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Contents:
Sodium/Chloride ("A. J. deLange")
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Date: Tue, 25 May 2010 09:41:21 -0400
From: "A. J. deLange" <ajdel at cox.net>
Subject: Sodium/Chloride
Dave asked whether there is an easy way to remove sodium and chloride
ions from softened water. Easy? Relatively. Inexpensive? Depends on your
definition but RO potentially qualifies. One can buy an 11 gallon per
day RO system from Home Depot for $279 (and lots of other
manufacturers, stores, websites... list lower priced units of around or
even greater than that capacity). But why would you want to use a
softener in the first place? It doesn't "remove" anything but rather
only replaces polyvalent ions (calcium, magnesium, iron, strontium...)
with equivalent amounts of (monovalent) sodium. Chloride is unaffected
(except perhaps for minimal amounts from residual brine) as are
bicarbonate, sulfate, nitrate.... If your goal is demineralized (or
largely demineralized) water the task is of the same magnitude at the
output of the softener as it would be if the softener were not used. Why
not just feed an RO unit directly from the mains? It turns out there is
a reason. If the water is hard (and we would'nt be having this
discussion if it weren't) there will be a gradient of calcium ions
concentration perpendicular to the membrane surface increasing in the
direction of the membrane. If the hardness is temporary (carbonate)
there will be a carbonate ion concentration gradient as well and,
depending on pH, hardness, alkalinity, and recovery rate the
concentrations of calcium and carbonate ions can exceed the solubility
product close to the membrane surface with resultant precipitation and
blockage. If the calcium has been removed by a softener then obviously
this problem is solved (and it will be some other salt which will limit
allowable recovery such as barium sulfate even though your water, we
hope, contains only miniscule amounts of barium).
So there is a reason to use a softener with an RO unit and, IMO, that's
the only job for a softener in a brewery (except perhaps boiler feed).
That said, if you go to HD, buy an RO unit and just hook it up to the
mains you will probably be fine. I had two of these units in service
(brewing only) for 4 years in one case and 5 years in the other on
nominally hard water (about 110 ppm as CaCO3 - mostly temporary) and
never changed out the membranes because I didn't have to. I'm on a well
so my pH is low (that helps) and these GE units have low, fixed recovery
(that also helps) of about 17%. On checking the GE manual for the
recovery I found that they too recommend a softener if you hardness is
greater than 10 gpg (about 179 ppm) and your pH > 7. That increases to
about twice that level of allowable hardness at pH 6.7; three times that
a pH 6.3 and so on. The manual also says the consequences of not using a
softener above these levels is shortened membrane life. In the system
which replaced those small units I feed softened water and feel quite
comfortable operating it at a recovery of around 40% (which should make
all you environmental types feel all warm and fuzzy inside).
I'm not sure I understand why Martin would favor nanofiltration over RO
for the likes of us. They will block the polyvalent ions (thus softening
the water and decarbonating it, getting rid of sulfate and nitrate) but
let the monovalents through so if deionized water is the goal additional
processing would be required to get rid of the monovalents. I guess they
are easier to clean but as I noted above I have never had to clean an RO
unit. On an industrial scale I'm sure things are dramatically different
and the processing methods chosen are determined by the particulars of
the feed water chemistry, the requirements for the processed water and a
whole lot of other considerations which include not only the capital
costs (which he mentioned as being high) but operating costs,
concentrate disposal and on and on. I know nanofiltration is becoming
popular among water authorities these days but don't see it for
homebrewers. But really, I know almost nothing about nano systems.
A.J.
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End of HOMEBREW Digest #5684, 05/25/10
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