BioCycle June 2004, Vol. 45, No. 6, p. 35
Living systems that rely on microbial populations to degrade compounds in waste gases, biofilters have a rich history and a bright future.
Sarina J. Ergas and Beatriz Cárdenas-González

OVER the last two decades, biofiltration has become a key component in the control of odorous emissions from composting facilities, wastewater treatment plants, confined animal facilities, food industry, and other sources of volatile organic compounds (VOCs) and odors. However, in the past, many decision makers at these facilities shied away from biofiltration technology because they had heard of system failures at other plants. Since that time, improvements in design and operation of biofilters have led to improvements in biofilter process performance and reliability. At the same time, we have begun to understand the limitations of biofilters and when other technologies may be more appropriate.
Biofilters are living systems that rely on microbial populations to degrade compounds in waste gases. A biofilter is a bed composed of compost, soil or peat media (Figure 1). Waste gases are blown through the bed, where soluble compounds in the gas phase are absorbed into a moist biofilm attached to the packing media. Inside the biofilm, the pollutants come into contact with the microorganisms and are biodegraded. The performance and reliability of biofilters have been improved by creating an environment in the biofilters that allows the microbial cells to grow and utilize the pollutants as a substrate (food and/or energy source) without producing undesirable by-products. Compared with other air pollution control technologies, biofiltration is considered economical, cleaner and greener because of following:
1) Low operating costs. Biofilters operate at ambient temperatures and pressures, so power consumption is minimal. Pressure drops are generally less than 10 cm of water column.
2) Absence of residuals. Compounds are usually degraded to harmless products such as CO2, H2O and inorganic ions such as SO42, and NO3-. Other control alternatives produce residuals such as spent activated carbon and chemical sludges requiring further treatment.
3) Unlike thermal oxidation, biofiltration produces minimal CO2 and NOx emissions.
Disadvantages of biofiltration include relatively high area requirements and moderate to high capital costs. In addition, biofiltration can only be applied to moderate concentrations of relatively soluble and biodegradable compounds.
ORIGINS OF BIOFILTRATION AND EXPANDED APPLICATIONS
A chronology of the development of biofiltration technology is shown in Table 1. The origins of biofiltration can be traced to a 1923 publication, which discussed the basic concept of control of odorous emissions from sewage treatment plants and composting facilities using soil beds1. The first successful applications and patents of biofilters were reported in the early 1950s in both the United States and Germany2. These simple systems consisted of open pits filled with porous soil underlain with simple air distribution systems made from perforated pipes. The systems were generally effective in the short term; however, they were prone to problems with drying and cracking, acidification and uneven air distribution, which often led to failures over the long term. In addition, these soil bed systems had low air permeability and required a large amount of space due to the low specific activity of soil.
During the 1970s, stricter air pollution regulations along with encroachment of residential areas on municipal and industrial facilities led to an expansion of the use of biofiltration systems for odor control. The use of compost based media along with improved moisture and pH control led to smaller reactor sizes and greater reliability of these systems. A summary of biofiltration improvements from the simple soil bed systems of the 1950s up to the present day is given in Table 2. Biofiltration success stories led to a greater acceptance by state and federal regulators, which allowed facilities greater ability to site these systems. A number of engineering firms began specializing in biofilter design.
During the 1980s, application of biofiltration expanded to the treatment of volatile organic compounds (VOCs) and hazardous air pollutants with the pioneering work of S. Ottengraf in The Netherlands5. Biofiltration, using compost based media, was found to effectively treat moderate concentrations of biodegradable VOCs such as benzene, toluene and xylene, ethanol, and gasoline vapors. This allowed biofiltration technology to expand to a number of new industries including adhesives production, breweries and bakeries, chemical manufacturing, electronics, film coating, flavors and fragrances, iron foundries, landfill gas extraction, painting and spraying operations, petroleum refining, plastics manufacturing, printing, pulp and paper manufacturing, soil and groundwater remediation, textiles and waste oil recycling.
