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Dechlorinator with RO water

Alex121

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24 Feb 2021
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Hi, I am just about to start using RO water from the LFS remineralised for a shrimp tank and was wondering whether you should still add a dechlorinstor such as prime when doing this?
 

X3NiTH

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Yes I would still add dechlorinator to shop bought RO water (not as much needed as to treat neat tap water) you have no control over the production and as such you need to make it safer.

No the whole idea of ro is it strips everything out of the water

RO doesn’t strip everything you need to pass it through a last stage De-Ionisation filter to remove everything. If the TDS reads Zero then it’s as good as pure you are going to have and as such safer to use without dechlorinator.

:)
 

Alex121

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Hi, Thank you both you have summarised both sides of my thoughts exactly! The RO has a TDS of 8 so there is something in there I just thought its a shame adding in water conditioner when you have paid for water with nothing in it. But I guess better safe than sorry approach is the way to go!
 

RudeDogg1

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Yes I would still add dechlorinator to shop bought RO water (not as much needed as to treat neat tap water) you have no control over the production and as such you need to make it safer.



RO doesn’t strip everything you need to pass it through a last stage De-Ionisation filter to remove everything. If the TDS reads Zero then it’s as good as pure you are going to have and as such safer to use without dechlorinator.

:)

DI is just a final polish I get zero TDS without a DI which is overkill for fresh water anyway
 

RudeDogg1

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Hi, Thank you both you have summarised both sides of my thoughts exactly! The RO has a TDS of 8 so there is something in there I just thought its a shame adding in water conditioner when you have paid for water with nothing in it. But I guess better safe than sorry approach is the way to go!

Are you buying it already remineralised or is that meant to be plain ro? In which case they need to replace their membrane
 

MichaelJ

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9 Feb 2021
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Minnesota, USA
Hi, Thank you both you have summarised both sides of my thoughts exactly! The RO has a TDS of 8 so there is something in there I just thought its a shame adding in water conditioner when you have paid for water with nothing in it. But I guess better safe than sorry approach is the way to go!
A TDS of 8 ppm out of the RO sounds be a bit high. I don't see a reason why it would hurt to add a bit of Prime to it, or if not let the water sit (perhaps with an airstone) for 24 hours if you suspect residual chlorine.

If your RO water would have been remineralized or treated somehow I would expect the TDS to be way higher anyway, so its probably just their membrane thats getting old.

Another thing could be your storage container. Could it be that you had some residual "something" in the container that could drive up the TDS? Happened to me once when I prepared RO water in a bucket that I didn't rinse beforehand and probably had some undissolved Seachem Equilibrium at the bottom!

No its meant to be pure RO, I had read RO can read non zero TDS is that correct?
Yes that is correct... just doing the RO without DI typically yield a non-zero TDS. For instance, my current RO water is 2-3 ppm (my membrane is fairly new -- probably only made 600 US gallons with it so far) - When brand new the TDS is 1-2 ppm.
 
Last edited:

sparkyweasel

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If the RO system is designed, built, operated and maintained properly, it should de-chlorinate the incoming water. Chlorine damages the RO membrane.
If your shop RO water has TDS of 8, it may not be from a well-maintained system.
 

ian_m

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Yes you should always add something like Prime to your RO water...or test for free ammonia and free chlorine before. These test kits will normally be quite accurate as there will be little other interfering ions, to mess up the results.

The reason for these two is due to the "failure modes" of RO systems.

If your flow rate is too high or carbon pre-filter exhausted or incorrect type then it will fail to remove chlorine from incoming water, which will then destroy the RO membrane and pass out into the RO water. So dechlorinate or test.

If chloramine is being used then it gets split into chlorine and ammonia by carbon pre-filter, the chlorine normally destroying the RO membrane and passing out into the waste water, but the ammonia passes through to the RO water. So add Prime or test.

The other way is to just to pre-store your RO water for a day or two (with air stone) and any chlorine and/or ammonia will de-gas.

Or ensure your pre-filters are not exhausted. This is where an incoming volume used gauge is useful/required.
 

Wookii

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A TDS of 8ppm is perfectly normal for RO water that’s doesn’t pass though a (largely unnecessary) DI final stage.

RO membranes work on a percentage removal basis. So the output TDS is entirely dependent on the incoming TDS, so if you have a TDS on the incoming water supply of, say, 350ppm, and an output from the RO membrane of 8ppm, that’s a 97.7% efficiency, which is pretty good. The efficiency is also dependent on water temperature and pressure at the membrane, so efficiency drops at this time of year when the mains water is colder.

My RO filters achieves an output of of around 4-5ppm on an incoming main supply with an incoming TDS of 270ppm, so an efficiency of over 98% which is very good.

As for the initial query, chlorine is usually absorbed by the carbon block pre-filters. Whether the LFS sourced RO water contains chlorine will depend on how on the ball they are at maintaining and replacing those pre-filters. It will also depend on whether they have a dedicated chloramine prefilter to ensure that is absorbed if added to the water supply by the water company. I personally would always add some Prime if I couldn’t confidently answer those questions.

On my own system I don’t need to add a dechlorinator, as I know I change my carbon block prefilters (one for chlorine and one for chloramine) regularly enough to not require it.
 

mort

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Completely agree with Wookii. I don't make ro anymore but my incoming tds was 400ppm plus and if I got 8ppm out before di, I'd have been delighted.

From my time in the lfs whenever we had a customer moan the tds was too high it nearly always came down to a dirty (can look sparkling clean but that doesn't mean it is) container, or slightly off tds meter. Some shops are crap at ro because for the average freshwater tank it doesn't matter that much but if they sell marines take a look at their coral tanks and it's normally clear if they know what they are doing or not.
 

MichaelJ

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A TDS of 8ppm is perfectly normal for RO water that’s doesn’t pass though a (largely unnecessary) DI final stage.

RO membranes work on a percentage removal basis. So the output TDS is entirely dependent on the incoming TDS, so if you have a TDS on the incoming water supply of, say, 350ppm, and an output from the RO membrane of 8ppm, that’s a 97.7% efficiency, which is pretty good. The efficiency is also dependent on water temperature and pressure at the membrane, so efficiency drops at this time of year when the mains water is colder.
Interesting. I thought water pressure and temperature only affected production ratio and not the residual TDS.
In my case with water @ 8-10 C (but fairly good water pressure) I get about 2 US gallons per hour which is about half of the 100 GPD the product promises at 25C / 60 PSI, and comes out at 2-3 ppm. My TDS seems unaffected by cold temperature at least - I reckon a 98% efficiency.
 

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