24-06-2014, 12:47 PM
BIOFILTRATION OF AIR
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INTRODUCTION
Biofiltration is a pollution control technique using living material to capture and biologically degrade process pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in air. Biofiltration refers to the biological transformation or treatment of contaminants present in the gas phase, usually air. The fact that air contaminants can be biodegraded by active bacteria has been known for quite some time. However, it is only in the last 10 years, that biofiltration has begun to emerge as an economically viable treatment process. Initially, biofiltration involved the use of naturally bioactive media, such as soil, peat, compost, etc. In naturally bioactive media, microorganisms present in the soil, peat or compost, have been known to biodegrade contaminants, and this has been successfully employed in bioremediation of contaminated sites. However, when contaminated air is passed through soil, peat, or compost, the naturally present microorganisms also begin to biodegrade the air contaminants. This led to the development of soil biofilters, in which soil with low clay and high organic carbon content was packed in a bed and contaminated air was passed through the soil bed to biodegrade the air contaminants. However, as more research was conducted on this simple process, it became clear that the biodegradation rates were low and hence the size of the biofilter bed required to achieve high destruction efficiencies was very large. Since, compost had a higher concentration of microorganisms; compost became the media of choice for biofilters. Major problems encountered were settling of the compost, resulting in increased gas-phase pressure drop, availability of nutrients, such as nitrogen and phosphorus, pH maintenance, and drying of the compost material due to moisture transferring to the flowing gas phase. These problems were countered to some extent by adding wood chips, which provided mechanical support to minimize settling, humidifying the inlet air to maintain proper water content in the compost material, adding lime pellets for pH control, and fortifying the compost with fertilizers containing nitrogen and phosphorus compounds. Further, in compost beds, it was necessary to have shallow beds (height < 1.5 m or 4.5 feet), to prevent compaction of the material and drying of the bed from the top surface. This required the beds to have large cross-sectional areas, and in many cases was simply left completely open from the top. In some cases, powdered activated carbon was also added to buffer the concentration changes, since activated carbon is known to adsorb
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 biofilter’s 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.
BACKGROUND
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
PROCESS DESCRIPTION
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.
BIOFILTER MEDIA
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.
TYPES OF BIOFILTER SUPPORT MEDIA
There are two kinds of air contamination problems: (1) when the air contaminants are present at low concentration (< 25 ppmv); and (2) when the concentration of the air contaminants is higher (> 25 ppmv). The reason for making this distinction is that soil, peat and compost materials exhibit low biodegradation rates, have limited supply of nitrogen and phosphorus, eventually begin to plug due to growth of microorganisms, and have limited capacity to neutralize acidic products of degradation. Hence, compost biofilters are capable of treating low concentration contaminants (< 25 ppmv) and are not ideally suited for treating air contaminated with high concentration organics
MECHANISMS OF BIOFILTRATION OPERATION
There are various transport mechanisms which operate simultaneously or sequentially in a biotrickling filter and these mechanisms, schematically shown in Figure 2, include: (1) diffusion of the contaminant(s) from the bulk gas flow to the active biofilm surface; (2) sorption of the contaminants directly on the biofilm surface; (3) solubilization of the contaminant(s) into the water content of the biofilms; (4) direct adsorption of the contaminant(s) on the surface of the support media; (5) diffusion and biodegradation of the contaminant(s) in the active biofilm; (6) surface diffusion of the contaminant(s) in the support media surface; and (7) back diffusion of the adsorbed contaminant(s) from the support media surface into the active biofilms. The effect of adsorption of contaminant(s) on support media surface, surface diffusion, and back diffusion of the adsorbed contaminant(s) from the support media surface into the active biofilms, predominantly occurs in activated carbon-coated support media and contaminant(s) which have affinity for the support media surface
DESIGN PARAMETERS
SPACE CONSTRAINTS
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
COMMERCIAL APPLICATIONS
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:
Chemical and petrochemical industry
Oil and gas industry
Synthetic resins
Paint and ink
Pharmaceutical industry
Waste and wastewater treatment
Soil and Groundwater remediation
COMMERCIAL POTENTIAL OF BIOFILTRATION
Biofiltration is capable of biodegrading a wide variety of air contaminants. Table 2 shows a list of the types of organic and inorganic air contaminants that can be treated in a biofilter. Table 3 shows the applicability of various air pollution control technologies, including compost biofiltration and biotrickling filters. Figure 4 shows the differences in investing and operating costs for biofiltration, when compared with catalytic oxidation and adsorption
CONCLUSION
The future of biofiltration (compost and biotrickling filters) depends on the regulatory requirements placed on industry. However there are specific trends which will impact the market for biofiltration technology, and these trends are:
1. Increased regulatory concern about emission of nitrogen oxides, which are emitted from thermal treatment processes. Biofilters do not create any additional nitrogen oxides;
2. Increased public complaints about odorous emissions from public owned wastewater treatment plants, manufacturing industries, solid waste treatment facilities, etc.;
3. Implementation of pollution prevention methodologies which has resulted in greater use of biodegradable solvents, reduced concentration of air emissions, and emphasis on achieving zero discharge processes; and
4. Increased concern about emission of air contaminants, worker exposure to organics, emphasis on environmentally friendly and low-cost treatment technologies.