United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-89/005 Jan. 1990
v°/EPA Project Summary
An Evaluation of the
Secondary Effects of
Air Stripping
Mark D. Umphres and James H. Van Wagner
At a 2.9 million gallon per day
(mgd) well contaminated with several
volatile organic compounds (VOCs),
principally trichloroethylene (TCE), a
packed tower aerator was pilot
tested, designed, constructed, and
monitored during its first 7 mo of
operation. Pilot testing was based on
gas/liquid mass transfer theory- Cal-
culated mass transfer coefficients
coupled with this theory were used to
design the full-scale aerator for TCE
control. Modeling of VOC off-gas
dispersion was required to obtain a
construction permit in Southern
California. In addition to liquid-phase
VOCs, other parameters including
bacteria, temperature, pH, dissolved
oxygen, calcium, alkalinity, turbidity,
particle counts, noise, and air-phase
VOCs were monitored to assess the
secondary effects of aeration.
Secondary effects refer to the air,
water, and ambient quality that might
be affected by tower operation for the
control of VOCs. Parameters such as
calcium carbonate deposition, corro-
sion, and Legionella were examined.
Of the many parameters, only cal-
cium deposition and standard plate
count (SPC) bacteria required con-
trol. The full-scale aerator was modi-
fied to improve VOC control. A capital
and operation and maintenance
(O&M) cost analysis indicated packed
tower aeration (PTA) to be cost-
effective.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Since 1974, researchers have observed
widespread contamination of finished
drinking water with trace amounts of
synthetic organic chemicals, many of
which have been linked to adverse health
effects. Recent surveys of groundwater
quality have discovered equally wide-
spread contamination of groundwaters
once thought to be protected. California
groundwater has been contaminated
principally with TCE and dibromo-
chloropropane.
The Valley County Water District
(VCWD) provides groundwater to the City
of Baldwin Park and a portion of the City
of Irwindale. The District lies in the San
Gabriel Valley, roughly 30 miles east of
the City of Los Angeles. In December
1979, TCE exceeding 1 mg/L was
discovered in one of the District's wells.
The California Department of Health
Services (CDHS), responsible for enforc-
ing drinking water standards, immediately
began an extensive monitoring program.
TCE contamination was found throughout
the San Gabriel Valley; however, the
concentrations observed elsewhere were
considerably less than those found in
some VCWD wells. In 1980, the CDHS
established an action level for TCE of 5
ng/L; wells exceeding 50 ug/L TCE were
shut down.
Granular activated carbon (GAG)
adsorption and air stripping are the most
common technologies for removing
organic contaminants. Air stripping by
PTA is considerably less expensive than
GAC. This study covered the pilot testing,
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design, and monitoring of a full-scale
PTA at the VCWD Lante Well. In addition
to TCE, the well contained tetrachlo-
roethylene (PCE) and 1,1,1-trichloro-
ethane (TCA).
Objectives
The viability of PTA has been com-
pared with that of GAC and other
processes. Although economics and sim-
plicity of operation and maintenance
favor aeration, further work was needed
to identify other aspects that could affect
its performance and cost. Several states
have established policies regarding the
emissions from tower aerators in re-
sponse to the concern over air quality
and the issue of trading a water contam-
ination problem for a potential air
pollution problem. Some states require
that controls be provided regardless of
the levels of VOCs in the off-gas. Other
states such as California require a study
of the effect of VOC dispersion to
determine if the level of air pollution is
significant and if controls are needed.
The primary objectives of this study
were to evaluate the secondary effects of
PTA. Secondary effects are those affect-
ing water, air, and ambient quality, and in
this study include PTA's influence on
mineral scaling, corrosivity, microbiolog-
ical quality, noise, air quality, and
particulate concentration. Pilot-scale tests
were conducted to develop design
criteria for the full-scale packed tower
aerator and to assemble information for
an air pollution control permit. The
aerator was then constructed, and once
in operation, it was monitored to assess
secondary effects.
Pilot-Scale Testing and Full-
Scale Design
Approach
The design of the pilot-scale testing
and the analysis of the data was based
on conventional gas/liquid mass transfer
theory. This theory established the rela-
tionship between a number of factors that
affect the design and performance of
packed tower aerators — factors includ-
ing the Henry's coefficients of TCE and
the other VOCs, packing depth, air load-
ing rate, water loading rate, air-to-water
ratio, and air pressure drop through the
packing.
Nineteen pilot runs were conducted.
Influent and effluent samples were
analyzed for VOCs, calcium, alkalinity,
total dissolved solids, turbidity, particle
counts, pH, chlorine residual, and
temperature. Each run corresponded to a
unique set of operating conditions for
packing depth and water loading rate.
