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Control of VOC and HControl of VOC and HAP by BiofiltrationAP by Biofiltration

朱 信朱 信Hsin ChuHsin ChuProfessorProfessor

Dept. of Environmental EngineeringDept. of Environmental EngineeringNational Cheng Kung UniversityNational Cheng Kung University

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1. 1. IntroductionIntroduction

A biofilter consists of bed of soil or compost beA biofilter consists of bed of soil or compost beneath which is a network of perforated pipe. neath which is a network of perforated pipe. Contaminated air flows through the pipe and oContaminated air flows through the pipe and out the many holes in the sides of the pipe thereut the many holes in the sides of the pipe thereby being distributed throughout the bed.by being distributed throughout the bed.

The microorganisms are the same that degradThe microorganisms are the same that degrade organic wastes in nature and in wastewater te organic wastes in nature and in wastewater treatment plants.reatment plants.

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In soils the pores are smaller and less permeablIn soils the pores are smaller and less permeable than in compost. Therefore, soil requires larger e than in compost. Therefore, soil requires larger areas for biofiltration.areas for biofiltration.

This technology has been used in Europe for maThis technology has been used in Europe for many years and is considered to be a Best Availablny years and is considered to be a Best Available Control Technology (BACT) for treating contame Control Technology (BACT) for treating contaminated gaseous streams.inated gaseous streams.

Biofilters function efficiently and economically for Biofilters function efficiently and economically for removing low concentrations (less than 1,000 to removing low concentrations (less than 1,000 to 1,500 ppm as methane) of VOCs, air toxics, and 1,500 ppm as methane) of VOCs, air toxics, and odor.odor.

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Advantages:Advantages:

low installation and operation costslow installation and operation costslow maintenance requirementslow maintenance requirementslong lifelong lifeenvironmentally safeenvironmentally safeeconomically applied to dilute gas streameconomically applied to dilute gas stream

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For odorous compoundsFor odorous compounds98 to 99% removal has been reported98 to 99% removal has been reported

For VOCsFor VOCsgenerally in the range of 65 to 99% removalgenerally in the range of 65 to 99% removal

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Characteristics of the biofilterCharacteristics of the biofilter

mediamediatemperaturetemperaturepHpHmoisture contentmoisture contentgas residence timegas residence timeproperties of the compounds being removedproperties of the compounds being removed

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Biofilters can also remove particulates and liquidBiofilters can also remove particulates and liquids from gas streams. However, care must be takes from gas streams. However, care must be taken because particulates or greasy liquids can funn because particulates or greasy liquids can function to plug the biofilter.ction to plug the biofilter.

Industries including chemical manufacturing, phaIndustries including chemical manufacturing, pharmaceutical manufacturing, food processing, warmaceutical manufacturing, food processing, wastewater facilities, and compost operations have stewater facilities, and compost operations have successfully used this technology for odor controsuccessfully used this technology for odor control. l.

Biofiltration has also been used to reduce VOC eBiofiltration has also been used to reduce VOC emissions in aerosol propellant operations. missions in aerosol propellant operations.

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2. Theory of Biofilter Operatio2. Theory of Biofilter Operationn

Biotransformations act along with adsorption, aBiotransformations act along with adsorption, absorption, and diffusion to remove contaminantbsorption, and diffusion to remove contaminants from the gaseous stream.s from the gaseous stream.

The gas passes upward through perforated pipThe gas passes upward through perforated pipes and the biofilter media bed.es and the biofilter media bed.

The contaminants in the gas are either adsorbThe contaminants in the gas are either adsorbed onto the solid particles of the media or absoed onto the solid particles of the media or absorbed into the water layer that exists on the medrbed into the water layer that exists on the media particles.ia particles.

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The media of the filter functions both to supply inThe media of the filter functions both to supply inorganic nutrients and as a supplement to the gas organic nutrients and as a supplement to the gas stream being treated for organic nutrients.stream being treated for organic nutrients.

The sorbed gases are oxidized by the microorgaThe sorbed gases are oxidized by the microorganisms to COnisms to CO22. The volatile inorganics are also so. The volatile inorganics are also so

rbed and oxidized to form calcium salts.rbed and oxidized to form calcium salts.

