Figure 1 shows a generic gas treatment bioprocess along with the nomenclature to be used in the subsequent analysis. Gas treatment bioreactors are sometimes described as gas-phase bioreactors, but this term is inexact in the sense that the actual reactions happen inside a microbial cell, which is necessarily an aqueous phase. Some reactions carried out in immobilized enzyme reactors do seem to involve direct interaction between the gas and the enzyme (33), although even here the humidity remains a critical variable and the enzyme may be covered with a layer of water a few molecules thick. For our purposes, the bioreactor is divided into a gas phase, a solid phase consisting of any solid packing or biofilm support particles, and a biophase, which contains the water and the microorganisms. The fraction of the bioreactor volume occupied by each phase is called its holdup, and the three values, eg, es, and eb, must add up to 1.
Effluent gas Contaminant partial pressure = p Volumetric flow rate = g Temperature = T Pressure = 1 atm
Nutrient or product concentration = Sji Flow rate = f + w
Unit volume of bioreactor
Gas volume = eg Biophase volume = eb Solids volume = es Viable cell concentration in biophase = Sx
Influent gas Contaminant mole fraction = ni Pressure = 1 + h atm
Liquid outflow Flow rate = f Viable cell concentration = zSx Nutrient or product concentration = Sj
For convenience, we think in terms of a bioreactor of unit volume and express the gas flow rate as the volume per volume per minute (VVM) flow rate, g, at the effluent gas conditions, taken as temperature T and atmospheric pressure. Others choose to work in terms of the molar outlet gas flow rate (g/RT, where R is the gas constant) or a gas residence time, either the empty bed residence time, 1/g, or the average time an element of gas actually spends in the reactor, eg/g. The flow rate of the influent gas is not the same as g because its temperature may not be T, it is at a higher pressure, (1 + h atmospheres, where h is the pressure drop through the reactor), and because various components (contaminant, oxygen, metabolic products, water vapor) are added to, or removed from, the gas stream as it flows through the reactor. When these effects are significant, it is essential to specify exactly what is meant by "gas flow rate."
The objective of some processes is to remove a gaseous contaminant by converting it into a soluble, nonvolatile metabolic product. Some such products, H+ for example, can be neutralized in the bioreactor, but others, such as the Cl~ from the degradation of chlorinated organics, must be removed in the liquid if they are not to accumulate to concentrations that inhibit the microorganisms. There may therefore be a liquid outflow, f per unit bioreactor volume, and an inflow that must be greater than f to allow for the evaporation rate, w. This outflow tends to wash microorganisms out of the bioreactor, and it is usually beneficial to retain them either by cell recycle or immobilization, making the parameter z in Figure 1 less than 1.
The question is what type of bioreactor should be put in the "black box" in Figure 1. Almost every possible configuration by which a gas can be contacted with a biofilm or a suspension of microorganisms has been employed in some application, and these can be roughly divided into four categories.
The earliest gas-treatment bioreactors were used to control odors in air coming from slaughterhouses and rendering plants and consisted of porous pipes buried in the ground, the noxious compounds being removed by a combination of adsorption and biodegradation as the air flowed up through the soil (26). Applications of the idea have since expanded to include the removal of hazardous, volatile compounds from chemical plants, paint shops, foundries, and a variety of agricultural and food-processing operations. Although often effective, the operation of these simple soil beds depends critically on the local soil and weather conditions. Unless they are covered and/or periodically sprayed with water, they may stop working, either due to becoming waterlogged or to drying out. They have largely been replaced by custom-made biofilters, which are essentially boxes made of plastic, wood, sheet metal, concrete, or even stainless steel, containing a bed ofmedia and a gas distribution system to ensure uniform flow of the gas through the bed.
In terms of the variables defined in Figure 1, biofilters are the small eb, small f and small z approach. The biophase consists of individual microorganisms attached to the media, perhaps enough to constitute a thin biofilm. The amount of moisture in this layer is critical, and is controlled by humidification of the inlet gas and/or by spraying water directly over the bed. Little or no water drips out of the bed, making it difficult to provide soluble nutrients and pH control chemicals, or to wash out nonvolatile metabolic products. Biofilters are thus best suited to applications that generate no such products and involve no pH swings, the treatment of air streams containing low levels of volatile organics being the obvious example.
The great advantage of biofilters over the early soil beds is that they allow the choice of the nature and particle size of the bed media. This media must support a high density of attached microorganisms suited to the particular application, perhaps even providing some of the nutrients needed for their growth. The ability to adsorb the contaminant is an advantage because adsorption can provide some contaminant removal during the start-up period, before a large population of well-acclimated microorganisms has developed (sometimes called "the bed-ripening period"), and during any sudden slugs of concentrated contaminant in the influent gas. Also, although the interactions between such adsorption and biodegradation are complex and poorly understood, they are generally favorable to biofilter performance. Finally, the media must be mechanically strong and resistant to disintegration, compacting, and the resulting channeling of gas flow. Soil, compost, peat, and wood-chip mixtures are all inexpensive media that have been used successfully. Materials such as activated carbon and limestone, although more expensive, may be mixed in to provide extra adsorption and pH buffering.
