95-TA9B.04
EPA/600/A-95/064
Performance of Trickle Bed Biofilters Under High Toluene Loading
George A. Sorial
Francis L. Smith
Amit Pandit
Makram T. Suidan
Pratim Biswas
Department of Civil and Environmental Engineering
University of Cincinnati
Cincinnati. OH 45221-0071
and
Richard C. Brenner
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
26 W. Martin Luther King Dr.
Cincinnati, OH 45268
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INTRODUCTION
v
The enactment of the 1990 amendments to the Clean Air Act has generated a new demand for
cost-effective technologies to control the emissions of volatile organic compounds (VOCs). The
concept of using biological processes for controlling undesirable compounds in different kinds of
wastes has increasingly been applied. The two major biological processes for treatment of
contaminated air are bioscrubbing and biofiltrauon. In the bioscrubbing process, the contaminants
are scrubbed from the waste gas in an absorption unit and then passed to a separate oxidation
reactor employing a standard water treatment method to aerobically degrade the contaminants. On
the other hand, the biofiltration process utilizes a biological microbial film fixed on support media
within a single process component where the contaminants are both scrubbed from the waste gas
and converted to benign end products such as H.O and CO;. The simplicity of the biofiltration
process has resulted in its emergence as a practical, cost-effective technology for the treatment of
large volumes of air contaminated with low concentrations of biologically degradable compounds,
as compared to other traditional VGC control technologies such as incineration and carbon
adsorption'. The low operating cost is mainly due to the utilization of microbial oxidation at
ambient conditions, rather than oxidation by thermal or chemical means.
Biofilter systems for VOC control are strongly affected by the choice of the attachment media.
Ideal attachment media are characterized by a high specific surface area, minimal back pressure,
and a suitable surface for the attachment of microorganisms. Biofilter media are mainly of two
types. The first type is a natural organic medium composed of peat, compost, leaves, wood bark,
and/or soil". The second type is inert synthetic media. Sometimes a combination of both types is
used. In addition to the above media types, activated carbon packing media could be used for both
supporting biofilms and providing a buffering treatment capacity'. The natural organic medium is
provided with solid nutrients and buffers incorporated directly in the media, and the bed moisture
level is maintained at a constant level by humidifying the air. Conversely, the synthetic media
require the delivery of liquid nutrients and buffers to the microbial population through a nozzle
system on the top of the bed. The delivery of the nutrient and buffer liquid flow is usually
sufficient to provide the necessary moisture levels for microbial activity. This type of biofilter is
usually referred to as Trickle Bed Air Biofilter (TBAB).
Preliminary investigations performed by the authors4 were made on three media: a proprietary
compost mixture; a synthetic, monolithic, a straight-channeled (channelized) media; and a
synthetic, randomly packed, pelletized media. These media were selected to offer a wide range of
microbial environments and attachment surfaces and different air/water contacting geometries.
After 18 months of testing, the pelletized media (6-mm Celite® R-635 Bio-Catalyst Carrier) was
demonstrated to be significantly superior to the other media investigated. Subsequent research for
evaluation of the pelletized media indicated that an increase in biofilter operating temperature
permits a significantly higher practical VOC loading and biofilter performance decreases
substantially with build up of back pressure due to accumulation of excess biomass within the
system5 6. In order to maintain consistent long-term efficient and reliable performance, further
investigations were conducted for controlling the biomass build-up within the biofilter5. The
biomass control strategy developed for this purpose was in-suit upflow washing with water, or
backwashing. at a rate sufficient to fluidize the media and permit rapid removal of excess biomass
growth.
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95-TA9B.04
During our early investigations with the pelietized media, it was noticed that the use of ammonia-
nitrogen (NHrN) as the sole nutrient nitrogen source resulted in the development and
accumulation of a sizable mass of nitrifying bacteria within the media bed. Nitrifying bacteria are