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The biofilters are actually a mixture of activated The biofilters are actually a mixture of activated carbon, alumina, silica, and lime combined with carbon, alumina, silica, and lime combined with a microbial population that enzymatically catalyza microbial population that enzymatically catalyzes the oxidation of the sorbed gases. es the oxidation of the sorbed gases.

The sorption capacity is relatively low, but the oxiThe sorption capacity is relatively low, but the oxidation regenerates the sorption capacity.dation regenerates the sorption capacity.

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Half-lives of contaminants range from minutes to Half-lives of contaminants range from minutes to months.months.

Next slide (Next slide (Table 15.1)Table 15.1)Compounds in order of their degradabilityCompounds in order of their degradability

Aliphatics degrade faster than Aliphatics degrade faster than aromaticsaromatics. .

HAPs

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Diffusion occurs through the water layer to the Diffusion occurs through the water layer to the microorganisms in the slime layer on the surface microorganisms in the slime layer on the surface of the media particles.of the media particles.

Through biotransformation of the food source, Through biotransformation of the food source, end products are formed, including carbon end products are formed, including carbon dioxide, water, nitrogen, mineral salts, and dioxide, water, nitrogen, mineral salts, and energy to produce more microorganisms. energy to produce more microorganisms.

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Oxidation of adsorbed compounds allows the bioOxidation of adsorbed compounds allows the biofilter to self regenerate. Adsorption sites are contfilter to self regenerate. Adsorption sites are continually becoming available as oxidation by microinually becoming available as oxidation by microorganisms occur.organisms occur.

Overloading of the biofilter results when adsorptiOverloading of the biofilter results when adsorption is occurring faster than oxidation. The result oon is occurring faster than oxidation. The result of overloading is to allow the contaminants to pasf overloading is to allow the contaminants to pass through the biofilter.s through the biofilter.

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3. Design Parameters and 3. Design Parameters and Conditions Conditions

A biofilter can be open or enclosed, it can be builA biofilter can be open or enclosed, it can be built directly into the ground or in a reactor vessel, at directly into the ground or in a reactor vessel, and it can be single or multiple bed.nd it can be single or multiple bed.

Next slide (Next slide (Fig. 15.1Fig. 15.1))A typical biofilter configuration A typical biofilter configuration Optional components include a heat exchange cOptional components include a heat exchange chamber to cool or heat the gas stream to optimal hamber to cool or heat the gas stream to optimal temperature for the filter bed and a water sprinkltemperature for the filter bed and a water sprinkler system to apply moisture directly to the filter er system to apply moisture directly to the filter media media surfacesurface..

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3.1 Depth and Media of Biofilter 3.1 Depth and Media of Biofilter BedBed

The depth of biofilter media range for 0.5 to 2.0 The depth of biofilter media range for 0.5 to 2.0 m, with 1 m being the typical depth of a biofilter.m, with 1 m being the typical depth of a biofilter.

Many different media types include soil, compost,Many different media types include soil, compost, sand, shredded bark, peat, heather, volcanic as sand, shredded bark, peat, heather, volcanic ash, and a mixture of these components have beeh, and a mixture of these components have been used. n used.

Next slide (Next slide (Fig. 15.2Fig. 15.2))A typical biofilter bedA typical biofilter bed

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Often polystyrene spheres or peat granules may Often polystyrene spheres or peat granules may be added to increase the structural support of be added to increase the structural support of the system and to increase the adsorptive the system and to increase the adsorptive capacity of the media.capacity of the media.

The two most commonly discussed media in the The two most commonly discussed media in the literature are soil and compost. literature are soil and compost.

Typical parameters include a neutral pH, pore Typical parameters include a neutral pH, pore volumes of greater than 80%, and a total organic volumes of greater than 80%, and a total organic content of 55% or greater.content of 55% or greater.

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Soil is a stable choice for media in that it does Soil is a stable choice for media in that it does not degrade. However, it contains fewer and less not degrade. However, it contains fewer and less complex microorganisms than compost media.complex microorganisms than compost media.

Compost has higher air and water permeability. Compost has higher air and water permeability. The buffering capacity of compost is also very The buffering capacity of compost is also very good. However, with time compost decomposes, good. However, with time compost decomposes, and the average particle sizes of the filter media and the average particle sizes of the filter media decrease.decrease.