The particle size of the media is critical. Small particles give a huge gas-biophase interfacial area per unit bed volume (a quantity subsequently called "a"), thus eliminating concerns about mass-transfer limitations between the two phases. But beds of small particles have a higher resistance to gas flow, are more prone to waterlogging and plugging by excess biofilm, and if too light, may get blown out of the reactor altogether. A general guideline is to choose the smallest available particles that avoid the latter problems, and then ensure that the shape of the bed is optimized to give the desired contaminant removal with a reasonable pressure drop (see "Scale-Up").
If the life of the bed is found to be finite due to compaction, loss of pH buffering capacity, accumulation of salts from the evaporation of added water and so on, one solution is to employ two beds in series. Bed 1 can provide most of the contaminant removal, while bed 2 polishes the effluent, mainly by adsorption, while developing its own population of well-acclimated microorganisms. When bed 1 is exhausted it can be replaced by bed 2, which is itself replaced by a bed of fresh media. The arrangement of the media on a series of trays in the bioreactor facilitates this mode of operation.
Conventional bubble-aerated fermenters make poor gas treatment bioreactors because they are designed for a high rate of transfer of gaseous nutrients and products (O2 in and CO2 out), rather than a high fractional removal of a component from the air stream. This latter objective requires taller bioreactors with longer gas residence times, but tall reactors mean higher pressure drops due to the hydrostatic head. Nevertheless, tall, mechanically agitated bioreactors with multiple impellers have been proposed for removing H2S from industrial gas streams (29), and bubble columns with no mechanical agitation have been demonstrated for the removal of TCE from air using a toluene-oxidizing organism (30).
These bioreactors are in many ways at the opposite extreme from biofilters. The biophase occupies most of the reactor (et, r 1 in Fig. 1), and it is dilute and well-mixed rather than dense and immobilized. The large volume of water makes it easy to add soluble nutrients, control the pH, and remove water containing nonvolatile products. However, care must be taken to not wash out the biomass, because z = 1 (Fig. 1) unless there are extra surfaces in the bioreactor for the attachment of a biofilm, or if a cell separation and recycle loop is added. Another major difference is that the gas-biophase interfacial area, a, is not only much smaller than in a biofilter, but is not even a constant, varying with the gas flow rate. Bubble gasified bioreactors are therefore best suited to contaminants like NOx that are very soluble in water (thus reducing concerns about the gas-liquid mass-transfer rate), whose dissolution in water cause pH swings, and whose metabolism requires large amounts of dissolved nutrients.
Trickling filters for gas treatment are similar to, but usually taller than, those in use for decades for the secondary treatment of municipal and industrial wastewaters. Like biofilters, they consist of a bed of media through which the gas flows either upward or downward, but unlike biofilters, the microbial culture is recirculated continuously over the bed from a reservoir beneath it. Stone aggregate or wood chips are the traditional media, but even seashells have been used (27). Most modern filters employ ceramic or plastic media, Pall rings, Raschig rings, and so on specifically designed to make a bed with a high surface area to maximize gas transfer, and a high porosity to minimize the pressure drop and the chance of clogging or flooding (28).
These bioreactors can be thought of as intermediate between biofilters and bubble-gasified systems and are best suited to applications between those already described. If the characteristics of the media and the microorganisms are such that most of the biomass stays attached, then a trickling filter is essentially a biofilter with a higher liquid flow and a smaller gas-biophase interfacial area, a, due to the larger media. If on the other hand, most of the biomass stays suspended and the recycle ratio (RR = flow of recycled liquid//) is high enough to keep the biophase well mixed, then it is functionally similar to a bubble-gasified bioreactor. The main differences are that the pressure drop, h, is usually much smaller, and the interfacial area, a, is a constant that is independent of the gas flow rate.
The systems just described essentially combine two functions, stripping the contaminant from the gas into the aqueous biophase and then subjecting it to biodegradation. But what if the physicochemical conditions in the biophase resulting from the stripping are not suitable for the micro-bial metabolism? The removal of sulfur dioxide from stack gas is a good example because stack gases contain a small percentage of oxygen that although relatively insoluble, inhibits the bacteria responsible for the reduction of the sulfite formed by the dissolution of SO2. One solution is the bioscrubber, which consists of an aqueous stripper in which the contaminant is transferred from the gas to water, and a separate bioreactor, in which the liquid effluent from the stripper is essentially treated as wastewater. The bioreac-tor should be a completely mixed, immobilized-cell type so that a large population can grow up in a well-controlled environment without continuously being recirculated through the potentially toxic environment in the stripper. Several configurations are possible, the simplest being that in which the liquid reservoir at the bottom of a packed-bed stripper is used as the bioreactor. Bioscrubbers are not considered further here because building a separate bioreactor and stripper is necessarily more expensive than combining them in a single unit, and when it is necessary, the two unit operations can be designed by conventional methods.
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