of no known utility in bio filter performance and may interfere with the availability of NHrN for
the VOC degrading microbes. Another study 7 was conducted, therefore, evaluating the relative
performance of biofilters under similar operating conditions varying only the form of nutrient
nitrogen. Using nitrate-nitrogen 
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95-TA9B U4
salt solution, a 2.3-mL vitamin solution, and a 4-mL of 0.01 M FeCI, solution, A nutrient spike
solution (2M NaNOj, 0.22M NaH:P04.H:0) was added so that the COD (g COD toluene) to
nitrogen ratio was 50:1. One molar sodium bicarbonate was used as a buffer to maintain the
desired biofilter operating pH (the volume added depended on the volume of spike used). Both
biofilters were operated in a co-current mode with the air and nutrient flows directed downwards.
Materials
Reagent grade toluene (99.9%. Fisher Scientific Co.. Inc.. Fair Lawn. NJ) was used as the sole
VOC contaminant in this study. The microbial seed for the biofilters was effluent liquid collected
from previously operated biofilters in the system.
Analytical Methods
Concentrations of toluene were measured by chromatographic separation on a 30-m megabore
column (DB 624, J&W Scientific. Folsom. CA) using a gas chromatograph (GC) (HP 5890. Series
II, Hewlett-Packard, Palo Alto. CA) equipped with a liquid sample concentrator (LSC 2000.
Tekmar. Cincinnati. OH) and a photoionization detector (PID) (Model 4430. OI Corp.. College
Station. TX). The liquid sample concentrator was programmed according to U.S. EPA Method
601, and a Tenax trap was used with a helium (He) purge flow of 40 mL/min. The GC oven
temperature was programmed from 40 to 120 "C at 5 degrees min with a 4-min hold at 40 °C and
a 6-min hold at 120 °C. The carrier gas (He) (low rate was set at 8 mL/min. and the PID detector
was used with He make-up gas at a flow rate of 20 mL.'min. a sweep gas flow rate (H,) at 100
mL/min. and a base temperature of 250 "C.
Gas phase samples for VOC analysis were taken with gas-tight syringes through low bleed and
high puncture tolerance silicone GC septa (replaced every week) installed in the sampling ports at
the gas inlet and outlet from the biofilters. Samples from the liquid phase for VOC analysis were
removed in a similar way from the liquid outlet from the biofilters. Both gas and liquid phase
samples were introduced to the GC through the liquid sample concentrator accessory. The gaseous
phase VOC analysis was conducted by introducing 5 mL of purged distilled deionized water into
the purge vessel of the liquid sample concentrator prior to the injection of the gas sample.
Effluent gas phase samples for carbon dioxide analysis were taken also with gas-tight syringes
through effluent sampling ports in the biofilters. A gas chromatograph equipped with a thermal
conductivity detector (TCD) was used for determining the carbon dioxide concentrations in the
effluent gas phase. The GC oven temperature was programmed from 50 to 80 °C at 10
degrees/min with a 3.2-min hold at 50 "C and a 1.5-min hold at 80 "C. The carrier gas (He) flow
rate was set at 30 mL/min. and the TCD detector was used with He make-up gas at a flow rate of
35 mL/min.
Liquid phase samples were also analyzed for nitrate and ammonia nitrogen concentrations by using
the electrode method of analysis according to Standard Methods9 4500-D and 4500-F, respectively.
Liquid phase samples for carbon dioxide analysis were also conducted by using an electrode
method of analysis. Samples were filtered through 0.45-,um nylon filters (Micron Separation, Inc.
Westboro. MA) prior to analysis.
pH determinations were conducted using a Fisher Accumet pH meter. Model 50 (Fisher Scientific
4
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95-TA9B.04
Co.. Inc.. Fair Lawn, NJ), The pi 1 meter was calibrated before use with buffers (pH of 3.0 and
7.0) supplied by the manufacturer.
RESULTS AND DISCI SSION
Each bio 11 her received a nutrient solution at a feed rate of 20 L/day, and the pH and operational
temperature were maintained constant at 7.7±0.2 and 32 °C (90 °F), respectively. Each biofilter
was loaded with a 50/50 mixture of fresh pellets and conditioned pellets used in previous runs.
These conditioned pellets had been washed by hand, batchwise, in hot water, soaked in hot
chlorine bleach solution for half an hour, drained, and rinsed overnight with deionized water.
Biofilter "A": This biofilter was started up at 50 ppmv toluene influent concentration, 1.0 minute
EBRT. and 21 mM N05-N per day as the sole nitrogen source. On day 3, the removal efficiency
had reached 99.9+% and was stable at this loading through day 17. On this day, the pressure drop
