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12.20 CHAPTER TWELVE

240
+++
+ +
20O
HCO 3 + +
++
+ +++++++++++++++-t
+
L) 160
e~
+ S040 o o o o
0 o
~ 120
oo
+ 0 0000000 (
0
"~ 80
Influent NO 3, 62 ppm
........... ;- ...........
a m-
+ . NO3, "*
Z 40
ni le
-.-~-.~A-tl:n iB I I I
0-
0 100 200 300 400 500 600
Bed volumes
FIGURE 12.4 NO3 breakthrough pattern for a nitrate-selective resin.
lective resin. Notice that with the nitrate selective resin the sulfate breaks through before
the nitrate. Also, the sulfate level will rise above the influent level if the resin is run be-
yond its usable capacity. When a nitrate selective resin is run past its usable capacity, the
nitrate level will gradually rise to the influent level, but not above.
Nitrate Removal by Selective Resins. Nitrate removal by so-called nitrate selective an-
ion exchangers is becoming widely practiced around the world and in the United States.
These ion exchangers are similar in composition and structure to type I strong base resins.
However, their functional groups are based on larger amine compounds than the trimethyl-
amine used in ordinary type I resins. Ion exchange resins that use triethylamine, tripropyl-
amine, and tributylamine have been offered commercially as nitrate selective. The larger
amine groups are more widely spaced compared with the trimethylamine groups used in
standard resins. This makes it more difficult to exchange with multivalent ions such as
sulfate. Actually, the term nitrate selective is a misnomer. These resins are effective be-
cause they are less selective for multivalent ions such as sulfates. Because of this, sul-
fates are not able to cause the massive dumping of nitrates when these resins are overex-
hausted as is the case with ordinary resins. These resins therefore offer greater safety in
high-sulfate waters. At levels of about 25% or less sulfates, the standard resins will have
a higher operating capacity than the so-called selective types. Due to the larger size of
the amine groups, the selective resins have lower total capacities so the overall operating
capacity is smaller than for the standard-type resins. When sulfate levels are greater than
25% the selective resins will give higher throughputs. The difference in operating capac-
ity with changing sulfate levels, comparing selective and nonselective nitrate resins, is
shown in Figure 12.5.
Sodium chloride is very effective in regenerating both standard and selective types of
resins. Average nitrate leakages are about the same for either type of resin at similar re-
generant levels. The biggest difference between the selective and nonselective resins is
that sulfates are dumped by nitrates from the selective resins instead of the other way
round in standard resins.