Biotechnology for Air Pollution Abatement and Odour Control
Appeared in Chemical Weekly on 12th January 1999
Sulfur oxides, nitrogen oxides, carbon monoxides, hydrogen sulfides, hydrocarbons, particulate matter, are the major components of air pollution and are responsible for health hazards and environmental hazards. But equally important are the substances which cause unpleasent offensive odour. The range of malodorous substances like phenol, styrene, TCE (trichloroethane), VOCs (volatile organic compounds), amines, H2S, methyl mercaptans, ammonia etc. are present in gaseous effluents of various industries, treatment plants, animal rendering activities etc. Deodourisation technology makes use of physical, chemical and biological means for odour removal. Biodeodourisation , though applied since 1923, is still less studied, less discussed and less applied area. The importance and application of biodeodourisation is however, increasing.
The Problem –
Increase in the environmental awareness has resulted in more attention of people to the pollution problems. Pollution is sensed by people by offensive odour far before their receiving the damage therefrom. Administrative regulations are obviously more and this leads to the development of deodourisation technology.
There are variety of industries which produce offensive waste gases. These are the pesticide industry, petrochemical industry, explosive industry, mining, meat processing industry, resin production units, paints and varnishes industry, textile industry, chemical industry, pharmaceutical units, animal rendering units, fermentation plants, broiler chicken house etc.
Range of malodorous substances that may be there in their gaseous effluents are alcohols, amines, ammonia, aldehydes, sulfur dioxides, sulphides, hydrocarbons, phenol, styrene, TCE (trichloroethane), VOCs (volatile organic compounds), H2S, methyl mercaptans, esters, ketones etc. Waste gases is really a major pollution problem for many industrial processes.
Waste gases which have an offensive odour –
(a) may be generated during the production process, or
(b) may originate from storage area, or
(c) may come from pumps and compressors which may have leakages, or
(d) may come during transfer of material, or
(d) they may be coming from open waste-water treatment plants and garbage composting plants.
Abatement of unpleasesnt odours is difficult because –
(i) A considerable number of different compounds are often involved and
(ii) Odour producing compounds may be present in very low concentrations. Some compounds have extremely low olfactory perception level. Some mercaptans, for example have an odour threshold of below 1 ppb.
(iii) Sources are often complex, multipoint and difficult to be traced correctly, with several compounds contributing simultaneously.
(iv) Odours that escape from transfer, filtration and drying operations are more difficult to contain.
Deodourisation processes –
Preventive as well as corrective methods are useful for control of odour. In preventive category process modification, equipment modification can be included. Deodourisation processes of corrective nature are roughly classified into physical, chemical and biological methods. Dispersion, water washing, adsorption, thermal incineration, catalytic incineration are amongst the dominant physical methods while chemical methods include catalytic oxidations. Physical and chemical methods in general are not flexible for volume, concentrations, or composition of gas changes that may occur. This can be overcome by biological control of gases. Biological processes earlier required skilled control and large space but the recent developments have made the biological processes more interesting. Recent biological deodorisation processes are characterized by low running costs (1/3 of other processes), easy operation / maintenance and control, energy conservation and treatment at room temperature.
There are three types of biological waste gas purification systems in operation. These are :
(2) Biofilters, Biobeds
(3) Biotrickling filter
|Immobilised||Biotrickling filters||Biofilters (Biobeds)|
Applications of biological processes depend on the physical phenomena and microbiological phenomena. Physical phenomena include : (a) Mass transfer between gas and liquid phase; (b) Mass transfer to microorganisms; (c) Average residence time of mobile phase. Microbiological phenomena include : (a) Rate of degradation; (b) Substrate / product inhibition; (c) Diauxy etc.
Biological purification of waste gases was discussed as early as 1923 for H2S emissions. In 1934, the earliest patents were filed. In the early 1950s large scale applications started. Biofiltration process has been exhaustively described by Ottengraf and coworkers (1983). Wheatley (1985), suggests that prototype units for waste gases will most likely become part of existing waste-water treatment plant. There is a lot in literature on laboratory experiments and successful field applications. Although biological deodourisation is considered to be an effective tool, applications are relatively limited. At present, biological deodourisation systems are treating odours from other treatment units; however, it may be some day possible to seed reactors with specially-cultured microorganisms so that odourous substances / gas will not be produced.