had reached 0.8 inches of water and. therefore, the biofilter was backwashed. The procedure used
was to recycle 70 L of water through the bed, bottom to top, at 32 °C (90 °F) for 1 hour at a flow-
rate of 15 gpm to induce full media fluidization and then to flush with 50 L of clean 32 °C (90 °F)
tap water at the same flow rate. The schematic of the backwash system is shown in Figure 2. The
backwash frequency was set and maintained at twice per week. Following backwashing, the
influent concentration was increased to the target value of 250 ppmv toluene (4.1 kg COD/m3.day)
with the NO--N feed rate at about 103 mM/day. The removal efficiency was initially 67%, but by
day 20 the efficiency had dropped to 59%. Due to this poor and declining performance, the
influent concentration was reduced to 100 ppmv on day 23. With this change, the N03-N feed rate
was lowered to 14 mM/day. The efficiency rose rapidly to about 96%, but then dropped to 91%
by day 25. At this point, it was noticed that the COD:NOrN ratio was low and, therefore, on day
26. the NOj-N feed rate was raised to 42 mM/day ill order to provide a COD:N03-N ratio of about
50:1. The efficiency rose to 99.9*% on that day and retained this level until day 34. On day 35.
the influent concentration was increased to 225 ppmv toluene and the NOrN feed rate was
increased accordingly to about 98 mM/day. The removal efficiency ranged between 99,9% and
97%. with the low value observed just prior to backwashing. On day 53, the influent
concentration was increased to the target value of 250 ppmv toluene and the NOrN feed rate was
increased to about 124 mM/day. The removal efficiency ranged from 99.9% to 97%, with the low-
value again typically observed just prior to backwashing. The performance of this biofilter is
shown in Figure 3. Continued operation after day 77 had shown that the removal efficiency
started to decline to as low as 88% between periods of backwashing. This is an indication that
excess biomass was still being held within the system and was causing channeling or short
circuiting. On day 129. the backwash duration was increased to 2 hours for the recycle period with
the frequency being maintained at two backwashes per week. Improvement in performance
between periods of backwashing was noticed after day 147. The maximum drop in removal
efficiency was to only 93% just prior to backwashing. The pressure drop for the period of
operation after the 2-hour recycle period during backwashing remained below 0.5 in. of water.
Effluent samples were collected after backwashing at time intervals for determining the response of
the biofilter. Figure 4 shows two typical effluent recovery curves determined for the two
backwash duration periods studied. In Figure 4, it can be seen that, for the 2-hour backwash
period, biofilter removal efficiency was stable for up to 2 days after backwashing at 99% and then
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95-TA9B.U4
dropped to 93% on the third day. On the other hand, tor the I-hour backwash period, biofilter
performance steadily dropped after the first day from 99% efficiency down to 90% efficiency on
the third day. It is further seen [hat the initial elimination efficiency of the biofilter for the 2-hour
period was lower than that for the I-hour backwash period. However, after 220 minutes, biofilter
efficiency recovered to 95%. while the recovery was 90% after 200 minutes for the 1-hour
backwash period.
The performance data shown in Table IV are for samples collected 1 day after backwash. The
data are shown separated by a double horizontal line. Abo\e the line are the data for the 1-hour
backwash period and below it for the 2-hour backwash period. The performance of the biofilter I
day after backwashing was essentially the same for the two backwashing strategies studied, with
an average toluene removal efficiency of about 84% at a depth of 0.71 m and 98%-99% efficiency
achieved through the full media depth.
Biofilter "B": This biofilter was started up at 50 ppmv toluene influent concentration, 0,67 minute
EBRT. and 21 mM NOrN per day as the sole nitrogen source. On day 3. the removal efficiency
had reached 99.9+% and was stable at this loading through day 17. On this day, the pressure drop
had reached 3.1 in. of water and. therefore, the biofilter was backwashed using the same initial
procedure described above for biofilter "A". Following backwashing. the influent concentration
was increased to the target value of 250 ppmv toluene (6.15 kg COD/m3.day) with the NO;-N feed
rate at about 103 mM/dav. The removal efficiency was initially 65%. but. by day 20, the efficiency
had dropped to 53%. Due to this poor and declining performance, the influent toluene
