BIOFILTRATION OF AIR
Selvi B. Anit
Robert J. Artuz
WHAT IS BIOFILTRATION?
ADVANTAGES AND DISADVANTAGES OF BIOFILTRATION
WHAT IS BIOFIFLTRATION?
Biofiltration is a relatively new pollution control technology. It is an attractive technique for the elimination of malodorous gas emissions and of low concentrations of volatile organic compounds (VOCs).
The most common style biofilter is just a big box. Some can be as big as a basketball court or as small as one cubic yard. A biofilters main function is to bring microorganisms into contact with pollutants contained in an air stream. The box that makes up this biofilter contains a filter material, which is the breeding ground for the microorganisms. The microorganisms live in a thin layer of moisture, the "biofilm", which surrounds the particles that make up the filter media. During the biofiltration process, the polluted air stream is slowly pumped through the biofilter and the pollutants are absorbed into the filter media. The contaminated gas is diffused in the biofilter and adsorbed onto the biofilm. This gives microorganisms the opportunity to degrade the pollutants and to produce energy and metabolic byproducts in the form of CO2 and H2O.
This biological degradation process occurs by oxidation, and can be written as follows:
Organic Pollutant + O2 à CO2 + H2O + Heat + Biomass
While the use of biofiltration in the United States is not wide spread, hundreds of these systems are successfully operating in Europe, Netherlands, New Zealand, Germany, and Japan. Biofilters have been designed primarily for odor control at wastewater treatment plants, rendering plants, and composting operations. However, biofilters are becoming more popular in the treatment of VOCs and other organic compounds.
The following is a brief timeline of the development of biofilters:
1923 -- Biological methods were proposed to treat odorous emissions.
1955 -- Biological methods were applied to treat odorous emissions in low concentrations in Germany.
1960s -- Biofiltration was used for the treatment of gaseous pollutants both in Germany and US.
1970s -- Biofiltration is used with high success in Germany.
1980s -- Biofiltration is used for the treatment of toxic emissions and volatile organic compounds (VOCs) from industry.
1990s -- Today, there are more than 500 biofilters operating both in Germany and Netherlands and it is widely spreading in US.
Applications for odor control have occurred since the 1950s and ranged from soil filters to large biological trickling filter plants. Odorous air emissions generally can be characterized as having relatively low pollutant concentrations that include hydrogen sulfide, mercaptons, and other reduced sulfur compounds. In addition, applications for VOC control have just become popular in the past decade and are still being researched extensively. For example, research has shown that biofilters can be used to remove a variety of airborne contaminants, including aliphatic and aromatic hydrocarbons, alcohols, aldehydes, organic acids, acrylate, carbolic acids, amines and ammonia.
Biofiltration utilizes a supported media for microbial growth to remove odors and organic contaminants from air streams. The filter consists of a closed chamber containing contaminant degrading microbes and absorbed water suspended in a filter medium. The filter medial is designed to provide a high capacity for water uptake, have a long working life, and provide a low pressure drop for the gases passing through the media.
A large (6000 ft2) single layer biofilter is shown below which is in operation at a Monsanto plant:
Smaller and more popular multi-layer biofilters are shown below:
These units, called "Biocubes" were designed by EG&G Biofiltration and are approximately 7 feet high and 6 feet in diameter. This multi-layered system prevents the compaction of the filter media and helps eliminate the risk of channeling by the contaminated air stream. In addition, the stacked discs allow for easy maintenance when it comes time to change the filter media.
In the biofiltration process, contaminated air is moistened by a humidifier and is pumped into the biofilter through a chamber below the filter medium. While the air slowly flows upward through the filter media, the contaminants in the air stream are absorbed and metabolized. The purified air passes out of the top of the biofilter and into the atmosphere. Most biofilters that are in operation today can treat odor and VOCs add efficiencies greater that 90%. However, the drawback to this technology is that it can only handle low concentrations of pollutants (<1000 ppm) and loading rates between 300-500 ft3/ft2-hr.
The moist filter medium provides physical and chemical conditions appropriate for the transfer of contaminants from the air to the liquid phase and the biodegradation of the contaminants in the biofilm layer. The mechanism of the biofiltration process includes a combination of adsorption, absorption and microbial degradation. Microorganisms contained in the biofilm layer continually metabolize the contaminants, as they are absorbed, converting them ultimately to water, carbon dioxide and salts.
Typical biofilter media material includes compost-based materials, earth, heather, plastic, or wood-product based material. The purpose of the biofilter media is to provide a large surface area for the absorption and adsorption of contaminants. The media also serves as a nutrient source for the microbial population. In fact, some types of media lack proper nutrients and will require the manual addition nutrients (e.g. nitrogen and phosphorous compounds) in order to sustain microbial life. Most biofilters will operate for 5-7 years before it is necessary to renew the filter media.
Major considerations when determining the appropriate filter material include:
Space at a site is the greatest concern during design of a biofiltration system. A small biofiltration unit can be designed to handle approximately 30 cubic-feet-per-minute in as little space as 25 square feet, similarly, a biofiltration system designed to treat large air volumes and require space as large as a basketball court.