Biologically active materials like peat, compost, humus, woody heather, bushwood carrying microorganisms, activated sludge of effluent treatment units or mixture of organisms or a single organism immobilised as a biofilm on an inert material or in suspended form is used in biological oxidation of gases. In the biological deodourisation process, bad smell ingredients are decomposed by exploiting the metabolism of microorganisms. Elucidation of deodourisation mechanisms is not clear in many cases.
A typical bioscrubber consists of an absorption column and one or more bioreactors. Biological oxidation takes place in these bioreactors. The reaction tanks are aerated and supplied with a nutrient solution. The microbial mass mainly remains in the circulating liquor which passes through the absorption column. Circulation rate is fast enough and not much of the biofilm will develop in the absorption column. If any biofilm develops in the packing of the column, then it has to be removed from time to time.
Waste air to be treated is first brought to a temperature range (10 – 430C) suitable for micro-organisms. Dust in the air, if any, should be removed by the filter in the line. Construction of the bioscrubber is such that air velocity will be 0.8m/s, residence time in packing is 1.8 seconds, liquor circulation rate is 5-6 kg / (h).(m2) and residence time of liquor in reaction tank is 50 minutes.
Bioscrubbers require a lot of skilled attention. They are reported to be successful in experimental works and at places where skilled attention is possible.
Bioscrubbers are applied in the food industry, livestock farming, foundries. Bioscrubbers are more suitable for water-soluble hydrocarbons. The use of activated carbon in the absorber improves mass transfer, buffer capacity and immobilisation of microorgansms. The ventury scrubber has 0.2 to 1kg TSS biomass /m3 and gas flow 0.5-1m/s and gives 90% conversions. Concentration of biodegradable compound is <100-500mg/m3 air, where bioscrubbers are applied.
Emissions of microorganisms is considered to be the risk involved. 103-104 organisms per m3 are present in the treated emission but this is the same as that of the normal air. It is still considered to be of concern by the food and pharmaceutical industries.
Biofilters is the most accepted techniques amongst the three. The soil contains many microorganisms with ability to oxidise VOCs and other odour compounds. Soil, compost, peat, heather, bark etc. is used in combination in biobeds. Moisture contents and weed growth are the main problems of biobeds. Uniformity, permeability of biobeds will decide the proper gas treatment and bypassing or chocking should not occur. Beds require a lot of space (7.5M3/hm3). Beds are turned 2-3 times in a year. Proper drainage is essential at the bottom. Weedicides cannot be used to control the weeds. In Europe, the process designs requiring much less space are developed which are based on specially-prepared media with a high percentage of void space and very large microbiologically-active material surface. Distribution of gas, should be proper through the bed. The residence time for biobed depends on the substrate and is about 28s to 56s. Height of packing bed is 1m and flow rate of gases is 130m3/hm3.
Britain and Ireland use peat and heather for biobed while Germany uses municipal waste compost. Heather:peat ratio is 2:1. Bark may be used instead of heather but it disintegrates early and requires repeated replacements. Acidic pH is maintained (pH 3) for peat for better functioning. After an initial acclimatization period of 3 months most of the VOC components are reduced by 99% with residence time of 51s. Results from large-scale applications are not available. Acetone, 2 butanol, n-butanol, butyl acetate, butyl benzene, ethyl benzene, n-heptane, methyl ethylacetone, 2 propanol, styrene, TCE, toluene, xylene show 99% reduction while n-octane and n-pentane show 70% and 20% reduction respectively.
The medium size biofilter treats 40000m3 of air per hour. Cost of installation in Dfl is 5,00,000 to 1,000,000. Future expected developments in biofilters are : (a) the use of specific microorganisms; (b) reduction in cost; (c) process control (e.g. pH, moisture, rate limiting nutrients); (d) more standardization; and (e) use of air flows over 100,000 m3/h.
Biofilters used may be of open-type (subject to weather conditions changes) or closed-type (are costly). Volatile compounds plus oxygen come in contact with the wet biofilm.
Microorganisms used in biobeds are mesophilic. Temperature 15-400C, moisture 40-60-% and gas contact time 10-30s are maintained for biofilters.
