Clean Techn Environ Policy (2005) 7: 285–293
DOI 10.1007/s10098-005-0003-x
O R I GI N A L P A P E R
N. D. V. N. S. Murali Krishna Æ Ligy Philip
Thiobacillus denitrificans immobilized biotrickling filter for NO2 removal
Received: 16 June 2004 / Accepted: 22 April 2005 / Published online: 6 August 2005
Springer-Verlag 2005
Abstract Nitrogen dioxide (NO2) removal efficiency of a
biotrikling filter was evaluated under different operating
conditions. Activated alumina (AA) was used as the
immobilization matrix for Thiobacillus denitrificans (T.
denitrificans) in the biotrickling filter. Batch studies were
conducted to find out the degradation kinetics of nitrate
and nitrite for a concentration range of 600–10,000 mg/
L expressed as nitrogen. Nitrite exhibited maximum
degradation rate followed by nitrate. Electron acceptor
in the form of NO2 gas showed least removal efficiency.
Bio-kinetic parameters for T. denitrificans, by utilizing
nitrate and nitrite as electron acceptors, were also evaluated. The lmax (Maximum specific growth rate) and YT
(Yield coefficient) values for T. denitrificans in the
presence of nitrate and nitrite were 1.03 h 1, 0.275 and
0.63 h 1, 0.1316 respectively. Column study was conducted to find the adsorption and desorption potential
of activated alumina. The adsorbed NO2 from AA could
easily be desorbed using distilled water with an efficiency
of 76±0.8%. Once fed batch studies were conducted to
evaluate the NO2 removal efficiency by a biotrickling
filter. With an influent NO2 gas concentration of
2,735 ppm, the reactor could achieve a removal efficiency of 99% within 2 min from gas phase and within
96 h from the liquid phase, with an average biomass
concentration of 200 mg/g of AA. The mechanism of
NO2 gas removal in the biotrickling filter seems to be the
dissolution of NO2 in water to form NO3 , conversion of
NO3 to NO2 , and finally to N2 gas.
Introduction
The presence of oxides of nitrogen (NOx) in the ambient
air has been, and still is, of great concern because of the
toxicity of individual compounds or the secondary
N. D. V. N. S. M. Krishna Æ L. Philip (&)
Department of Civil Engineering, Indian Institute of Technology
Madras, Chennai, 600 036 India
E-mail: [email protected]
pollutants produced by the reaction of NOx with
hydrocarbons and other chemicals such as ozone in
presence of sunlight (Wark and Warner 1981). NOx is a
collective name used to represent various compounds of
oxides of nitrogen. From the air pollution point of view
NO and NO2 are of major concern. NO exists in very
low concentrations in the atmosphere and is converted
to NO2 at higher concentrations. About 44% of NOx
pollution is contributed by mobile sources, 55% by
stationary sources and the remaining by solid waste
disposal and miscellaneous processes. Various stationary
sources that emit NO2 are power plants, utility boilers,
steel industries, ceramic industry, nitric acid manufacturing industry, oil refineries, ammonia manufacturing
industry, fertilizer manufacturers, pickling operations in
anodizing plants and nylon intermediate plants (NAPCA 1970).
To curb gaseous pollutants, two methods viz., modification of original source and removal of gaseous
pollutants from the flue gas stream are commonly employed. Because of various reasons, the modification
process may not be always feasible to attain desirable
levels of emissions. The latter method involves methods
like absorption, adsorption and combustion controls.
The major problems associated with sorption methods
include the high cost of adsorbents and the attention
required for treatment and/or disposal of spent adsorbent. Removal by combustion controls requires high
temperature; moreover, high cost of fuel and catalysts
make the processes non-viable. Because of the prominent disadvantages of conventional treatment methods,
attention has been shifted to biological alternatives for
air pollution control.
Nitrogen oxides are facing increasingly stringent
regulations due to Clean Air Act Amendments of 1990,
because of its active role in photochemical reactions
and the smog formation. Though many technologies
are available for the reduction of NOx formation at
source or control of generated NOx, most of them are
expensive or having other drawbacks. As an alternative
technology for the control of NOx, biofiltration is
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receiving increased research attention. (Apel and Turick 1993; Shanmugasundaram et al. 1993; Davidova et
al. 1997; Chou and Lin 2000). Biofiltration has been
proven an effective and inexpensive method of gaseous
waste treatment (Leson and Winer 1991; Tiwaree et al.
