WHEN I first joined the Philadelphia Water

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1 54 Biofiltration Basics: Biofilter Design, Operational Overview William E. Toffey WHEN I first joined the Philadelphia Water Department s biosolids management unit, I reviewed the composting procedures and practices being used. The biofilters were among the more curious components of our processing system. At the time, nothing I read could persuade me that biofilters were not akin to a hoax, which worked through dispersion and dilution, rather than through genuine treatment processes. I can now say that I am 99 percent sure that biofiltration is a legitimate treatment technology. The remaining one percent is a residue of doubt, which I find difficult to shake. But I am mostly convinced that biofiltration is an effective technology for destroying contaminants in composting process air. Scientists have demonstrated unequivocally that biofiltration works. Furthermore, they have demonstrated that the process is largely biological action upon the contaminant by microorganisms residing in biofilms. Biofilms are a recently introduced area of scientific inquiry. It is one which spans several different fields, involving biology, chemistry, engineering and medicine. A biofilm is the thin layer of moisture, mixed with organic substrate such as polysaccharides, and inhabited by a diverse ecosystem of bacteria and related microorganisms. This layer acts upon gases absorbed from the passing air stream. Scientists have shown, with radioactivelymarked organic compounds, that the biofilm converts organic compounds to CO 2 and water and to additional biofilm biomass. Biofilms are at work in such diverse locations as the water mains under the street and in the human gut. Biofilms are new to science. But if you perform a search on your favorite Web browser, literally hundreds of sites await your visit, and from them you can learn of the recent work. Science is at work to transform the black box which was at one time the biofilter, into a well understood and reliable technology. The research and science is still in its infancy, and substantial areas of inquiry are possible, particularly for the processing of emissions from composting facilities. Biofiltration, from a broad view, is a control technology ideally suited for control of emissions from composting facilities for these reasons: The natural biofilm surfaces in the biofilter assimilate the contaminants; Energy or chemical needs for processing are low, as a biofilter does not need supplemental energy to burn or to neutralize air streams; Biofilters decompose odorous, volatile organic compounds and hazardous air pollutants, whereas alternative technologies, such as scrubbers, do not; Biofilters are not as sensitive as the more highly engineered solutions on the rate of air flow and the concentration of contaminants in the air stream; and Open biofilter systems can be made with off-the-shelf equipment and are adaptable for many different scales of air flow. Design Components The biofilter is a device with two principal elements the porous media through which contaminated air passes, and the air distribution system. Biofilters are usually characterized as open or closed. A closed biofilter is one in which the media is contained in a structure, with an inlet for dirty air and an outlet port for the cleansed air. The open system is open to the atmosphere, usually on the sides as well as the top, and emissions are from all exposed surfaces. Several factors may influence the choice of biofilter type. First is cost, as open biofilters are considerably less costly than closed systems. Choice of biofilters will be influenced by the type of process air being handled. For composting of biosolids and

2 BIOFILTRATION BASICS: BIOFILTER DESIGN, OPERATIONAL OVERVIEW 55 other organic residuals, where the process air is dilute, an open type system is usually employed; it is also inexpensively constructed. For process air from industrial activities with high concentrations of organic compounds, enclosed systems are employed more often. Regulators may be more comfortable with enclosed systems, which can be monitored easily, and may be necessary for air streams with high strength contaminants. The reliability and reuse capabilities of enclosed systems need to be assessed for applicability to composting. The other design component critical to effective biofiltration is air distribution. As is true for the composting process itself, even air distribution beneath the biofilter is a key to good processing. Without it, channels can erupt through the media, allowing air to bypass filtration. The challenge in the design is to accomplish minimal pressure loss to friction as the air is distributed below the biofilter, and to properly design the ports through the filter to achieve even flow in all areas. Many designers fail to properly consider the size of pores or slots in the plenum, as too large a slot may contribute to uneven air distribution. Design of an effective air distribution plenum has been the source of fatal flaws for many biofilters, including Philadelphia s, as the distribution equipment can become clogged and broken. Choice of air pipes is particularly important because the diameter and texture of the pipe influences pressure loss. For instance, at Philadelphia s facility, the use of six inch corrugated pipe introduced frictional pressure losses which were nearly impossible to overcome. The early design concept of a gravel enclosed aeration hose, covered by a filter fabric, is also not as widely recommended as they once were, as