Performance Evaluation of Biofilters for Wastewater
Lift Station Emissions
Jose Cabezas, Alvaro Martinez, Kim Jones, Sneha Rathibandla, James Boswell*
Department of Environmental Engineering, Texas A&M University-Kingsville, Kingsville, TX 78363, USA
*BioReaction Industries LLC Tualatin, OR 97062 USA
ABSTRACT
Biofilters are used in some locations for the treatment of odors emitted from wastewater
lift stations. Often, biofilters are fabricated as simple open cylinders packed with
compost and wood chip media with water sprinklers for humidification. In some cases,
addition of a chemical base, such as magnesium hydroxide, is used to prevent stripping of
odorous compound in the wet well. Such systems are usually evaluated in the field
focusing almost exclusively on the removal of hydrogen sulfide; however, many other
volatile organic compounds may also be emitted and their fate in the biofilters has not
been studied. Engineered biotrickling-biofilter systems have been proposed as an
alternative to the simpler designs, promoting the ability to handle higher concentrations
of contaminants. This research reports on a field evaluation of the performance of the
two design approaches. For this purpose, a pilot biotrickling-biofilter has been installed
in a wastewater lift station beside the compost biofilter currently in operation. The
characterization of the wet well gases was accomplished using field instruments and lab
GC-MS and GC-FID analyses of gas samples. The simultaneous removal of H2S and
specific VOCs was studied for the two technologies in a field application treating exactly
the same gases.
INTRODUCTION
Biofiltration is a new environmentally friendly alternative for treatment of odorous and
toxic air emissions in municipal wastewater lift stations.1 In these stations, the pollutants
are generated in wastewater collection/pumping chambers called wet wells. Wet wells
store wastewater and contaminated air at atmospheric pressure. Fans are used to extract
the contaminated air from the wet well and impel it into the air treatment system. The
contaminated air contains reduced sulfur compounds and volatile organic compounds
(VOCs). Hydrogen sulfide (H2S) is one of the main contaminants present in
concentrations that generally fluctuate from 100 ppm to more than 1000 ppm. Usually,
the contaminant concentration increases with temperature and daylight. But, after
treatment, the H2S concentration must be sufficiently low to avoid odor problems,
especially in urban areas.2
Biofilters can be broadly classified in two types: simple and engineered biofilters. A
simple biofilter is an open container filled with appropriate packing materials like
compost and wood chips. Contaminated air is injected into the bottom of the biofilter. It
moves up by advection all the way through the wet porous media where the pollutants are
converted into innocuous gases and liquid products. Because of its large size, this type
of biofilter can digest air pollutants even at concentrations as high as 1000 ppm.
However, there is not enough control over gas flow through the media; the biofilter
eventually can be saturated by products of reactions of pollutants inside the media. As a
result, the biofilter efficiency decreases and eventually the media have to be replaced.3
The other type of pollutant removal equipment is the engineered biofilter. Most
engineered biofilters are designed for low to moderate concentrations.4,5 Therefore, if the
pollutant concentration is too high, it can be reduced by mixing with fresh air. For
convenience, the removal process is carried out in two sequential stages: first in a wet
scrubber or a biotrickling filter, and then in a biofilter itself, which acts as a polishing
step. The overall biofiltration efficiency may be higher than 90% depending on the
operating conditions of the system and the characteristics of the incoming gases.5
This report is a field evaluation of the engineered biofiltration system installed in
September 2004, at the wastewater Lift station 64 in Brownsville, Texas. It includes a
short background, the methodology for field data collection and sampling, the description
of the system, results and discussion, conclusions, and future work.
BACKGROUND
A simple biofilter has been in operation at Brownsville wastewater Lift station 64 since
2002. This biofilter processes contaminated air from the wastewater wet well located in
the station. It is equipped with water sprinklers for humidification. Its efficiency is
apparently very high; however, the biofiltration media is required to be replaced annually.
Despite its high removal efficiency, the biofilter also requires deodorant to control odor
near the biofilter.
Previous research work 4, 5 has shown that the compost and wood chip material used for
the simple biofilter is capable of removing H2S in the presence of ammonia, and that the
engineered material is also effective offering a better utilization across the length of the
biofilter bed.
A Texas A & M University-Kingsville field team started data collection on the operation
of Lift station 64 in May 2004. The objective of this preliminary work was to set an
environmental baseline prior to the installation of one pilot scale engineered biofilter.
The new engineered biofilter was installed in September 2004, and started operation in
the following month. After two months of continuous operation, the biofilter reached
80% H2S removal efficiency.