1992; Zilli et al. 1993; Deshusses 1997; Chung and
Huang 1998; Hayens 1999). Chung and Huang (1998)
were successful in treating ammonia in air by an
immobilized Nitrosomonas europaea biofilter with removal efficiencies greater than 97%. Du Plessis et al.
(1988) were successful in removing an NO concentration of 60 ppm at an EBRT of 3 min in an aerobic
toluene treating reactor. Chou and Lin (2000) demonstrated the suitability of a biotrickling filter for the
removal of NO.
Many people have tried different immobilization
materials for biomass varying from alginate beads to
compost (Ottengraf and Van Den Oever 1983; Zilli et
al. 1993; Tiwaree et al. 1992; Chung and Huang 1998;
Hayens 1999). Even though alginate beads can immobilize very high concentrations of biomass, mass
transfer is the problem (Philip et al. 1993; Knippschild
and Rehm 1995).The reaction rates are higher for
surface immobilized compared to those entrapped in
polymeric networks as the contact with the microbes
and the pollutant molecules are better in case of surface
immobilization. Though compost is a good biofilter
matrix, it cannot be used effectively in biotrickling filters due to the compaction and excessive pressure
drops. Activated alumina (AA) is known as a very
good bacterial immobilization matrix (Philip et al.
1996). Chagnot et al. (1998) used activated carbon as
pre-concentration medium while treating oxides of
nitrogen in biotrickling filter.
Even though a few researchers demonstrated the
treatment of NOx using biotrickling filters, the information available on the biokinetic parameters of the
microbes and the fate of pollutants in the leachate are
scanty. In the present investigation, an attempt was
made to develop a biotrickling filter for the removal of
NO2 gas. Studies were conducted to evaluate the kinetics
of NO2 degradation by Thiobacillus denitrificans, isolated from sulfate containing wastewaters and the biokinetic parameters of the system. The suitability of
activated alumina as biofilter media was also evaluated.
Finally, the performance of the biofilter for NO2 removal was monitored under different feed conditions,
which gave an insight into the mechanism of NO2 gas
removal in the bio-system.
Materials and methods
Microbe
In this study, T. denitrificans, isolated from sulfate
containing wastewater and obtained from Regional
Research Laboratory, Trivandrum, Kerala, India was
used.
Media
The growth medium (M1) for bacteria consisted of
KNO3, 5 g; NH4Cl, 0.5 g; MgSO4, 0.02 g; NaHCO3,
1 g; KH2 PO4, 1.8 g; Na2 HPO4, 1.2 g; CaCl2, 0.03 g;
MnSO4, 0.02 g; FeCl3, 0.02 g and glucose with a C: N
ratio of 2.5:1 and 1 mL of trace metal solution in 1 L of
distilled water and the pH was adjusted to 7±0.1 with
0.1 HCl and/or 0.1 N NaOH. The medium used for
NO2 removal studies contained all other compounds
except KNO3 and NH4 Cl. The glucose concentration
varied as per the nitrate/nitrite nitrogen concentration in
the gas stream. During the batch and continuous
experiments, pH of the system was monitored regularly
and adjusted to 7–8 using NaHCO3 solution.
Bacterial cultivation
Cells were grown overnight in previously sterilized
nutrient broth at 30C and harvested by centrifuging at
6,000·g for 15 min, resuspended in physiological saline
water. The cell suspension thus prepared was used for
the batch studies.
Biofilter media
It consists of mixture of activated alumina (AA), sand
and low bulking thermocol balls. Sand and thermocol
balls were added to increase the porosity of the media.
The specific surface area of AA used was 320 m2/g, pore
volume 0.38 cm3/g and bulk density 820 kg/m3. Activated alumina was of 0.85 to 1mm, sand was of size 1–
2 mm and thermocol of size 2–4 mm were mixed in a
proportion of 40:20:40% by volume. This media was
overlaid on glass wool of 1 cm height, gravel bed of
2 cm height, and finally supported on a perforated sieve
plate. Out of a total weight of 2.07875 kg, 1.0575 kg and
0.9 kg were the weights of AA and sand respectively.
The total volume of the media was 4,712 cm3 with a
height of 60 cm. The bulk density of the medium was
0.4412 g/cm3.