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The useful life of the media is typically up to 5 yeThe useful life of the media is typically up to 5 years.ars.

Fluffing, or turning, of the media material in the bFluffing, or turning, of the media material in the biofilter may be required at shorter intervals to preiofilter may be required at shorter intervals to prevent excessive compaction and settling.vent excessive compaction and settling.

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3.2 Microorganisms3.2 Microorganisms

Three types of microorganisms are generally preThree types of microorganisms are generally present in a biofilter. These include fungi, bacteria, sent in a biofilter. These include fungi, bacteria, and antinomycetes.and antinomycetes.Antinomycetes are organisms which resemble bAntinomycetes are organisms which resemble both bacteria and fungi.oth bacteria and fungi.

The growth and activities of the microorganisms The growth and activities of the microorganisms is dependent on ample oxygen supply, absence is dependent on ample oxygen supply, absence of toxic materials, ample inorganic nutrients for tof toxic materials, ample inorganic nutrients for the microorganisms, optimum moisture conditionhe microorganisms, optimum moisture conditions, appropriate temperatures, and neutral pH rans, appropriate temperatures, and neutral pH range.ge.

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Start up of a biofilter process requires some accliStart up of a biofilter process requires some acclimation time for the microorganisms to grow specmation time for the microorganisms to grow specific to the compounds in the gaseous stream. ific to the compounds in the gaseous stream.

For easily degradable substances, this acclimatiFor easily degradable substances, this acclimation period is typically around 10 days. on period is typically around 10 days.

The acclimation process also allows the microorThe acclimation process also allows the microorganisms to develop tolerance or acceptance for ganisms to develop tolerance or acceptance for compounds they may find to be toxic in nature.compounds they may find to be toxic in nature.

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Often, biofilters are not used continuously in the Often, biofilters are not used continuously in the treatment process. They may be employed intertreatment process. They may be employed intermittently or seasonally, depending on the treatmmittently or seasonally, depending on the treatment process.ent process.

The biomass has been shown to be able to be viThe biomass has been shown to be able to be viable for shut downs of approximately 2 weeks.able for shut downs of approximately 2 weeks.If inorganic nutrient and oxygen supplies are conIf inorganic nutrient and oxygen supplies are continued, the biomass may be maintained for up to tinued, the biomass may be maintained for up to 2 months.2 months.

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3.3 Oxygen Supply3.3 Oxygen Supply

Typically, a minimum of 100 parts of oxygen per Typically, a minimum of 100 parts of oxygen per part of gas must be supplied.part of gas must be supplied.

Anaerobic zones need to be avoided to ensure tAnaerobic zones need to be avoided to ensure that the compounds are biotransformed and to prhat the compounds are biotransformed and to prevent any anaerobic zone odors (primarily hydroevent any anaerobic zone odors (primarily hydrogen sulfide) from forming.gen sulfide) from forming.

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3.4 Inorganic Nutrient Supply3.4 Inorganic Nutrient Supply

These are typically nitrogen, phosphorous, and These are typically nitrogen, phosphorous, and some trace metals.some trace metals.

Trace metals are almost always well supplied in Trace metals are almost always well supplied in the media material. Nitrogen and phosphorous the media material. Nitrogen and phosphorous may need to be added, depending on the media may need to be added, depending on the media characteristics.characteristics.

For aerobic microorganisms, the O/N/P ratio is For aerobic microorganisms, the O/N/P ratio is estimated as 100/5/1.estimated as 100/5/1.

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3.5 Moisture Content3.5 Moisture Content

Moisture content is the most critical operational Moisture content is the most critical operational parameter for the successful operation of a biofiltparameter for the successful operation of a biofilter. The gaseous streams tend to dry out the biofier. The gaseous streams tend to dry out the biofilter media. lter media.

Too little water will result in decreased activity of Too little water will result in decreased activity of the microorganisms, and perhaps transfer of the the microorganisms, and perhaps transfer of the adsorbed contaminants out of the filter and into tadsorbed contaminants out of the filter and into the atmosphere.he atmosphere.

