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Development of a groundwater biobarrier for the removal
of polycyclic aromatic hydrocarbons, BTEX, and
heterocyclic hydrocarbons
A. Tiehm, A. Mu
¨ller, S. Alt, H. Jacob, H. Schad and C. Weingran
ABSTRACT
A. Tiehm
A. Mu
¨ller
Department of Environmental Biotechnology,
Water Technology Center, Karlsruher Str. 84,
Karlsruhe 76139,
Germany
E-mail: tiehm@tzw.de;
a.mueller@tzw.de
S. Alt
H. Jacob
CDM Consult GmbH, Neue Bergstr. 9-13,
Alsbach-Ha
¨hnlein 64665,
Germany
E-mail: helmut.jacob@cdm-ag.de
H. Schad
I.M.E.S. GmbH, Martinstr. 1, Amtzell 88279,
Germany
E-mail: hermann.schad@imes-gmbh.net
C. Weingran
HIM GmbH, Waldstrasse 11, Biebesheim 64584,
Germany
E-mail: christian.weingran@him.de
A full scale funnel-and-gate biobarrier has been developed for the removal of tar oil pollutants at
an abandoned tar factory site near the city of Offenbach, Germany. Laboratory and on-site
column studies were done to determine the operation parameters for microbiological clean-up of
the groundwater polluted with 12,000 mg/L mono- aromatic hydrocarbons such as benzene and
the xylenes, 4,800 mg/L polycyclic aromatic hydrocarbons such as naphthalene and
acenaphthene, and 4,700 mg/L heterocyclic aromatic hydrocarbons such as benzofuran and
benzothiophene. In the laboratory study, a residence time of approx. 70h proved to be sufficient
for aerobic pollutant biodegradation. Up to 180 mg/L H
2
O
2
were added and did not lead to any
toxic effects to the degrading bacteria. The feasibility of the concept was confirmed in an on-site
pilot study performed with a sedimentation tank (removal of ferric iron) and two bioreactors. In
the bioreactors, .99.3% of the pollutants were degraded. Biodegradation activity corresponded
to a significant increase in numbers of pollutant degrading bacteria. In the bioreactors, a fast
dissociation of H
2
O
2
was observed resulting in losses of oxygen and temporary gas clogging.
Therefore, a repeated addition of moderate concentrations of H
2
O
2
proved to be more favourable
than the addition of high concentrations at a single dosing port. The full scale biobarrier consists
of three separated bioreactors thus enabling extended control and access to the reactors. The
operation of the funnel-and-gate biobarrier started in April 2007, and represents the first
biological permeable reactive barrier with extended control (EC-PRB) in Germany.
Key words
|
biobarrier, BTEX, heterocyclic compounds, H
2
O
2
, microbial numbers, PAH, toxicity
reduction
INTRODUCTION
At an abandoned tar factory site near the city of Offenbach,
Germany, the groundwater is contaminated with tar oil
pollutants. A plume of dissolved contaminants extends in the
aquifer from the hot spot area downgradient towards the
Main river. Under the strictly anaerobic, i.e. methanogenic,
conditions in the groundwater plume at the site, biodegra-
dation of the pollutants is limited resulting in a long plume.
However, most low molecular weight polycyclic aromatic
hydrocarbons (PAH), the BTEX (benzene, toluene, ethyl-
benzene, xylenes), other mono-aromatic hydrocarbons
(AHC) and heterocyclic aromatic hydrocarbons (HCY)—
toxic pollutants often occuring at tar oil polluted sites—are
degradable in the presence of oxygen (Cerniglia 1992;Fritsche
& Hofrichter 2000;Ka¨ stner 2000;Sagner & Tiehm 2005). In
the presence of nitrate, biodegradation of some specific tar oil
pollutants also has been demonstrated (Hutchins 1991;Leduc
et al. 1992;Tiehm et al. 1997;Lovley 2000;Rockne et al. 2000;
Tiehm & Schulze 2003;Sagner et al. 2005).
As part of the RUBIN research and development
network (“RUBIN” 2007) focusing on permeable reactive
doi: 10.2166/wst.2008.730
1349 QIWA Publishing 2008 Water Science & Technology—WST
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58.7
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2008
barriers (PRB), a funnel-and-gate system was constructed at
the site (Figure 1). The concept for water clean-up is based
on a three step process including (i) sedimentation of ferric
iron, (ii) aerobic biodegradation of the aromatic hydro-
carbons and heterocyclic compounds, and (iii) a subsequent
zone with granular activated carbon (GAC) removing
remaining pollutants. This paper focuses on the studies
leading to the operation and design parameters of the
microbiological in-situ reactor.