Experimental set-up
The experimental setup consists of NO2 gas generator,
sampling arrangement, biofilter and a recycling
arrangement as shown in Fig. 1. The biofilter was made
of polyacrylic tube of 10 cm internal diameter and
100 cm height. The NO2 gas generated by the chemical
reaction between nitric acid and copper foils was passed
through a filter of glass wool to prevent the acid mist
from entering in the bioreactor. The reactor was operated in the upflow mode. The nutrient medium was
sprinkled continuously in a downward direction at flow
rate of 50 mL/min. The entire reactor was maintained in
anoxic condition. Because of the continuous nutrient
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medium sprinkling, the humidity of the reactor was always 100%. Glucose was added along with the nutrient
medium as sole carbon source. The entire experiment
was performed under sterile conditions. Spent nutrients
were replaced periodically by sterilized fresh medium.
Chemical reaction was employed for the NO2 generation. The gas thus generated was directly supplied to the
reactor through a connecting tube. Hence, there was no
bacterial contamination from that source. The purity of
the culture in the reactor was checked periodically by
sub culturing.
made upto 10 ml using fresh saline water. From each
dilution, 3 ml of sample was filtered through Millipore
filter paper (0.45 lm), in triplicate and the filter paper
was kept in the oven at 104 C for 3 h. The difference in
weight (average weight) of the filter paper was recorded
as bacterial cell density. The optical density of the corresponding dilutions was found by UV digital spectrometer (Shimanzu, Japan) in fixed-point measurement
at a wavelength of 550 nm. Using bacterial cell density
and the optical density a standard graph was prepared
and thereafter, the bacterial cell density in the samples
was calculated using the standard graph.
Batch experiments
Growth kinetics of T. denitrificans
Adsorption column study
In this study, growth curves were prepared for T. denitrificans using nitrate and nitrite individually as electron
acceptors at concentrations varying from 500 ppm to
5,000 ppm. Bacterial concentrations at different time
intervals were measured. The substrate concentration
was also measured at 2 h intervals up to 24 h for each
sample. All batch experiments were performed by
keeping triplicates and the average values are presented
in the results.
In this study, a glass column of 50 cm height and 1.5 cm
internal diameter was filled with 16 g of AA. NO2 gas
was passed at a flow rate of 0.4 L/min in the upward
mode through the column. Influent and effluent NO2
concentrations were monitored continuously. After the
exhaustion of the bed (influent concentration is almost
equal to effluent concentration), the adsorbent was
regenerated using distilled water. The concentrations of
nitrate and nitrite in the eluents were measured to find
out the feasibility of NO2 desorption from the media.
Bacterial cell density measurement
The bacterial culture was grown to midlog phase in NB
and the cells were centrifuged at 5,000·g for 10 min. The
bacterial pellets were washed twice and suspended in
100 ml of saline water (0.8% NaCl). About, 0.1, 0.2, 0.3,
0.4 and 0.5 ml of the above set solution was diluted and
Fig. 1 Schematic of biotrickling filter
Batch study on the biotransformation of NO2
This study was conducted to find the NO2 removal
efficiency in a suspended growth system. In a closed 5-L
bottle, 500 mL of nutrient broth was taken and initial
bacterial concentration of 1,500 mg/L was added. NO2
gas of concentration 100 mg/L was injected into the
bottle. The bottle was tightly closed and shook well to
ensure complete dissolution of the NO2 gas. This setup
was kept at 35C in a BOD incubator shaker. Nitrite and
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nitrate concentrations were measured at 2 h intervals.
Vent pipe was opened at suitable time intervals to release
the gases produced in the bottle.
Lowry’s method (Lowry et al. 1951) so as to suit for
bacterial cells. Dehydrogenase activity of the viable cells
was measured using standard biochemical procedure
(Philip and Venkobachar 2001)
Once-fed batch reactor
Batch studies were conducted to find the NO2 removal
efficiency in a T. denitrificans immobilized biofilter. First
T. denitrificans, was immobilized on the media. After
attaining sufficient bacterial growth on the media
(measured in terms of protein), NO2 gas was passed
through the media in upward direction. The inlet and
outlet NO2 concentrations were monitored. As soon as
NO2 started appearing in the outlet, NO2 supply to the
bioreactor was stopped. The mineral media along with
glucose, excluding KNO3, was sprinkled at a rate of
0.38 m3/m2/h (50 ml/min). After passing the gas, pH
was monitored and maintained between 7 and 8 by
adding NaHCO3 as and when required. After every 24 h
of operation, the nitrite/nitrate concentration in the
recycling liquid was measured and nutrients were added
to the recycling liquid. The above process was continued
until the removal efficiency of nitrite and nitrate ions
reached above 90%. The entire reactor was operated in
anoxic conditions with glucose as external carbon
source. Oxygen was removed from the system by nitrogen flushing at 2 min.