fabric can become clogged, and vents can appear where the fabric separates under the force of air. New products on the market include prefabricated aeration plates, but these may be costly. Capacity, Media And Bed Moisture The design of a biofilter is usually guided by the parameters of loading rates and residence time. These are usually fairly simple calculations, made with the simplifying assumptions that the filter bed is empty (porosity is 100 percent). The most common factor is the rate of air flow per surface unit of biofilter, expressed either as cubic meters of air per second for each square meter of biofilter surface (in the literature reportedly ranging from 0.3 to 9.5) or, in English units, cubic feet per minute of air flow per square foot of biofilter surface (in an equivalent range of one to 30). This design parameter can also be expressed, when multiplied by the volume of the biofilter, into a unit of residence time. A typical recommendation for residence time is 30 to 60 seconds. These times are based on air from composting, and longer residence times may be necessary for air containing higher contamination or if the media is so porous it reduces the exposure of the air to active surfaces. Because biodegradability of compounds differ, the detention time ought to be established for removal of the compounds of most concern. For instance, terpene compounds may need several minutes of treatment for effective removal. Another means of expressing loading rate is in the mass of air pollutants loaded to the biofilter daily, per unit mass or unit volume of biofilter. The metric terms are grams of pollutant (as carbon) per kilogram of dry media per day, or grams per cubic meter of media per day. This calculation is more typically made for industrial air processing, rather than compost process air, because of higher concentrations in the air flow. The filtration media is usually comprised of organic materials, such as wood chips and compost, but also possibly soil and peat moss. A substantial rethinking of the nature of the media is reported in the literature. Early recommendations emphasized the importance of media surface area for providing activity sites for biofiltration, and, to achieve this goal, a filter media with fine particles, such as leaf mold and soil, was prepared. This media proved to be too dense, became too soggy and tended to lose its porosity quickly. Revised formulations focused on choice of media which would retain its structure over a several year period, such as hardwood chips. Recent studies have examined the use of inert structural elements made, for instance, of plastic, to provide porosity and support. A lower proportion of fine organic material than earlier prescribed seems to be effective in emissions removal when treating air streams with low concentrations of contaminants. Additional research is needed to correlate biofilter effectiveness and media effective surface area. A key parameter influencing the effectiveness of biofilters is moisture content. The biofilter operates at moisture contents consonant with the requirements of microbial life, typically in the 40 to 60 percent range. An early finding into research of biofilters showed that heated air put into biofilters tended to dry them out, and that the moisture in the biofilter media had to be replaced continually. A common recommendation today in biofilter operation is for a mechanism of adding moisture to the biofilter, both internally and from the surface.

3 56 ENVIRONMENTAL PROTECTION - AIR, WATER AND FIRE Recommendations are also common for the installation of control equipment to sense the moisture content of the biofilter, and to add moisture when the media falls below a minimum moisture content, e.g. 40 percent. There is the danger, when a biofilter is irrigated, that excess moisture may cause a drop in biofilter porosity. This would, in turn, cause a backflow pressure buildup, a risk of venting of air through bypass channels, and a build up of slime which would contribute to, rather than remove, odors. This problem can be addressed largely through changing the formulation of the biofilter media, making it coarser and more porous. A means of draining excess water from the biofilter is often recommended today. Drainage water may be recirculated back to the biofilter, but this is not standard practice. Pressure Drop, ph And Nutrients Biofilters fail when they become so compacted and low in porosity that the process air breaks through in vents which allow air to bypass the filtering media. Actions need to be taken when pressure losses are seen to increase over time. Compacting media may require mechanical turning or replacement, even if it s only been in operation for less than one year. Some facilities are experimenting with new types of media which provide dependable porosity and stable pressures. Composted organic matter may be supported by a larger volume of inerts such as ceramics, plastics, granular activated carbon or large hardwood chips. Recommended design criterion is less than one inch as water column loss per foot of biofilter depth. A common biofilter recommendation is that the biofilter have a ph between 6.5 to 7.5 for proper microbial activity. The biofilter may need to be maintained to correct for acidic by-products resulting from the treatment of hydrogen sulfide, and perhaps ammonia. Periodic monitoring for acidity in the biofilter should be undertaken; those biofilters being used to treat air streams high in these compounds may have a buildup of acids in the biofilter, and the acids have to be periodically washed out. Adjustment