In the bioreactor, the availability of suitable electron
acceptors and nutrients has to be considered as important
factor affecting biodegradation efficiency. Due to the high
pollutant concentration in the groundwater at this site (approx.
25 mg/L tar oil pollutants), hydrogen peroxide was selected
as oxygen carrier due to its higher water solubility as compared
to oxygen. Additionally, nitrate was added as alternative
electron acceptor. Laboratory and pilot scale experiments
were done to determine (i) the site-specific efficiency and
sequence of pollutant biodegradation, (ii) the locations and
concentrations of hydrogen peroxide addition, and (iii) the
suitable operation parameters of the in-situ biobarrier.
MATERIALS AND METHODS
The laboratory column experiments were performed at
groundwater temperature (12–148C) with groundwater
samples taken from well GWM19. The groundwater
samples were filled into 30 L Teflon bags enclosed in
nitrogen flushed vessels and stored under anaerobic
conditions. The laboratory column (B25 cm, length
100 cm, volume 12.7 L) was filled with silica sand (grain
size 1– 2.5 mm), and the flow direction was top—down. In
order to remove precipitated ferric iron, reversal flushing
was performed discontinuously. Hydrogen peroxide (H
2
O
2
)
and nitrate were added at the column inlet. During column
operation a second dosing port of H
2
O
2
in the upper part of
the column was installed. Column experiments were carried
out for a period of 300 d.
The pilot plant columns were operated on-site in an air-
conditioned container. Two bioreactors (stainless steel
columns, filled with silica sand) were operated with
groundwater taken from well GWM19. A sedimentation
tank with a lamella separator to remove ferric iron after the
first H
2
O
2
-addition was installed in front of the bioreactors.
H
2
O
2
(35 Vol.-% H
2
O
2
) was added to the sedimentation
tank and to the two bioreactors. Nitrate was added to the
first bioreactor. Phosphate (KH
2
PO
4
/Na
2
HPO
4
-buffer) was
dosed discontinuously at both bioreactors. A GAC column
was installed at the bioreactor effluent to remove remaining
pollutants. The experimental set-up is shown in Figure 2.
The on-site column experiments were operated for a
period of 270 d.
Determination of microbial numbers
Microbial numbers were determined by the most probable
number (MPN) microplate technique with groundwater
samples (Stieber et al. 1994). Total heterotrophs and
denitrifying bacteria were determined in complex media
containing easily degradable compounds under aerobic or
anoxic conditions, respectively. Pollutant degrading bac-
teria were determined under aerobic conditions in mineral
medium with BTEX or 2-/3-ring PAH as sole carbon
source. Samples are diluted in microplates. Increased
turbidity or the formation of coloured metabolites indicated
the growth of aerobic heterotrophs and aerobic pollutant
degraders. Increased turbidity and consumption of nitrate
indicated denitrification. Incubation was done at 20 8Cin
the dark for 7 days (total heterotrophs), 21 days (2-/3-ring
Figure 1
|
Site map with location of the funnel-and-gate biobarrier.
1350 A. Tiehm et al.
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PAH- degrading bacteria, denitrifying bacteria) and 42 days
(BTEX- degrading bacteria).
Chemical analysis
Benzene, ethylbenzene, xylenes, naphthalene, and methane
were routinely analysed with a gas chromatograph (HP 5890,
Germany) with flame ionization detector (FID) equipped
with a headspace autosampler. Analysis of groundwater
extracts was performed by gas chromatography-mass spec-
trometry (HP 6890N, Germany). Extracting agents were
pentane to quantify monoaromatic hydrocarbons and PAH,
and methyl tert-butyl ether (MTBE) for HCY. Chemical
oxygen demand (COD) was determined with the Nano-
color test kit (Macherey & Nagel, Germany) according to
ISO 15 705.
Toxicity test
The aquatic toxicity tests were carried out according
to ISO11348-3 L34 with a LUMIStox-Photometer and
LUMIStherm-incubation unit (Dr. Lange, Germany) using
lyophilized Vibrio fischeri as test organism.