Analytical methods
Determination of NO2 gas
Diazotization method (IS: 5182 1975), for the analysis of
oxides of nitrogen as nitrite is used for the present study.
This involves diazotization of sulphanilic acid by nitrous
acid derived from nitrogen dioxide, followed by a coupling reaction with N (1-naphthyl) ethylene diamine dihydrochlride to form the dye. The intensity of the color
was quantified by a spectrophotometer.
Determination of nitrites and nitrates
For nitrite analysis, the sample collected was first filtered
and diluted to low concentration. Then compare the
colors produced by treating the sample and standards
with sulfanilamide acid and N (1-naphthyl) ethylenediamine dihydrochloride. Determination of nitrates was
carried by disulfonic method as per Standard Methods
(APHA 1998).
Results and discussion
Growth kinetics of T. denitrificans in the presence
of nitrates
The reaction of nitrogen dioxide with water forms nitrite
and nitrate ions. To design a treatment system like
biotrickling filter working on the principle of scrubbing
and biotransformation of pollutants from contaminated
air, it is essential to know the pollutant degradation
rates as w ell as the microbial growth kinetics. Flue gas
from any combustion process contains oxides of nitrogen and oxides of sulfur along with other pollutants like
unburned hydrocarbons, particulate matter and trace
metals. The concentration of pollutants varies based on
the quality of the fuel and the process design. Any
treatment system proposed for flue gas treatment should
be able to handle both NOx and SO2. It is reported that
T. denitrificans are able to oxidize hydrogen sulfide and
convert nitrate to nitrogen gas. Hence, this microbe was
selected for the present study.
Growth kinetics studies of T. denitrificans using nitrate as electron acceptor was conducted under different
nitrate concentrations and the result is presented in
Fig. 2. The growth curves have three distinct phases i.e.,
lag, log and stationary. There was slight increase in the
lag time with increase in concentration of nitrate. It was
also evident from the graph that concentration of biomass increased with increase in concentration of nitrates. The residual nitrate concentrations were also
monitored from the systems. From the curves, specific
growth rate (l) was determined by taking the slope of
the logarithmic growth phase. The results are presented
in the Table 1. Maximum specific growth rate (lmax,
maximum specific substrate utilization rate (qmax and
yield coefficient (YT) were calculated for the system
using the batch study results (the data is not
shown). lmax value of 1.03 h 1 and a yield coefficient of
0.275. was obtained for T. denitrificans using nitrate as
electron acceptor. These values match with the biokinetic constants of other heterotrophic denitrifiers
(Benefield and Randall 1977).
Growth kinetics of T. denitrificans in the presence
of nitrites
Determination of total protein
The attached microbial concentration in the biotrickling
filter was measured in terms of dehydrogenase activity
and protein content. The total protein of intact cells was
determined according to the method of (Herbort et al.
1971). This method is essentially the modification of
Similar studies as explained earlier were carried out
using nitrate as electron acceptor. The growth curves for
T. denitrificans in the presence of nitrites are shown in
Fig. 3. The concentration of biomass increased with
increase in concentration of nitrites up to 5,000 ppm and
there onwards it decreased. The specific growth rate (l)
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Inhibition of nitrite on bacterial growth
A plot of the specific growth rate versus nitrite concentration is given in Fig. 4. A concentration up to
1,000 mg NO2 _N/L, the substrate was rate-limiting for
the system. Nitrite concentration in the range of
1,000 mg NO2 _N/L to 5,000 mg NO2 _N/L, the
growths of microbes were independent of substrate
concentration. Nitrite concentration higher than 5,000mg NO2 _N/L retarded the growth of microbes. From
the study it is clear that nitrite concentration beyond
5,000 mg NO2 _N/L inhibits to the microbes. Any T.
denitrificans reactor used for the treatment of NO2 can
work within a range of 1,000–5,000 mg NO2 _N/L,
without any substrate level inhibition.
