January 30, 2004 | General

Compost-Based Biofilters Control Air Pollution

Jim Boswell
BioCycle January 2004, Vol. 45, No. 1, p. 31
Use of biofilter technology to control volatile organic compounds (VOC) and odors in municipal, commercial and industrial applications has continued to increase in North America. Applications can range from odor control in sewage collection systems to VOC removal at a print shop.

Many, if not most, biofilters use some type of organic biomass and/or compost as the substrate, media or “bed” for supporting and growing the microbial biofilm that accomplishes the actual capture and degradation of the targeted compounds. Use of compost and organic media in biofiltration represents another area of emphasis for the composting and organics recycling industry. The typical size for a single, three-foot deep, compost media-bed biofilter to treat a 50,000 cfm (85,000 m3/hr ) air emission stream would be approximately 80 by 100 ft (25 by 30 m). This footprint requires a substantial amount of compost or organic biomass media (750m3, 981 cu.yds), which would weigh between 525 and 675 tons.
The large size and mass have been one of the negative factors on expanding the use of bio oxidation (biofilter) systems in industrial/commercial settings, especially at existing facilities with limited property. In addition, regulatory agencies with air emission control responsibilities believed that because of the seemingly fragile nature of these microbial systems in the biofilters combined with the variable concentrations of contaminates in air emission streams, biofilter systems were not reliable enough to be considered as applicable air pollution control technology. Industry, with a specific air emission limit to meet, also had a certain reluctance to add a control device that was looked upon by the regulatory agencies with skepticism. Another confounding factor was the compliance assurance monitoring (CAM) regulations of the 1990 Clean Air Act Amendments that required verification of control equipment performance on a continuous basis. Few manufacturers have been comfortable installing this equipment because of the lack of experience with biofilter equipment, plus the relatively simple methods for monitoring combined with the inherent biological variability (metabolically and by species of microbes).
However, the landscape has changed for the biofilter industry, and that means increased opportunities for the utilization of compost and green waste as substrate for these units. Recently a new generation of biofilters has come into the marketplace, using the same biological oxidation principles as earlier ones but extensively engineered. New types of media provide increased surface area per unit volume (air to biofilm contact area) and include structural components that virtually eliminate problems of compaction and collapse. In addition, designs that combine the various types of bio-oxidation reactors (biofilters, bioscrubbers, and biotrickling filters) achieve multiple stages of treatment for mixed contaminant air emission streams. In these combined bio-oxidation treatment systems, the use of green waste and/or compost is extremely beneficial to overall microbial growth and biofilm development.
These bio-oxidation systems have several things in common. Each needs virtually 100 percent relative humidity in the air emission stream to prevent the media, or sections of it, from drying. Relatively long contact times are necessary (seconds to minutes) so that the contaminated air remains in close proximity to the microbes so sorption can occur. Each biofilter device must contain a community of microorganisms growing in the air-water interface or biofilm, which is capable of biodegrading the pollutant compounds found in the air stream. The products of this bio oxidation/biodegradation process are carbon dioxide, water, mineral salts and microbial biomass. This process is much like natural, aerobic decomposition of organic matter in soils and aquatic and marine environments.
Many early (1970s and 80s) biofilters were applied to wastewater treatment plant (WWTP) off-gases. This practice continues today with placement of compost and bark in open-topped, in ground units, three to seven ft.(1 – 3 meters) in depth, utilizing a series of perforated pipes set in coarse gravel for the air distribution system. Usually a series of spray nozzles are placed in the “header” leading to the air distribution pipes for humidification. Additional water may be added through soaker hoses or surface sprinklers. The biofilter media is usually some type of compost or bark mulch, layered over a base of bark or wood chips (but it may be porous lava rock) to prevent collapse and filling of the gravel in the distribution system base. The WWTP odors being controlled are primarily H2S, mercaptans and scatols from the headworks and digesters with flows ranging from about 2,000 to 45,000m3/hr (1,250 to 25,500 cfm). When properly designed and installed, these units should operate for many years. Replacement of the compost, bark mulch, bark and wood chips is required every two to three years, representing a rather large quantity of material (several 100 m3 or cubic yards) to be provided by green waste composters and recyclers. The large amount of material required can be handled with backhoes, front-end loaders and a minimum of individual manual labor, primarily because of the in-ground, open-topped biofilter systems at these facilities. Potential suppliers must be attuned to the type, size and moisture requirements of the material that may be specified by the WWTP or its engineering firm.
