The resulting problems are illustrated
by the following examples.
At a styrene/butadiene rubber production plant, PPC Biofilter installed a
commercial pilot unit to remove styrene emissions from a rubber dryer. Past
pilot research at other facilities had shown excellent removal of styrene.
However, daily tests with a portable FID indicated removal efficiencies for
total VOCs of only 60-80%, far below the projected 95%. Further investigation
revealed that the off-gas from the direct-fired rubber dryer contained significant
concentrations of uncombusted methane. While methane is detected efficiently
by FID, its removal in biofilters is extremely poor, primarily due to its
high Henry's Law constant. It also is not considered a VOC for regulatory
purposes. Thus, when methane is present in an off-gas, the use of a FID tends
to underestimate the removal efficiency of a biofilter for total VOC or for
specific hazardous air pollutants (HAP), such as styrene.
In order to verify the removal of styrene
in the dryer off-gas, grab samples were collected and analyzed by gas chromatography/mass
spectrometry (GC/MS). The results indicated styrene removal of 98%, compared
to 60-80% removal of total VOCs, as measured by an FID. The same problem
has been encountered with other biofilters treating off-gas from direct fired
ovens and dryers, including several installations on bakery ovens for ethanol
removal, fiberglass mat dryers for formaldehyde removal, and wood dryers
for the removal of alcohols, ketones, and pinenes.
The use of FID for measuring removal of
total VOCs has also presented problems when applied to complex gas streams
containing both, oxygenated and non-oxygenated VOCs. In a film manufacturing
plant, PPC Biofilter installed a pilot biofilter to remove VOC emissions
from a solvent mixing room. Solvents included both, non-oxygenated (toluene)
and oxygenated compounds (methanol, ethanol, NEK, ethyl acetate). When comparing
total VOC removal measured by an FID to that determined by GC analysis of
concurrent inlet and outlet samples, the GC results showed a higher removal
efficiency. This can be explained by the lower response factor of the FID
for oxygenated compounds, compared to aliphatic, olefinic or aromatic compounds,
relative to commonly used calibration gases (methane, propane). At the same
time, oxygenated VOCs are generally more efficiently removed in biofilters
than non-oxygenated compounds. As a result, a FID tends to particularly underestimate
the inlet VOC concentrations in off-gases containing a significant fraction
of oxygenated compounds, thus underestimating the percent removal efficiency
for total VOCS.
The same problem has been encountered in the wood industry where PPC has
installed commercial biofilters handling off-gas flows of more than 100,000
acfm. Alcohols and formaldehyde are present in these gas streams, in addition
to terpenes, which produce a comparatively good response in a FID but are
less efficiently removed in a biofilter than alcohols and formaldehyde. Again,
underestimation of the concentration of oxygenated VOC, particularly in the
biofilter inlet, results in underestimation of the removal efficiency for
total VOCS.
Off-gas streams exiting biofilters are
generally saturated with moisture, and their temperature frequently exceeds
ambient temperatures. As a result, condensation may occur in sampling lines
and/or the FID itself, causing flame-outs, retention of water soluble compounds
in the sampling lines and/or plugging of the lines. The use of sampling lines
heated to above 220'F may be required in these cases. In the wood industry
this has caused problems in obtaining accurate measurements of the VOC concentration
in the biofilter outlet. Gas streams may still contain sub-micron condensable
particulates, comprised of pine tars and long-chain organic acids. Depending
on the pore size of the particulate filter before the sampling line, some
of these particulates may pass through the filter, re-volatilize in the heated
sampling lines and be detected as VOCs by the FID, thus overestimating outlet
VOC concentrations.
PARTICULATE EMISSIONS
A thorough knowledge of the characteristics of the off-gas considered for
biofilter treatment is essential in achieving the desired pollutant removal
and avoiding operational problems. In some cases, full-scale biofilters have
been sized based on available off-gas monitoring results without conducting
a pilot test. This approach requires both, a thorough understanding of the
assumed off-gas composition and accuracy of the theoretical models applied.
In unfamiliar applications, neither requirement may be met, thus requiring
a pilot test. Even then, it is crucial that the properties of the tested
slipstream are well known and representative of the target stream. Yet, experience
with industrial biofilters has shown, that comprehensive understanding of
an off-gas stream with respect to gaseous pollutants, particulates and temperature
is often lacking.
In addition to the nature and concentration of the VOC species present, evaluation
of the potential presence of particulates is essential in order to avoid
media clogging by particulate carryover. Although not designed as a particulate
control device, the biofilter will act as a very effective particulate filter.
While many industrial facilities may have collected PM 1O results, it is
unusual to obtain both, particulate concentrations according to EPA Method
5 (front-half) and condensables, using EPA Method 202 (back-half). The latter
have repeatedly caused media clogging in biofilters. This suggests, that
it may not only be important to know the total particulate loading to the
filter, but also their physical and chemical nature.
