DI resin when it gets to about 2 TDS or sooner.

NOW.. If I was you I'd get my TDS meter checked. even if you have a membrane with 99% rejection rate you will still have some TDS's coming out of your membrane filter. I just don't think you are actually getting 0 TDS out of your membrane.

[mg426]

I change the sediment and carbons every 6 months, the DI when I get 1 TDS.

[GrandeGixxer]

Your final TDS shouldn't have anything to do with your prefilters. The sediment filter is to do just that, remove sediments from the incoming water. The Carbon filter is to take chlorine/chloramines out of the water before it gets to the membrane(chlorine/chloramine will wreck your membrane in short order. The membrane is to take out the smaller particulates before it gets to the DI. Your TDS should be at 2-10% of your initial incoming TDS at this point. If you have a 100% flow through DI(vertical) chamber on your RO/DI and your resin is good, your final TDS should be at 0. Changing the prefilters is really just there to protect your membrane. For the sediment filter, the pressure change is a good way to measure. To know when to change the carbon, you would actually need a test kit to test for chlorine/chloramines. When they are getting through to the membrane, it is time to replace it. I change my carbon about every 2-3 sediment filters depending on if I have 1 on hand or not. This is not the best way, but it is easier than testing water and such. Hope this helps.

[BuckeyeFS]

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 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 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 then would otherwise have been the case. Sometimes the problem is poor quality resin – remember that all resins are not created equal!

Russ

[tony13]

Thanks Russ. You always explain things clearly.
I checked the tap water and my TDS is 81ppm. I have well water going thru a house water softner system. So I take it that that is on the lower side and should be happy. Again the readings are zero on the in and out of the DI filter so things are good.
Yeah I wasn't thinking about the RO membrane being protected by the prefilters and what the carbon block actually does. I'll continue to change them out every 6 months like I have been.
Always looking for a way to save some bucks but this ain't one of them.

[BuckeyeFS]

81 ppm for tap water is outstanding. However I'd be a little suspect of the ) ppm reading coming out of the RO - I suspect that in reality your RO water is no less than about 1 or 2 - but because we are talking about such low numbers, I'm really splitting hairs here. My point is that your RO membrane rejection rate might be as high as about 98%, but not 100%.

I do have a way for you to save some money. With your very low TDS, and with your softener feeding the RO system, if I were you I'd shoot for a 3:1 waste to permeate ratio rather than a 4:1 - that will yield about a 20% savings on the cost of water to feed the system.

Russ

[tony13]

What do you have in mind that I could do for changing the waste ratio?

[BuckeyeFS]

You could:
1.Use a capillary flow restrictor intended for a membrane of a lesser capacity and trim it shorter, or

2. Get an untrimmed capillary flow restrictor and trim it to length, or

3. Use an adjustable flow restrictor.

Russ

[tony13]

Never heard of any of these things. Are they on your site where I could read up about them?

[atvdave]

Here's what I use as my adjustable flow restrictor.

http://www.lowes.com/lowes/lkn?actio...4-A140/PB9106C

I just set it at 4:1 ratio. (wast water : good water)