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Subject: Water purification

Water purification

From: Geoffrey I. Brown <geoffrey.i.brown>
Date: Friday, November 11, 1994
Although there are several different types of water treatment, they all
produce different products, i.e. they vary in what impurities are
removed and how effectively they are removed.  They all vary in cost to
install, but most importantly, they vary in cost to operate and repair
or replace.

Distilled water requires expensive equipment up front, with a high
replacement/repair cost and high energy costs to operate.  Distilled
water is quite effective on removal of organics but relatively poor in
removal of inorganics.  The latter are typically what causes problems
with solution mixing, cleaning, or causing stains.  Consequently, I do
not recommend Distilled water for conservation installations.

Deionized water has reverse capabilities.  It can be extremely effective
at removal of inorganics (salts, metallic ions) but is less effective at
removal of organic compounds.  The basic process usually utilizes a
series of filters and resin tanks.  A carbon tank can be added to help
with removal of organics and a UV sterilization cell can be incorporated
to kill microorganisms.  The cost of operation depends on local costs
for exchange resin tanks which are typically provided on a "rental"
basis.  Flow rates can be very high if you select large tanks for the
system.  Particulate and carbon filters are disposable so check the
prices, the carbon can be expensive.  Installation costs are typically
low to moderate, depending on what type of monitoring equipment you
select and what your flow rate needs to be.  Opt for a lab-type system
that will give you about 3 megohms resistance but don't go for any
greater purity because ultra-pure water is very corrosive--it is
ravenously ion-hungry.

Reverse osmosis has become popular in the last 10 years, due to
decreasing costs and increased reliability.  Installation costs are
high, especially for high flow-rate systems.  RO uses particulate
filters and sometimes carbon filters as well as the pump and membrane.
As a process, it can be very effective at removal of both organics and
inorganics.  Operation costs can be moderate to high in both electricity
and water use, as the impurities filtered out by the RO membrane are
removed with a stream of water that operates more or less continually.
The membranes require replacement periodically, with a working life of
about 3 years.  Replacement costs for the membrane can be extremely
high, depending on the size of the system and the manufacturer.

In general, for typical conservation needs, both RO and Deionization can
produce excellent quality water.  RO tends to be significantly more
expensive to install and operate.  Deionization systems can get
expensive if many stages are required to deal with the local water.
Reasonably high-purity water has a pH of about 6.7, so some paper
conservators install a filter can filled with marble-chips in the
treated water line to raise the pH a bit.  It makes the most sense to
split the line and have two faucets--one with unmodified treated water
for solution mixing and other needs, and the other for higher pH water
for washing.

    **** Moderator's comments: The marble chips are used not to raise
    the pH but to 'backfill' the water with (calcium) ions, in an
    attempt to slake the ion-hunger discussed above. (See Tang and
    Jones, "The Effects of Wash Water Quality on the Aging
    Characteristics of Paper JAIC 18, No. 2, Spring 1979, pp. 61-81)

I tend to favor Deionization because it is lower "tech" and can be
monitored and maintained easily by in-house people.  Maintenance costs
are usually fairly predictable.  In most applications, control of
organics is probably not necessary, reducing costs to a great degree.
Remember that you may have funds now in a construction budget, but you
may not have the same level of operating funds later.

Also be sure that all piping, valves, faucets and other devices in the
water stream are PVC or stainless steel.  High quality water will
extract ions from any other materials.  "Season" the plumbing system by
running the treated water through it for 1 or 3 weeks and figure the
cost of doing this as part of the installation.  It is not uncommon to
see a good quality water system compromised by the use of a brass faucet
at the sink.  Don't be afraid of PVC as it has proved to be very stable
in this application.

Feel free to call or message me directly to discuss these options.

Geoffrey Brown
Curator of Conservation
Kelsey Museum
University of Michigan

                  Conservation DistList Instance 8:37
                 Distributed: Sunday, November 13, 1994
                        Message Id: cdl-8-37-005
Received on Friday, 11 November, 1994

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