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RO: Tap Water Ratios
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Jimbob
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Joined: 27 Feb 2004
Location: London, England

PostPosted: 2010.10.16(Sat)0:45    Post subject: RO: Tap Water Ratios Reply with quote

Hi,

I have a freshwater tank set up with available capacity of 170 litres that I want to stabilise at pH 7.0. I can't understand why different volumes of tap water at an RO/Tap ratio of 50:50 produces a different pH.

I live in a hard water area with water out of the cold tap reading:

pH 7.4, KH 8, Gh 10, TDS 290, Nitrate 50

I am filtering the tap water though a resin canister to remove nitrate that results in the following readings at a ratio of 50:50 RO/Tap water at 25c:

30Ltrs produces pH 7.0 KH 2, Gh 6, TDS 120, Nitrate 0

36Ltrs produces pH 7.2 KH 4, Gh 7, TDS 122, Nitrate 0

The resultant mix at 30litres is the desirable target but when I try increasing the volumes above 15ltrs pH remains at 7.2, even when increasing the volume of RO water to ratios of 40:60 and 30:70 (Tap:RO). I have taken the water samples to my LFS who confirm the readings so am confident my test kits are stable.
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diademhill
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Joined: 18 Apr 2007

PostPosted: 2010.10.16(Sat)1:20    Post subject: Reply with quote

The number don't add up.
Where is the nitrate reading form the tapwater you are mixing in?

What resin canister are you using?

With so much nitrate I would consider using full RO with supplements.
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Jimbob
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Joined: 27 Feb 2004
Location: London, England

PostPosted: 2010.10.16(Sat)1:30    Post subject: Reply with quote

Agreed my tap water is very poor quality.

I remove the nitrates by running the water through a tube containing nitrate absorbing resin resulting in zero nitrates. I have used this system for over five years resulting in tank water nitrates being no more than 10ppm if I maintain 25% water changes once a week.

I mix the filtered tap water with RO water resulting in the readings as described above. I guess you could disregard the nitrate readings as it is the pH and hardness that are of real interest to me.


What I don't get is when I mix the filtered tap water and RO water I end up with a different pH when the overall volume increases above 30ltrs.
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unissuh
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Joined: 29 Mar 2005
Location: Melbourne, Australia

PostPosted: 2010.10.16(Sat)12:59    Post subject: Reply with quote

Have you tried letting the tap water sit to let out any CO2 that could be interfering with the pH?
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Jimbob
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Joined: 27 Feb 2004
Location: London, England

PostPosted: 2010.10.16(Sat)17:42    Post subject: Reply with quote

Quote:
Have you tried letting the tap water sit


I run the tap water and RO water into a reservoir tank, add a dechlorinator then run a power head for 90mins to circulate until the heater gets the water up to temperature (25c).

I can
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unissuh
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Joined: 29 Mar 2005
Location: Melbourne, Australia

PostPosted: 2010.10.17(Sun)8:48    Post subject: Reply with quote

Honestly can't see any way that volume should make a difference if it's properly mixed & aerated. Perhaps try leaving it overnight once to see if the pH changes but thats the only suggestion I have.

The nitrate resin is one thats regen'd with saltwater right?
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Jimbob
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Joined: 27 Feb 2004
Location: London, England

PostPosted: 2010.10.18(Mon)1:25    Post subject: Reply with quote

Quote:
The nitrate resin is one thats regen'd with saltwater right?


That's the one - I drip a salt solution of 250g table salt mixed in 10litres of water through an airline so it takes about three hours to recharge.

Works like a dream and I can get 250Litres of tap water through it before having to recharge (that includes a 10litre run-off to clear the salt solution). I checked the intructions and there is no brand name, supplier or manufactoring details.

Thanks for the tip on letting the water stand longer; I will be trying that at the weekend.
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