Biofilters reduce 72% emission of broiler chicken house. Disadvantage of biofilter is, at high loading and degradation rate, humidification is problematic. Chlorinated hydrocarbons can not be removed by biofilters as dechlorination causes acidification of packing material. Membrane reactors (porous and hydrophobic) may be used for this purpose. It separates the gas and liquid phases. Microorganisms are located on the liquid side. Diffusion occurs at partition towards wet biofilm.
Recently many applications are reported of biofilters due to more knowledge of process conditions, improvement in filter characteristics, composition of filters etc. Various organisms are used for xenobiotic degradation by biofilters. Actinomyces globisporous, Penicilium spp, Cephalosporium spp., Mucor spp., Micromonospora albus are useful. Organisms from waste water treatment plant may be used. Compost filters are used in industrial waste gases. Biofiltration of other volatile xenobiotic compounds used activated sludge from municipal sewarage treatment plant. Microbial flora used is heterogenous. Dichloromethane is eliminated by Hypomicrobium from activated sludge. 1,2 dichloroethane is eliminated by Xanthobacter autiotrophicus. Xylene and styrene by Nocordia.
The Federal Republic of Germany, Netherlands are leading in the field of biofiltration. A number of biofilters sold by the two main companies (Dutch) is 90. Netherlands in its ‘Hydrogen 2000’ programme is planning to bring about 65% reduction in emissions by the year 2000 AD as compared to that in 1981.
BRI has contributed to the development of a biotechnology aimed at the biodegradation of air stream volatile hydrocarbons. According to BRI, biofiltration of air emission is more promising than conventional adsorption processes. Recent work at BRI focused on understanding the microbial ecology of biofilms as well as on the engineering aspects of the bioprocess.
These have limited applications. Degradation of halogenated hydrocarbons, NH3, H2S etc., encounters situation of acid production. This will have to be neutralized. Acid produced can have an inhibitory effect on the microbiological process. Trickling filters can be used to solve the problem of acid becoming inhibitory
e.g. CH2Cl2 + O2 ———————-> CO2 + 2HCl
Recently, the number of applications for biological deodourization or biological purification of air have been increasing. This is due to more knowledge of process conditions, improvements in biofilters’ characteristics, composition of filters etc.
The Envirogen, Inc., has developed a biocatalytic route for degradation of trichloroethylene (TCE). Here instead of naturally occurring microbes, a pure culture of Pseudomonas strain is used. In the first field trial carried out in New York, 90% of TCE in contaminated air from air stripper treating ground water was successfully degraded. Pure culture though can degrade TCE, prefers phenol and toluene. So in the process, bacteria are kept alive on a subsistance diet of phenol and toluene and are made to degrade TCE. Careful control of conditions is required as far as temperature, pH and exclusions of microbial predators are concerned. The company also has a process where genetically engineered E.coli is used as an efficient TCE degrader. E.coli can be fed on glucose and is not competitive substrate as phenol and toluene are for Pseudomonas.
The EG and G Rotron (New York) and EG & G Idaho and US Department of Energy’s Idaho National Engineering Laboratory have developed a process of aerobic biofiltration. The method is called ‘Biocube’ and employs naturally occurring microorganisms, mostly actinomycetes and Pseudomonas to remove more than 90% of aliphatic and aromatic substances and their derivatives from gas streams. Biocube’s filter beds are modular trays filled with soil; compost mixture containing miroorganisms. Beds are kept moist and at proper temperature so that biofilm develops on the surface. The volatile organic compounds (VOCs) are degraded and end products are CO2, water, biomass and inorganic salts. The process is cheaper than thermal and catalytic oxidation alternatives. Biocube can also handle malodourous gases such as H2S.
An exhaust gas treatment system for H2S and SO2 , based on bacteria, is already mooted in Japan. The Dowa mining company uses Chilobacillus ferroxidans which is found in natural environment of mines, which oxidizes Fe+2 to Fe+3 for energy and gives solid sulfur from H2S. Bacteria are circulated in intimate contact with ferrosulfate and ferric sulfide. The new system of Dowa mining consists of H2S reaction tank, bacteria tank, a sulfur recovery unit and a simple closed circuit. Compared to conventional process which uses caustic soda for neutralization, the biological system works at only 1/3 cost. Energy conservation, compact system, room temperature treatment and easy operational control are the other advantages. The biological system has potential applications in petroleum plants, chemical processing plants.