Fig. 2 Growth kinetics of T. denitrificans using nitrate as electron
acceptor
Performance of the system with nitrogen dioxide as
electron acceptor
In this study, NO2 gas of 600 ppm was used as a nitrate/nitrite source for bacteria. Immediately after
passing the gas, nitrite concentration in the reaction
mixture was about 44 mg NO2 _N (Fig. 5) and the
corresponding nitrate concentration was 107 mg
NO3 _N/L. After 24 h of incubation at 37C, nitrite
concentration increased to 52 ppm. This increase in
concentration may be due to the biological conversion
of nitrate to nitrite. After 48 h of incubation, nitrite
concentration was about 22 ppm. The slow response
on the first day as incubation is possibly due to the lag
face of microorganisms in the new environment. After
72 h, the nitrite concentration was found to be
16 ppm. After 96 h (4 days) of incubation, nitrite
concentration was 7.67 ppm, corresponding to a to
removal efficiency of 82.5%. The average biomass
concentration in the system was 2,000 mg/L. Nitrate
concentration in the system was non-detectable
throughout the study.
Fig. 3 Growth kinetics of T. denitrificans using nitrite as electron
acceptor
of T. denitrificans in the presence of varying concentration of nitrites are given in Table 2. lmax value and yield
coefficient (YT) for T. denitrificans using nitrite as the
electron acceptor were calculated using batch study results. The calculated lmax and YT values are 0.63 h 1
and 0.1316, respectively. These values are lower than the
values obtained while nitrate was used as the substrate.
This shows that T. denitrificans prefers nitrate over nitrite as electron acceptor.
Kinetics of nitrate, nitrite and NO2 gas removal by T.
denitrificans
Kinetic studies were conducted to find out the rate of
nitrate and nitrite removal by T. denitrificans. The result
is presented in Fig. 6. The removal of both nitrate and
nitrite were very slow at the beginning. With time, nitrite
removal rate started increasing. At the end of 24 h,
around 85% of nitrite was removed from the system
whereas nitrate removal was only 70%. In the case of
NO2 gas, microorganisms took about 48 h to achieve
50% reduction. Nitrite present in the liquid medium
Table 1 Specific growth rates of T. denitrificans at different nitrate concentrations
Nitrate_N concentration mg/L
Specific growth rate (l) h
1
600
1,000
1,500
2,000
3,000
0.38±0.02
0.54±0.03
0.61±0.02
0.66±0.03
0.78±0.03
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Table 2 Specific growth rates of T. denitrificans in presence of different nitrite concentrations
Nitrite_N concentration (mg/L)
Specific growth rate (l), h
1
600
1,000
5,000
10,000
15,000
0.335±0.021
0.424±0.018
0.562±0.031
0.279±0.009
0.04±0.001
might have converted directly to nitrogen gas whereas
nitrate might have first changed to nitrite then to
nitrogen gas. This may be the reason for the slow removal rate of nitrate compared to nitrite. When nitrite is
supplied in gaseous form, it has to convert to nitrate,
then to nitrite and finally to nitrogen. This may be
attributed to the lowest rate of removal when the electron acceptor is supplied in gaseous form. In all the
studies glucose was used as carbon source (electron
donor) at a concentration 2.5· that of the total nitrogen
present in the system.
Adsorption of NO2 on activated alumina and its
desorption
The adsorption pattern of NO2 gas with time was plotted and is shown in Fig. 7. Total NO2 gas adsorbed was
11 mg/g of AA with a bed depth of 50 cm. It was observed that activated alumina adsorption capacity was
exhausted within 20 min. when a gas stream with
700 ppm of NO2 passed through the column at a flow
rate of 0.4 L/min. The results of subsequent desorption
using distilled water as desorbent is given in Fig. 8.
The total desorbed nitrite concentration in the eluents
was 1,180 mg/L and that of nitrate was 2,655 mg/L.
These values correspond to NO2 gas concentration of
4,730 ppm, which gives a desorption capacity around
76% with a throughput volume of 500 mL (5.6 bed
volumes). This shows that the adsorbed gas on AA
could easily be desorbed in distilled water in the form of
nitrates and nitrites. The bacteria can utilize the desorbed nitrates and nitrites. This study reveals that
activated alumina acts as a good adsorbing material for
NO2 gas. The advantages of this matrix are high
immobilization capacity, less mass transfer resistance
and low head loss. Moreover, it can act as an adsorbent
for the pollutant and thus eliminate the immediate toxicity to microbes, in case of shock loads.
