TM 5-813-3/AFM 88-10, Vol 3
where
the temperature of the influent water, so the flow rate
THC = compensated hardness in mg/L as CaC03
must be decreased as water temperature decreases if a
TH = total hardness in mg/L as CaC03
constant bed expansion is to be maintained,
(4) Blending, An ion exchange softener operating
TC = total cations in mg/L, all expressed as CaC03
properly will produce a water having a hardness ap-
proaching zero. Inasmuch as it is not generally eco-
grains per gallon by multiplying by 0.0584 or dividing
nomical nor desirable to soften all water to this low
by 17.1.
hardness level, provisions, for blending the softened
(6) Removal of noncarbonated hardness following
water with the unsoftened water are desirable.
lime softening. In some cases, it is more economical to
(5) Other factors affecting ion exchange soften-
remove noncarbonated hardness in cation exchanges
ing,
than by application of soda ash. This method involves
(a) Turbidity, Turbidity particles present in the
the use of lime for reduction of carbonate hardness.
water influent to the softener are deposited on the
Following recarbonation, the water is filtered. Then all
softening medium and may cause losses of exchange
or part of the water, depending on the final hardness
capacity and excessive head losses through the soften-
desired, is treated in cation exchange softeners for the
er. If turbidity levels are excessive, the particles must
removal of noncarbonated hardness. The technique is
be removed from the water prior to softening or spe-
most suitable to those areas where regeneration salt
cial backwashing procedures must be implemented.
can be obtained at a low cost.
(b) Bacterial slimes. Unless proper disinfection
(7) Comparison on lime-soda ash and cation ex-
is practiced, bacterial slimes can form in the softening
change processes. Although the purpose of both the
medium and cause excessive head losses and loss of ex-
lime-soda ash process and the cation exchange process
change capacity. These slimes can be prevented or re-
is to achieve removal of calcium and magnesium ions,
moved through chlorination of' feedwater or regenera-
the modes of operation and the quality of the resultant
tion water.
water are somewhat different.
(c) Temperature. The loss of head through a wa-
(a) Turbidity, iron, and manganese. Lime-soda
softening also effects removal of turbidity and iron
ter softener is strongly affected by water temperature,
and manganese, whereas cation exchange softening
with lower head losses occurring at higher tempera-
may have to be preceded by conventional treatment
tures. For example, at similar flow rates the head loss
for removal of suspended matter and iron and manga-
through a softener at 1220 F. is only about 35 percent
nese.
ture affects the exchange capacity of the softener,
with a 10 to 15 percent increase at high operating tem-
peratures (>860 F.) over the exchange capacity at low
the water. In contrast, the water entering a cation ex-
change softener must be disinfected in order to pre-
temperatures (32 to 50 F.)
( d ) I r o n , m a n g a n e s e a n d a l u m i n u m . If iron,
vent the growth of bacterial slimes within the soften-
manganese, and aluminum are present in the influent
ing resin.
water, precipitates may be formed which coat the me-
(c) Total dissolved solids. Total dissolved solids
dium particles and cause a loss of exchange capacity.
concentrations of water are usually lowered by lime-
This problem can be avoided through treatment to re-
soda ash softening, especially if most of the hardness
move the iron, manganese, and aluminum from the
initially present is carbonate hardness. However, ap-
water prior to softening. If iron fouling occurs it may
plication of soda ash to remove noncarbonated hardness
be possible to overcome it by periodic applications of
results in a slight increase of TDS concentrations.
sodium bisulfite, sodium hydrosulfite, hydrochloric
Softening of water by cation exchange processes al-
acid, or sulfuric acid to the softening media. However,
ways results in an increase in TDS levels, because the
these treatments should be implemented only after a
sodium required to replace calcium and magnesium in
thorough study of the problem by someone experi-
the water has a mass 1.15 times as large as the calcium
enced in this area.
replaced and 1.89 times as large as the magnesium re-
(e) Total hardness and sodium concentration. If
placed.
the total hardness exceeds 400 mg/L or the sodium
2-11. Iron and manganese control.
a. Occurrence of iron and manganese. Dissolved
softener should be sized on the basis of the "compen-
sated total hardness" rather than the total hardness.
iron and manganese are encountered principally in
Compensated hardness is calculated as follows:
ground waters devoid of dissolved oxygen, Normal,
oxygenated surface waters do not contain significant
concentrations of these metals; however, stagnant
water, found in the bottom of thermally-stratified
2-24
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