Walshe (1988) has described biofiltration unit to remove offensive odours produced at animal rendering plants in south Tippeary using 25% peat and 75% heather. The odourous air is sprayed with water to lower its temperature and increase its humidity before pumping it through the filter. Efficiencies of such plants in West Germany are 95%. There are over 70 biofilters in operation to treat sewage treatment plants’ gaseous emission. Animal rendering units collect and process animal bodies, slaughterhouse offal, blood etc. which produces odourous emissions. C2-C11 straight chain alkanals, methyl propanal, 2,3 methyl butanal, C2-C6 straight chain organic acids, furans, sulfur compounds, thiophene, H2S, NH3 etc. are present. Degradation of aldehydes and ketones is better while that of sulfur compounds, thiphene, H2S, NH3 is poor.
Biological elimination of ammonia gas in the exhaust air from livestock production is reported. It is a two-step process converting NH3 —-> NO2 —–> NO3. Packing material is gradually acidified due to the process. Acidification inhibits elimination process. Acid buffers keep pH constant. Diffusion and reaction rate of biofilter decide the overall elimination capacity. Maximum elimination capacity at laboratory scale is 9 gram / m3 packing material per hour while at pilot plant scale, it is 2 gram / m3 of packing material per hour.
Rapid microbial deodourisation of agricultural and animal wastes is reported in which animal house feces of pigs, cows, sludge, domestic garbage is mixed with seed culture (5:1 W / W) and is blended with rice hulls or rice straw or saw dust. It is then left in a wooden box. Temperature rises on its own to 700C. Deodourisation occurs. Sulfites, hydrogen sulfite, mercaptans, low molecular weight fatty acids etc. are metabolized. Organisms active are Streptomyces griseus, Streptomyces antibioticus, Thermoactinomyces spp. There are many advantages of biological deodourisation of this kind : (1) simple and rapid process; (2) No aeration, no heating; (3) Low cost; (4) Application at small or large scale; (5) Deodourised wastes are recycled as seed; (6) Coliforms decrease; (7) No flies; (8) Drying in sun; (9) Deodourised wastes are useful as fertilizer and fodder.
An efficient and economical bioscrubber system to remove styrene and volatile organic compounds (VOC) from industrial waste gases has been developed. The process uses water to strip styrene and VOCs from industrial waste gases in a packed column scrubber. The styrene-laden wastewater is pumped to fermenter, where selected strains of naturally occurring bacteria decompose styrene to CO2 and water. Clean wash water is then recycled to the scrubber. Since 1994, the first commercial unit has run continuously on 20,000 m3/hour waste gas stream at German automotive parts manufacturer, cutting the styrene concentration from 400 ppm to 5 ppm. The operating costs are reported to be only about 20% of those of comparable biofilters, while the capital costs is at least 40% less.
The Envirogen Inc. (Lawrenciville, N.J., USA) is scaling up their biofiltration systems to permit handling of 2000 – 200,000 Scfm of airflow. Styrene is hazardous air pollutant and manufacturers of poystyrene are required to achieve 90% reduction in its release by the year 2000 AD In Envirogen’s system, naturally occuring microbes are immobilized on a porous filter substrate such as compost or peat. Concentrated vapour stream passes through the filter bed, pollutants from vapour phase are transferred to the immobilized biofilm and are oxidized to CO2 and water. The biological route has 30-70% lower operating costs than other physical / chemical methods.
TNO was one of the first organizations to appreciate the true potential of biological treatment. They have developed a special low-cost biofilter containing compost and wood bark which treats VOCs. They have also developed a fast-acting biofiltration system which removes toluene, xylene, propene and styrene. Fungi are used in this system well dispersed on ceramic carrier. TNO has also developed two stage system in which a photoreactor with UV radiation improves the biodegradability of off-gases containing hydrophobic pollutants which are then biotreated. Styrenes, NOx and alkanes are being treated this way. The last compound is tried by improved bioscrubber.