Performance of once-fed batch reactor
Fig. 4 Inhibition model for T. denitrificans in presence of nitrite
Fig. 5 Kinetics of NO2 removal by T. denitrificans
Performance of the T. denitrificans immobilized biofilter
for NO2 removal was studied using a bench scale once
fed batch reactor. The reactor was run under batch
conditions because of the unavailability of continuous
supply of NO2 gas.
Biotrickling filter was able to remove 100% NO2
from the gas stream within 2 min irrespective of the
initial concentration of NO2. But the removal of nitrite
and nitrate from leachate/trickling water was not very
fast.
The performance of the biotrickling filter in three
cycles of operation is shown in Figs. 9 and 10. In the first
cycle, the biotrickling filter was operated for nine days.
NO2 concentration of 2,640 ppm was passed into the
biofilter at a flow rate of 1.5 L/min. for 5 min. A removal efficiency of 76% nitrite from liquid phase was
achieved at the end of the cycle. The second cycle lasts
for 5 days, with an overall removal efficiency of 99%
with a reduced influent concentration of 1,545 ppm. The
third cycle lasts for about four days and removal efficiency of 99% was obtained even though the influent
concentration was increased two times (2,735 ppm) that
291
Fig. 6 Kinetics of nitrate and nitrite removal by T. denitrificans
Fig. 8 Desorption kinetics of nitrogen dioxide (NO2) by activated
alumina (AA)
Fig. 7 Sorption of nitrogen dioxide (NO2) by activated alumina
(AA)
of the second cycle. This concentration range was selected based on the NO2 concentration in flue gases, of
most of the stationary sources.
In the first cycle, NO2 gas concentration of about
2,640±0 ppm was passed into the biofilter at a flow rate
of l.5 L/min for 5 min. the NO2 concentration in effluent
air was non-detectable It took 5 min for traces of NO2
to occur in the exit gas. After the passing of the gas, pH
was reduced to six. This may be due to the formation of
nitric acid and nitrous acid after the dissolution of NO2
gas in the re-circulating liquid. The pH of the system was
raised to 7.5 by the addition of NaHCO3 as it was
optimum for the bio-system. Even though no nitrate was
added in the nutrient medium externally, NO3 concentration was found to be 305±12 mg NO3 _N/L in
recycling liquid. This shows that the NO2 gas is converted to nitrate first. Nutrient broth was supplied and
Fig. 9 Performance of T. denitrificans immobilized biofilter
recycled continuously. In this cycle regular pH adjustments were not done. From the graph it is clear that
there was an increase in the NO2 concentration after
one day. This may be due to the dissolution of NO2 gas
adsorbed on the AA and the conversion of nitrate ions
to nitrite ions.
In the second cycle, NO2 gas concentration of about
1,545±8 ppm was passed, at a flow rate of 1.5 L/min
for 5 min, into the biofilter. After passing the gas, pH
was reduced to 6.5. The pH was monitored and adjusted
regularly. This reduction in pH was more predominant
in the initial stages of operation. Nitrate concentration
was 204±3 mg NO3 _N/L in the recycling liquid within
30 min. of NO2 gas feeding. Nutrient broth was supplied
292
the biofilter is possibly due to desorption of NO2 from
the media. Once desorption of NO2 from AA was almost
ceased, the pH of the system remained a constant.
Policy implications
Fig. 10 Comparison of NO2 removal efficiency in various cycles of
operation
and recycled continuously. At the end of 5 days, nitrate
concentration was non-detectable. Immediately after a
few hours of completion of nitrite and nitrate, the denitrificans were taken over by anaerobic microbes. The
reactor color changed from yellow to jet-black within
few hours of feeding.
In the third cycle, NO2 gas concentration of about
2,735±10 ppm was passed, at a flow rate of 1.5 L/min
for 5 min, into the biofilter. The pH was monitored and
adjusted regularly. NO3 concentration was 340±5 mg
NO3 _N/L in recycling liquid within 30 min. of feeding.