Expansion of biofiltration research led to the development of two new bioreactor configurations for air pollution control, biotrickling filters and bioscrubbers. The primary difference between biofilters and biotrickling filters is that in biotrickling filters, the biofilm is developed on an inert packing material such as polyurethane foam, fibrous packing, ceramic pellets, granular activated carbon, porous lava rocks or structured plastic. A liquid nutrient/buffer solution is sprayed over the top of the packing material that moves (trickles) through the bed. In bioscrubber systems, absorption of the pollutants takes place in one unit, a packed column, spray tower or bubble column scrubber, while biodegradation primarily takes place in a separate bioreactor, similar to an activated sludge reactor. The development of biotrickling filters and bioscrubbers allowed for greater control over process variables such as pH and biofilm thickness, and could be operated at higher VOC loading rates than biofilters.
RESEARCH DEVELOPMENTS
During the 1980s, most of the research and application of biofiltration technology took place in a few countries including The Netherlands and Germany. In the U.S., it was not until the 1990s that research and development of biofiltration were extended. Since 1990, interest in biofiltration has increased considerably worldwide as is evident by the extensive amount of literature produced and the large number of applications. The annual meeting of the Air & Waste Management Association now serves as a major forum for the biofiltration research community. In addition, the University of Southern California and The Reynolds Group (USC-TRG) has a conference devoted to biofiltration in Southern California every two years. The Batelle In-Situ and On-Site Bioremediation conference also includes sessions on biofiltration technology. Books about biofiltration include Biofiltration for Air Pollution Control by Devinny and coworkers7 and Bioreactors for Waste Gas Treatment edited by Kennes and Veiga8. Research on biofiltration has included the development of novel biofilter designs such as membrane bioreactors and rotating biological contactors, as well as improved understanding of microbiological pathways, treatment of recalcitrant compounds and mixtures, biomass control, high and low temperature operation, dynamic loading, transient conditions and process modeling. In terms of biofilter media, research has focused in the characterization of the performance of different materials (including different types of compost) in the search for the optimal media.
APPLICATION OF TECHNOLOGY IMPROVEMENTS TO COMPOST ODORS
Improving biofiltration technology has been a focus of the research section of the Los Angeles County Sanitation Districts (LACSD) for more than a decade. At LACSD’s Joint Water Pollution Control Plant (JWPCP), two large biofilters are being constructed to treat odors from the biosolids management operations. In addition to the biofilters, the $25 million project involves modifying the top of the biosolids silos building, fully enclosing a truck loading station and covering the conveyor belts moving biosolids to the silos building to better capture fugitive odors. One of the biofilters is three-quarters of an acre in size (five cells); the second is about three-fifths of an acre (four cells). Combined, they will treat 175,000 cfm of air from the silos, conveyors and truck loading station. The JWPCP produces 1,600 wet tons/day of biosolids, all of which are managed off-site (about half composted, one-third land applied and the remainder either processed in a cement kiln or landfilled).
LACSD is using air plenum plate technology (supplied by Bac-Tee) to help distribute the air flow evenly throughout the biofilter media. It is hoped that by using an engineered air distribution system, instead of lateral pipes embedded in asphalt or rock, more of the media in the biofilter will be used. The media – to be placed at a minimum depth of four feet – will be comprised of wood chips and LACSD biosolids compost. Loading rate into the biofilters is designed for 3-cfm/sq ft of media, with a residence time of 75 seconds. Biofilter construction is complete, except for placement of the media, air plenum plates and mechanical equipment. The duct work that connects the biofilters to the storage and loading area is being installed. LACSD expects that some of the biofilter cells will be operating by the end of 2004.
Sarina J. Ergas is in the Department of Civil and Environmental Engineering, University of Massachusetts, Amherst. Beatriz Cárdenas-González is in the National Center for Environmental Research and Training at the National Institute of Ecology, Mexico City. The authors extend their thanks for information suppled by Robert Morton of the Los Angeles County Sanitation Districts, Tracy Barton of Bio-Reaction Industries, and Jorg Sattler of BioSal Biofiltration.
REFERENCES
1. Leson, G., Winer, A.M.(1991). Biofiltration: An innovative air pollution control technology for VOC emissions. J. Air & Waste Mgmt. Assoc., 41, 1045-1054.
2. Pomeroy, R. D., (1957). Deodorizing gas streams by the use of microbiological growth. US Patent 2,793,096.
3. Carlson, D.A., Leiser, C.P. (1966). Soil beds for the control of sewage odors. J. Water Pollution Control Fed., 38, 829-835.
