Alkaline RO water? - The Planted Tank Forum
 
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post #1 of 9 (permalink) Old 05-24-2015, 02:06 PM Thread Starter
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Alkaline RO water?

I just had a whole house carbon filter (dechlorinator), a softener for half of the lines, and from the softener I had an RO unit installed. A few hours after they installed everything and after me flushing the RO unit and tank, I decided to test everything to see where to start in preparation of figuring out my holding barrel for my Amazon tank. Which I was shooting for a 6.5ph and 50ppm TDS by mixing RO and hard dechlorinated tap.

My city water to begin with is very hard, reading at least 8.4ph or above. TDS was 435ppm. Obviously the carbon filter didn't change that. From the softener it came out as 7.6ph and 445ppm TDS, which I was kind of disappointed that it wasn't acidic (but at least it's an improvement for cleaning). Now the strange part, from the softener and in to the RO, it came out as 25ppm TDS and went back up to at least 8.4ph.

I was under the impression RO produces acid/soft water? I looked a bunch of places online trying to figure this out. I ended up just being even more confused. One place mentioned that RO takes on the Ph of the water used, even though mine for some reason went up. I grabbed two cups, put an old peat moss filter in one, and left both sit over night. The one without the peat didn't change. I kind of screwed up the other, and the old peat moss (which I used for months before) turned the RO water in to tea (950ppm TDS), but at least it read acidic (not sure how acidic though).

Have I messed something up? Do I need to check something that might be wrong or add anything? Will I just have to deal with putting peat in the holding tank, and letting it acidify before pumping it in to the aquarium? If the RO TDS is still 25ppm, do I have to worry about the fact of sodium from the softener and should I either consider moving the line to use the non-softened water instead or add an DI unit? If I'm going to be adding peat (which I assume raises TDS), will I end up having to use almost all RO water and is that even safe without adding back in trace minerals (which I assume also raises TDS)?


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post #2 of 9 (permalink) Old 05-24-2015, 02:26 PM
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It seems slightly odd to attach the RO unit to the softener - I think usually you'd attach it directly to the tap water.

A TDS of 25 is a bit high for RO, you might need to flush it through more, or it could be related to the softener - I don't know what effect having that first will do.

ROs got no buffering ability at all so it will swing depending on tiny amounts of acid/alkaline - it's usually acidic because of CO2 not the source water, but I don't know what the TDS25 is made up off. You'll get a better idea of what it will be once you've buffered it.
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post #3 of 9 (permalink) Old 05-24-2015, 02:38 PM
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Good to attach the RO after the softener. RO membranes do not do so well on calcium, but can handle sodium better.

Yes, TDS will go UP when you run high GH water through a sodium exchange softener. Calcium is Ca++ and sodium is Na+. The softener adds 2 sodium ions to replace one calcium.

When you then run this water through the RO it ought to remove pretty much everything. The end result is water like tamsin says: so little buffer that it can swing either way with just the littlest trace of anything.

I think the peat was contaminated by whatever you had used it for before. Peat moss can act like an ion exchange water softener: Donating H+ to the water and removing any of several positive ions. (cations). If it has already done this and reached capacity it might be releasing these to the water. Your TDS reading seems to show that it is doing this.
Run this test again with new peat moss.

It might be that your tap water won't work to supply the minerals the water needs. But try it again, testing GH, KH, TDS and pH.
If you can get the first 3 in the right range, then filtering through peat moss ought to bring the pH more in line. If not, go ahead and use it anyway. The first 3 are more important than the pH.
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post #4 of 9 (permalink) Old 05-24-2015, 02:40 PM
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if possible hook up the ro unit directly to the tap water. how long have you had your ro unit running? was it used or brand new unit? if the membrane is bad then you will not get a good water out of that unit, maybe your unit need some filter/membrane replacement.

on my system i get 0-1ppm of tds and ph of 6.2 and my tap water is also very hard 300-400tds.

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post #5 of 9 (permalink) Old 05-24-2015, 03:36 PM Thread Starter
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As Diana mentioned, I've read several places that using softened water is less stressful on the membrane than hard water.

I flushed the RO unit and tank again. This time TDS read as 40ppm. The system I bought new, and it was an offer that a Kinetco supplier threw free in with my softener. Is perhaps this RO (Kinetco AquaKinetic with an extra large tank) unit just a bad quality unit? I do find it strange that the carbon cartridge is after the membrane, as one would think that should go between the sediment and membrane. Switching over to tap might be tricky, since it's already drilled in to a brick wall and plumbed already from the pipes.

I'll have to go pick up some new peat moss, and then I'll try playing around with it, seeing if I can get the pH/gH/kH/TDS right.
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post #6 of 9 (permalink) Old 05-25-2015, 09:39 PM
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First, the comment on TDS of 0-1 is for RODI water, and it sounds like you have RO water.

RO water should be removing 95% or better (usually better) of what's in the water, so if your TDS of incoming water is 300, expect to see around 15 TDS. Note I don't know how that plays with water softeners. But I think 25-40 is way too high.

Now for the "however". This sounds like it is a drinking water unit? Often a drinking water system may actually have a "filter" for adding mineral content back in to make the water taste better. Make sure yours doesn't.

Also, the ones with a pressure tank are not great in terms of keeping TDS low. An RO unit by its nature has relatively high TDS water come out when it first has water flow through it from any stoppage. What happens is the normally reverse osmosis process is reversed when the pressure drops by either turning off the input water, or (in some ways worse) just letting pressure build on the output line. This reversal (i.e. forward) pumps contaminants back into the filtered water. They do this on home water systems so it is always ready, feeding a tank, you take some from the tank, it puts more in, and for a home system who cares if it is 10 or 40 or 80 -- in fact some water companies selling RO water set their target around 50-80. For people that's not only pure enough, but tastes better, and doesn't corrode things as much.

In general for the cleanest water you want to run water through the RO membrane for a while before you start using water from it. Same with RODI -- it's helpful to divert RO water for the first 5-10 minutes before running it through the DI filter.

If you are dealing with a tank, that's harder, but the thing you want to find out is whether the outflow from the tank is FASTER than the RO production. If so that's good -- it means the RO membrane is not starting and stopping. If it's slower, however, that is bad -- you do not want (when making a bunch of water for your tank) to have it stop/start/stop/start/etc. It's not terrible, but the TDS content will be easily 2-5 times as high in those first minutes. But just make a bunch of water all at once and the later hours overshadows the first few minutes.

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post #7 of 9 (permalink) Old 05-26-2015, 02:24 AM
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i do get 1 tds on just RO system without using any DI. btw i hope you have your filters hooked up correctly, the membrane should be after carbon and sediment filter, first one should be sediment filter then carbon and then membrane.

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post #8 of 9 (permalink) Old 05-26-2015, 02:26 AM
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Originally Posted by happi View Post
i do get 1 tds on just RO system without using any DI. btw
That's impressive. Don't take this the wrong way, but are you sure your TDS meter is accurate? Or do you have particularly clean water feeding it?

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post #9 of 9 (permalink) Old 05-26-2015, 03:27 AM
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am 100% certain its accurate, i get 0 tds if i use DI with it, otherwise i do get 1 tds, it does go up if filters started to get old. hard water at 300+ tds go right through it.

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