Page 884 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Appendix F: Alum Data and Conversions 839

not be unreasonable. Allowing for losses, such a specific gravity, concentration in g Al 2 (SO 4 ) 3 14H 2 O=L solu-
motor size would be reasonable. tion. The latter is most important in metering the neat solution
. The power number approach, while rational, to the flow of a water treatment plant. Also important are the
gives unreasonable results. The empirical approach, capacities of trucks, common sizes of storage tanks, pH, and
i.e., using the P=V criterion for intense mixing, freezing temperature. Note that the Baumé is a hydrometer
remains the favored method (other than CFD)
to estimate the power requirement for mixing. reading; the scale is calibrated to give an equivalent specific
gravity of the solution being tested.
A slightly higher mixing power is preferred over
undersizing. Other properties of interest include pH, given in
Figure F.3a as a function of mass concentration. As seen,
pH 2.8 for a mass concentration of about 25%, which is
F.4 LIQUID ALUM equivalent to a 1:1 dilution. With pH 3.5, there should be
no hydrolysis products which could reduce the effectiveness
Liquid alum came on the scene in water treatment in the about
of the liquid alum.
the 1950s (based on word of mouth recalled from the 1960s). In
The viscosity of liquid alum is given in Figure F.3b
the United States, practice has shifted during the period from
as a function of temperature for different mass concentra-
1960 to 1990 from dry alum to mostly liquid alum. By 1990,
tions of alum. The relationships are useful in hydraulic
for example, the dry alum equivalent of the liquid alum used
calculations.
was 500,000,000 kg (1.1 million tons) annually while the dry
alum used (i.e., solid alum as Al 2 (SO 4 ) 3 14H 2 O crystals) was
33,000,000 kg (70,000 U.S. tons) annually. One reason for the
F.4.4 EXPRESSIONS FOR ALUM MASS
shift has been that the production centers have been dispersed
geographically, reducing the transport distances with conse- Concentrations of alum have been expressed in five equiva-
quent reduction in cost. The appeal of liquid alum is that it is lent forms. They are enumerated in the paragraphs that follow.
more convenient to use than the solid form, labor costs are less Al 2 (SO 4 ) 3 14H 2 O: As a rule, alum concentration is
than handling solid alum, and quality control is more easily expressed as g=LofAl 2 (SO 4 ) 3 14H 2 O, called ‘‘dry alum’’
assured. Liquid alum may be delivered in tank trucks or by rail in the industry. This is the commercial grade and the form
and pumped or fed by gravity to storage tanks at the plant site. manufactured for water treatment. The rationale for express-
ing concentration in this form is that the mass added to a given
F.4.1 DEFINITION volume of water is as Al 2 (SO 4 ) 3 14H 2 O.
Al 2 (SO 4 ) 3 18H 2 O: If pilot plant experiments, or laboratory
The term ‘‘liquid alum’’ refers to the specifications of a
experiments, are conducted using reagent grade alum, i.e.,
manufactured product, i.e., a solution of dissolved
Al 2 (SO 4 ) 3 18H 2 O, then this is the expression usually
Al 2 (SO 4 ) 3 14H 2 O in water that has a specific gravity of
adopted. Alternatively, such concentration may be converted
1.335 ( 0.002). The concentration of the solution is expressed
to Al 2 (SO 4 ) 3 14H 2 O (using the ratio of molecular weights).
as mass of Al 2 (SO 4 ) 3 14H 2 O per unit volume of solution. For
SG ¼ 1.335, C(alum) ¼ 647 g Al 2 (SO 4 ) 3 14H 2 O=L solution. Al 2 (SO 4 ) 3 : Alum concentration may be expressed also
This particular solution strength is specified merely because as Al 2 (SO 4 ) 3 with the rationale that this is the only solid
the freezing point is lowest. that exists.
Al : Increasingly, Al 3þ is the favored expression since this is
3þ
the only constituent from the alum that participates in the
F.4.2 PRODUCTION
coagulation reaction. Expressing unit cost as dollars=kg Al 3þ
Liquid alum is manufactured as indicated by the schematic is a means to ‘‘normalize,’’ i.e., finding a basis for compari-
diagram, Figure F.1. The chemical reaction in the ‘‘reactor’’ in son, alum costs.
Figure F.1 is given by Equation F.1.
Al 2 O 3 : Finally, the Al 2 O 3 equivalent is used because
The intended specific gravity is variable, depending on the
the expression is a holdover from the days when chemicals
specifications of the finished product. Alum is manufactured
were ignited to create their equivalent oxides and with
to conform to the American Water Works Association Stand-
gravimetric determination of the product. Thus, one sees iron
ard B403-98 for aluminum sulfate, which specifies ranges of
salts as ferric oxide, i.e., Fe 2 O 3 , caustic soda and soda ash as
strength for Al and Al 2 O 3 of alum products. The industry
Na 2 O, etc. The industry is moving away from the oxide form of
controls on strength is based upon the specific gravity test.
expression and toward the active metals form, as seen by
The SG standard stated above does not seem to be a common
AWWA standards for all chemicals used in water treatment.
target industry wide.
F.4.3 DESCRIPTION F.4.5 ALUM CONVERSIONS
Selected properties of liquid alum are listed in Table F.2. Alum concentrations are expressed in a number of different
Among the data given, some are for reference and others are forms. Guidance on how to convert between forms is
requisite to calculations. Data include the molecular weight, given here.