The operating conditions during the pilot
study ranged from 1.75 to 9.75 ft of
packing depth, 4.3 to 28 gpm/ft2 water
loading rate, and 29 to 160 cfm/ft2 air
loading rate.
Equipment
The pilot-scale unit was designed for
countercurrent air and water flow with
forced draft air supply. It was constructed
of 11.5-in. diameter clear Plexiglas", sup-
ported by a plenum. The shell was
assembled in 2-ft sections separated by
flow redistributors. The tower was packed
with 1-in. polypropylene Super Intalox*
saddles. Air was supplied by a centrifugal
blower. The blower intake was extended
15 ft from the tower to reduce recir-
culation of the off-gas. Air flow was
adjusted to a predetermined pressure
drop across an orifice plate in the air flow
meter. An inclined-tube manometer
measured differential pressure. Influent
water was supplied directly from the well
pump discharge. Influent water entered
an 18-ft standpipe that maintained a
constant influent pressure and water flow.
From the standpipe, flow was diverted to
a rotameter and then to the packing.
Treated water flowed from the plenum to
an air break. The height of the air break
was adjusted to keep water in the plenum
so that gasses would not be lost through
the water discharge line.
Testing and Design
Pump tests and pilot-scale studies took
place in November and December 1982.
Pump tests indicated TCE typically
increased from near 70 to near 400 ug/L
in 6 hr and leveled off near 500 pg/L at
24 hr. During pilot tests, the pump was
operated 4 hr before testing began with
the result that influent TCE ranged from
200 to 400 ug/L.
The Sherwood and Holloway corre-
lation was used to determine mass
transfer coefficients (KLa's). The data
analyses discounted the contribution of
end effects, i.e., volatile losses in the
distribution and plenum collection of
water. For each of the 19 runs, Kua's
were determined for TCE, PCE, TCA and
for four trihalomethanes that were spiked
into the influent water. A computer model
•Mention of trade names or commercial products
does not constitute endorsement or recom-
mendation for use
(utilizing the theory and design crit
described in the Project Report) analy
the data for design purposes.
As inputs to the computer model,
ug/L influent TCE and 3 pg/L efflt
TCE were conservatively chosen i
yielded a design of 99.4% remo
based on countercurrent operation. '
1-in. saddles used during piloting w
also chosen. The least-cost desi
based on a 1400 gpm (2.9 mgd) flowr,
included: 30 gpm/ft2 water loading r;
5600 cfm air loading rate, 30:1 air
water ratio, 18-ft packing depth, 7.
tower diameter, and an air pressure d
of 0.05 in./ft of packing. The design v
done in early 1983.
A unique feature of the design v
elevating the packed tower aerator 01
structural platform to allow treated we
to flow directly to an existing, surfa
level, 2-mil-gal storage reservoir. Ad
tionally, the design called for the cai
bility to feed chlorine and sodium he
metaphosphate (SHMP) to the aera
influent or effluent, as needed, for mic
biological or stability control. The aera
incorporated ports at its top and botti
to allow access to water and
distributors and packing material.
Secondary Effects
Pilot testing included an analysis
secondary effects. The chlorine dema
was found to be less than 0.5 mg
Turbidity and particle count data show
insignificant change; these data su
gested that, even at higher air-to-wa
ratios, any entrainment of dust would r
measurably affect suspended solid cc
centration. Aeration raised the pH abc
0.3 units. Calcium and alkalinity lev*
showed very little change. The calculat
Langlier Index indicated a slight increa
as a result of aeration, but suggest
only slight calcium carbonate scalii
tendencies. Influent water temperatun
were near 64°F and dropped less th;
1°F with aeration. Mean effluent c
temperatures were the same as efflue
water temperatures.
Ambient air was sampled for VOCs
the pilot unit when it was not in operatic
TCE concentrations between 6 and (
ng/L were found, with the higher lev
attributed to nearby industrial activit
Using Henry's law and the pilot-sea
hydraulics, the worst-case scenario
entraining these VOCs into the treat*
water was calculated and found to t
between 0.02 and 0.2 ug/L. The Ce
culated levels were lower than tho:
typically found during pilot testing ar
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suggest that ambient air would have no
detrimental impact of full-scale PTA
performance.
Air Quality Modeling
The South Coast Air Quality Manage-
ment District (SCAQMD) did computer
modeling of the dispersion pattern of
expected VOC discharges from the
designed packed tower aerator. The
modeling produced estimates of annual
average ground-level concentrations of
TCE within 2 mi of the Lante Well. The
model utilized projected concentrations of
the VOC off-gases and meteorological
data from nearby airports. Results indi-
cated that PTA off-gases would create a
maximum annual average TCE concen-
tration of 21 ng/L northeast of the well
outside a residential area. A maximum
annual average in the nearby residential
area would be 5.8 ng/L. The CDHS, in
reviewing the results, used a 106 risk
factor of 52 ng/L for TCE. The VCWD
received a permit to construct.
