Page 243 - Water and wastewater engineering

6-20 WATER AND WASTEWATER ENGINEERING

b. The amount of alkalinity that will be consumed is found by using Equation 6-8 , which

shows that one mole of alum consumes six moles of HCO 3 . With the molecular weight
of alum equal to 594, the moles of alum added per liter is:

3
12 5 10 g/L 5
.
.

21 10 moles/L
594g/mole
which will consume

5

.
.
(
6 2 1 10 moles/L ) 1 26 10 4 moles/LHCO 3
The molecular weight of is 61, so
3
4

(126 10 moles/L )(61 g/mole )(10 mg/g ) 7.77 mg/LHCO 3
.
are consumed, which can be expressed as CaCO 3 :
.. CaCO
E W 3
.
(77 mg/L HCO 3 )
EW
.. HCO 3
50 g/equivalent
.
(77 . mg/L HCO 3 ) 6 31 mg/L HCO as CaCO 3
3
61 g/equivaleent
As noted earlier, the lack of sufficient alkalinity will require the addition of a base to adjust
the pH into the acceptable range. Lime (CaO), calcium hydroxide Ca(OH) 2 , sodium hydroxide
(NaOH), and sodium carbonate (Na 2 CO 3 ), also known as soda ash, are the most common chemi-
cals used to adjust the pH. Table 6-3 illustrates the neutralization reactions.

TABLE 6-3
Neutralization reactions
To neutralize sulfuric acid with
Lime: H 2 SO 4 CaO CaSO 4 H 2 O
Calcium hydroxide: H 2 SO 4 Ca(OH) 2 CaSO 4 H 2 O
Sodium hydroxide: H 2 SO 4 NaOH NaSO 4 2H 2 O
Soda ash: H 2 SO 4 Na 2 CO 3 Na 2 SO 4 H 2 O CO 2
To neutralize hydrochloric acid with
Lime: 2HCl CaO CaCl 2 H 2 O
Calcium hydroxide: HCl Ca(OH) 2 CaCl 2 H 2 O
Sodium hydroxide: HCl NaOH NaCl H 2 O
Soda ash: HCl Na 2 CO 3 NaCl H 2 O CO 2
Note: a stoichiometric reaction will yield a pH of 7.0.