concentration was reduced to 100 ppmv on day 23. With this change, the NO;-N feed rate was
lowered to 24 mM/dav. The efficiency rose steadily to about 99.9% by day 34. On day 35. the
influent concentration was increased to 225 ppmv toluene and the N05-N feed rate was increased
accordingly to about 114 mM/day. The removal efficiency ranged between 98% and 85%. with
the efficiency typically declining between backwashings. On day 53. the influent concentration
was increased to the target value of 250 ppmv toluene and the N03-N feed rate was increased to
about 155 mM/day. The removal efficiency ranged from 94% to 73%. with the efficiency again
typically declining between backwashings. The performance of this biofilter is shown in Figure 5.
Continued operation had shown that the performance started to decline further to as low as 71%
between periods of backwashing. This is an indication that excess biomass was still being held
within the system and was causing channeling or short circuiting. On day 129. the backwash
duration was increased to 2 hours for the recycle period while maintaining a frequency of two
backwashes per week. Improvement in performance between periods of backwashing was noticed
after day 147. The maximum drop in removal efficiency was to only 79% just prior to
backwashing. The pressure drop for the period of operation after the 2-hour backwash period
remained below 0.8 in. of water, with the highest value obtained just prior to backwashing.
Similar to biofilter "A", effluent samples were collected after backwash at time intervals for
determining the response of the biofilter. Figure 6 shows two typical effluent recovery curves for
the two backwash duration periods studied. The impact of the 2-hour backwash period was more
pronounced in this biofilter than in biofilter "A". The recovery of the biofilter was better at all
times, and. furthermore, the drop in efficiency after the first day following backwashing was less
pronounced than that for the 1-hour backwash period. This indicates that the 2-hour backwash
period achieved greater removal of excess biomass from the media than did the 1-hour backwash
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95-TA9B.04
period.
Tabic V shows performance data for samples collected 1 day after backwash. The layout of the
table is similar to that stated previously for Table IV. Similar to biofilter "A", essentially no
difference is seen in the data collected for the two backwash strategies studied at any of the media
depths. Since the EBRT for biofilter "B" was 2/3 that of biofilter "A" and the influent toluene
concentrations were the same for both biofilters, the toluene or COD loading was 50% higher for
biotlIter "B" than for biofilter "A". If we consider the effects of the different space velocities to
be negligible then the overall performance of biofilter "B" 1 day after backwashing should be
expected to be similar to the performance of biofilter "A" at 2/3 the depth of the media in biofilter
"A", i.e., at 0.67 of 1.14 m. Alternatively, the performance of biofilter "B" should be expected to
match the overall performance of biofilter "A" if the depth of the media in "B" were to be
increased by 50%.
CONCLUSIONS AND FUTURE WORK
The performance of two pelletized media biofilters, highly loaded with toluene, was evaluated in
this study. Both biofilters were operated at the same influent concentration of 250 ppmv toluene.
Biofilter "A" was operated at I minute EBRT and biofilter "B" at 0.67 minute EBRT. The impact
of backwash duration on performance was studied for both biofilters, primarily the stability of
performance between backwashings. For a backwash period of 1 hour, both biofilters showed a
decline in performance for the days following backwashing. For biofilter "A", the performance
dropped from 99% to 90% on the third day following backwashing. For biofilter "B", the
performance dropped from 90% to as low as 71% on the third day. On the other hand, for a
backwash period of 2 hours, both biofilters showed significantly less drop in performance for the
days following backwashing. The performance of biofilter "A" was stable at a removal efficiency
of 99% up to the second day following backwashing, but then dropped to 93% on the third day.
For biofilter "B". the performance dropped from 90% to 79% on the third day following
backwashing. This behavior indicated that increasing the backwash period from 1 to 2 hours was
effective in improving the performance stability of both biofilters. This improved performance
stability is due to reduction of short circuiting within the media, caused by accumulating biomass.