As nitrogen dioxide concentration was high there was
low response in the initial stages. Once the bacteria
acclimatized to the new conditions, removal efficiency
was improved. Nitrate concentration at the end of the
fourth day (96 h) of operation was found to be nondetectable. At the end of the third cycle a bacterial
concentration of 200 mg/g of AA was observed in the
reactor.
The possible mechanism involved in the biofiltration
process for NO2 removal using T. denitrificans immobilized activated alumina may be as follows. When the
NO2 gas passes through a biotrickling filter, it might be
adsorbed by the activated alumina. When the nutrient
broth was sprinkled over the media, adsorbed gas may
be desorbed in the form of nitrites and nitrates, which
was clear from the elevated concentrations of nitrites
and nitrates in the re-circulating medium. The nitrites
and nitrates thus formed, were utilized by microbes in
the presence of glucose. In this denitrification process,
nitrates might have converted to nitrites, subsequently to
NO and finally to N2 gas, which was evident from the
concentration profile of nitrate and nitrite in the system.
In the third cycle, gas concentration of about 2,735 ppm
was passed through the filter. Stoichiometric calculations show that 25% of NO3 was not desorbed. pH
reduction was due to H+ ions from HNO3 or HNO2.
The reduction in pH in subsequent operating hours of
Oxides of nitrogen (NOx) is a group of nitrogen containing compounds emitted from combustion process.
Though at low concentrations, NOx may not cause any
adverse health effects, it actively participates in photochemical reactions and produce many secondary pollutants that are harmful even at minute concentrations. In
order to control the secondary pollutant formation,
more and more stringent emission standards are implemented for NOx emission in the recent past. One of the
options practiced throughout the world to reduce NOx
emission is to change the combustion system in such a
way that less amounts of NOx may be generated.
However, it affects the efficiency of the system. The flue
gas treatment option for NOx is either catalytic oxidation or chemical scrubbing. Both the processes have its
own disadvantages. Efficiency of the catalytic converter
reduces drastically if the flue gas contains particulate
matter or other chemicals like heavy metals, which can
react with the catalyst. The scrubbing process generates
liquid or solid wastes, or both in some cases, which need
further attention. In other words, the pollution problem
is changed from one phase to another.
The new process discussed here is environmental
friendly. Complete treatment of flue gas takes place and
the final products are nitrogen gas and more biomass.
Hence, no secondary treatment problem arises. The
material required for the treatment, ie. microbes and
carbon source are readily available. Though in the
present study, glucose was used as a carbon source, it
can easily be replaced by either domestic wastewater or
food processing industrial waste. The time required for
the complete treatment varied from 3 days to 5 days. It
can be reduced to a few minutes or seconds by making a
two-stage system. The pollutant from the gaseous phase
can be removed in one system and the liquid treatment
in the other. By doing so, the volume of the treatment
units can be reduced considerably.
Though cost comparison or economic analysis of the
system is not yet done as the process is only in the laboratory stage, it is clear that this technology has tremendous scope in the field of NOx control
Conclusion
Nitrogen dioxide removal efficiency of T. denitrificans
immobilized activated alumina biofilter was evaluated.
Experiments were conducted to find degradation kinetics of T. denitrificans in the presence of nitrate and nitrite, to find the potential of the activated alumina to
adsorb NO2 gas and finally to find the removal efficiency
of NO2 in a once bed batch reactor. T. denitrificans was
293
able to utilize nitrite and nitrate as electron acceptors for
a wide concentration range. The lmax and YT values for
T. denitrificans as nitrate and nitrite as electron acceptors were found to be 1.03 h 1, 0.275 and 0.63 h 1,
0.1316 respectively. The working rage of NO2 was
found to be 1,000 mg NO2 _N/L to 5,000 mg NO2 _N/
L, beyond which there seems to be substrate-level inhibition, whereas NO3 did not exhibit any inhibition up to
a concentration of 3,000 mg NO3 _N/L. Activated alumina acted as a good adsorbing material for NO2 apart
from being a good bacterial immobilization medium.
The biofilter could treat 1,500 to 2,735 ppm of NO2 gas
with an efficiency of 99% within 2 min from the gaseous
phase and within 4 days from liquid phase. The mechanism of NO2 gas removal in the biofilter seems to be the
dissolution of NO2 in water to form NO3 , conversion of
NO3 to NO2 _, and finally to N2 gas.
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