4. Hartenstein, H. V. (1987). Biofiltration and odor control technology for a waste water treatment plant. M.S. Thesis. University of Florida, Gainsville, FL.
5. Ottengraf, S.P.P., van den Oever, A.H.C. (1983). Kinetics of organic compound removal from waste gases with a biological filter. Biotechnology & Bioengineering, 25, 3089-3103.
6. van Lith, C., Leson, G., Michelsen, R.(1997). Evaluating design options for biofilters. J. Air & Waste Mgmt. Assoc., 47, 37-48.
7. Devinny, J.S., Deshusses, M.A., Webster, T.S. (1999). Biofiltration for Air Pollution Control, Lewis Publishers, Boca Raton.
8. Kennes, C., Veiga, M.C. (2001). Bioreactors for Waste Gas Treatment, Kluwer Academic Publishers, Dordrecht, The Netherlands.
9. Cárdenas, B., Revah S., Hernández, S., Martìnez, A., and Gutiérrez, V. (2003) Tratamiento biológico de compuestos orgánicos volátiles de fuentes fijas. Instituto Nacional de Ecologìa. Mexico. ISBN. 968-817-499-8.
10. Vladimir, P., Bezdorodov, A., Cross, p., Jackson, W. (2002). Design, construction and long-term performance of novel type of industrial biotrickling filters for VOC control. Proc. Air & Waste Mgmt. Assoc. 95th Annual Conference & Exhibition June 23-27. Baltimore, MD.
11. Revah, S., Hugler, W. (1998). La biotecnologìa ambiental, una oportunidad de vinculación. Revista de vinculación. UAMI. pp. 26-32.
12. Cárdenas, B., Munguìa, J., Martìnez, D., Herrera, L., Hernández, S., Revah, S. (2003). Operación de un sistema de biofiltración para el control de una mezcla de compuestos orgánicos volátiles por largos periodos. X Congreso Nacional de Biotecnologìa y Bioingenierìa, , Puerto Vallarta, Jalisco, México.
13. Ergas, S.J., Kinney, K.A. (2000). Biological Control Systems. In: Air Pollution Engineering Manual (W.T. Davis ed.), pp. 55-65, Air & Waste Mgmt. Assoc.-John Wiley and Sons, NY.
BIOFILTRATION IN MEXICO
AT PRESENT, only a few industrial processes in Mexico are required to have VOC emission controls; however, the high ground level ozone concentrations in some of the main cities in Mexico as well as the need for air toxics control may lead to implementation of stricter and wider VOC and odor control in the future. By the beginning of the 1990s, a relatively small group of Mexican researchers saw biofilters as a feasible technological option for odor and VOC emission control. Over the last decade, research has focused on basic and applied research on the physical, chemical and biological phenomena of biofiltration, as well as design and application.
There are now several groups working on aspects of biofiltration both in academic institutions and research institutions such as the Metropolitan Autonomous University, the Mexican National Autonomous University, the Aguascalientes Autonomous University, the National Polytecnic Institute, the Mexican Petroleum Institute and the National Institute of Ecology. At present, studies are being conducted both at laboratory and pilot-scale on biofiltration of different types of compounds including CS2, H2S, toluene, ethyl acetate, BTEX, methanol, isopropranol, hexane, gasoline vapors, MTBE and ethanol.
The availability of media materials with optimal characteristics for biofiltration has also been studied, including agricultural waste and compost from different sources. Results of these projects have been disseminated in international and Mexican scientific and technical publications, including a recent booklet on biofiltration in Spanish9. The National Congress on Biotechnology and Bioengineering has become the main Mexican forum for biofiltration research.
Industrial biofilters installed in Mexico include a 300 m3/min capacity biofilter for odor control at a wastewater treatment plant, an 800 m3/min capacity biotrickling filter for sulfur, CS2 and H2S emissions at a cellophane and rayon plant, a biotrickling filter for high CS2 concentrations, and a biotrickling filter for VOC removal emitted from a printing process 10,11. A pilot-scale compost based biofilter (1 m3/min) for the treatment of solvent vapors has been in continuous operation over the last four years12.