Biofiltration for the Treatment of Recalcitrant Volatile Organic Compounds
in Polluted Air Streams
Sarah Mahon
MANE 6960H01
Air & Water Pollution Control Engineering
Fall 2013
Table of Contents
1.0 Introduction
2.0 Conventional Biofilter Systems
3.0 Chlorinated Solvents / Indoor Air Quality
4.0 Treatment Technologies with the Potential to Treat Chlorinated Solvents
4.1
Dehalogenation
4.1.1
Cometabolism
4.1.2
Dehalorespiration
4.2
Bioaugmentation
4.3
UV Photodegredation
5.0 Conclusions
1.0 Introduction
Soil vapor extraction is an in situ remedial technology that reduces concentrations of volatile
constituents adsorbed to soils in the unsaturated zone. A vacuum is applied to the contaminated soil
matrix, creates a negative pressure gradient that causes movement of vapors towards the wells, and
extracted vapor is then treated before being released to the atmosphere.
Conventional volatile organic compound (VOC) treatment systems include granular activated
carbon adsorption, liquid/vapor condensation, incineration, and catalytic conversion. Each of these
systems have advantages and disadvantages. Based on the high volume of air generated by soil
vapor extraction and the low concentrations of VOCs typically generated, each of these treatment
options typically have significant disadvantages; granular activated carbon requires periodic
replacement since the absorption efficiency decreases with loading. Thermal oxidation risks the
creation of dioxins, and needs a constant fuel source to keep temperatures ranging from 600F-800F
for catalytic oxidation and 1350-1500F for thermal oxidation. Vapor condensation requires
cryogenically cooling the vapor stream to below 40C such that VOCs condensate out of the vapor
stream
Due to the disadvantages of conventional soil vapor extraction treatment technologies, remediation
of extracted soil vapor using a biofilter may be advantageous from both an environmental and
economic standpoint. Biofilters have been successful in industry mitigating odorous VOCs from
wastewater treatment plants and abating low-level VOCs generated during surface coating
operations. They have lower capita costs than conventional treatments, lower operating costs than
conventional treatments, and require no combustion sources to operate. However, biofilters are not
typically used in soil vapor extraction systems for the abatement contaminated air streams. This
paper explores how conventional biofiltration systems work, why conventional biofilters are not
suited to treat typical soil vapor extraction air streams, and research that is being conducted which
may allow biofilters to become more widely used in soil vapor extraction.
2.0 Conventional Biofiltration Systems
Traditional biofilters involve a filter material that serves as a breeding ground for microorganisms.
The microorganisms live in a thin layer of moisture (biofilm) that surrounds the filter media. A
polluted airstream is pumped through the biofilter and pollutants are absorbed into the filter media.
Contaminated gas is diffused in the biofilter, adsorbed into the biofilm, and allows the
microorganisms to degrade the pollutants in an aerobic atmosphere. The biological degradation
process occurs by oxidation, and can be written as:
Organic Pollutant + O2 - CO2 + H2O + Heat + Biomass
Some systems (known as biotrickling filters) continuously or intermittently trickle water over the
packed bed. The water provides nutrients to the microorganisms, leach by-products, and maintain
favorable conditions for the process culture.
Most biofilters are suited to treat air contaminants in low concentrations; this is because of low
biodegradations rates, clogging of filter media with biomass growth, and limited capacity to
neutralize acidic products of degradation.
3.0 Chlorinated Solvents and Indoor Air Quality
Multiple VOCs are regulated to certain concentrations in soil vapor due to their harmful effect on
human health. In the state of Connecticut, these concentrations thresholds are known as
“volatilization criteria” and can be found in regulations 22a-133k Appendix F. The VOCs with the
lowest criteria are frequently chlorinated solvents, including tetrachloroethylene (PCE),
trichloroethylene (TCE), vinyl chloride (VC), carbon tetrachloride, 1,1,2,2-tetrachloroethane, etc.
The chemical properties of chlorinated solvents lead them to disperse widely in the environment and
their volatile nature leads to formation of vapor plumes in soil. At industrial facilities, chlorinated
solvents are frequently the driver for the installation of soil vapor extraction systems. Figure 1
depicts the formation of a vapor plume from a chlorinated solvent release.
Chlorinated VOCs are frequently referred to as “recalcitrant” VOCs in terms of biofiltraion. These
substances are volatile, non-water soluble, have low adsorption and or/degradation rates, have a
molecular structure that is hard to break down, and do not promote bacterial growth. For these
reasons, conventional biofilters have not been effective at mediating recalcitrant VOCs.
