Several abiotic engineering methods are commonly used to treat contaminated air-streams, including those of incineration, carbon absorption, condensation, and scrubbing (i.e., either with water, chemicals, or combinations thereof). However, within the past few decades the idea of using biofiltration columns has proven to be a highly attractive,
Parallel Air B ¡of i Iter Reactors
Parallel Air B ¡of i Iter Reactors
complementary treatment strategy for air quality remediation. Biofiltration consists of a closed vessel containing a porous support medium with an attached biofilm through which a contaminated gas is passed. The attached biofilm will first sorb, and then degrade, the incoming contaminants. This technology first gained significant attention in Europe during the late 1970s, and there are now a number of relatively recent applications in the United States. However, as with constructed wetlands and phytoremediation systems, the involved technology is still in the process of being clarified and optimized.
Biofiltration technology is, in fact, rather similar to that of the attached-growth media-filled biofilters (e.g., trickling filters) described earlier, although in this case the stream to be treated is that of contaminated air rather than wastewater. Similarly, the mode of construction and operation used for biofiltration columns is also comparable to that of bioso-lids composting using aerated piles, although with a goal of treating the incoming gas rather than a solid-phase (sludge) material blended into the pile.
In some cases, the main objective of gas-phase biofiltration processing may simply be that of remediating a malodorous gas stream by removing the chemicals responsible (e.g., hydrogen sulfide, mercaptans, free ammonia). Figure 16.57 depicts one such full-scale application used to effectively cleanse these types of contaminants from the off-gas discharge generated by an aerobic thermophilic biosolids digester. Several comparable wastewater-related applications exist for handling contaminated, odor-bearing off-gas streams, including those released by attached-growth wastewater treatment towers, activated-sludge reactors, biosolids digesters, and even sewer force mains.
However, biofiltration may also be used to biochemically degrade one or more regulated volatile organic compound air contaminants such as those generated within a specific industrial airstream. High-level removals have, in fact, been observed with a number of industrial organic contaminants, including phenol, styrene, formaldehyde, BTEX, and methanol as well as various aldehydes and ketones. Many of these biofiltration applications have been tested within industrial applications in which the current abiotic off-gas treatment procedures are too mechanically complex, too expensive, or simply too inefficient.
Figure 16.58 depicts a typical biofiltration system. The various design strategies for biofiltration systems cover a wide range of options, some of which depend solely on
the biofilter for contaminant removal (e.g., typically involving low-level contaminant concentrations), while in other instances the process train will include a pretreatment abiotic scrubber. Abiotic scrubbers are, in fact, often included as an integral part of many systems, helping to moderate influent gas temperatures and ensuring full humidification as well as securing preliminary sorption and partial removal of highly soluble contaminants such as ammonia. Four options are typically seen with these scrubbers: (1) water-only scrubbing, to humidify and cool an incoming gas stream as well as to initiate desired gas-to-liquid sorption; (2) acid spray scrubbing, to adjust pH in a downward fashion that shifts gaseous ammonia to ionic, liquid-phase NH^ ; (3) caustic spray scrubbing, to adjust pH upward both to shift gaseous hydrogen sulfide to ionic, liquid-phase HS~ and to buffer both the scrubber water and gas-phase moisture against the acidity of incoming gaseous CO2; and (4) oxidant (i.e., typically bleach) spray scrubbing, to oxidatively attack any chemically amenable contaminants.
Once this prescrubbed gas stream reaches the biofiltration zone, the targeted contaminants must be readily transferable from the gas phase to the water phase within which the biomass is growing. The success of this sorptive phenomenon depends on several factors, including the Henry's constant (KH) of the involved contaminant species, ambient temperature, media-film pH, and the media-biofilm surface area available for interfacial gas-liquid exchange. Biofiltration-based degradation of contaminants with high KH values may therefore be difficult, since it is likely that these species will remain in the gas phase rather than being sorbed and degraded.
Media temperatures between —20 and 40° C appear to work best, avoiding low or high extremes that would retard the desired metabolic activity of the biofilm or escalate water evaporation to an unacceptably high level. For cold incoming gas streams, steam might be used to raise a reactor's operating temperature, and hot incoming gases can be cooled by means of the initial water scrubbing.
In terms of flow schemes and design retention times, both up- and down-flow regimes have been used, although the up-flow option is probably the most common. The contact time for gas throughput also represents another key factor, with most units sized to provide ^45 seconds. Pile moisture levels in the neighborhood of ^50% are considered optimal, and at the same time, the relative humidity in the gas stream will need to be above ^95+% to avoid excessive levels of water evaporation from the pile. Many biofiltration vessels are fitted with internal humidifying spray misters as a means of ensuring desired wetting and moisturization of the biofilm, and if necessary, supplemental nutrient or buffering chemicals may be blended into these wetting streams to enhance biological activity.
Extending beyond these heuristic details, though, there are at present relatively few specific criteria for designing or operating biofilter units, such that their design and construction represents, more a heuristic art than a codified engineering science. Indeed, the state of the art for this technology is still being actively refined, covering a wide range of design and operational factors (e.g., optimal bed depths and configurations relative to minimizing undesired short circuiting, airflow per unit volume of media, throughput velocities, necessary rates of water addition and scrubbing, nutrient and buffer requirements, bed media longevity and replacement practices, tolerance levels with loading variations, microbial seeding and startup requirements).
Undoubtedly, though, one of the most important variables with any biofiltration system is that of its media form and composition. In general, three different types of media are used in biofiltration reactors: (1) synthetic random-packed plastic media, (2) inorganic rock-type media (e.g., lava rock, limestone, clay, shale), and (3) organic plant- or tree-derived media (e.g., shredded or chipped roots and stems, as well as bark). The issue of media packing itself can impose a considerable resistance to the passage of the gas stream, such that it will have to be effectively pushed through the bed using a mechanical air pump or rotary blower. In many instances, though, coblended columns (see Figure 16.59) have been constructed successfully with various blends of organic and inorganic materials (e.g., shredded roots, stems, and bark plus limestone and clay admixtures), with the overall goal of providing an enhanced surface area conducive to biofilm growth, as well as a higher moisture retention capability and enhanced buffering capacity. In addition, these blended natural ingredients may contribute secondary benefits that further enhance metabolic efficiency. For example, the concomitant breakdown of plant- and tree-based support materials may beneficially serve as secondary organic substrates that promote complementary microbial (e.g., fungal) growth as well as potentially supplying a fraction of the metabolic nutrients required. Hardwood materials passed through shredders and grinders probably appear to represent one of the more desirable natural options, given their perceived durability and resistance to rotting associated with prolonged wetting while favoring biofilm attachment and retention. In addition, inorganic media additives, such as limestone and clay, may also provide a beneficial buffering capacity, which in the case of ammonia or sulfide hydrolysis may comprise a particularly important benefit.
With time, though, the historical pattern observed with all of these natural media is that they tend to degrade in terms of media depth and overall performance, at which point they would have to be supplemented with fresh media to rejuvenate the bed. During large-scale bed replacement with new media, partial blending with some of the older media would help to secure beneficial recycling of microbial seed. Here again, the science behind media replacement is still in a research stage, but 5 years is generally believed to be a reasonable time frame.
Yet another aspect that has not yet been fully explored is that of the presence and role of microscale metabolic niches that develop inside the biomass contained within biofilters. It
is highly likely that metabolically complementary anaerobic-aerobic mechanisms will develop in these sites, whereby special biodegradation pathways could be developed to attack normally recalcitrant chemicals (e.g., reductive dehalogenation of TCE), followed by aerobic degradation of the partially dehalogenated products).
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