METHODOLOGY
The field work was planned in two stages. From May to August 2004, a field team was
dedicated to familiarize themselves with the operation of wastewater Lift station 64,
including the biofiltration system. The source of air pollution in the station is the wet
well wastewater. Magnesium hydroxide is injected into the wastewater in the wet well to
control the generation of hydrogen sulfide. In order to get a representative outlet
concentration and flow rate, the biofilter was covered with a tarp. A piece of plastic
hose was connected from biofilter to a PVC pipe and to the vent. The 0-50 ppm range
logger and a velocity pressure manometer were connected to the PVC pipe for measuring
the H2S concentration and the flow rate. Figure 1 shows the biofilter with the plastic tarp,
and the biofilter inlet and outlet connections on back.
Figure 1: Biofilter with plastic tarp.
In September 2004, a pilot scale engineered biofilter was installed at the site. This
biofilter has been in continuously in operation since October 2004. After several trials,
this biofiltration system attained stable conditions treating approximately 60% of the total
flow rate of contaminated air. The remainder is treated in the simple biofilter. The
objective of this work is to evaluate the performance of the engineered biofilter.
SYSTEM DESCRIPTION
Brownsville wastewater Lift station 64 has three submerged water pumps enclosed in a
concrete chamber, an automatic control room, a chemical injection arrangement and a
biofiltration system. Wastewater is collected in the chamber, also called wet well, and
periodically pumped to the North Waste Water Treatment Plant. Magnesium hydroxide
solution is injected into the wet well to control generation of H2S. An electric fan blows
the wet well gases to the biofiltration system.
Currently, there are two biofilters in operation: a simple biofilter and an engineered
biofilter. The simple biofilter is a 12’ diameter, 6’ deep cylinder, open on the top and full
of compost and wooden chips. The contaminated air is injected at the bottom of the
biofilter through a 6” PVC pipe arrangement. The biofilter has a 1” diameter PVC pipe
water sprinkler system that operates half an hour every day.
The engineered biofilter is a vertical metallic enclosure that houses a biotrickling filter in
the middle and a compost/ball media biofilter located in the upper part of the enclosure.
The enclosure is a vertical rectangular parallelepiped of 6’ x 8’ wide and 12’ tall. A
recycling pump spreads water into the biofiltration media for humidification and into the
wet scrubber for mass transferring. Water carries reaction products down to the sump.
Currently, 300 gallons of fresh water per day are added into the recycling system.
Certified instruments are used for measuring operational parameters. Biofilter inlet and
outlet H2S concentrations are recorded with two Odalog portable gas loggers. One 01000 ppm and another 0-50 ppm H2S loggers are connected to the biofilter inlet and
outlet respectively. An Odalog gas logger can store up to 32,000 readings of
concentrations in a period of 5 days up to three months. Figure 2 below shows the
engineered biofilter and the gas logger case. A Dwyer Pitot-tube/inclined manometer
device is used for measuring the volumetric flow rate at several points within the
treatment system.
Figure 2: Engineered biofilter, inlet piping arrangement and two gas loggers
Recycling water acidity and conductivity are monitored at the sump with a pH meter.
This instrument can record pH, conductivity, oxygen content, temperature and date
simultaneously. The water recirculation system of the biofilter includes an electric pump,
a ball flow meter and two flow totalizers. The flow totalizers register fresh water income
and drainage from the sump.
The field team is monitoring the engineered biofilter every week to maintain reliable
operation conditions. The work includes gas sampling and biofilter media sampling for
laboratory analysis.
RESULTS AND DISCUSSION
The concentration of pollutants coming out from the wet well is variable. It is affected
by the pumping rate and the ambient temperature. The ambient temperature varies daily
and seasonally. It approximately varies from 90F to 110F daily during hot seasons, and
from 50F to 70F during cold seasons. Water pumping rates depend on water
consumption of the area. Wastewater is continuously collected in the wet well, but the
pumping operation is intermittent. One or more pumps are on every five minutes
approximately. As an example, Figure 3 shows the effect of pumping operation on the
H2S concentration during few hours in August 17, 2004. According to this figure, the
H2S concentration decreases when one pump was on.
Figure 3: Biofilter inlet H2S concentration and the on/off main water pump operation.
700
LS 64 Biofilter Inlet Concentration Vs Pump Operation 08/17/2004 3pm to 6.30 pm
Pump Operation - ON: 200/OFF: 0
Pump Operation
Inlet Concentration
600
500
500
400
300
200
In let Concentr atio n (ppm)
300
200
Pum p Oper ation ON/OFF
400
0
0
15:00
100
100
15:15
15:30
15:45
16:00
16:15
16:30
16:45
17:00
17:15
17:30
17:45
18:00
18:15
18:30
Tim e(m in)
The engineered biofilter is in operation since October 2004. The biofilter acclimation
period may last from few hours to more than a year.5 In the present case, the removal
efficiency increased to more than 80% in less than three months which is acceptable
under current operating conditions. The current operating conditions of the engineering
biofilter are the following:
 Total flow rate of the system: 80 cfm









Contaminated air flow rate before mixing with fresh air: 64 cfm
Biofilter inlet flow rate after mixing with fresh air: 180 cfm
Inlet line pressure: 2 in wc
Inlet pipe diameter: 4 in
Average sump water conductivity: 2000 µS
Average sump water pH: 2.5
Water recycling flow rate: 14 gpm
Fresh water injection rate: 300 gpd
Sump water volume: 300 gallons
Figure 4 shows the inlet and outlet H2S concentrations of the engineered biofilter under
the above conditions. The concentration decreases at night due to lower temperature and
probably lower water consumption in the neighborhood.