Chemical Constituents and Concentrations:
Analysis of chemical constituents and their concentrations are required to determine if biofiltration is a plausible alternative. Biofilters performed best when treating hydrophilic compounds in low concentrations (<1000 ppm). Some chemicals biologically degrade at low rates, such as chlorinated compounds, which require units to be oversized.
Residence Time represents the amount of time the microbes are in contact with the contaminated air stream, and is defined by (Void Volume/Volumetric Flow Rate). Consequently, longer residence times produce higher efficiencies; however, a design must minimize residence time to allow the biofilter to accommodate larger flow rates. For most biofilters, residence times range between 30 seconds to 1 minute.
The humidity of gas stream is important for maintaining the moisture content of the biofilter media. Gas streams introduced to the biofiltration system are usually pumped through a humidifier prior to entering the biofilter. The gas entering the biofilter should be humidified to greater than 95% relative humidity.
The by-products of microbial degradation are organic acids. In order to maintain the pH of the vessel around neutral, or a pH of 7, buffering material may be added to the organic media.
The media used in biofilters can include peat, heather, bark, composted sewage sludge, granular carbon or other suitable materials. Generally, the media should be capable of providing nutrients to the microorganisms and minimizing pressure drop. In addition, the moisture content of the biofilter media must be maintained between 30% and 60% in order to support the microbial population. In addition to humidifying the airflow, sprinkler systems are frequently installed inside the biofilter that can be controlled to maintain a suitable bed moisture.
Pressure drop across the biofilter reactor vessel should be minimized since an increase in pressure drop requires more blower power and can result in air channeling through the media. Pressure drop is directly related to the moisture content in the media and the media pore size. Increased moisture and decreased pore size result in increased pressure drop. Consequently, media filter selection and watering is critical to biofilter performance and energy efficiency. For a typical biofilter pressure drops range between 1 and 10 hPa.
The operation and maintenance of the biofiltration system would require weekly site visits during initiation of operations for emiss. However, after acclimation and all system problems are resolved the frequency of site visits could be reduced to the biweekly or monthly.
ADVANTAGES AND DISADVATAGES OF BIOFILTRATION
Advantages of Biofiltration:
Disadvantages of Biofiltration:
There have been over 50 commercial biofilters using compost-type material installed in Europe and the United States over the past 15 years.
VOC applications to date have included the following industries:
Odor abatement applications to date have included the following industries.
Here are some companies and institutions that sell and/or have research on biofiltration systems:
MBP TECHNOLOGIES INC.
Air-Science Technologies. http://www.enviroaccess.ca/fiches_2/F2-04-96ba.html Canada, Quebec.
Andreoni V., et. al. "Characterization of a Biofilter Treating Toluene Contaminated Air", Biodegradation. Kluwer Academic Publishers, Netherlands: 1997. Vol 7, p397-404.
Aycaguer, A., et. al. "Biofiltration for Air Pollution Control"
http://www.usc.edu/dept/civiil_eng/biofilters/intro.html University of Southern California.
Baruch, A. Microbes and Air Pollution. http://www.bact.wisc.edu/scienceEd/microldoc Aug 24 1997.
Biogene Inc. http://www.enviroaccess.ca/fiches_2/F2-04-96ca.html Canada, Quebec.
Bruns, B.S., "History and Development of Biofiltration", 1995 Atlantic Enviro Conference, NJ. Air & Waste Management Association: 1993. Ambient Engineering Inc., NJ.
Chou, M., Shiu, W. "Bioconversion of Methylamine in Biofilters", Journal Air And Waste Management Association: 1997. Vol 47, p58-65.
Felder, R. M., et. al. Elementary Principles of Chemical Processes. John Wiley & Sons, NY:1986. p243-371.
Kennes, C., et. al. "Design and Performance of Biofilters for the Removal of Alkylbenzene Vapors", Journal of Chemical Technology and Biotechnology.SCI, Great Britain: 1996. Vol. 66, p300-304.
MBP Technologies, Inc. http://www.mbptech.com, Agowam, MA.
Mohsani, M. The Enhancement of VOC Removal from the Pulp & Paper Air Emissions with Wood Wastes and Activated Carbon Mixtures. Pulp & Paper Centre, University of Toronto: 1995.
Scotford, I.M., et al. "Minimim Cost Biofilters for Reducing Odours and Other Aerial Emissions from Livestock Buildings: Part 2, A Model to Analyze the Influence of Design Parameters on Annual Costs", Journal of Agricultural Engineering Res. Silsoe Research Institute, UK: 1996. Vol 64, p155-164.
Ziminski, R., et. al. "Control of Hazardous Air Pollutants Using A Commercial Biofilter", 86th Annual Meeting & Exhibition, Denver, Colorado: 1993. Air and Waste Management Association: 1993. Ambient Engineering Inc, NJ.