Figure 1 – Vapor Plume Formation
4.0 Treatment Technologies with the Potential to Treat Chlorinated Solvents
4.1 Dehalogenation
In conventional biofiltration, microorganisms directly consume target contaminants. The polluted
air stream can be passed continuously through the biofilter as the microorganisms directly
metabolize the contaminants as sources of food. Contaminants easily destroyed by conventional
biofilters include methyl ethyl ketone, toluene, xylene, benzene, styrene, methylene chloride,
hydrogen sulfide, and ammonia. However there are certain contaminants, including chlorinated
solvents, that cannot be metabolized directly by naturally occurring aerobic microorganisms. To
degrade chlorinated solvents, they need to be broken down into degradation products.
Dehalogenation is the stripping of halogens (usually chlorine) from organic molecules such as PCE
or TCE. This process can occur through cometabolism (aerobic) or dehalorespiration (anaerobic)
processes. An example of dehalogenation of PCE is shown below:
Figure 2 – Degradation Products of PCE
4.1.1Dehalorespiration
There are certain types of bacteria that can utilize PCE and TCE in dehalorespiration; however most
of these bacteria cannot completely dechlorinate the compound to either ethene or ethane (which
are readily biodegradable in biofilters). Breakdown products such as cis-dichloroethene (cis-1,2DCE) and vinyl chloride (VC) are more toxic than the parent compounds. Dehalorespiration also
requires an oxygen-free environment, which can be difficult to produce. Due to the high toxicity of
breakdown products and inability of bacteria to fully complete dechlorination, dehalorespraiton as
the sole treatment technology has not been widely pursued.
4.1.2 Cometabolism
Cometabolism relies on the transformation of a compound by a microbe relying on another primary
substrate. Multiple investigations have been completed performed into cometabolism, specifically
for the treatment of TCE. Biofilters using methane, butane, propane, propylene, phenol, toluene, or
ammonia as the primary substrate have shown success in the treatment of TCE. Investigations
using propane, methanol, and toluene as primary substrates are summarized below.
In an investigation by Lackey et al., TCE contaminated air was fed into a biofilter which used
propane as a primary substrate. The TCE was introduced into the biofilter in two modes; the first
involved feeding TCE and propane to the biofilter at the same time, the second involved cycling the
propane and the TCE is a step-wise fashion. Feeding both the propane and the TCE into the
biofilter at the same time achieved 25% destruction of TCE; the cyclic process achieved greater than
98% removal of TCE. The TCE was destruction rate was dependent on the feeding scheme; for fullscale purposes, the authors proposed a two-biofilter system which would alternate TCE
contaminated air and primary substrate.
Investigations have also been done into using multiple species of bacteria for cometabolism instead
of one sole microorganism. In a study by Shukla et al., biofilters used methanol as a primary growth
substrate for cometabolism of TCE using a mixed culture of methantropic bacterial species. It was
anticipated that the mixed culture would last for a longer duration and treat a wider range of TCE.
Removal efficiency was found to be higher than 90% throughout the experiment.. Although this was
successful, the authors pointed out that it would be difficult to replicate the efficiency found in the
laboratory to full-scale field usage.
Cometabolism is dependent on the ratio of primary substrate to contaminant. An investigation by
Jung and Park looked at the cometabolism of TCE using toluene as a primary substrate in both stage
feeding and cyclic feeding methods. The goal was to analyze the effects of inlet TCE concentration,
inlet primary substrate concentration, and TCE/substrate ratio on the destruction efficiency of
TCE. It was found that at higher concentrations of TCE at the inlet, TCE degradation was less
effective due to the toxic effects of the TCE. The TCE destruction rate decreased after a
TCE/toluene loading ratio of 0.3 (TCE/toluene). It was also found that the presence of toluene
inhibits the degradation of TCE, due to the microorganism’s preference for the primary substrate.
To maximize biofilter efficiency, it is necessary to provide minimum amount of toluene to maintain
activity of the microorganisms.
Although multiple investigations have shown promise in the field of cometabolism, most research
available is specific to the degradation of TCE. Some chlorinated solvents (such as PCE) can not be
cometabolized. Therefore, cometabolism will only be useful in certain applications. In addition,
depending on the waste air stream, primary substrates may not be present and will need to be added.
4.2 Bioaugmentation
A major concern when reducing chlorinated solvents such as TCE is the breakdown products (such
as cis-DCE and VC) which can occur. In a study by Popat, Zhao, and Deshusses, a biofilter was
setup with a mixed culture capable of reducing TCE. After a significant accumulation of cis-DCE
and VC was observed, an additional strain of bacteria specifically for the degradation of cis-DCE
and VC was added to the biofilter. The study found that after bioaugmentation, the conversion of
TCE to ethene increased to 45% from 10% prior to bioaugmentation. The study demonstrated that
it is possible to enhance the performance of biofilters by adding bacteria to target specific
constituents that have built up and are inhibiting the biofilters performance. However, it may be
impractical to implement this in full-scale mode based on the amount of monitoring required.