for ph can be done by adding alkaline buffers to the media, or introducing lime with the supplemental water. Operating experience is necessary to determine whether acid by-products are a concern, and thus ph monitoring of the biofilter should be undertaken periodically. Nutrients are necessary to support the growth of microbes responsible for biofiltration. A compostbased media generally supplies adequate nutrients for biofilm growth, and the process air from biosolids composting would itself carry nutrients to the biofilter. If an inert fraction is used as a filtration media, it is possible that nutrient additions may be necessary, particularly nitrogen. But too much nitrogen could be a problem in itself, as it might cause excessive biofilm growth and slime build up, which could impede air flow. Humidification And Temperature Of Air Flow Humidification of air flow into the biofilter is now nearly universally recommended for biofilter operations. The principles of thermodynamics predict that a nonsaturated air flow, when heated within the biofilter and when added at a high velocity, will thoroughly desiccate a biofilter. Desiccation kills the microbes responsible for removal of air contaminants. A fine mist of water, sprayed in the process air just before it enters the biofilter, has been used to create a saturated air stream to the biofilter. Whereas the biofilter often gives off heat and can raise the temperature of the air flowing through it, some biofilter experts may recommend internal irrigation of the biofilter as well, so that the air flow can be supplied with additional moisture. Surface irrigation can be done on an as needed basis, determined by monitoring. In the future, process control techniques should be developed for automatic adjustments to rates of humidification and irrigation. The temperature of the air flow has been generally shown to influence the effectiveness of biofiltration. Organisms shown to be responsible for biofiltration are mesophyllic bacteria, and biofilter performance seems to drop when temperatures exceed 40 C. Some biofiltration experts hold a contrary viewpoint, and assert that thermophyllic bacteria are capable of biofiltration, and that the apparent drop in effectiveness is induced by desiccation, which can be overcome with humidification. The answer to the possible operation of biofilters in the thermophyllic range must await new word from the scientists. What is clear is that different composting processes present very different kinds of air flow to the biofilters. In-vessel composters have dilute, cool air streams, as their flow includes the air of the enclosure surrounding the compost vessel. Static pile composting, which draws air down through the compost mix, will be stronger in concentration of air contaminants and comparatively hot, and can benefit from drawing in atmospheric air. Reference to a textbook on thermodynamics will reveal that major modification of process air temperature can be extraordinarily difficult. Introducing

4 BIOFILTRATION BASICS: BIOFILTER DESIGN, OPERATIONAL OVERVIEW 57 atmospheric air to the compost exhaust for the purpose of cooling it complicates the challenge of humidification of the air flow and increases the required biofilter size. Heat exchangers for preprocessing the process air to reduce moisture requires expensive equipment and produces significant wastewater effluent. A few biofilter experts have actually recommended preheating, in addition to humidification, of process air prior to introduction into the biofilter. The concern is that biofilters generate heat, thereby raising air temperature, and drying out the air stream once it enters the biofilter. The thermodynamic and moisture balances in biofilters need to be more thoroughly researched to address these kinds of concerns. Ammonia And Hydrogen Sulfide Removal Two traditional compounds of concern for odors, and present in significant concentrations in compost process air, are ammonia and hydrogen sulfide. When filtered, both may create acidic compounds, essentially nitric and sulfuric acids. There is concern that these compounds will acidify the biofilter, interfering with microbial action and filtration effectiveness. One of the lively debates in biofiltration is whether a practicable means can be demonstrated for preprocessing the compost air, e.g. through a wet scrubber, to remove excess ammonia and hydrogen sulfide prior to biofiltration. It should be noted that composts with different raw materials give off strikingly different levels of ammonia and hydrogen sulfide, so part of the equation in control of these compounds is in the selection of composting inputs. For example, hydrogen sulfide emissions may not be excessive if stabilized biosolids are composted. Another point is the very small contribution of either ammonia or hydrogen sulfide to off-site odor nuisances. In addition, the metabolic processes at work within the biofilter to treat these compounds have not been well explained, so extraordinary actions to pretreat the air stream may not yet be warranted. Again, we are waiting for the results of scientific and engineering study of this issue. Study also is required of the relationship between ammonia and hydrogen sulfide loading rates and changes in biofilter ph. Process Monitoring And Media Replacement The effectiveness of biofiltration has been well demonstrated under laboratory conditions, but effectiveness in field applications is seldom studied. A common recommendation given today for biofiltration operation is the use of process monitoring equipment to give operators information on the performance of the biofilters. Process parameters which are usually recorded are: Offgas temperature to determine whether operation is within effective range of biofilter organisms; Relative humidity of gases in the biofilter to judge the adequacy of irrigation quantities; Pressure drop within the biofilter to detect loss of media porosity, moisture content of the filter so that additional moisture may be added; ph of drainage to assess need for liming to counteract acidity induced by ammonia and hydrogen sulfide; Ammonia, hydrogen sulfide, odor units (dilutions to threshold) and Total Organic Compound concentrations measured on inlet and outlet to determine effective removal rates. The emphasis of monitoring should be two fold measurements which demonstrate the healthy and steady performance of the biofilter and measurements which document compliance with regulations or guidelines. But another emphasis may be justified, namely the collection of data which can contribute to the scientific understanding of biofilter performance. It would be helpful to look at pile performance across a variety of facilities. Comparison of odor panel evaluation of D/T to the measurement of indicator compounds (dimethyl disulfide) would be useful, as would the study of the relationship between media texture, moisture, and pressure loss. A major source of controversy with biofilters is their anticipated useful life. Typical recommendations for replacement of open biofilters at composting facilities have embraced a range of two to seven years. Many operators have concluded that biofilter replacement should be planned for every two to three years. Innovative approaches to biofilter design have focused on methods of convenient media replacement, allowing the air distribution system to be unaffected. In open systems, cast concrete air distribution elements can remain in place as equipment works over it to fluff up old media or to remove spent filter media and replace it with new. The experimentation with nondegradable plastic units as a component of biofilter media, could provide the permanent, nonsettling structure of the biofilter, obviating the need for frequent replacement of the media. Siting Considerations And Regulatory Performance Biofiltration should be one of several odor minimizing practices at a composting facility. Others might include control of the mixing area and

5 58 ENVIRONMENTAL PROTECTION - AIR, WATER AND FIRE choice of days and hours of operation. If composting operations are successful at stabilizing organic residuals during the aeration stage, then biofilters will be the principal source of odor emissions. It is prudent planning to consider the location of this odor source as far away as practicable from homes, public roads and other sensitive receptors. Biofilters designed to the capacities given above typically become major features of a composting facility, taking substantial space otherwise devoted to processing and product storage. Redundant biofilter units may need to be installed to allow for air treatment during periods of biofilter replacement or repair, further consuming processing space. If it is necessary to use an enclosed biofilter, steel or plastic containers for biofilters may be selected in lieu of the more common concrete boxes when the filter is planned to be situated above the process. This approach has been used in Europe. Biofilter boxes usually have a port through which the treated air is exhausted; dispersion fans may be used to minimize the off-site impact. Such engineered approaches to biofilters may be warranted for sites where neighborhood odor nuisances are predicted. Regulation of biofilters raises the prospect of standards which are not practical. This includes specific removal percentages or attaining specific low odor levels (as D/T measurements off the biofilter). The variation in regulatory approaches is great, which might suggest that regulators use caution in adopting hard and fast regulations. A balanced set of metric standards may be devised for biofilter performance. Generally speaking, well designed and operated biofilters should be capable of achieving a D/T of 50 for emissions at the surface of the biofilter. A growing number of reports confirm that modeling of odor nuisances can be employed by the operator or regulator to demonstrate the potential effect of odor emissions at this level on sensitive receptors, or at the property line. There is no commonly agreed to regulatory approach. And that fact is understandable, and arguably preferable. Regulatory approaches may reasonably differ. A facility located in an industrial setting may not need to meet the same kinds of standards as a facility in a residential zone. Offgases from a functioning biofilter, in 50 to 100 odor units, are rarely noticed if the site has a large buffer zone and is located in a rural or industrial sector. If the biofilter is being used for VOC control, then compliance with quantitative regulatory standards may need to be verified by emission testing in raw and treated offgases. In this case, the choice of biofilter design influences control effectiveness. For instance, open biofilter systems will impose challenges to verification compared to closed biofilters, which can be more effectively controlled and monitored. Regulatory concern legitimately embraces biofilter operation and maintenance. Ultimately, regulators may want to use evidence of biofilter maintenance in lieu of specific performance measurements to ensure compliance. Until recently, Bill Toffey was manager of the Biosolids Utilization section at the Philadelphia Water Department.