Many biofilters treat off-gases which are at temperatures
above the mesophilic range (> 105 F), thus requiring cooling prior to
entering the biofilter. Depending on whether the off-gas has a wet bulb temperature
above or below 105 F, cooling may be accomplished by adiabatic cooling during
prehumidification (see below) or by forced cooling. In either case, cooling
is likely to cause the condensation of particulates present in the off-gas.
The condensables can be high molecular weight organic compounds (oils, fats,
grease) or polymers. While they may be biodegradable, degradation may be
too slow to prevent accumulation of particles at the biofilter inlet, i.e.
the top of the media in case of a down-flow system. Clogging may result and
cause a rapid increase in pressure drop across the media.
PPC Biofilter installed a pilot unit on a fiberglass oven for abatement of
formaldehyde emissions. In the production process, glass fibers are rolled
into a mat, to which a liquid urea formaldehyde resin is applied as a binder
for the fibers. The fiberglass mat is then sent to a curing oven and heated
to 400 F. During curing, excess formaldehyde and other VOCs are emitted from
the oven. The pilot unit incorporated a packed humidification tower, acting
as a direct contact cooler. The recirculation water passed through a plate
and frame heat exchanger and was cooled by the facility's cooling system
to 80 F, thus lowering the 400 F exhaust temperature to 100 F. The forced
cooling caused two problems that will have to be addressed in a full-scale
installation. First, condensate containing formaldehyde was formed in the
heat exchanger, requiring discharge. Secondly, uncured urea formaldehyde
resin formed small condensed globules on the surface of the media. Thus,
a full-scale installation would require POTW approval before discharging
the condensate, and a pre-treatment filter for removal of condensed resins.
PPC Biofilter also designed and installed a full-scale
APC system, including a commercial biofilter for treatment of the exhaust
from a corn dryer. In this process, the corn is injected with SO2 to loosen
the hull/germ before processing. Drying the processed corn generates methanol
and ethanol emissions. The dryer exhaust has typical dry and wet-bulb temperatures
of 180 and 120 F, respectively. PPC installed a fiberglass SO2 scrubber with
caustic injection to also act as a direct contact cooler and pre-humidifier.
The recirculation water passes through a heat exchanger which uses cooling
tower water for cooling. The scrubber achieves high removal for SO2 and the
gas stream is cooled to 100 F. Blowdown is primarily related to condensate
formation and discharged to the waste water treatment system.
This commercial installation experienced pressure drop
problems in both the SO2, scrubber and the filter media after one month of
operation. The cause of the media pressure drop was investigated first. It
was initially assumed that a high media moisture content promoted slime growth
and caused a higher pressure drop. The moisture control logic was adjusted
to increase the dry matter percentage of the media. Slime growth and pressure
drop decreased, yet heavy fungal growth occurred on the media surface of
this down flow system, and the pressure drop increased again. Further research
showed that fungal growth was enhanced by low pH conditions, however, the
media retained a neutral pH. Two potential explanations were identified.
The scrubber may not have been removing all of the SO2, thus causing acidic
deposits on the media. Alternatively, a source of nitrogen in the off-gas
may have been overlooked. The SO2 scrubber maintained recirculation water
at a pH of 9. It was concluded that acidic conditions were not caused by
SO2 accumulation on the media. Further investigation revealed that deposits
and acidification were likely due to emissions of proteins and fatty acids.
Proteins could be emitted from the heating of the corn. In order to maintain
production levels, the gluten dryer, which was believed to be the source
of the proteins had to be operated at current levels. Evidence from other
facilities indicated that high temperatures in the gluten dryer could cause
some of the corn protein to pyrolize into smaller peptide and/or polypeptide
fragments. These fragments would then be carried in the dryer exhaust as
particles with molecular weights ranging from several thousands to several
millions. PPC sampled the dryer exhaust before and after the SO2 scrubber
into three consecutive chilled impingers filled with deionized water. The
water was analyzed for nitrogen as in indicator of protein concentration.
The results suggested protein concentrations in the exhaust of between 0.31
and 0.94 g/m3. This far exceeds our design specifications for particulate
loading to the filter media. However, the potential removal of the protein
particles from the exhaust by a high energy venturi scrubber or a wet electrostatic
precipitator, would have required considerable additional expenditures in
capital and operating costs.
