Technology Status Report: In Situ Flushing - CLU-IN

In Situ Flushing Project Summaries
GWRTAC Case Study Database
was found to coincide with elevated surfactant concentration. Also, the tailing of surfactant
recovery continues to enhance contaminant concentrations in the recovered groundwater. Average
enhancements of contaminant concentrations in the recovered groundwater ranged from a factor of
10 to 20. It should be noted that, to ensure hydraulic control, the recovered groundwater stream
was being diluted within the well bores by factor of 4:1. Actual enhancement of contaminant
concentration in the flushed zone is estimated to range from 40 to 100 times the normal aqueous
contaminant concentration.
The enhancement of contaminant recovery was not consistent for the BTEX compounds. The
results indicate that the enhancement of benzene and toluene was consistently higher than that of
xylenes. This may be due to the relative low concentrations of xylenes at this particular site or
preferential solubilization.
Air Stripper Results
Three air strippers were utilized for contaminant removal, a packed column (PC) and two hollow
fiber membranes (HF1 and HF2). These strippers were operated in a number of different
configurations and using a wide range of operating parameters. All air strippers were operated
effectively at surfactant concentrations well above the CMC. The packed column was operated at
varying air to water ratios without foam production. Stripper required minmal adjustment and
maintenance.
As noted by the results the stripping effectiveness of the hollow fiber membrane HF2 surpassed
that of HF1. It should be noted that HF2 was new, whereas HF1 was used and did not perform as
well as HF2. Also, recovered surfactant concentrations fluctuate throughout the air stripping
sensitivity analysis, and could introduce some small differences in TPH removal. Baseline
removal was evaluated for each of the air strippers before surfactant was introduced into the
system. These values were used to determine the extent that surfactants inhibit contaminant
removal using these strippers. The baseline values were calculated by running the strippers in
parallel and using the same operating parameters.
Once the baseline had been established and surfactant had been introduced into the system at .5
% by weight,the strippers were operated under similar conditions (air to water ratio of 5.2:1 instead
of 4:1) and the removal efficiency calculated. Results from operating the P.C., HF1 and HF2 in
parallel with water flow at .25 gpm and air flow at 1.3 cfm to each were given in tabular form. It
can be seen that the surfactant inhibited contaminant removal by as much as 32% in HF1 (the
used hollow fiber unit) and little as 17% in the packed column.
The hollow fiber membranes were operated in series and parallel and under varying flow rate ratios
of air to water. The results from operating HF1 and HF2 in series with a 4.5:1 air to water ratio
were shown in a table. The average removal for HF1 and HF2 in series is the percentage removed
from total TPH entering the system. That is, HF2 removed 61% of the TPH going into HF1, not
61% of the TPH leaving HF1. Table 4 also shows the same units in parallel with a ratio of 5:1 air
to water. Comparison of the two tables indicate that running the HF units in parallel is somewhat
more efficient. In comparison with Table 5, not much improvement was realized from by
increasing the ratio from 3:1 to 5:1. Another table provides results from operating the three units
in parallel at an air to water ratio of 8:1. It was seen that some improvement was realized for the
HF units but none for the Packed Column.
Ground-Water Remediation Technologies Analysis Center
Operated by Concurrent Technologies Corporation
Appendix - Page 148 of 164
Copyright GWRTAC 1998
Revision 1
Tuesday, November 17, 1998