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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.15

affinities for the hydrogen ion and are fully converted to the hydrogen form with little
more than the stoichiometric amount of acid during regeneration. The degree of exchange
during the service cycle, or operating capacity, is limited by the amount of alkalinity, the
ratio of divalent cations to alkalinity, and total ionic concentration. The key factors in de-
termining the potential operating capacity are the hardness-to-alkalinity ratio and the leak-
age endpoint. The base operating capacity can vary from about 10 to 60 kgr/ft 3 of alka-
linity as the hardness-to-alkalinity ratio varies from 0 to 1.2 or higher. The regeneration
dose is usually calculated as 120% of the operating capacity. Flow rate and temperature
during the service cycle also affect the operating capacity and can reduce the operating
capacity by 35% as the temperature drops from 70 ° to 35 ° F (21 ° to 2 ° C) or as the flow
rate doubles from 2 to 4 gpm/ft 3.
Hardness is removed and bicarbonates are converted to carbonic acid, which can be
removed by a degasifier. In effect the effluent is partially demineralized. The degree of
softening depends on the hardness-to-alkalinity ratio, and any hardness in excess of al-
kalinity remains. The regeneration equipment and all process and tanks, piping, and valves
need to be acid-resistant. Sulfuric acid is commonly used as the regenerant because it is
less expensive, but calcium sulfate precipitation is a very real concern. When sulfuric acid
is used, the regenerant concentration should not be allowed to go above 0.75%. It is usu-
ally held between 0.5% and 0.75%. Hydrochloric acid can be used at higher concentra-
tions, with 6% to 10% the common range. This gives much lower waste volumes. Hy-
drochloric acid is not as widely used because it is more expensive and more corrosive and
gives off hazardous fumes. Equipment costs are usually more expensive also, because of
the increased material costs associated with the increased corrosivity of HC1.

pH Effects. Weakly acidic resins have a small amount of strong acid capacity and will
initially convert all salts to acids in the beginning of the service cycle. Initial pH values
can be as low as 2 and rise gradually, but remain under 7 for over three-fourths of the
service cycle. When this process is used to treat boiler feedwater, the carbon dioxide gen-
erated by the exchange reaction has to be removed by aeration or vacuum degasification;
otherwise, it will be converted back to bicarbonate when caustic is added to raise the pH.
Dealkalization by Dual Strong Acid Columns Operated in the Sodium and Hydrogen
Cycles. In this scheme two vessels, each loaded with strong acid resins, operate in par-
allel; one is regenerated with salt and the other with an acid. The two effluents are com-
bined in a ratio based on the composition of the untreated inlet water so that acid gener-
ated in the hydrogen cycle vessel is just sufficient to neutralize the alkalinity in the effluent
from the salt regenerated vessel. This process produces partially demineralized, fully soft-
ened water with zero alkalinity.
The sodium cycle vessel is sized and designed as a traditional salt regenerated soft-
ener. The hydrogen cycle vessel is typically designed to operate at the highest acid effi-
ciency, usually about 3 lb/ft 3 of sulfuric acid. The throughput capacity and leakage and
blending ratios will depend on the water analysis. When it is practical, the two vessels
are sized to exhaust simultaneously.

Arsenic
Arsenic occurs widely in the earth's crust, usually in the form of insoluble complexes
with iron and sulfides. Another source of arsenic in potable water supplies comes from
its extensive use in the past as a pesticidal agent. It also exists in soluble form, primarily
as arsenites (AsO3) and arsenates (AsO4). Ingestion of as little as 100 mg of arsenic per
day can cause severe poisoning in humans.