Author Topic: Should salt be getting salt through my RO filter?  (Read 464 times)

Offline Joe_Beer

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Should salt be getting salt through my RO filter?
« on: February 13, 2022, 06:01:08 am »
I picked up an RO filter from Amazon and have it hooked up to softened water(pic here). The filter was only ~$30 bucks and I was a bit skeptical due to the price so I added the TDS meter to check output periodically. I've noticed the TDS reading bounces around sometimes but the display doesn't stay on constantly so not able to get a feel if it's a spike or gradual or what. if I'm filling jugs, and turn off the water while changing jugs, the TDS reading will start out around 30 or 20 and gradully drop to 1 or 2 over the course of about a minute. I was curious so put about 8oz of water in a dish and let it evaporate over the course of about 5 days. I don't remeber if the sample was from a high or low TDS reading (I'm running the test again with 0 on the meter this time) This is what was left behind. Definintely salty. Is this RO filter defective or is this typical output you would expect from softened water? I put the filter on the softened water because somewhere I read that hard water (you can almost taste the gravel in the tap here) will destroy the membrane pretty quickly.

 

Offline hopfenundmalz

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Re: Should salt be getting salt through my RO filter?
« Reply #1 on: February 13, 2022, 07:15:05 am »
What is the TDS of the feed water? An RO filter removes about 98-99% of the dissolved minerals.

I've never tested the TDS at start up, but measure in my 5 gallon jugs after those are filled. My results are <10ppm. Would that leave a film? Probably.

Edit $30 seems to good to be true.
« Last Edit: February 13, 2022, 07:16:57 am by hopfenundmalz »
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Offline mabrungard

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Re: Should salt be getting salt through my RO filter?
« Reply #2 on: February 13, 2022, 08:35:53 am »
Mono-valent ions do pass through a RO membrane at a higher rate than di- and poly-valent ions. If the feed water is ion-exchange softened water, that further boosts the likelihood that sodium or potassium (both mono-valent) will end up in the treated RO water.

Those mono-valent ions pass a typical RO membrane at up to about 4 to 5 percent of their feed water concentrations (95 to 96 percent removal). For my softened water with over 300 ppm sodium, my treated RO water might have 10 to 15 ppm sodium in it in normal operation.

I notice that my in-line TDS meter reports between 5 and 25 ppm routinely on a day to day basis. But the quality does usually settle down to the lower end with operation. Don’t worry too much about brief TDS excursions, but do be ready to replace the membrane if TDS jumps into the +50ppm range consistently.
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Offline MDL

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Re: Should salt be getting salt through my RO filter?
« Reply #3 on: February 13, 2022, 10:58:31 am »
TDS bleeds through the membrane when the unit isn’t running. There will be a spike on startup and as the membrane rinses it’ll come back down. I run mine to waste for several minutes before collecting product water because the unit sits for weeks between use.

There are auto flush setups that flood the membrane with product water before shutdown. This helps reduce tds creep and prolong membrane life. Likely overkill for most situations.

Offline hopfenundmalz

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Re: Should salt be getting salt through my RO filter?
« Reply #4 on: February 14, 2022, 06:57:55 am »
TDS bleeds through the membrane when the unit isn’t running. There will be a spike on startup and as the membrane rinses it’ll come back down. I run mine to waste for several minutes before collecting product water because the unit sits for weeks between use.

There are auto flush setups that flood the membrane with product water before shutdown. This helps reduce tds creep and prolong membrane life. Likely overkill for most situations.

My RO System has a Fast Flush set up, recommended to run it for 3-5 minutes before making product water. Now I know why.
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Offline Joe_Beer

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Re: Should salt be getting salt through my RO filter?
« Reply #5 on: February 15, 2022, 04:07:30 am »
Thanks for the input everyone. Mine goes weeks without running as well. I do run it up for a minute or two every couple days to keep things wet. I don't know if it matters, just thought it made sense for the RO membrane anyway. Good to know that the TDS output spike is common on startup.

I just checked the TDS on the feed and it went up to 418 (salt pellet softener). The output side dropped from 25 to 1 after about a minute of running.

This membrane looks like the same membrane used in other systems costing 3-5 times as much but yeah, $30 seems sketchy. The output TDS does get down to zero though, so it must be working? Also, I'm not sure what the TDS was when I pulled that first sample. It could have been on startup when output TDS was in the 25ppm range.

Offline BrewNerd

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Re: Should salt be getting salt through my RO filter?
« Reply #6 on: February 24, 2022, 04:52:37 am »
TDS bleeds through the membrane when the unit isn’t running. There will be a spike on startup and as the membrane rinses it’ll come back down. I run mine to waste for several minutes before collecting product water because the unit sits for weeks between use.

There are auto flush setups that flood the membrane with product water before shutdown. This helps reduce tds creep and prolong membrane life. Likely overkill for most situations.

My RO System has a Fast Flush set up, recommended to run it for 3-5 minutes before making product water. Now I know why.

Slightly disappointed that an RO system doesn't always remove everything. I'll write a letter to the internet to have them correct the error.

Also glad to know that there's a way to compensate for things creeping through the membrane/ filter.

Offline Joe_Beer

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Re: Should salt be getting salt through my RO filter?
« Reply #7 on: February 26, 2022, 05:08:19 am »
The second (8oz) sample I pulled last week at 0 TDS finally evaporated off and left behind next to nothing. To split hairs, there is a slight film on the glass but needs to be at just the right angle to see it. It could be from whatever settled in the water during the week it was drying too. The picture is a little deceiving due to the scratches and hard water film on the outside of the glass but including it anyway for comparison. It's quite a difference from the first sample which I suspect I pulled when the TDS was in the 20s. Seems to be working well enough for me. As someone else mentioned in this thread, a TDS meter can come in super handy for brewing with RO.


 
« Last Edit: February 26, 2022, 05:12:07 am by Joe_Beer »