At least three companies are manufacturing small, compact biofilters for odor control along wastewater collection systems, reducing the odors to nondetectable levels, as judged by our human olfactory sense. These units can be used to control odors from manholes, pressurized sewer vents, transfer and lift stations and virtually any other odor source that can be collected and routed to the units. They are relatively inexpensive and can be set in place and made operational with little manpower. They generally need only 115-volt electric service, a “freshwater” connection, air collection duct (four to six inches; 10 – 15 cm) from the offending source and a wastewater return to the collection point. These units use compost/mulch type media as the matrix for growing the biomass of organisms that affect the treatment.
Air emission controls using biofilter systems with a compost matrix for VOC and hazardous air pollutants (HAPs) have been applied to several industries; and the list is expanding. Small, compost/mulch-based biofilters have been designed and installed to control gasoline vapors from remediation of contaminated water and soil. Another “small air flow” example ( While the overall quantity of compost, mulch and “green waste” used in these units is small on an individual basis, the increased use of this type of control can mean increased overall compost/mulch utilization. These units vary in cost, ranging from approximately $10,000 up to $100,000 (US). Operating costs are generally low, ranging from less than $100 to several hundred dollars per month. Generally, capital costs of other VOC/HAP control devices such as thermal oxidizers or GAC systems may be similar or slightly more than a biofilter unit, but the operating costs (fuel for the thermal oxidizer and GAC regeneration or disposal) are 2 to 6 times that of a biofilter.
A number of large, compost/mulch-based biofilter devices (>10,000 cfm; 17,000m3/ hr) are in industrial applications for VOC/HAP or other air emissions control. These units, based on a European design, were built and installed in the 1990s for panel board plants in the wood products industry and for manufacturing fragrance and housekeeping products. These biofilters are large, many almost the size of a football field. Single or two-bed (two compost layers) units are being operated to control VOCs, HAPs and odors . Although these biofilters are successfully operating, some have had recurring issues with bed drying and short circuiting, causing a reduction in treatment efficiency. A new 450,000 cfm (765,000m3/hr) biofilter was completed in 2001(by PPC Biofilter) for a wood products application, biodegrading primarily methanol and formaldehyde, using a bark/mulch type media for microbial growth. BioReaction Industries (BRI) has designed a 200,000 cfm (340,000m3/hr) unit for treatment of methanol and formaldehyde plus terpenes and other hydrophobic VOCs from a wood products operation. This unit, which is currently being installed, will have approximately 1,570 cu. yards (1,200 m3) of the compost-filled, plastic balled media.
Other applications of industrial biofilters may be found in the printing industry for control of VOCs emitted during printing and ink drying. A two-unit system at a Wisconsin facility allows plant operations at all times with continuous emission controls. Removal efficiency has been shown to be generally in excess of 80 percent. PPC Biofilter installed these insulated, concrete units in 1997. Recent installations have also been made in the paint manufacturing industry and in coating applications for a paint booth exhaust. These units installed by BRI in 2000 and 2001, respectively, have provided more than 85 percent removal efficiency for total VOCs. Both units have small steam generators for maintenance of optimum temperature. The units are both operating at their design capacity of 10,000 cfm (17,000m3/hr), with variable VOC loading rates. One new application is currently in the installation stage at a casting facility for the control of VOC, oily vapor and oil mist from the metal casting process.
Whether the bio-oxidation system is large or small, a critical element is the media or matrix, which provides the supportive structure for the growth of the biotic community. The matrix should provide maximum surface area for growth of the biota that will perform the biodegradation of the VOC, HAP or odors in the emission stream. This media should be resistant to channeling and compaction over a specified time period, at least two years, and preferably four to five years, for obvious reasons – maintenance, operational and replacement costs. The media might need to be relatively light and porous to allow for increased depth of the matrix to provide a longer retention time in a unit with a smaller foot print, where space for control equipment is limited. The ability of the media to avoid compaction, especially at the bottom is critical if extra matrix depth is part of the biofilter design. A variety of materials have been used in the past, with various levels of effectiveness. These materials include: compost, wood chips and bark, peat, soil and sand mixtures, activated carbon, lava rock, and synthetic organic (e.g. plastic rings and spheres, and poly-foams) and inorganic materials (e.g. ceramics and glass wool). In addition, various nutrients (N, P, K), micronutrients (like Se, S, Mn) and buffering chemicals may be added to the matrix or to the waters in the humidification unit(s) and/or sump(s) of the bio-oxidation systems. Unlike compost-based biofilters, nutrients are required for any biofiltration system that has only synthetic organic or inorganic media as there is no release of any nutrient constituents.