Biotechnology can clean up sulfur from gas streams and produces elemental sulfur. A flue gas desulfurization process called Biostar has been developed by Paques BV (Netherlands) and Hoogovens Technical Services Energy & Environment BV. First sulfur dioxide is absorbed and converted to sulfite by reaction with sodium hydroxide, then sulfate reducing bacteria convert it to H2S , which in turn is converted to elemental sulfur by Thiobacilli. Another Paque bioprocess, H2S for has been installed commercially in a Dutch paper mills and is producing 0.2 m.t /d of sulfur from a gas stream, reducing the H2S content from 12000 ppm to 40 ppm. NKK Japan uses Thibacillus ferroxidans bacteria in its Bio-SR process. A ferric sulfate solution absorbs H2S from a gas stream, producing elemental sulfur and ferrous sulfate solution. After sulfur is filtered, the solution is regenerated to ferric form by the T.ferroxidans. Sulfate reducing bacteria (anaerobic) may also offer a way to deal with the mountains of gypsum accumulated from wet scrubbing of SO2 from stack gases. In a process developed by Idaho National Engineering Laboratory, bacteria and slurried gypsum are mixed in stirred tank along with a nutrient of starch from potato wastes. The bacteria produce H2S which can be converted to elemental sulfur. INEL also is developing a way to treat stack gas directly by sparging it into a tank containing water, bacteria and starch. The SO2 dissolves to form sulfurous acid, from which the bacteria produce H2S. The process is yet to be scaled up. At present the technology to remove sulfur from process streams and effluents are fast developing and will provide sulfur in a big way. Recovered sulfur may one day exceed the total demand and there will be no native sulfur production from mining.
Idaho National Engineering Laboratory USA is reported to be experimenting with a bioreactor for removing nitrogen oxides from flue gas. Tests using a gas stream containing 250 ppm. nitric oxide showed that bacteria can remove upto 99%of the NO, leaving a residual conc. of only 2.5 ppm. Flue gas from coal typically contains 100-400 ppm. of NO. The flue gas is passed through a column of 100 mm. diameter and 1 meter long. Compost inside the column immobilises the Pseudomonas denitrificans bacteria and also serves as source of nutrients. A sugar solution dropped over the bed every few days provides a food supplement. With flow rates of 1-2 lit./min. the residence time is of the order of 1 min. Researchers attribute the impressive performance by bacteria to the fact that in gas phase the mass transfer is better than that in similar liquid system. The bacteria grow best at 30-450C so the system has to be in the coolest part of the flue gas duct.
Odours from food processing and sewage treatment plants that are caused by mercaptans, alkyl sulphides and hydrogen sulfide are removed by sulfur eating bacteria in a technique developed by Obayashi Corp. (Tokyo) and Hitachi Zosen Corp. (Osaka). The equipment costs almost same as conventional activated carbon systems, but operating costs are lower, because regeneration is not needed.
The bacteria grow on 2-30 mm diameter ceramic pellets that are packed in a tower. Exhaust gas passes through the tower from bottom in about 20 seconds which is sufficient for complete odour removal. Water is sprinkled from top periodically to keep the bacteria alive. Until now there is no single bacterial treatment method for all three types of compounds.
Odours from mercaptans and alkyl sulphides have been treated by bacteria that work at neutral pH and those from hydrogen sulfide by acid loving bacteria. The companies solved this problem by means of bacteria that work in neutral environment. The main factor in maintaining that environment is the development of ceramic pellets made by mixing process sludge with an undisclosed component.
Phenol derivatives are common constituents of gaseous effluents of resin production, petrochemicals, pharmaceuticals, pesticide, explosive , textile, colour, coffee industry . Phenol is toxic and malodourous. Phenol removal from waste gases with biological filters by Pseudomonas putida is reported. For Phenol other than Pseudomonas putida, Candida tropicalis, Fusarium flocciferium, Trichosporon cutaneum and heterogenous population is used.
The Tobacco industry emits odourous air during manufacturing. Biofilter system of Clean Air TechniQ Pty Ltd. Of Australia consists of one or more filter housings where the compounds in question are absorbed and oxidized by the selected bacterial species. The filter volume is calculated based on previous experience and using a computer aided model. The polluted air is delivered to the biofiltration system by a centrifugal fan. The air is then passed through an air washer where the particulates are removed and the gas is conditioned to the correct temperature and humidity (20-40oC, 100% RH). After leaving the air washer travels to the inlet plenum where it is distributed prior to moving through the biological filter. After passing through the filter the air is discharged to the atmosphere via a stack.