Full-Scale Monitoring
Following construction and start-up
testing, the packed tower aerator was
monitored during the 7 mo period begin-
ning in July 1985. For the first 3 mo,
operation was continuous and allowed for
corrosion testing. Normally, the VCWD
operates the Lante Well on an inter-
mittent basis to maintain distribution
system pressure and meet demand. Fol-
lowing the corrosion monitoring, the
aerator was operated intermittently, com-
ing on-line at midnight and shutting down
near 8:30 am. Samples were typically
collected at 7:30 am.
VOCs
Start-up tests began in January 1985.
During the first few months of testing,
TCE influent concentrations occasionally
exceeded 800 iig/L-significantly above
the 500 tig/L design criteria. The reason
for this is not understood. Because of this
increase, effluent TCE concentrations
sometimes exceeded the 5 \ng/L target
level. Control of TCE was as low as
96.9% removal. Several modifications
were made in an attempt to improve
performance. After start-up, 18 ft of pack-
ing had settled to approximately 17-1/2 ft.
Further, it was observed that water was
not evenly distributed over a series of
weir troughs that delivered water to the
packing. Adding 6 in. of packing material
and stainless steel wire mesh to the weir
troughs improved performance. The air
inlet was modified from one that
delivered air offset from the center of the
aerator to one that directed air more
symmetrically across the bottom of the
tower. This, however, did not improve
performance.
During the 7-mo monitoring period,
TCE control ranged from 99.1% to 99.5%
removal, and effluent concentrations were
typically below 5 pg/L. The difference
between pilot-scale prediction and full-
scale performance may have been due to
axial dispersion and side-wall channeling.
The consensus of packing manufacturers
was that it is not necessary to redistribute
water with packing depths less than 20 ft.
No redistributors were built into the full-
scale tower although they were used in
the pilot-scale tower upon which the
design was based. At full-scale, redis-
tributors may have improved perform-
ance.
Secondary Effects
Microbiological Quality
During the first 3 mo of monitoring,
chlorine was fed ahead of the aerator but
downstream of the influent sample tap.
When prechlorinating, effluent residuals
exceeded 0.6 mg/L, and little difference
was observed between influent and
effluent SPC when either R2A or plate
count agar media was used. To evaluate
the potential for microbiological growth in
the aerator, prechlorination was discon-
tinued. In the following months, effluent
plate counts were approximately 2 logs
greater than influent counts as a result of
aeration. Effluent densities were typically
103/mL. Results from the two media were
in reasonable agreement.
Legionella were not found in any of the
samples. Colonies with morphology simi-
lar to Legionella were observed in pre-
sumptive tests, however. Subsequent
confirmation attempts using fluorescent
antibody staining indicated that Legion-
ella were not present.
Similarly, coliforms were not found in
any of the samples using the most
probable number method. All samples
were reported at less than 2.2 colony
forming units/100 mL.
Water Temperature
The influent water temperature was
consistently in the range of 59° to 63°F.
Effluent temperatures were typically
within one or two degrees of influent, with
a few exceptions when effluent temper-
atures dropped 5° to 10°F. On these
occasions, correlation with dry bulb
temperature or relative humidity was not
apparent.
Calcium Carbonate Deposition
The VCWD observed calcium carbon-
ate scaling during an earlier project
studying spray aeration in the 2-mil-gal
reservoir. In this project, after 3 wk of
operation, the pump boosting water from
the reservoir to the distribution system
failed as a result of scaling within the
casing. Additionally, scaling was ob-
served on the bottom 3 in. of packing.
After SHMP was brought on line at 1
mg/L and the pump was acid cleaned,
further scale buildup in the pump and on
the packing in subsequent weeks was not
evident.
Analyses of the packing showed no
calcium scale at the top of the tower, but
an increase from less than 1 mg Ca/piece
at the bottom at start-up to 131 mg
Ca/piece at the bottom after 3 wk.
Analyses of dissolved calcium showed
very little change across the aerator.
Concentrations fluctuated between 66
and 72 mg/L. Coupled with the flow-rate
data over the 3 wk period, it was
concluded that scaling was limited to the
lower 3 in. packing. Analyses of the
packing following several weeks of add-
ing SHMP confirmed that further scaling
had not occurred.
Influent and effluent alkalinity were
nearly constant at 180 mg/L over time.
Aeration raised the mean dissolved oxy-
gen (DO) concentration from 5.5 to 9.3
mg/L. The pH level was typically elevated
from 7.6 to near 7.9 as carbon dioxide
was stripped. Although C02 was not
measured, its calculated loss, based on
carbonate chemistry, was approximately
5.2 mg/L.