In the United States and Canada, there are a number of bio-oxidation units in operation that contain compost, bark-mulch, wood chips and various mixtures of vegetable wastes. Some of these may also contain various percentages of composted manures from domestic animals. The value of a compost-based or other biodegradable substrate bio-oxidation system is found in its ability to provide virtually all of the necessary nutrients, as well act as a carbon source for the microbes that develop as a result of “feeding” on the VOC/HAP/odor laden airstream. Compost-based media, depending on the amount of biological activity, should last four to six years with adequate micronutrients available for release, while maintaining its structural integrity. Rock, granular activated carbon (GAC), polyurethane foams, and plastics cannot supply nutrients that must be added, usually N, P, K with a micronutrient mix similar to what is used in liquid plant fertilizer. In any biofilter application, it is prudent to add complete and balanced fertilizers on a regular basis, particularly with respect to nitrogen consumption and availability. Soluble or available nitrogen should be determined periodically in both the matrix and the water collected (and often recirculated), in the biofilter device. If organic nitrogen compounds are being oxidized (like amines), or ammonia is present in the air emissions stream, then sufficient nitrogen may be available throughout years of operation.
Periodic loss of the emission stream “food source” from manufacturing down time or production changes for days or even a few weeks does not cause a great loss of biomass in the compost-based biofilter because of its back-up food source in the compost. As long as the airstream is properly humidified and the temperature regime for the system is not upset, the biofilter biota will survive and be able to degrade the airstream contaminants once those contaminants reappear. Usually a short period of re-acclimation and regrowth of biomass will be necessary, but that should only require 48 to 96 hours. After that period, the system should be at 95 percent of its original capacity for reduction of the airstream contaminants, and within a few additional days complete capacity should return.
Organic media is subject to increasing pressure drop due to settling and compaction of the particles. “Lofting agents” like polystyrene beads, perlite, and bark “nuggets” are sometimes added to decrease the pressure drop across the matrix bed. Often the mixtures are sieved to remove the smallest particles ( Compaction of the media can increase as the matrix particles themselves decompose and shrink over time. To combat this, BioReaction Industries (BRI) has designed and patented media composed of compost-filled plastic spheres. The plastic sphere provides structural strength to prevent compaction and a uniform size to the bed matrix that minimizes channeling of the airstream. The compost supplies long-term nutrient and micronutrient release, a maximum surface area-to matrix volume relationship for microbial biofilm development and a refuge (inside the compost packed sphere) for microbes should a catastrophic accident occur (flash fire). The BRI compost formula uses only “green waste material” (no manure) that is composted for a specific time period then “packed” into the plastic spheres.
All biofilters share certain operational factors that determine the overall effectiveness of the application. The three most important parameters are moisture, temperature and pH. Other factors that have variable influence include: nutrients, particulate material in the emission stream (including oil and grease aerosols), direction of air flow, type of contaminant(s), and available oxygen. These factors are especially true for compost-based bio-oxidation units.
Moisture: Control of moisture in a biofilter is the most critical factor, after establishing the biomass/biofilm capable of converting VOCs to organic carbon, carbon dioxide and water. In compost-based bio-oxidation systems, the compost material needs to have a specific moisture content, depending on the application, but generally in the range of 40 to 65 percent. After the biofilter “bed” is in place, the biologically active surfaces must not dry out, nor be too wet (flooded). If it becomes too dry, the microbes die or become dormant and the contaminated air moves rapidly through the system without adequate treatment. As drying occurs in one area airflow increases, that dry area allows additional air to pass, resulting in short circuiting and significant treatment loses. Conversely, too much moisture may “wash out” the microbial biomass, with loss of treatment capability. The airflow may also be restricted (increased back pressure) as a result of biomass and compost material being “washed out” from the top, then accumulating in the lower areas of the bed. This decreased flow can also create conditions that lead to poor capture (lowered face velocity) at the emissions source. Adequate humidification (with little direct bed watering) of the incoming air stream is often the key to proper moisture control of the media in a biofilter.