The biofilter reported by Clean Air TechniQ Pty Ltd. Of Australia is 300m3 in volume. The housing is normally 18 m long by 16 m wide and 5 m high. The biological bed is located on a grid approximately 1 meter from the ground and covers the entire housing to a depth of 1 meter. The entire bed is moistened by PLC operated spray system located above the bed. The spray system is controlled by the moisture content of the bed. Optimal performance is obtained between 50 and 60 % moisture (w/w). 90% odour removal and 98% particulate matter removal is reported. Air volume treated is 100,000m3/h. and inlet air odour concentration is 5,000-8,000 O.U./m3
Physical and chemical treatments for wastes results only in transformation of pollutants from one form to the other. It does not solve the basic problem of total removal of pollutant. Biotreatments are relatively simple, specific, work at ambient temperatures-pressure, and are less costly. As seen from the various examples cited, biotechnology can offer an effective solution to deodourisation. Biotreatment of waste gases is still considered to be a technology under an experimental state. It is through wider applications only, that a technology can improve. But initial faith and willingness is required for its practical use.
Table 1 – Air Pollutants Degradation for Various Source Industries
|Source Industry||Type of pollutants||System used and
|Technology by Company / Laboratory / country|
|(1)||food processing and sewage treatment plants||mercaptans, alkyl sulphides and hydrogen sulfide||Obayashi Corp. (Tokyo) and Hitachi Zosen Corp. (Osaka).|
|(2)||Coal industry||nitrogen oxides from flue gas||Pseudomonas denitrificans||Idaho National Engin-eering Laboratory USA|
|(3)||SO2 from flue gas||Sulfate reducing bacteria, Thibacillus ferroxidans||Paques BV, Netherlands & Hoogovens Technical Services Energy & Environment BV|
|(4)||Trichloroethylene (TCE)||Pseudomonas or genetically engineered E.coli||The Envirogen, Inc. (Lawren-civille, N.J.)|
|(5)||gas stream at German automotive parts manufacturer||styrene and volatile organic compounds (VOC)||Bioscrubber with naturally occuring bacteria|
|(6)||Polystyrene plant||Styrene||naturally occuring bacteria||Envirogen Inc. (Lawren-civille, N.J.)|
|(7)||mining, petrochemical, chemical units||H2S, SO2||Chilobacillus ferroxidans||Dowa Mining company|
|(8)||Agricultural & animal wastes||. Sulfites, hydrogen sulfite, mercaptans, low molecular weight fatty acids etc.||Streptomyces griseus, Streptomyces antibioticus, Thermoactinomyces spp.|
|(10)||Animal rendering units||C2-C11 straight chain alkanals, methyl propanal, 2,3 methyl butanal, C2-C6 straight chain organic acids, furans, sulfur compounds, H2S, thiophene, NH3 etc.||Biofilters||Germany|
|(11)||Resin, Pesticide Petrochemical, Pharmaceutical Textile industries.||Phenol derivatives||Biofilters using Candida tropicalis, Pseudomonas putida, Fusarium flocci-ferium, Trichosporon cut-aneum and heterogenous population|
Table 2 – Application of Biofilters
|(1)||Gelatin production||elimination of odour||
|(2)||Cocco and Chocklet processing||elimination of odour||
|(3)||Fishmeal factory||elimination of odour||
|(4)||Tobaco industry||elimination of nicotine||
|(5)||Waste-water treatment||elimination of odour, H2S, acetone||
|(6)||Flavour and fragrance||removal of odour, H2S, acetone||
|(7)||Paint production||organic solvent||
|(8)||Pharmaceuticals||aromatic, aliphatic, chlorinated compounds||
|(9)||Photofilm production||organic solvent||
|(10)||Food processing||elimination of oil, odour||
Table 3 – Cost Comparison of Biofilters
|For Gas flow – 10000 m3h-1
VOC – 100 – 2000 mg/m3
|Type of Treatment||Cost|
|Thermal incineration||7 – 9 Dfl / 1000 m3|
|Catalytic incineration||6 – 8 Dfl / 1000 m3|
|Adsorption||14 – 18 Dfl / 1000 m3|
|Biofilter||0.5 – 3 Dfl / 1000 m3|
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(4) Chemical Weekly, Sept. 16, 1997.
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