Figure 4: Inlet and outlet H2S concentrations of the engineered biofilter.
Inlet & Outlet H2 SConcentration
Lift S ta tion 64, Browns ville
December 2004
160.00
140.00
Concentrati on of H2S (ppm )
120.00
100.00
80.00
60.00
40.00
20.00
0.00
11/25
11/29
12/3
12/ 7
12/11
12/15
12/ 19
12/23
Date
Inlet Concentration
Outlet Concentration
In addition, the recycling water pH also affects the concentration of the contaminated air.
Water pH below 2.0 at the sump suggests that recycling water acidity increases probably
due to the generation of sulfuric acid in the biotrickling filter. In order to control pH, the
recycling flow rate was increased from 15 gpd 300 gpd. As a result, the average removal
efficiency rose from 75% to 85%. Figure 5 shows H2S removal efficiency of the
engineered biofilter during December 2004.
Figure 5: Hydrogen Sulfide Removal Efficiency
H2 S Re moval Effi ciency
Lift Stati on 64, B row nsville
De cember 20 04
100.0
90.0
% H2S Removal E fficiency
80.0
70.0
60.0
50.0
40.0
30.0
20.0
10.0
0.0
11/25
11/2 9
12/3
12/7
12/11
12/15
12/19
12/23
D ate
CONCLUSIONS
The engineered biofilter installed at Brownsville Lift station 64 is working satisfactorily.
Its removal efficiency increased to almost 85% after three months of operation. This
efficiency can be considered the baseline under actual conditions. However, the
efficiency will probably increase with growth of the microorganism population.
Sump water acidity increases probably due to the presence of sulfur oxidation reaction
products generated in the biotrickling filter3 and the biofilter. High acidity affects
microorganism growth in the biofilter. The injection of 300 gpd of fresh water into the
recycling system helps to control recycling water acidity. This is one way to protect
viability of the microorganism’s consortium in the biofilter.
The generation of H2S in the wet well is affected by the water pumping frequency /flow
rate. The H2S concentration of wet well gases is high and fluctuating. Addition of
Magnesium hydroxide helps to control stripping of H2S. Considering that engineering
biofilters are designed for treatment of moderated H2S concentrations, the wet well gases
are mixed with fresh air before treatment.
FUTURE WORK
Optimization of the contaminated air treatment operation parameters is the next step.
Some of the most sensitive parameters involved in the optimization process will be the
microorganism growth rate, recirculation water pH/conductivity and flow rate, and
chemical injection rate. Establishing advantages and disadvantages of these parameters
will help to determine the best efficiency of the biofiltration treatment.
ACKNOWLEDGMENT
This material is based upon work supported by the National Science Foundation under
Grant No. HDR-0206259 and from the Texas Higher Education Coordinating Board
Technology Development and Transfer Program Y04 We gratefully acknowledge the
additional in-kind support by Bio Reaction Industries LLC and the Brownsville Public
Utilities Board.
REFERENCES
1.
2.
3.
4.
5.
Standefer, Scot (n. d.). Evaluating biofiltration for air pollution control.
Retrieved January 2005 from: http://www.ppcbio.com/ppcbiopaper8.htm
US EPA. (September 2000). Collection systems technology fact sheet. EPA 832F-00-073
Wu, Ming. (n. d.). Tricking biofilters for hydrogen sulfide odor control.
Retrieved December 15, 2004 from:
http://www.cheresources.com/biofilters.shtml
Jones, K.D., Martinez, Rizwan, M., Boswell, J., “Sulfur Toxicity and Media
Capacity for H2S Removal in Biofilters Packed with both Natural and
Commercial Media,” accepted for publication, J. of Air & Waste Management
Association, (August, 2004)
Jones, K.D., Martinez, A., Maroo, K., and Deshpande, S., “Kinetic Evaluation of
H2S and NH3 Biofiltration for Two Media Used for Wastewater Liftstation
Emission,” J. of Air & Waste Management Association, 54, (January 2004).
KEY WORDS
Biofiltration efficiency, odor treatment, wastewater lift station