Continuing research is being conducted to evaluate the impact of backwash frequency on the
stability of the performance of these biofilters between backwashings. The results of these
investigations will be presented at the conference.
ACKNOWLEDGEMENTS
This research was supported by Cooperative Agreement CR-821029 with the U.S. Environmental
Protection Agency. The findings and conclusions expressed in this publication are solely those of
the authors and do not necessarily reflect the views of the Agency.
REFERENCES
1. Ottengraf. S.P.P, "Exhaust Gas Purification in Biotechnology, Vol. 8, Rehn, H.J.:
Reed, G. Eds., VCH Verlagsgesellschraft, Weinham, 1986.
7
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95-TA9B.04
2.	Leson. G. and Winer. A.M. "Biollltration: An innovative Air Pollution Control
Technology for VOC Emissions." J. Air and H aste Manag Assoc., 41(8): 1045-
1054 (1991).
3.	Utgikar. V.. Govind. R.. Shan. Y.. Saffcrman. S.. and Brenner. R.C. "Biodegradation
of Volatile Organic Chemicals in a Biofilter." in Emerging Technologies in
Hazardous Waste .Management II. Tedder. D.W. and Pohland. F.G. Eds.. ACS
Symposium Series. American Chemical Soeietv, Washington D.C.. pp. 233-260
(1*991).
4.	Sorial. G.A.. Smith. F.L.. Smith. P.J.. Suidan. M.T., Biswas. P.. and Brenner. R.C.
"Evaluation of Biofilter Media for Treatment of Air Streams Containing VOCs."
Proceedings of Water Environment Federation 66th Annual Conference and
Exposition. Volume 10 Facility Operations, pp. 429-439 (1993).
5.	Sorial, G.A.. Smith. F.L.. Suidan. M.T.. Biswas. P.. and Brenner. R.C. "Evaluation
of Performance of Trickle Bed Biofilters - Impact of Periodic Removal of
Accumulated Biomass." Paper No. 94-RA115A.05 presented at the 87th Annual .
Meeting and Exhibition of the Air and Waste Management Association, Cincinnati.
Ohio. June 19-24 (1994).
6.	Smith. F.L.. Sorial. G.A.. Suidan. M.T.. Biswas. P.. and Brenner. R.C. " Pilot-Scale
Evaluation of Alternative Biofilter Attachment Media for the Treatment of VOCs."
Presented at the EPA Symposium on Bioremediation of Hazardous Wastes:
Research. Development, and Field Evaluations. San Francisco. California, June 28-
30 (1994).
7.	Smith. F.L.. Sorial. G.A.. Suidan. M.T.. Biswas. P., and Brenner. R.C. "Trickle Bed
Biofilter Performance - Biomass Control and N-Nutrient Effects." Proceedings of
Water Environment Federation 67th Annual Conference and Exposition, Volume 10
Facility Operations, pp. 553-564 (1994).
8.	Sorial. G.A.. Smith. F.L.. Smith. P.J.. Suidan. M.T.. Biswas P.. and Brenner. R.C.
"Development of Aerobic Biofilter Design Criteria for Treating VOCs," Paper No.
93-TP-52A.04 presented at the 86th Annual Meeting and Exhibition of the Air and
Waste Management Association. Denver. Colorado. June 13-18 (1993).
9.	Standard Methods for the Examination of Water and Wastewater, 18th Edition.
American Public Health Association. Washington. D.C. (1992).
8
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95-TA9B.04
Tabic I. Stock Trace Salt Solution.
Component
Concentration, g/L
(NH4 )nMo70;j.4H;0
2.08
Na:B4O7l0H,O
1.15
MnCl,.4H,0
4.74
CoCI,.6H:0
2.86
ZnC12
3.27
CuCl,.2H,0
2.05
Table II. Stock Salt Solution.
Component
Concentration,
g/L
Trace salt solution
33.1 mL/L
MgCl,.6H,0
8.13
CaCUJHp
2.22
khso4
13.6
Table III. Stock Vitamin Solution.
Component
Concentration, j
g/L
p-Aminobenzoie Acid
0.01
Biotin
0.0039
Cyanocobalamin (B12)
0.0002 J
Folic Acid
0.0039
Nicotinic Acid
0.01
Pantothenic Acid
0.01
Pyriodoxine Hydrochloride
0.02
Riboflavin
0.01
Thiamin Hydrochloride
0.01
Thioclic Acid
0.01
9
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95-TA9B u4
Table IV. Biofiltcr "A" Removal Efficiency with Depth
Seq. Date,
days
Removal Efficiency. %
0.1 m
0.41 m
0.71 m
1.01 m
1.14 m
60
62
75
85
98
99
75
50
69
92
97
99
84
48
70
80
90
98
105
35
65
78
90
98
116
40
65
85
96
99
147
42
68
82
95
99
158
35
67
84
93
99
Table V. Biofilter "B" Removal Efficiency with Depth
Seq. Date,
days
Removal Efficiency. %
0.3 m
0.6 m
0.9 m
1.14 m
56
50
67
78
90
67
43
67
88
92
81
57
73
80
90
88
50
68
87
93
98
53
72
82
90
109
58
69
80
89
123
58
67
83
90
140
57
68
78
90
154
57
74
86
90
10
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Carbon dioxide, water vapor, particulates
-*VOCs
Raw air
Nutrient return
Nutrient supply
Qf ci
GAC
FCI
PCV
TCI
S
100 psig
Biofilter
10 psig
to other biofilters
TCI _ Syringe
pump
Effluent water
Electrical
Heater
granular activated carbon canister
flow controller indicator
pressure control valve
temperature controller indicator
sampling port
Effluent air
to atmosphere
to sewer
Figure 1. Schematic of the Experimental Setup
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Make-up
tap water
I
Clean