RESULTS AND DISCUSSION
Laboratory column studies
The microbiological investigations at the Offenbach site
revealed that sufficient numbers of microorganisms capable
to degrade BTEX and PAH under aerobic conditions were
present in the contaminated soil and groundwater. Deni-
trifying bacteria were also detected. Therefore, additional
inoculation of allochthonous microorganism was not
required to establish aerobic and denitrifying consortia in
the bioreactor.
The results of the laboratory column studies with
groundwater from the site showed that ethylbenzene
and naphthalene were already degraded under O
2
-restric-
ted/denitrifying conditions. In particular the degradation
of ethylbenzene was already stimulated by the addition of
nitrate (20 mg/L). The subsequent addition of increasing
amounts of H
2
O
2
up to a concentration of 180 mg/L
resulted in increasing biodegradation of the pollutants.
Methane degradation started after removal of the hydro-
carbons (Figure 3). The main pollutants were eliminated
in the following sequence: ethylbenzene, naphthalene,
m-/p-xylene, benzene, o-xylene. Also heterocyclic aromatic
hydrocarbons such as benzothiophene and benzofuran
Figure 2
|
Scheme of the experimental set-up of the on-site biosorption system.
1351 A. Tiehm et al.
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were degraded. The increasing consumption of electron
acceptors was corresponding to a decreasing chemical
oxygen demand (COD).
Pollutant biodegradation was also demonstrated in
additional batch studies (data not shown). In abiotic
controls, operated with field samples inhibited by addition
of HgCl
2
, high pollutant recovery was obtained even after
incubation with H
2
O
2
for periods of several months.
Therefore, our study does not indicate any significant
advanced oxidation processes, most probably due to the
groundwater pH values between 6.6 and 8.0.
In the lab column study, a flow rate between 2 and 4 L/d
proved to be suitable for complete microbiological pollutant
removal. The corresponding residence time of approx. 36 to
72 h was also chosen for the subsequent field study.
Pilot plant field study
The laboratory results were confirmed by a subsequent pilot
study performed at the Offenbach site. The flow rate was
adjusted to 22 –44 L/d resulting in residence times between
36 to 72 h. In the pilot plant, a ferric iron-sedimentation
tank with a lamella separator was installed prior to the
biodegradation step in order to avoid clogging effects by
precipitation of ferric iron (up to 25 mg/L ferrous iron in
groundwater) and to avoid degassing of supersaturated
gases (methane) in the bioreactor.
The addition of H
2
O
2
into the sedimentation tank also
resulted in aerobic biodegradation of part of the pollutants
in this area. However, degradation was most pronounced in
the bioreactors. After an adaptation and operation time of
270 d, concentrations decreased to 28 mg/l BTEX (10 mg/L
benzene), 16 mg/L other monoaromatic hydrocarbons,
32 mg/L heterocyclic hydrocarbons (17 mg/L benzofuran),
1.2 mg/L naphthalene and 2.6 mg/L other PAH in the two
bioreactors. Elimination of the main contaminants was
.99% at a residence time of 72 hours (Table 1). Toxicity
(inhibition of bacterial luminescence) was also significantly
reduced (Figure 4).
Overall approximately 20–25 mg/L of the monitored
organic compounds were degraded. The stoichiometrically
calculated oxygen demand for the complete mineralisation of
the hydrocarbons corresponds to 60– 75 mg O
2
/L (130–
160 mg/L H
2
O
2
). In the pilot plant, an influent COD of
142 mg/L was observed indicating the presence of other
oxidizable compounds that were not detected by the analysis
of the specific hydrocarbons. In the bioreactors, approximately
100 mg/L of the influent COD was removed. This removal
efficiency is consistent with the amounts of H
2
O
2
added, taking
into consideration biomass formation (Figure 5) resulting
in incomplete mineralisation of the organic substrates.
The stimulated aerobic biodegradation resulted in
increasing microbial numbers in the biobarrier system.
Figure 3
|
Effect of increasing H
2
O
2
addition on pollutant biodegradation.
