Page 188 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.24 CHAPTER FOUR

TABLE 4.7 Typical Cations and Anions Found in Water

Cations Anions
Calcium Carbonates
Magnesium Bicarbonates
Sodium Sulfates
Potassium Chlorides
Iron Nitrates
Manganese Silica

for the removal of ions to produce pure water, it is often referred to as deionization or
demineralization.
The deionization/demineralization process uses different types of resin to exchange first
anions and then cations, which will result in the removal of all ions from feedwater when
carried to completion. When all of the ionic components devolved in water are removed by
ion exchange, the water is said to be deionized or demineralized. The ion exchange process
is also used to remove dissolved inorganics. Water softening only exchanges some types of
ions for others less detrimental for the intended end use of the water. Table 4.7 lists the com-
mon anions and cations typically found in water. Ion exchange will not remove significant
amounts of organics, bacteria, pyrogens, or turbidity.

Regenerable Ion Exchange
Regenerable ion exchange is a batch process where ions in raw water are transferred onto a
resin medium in exchange for other ions bonded to that medium as the raw water percolates
through them. This is accomplished by having the ions in the raw water adsorbed onto a
bed of exchange resins and replaced with an equivalent amount of another ion of the same
charge. This action continues until the medium has reached its exchange capacity, where it
is no longer capable of exchanging ions. Water softening and deionization are the two most
common ion exchange processes.
There are two general types of deionizers, working and polishing. The working type is
used for the initial removal of the bulk of ions from feedwater or as an ion exchange process
alone (such as hardness removal) if the purification is a single process. A polishing type is
used to purify feedwater after an initial run through a working ion exchange system.

Resins
Resin exchange media include natural inorganic aluminum silicates (sometimes called
zeolites or greensands), bentonite clay, synthetic gelatinous resins, and synthetic organic
resins. Most processes use the synthetic resins. Resins are graded by purity and consistency
of bead size.
Resin is manufactured in the form of a large number of spherical beads, typically about
0.4-mm diameter. These beads have weakly bonded ions present on their surface that are
used for the exchange process. Because the process must exchange ions of the same charge,
ion exchange resins are composed of either anion or cation exchange resins. Manufacturers
are constantly making new resins for different ion-removal purposes.
+
−
Traditional deionization exchanges cations with H ions (acids) and anions with OH
ions (bases). Although not 100 percent effective, the end result of these two exchange pro-
cesses combines the ions remaining in the feedwater to create deionized or demineralized
water as the end product.

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