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Too much water can also cause problems, such Too much water can also cause problems, such as anaerobic zones, with the potential of producias anaerobic zones, with the potential of producing odors, and increases in the headloss of the sng odors, and increases in the headloss of the system.ystem.

Optimal water contents vary in the literature, but Optimal water contents vary in the literature, but generally the range of 20 to 60% by weight is acgenerally the range of 20 to 60% by weight is accepted.cepted.

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Moisture can be added to the system in two wayMoisture can be added to the system in two ways: humidification of the gas stream or direct applis: humidification of the gas stream or direct application of water to the biofilter surface.cation of water to the biofilter surface.

Typically, the degree of saturation suggested is aTypically, the degree of saturation suggested is at least 95%, with saturation percentages of 99% t least 95%, with saturation percentages of 99% and 100% quoted as the optimum.and 100% quoted as the optimum.

Typically, water droplet diameters of less than 1 Typically, water droplet diameters of less than 1 mm for surface sprays are suggested, in order to mm for surface sprays are suggested, in order to prevent compaction of the biofilter.prevent compaction of the biofilter.

The maximum water loading rate suggested is 0.The maximum water loading rate suggested is 0.5 gal/ft5 gal/ft22 ．． h.h.

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3.6 Temperature3.6 Temperature

The microorganisms’ activity and growth is The microorganisms’ activity and growth is optimal in a temperature range of 10 to 40optimal in a temperature range of 10 to 40ooC. C. Higher temperature will destroy the biomass, Higher temperature will destroy the biomass, while lower temperatures will result in lower while lower temperatures will result in lower activities of the microorganisms.activities of the microorganisms.

In winter, heating of the off gas streams may be In winter, heating of the off gas streams may be required. On the contrary, high temperature off required. On the contrary, high temperature off gases may need to be cooled.gases may need to be cooled.

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3.7 pH of the Biofilt3.7 pH of the Biofilterer The pH in the biofilter should remain near neutraThe pH in the biofilter should remain near neutra

l, in the range of 7 to 8. l, in the range of 7 to 8. When inorganic gases are treated, inorganic aciWhen inorganic gases are treated, inorganic aci

ds may be produced. ds may be produced. For example, treated HFor example, treated H22S will produce HS will produce H22SOSO44. Ot. Ot

her inorganic acids which can be formed include her inorganic acids which can be formed include HCl and HNOHCl and HNO33..

These acids can cause lowered pH in the media These acids can cause lowered pH in the media over time.over time.

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Carbon dioxide production by the Carbon dioxide production by the microorganisms can also lower the pH over time.microorganisms can also lower the pH over time.

The media typically has some inherent buffering The media typically has some inherent buffering capacity to neutralize small changes in the pH. capacity to neutralize small changes in the pH. However, lime may need to be added if the However, lime may need to be added if the buffering capacity is not sufficient.buffering capacity is not sufficient.

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3.8 Loading and Removal Rates3.8 Loading and Removal Rates

Loading rates can be expressed in three ways: Loading rates can be expressed in three ways: flow rates of gases through the bed, gas flow rates of gases through the bed, gas residence times, and removal rates.residence times, and removal rates.

Flow rates of gas into the bed range from 0.3 to Flow rates of gas into the bed range from 0.3 to 9.5 m9.5 m33/min-m/min-m22. The typical range is 0.3-1.6 . The typical range is 0.3-1.6 mm33/min-m/min-m22..

Off gas rates are typically around 1,000 to Off gas rates are typically around 1,000 to 150,000 m150,000 m33/h./h.

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Gas residence time, the time the gas actually spGas residence time, the time the gas actually spends in contact with the biofilter material, is the tiends in contact with the biofilter material, is the time available for adsorption and absorption to ocme available for adsorption and absorption to occur.cur.

Suggested gas residence times are a minimum Suggested gas residence times are a minimum of 30 s for compost media and a minimum of 1 of 30 s for compost media and a minimum of 1 min for soil media.min for soil media.

Sligtly longer residence times are suggested for iSligtly longer residence times are suggested for inorganic gases.norganic gases.

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Removal rates are typically reported in units of gRemoval rates are typically reported in units of g/kg of dry media/day. /kg of dry media/day. Generally, the lower-molecular-weight, less-comGenerally, the lower-molecular-weight, less-complex compounds are more easily degraded and plex compounds are more easily degraded and more quickly removed in a biofilter.more quickly removed in a biofilter.

