Page 237 - Water and wastewater engineering

6-14 WATER AND WASTEWATER ENGINEERING

where K w 10 14 (p K w 14):

10 14 4
[OH ] 10 10 moles/L
10
and

mg/L (10 4 moles/L)(17 g/mole)(10 3 mg/g 1.77
)
Now, the mg/L as CaCO 3 is found by using Equation 6-7 and taking the equivalent weight of
CaCO 3 to be 50:

2
.
CO 3 100 0 ⎛ 50 ⎞ 167
⎝ 60 ⎠
⎛ 50 ⎞
HCO 3 75 0 . 61
⎝ 61 ⎠

5
H 10 7 ⎛ 50 ⎞ 10 6
⎝ 1 ⎠

OH 17 ⎛ 50 ⎞ 5 0
.
.
⎝ 17 ⎠
The exact alkalinity (in mg/L) is found by
6
Alkalinity 61 167 5 0 5 10 ) 233 mg/L CaCO 3
.
O
(
It is approximated by 61 167 228 mg/L as CaCO 3 . This is a 2.2% error.
Aluminum. Aluminum can be purchased as either dry or liquid alum [Al 2 (SO 4 ) 3 · 14H 2 O].
Commercial alum has an average molecular weight of 594. Liquid alum is sold as approxi-
mately 48.8 percent alum (8.3 percent Al 2 O 3 ) and 51.2 percent water. If it is sold as a more
concentrated solution, there can be problems with crystallization of the alum during shipment
and storage. A 48.8 percent alum solution has a crystallization point of 15.6 C. A 50.7 percent
alum solution will crystallize at 18.3 C. The alternative is to purchase dry alum. However, dry
alum costs about 50 percent more than an equivalent amount of liquid alum so that only users of
very small amounts of alum purchase it in this form.
When alum is added to a water containing alkalinity, the following reaction occurs:

H
(
(
)
(
)
2
Al SO 4 3 14 H O 6 HCO 3 2 Al OH 3 3 H O s) 6CO 2 8 2 O 3SO 4 2 (6-8)
2
2
such that each mole of alum added uses six moles of alkalinity and produces six moles of carbon
dioxide. The above reaction shifts the carbonate equilibrium and decreases the pH. However, as
long as sufficient alkalinity is present and CO 2 (g) is allowed to evolve, the pH is not drastically