4.3 UV Photodegredation
To aid biofilter treatment of recalcitrant VOCs, ultraviolet light has been considered as a
pretreatment process. The UV irradiation is capable of oxidizing a wide range of contaminants
including non-biodegradable and recalcitrant VOCs. The UV treatment can create by-products more
toxic than the parent pollutant, but the by-products are more water soluble and biodegradable than
parent compounds.
Initial testing using chlorobenzene has shown promise when using UV and biofiltration in tandem.
In an experiment conducted by Can et al, controlled experiments demonstrated that the combined
process can result in higher removal efficiency than the biofilter process alone. In addition, the UV
pretreatment reduced inhibitory effects that can occur when pollutant concentration increases. The
biofilter is able to recover more rapidly after transient loading conditions when paired with UV.
Although the UV photodegradation of chlorobenzene can result in products having higher acute
toxicity, the toxicity was significantly reduced after bio filtration. Ozone (harmful by-product of UV)
was also eliminated by the subsequent biofilter.
A similar experiment by Moussavi and Mohensi compared the removal efficiency of a mixture of
toluene and o-xylene in a UV-bio filtration process and a reference biofiltration process. O-xylene is
an extremely recalcitrant VOCs; and o-xylene has been shown to significantly inhibit the
biodegradation of toluene. The UV pretreatment was able to partially oxidize a fraction of the
airborne contaminants, turning them into biodegradable intermediates (such as acetaldehyde and
formaldehyde)which were effectively removed in the downstream biofilter. The combined UVbiofilter was able to provide greater than 95% removal efficiency, greater than the sum of either the
UV treatment by itself or the biofilter by itself . In addition, the biofilter receiving the pretreated air
stream experience less biofilm accumulation and experienced less pressure drop. Although the UV
pretreatment generated ozone (a toxic gas), the ozone was effectively treated by the biofilter, it did
not negatively impact the degradation of VOCs, and it was credited with controlling the biomass
growth while maintaining a healthy population of organisms.
In a paper by Den et al., bench scale studies were able to consistently achieve contaminant removal
efficiencies of 99-100% under optimal conditions for PCE and TCE using a UV pretreatment
system. It was discovered in the initial UV pretreatment, photolysis products of TCE byproducts
included degraded into phosgene, dichloroacetyl chloride, trichloroacetic acid, carbon dioxide, and
hydrochloric acid, while those for PCE included phosgene, dichloroacetic acid, carbon dioxide,
hydrochloric acid. These toxic photooxidation products were completely destroyed in the
biofiltration phase.
Table 1: UV Photooxidation Products of TCE and PCE (Den et al.)
The UV pretreatment of contaminated airstreams shows potential for use in soil vapor extraction
systems. The UV pretreatment can degrade a wide range of chlorinated solvents (including PCE,
which is unable to be cometabolised). Although the degradation process through photolysis
generates a large range of toxic by-products, these by-products have effectively treated by the
subsequent biofilters. In addition, the ozone generated has been credited with less biofilm
accumulation, maintaining a healthy level or microorganisms, and less pressure drop across the
biofilter.
5.0 Conclusions
Biofiltration for the treatment of air pollutant mitigation is a complicated and evolving technology.
The mechanisms of how biofiltration work is still not well understood. A challenge in treating waste
gas is the variety of bacteria that can be used (pure cultures or mixed cultures), the variety of filter
media that can be used, the variety of contaminants that can be present (single contaminant or
multiple contaminants), the inhibitory multiple contaminants can have on each other during the
degradation process, and the toxic breakdown products that can be produced.
Bench-scale studies have shown promise in treating certain chlorinated solvents. Cometabolism has
shown promise specifically in the treatment of TCE using a variety of primary substrates, and UV
pretreatment has shown promise for multiple recalcitrant VOCs including PCE and TCE. Any fullscale system will need to be carefully tailored to the specific waste gas stream to be treated.
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http://www.bioremediation-specialists.com/Reductive-Processes.php (Figure 1)
http://www.washoecounty.us/water/PCE_background.htm (Figure 2)
http://www.rpi.edu/dept/chem-eng/BiotechEnviron/MISC/biofilt/biofiltration.htm#ADVANTAGES AND DISADVATAGES OF
BIOFILTRATION