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3.9 Pressure Drop3.9 Pressure Drop

The pressure drop through the filter bed dependThe pressure drop through the filter bed depends on the media type, porosity, moisture content, s on the media type, porosity, moisture content, and compaction of the media.and compaction of the media.

Fluffing or replacing the media over time can helFluffing or replacing the media over time can help to prevent compaction and higher pressure drop to prevent compaction and higher pressure drops.ps.

Typical pressure drops range from 1 to 3 in. of wTypical pressure drops range from 1 to 3 in. of water. ater. Typical power consumption for a biofilter is in the Typical power consumption for a biofilter is in the range of 1.8 to 2.5 kWh/1,000mrange of 1.8 to 2.5 kWh/1,000m33..

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3.10 Pretreatment of Gas 3.10 Pretreatment of Gas StreamsStreams

Besides humidification, heating, or cooling, other Besides humidification, heating, or cooling, other pretreatment necessary may include removing ppretreatment necessary may include removing particulates.articulates.

Though the biofilter is capable of removing particThough the biofilter is capable of removing particulates, the solid matter can cause clogging of thulates, the solid matter can cause clogging of the biofilter and gas distribution system.e biofilter and gas distribution system.

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4. Biofilter Compared to Othe4. Biofilter Compared to Other Available Control Technolr Available Control Technologyogy

Other control technology for the control of VOCs Other control technology for the control of VOCs and air toxics include incineration, carbon adsorand air toxics include incineration, carbon adsorption, condensation, and wet scrubbing. ption, condensation, and wet scrubbing.

The advantage that biofilters have over all of theThe advantage that biofilters have over all of these technologies is their ability to treat dilute gas se technologies is their ability to treat dilute gas streams in a cost-effective manner.streams in a cost-effective manner.

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Other technologies often take the pollution from Other technologies often take the pollution from one form and place it in another, for example, reone form and place it in another, for example, removing contamination from an air stream and plmoving contamination from an air stream and placing it in the wash water.acing it in the wash water.

Biofilters allow the biotransforming of the pollutioBiofilters allow the biotransforming of the pollution to less-or nontoxic forms and reduced volumes.n to less-or nontoxic forms and reduced volumes.

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Incineration works as a control technology for hiIncineration works as a control technology for highly concentrated waste streams. It is more expghly concentrated waste streams. It is more expensive to install and operate than a biofiller systeensive to install and operate than a biofiller system.m.

Carbon adsorption is a very effective technology. Carbon adsorption is a very effective technology. However, it is very expensive to use, which is esHowever, it is very expensive to use, which is especially prohibitive to small operations. If the carpecially prohibitive to small operations. If the carbon is regenerated on site, the costs will be less bon is regenerated on site, the costs will be less than if it is not regenerated on site.than if it is not regenerated on site.

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Condensation is an effective technology for treatiCondensation is an effective technology for treating concentrated and pure off gases. As with incing concentrated and pure off gases. As with incinerators, the treatment of dilute streams is too enerators, the treatment of dilute streams is too energy intensive to accomplish cost effectively.nergy intensive to accomplish cost effectively.

Wet scrubbing technology is also more expensivWet scrubbing technology is also more expensive than biofilter systems. e than biofilter systems.

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5. Successful Case 5. Successful Case StudiesStudies

Gases from an animal rendering plant process wGases from an animal rendering plant process were treated in a soil biofilter for odor removal. Reere treated in a soil biofilter for odor removal. Removal rates of 99.9% were obtained.moval rates of 99.9% were obtained.

Another application used a sludge compost biofilAnother application used a sludge compost biofilter to treat a gas stream containing volatile aminter to treat a gas stream containing volatile amine compounds. Removals exceeding 95% were oe compounds. Removals exceeding 95% were obtained.btained.

A prototype biofilter with soil media was used to tA prototype biofilter with soil media was used to treat light aliphatic compounds and trichloroethylreat light aliphatic compounds and trichloroethylene from aerosol propellant releases. Reduction ene from aerosol propellant releases. Reduction rates of 90% were obtained.rates of 90% were obtained.