Test Results

Calcium removal in fluidized bed crystallization bench-scale tests is exemplified by the Beverly Hills, California, results shown in Fig. 3.

Table 1 Source water characteristics and ZLD applications evaluated

(mg/L)

Hardness (mg/L as CaCO3)

Silica (mg/L)

Water type

Application

SNWA, NV

690

290

9

Surface

Desalination of treated water to reduce TDS and hardness

Beverly Hills, CA

720

300

41

Groundwater

Recovery of concentrate from existing desalination facility

Phoenix, AZ

1300

68

27

Groundwater

Development of new water source

Scottsdale, AZ

1100

340

34

Reclaimed

Recovery of concentrate from existing water reclamation facility

SAWS, TX

3500

1720

57

Groundwater

Development of new water source

Calcium removal was greater than 60% at a NaOH dose of 100 mg/L, and greater than 80% at a NaOH dose of 250 mg/L. The NaOH dose required for meeting the treatment goals varied among the water sources tested, but effective calcium removal was achieved at each site with fluidized bed crystallization.

Barium was also effectively removed in fluidized bed crystallization tests and there was a correlation between barium removal and calcium removal as illustrated in Fig. 4. Barium fraction remaining is plotted against calcium fraction remaining in this figure for the SAWS, Scottsdale, and SNWA sites.

Table 2 Summary of concentrate treatment goals and projected recovery by reverse osmosis

CL D

Table 2 Summary of concentrate treatment goals and projected recovery by reverse osmosis

Primary RO recovery (%)

Ca removal (%)

Ba removal (%)

SiO2 removal (%)

Secondary RO recovery (%)

Total RO recovery (%)

Beverly Hills, CA

73

26

33

84

85

96

SNWA, NV

85

20

63

0

76

96

Scottsdale, AZ

73

69

67

64

76

94

Phoenix, AZ

85

72

50

77

85

98

SAWS, TX

66

77

0

87

88

0.50

0.45 -

0.40 -

ro c

0.35 -

c

ra F

0.30 -

<i)

0.25 -

o

'S 2

0.20 -

m

0.15 -

O

0.10 -

0.05 -

0.00 -

100 200 300 400 500 NaOH dose (mg/L)

Figure 3 Calcium in fluidized bed crystallization tests at Beverly Hills, California.

Ca (fraction remaining)

+ SNWA o Scottsdale

Figure 4 Correlation between barium and calcium in fluidized bed crystallization test.

It was theorized that the removal mechanism was substitution of barium for calcium in the calcium carbonate crystal lattice.

Silica, however, was not effectively removed in the fluidized bed crystallization test at pH below 10, as illustrated in Fig. 5, and it was

J 060

0.20

0.00

Figure 5 Silica in fluidized bed crystallization test.

concluded that effective silica removal in each test occurred only after the pH was high enough to cause Mg(OH)2 precipitation. The need for Mg(OH)2 precipitation for effective silica removal in chemical softening is a long-recognized phenomenon, and as illustrated in Fig. 6, correlations between silica and magnesium in the fluidized bed crystallization tests confirmed that silica removal occurred due to magnesium hydroxide precipitation.

Rather than increasing pH for silica removal, tests were conducted with alum and sodium aluminate to evaluate silica removal by adsorption to aluminum hydroxide. The tests showed that effective silica removal could be achieved in the pH range of 8—9 by adding either alum or aluminum hydroxide to the fluidized bed crystallization process in addition to NaOH or lime.

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