Page 249 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

204 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

3. In the case of Al , however, there are two 9.5.2.1 Role of Alkalinity as a Buffer
3þ
atoms=molecule, that is,
For many years alum chemistry was described in terms of its
reaction with alkalinity. The well-known ‘‘classical’’ reaction
0:0001683 mol Al 2 (SO 4 ) 3 2 26 982 mg Al
between alum and the alkalinity in water is (Black,
L solution mol Al 2
1948, p. 142),
9:08 mg Al
¼
L solution
Al 2 (SO 4 ) 3 þ 3Ca(HCO 3 ) 2 þ 6H 2 O
Discussion ! 3CaSO 4 þ 2Al(OH) þ 6H 2 CO 3 (9:5)
3
An important point, sometimes overlooked, is that there
are 2 mol Al 3þ per mol Al 2 (SO 4 ) 3 14H 2 O, which is seen After omitting the ‘‘spectator’’ ions, Ca 2þ and SO 4 , the
2
in the factor ‘‘2’’ in (3). The conversion procedure is reaction becomes
relevant in understanding coagulation diagrams which
usually have two scales, that is, mols Al =L and mol
3þ
2Al 3þ þ 6HCO 3 þ 6HOH !þ2Al(OH) þ 6H 2 CO 3

Al 2 (SO 4 ) 3 14H 2 O=L, for example, 100 mg Al 2 (SO 4 ) 3 3
4
14H 2 O=L ¼ 0.0003366 mol Al =L ¼ 3.34 10 mol (9:6)
3þ
Al =L (since there are 2 mol Al 3þ per mol Al 2 (SO 4 ) 3
3þ
14H 2 O). Also, for later reference, concentration may 3þ
An equivalent depiction focuses on the idea of Al reacting
3þ 3þ 3þ
þ
be expressed as p[Al ], that is, p[Al ] ¼ log[Al ] ¼ with H 2 O to form Al(OH) 3 , releasing H as in Equation 9.7.
4
log[3.34 10 mol Al =L] ¼ 3.47.
3þ
Removing the H , by the HCO 3 buffer as in Equation 9.8,
þ

drives the reaction to the right, forming H 2 CO 3 . Summing the
two equations, that is, Equation 9.9, the result is the same as
9.5.1.3 Liquid Alum Equation 9.6, but the emphasis is on HCO 3 as a buffer.

Since its introduction in the 1950s, liquid alum has become
more widely used than the hydrated solid crystal form. The Al 3þ þ 3H 2 O ! Al(OH) 3 þ 3H þ (9:7)
reason is convenience and cost. Costs have been reduced due
to the dissemination of distribution centers in the United 3HCO 3 þ 3H ! 3H 2 CO 3 (9:8)
þ

States which has lowered the haul distance. Handling of a
Al 3þ þ 3HCO 3 þ 3H 2 O ! Al(OH) þ 3H 2 CO 3 (9:9)

solid requires storage, metering, mixing, and cleaning. Liquid 3
alum, on the other hand, is delivered by truck (or rail) to a
storage tank and then is metered directly into the rapid-mix. If The reactions are stoichiometric, meaning that the Al(OH) 3
precipitate will be produced in proportion to the availability
delivered at a specific gravity of 1.335, the corresponding
is pre-
alum concentration as Al 2 (SO 4 ) 3 14H 2 O is 647 mg=L. For of HCO 3 . Regarding carbonate equilibria, HCO 3
dominant in the range 4.35 < pH < 10.33 which are the
reference, the equivalent concentration expressed as Al 3þ is
from H 2 CO 3 and
(54=594) 647 ¼ 58.8 mg=L. respective pK a ’s that separate HCO 3
CO 3 , respectively.
2
9.5.2 ALKALINITY 9.5.2.2 Effect of Alkalinity on Demand for Alum
That ‘‘alum-demand’’ is proportional to alkalinity is seen in
Part of the lore of coagulation practice has been that
the experimental plot, Figure 9.9a, which is consistent with
alkalinity (Box 9.2) is necessary for coagulation. Both the
Equation 9.9. Alum-demand was defined as the ‘‘critical
traditional view and the modern view are described for
coagulant concentration’’ (CCC) to achieve a zeta potential
reference.
of 5 mV for the suspension being treated, which was the
zeta potential that corresponded to minimum settled water
turbidity. Figure 9.9b shows that as the alkalinity increases
the settled water turbidity declines toward an asymptote and
that the residual pH hovers near neutral (Tseng et al., 2000).
BOX 9.2 ALKALINITY DEFINED
Alkalinity is defined as the sum of HCO 3 ,CO 3 , and 9.5.2.3 Effect of Alum on pH

2
OH . By convention, alkalinity is expressed in terms of Alum and ferric iron act as ‘‘Bronsted acids,’’ which means

the CaCO 3 equivalent with molecular weight 100. To that they may donate a proton (H ion) to the solution, thus
þ
illustrate, let the HCO 3 concentration be say 48 mg=L depressing the pH. The effect is marked for a low alkalinity

as HCO 3 , which is (100=61) 48 ¼ 78.7 mg=Las water (i.e., a water that has little buffer capacity), as demon-

CaCO 3 . The total alkalinity then is the sum of the strated in Figure 9.10. Figure 9.10 shows a pH depression
concentrations of all three, expressed as CaCO 3 . from pH ¼ 5.0 at 40 mg=L alum to pH ¼ 4.2 at 120 mg=L
The CaCO 3 expression is also used for Ca 2þ and alum. As seen, alkalinity will react with the H to maintain
þ
Mg , the sum of which is defined as ‘‘hardness.’’ pH levels. Without alkalinity as a buffer, the H generated
þ
2þ
acts to depress the pH. In another example, for pilot plant