Page 712 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Precipitation 667

100–200 is a nuisance perhaps tolerable, and >300 is usually
TABLE 21.3 considered not tolerable. Usually, about 40–60 mg=L hard-
Solubility Rules for Ionic Compounds in Water ness as CaCO 3 is considered desirable, and is useful in order
to have a ‘‘depositing’’ water, that is, water with a Langelier
Soluble Ionic Compounds Insoluble Ionic Compounds
Index 0. Snowmelt waters, with total dissolved solids,
1. All common compounds of Group 1. All common metal hydroxides are
TDS 50 mg=L, C(Ca ) 30 mg Ca =L, and pH 7.0,
2þ
2þ
1A(1) ions (Na ,K , etc.) and insoluble, except those of Group
þ
þ
may be corrosive in the distribution system with consequent
ammonium ion NH 4 Þ are 1A(1) and the larger members of
þ
ð
leaks in water mains after decades of operation. The City of
soluble Group 2A(2) (beginning with
Fort Collins, in fact, adds lime, just following disinfection, to
Ca )
2þ
give C(Ca ) 60 mg=L as CaCO 3 , which results in a
2þ
2. All common nitrates (NO 3 ), 2. All common carbonates CO 3 2 Þ

ð
‘‘slightly depositing’’ water, desired to protect the water
acetates (CH 3 COO ), and most and phosphates PO 4 3 Þ are

ð
perchlorates ClO 4 Þ are soluble insoluble, except those of Group mains.

ð
þ
1A(1) and NH 4
3. All common chlorides (Cl ), 3. All common sulfides are soluble, 21.2.2.2 Expressing Hardness as CaCO 3

bromides (Br ), and iodides (I ) - except those of Group 1A(1),

By long-standing convention, hardness is nearly always
are soluble, except those of Ag , Group 2A(2), and NH 4 þ
þ
expressed as calcium carbonate, that is, CaCO 3 ,MW
2þ þ
2þ
Pb ,Cu , and Hg 2
(CaCO 3 ) ¼ 100. Other units may be as mg=L of the particular
4. All common sulfates SO 4 2 Þ are
ion or as mol=L of the ion (which should be the same as
ð
soluble, except those of Ca ,
2þ
mol=L as CaCO 3 ). Conversion from any units to any other
Sr ,Ba , and Pb 2þ
2þ
2þ
units may be in terms of mol=L as the intermediate expression.
Source: Adapted from Silberberg, M., Chemistry—The Molecular Nature of For example, to convert from mg Ca =LtomgCa 2þ as
2þ
Matter and Change, Mosby-Year Book, Inc., St. Louis, MO, CaCO 3 =L, first convert to mol Ca =L. The second step is to
2þ
p. 147. convert to mg Ca 2þ as CaCO 3 =L, that is, there are 100,000 mg
Group 1A(1) elements: H, Li, Na, K, Rb, Cs, and Fr.
CaCO 3 =mol.
Group 2A(2) elements: Be, Mg, Ca, Sr, Ba, Ra.
Consider a concentration of 10 mg HCO 3 ; the first step is

to convert to molar concentration, that is,
For such conditions, if iron is present, Fe 2þ will be the species
10 mg HCO mol HCO 3 1:64 10 4 mol HCO 3
expected, as indeed occurs. As the water emerges from the 3 ¼
mine and if exposed to the atmosphere, ambient oxygen will L 61,000 mg HCO 3 L
dissolve in solution, which will increase the redox potential
(or pe). In treatment, if the pH is increased, the (pH, pe) The second step is to convert to mg as CaCO 3 =L, that is,
coordinates will cause a shift toward Fe(OH) 3 (s) becoming
the predominant species, which may be removed as a solid, 1:64 10 4 mol HCO 3 100,000 mg CaCO 3 16:4 mg CaCO 3
for example, by settling and filtration. ¼
L mol CaCO 3 L
21.2.1.6 General Rules of Solubility
Examples 21.3 through 21.5 illustrate conversions based on
Table 21.3 summarizes solubility rules for common ions; the
the same principle, but applying ‘‘short cut’’ to each.
left-hand columns show the rules for solubility, while
the right-hand columns show the rules for insoluble com-
pounds. The rules provide general guidelines for predicting Example 21.3 Conversions of Carbonate Hardness
solubilities.
Given
C(Ca ) ¼ 62 mg=L
2þ
21.2.2 HARDNESS
Required
Hardness is defined as the soap consuming capacity of a
Hardness expressed as CaCO 3
water. Its cause is Ca ,Mg ,Fe ,Mn , or any other
2þ
2þ
2þ
2þ
Solution
ion or substance that may consume soap or cause deposits, for
62 mg Ca =L 100 mg CaCO 3 =40 mg Ca 2þ ¼ 155 mg=L
2þ
example, scale in boilers or precipitates in pipes. Most hard-
hardness as CaCO 3 .
ness that occurs in ambient waters is Ca .
2þ
21.2.2.1 Occurrence of Hardness
Example 21.4 Conversion of Ca 2þ Concentration
Hardness is common in ambient waters, especially when
groundwater is the source and in the lower reaches of river Given
systems. Calcium is one of the most common anions and Calcium ion, Ca , is a hardness component in a water;
2þ
bicarbonate is one of the most common anions. Generally, 2þ
its concentration in the ambient raw water is C(Ca ) ¼
<100 mg=L of hardness as CaCO 3 is not grounds for removal, 33 mg=L as CaCO 3 .