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Table 1
|
Bioreactor elimination performance after an operation period of 270 days (d)
Influent concentration
bioreactor 1 [mg/L]
Effluent concentration
bioreactor 2 [mg/L]
Bioreactor elimination [% of input concentration]
Reactor 1 Reactor 2 Reactor 1 12
Compounds
Naphthalene 2,400 1.2 97 3 99.95
Ethylbenzene 550 0.54 94 6 99.9
Benzene 5,300 10 89 11 99.8
Carbazole 150 0.5 85 14 99.7
1-benzothiophene 570 9.4 83 15 98.4
Acenaphthylene 12 0.16 81 18 98.7
Dibenzofuran 171 0.9 73 27 99.5
Toluene 66 0.88 73 26 98.7
Acenaphthene 110 1.8 72 27 98.4
2-methyl-benzofuran 165 3.9 70 27 97.6
m- þp- xylene 1,500 10 68 31 99.3
Benzofuran 2,500 17 67 33 99.3
Phenathrene 13 0.078 59 40 99.4
Fluorine 26 0.27 58 41 99.0
2,3-dimethylbenzofuran 16 ,0.1 58 .42 .99.4
o-xylene 660 7.0 50 49 98.9
Dibenzothiophene 9.1 0.4 33 63 95.6
Fluoranthene 0.66 ,0.02 11 .86 .97.0
Anthracene 1.0 0.092 5 .86 .90.8
Pyrene 0.40 ,0.02 0 .95 .95.0
Methane 5,140 2,320 3 52 55
Classes of pollutants
BTEX 8,076 28 82 17 99.6
other AHC 5,588 16 83 16 99.6
HCY 3,575 32 70 29 99.1
PAH w/o NAP 163 2.6 68 30 98.5
COD [mg O
2
/L] 142 37 42 21 63.4
Figure 4
|
Toxicity reduction before treatment and after passage of the iron
sedimentation, the bioreactors and the activated carbon column. (
p
G
L20
value: dilution factor resulting in an inhibition ,20%).
Figure 5
|
Numbers (MPN) of pollutant degrading microorganisms in the original
groundwater and after passage of the lamella separator and the biobarrier
components.
1353 A. Tiehm et al.
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2008
The highest density of BTEX- and PAH-degrading bacteria
was observed in the first bioreactor (Figure 5) where most
of the pollutants were eliminated (Table 1).
In the on-site bioreactors and also during the laboratory
bioreactor study, the addition of high concentrations of
H
2
O
2
resulted in oxygen gas bubble formation and a
reduction of hydraulic conductivity, due to the fast
dissociation of H
2
O
2
and limited solubility of O
2
. Therefore,
the column experiments revealed that moderate additions
of H
2
O
2
at several dosing ports are favourable as compared
to the addition of a high amount of H
2
O
2
at a single dosing
port. Taking into consideration these findings and—in
general—the better ability to control the system, a modular
full scale system was developed.
CONCLUSIONS AND FULL SCALE BIOBARRIER
DESIGN
The laboratory experiments and the pilot plant field study
confirmed the feasibility of the biobarrier system. Within the
bioreactor 99% of the monitored influent contaminants
were eliminated at a residence time of 72 h. Due to a fast
dissociation of H
2
O
2
, a repeated addition of moderate
concentrations is recommended in order to avoid losses of
oxygen and gas clogging in the biobarrier.
In the full scale funnel-and-gate biobarrier (Figure 6)
the contaminated water is collected from the aquifer
through a gravel filter. An open water area is separated
from the gravel through a steel screen. Within the open
water area (open water zone 1), the groundwater flow is
focused to a connecting pipe, where hydrogen peroxide is
added (dosing port 1). The following lamella separator is
needed for the sedimentation of precipitated iron. At the
outlet tube from the lamella separator (inlet of the first
bioreactor), hydrogen peroxide and nutrients are added
(dosing port 2). Distribution of the water to the entire cross-
section of the bioreactor is accomplished through the open
water zones. All bioreactors are filled with coarse sand. In
between the different bioreactors again hydrogen peroxide
and nutrients are added at the connection of the open water
areas for complete degradation (dosing ports 3 þ4).
Samples for monitoring are taken in front of the dosing
ports and in several sampling wells. If pollutant elimination
in the full scale system is not sufficient to reach the
compliance values, activated carbon will be filled into the
last chamber to remove the remaining pollutants.
The modular design of the biobarrier with three separated
bioreactors follows the concept of permeable reactive
barriers with extended control (EC-PRB) which enable
access to the reactors in case of malfunction. A successful
operation of EC-PRBs has been reported for Zero Valent
Figure 6
|
Longitudinal section of the full-scale biobarrier system.
1354 A. Tiehm et al.
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Iron (ZVI) and GAC barriers (Birke et al. 2005). The
operation of the funnel-and-gate biobarrier started in April
2007, and represents the first biological EC-PRB in Germany.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge financial support by the
HIM GmbH and the German Ministry of Education and
Research (RUBIN funding priority, BMBF, grant no
02WR0293).
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