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2.4 Ion exchange softening (Water for coffee)


This article is from the Water for coffee FAQ, by Jim Schulman with numerous contributions by others.

2.4 Ion exchange softening (Water for coffee)

Ion exchange runs the water through a polystyrene resin or
xeolite bed saturated with sodium and/or potassium chloride. The resin
bed is usually cationic, that is, formulated so that highly solvent
univalent cations (sodium or potassium) replace the less solvent
multivalent ones in the water (calcium, magnesium, iron, lead and
other heavy metals). This version reduces hardness without reducing
alkalinity. Alternatively, an anion resin will replace the bivalent
bicarbonates with more soluble univalent hydroxide, chloride or
sulphate ions. This version reduces alkalinity without reducing
hardness or heavy metals. Dual systems that produce completely
softened water are also available.
The amounts of sodium or potassium introduced by cation
softeners are below the taste threshold, and are not a health factor
at coffee consumption levels. For each 100 mg/l CaCO3 hardness
removed, 46 mg/l sodium or 78 mg/l potassium are added. However,
sodium restricted heart patients are sometimes advised not to use
sodium exchangers for drinking water. In no case should cation ion
exchange water be used to water plants, since sodium and potassium
accumulate in soil.
Plumbed in, self recharging ion exchange systems generally
reduce hardness (and/or alkalinity) to below 20 mg/l. They are cheaper
than RO systems of the same capacity and require less maintenance. To
get the same performance from pourover espresso machine hose units,
the filters have to be recharged regularly.
Both Brita and Pur jug filters have ion exchangers designed to
eliminate heavy metals, but which also remove some hardness as a side
effect. They are of a non-rechargeable variety that add hydrogen,
rather than sodium, to the water. The water emerges slightly and
temporarily acidic. The Claris filters in Jura/Capresso machines
apparently also work this way. They do not completely remove hardness,
rather reduce it by 33% to 67%, depending on the age of the filter.
Neither company's water tap filters use ion exchange, instead relying
on a catalyst layer to remove heavy metals. They have no effect on
It proved impossible to get accurate information on
performance of either the Brita or the (generic?) hose end softener
sold with Sylvias. So I decided to do my own testing. Here are the
BRITA JUG FILTER I tested the filter new, and after 3
weeks, or 75 liters use (about half the recommended cartridge life) on
150mg/l hardness, 100 mg/l alkalinity water. As expected, the filter
left the alkalinity unaffected. Only about 2/3 of the hardness was
removed when the filter was a few days old, and its performance
dropped to 1/3 removal after a three weeks. This probably is due to it
being designed to remove heavy metals, rather than calcium. The NSA's
refusal to allow Brita to advertise in the U.S. as a water softener is
therefore not just bureaucratic gobbledygook, but justified by the
facts. I have not tested the Pur jug filter or the Claris system; the
identical design principal probably results in similar performance.
RANCILLIO SYLVIA SOFTENER This is the small softener
sold along with the Sylvia that fits on the intake hose end. The
softeners sold with other machines look identical, so this may be a
generic product. I was told to recharge mine weekly with a few
tablespoons of pure salt in a highball glass. The test result is based
on this procedure. This softener also does not completely eliminate
hardness. In my case, it reduces hardness from 150 mg/l to about 50
mg/l. The performance is about equal to a new Brita jug filter. Weekly
recharging keeps it at this level. At this level of softening, my
water still generates scale, but at about half the rate of unsoftened
water. The softening creates close to zero LI water at coffee brewing
temperatures, albeit with higher alkalinity than hardness. The
combination of a new Brita and hose end did reduce the water to a 20
mg/l nonscaling level. All in all, neither the Brita or the hose end
unit will produce boiler safe water either alone or in combination
unless your tap water is already fairly soft.
It should be noted that the performance of ion exchange
softeners is affected by temperature. In particular, their performance
degenerates when the water is above 15C. Since the water in a pourover
semicommercial machine is usually closer to 35C, this may have
affected the test results.


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