Civil Engineering Reference
In-Depth Information
pH.
The optimum pH for As removal by activated alumina was reported by Clifford
and coworkers
10
to be in the range of 5.5 to 6.0. However, that study also noted that
reducing the feedwater pH to this value consumes nearly all of the alkalinity, increases
the TDS through addition of acid, and requires subsequent neutralization of the treated
water. The benefit of lowering the pH is that the column runs are 5 to 20 times longer
than those at natural pH. The amount of acid required to lower the pH of natural water
depends on the initial pH and alkalinity of the water.
Because the feedwater must have reduced pH for the activated alumina system to
operate, it will be necessary to adjust the product water pH prior to placing in the
distribution system. This would likely be accomplished with caustic soda.
Empty Bed Contact Time.
Clifford et al.
11
investigated EBCT values ranging from
1.5 to 10 minutes at pH 6; Amy et al.
12
(1998) investigated EBCT values ranging from
2.5 to 15 minutes at pH 7. Both investigations found that EBCT had a significant
effect on run length.
Selection of the operating EBCT represents a compromise between the improved
As removal versus the added cost of extra alumina and a bigger reactor vessel for a
long EBCT. While neither investigator makes an explicit recommendation regarding
the EBCT, the Clifford et al. report
11
implies that EBCT of 5.0 minutes is appropriate
for this system. Alcan, a manufacturer of activated alumina, has also confirmed this
recommendation of EBCT.
A second factor to consider in the hydraulic design of the sorption column is its
aspect ratio (ratio of length to diameter). The Clifford et al. study recommends bed
depth of at least 2.5 feet. For an EBCT of 5.0 minutes, this correlates to a surface
loading of 3.7 gal / min-ft
2
, which is comparable to ion-exchange loading rates.
Process Run Time.
A common method of tracking the performance of any sorption
process is to report the number of bed volumes (BVs) of water that have been treated
to reach either breakthrough or exhaustion. Breakthrough is typically identified as
when the effluent concentration increases to within 10 percent of the influent concen-
tration or when it reaches a target concentration. Exhaustion is achieved when the total
sorption capacity of the media has been consumed. Since the effluent concentration
1.0
0.9
0.1
Number of Bed Volumes Treated
Fig. 18-8.
Example of typical breakthrough curve for activated alumina