Further analysis of the particulate samples showed
that the off-gas also contained typically 13-18 mg/m3 of fatty acids. They
formed, when reacting with the caustic in the SO2 scrubber, a soap sodium
carboxylate which resulted in foaming in the scrubber vessel and caused additional
pressure drop. A reduction in scrubber pH provided some relief. Eventually,
a second scrubber was installed in parallel to lower off-gas velocity and
compensate for the effect of foaming. The protein accumulations on the filter
media were found to be very biodegradable. However, they tended to form deposits
of egg-white-like consistency on the media surface which prevented good access
for bacteria. Thus, the bed irrigation schedule was altered to spray the
media for short periods, several times a day. This assisted in the break
up and flushing of the proteins deeper into the media and their more efficient
biodegradation. As a result, bed pressure drop returned to design conditions.
Additional evidence of protein accumulation was found when the system demisters
were removed for inspection and cleaned after approximately one year of operation.
Large sheets of material were removed from the demisters, in some cases almost
one half inch in thickness, which covered most of the exposed area of the
demisters. This material had a slimy texture, was almost leathery in some
places and appeared to be proteinaceous material.
Condensables have also been found in off-gases emitted
from wood products operation, ranging from wood acids to hydrocarbon wax.
While the risk of clogging the media by forming slowly degradable deposits
on the media may vary considerably with their concentration and chemical
nature, condensables have been found to increase the dissolved and suspended
solids level of the humidifier recirculation water. Without adequate blowdown,
the higher solids concentration have created foaming problems in the humidifier
sumps, requiring the addition of defoamers. The recirculation water must
be checked periodically to insure proper solids levels are maintained. Establishing
a sufficient humidifier blowdown creates a particular problem for wood products
operations because they classed as zero discharge facilities.
OFF-GAS HUMIDIFICATION
A consistent filter media moisture content is the single most important parameter
affecting biofilter performance. Maintaining this moisture content is simplified
by pre-humidifying the off-gas strewn. Without prehumidification, media dry-out
is usually rapid, requiring frequent spray-irrigation of the media. This,
in turn, causes vacillation between wet and dry conditions, fluctuations
in the populations of resident bacteria and fungi, and contributes to media
decomposition. Prehumidification can take on several different designs ranging
from a quench duct to a packed tower.
PPC installed a full-scale biofilter to remove odors
from 12,500 cfm of off-gas from a flavor manufacturing process. A major fraction
of the off-gas results from a spray dryer which causes emissions of particulates,
which are only partially removed in a venturi scrubber. The particulate is
formed from organic materials which provides, when discharged with the warm
off-gas stream (80 F) into the recirculation water of the humidifier, an
optimum environment for slime growth. The humidifier consists of a packed
bed of pall rings over which water is recirculated. After eight months of
operation, slime growth had reached a level that created a flow restriction
and pressure drop across the packing. The packing was removed, spray washed
and reloaded with the pressure drop returning to design conditions. Slime
growth occurred again in two months.
Due to the potential for carryover into the media,
the addition to the humidifier of biocides, as used in cooling towers, may
affect biological activity in the bed. Consequently, hypochlorite has been
added periodically to the humidifier water during non-production days. Water
continues to be recirculated, with the biofilter fan turned off. Before startup
of production, the humidifier sump is drained and refilled with clean water.
This has provided for excellent slime removal. However, growth will reappear
requiring the same exercise once every 4-8 weeks. Disinfectants that are
effective at removing slime molds but do not affect the prevailing pseudomonas
in the media have been tested by PPC, but not been used in the field, yet.
Quench ducts can also be employed for prehumidification
and eliminate the tight void space found in humidifier packing. However,
the less efficient contact between water and gas stream may result in incomplete
humidification. The use of air assisted atomizing nozzles has been found
to provide superior humidification at somewhat increased capital and operating
cost. In our experience, almost all off-gases containing organic constituents
result in some biological growth on the packing, in effect turning it into
a bio-trickling filter that provides some pollutant removal. With clean solvent
emissions, film growth can be controlled to a level that does not cause clogging
and flow restriction. However, if organic particulates are present in the
gas stream, the growth can become excessive and result in clogging of the
packing. Installation of efficient particle removal systems (venturi scrubber,
electrostatic precipitator, high energy filter) has been found to reduce
the rate of slime growth considerably but also results in additional capital
and operating costs.
We have also found that all humidification systems
require the installation of demisters to prevent excess water droplet carryover
onto the filter media. While water droplets of 1000 microns in diameter have
a terminal velocity of 14 ft/sec, duct work is typically designed for off-gas
velocities of 33 to 83 ft/sec. Consequently, the off-gas velocity must be
reduced to eliminate droplet carryover. However, the frequently used mesh
pad demisters may, when installed in gas streams containing organic particulates,
promote slime growth and clogging, and cause considerable operating problems.
TEMPERATURE EFFECTS
Today, essentially all industrial biofilters rely for pollutant removal on
mesophilic microorganisms, with a preferred temperature range of 50-105 F
Since biological activity typically doubles with a temperature increase of
12 F, PPC Biofilter typically specifies a minimum wet-bulb temperature of
70 F for commercial installations which have to achieve consistent performance,
even during the cold season. However, we have observed two distinct responses
of biofilters to temperature changes.
