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12.18 CHAPTER TWELVE

Sulfate
Sulfates at high concentrations in potable water supplies cause diarrhea in humans and
livestock. They can be removed by strong base chloride cycle anion exchange. Type II
resins are typically used because of their lower odors. Removal rates are typically in the
range of over 95% to 100%. A salt dosage of 5 lb/ft 3 is sufficient, and operating capaci-
ties of 20 to 25 kgr of sulfate, expressed as calcium carbonate per cubic foot of resin, are
typical for gel-type strong base resins. Sulfate displaces the other common ions includ-
ing chlorides and nitrates during the exhaustion cycle, so the operating throughput is cal-
culated based only on the sulfate concentration in the raw water.

Nitrate

Nitrates are considered to be toxic to human infants and ruminant animals such as cows.
They occur naturally as a result of the decomposition of nitrogen-containing waste mat-
ter, also as a result of farm runoff from fertilizer. The maximum level of nitrates in potable
water is usually limited to 10 ppm as nitrogen. At levels above 50 ppm nitrates as nitro-
gen (N) can cause methemoglobinemia in infants, also known as blue baby syndrome.
Type I or type II strong base gel anion resins are often used for nitrate removal. They
are selective for nitrates over all the common ions in tap water with the exception of sul-
fates. During the service cycle, sulfates will load first and can displace nitrates at con-
centrations equal to the sulfates plus nitrates if the vessel is overrun. This is discussed be-
low under "Nitrate Dumping" (see Figure 12.2).
Sulfate, being divalent, undergoes selectivity reversal at the higher ionic concentra-
tions during regeneration. Therefore sulfates are more fully removed from the resin than
nitrates during regeneration.

ResinTech SG2 (Type II gel)
24
22
20
18
c7
16
14
~ 51b
er~
12
©
Z
10
8
/~ Sodium chloride regeneration
6
3
f/~ 2 gpm/ft j exhaustion rate
4
p' 10% breakthrough end of cycle
2
I I t I I I I I [
0,
10 20 30 40 50 60 70 80 90 100
0
Nitrate percent of anions
FIGURE 12.2 Nitrate removal capacity.