Rinse
Water

Recycle

Water
Supply

Tank

Receiving
Tank

(70 L)

Tank
(50 L)
—c>-
Backwash
Pump
+ 40%
<	
Fluidized
height
Normal
packing
height
Biofiiter
Figure 2. Schematic of Backwash System
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300
200
rruJliu i.rr

1 ^ i
100
• Influent Cone.
~ Effluent Cone,
a Percent Removal
60
40
/
/
¦pVM|N


) 20 40 60 80 100 120 140 160
Sequential Date, days
Fiuure 3. Performance of Biofilter "A" with Respect to Toluene Removal
at 1.0 Minute EBRT
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£
w
>
o
6
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95-TA9B 04
400
"XIXXPID-t
300
200


• Influent Cone.
~ Effluent Cone,
a Percent Removal
/
/

/
60

40
\<=
©x

>>

O

c
20
*G

s

w

15

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o

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1-hour	backwash
2-hour	backwash

y
0	100	200	300 2000 3000 4000 5000
Time, min
O
Figure 6. Typical Effluent Response Following Restart from Backwashing
for Biofilter "B"	s
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TECHNICAL REPORT DATA
{Please read Instructions on the reverse before completingj
—
1 Rm76°0O/A-95/O64
3. RECIP
4. title AND subtitle Performance of Trickle Bed Biofilters
Under High Toluene Loading
5. REPORT OATE
6. PERFORMING ORGANIZATION CODE
i. author(s) Richard Brenner, George Sorial, Francis
Smith, Amit Pandit, and Makram Suidan
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. EPA, REEL—26 Martin Luther King Dr.-Cinti, OH
University of Cincinnati, Dept. of Civil & Envl Engrg
Cincinnati, OH 45221-0071
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
CR-821029
12. SPONSORING AGENCY NAME AND ADDRESS
Risk Reduction Engineering Laboratory— Cincinnati, OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOO COVERED
Published Paoer
14. SPONSORING AGENCY CODE
EPA/600/14
supplementary notes Project officer = Richard Brenner (513) 569-7657
To be presented and published in the proceedings of the AWMA Conference, San Antonio,
Jnnp 1
is. abstract The performance of two penetized meaia oi or liters, highly loaded with
toluene, was evaluated in this study. Both biofilters were operated at the same
influent concentration of 250 ppmv toluene. Biofilter "A" was operated at 1 minute
EBRT and biofilter "B" at 0.67 minute EBRT. The impact of backwash duration on perform
ance was studied for both biofilters, primarily the stability of performance between
backwashings. For a backwash period of 1 hour, both biofilters showed a decline in
performance for the days following backwashing. For biofilter "A", the performance
dropped from 99% to 90% on the third day following backwashing. For biofilter "B", the
performance dropped from 90% to as low as 71% on the third day. On the other hand, for
a backwash period of 2 hours, both biofilters showed significantly less drop in perform
ance for the days following backwashing. The performance of biofilter "A" was stable at
a removal efficiency of 99% up to the second day following backwashing, but then drop-
ped to 93% on the third day. For biofilter "B" the performance dropped from 90% to 79%
on the third^day following backwashing. This behavior indicated that increasing the
backwash period from 1 to 2 hours was effective in improving the performance stability
of both biofilters. This improved performance stability is due to reduction of short
circuiting within the media, caused by accumulating biomass.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c, COSATl Field/Group

trickle bed biofilters

18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (This Report)
UNCLASSIFIED
21. NO. OF PAGES
17
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
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