Is zero tds possible with RO only?

lastlight · #1 11-02-2009, 03:16 PM

I'm in Calgary btw. My dual stage meter shows the source water to be about 280 (though I just dipped the inline probe in a glass of water to avoid making more cuts in my pipes to make sure the meter showd anything other than 0). I'm getting a reading of 0 without running DI. The unit has 5 micron sediment, 5 micron MATRIKX CTO/2 carbon and then dow filmtec 75 gpd membrane.

With 98% rejection rate I was expecting around 280 x 0.02 = 6 or so

When I first set things up btw I was getting a reading of 1 after the RO but after 10 min or so it read 0.

sphelps · #2 11-02-2009, 03:28 PM

Yeah it's possible the DI actually does very little, will usually drop the TDS by around 10ppm at most. I've setup RO systems in Calgary before and got 0 without the use of DI.

280 does seem awfully high for Calgary though, but it could just be your location. It likely depends on what kind of solids you're dealing with, perhaps the pre-filters are doing a lot of the work prior to it hitting the RO membrane. It would be interesting to compare TDS before and after the pre-filterers.

mike31154 · #3 11-02-2009, 03:30 PM

Hmm, my source water shows a TDS in the mid 200's as well and my system is similar to yours, 75 GPD. RO TDS gets down to 1, occasionally 2, but I've never seen it at 0. Source water comes from a local lake and it's considered somewhat 'hard' due to the makeup. I've checked my source water both with the Dual TDS meter and a cheap handheld. There's a discrepancy of about 30 to 40 with the Dual meter having a higher reading than the handheld for source water TDS. I'm inclined to put more trust in the Dual meter, since I used both probes and they were within 1 TDS in their readings.

Did you check the source water with both probes? If not, might be worthwhile as a comparison and to instill confidence in the reading, although an accurate reading at a TDS of 280 does not necessarily translate into an accurate reading at the lower end of the scale. That certainly seems to be a concern with refractometers in many posts & threads. Pretty sure the Dual TDS meters have a calibration adjustment on them, so it may be an idea to look into that.

Or, maybe the makeup of your source water is such that the solids are readily separated, who knows. Hopefully some fellow Calgarians will check in with their experiences.

Lance · #4 11-02-2009, 03:44 PM

Brett, I always get readings of 0 to 1 ppm using R/0 only. Mind you our water is pretty good; 16 to 20 right out of the tap.

lastlight · #5 11-02-2009, 03:55 PM

Whoa 16 to 20! Your membrane will last a lifetime lol.

The meter I think is much more acurate with water flowing past the probe contacts. Like I said I only dipped the probe into a glass of tapwater so there was no flow. The unit is not quick conenct fittings so I *really* didn't want to bother with making another cut and then redoing the connection later.

I'll Continue as is for now since it seems that things are ok. I'm sure these meters are not THAT accurate but whatever I'm getting out is very low in tds compared to the source. Good enough right now I need SALTWATER!

Lance · #6 11-02-2009, 04:00 PM

Quote:

Originally Posted by lastlight

Good enough right now I need SALTWATER!

YES you/we do!

lastlight · #7 11-02-2009, 04:12 PM

Re: water.

I'm at 5" right now. My wife tells me that's not gonna cut it.

ONWARD!

Lance · #8 11-02-2009, 04:18 PM

[quote=lastlight;460267]

I'm at 5" right now. My wife tells me that's not gonna cut it.

Hehe Does she wear glasses? If so, switch out the lenses to macro.

JDigital · #9 11-02-2009, 04:21 PM

^^ She doesn't like the shallow look?

As for the question.. up here in the NW, I'm only running RO, and as of today, I am getting 130 IN... 0 OUT... that's on a 7-8month old membrane.

Delphinus · #10 11-02-2009, 04:22 PM

Looks like you have your answer already but since this is a topic I feel I have a tiny bit of knowledge on, I'm compelled to post anyhow.

Yes 0 TDS is possible in Calgary water with RO alone, but typically it's better still to use the DI because there tends to be TDS surges and the DI will catch those. It depends on the membrane, some brands have better rejection rates than others. With the membrane I have right now, the best I can get after a 20 minute flush is about 3-4 TDS (it will go down to zero after about an hour or so). The membrane I had before had such a great rejection rate that it would be zero TDS within a 2 minute flush. I've kept my old one so I can remember what kind to order when I've had enough of this one.

Keep an eye on your RO output TDS, when it stops going below 10 on a 5 minute flush it's pretty much time to let it go. Expect to replace them around 12 to 18 months around here.

If you don't have a flush kit, get one.

And and whatever you do, even if you have a auto-shutoff on your RO, don't hook it up to a float valve or float switch in your tank, because constant on/off's are the fastest way to kill a membrane. They are designed for long runs, so turn it on, flush it for 2-5 minutes or whatever, then let it run for say 12 hours. Shut it off and leave it off until you need more again.

The ONLY thing those shutoff kits should be used for is preventing overfills. As soon as whatever it is you're filling is full, turn off the source water.