Temperature: Microbes are dependent upon the surrounding temperature for the operating regime that controls their metabolism. A biofilter can operate at a wide range of temperatures, however a warm (77 – 95°F; 25 – 35°C,) bio-oxidation unit generally supports more organisms, both in absolute numbers and in species, and at higher activities (metabolic rates). The drawback to “higher temperature” operation is that less of a specific contaminate dissolves in water and adsorbs to the media. Microbial activity, metabolism, doubles (approximately) with each 10°C (18°F) increase in temperature, as long as the organisms remain in their thermal tolerance zone. [There is an upper thermal limit where metabolism begins to decrease and, if temperatures are sufficiently elevated, the organism(s) die.] Therefore, generally, a warmer biofilter oxidizes incoming organic compounds faster, thereby having the ability to handle greater loadings of contaminants per unit time. However, the microbial communities in these biofilters contain so many different species with various thermal tolerances that relatively good bio-oxidation can be maintained over a wide temperature range. Data from pilot biofilters treating the water-soluble compounds formaldehyde and methanol from particleboard press vent emissions have consistently shown Dre’s in excess of 92%, at bed temperatures ranging from 65 to 88°F (18 to 31°C). Although this example provides credible evidence of a biofilter operating successfully over a wide temperature range, there are a number of reasons to control temperature within a narrower operating range. Sudden and extreme temperature changes can be very detrimental to bio-degradation of incoming contaminants. Finally, moisture control is more easily handled if the temperature can be maintained within a narrow range.
pH: The pH range for specific microbes is somewhat analogous to their temperature range, with defined upper and lower limits and an optimal operating range. Biofilters can function at pH’s ranging from as low as 2 – 3 (H2S removal) to as high as eight or nine, with pH often changing (usually decreasing) after initiation of operation. To counteract the acidification that occurs with the biodegradation of H2S and organo-sulfur compounds in wastewater odor control applications (sulfuric acid production), increased irrigation of the media bed may be required and increased blowdown of sump waters must occur to prevent unacceptably low pH in the system. However, H2S biodegradation operates best at a relatively low pH (2 – 4), while organo-sulfur compounds are best oxidized at a pH of 5 – 7. Should both types of sulfur compounds occur together, as in emissions from pulp and paper mills, a multi-stage biofilter is the best choice with the initial treatment section at the entry of the emissions operating at low pH (3 – 4) for H2S removal while those beds further downstream in the system are maintained at higher pH (5 – 7) for best biodegradation of the organo-sulfur compounds and other VOCs. The pH can also be managed by the addition of crushed limestone to a compost-based media, adding a buffering solution to the irrigation/humidification water, and by more or less frequent water removal and addition. The tendency of sulfur and chlorinated compounds, and some other organics, to produce acids upon biological decomposition makes attention to changes in pH critical to the long-term successful operation of a biofilter. With a compost-based biofilter system, pH changes are generally very slow to occur when compared to synthetic substrate units. The compost provides leachable compounds that act as natural buffers and provide some manner of pH stabilization to the system.
With the advent of well-designed and engineered bio-oxidation systems, combined with tacit acceptance by regulatory agencies and the increasing cost of fuels for alternate control technology, control of air emission sources utilizing bio-oxidation systems has been increasing and should remain strong for the foreseeable future. While this use for “green waste” and compost may not be the largest for these materials, it is a growing market. Additionally, because of the weight of this material, sources near the ultimate location of the bio-oxidizer system will be used to provide the necessary material. Potential compost suppliers should contact individual biofilter manufacturers to get an idea of what they require. Those companies in or near major metropolitan areas may be situated to better take advantage of local industries that may choose bio-oxidation as an air emissions control option. Compost needs for individual units could range from as small as 13 cubic yards (10m3) to as large as 2,600 cu.yds. (2,000 m3). Each biofilter manufacturer will have individual preferences of specific materials, mixtures and additives. It is certainly not expected that compost producers will have on hand the exact mix needed, but rather the ability to produce it in four to six months.
Jim Boswell is a biologist and senior scientist for Bio-Reaction Industries in Tualatin, Oregon.

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