Water purification

Winston Atkins <winston.atkins [at] duke__edu> writes

>Water Filtration for Conservation Lab
>
>Duke University Libraries is planning a new book and paper
>conservation lab and one of the issues we need to address with our
>architects is how to spec out the water filtration system. I'd
>appreciate advice from you all regarding:
>
>    *   merits of deionization vs reverse osmosis,
>
>    *   volume (gals/minute) to request for a large washing sink
>
>    *   degree of purity (see paragraph below--our architects'
>        consultants tell us that getting 3 megohms resistance will
>        be difficult; RO will give us 0.5 to 1.0 megohm resistance,
>        putting in DI beds will give us up around 18 megohm)
>
>    *   issues of cost (initial purchase and ongoing maintenance)
>
>    *   ease of maintenance
>
>I've scouted the Cons DistList for information and have a 1994
>exchange, which includes a recommendation not to go for greater
>purity than 3 megohms--too corrosive. I've also reviewed Season
>Tse's CCI publication on water quality. Unhappily, it is not
>prescriptive enough for me. So--I'm going public and would
>appreciate any thoughts or guidance you all can provide.

The British Library has recently gone through this process in
designing the studios in our new Centre for Conservation, so I can
share our conclusions with you.

The flow rate you need will be determined by the size of your sink
and the amount of work you intend to put through.  Remember that the
water purifier doesn't have to produce water at the same rate as you
use it, since it can fill up a storage tank overnight.  We have
specified units capable of producing 6 litres per hour, which feed
into 100 litre storage tanks, to serve sinks which are approximately
60 x 75cm and 10cm deep.

I don't know what the quality of the tap water is like in North
Carolina, but in London it is very hard and heavily chlorinated. The
units we specified have a prefilter to remove particulates and most
of the chlorine, a reverse osmosis unit to remove the bulk of the
soluble salts, and a deioniser unit to remove the remaining organic
and inorganic ionic impurities.  You will need to take advice on
what is appropriate for your feedwater.

You should resist the temptation to over-specify the degree of
purity you want to attain.  The main criteria for paper conservation
are that the water should be free of particulates (scale etc), free
of metal ions, and free of chlorine.  Water conforming to ASTM
D1193-06 Type III should be adequate--this is suitable for
non-sterile analytical work. You do not need the same degree of
purity as an analytical or genetic research lab.  My experience in
chemistry taught me that you cannot reach a resistance greater than
10 megohms except in a closed system with continuous nitrogen
flushing.  An open bath of water will rapidly absorb carbon dioxide
from the air, the pH will drop to around 6 and the resistance will
drop to less than 1 megohm, no matter how pure it was to start with.

The units we purchased were Purite Analyst 40, however, I do not
know whether these are available in the US.  The costs of purchase
and maintenance will obviously be different from the UK.

Dr Barry Knight
Head of Conservation Research
The British Library
96 Euston Road
London NW1 2DB
+44 20 7412 7229
Fax: +44 20 7412 7658


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                  Conservation DistList Instance 21:3
                  Distributed: Thursday, May 10, 2007
                        Message Id: cdl-21-3-003
                                  ***