Page 238 - Water and wastewater engineering

COAGULATION AND FLOCCULATION 6-15

reduced and is generally not an operational problem. When sufficient alkalinity is not present to
neutralize the sulfuric acid production, the pH may be greatly reduced:

)
(
Al SO 4 3 14 H O 2 Al OH 3 3 H O s) 3 HSO 4 2HO
)
(
(
2
2
2
2
2
(6-9)
If the second reaction occurs, lime or sodium carbonate may be added to neutralize the acid
formed because the precipitate will dissolve.
Example 6-2 illustrates the destruction of alkalinity.
Example 6-2. Estimate the amount of alkalinity (in mg/L) consumed from the addition of 100
mg/L of alum.
Solution:

a. Using Equation 6-8 , note that 6 moles of HCO 3 are consumed for each mole of alum
added.
b. Calculate the moles/L of alum added.
100 mg/L of alum 100 mg/L 4
168 . 10 moles/L
GMW of alum ( 594 g/mole)(10 3 mg/g)
m

c. Calculate the moles/L of HCO 3 consumed.

.
(
6 1 68 10 4 moles/L ) 1 01 10 3 moles/L
.
d. Convert to mg/L
3

)(
.
(101 10 moles/L GMW of HCO )
3

.
.
.
(1 01 10 3 moles/L )(61 g/mole ) 6 16 10 2 g/LLor 61 6 mg/L asHCO
3
Comment. A rule of thumb used to estimate the amount of the alkalinity consumed by alum is
that 1 mg/L of alum destroys 0.5 mg/L of alkalinity as CaCO 3 .
3
An important aspect of coagulation is that the aluminum ion does not really exist as Al ,
and that the final product is more complex than Al(OH) 3 . When the alum is added to the water,
it immediately dissociates, resulting in the release of an aluminum ion surrounded by six water
molecules. The aluminum ion starts reacting with the water, forming large Al · OH · H 2 O com-
4
plexes. Some have suggested that [Al 8 (OH) 20 · 28H 2 O] is the product that actually coagulates.
Regardless of the actual species produced, the complex is a very large precipitate that removes
many of the colloids by enmeshment as it falls through the water.
Iron. Iron can be purchased as either the sulfate salt (Fe 2 (SO4) 3 · x H 2 O) or the chloride salt
(FeCl 3 · x H 2 O). It is available in various forms, and the individual supplier should be consulted for