MARLO MATD 9500 XT, Manual

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Page
6
DEALKALIZATION
Alkalinity is a measure of a waters ability to neutralize an acid. There are three
components to alkalinity including, bicarbonate (HCO
3
), carbonate (CO
3
) and hydroxide (OH).
The existence of these ions is pH dependent. HCO
3
will be the predominant constituent of
alkalinity at a pH of 4.3 to 8.3. The CO
3
ion will begin to form at a pH greater than 8.3. At a pH
of approximately 1O the OH ion will start to exist.
The relationship between the components of alkalinity and pH allows for the accuracy of
the water analysis to be verified. If water has a pH of 9.0 and only the bicarbonate alkalinity was
reported on the analysis, the analytical results are deficient. Carbonate alkalinity should have
also been reported on the analysis. In order to eliminate the potential for error in the water
analysis a total alkalinity test can be performed. Total alkalinity is run by testing for P and M
alkalinity. P alkalinity or phenolphthalein endpoint is a measure of the amount of acid required
to neutralize the alkalinity down to a pH of approximately 8.3. M alkalinity or methyl orange
endpoint is a measure of the amount of acid required to neutralize the alkalinity down to pH of
approximately 4.3. The actual concentrations of HCO
3
, CO
3,
and OH are derived from
calculations using the P and M alkalinity; see Standard Methods Edition 17 Method 2320 B.
The most common question asked is why when caustic (NaOH) is added to acidic water
and the pH is elevated to 8 that no OH alkalinity exists? The answer goes back to pH. No OH
alkalinity can exist at this pH. The OH will react with CO
2
to form bicarbonate, at this pH.
If water’s alkalinity concentration becomes too high, it can have a bitter taste. Anion ion
exchange resin can be used to remove alkalinity, thereby reducing the pH and bitter taste
associated with high concentrations of alkalinity. If the alkalinity concentration in the influent
water is high, the replacement of alkalinity with chloride may result in a salty taste. The salty
taste may be noticed at levels of 600 to 800 ppm TDS. An R/O system should be used to
eliminate the salty taste.
Alkalinity is also a problem with boiler feed water. The bicarbonate and/or carbonate ions
can be broken down to form OH and CO
2
when water is converted to steam in the boiler. The
OH ion remains in the water while the steam contains CO
2
. When this steam condenses the
CO, forms carbonic acid (H
2
CO
3
), which will attack most metals, it will shorten the life of the
condensate return system. For this reason dealkalization of water used in boilers may be
required, depending on the conditions of operation. Consult your local water doctor.
A type II anion resin is generally used to dealkalize water. There are two types of
regeneration. The first utilizes only salt, the second uses a combination of salt and caustic
(NaOH). If salt is used, the hardness in the influent water should be less than 10 grains / gallon
to prevent the precipitation of CaCO
3
. In most cases, where dealkalization is necessary, the
hardness will have to be removed, since scaling may occur. If salt and caustic are used, the
water that is fed into the dealkalizer, as well as the water used to regenerate it, should be soft.
The biggest difference between the two regeneration processes is that the salt and caustic will
achieve higher capacities than salt alone.