PPC Biofilter is currently constructing a commercial
filtration system for solvent removal in a 45,000 cfm exhaust stream from
a printing operation in Wisconsin. The off-gas contains low concentrations
of various solvents, primarily alcohols, ketones, esters, and aromatic VOCs.
Cold Wisconsin winters posed a potential problem in maintaining an off-gas
temperature of 70 F A pilot unit was installed on the exhaust to confirm
previously estimated removal efficiency and full-scale size. For two weeks,
the biofilter inlet was maintained at a wet-bulb temperature of 80 F After
the temperature was reduced to 75 F and the removal efficiency increased
slightly. A further reduction in inlet temperature to 70 F did not, contrary
to our expectations, cause any loss in performance over a thirty day period.
In this case, pollutant removal is, due to their low concentrations in the
off-gas, limited by the rate of transfer from the gas to the liquid phase,
rather than the activity in the biofilm. Apparently, a decrease in temperature
and the resulting decrease in the Henry's Law constant improves mass transfer
enough to compensate reduced diffusion in the biofilm and biological activity.
However, depending on the contribution of each factor
to pollutant removal, a reduction in temperature frequently causes a negative
effect on overall performance. For example, PPC Biofilter has provided two
commercial biofilters to treat press vent emissions from wood products operations,
with flow rates ranging from 100,000 to 140,000 acfm. Dry-bulb temperatures
at the presses range between 120 - 145 F and remain fairly constant, while
wet bulb temperatures of typically 95 F were assumed, based on previous measurements.
Both installations were completed during the winter months. It was found
that on cold days with ambient temperatures of 20-40 F and low humidity,
actual wet bulb temperatures after humidification were as low as 60 F, while
dry-bulb temperatures at the press were maintained at 120-140 F In this case,
the lower off-gas temperature was found to result in a reduced removal efficiency
for total VOCs, primarily pinenes. Conversely, an increase in wet-bulb temperature
by 1 F increased the removal efficiency by typically 1%. Consequently, steam
is now injected into the off-gas during the winter months. While the biofilter
achieves a total VOC removal, as measured by an FID, of 80% at off-gas temperatures
of 75 F, it increases to 94% at 91 F
These examples demonstrate that the impact of temperature
on the removal of complex mixtures of VOCs at lower concentrations is difficult
to predict. Thus, in order to avoid over- or under sizing a full-scale biofilter,
such impact should be studied under controlled conditions during a pilot
test.
MEDIA MOISTURE CONTENT
Media moisture plays a crucial role in the performance of a biofilter. Insufficient
moisture will reduce biofilm thickness and impair bacterial activity and
biofilter performance. Extremely dry conditions may create channeling and
discharge of partially treated off-gas. Excess fungal growth has also been
noticed when operating the media at a low moisture content. On the other
hand, over wetting of the media can create moisture saturated, anaerobic
zones which do not receive sufficient air flow. As already mentioned, it
may also result in excess slime formation and pressure drop. As described
earlier, gas streams may have unique characteristics that require modification
of spraying schedules and other optimization of the respective moisture control
strategies. Relevant factors include the pollutant mass loading, the specific
compounds present, and the type of particulate emissions that may enter the
bioreactor.
It has been argued that the moisture content of the
media may also critically affect the removal of more hydrophobic VOC, such
test as styrene, in a biofilter. Their removal is, particularly if present
at lower off-gas concentrations, limited by the comparatively poor transfer
into the biofilm surrounding the media particles. Previous research on the
removal of styrene had suggested that its removal can be improved by operating
the filter media at a lower moisture content. PPC provided a commercial biofilter
system for the treatment of emissions from storage tanks containing styrene
covered with a nitrogen blanket for explosion protection. To meet electrical
requirements for Class 1 Div. 1 areas, the skid mounted system included an
explosion proof pump, fan, differential pressure transmitters, thermocouples,
and starters controlled by a programmable logic controller (PLC). Previous
test results had indicated that high removal efficiencies for styrene can
be accomplished by maintaining a moisture content in the filter media at
35% by wet weight. In this system, with styrene concentrations as high as
800 ppm, removal efficiencies stabilized between 65-80%. In order to assess
the impact of moisture content on styrene removal, the media moisture content
was subsequently raised to 70%. At this higher moisture level the styrene
removal efficiency stabilized, contrary to our expectations, at 93%. No noticeable
increase in pressure drop was caused by the increase in moisture content.
In several other biofilter pilot tests, PPC treated off-gases containing
considerable fractions of poorly water soluble VOCS, including aromatic compounds
and terpenes, and evaluated the impact of moisture content on pollutant removal.
These tests suggest the following conclusions:
