Nature’s Own - Biofiltration process offers promise of (near) energy-free pollution abatement

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A filter could be described as any substance through which liquid is passed to remove suspended impurities. Although a “biofilter” is actually not a filter, “biofiltration” is on the threshold of finding a market as a compliance solution for air pollution regulations after a number of false starts. Biofilters have long been used as a means for reducing objectionable odors in Germany, the Netherlands, the United Kingdom, Japan and, to a limited extent, even in the United States[1].

Their use is sufficiently common in Europe that the Netherlands, a country smaller than the combined areas of New Hampshire and Vermont, has long been home to five large manufacturers of biofilters. Their use in the United States goes back to as early as 1953 when Long Beach, Calif., is reported to have used a soil biofilter system for odor reduction[2]. Their success in controlling odors, detectible at only a few parts per billion, would make their success in compliance matters almost a foregone conclusion since most air pollution regulations allow emissions of a few parts per million. 

Yet, the road to engineering an efficient and dependable biofiltration system has not been smooth. Even the name may have been an impediment. How and why this fascinating technology of nature was termed biofiltration is unknown. It is possible that the early construction techniques contributed to a misconception of the principles involved.

The major component of the traditional biofilter was a large, shallow basin. Usually these were formed of concrete, although some were merely shallow excavations (with or without a liner[3]) in the soil. Perforated piping arrayed across the bottom of the basin was used to deliver noxious air near its floor. The piping was covered with truckloads of compost leveled to a uniform depth of three or four feet above the pipe. Air sparged through the pipe was freed of objectionable odor as it passed up through the compost.

Is it possible that early investigators believed that the compost physically filtered odorous compounds from the air? The prefix “bio” makes clear that they recognized life forms played a role. Were they aware, however, that the process depended on a healthy community of invisible, microscopic life forms; that bacteria and fungi, abundant in compost, were responsible for actual cleansing of the air? Did they understand that, in turn, those life forms depend on a readily available source of carbon-containing material for food and, given the choice, readily choose vapor phase organics because they are more easily metabolized than the solid compost materials?

The comeback

Regardless of who knew what in the early years of this technology, biofilter is now so firmly entrenched it is unlikely that a more descriptive terminology (bio-reactor, bio-catalysis, bio-oxidizer, fungal-reactor, fungal-catalysis) will replace it. The reality, however, is that for practical purposes, a biofilter is, in fact, a low-temperature catalytic oxidizer.

Unlike traditional oxidizers that must operate at 1,600 degrees F or catalytic oxidizers that operate at above 600 degrees F, the optimum operating temperature for a bioreactor is 80 to 100 degrees F, that of a warm summer day. Especially important as the nation turns its attention to better control of nitrogen oxide emissions is, unlike thermal and catalytic oxidizers, bio-oxidizers form no nitrogen oxides. They convert (metabolize) volatile (or hazardous) organic gases directly to carbon dioxide and water without creating any other aggravating pollution issues.

It was their low energy demand that brought biofilters to the attention of the U.S. Environmental Protection Agency in the late 1980s[4]. The Agency’s investigation of the technology exposed several shortcomings and, as a result, biofilters were neither the basis for a regulation nor accepted as a dependable compliance technology. In retrospect, the technology of those days appears somewhat primitive. The extensive compost beds were subject to channeling and, on occasion, complete collapse of part of the bed. The huge units, which often approached soccer-field size, were open and the filtered air discharged from the entire surface directly to the atmosphere (an acceptable practice when the neighbor’s nose is the monitoring device).

Because of the expansive surface of compost, there was no meaningful way to measure or monitor the pollutant concentration from the open discharge. Enforcement of a regulation would have been impossible. Similarly, their construction and large size would make temperature or humidity control difficult, at best. Finally, their large “footprint,” made them unsuitable for retrofit installations. Many industrial sites would be unable to find sufficient open land near the source of pollution.

Perhaps their most unacceptable characteristic, however, was the fragile nature of the catalyst. As with traditional catalysts, contamination would kill the activity of the microbe population. Their activity is proportional to their numbers and their numbers were very vulnerable to changes in either the composition or concentration of the feed stream. Further, when the microbe population suffered significant loss, the time required to recover the activity of the bacterial-based catalyst system was measured in weeks, even when concentrated volumes of microbes were injected (or seeded) directly into the compost system. It was clear that the Agency could not endorse a technology whose destruction efficiency could not be accurately determined and that could founder for weeks after a process upset.

For decades, biofilters had proven effective in oxidizing odorous volatile organic compounds (VOC) as well as sulfur- and nitrogen-containing compounds. Yet, those rather primitive units were not sufficiently reliable for consideration as a compliance device. Needed was innovation in design, monitoring techniques, development of hardier microbes, more stable compost beds and long-term performance data.

After five or more decades of extensive use for odor control, only now are highly engineered bioreactors finally becoming a viable option for abating air pollution. The early 1990s[5] initiated a period of transformation for biofiltration technology. Since then, significant effort has been invested in research into this infinitely sustainable miracle of nature. Kept warm and moist, the ubiquitous natural microbes convert volatile organic hydrocarbons to carbon dioxide and water (and new microorganisms, biomass).

Success in harnessing the technology into a viable and competitive air pollution control system could have a dramatic impact not only on the cost of air pollution control but on the flexibility of plant operations as well. Such a low-cost compliance option, for example, would allow coatings industries to select paints and other coatings based on performance, appearance, or ease of application rather than their compliance status since excess emissions would be readily destroyed at reasonable cost.

An innovation

At least one manufacturer has made significant progress in overcoming each of the shortcomings so obvious a decade ago. BioReaction Industries, LLC, of Tualatin, Ore., after more than seven years of applied innovation, is now marketing an engineered bioreactor that appears to have addressed all of the problematic areas evident in traditional biofilters. The units are built vertically rather than horizontally to minimize the “footprint” making them a viable candidate for retrofit in almost any circumstance. The units’ design is modular with mix-and-match components, and rigid compost beds are so unique that patents are pending on the manufacturing process; they cannot collapse. Their homogeneous nature is conducive to uniform air flow across the face and through the body of the compost, thereby precluding channeling while ensuring maximum contact between the air and the boundary-layer moisture around the microorganisms.

Perhaps of maximum importance, however, are the developed community of hardy microorganisms on which success depends. All biofilters are based on selective breeding from the tens of thousands of different organisms that inhabit a typical compost. A biofilter manufacturer will generally precondition its compost by exposing it to a synthetic feed stream that approximates in composition the process to which the microbes will be dispatched. Those that find the feed harmful quickly decline in number. Others multiply rapidly when the volatile organics prove a desirable food source. The result is a compost bed rich in concentration of microorganisms that thrive on the artificial feed stream. These pre-selected, naturally occurring organisms are then delivered to the client as an integral part of the bioreactor system.

The units also utilize a proprietary process for selecting its community of microorganisms that has proven to result in a much hardier biocatalyst. Their microbial community relies heavily on fungi, a higher form of life than bacteria, to provide a hardiness to the community that bacteria alone cannot achieve. The fungi are reported to adsorb and store some of the volatile organics which can be made available to the bacteria in the bed in case of famine. Further, the fungi are not dependent on the volatile organics delivered by the contaminated air stream. When the manufacturing process is down and not delivering the volatile organic feed, they can survive using the compost as their sole food source.

BioReaction Industries reports that with the hardy community of microorganism, their bioreactors have recovered in a matter of hours from sudden and dramatic changes in feed concentration. While controlling a low-volume vent from a glycol dehydrator (used in natural gas fields to remove water and organics from natural gas), the nominal 17,000 ppm stream surged to over 600,000 ppm. The overall control efficiency dropped briefly from 99 to 95 percent while the efficiency for destruction of BETX (benzene, ethylbenzene, toluene and xylene) remained at about 99 percent. Other organics present included large amounts of CO2 and methane[6].

