Page 890 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Appendix F: Alum Data and Conversions 845

L
F.5.2 STORAGE OF LIQUID ALUM Q(alum solution) ¼ 0:002106
min
Under favorable storage conditions, i.e., no evaporation and mL
¼ 2:106
temperatures above 108C (108F) to prevent freezing, liquid min
alum will remain chemically intact and usable. In other words,
Discussion
the ‘‘shelf life’’ of liquid alum is indefinite under such condi-
Some metering pumps (syringe or peristaltic) can achieve
tions. While temperature may be controlled, the storage tanks
such low flows. Another problem is in the distribution of
should be vented and so there is loss of water by evaporation.
the alum throughout the raw-water flow. Therefore, dilu-
According to one person experienced in the field, the storage
tion should be considered.
should be not so long then that the density of the alum
changes appreciably.
Except for very small plants, the alum storage volume should F.5.4 METERING AND CALIBRATION
be sufficient to handle a full load of alum from a truck or for
In any plant, e.g. full-scale or pilot plant, the metering flow
several days’ supply, whichever is the larger volume. As with
should be confirmed by volumetric measurement. Figure F.7
most facilities, storage and feed facilities should be redundant,
shows the setup for volumetric calibration of a metering
i.e., with duplicate facilities to ensure reliability and to provide
pump. A graduated cylinder is placed in the line from the
for cleaning and maintenance.
main alum storage to the rapid mix. For pump calibration
valve b is opened and valve a is closed. At time t 1 the volume
of alum in the graduated cylinder is measured, i.e., V(alum) 1 .
F.5.3 MASS FLOW CALCULATIONS
At time t 2 the volume of alum in the graduated cylinder is
Alum flow into the raw water is determined by the materials measured again, i.e., V(alum) 2 . Therefore,
balance principle, i.e.,
2
1
½ V(alum) V(alum) Š
Q(alum solution) C(alum) Q(metering pump) ¼ (t 2 t 1 ) (F:9)
¼ Q(raw water) C(required alum in raw water) (F:8)
In a pilot plant, such as a 75.7 L=min (20 gpm) size,
Example F.9 illustrates the calculation. the graduated cylinder may be a 100 mL burette. For a
full-scale plant, the size would depend upon the rate of alum
flow; the cylinder should be large enough to minimize the
Example F.9 Calculate Metering Rate for Dilute error of measurement. Note that after valve b, a waste line
Liquid Alum Solution to a Raw-Water Flow
should be inserted with needed valves in order to clean
the cylinder and lines between uses.
Given
The rapid mix for a 20 gpm pilot plant is to receive an
alum (as Al 2 (SO 4 ) 3 14H 2 O) dosage of C(alum in raw F.5.5 CLEANING LINES
water) ¼ 18 mg=L). The alum-feed solution has a concen-
tration, C(alum feed) ¼ 647 g=L. In maintaining the alum-flow system the storage and feed
should be in duplicate, such that one system can be rotated
Required
Determine the flow of alum-feed solution Q(alum) to the in service with another. Cleaning should be facilitated by
rapid mix. inclusion of valves and tees so that the lines can be flushed
easily with suitable provision for handling the waste flow.
Solution
Convert all U.S. Customary Units to SI=Metric. The SI units
are always preferred, but not always convenient, so apply
appropriate metric units for this problem. The pilot plant
flow is the only conversion needed, i.e., Q(raw water)
Alum Graduated cylinder
gal 3:785 L L
20 ¼ 75:7 storage
min gal min C(alum) Valve b Q(alum)
P Rapid
Apply Materials Balance Principle Valve a Metering pump mix

Q(alum solution) C(alum)
C(required alum in raw water)
¼ Q(raw water) C(required alum in raw water)
mg L mg FIGURE F.7 Volumetric measurement of alum flow—calibration
Q(alum solution) 647,000 ¼ 75:7 18
L min L of metering.