Page 889 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

844 Appendix F: Alum Data and Conversions

Example F.7 Calculate Cost of Al in Liquid Alum Required
Solution Determine the quantities of liquid alum and water,
respectively, to formulate the dilute solution.
Given Solution
The cost of liquid alum at Denver, Colorado in 1999 was Rationale: Keep in mind the 10% specification, or 1:10
$181=ton liquid alum. dilution, are expressions that should be defined if they are
Required used. Therefore, the term, 1:10 dilution is defined as add-
Determine the cost of Al in liquid alum in $=kg Al. ing x g of alum contained in 1.000 g of liquid alum, to
9.000 L of water, to give a solution of x=10 g=L concentra-
Solution
tion. In other words, express the new solution as a 0.1x g=L
Table F.3 outlines the solution.
solution (i.e., as a concentration rather than as a ‘‘10%’’
Rationale: Again, the approach is to use the chain of alum solution). The latter is nebulous, having no agreed
conversions to achieve the result desired. upon meaning.
Sequence of Steps
F.5 USING ALUM IN WATER TREATMENT 1. Conditions specified for the dilute solution.
Suppose that the concentration of the diluted
Some concerns in using alum relate to dilution, storage, mass alum solution is to be
flow, calibration of metering pumps, and cleaning lines. The
issues of dilution and storage come up frequently in the
64:7gAl (SO 4 ) 3 14:0H 2 O
operation of pilot plants. The others issues are included to 2 and that 10:0 L is needed:
L
emphasize their importance and to provide guidelines for
operation.
2. Mass of alum needed for the 10.0 L is
F.5.1 DILUTING ALUM
64:7gAl (SO 4 ) 3 14H 2 O
2
In practice, liquid alum is metered into the rapid mix as a 10:000 L
L
‘‘neat’’ solution. In pilot plants, however, the required alum
¼ 647 g Al (SO 4 ) 3 14H 2 O
2
flow as a neat solution may be less than that of the minimum
flow of the metering pumps available. In addition, the distri-
bution of the alum flow throughout the raw water may be 3. Volume of neat liquid alum that contains 647 g
Al 2 (SO 4 ) 3 14H 2 O is 1.000 L.
more difficult with only a small point flow of neat alum.
4. Dilution protocol.
To alleviate both problems, the neat alum solution may be
Measure 1000 mL of neat liquid alum solution.
diluted. According to persons knowledgeable in the field, there
Measure, for example, 5000 mL of water, with pH
is no limit to the dilution ratio as long as the pH of the diluted
adjustment to pH 3.
solution is less than 4.0, keeping in mind that the pH of the neat
Add water to result in about 9.500 L of solution
alum solution is about 2.4. The idea is that the pH should be low
Make sure that pH 4.0
enough such that the equilibrium is toward Al 3þ as the predom- Mix neat liquid alum and water
inant alum species vis-à-vis hydrolysis products. Nevertheless, Add water to result in 10.000 L
in pilot plant and full-scale operations, most persons the author Mix
knows use neat alum as long as it is feasible to do so with the 5. pH check.
metering equipment, in order to avoid any uncertainty. After mixing, check the pH of the dilute alum
Another aspect of dilution is that the terminology should solution to make sure that it is much less than 4.
be clear. Dilution by volume should state, for example, that 6. Resulting concentration.
10 mL Al 2 (SO 4 ) 3 14H 2 O solution is diluted to result in The resulting alum solution concentration is
100 mL solution. The terminology 1:10 dilution may cause
confusion as to whether the dilution is 10–100 mL solution or 647 g Al (SO 4 ) 3 14H 2 O 64:7gAl (SO 4 ) 3 14H 2 O
2
2
10 mL added to 100 mL water. Example F.8 illustrates the 10:000 L alum solution ¼ 1:000 L alum solution
rationale for dilution calculations.
Discussion
Example F.8 Calculate Quantities of Liquid Alum The resulting alum solution should be referred to as a
to Obtain a Dilution of 10% ‘‘64.7 g Al 2 (SO 4 ) 3 14H 2 O=L alum solution.’’ If one wishes
to state that it is a 10% alum solution, to indicate the
Given amount of dilution, then this may be understood as an
Suppose that a neat liquid alum solution is to be diluted, approximation of what was done. But it has no precise
say about 1:10, in order to increase the effectiveness of meaning upon which calculations can be based, which
alum mixing in the raw water. should be conveyed in communication.