The calculated Langlier Index of the
water influent to the aerator was near
zero. With the increase of pH during
aeration, however, the Index was slightly
positive, suggesting the treated water had
mild scale-forming properties. This could
account for the calcium scaling in the
bottom of the tower and in the down-
stream pump. Both of these waters
experienced the highest pH.
Corrosion
Mild steel and copper corrosion rates
were evaluated during the first 3 mo
when the aerator was in continuous
operation. Rates were measured with
Rohrbach corrator probes and an ASTM
procedure for metal coupon weight loss.
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Both metal coupons and corrator probes
were installed in corrosion test racks
located on the influent and effluent sides
of the aerator. Coupon analyses indicated
a decrease in the rates of both metals
over time. Copper displayed little
difference in rates between influent and
effluent waters, whereas mild steel rates
decreased slightly across the aerator.
Weekly corrator readings indicated a
similar drop in rates for both metals over
time, and generally, the readings were
within 1 log agreement of rates deter-
mined by coupon weight. The slightly
higher mild steel corrosion rates in the
aerator influent may possibly be a result
of the lower pH and lower Langlier Index
of that water.
The addition of SHMP after the third
week of operation appeared to have little
effect on the effluent corrosion rate of
either material.
Particulates
Both turbidity and particle counts in the
range of 1p to 60ii were measured.
Despite the fact that a residue gradually
built up at the blower inlet and at the air
inlet within the aerator, no significant
difference between influent and effluent
turbidity or particle counts were
observed. Effluent turbidities averaged
0.29 nephelometric turbidity unit.
Air Quality
24-hr, continuous air samples were
collected by a SCAQMD contractor up-
and downwind of the aerator. Based on
the model and ambient conditions, the
downwind sampler was located at a
distance suspected of having the highest
concentrations of TCE. The detection
level provided by the contractor was near
5.8 jig/L for TCE. A lower detection level
would have required technologies not
routinely used by the SCAQMD. TCE was
not found in any of the samples. Because
these levels were appreciably higher than
the 21.8 ng/L annual average TCE
concentration predicted by the model
before the construction permit was
granted, model predictions could not be
checked directly.
Noise
With the aerator operating, noise levels
were near 80 decibels (dB) at the blower.
At a distance of 100 ft, however, levels
dropped to near 55 dB, which was the
background level at that distance with the
aerator shut down. OSHA restricts 8-hr
workplace noise to no greater than 90 dB.
Costs
The capital cost of the tower was near
$218,000. This included connecting of
the existing reservoir and the chlorine
and SHMP feed systems. The amortized
capital cost was $0.062/1000 gal. O&M
costs, based on the 7-mo monitoring
period, amounted to $0.031/1000 gal. A
comparison of the capital cost of the
elevated tower versus a wet well and
repumping indicated comparative costs
within 10%. This suggests that the total
cost of $0.093/1000 gal can be con-
sidered representative of PTA facilities.
Conclusions
The use of gas/liquid mass transfer
theory together with pilot testing data
provided for the design of a full-scale
PTA. Factors such as water distributic
over the packing, redistribution onto tt
packing, and changes in packing dep
can influence full-scale performance fi
control of VOCs. The secondary effec
observed during pilot testing were a goc
predictor of the secondary effed
observed during full-scale operation. Th
changes in water chemistry as a result <
aeration, such as pH, DO, and C02, ma
affect calcium carbonate deposition c
corrosion of some metals. Corrosion <
mild steel and copper was insignificant i
these studies; however, SHMP wa
required to prevent deposition. Bacteri;
densities, as measured by SPC, ir
creased as a result of aeration, bt
Legionella and coliform bacteria were nc
observed. Moderate levels of chlorin
were sufficient for bacterial contro
Aeration had no significant effect oi
waterborne particulates, water tempera
ture, or ambient noise levels. The impac
of VOC off-gas dispersion and centre
must be considered. In this study, m
TCE was found in air at low part-per
billion levels during tower operation
Modeling of PTA off-gases indicated TCf
levels in the low part-per-trillion range
which were not significantly different thai
measured ambient TCE levels. A PT>
total cost of less than $0.10/1000 gal a
2.9 mgd was demonstrated and deeme(
cost-effective for VOC control.
The full report was submitted in ful
fillment of Cooperative Agreement CR
809974 by James M. Montgomery Con
suiting Engineers, Inc., under the spon
sorship of the U.S. Environmental Protec
tion Agency.
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Mark D. Umphres is with James M. Montgomery Consulting Engineers, Inc.,
Pasadena, CA 91101; and James H. Van Wagner, now retired, was with the Valley
County Water District, Baldwin Park, CA 91706.
Richard J. Miltner is the EPA Project Officer (see below).
The complete report, entitled "An Evaluation of the Secondary Effects of
Air Stripping," (Order No. PB 89-161 517I AS; Cost: $15.95, subject to change) will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-89/005
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