These hardy communities of microorganisms also withstand plant shutdowns. A higher airflow biofilter of an earlier design was used to reduce volatiles from a facility that manufactures paint. During the week, the 10,000 cfm feed to the bioreactor averaged about 10,000 ppm. On weekends when the plant was not operational, the air flow to the bioreactor was automatically reduced to about 2,000 cfm. Because of the many emission sources within even when the plant was idle, the inlet concentration to the bioreactor remained about 200 ppm. Tests revealed that the control efficiency on Monday morning quickly recovered to levels of the previous Friday[7].

Part of the success is due to the precision with which they control both temperature and moisture conditions for the community of microorganisms. One impressive innovation was to precede each bioreactor with a trickling filter that serves to humidify the incoming air stream, knock out entrained particulate and aid in temperature control. Precisely placed water sprays circulate water from the trickling filter’s sump to help ensure maintenance of 100 percent humidity throughout the reactor. Trace amounts of steam can be injected to aid temperature control. Collectively, the bioreactor’s process control system insures that the community of microorganisms enjoys optimum “comfort” to ensure their maximum vitality and activity.

Another innovation is the patent-pending biocatalyst support system. The company has perfected a means for structuring compost into uniformly round pellets approximately an inch in diameter. Each “pellet” is so rigid that bed collapse is impossible; there seems no limit to their ability to support bed heights far greater than can the maximum that can be maintained uniform in humidity and temperature. Yet, each pellet is sufficiently porous that it easily adsorbs moisture which serves as a backup source for the microbial community.

The design for the units is structured around a four-element concept. Sized to handle a nominal 10,000 cfm, the elements can be assembled in a variety of ways that result in different exposure (residence) times. The base unit is normally a trickling filter. Joined above it, depending on the configuration of the unit, are “stack modules,” “air management inserts,’ and at the top, a “discharge head.”

Figure 1 provides flow diagrams for four of BioReaction’s most advanced designs that were recently installed to remove formaldehyde and methanol (hazardous air pollutants) from the press vent gases at a particle board plant in the Pacific Northwest. A Federal air pollution regulation for this industry is scheduled for promulgation within the next year.

In the meantime, the wood products industry is searching for an alternative to the high energy and operating costs of incineration, the currently accepted technology. Many similar facilities across the nation face the future compliance deadlines. Each of the nominal 10,000 cfm units is equipped with a 30 horsepower blower and a 7 horsepower pump. The pump circulates water from the sump of the trickling filter to the compost bed. A small amount of additional energy may be required to maintain the compost bed at 90 degrees F.

Data from the four varied configurations will help optimize design and operating conditions for future bioreactors in these applications. Early indications are that three of the four units are removing both methanol and formaldehyde to levels below detection limits throughout the range of inlet concentrations (16 to 60 ppm). The efficiency of the fourth unit has been less consistent, ranging from 73 to 96 percent.

Unit Number 1 contains an internal trickling filter upon which are stacked three bioreactor modules in series.

Unit Number 2 also has an internal trickling filter. The discharge from the trickling filter is split and each of the two 5,000 cfm streams passes through two bioreactor modules in series.

Unit Number 3 has an external trickling filter whose discharge is divided into three factions. Approximately 3,300 cfm passes through each of three bioreactor modules.

Unit Number 4 is identical to Unit 3 except that the speed of its blower can be varied. This provides near-infinite flexibility in evaluating the effect of residence time on destruction efficiency.

Additional testimony regarding this new chapter in the history of biofiltration technology will become available in future weeks as units have been contracted at two additional industries with very diverse emission problems. One will minimize emissions of oil mist at an aluminum casting facility while a second will destroy emissions (largely isopropyl alcohol), from a wafer-manufacturing facility.

Because of the potential for huge savings in energy costs (80 percent less than that of a catalytic incinerator[8]), absence of nitrogen oxide byproducts, sustainability of the technology and ease of operation, demand for this new generation of engineered biofilters to reduce emissions of VOC is expected to grow dramatically in future years. The industry seems on the threshold of reaping the benefits of these low-energy systems. Organic catalysis of volatile organic compounds appears finally positioned for significant growth.

Stay tuned for future articles as a variety of industries verify how effectively these state-of-the-art devices harness nature’s most basic forces.

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