Bioreactors

In air pollution control, bioremediation is the use of bacteria and fungi to consume pollutants from an air stream contaminated with organic compounds that are soluble in water or can be solubilized in water. Bioremediation also includes chemical transformation for some inorganic compounds like sulfides and nitrogen oxides.

Device Types

Biofilters, biotrickling filters, aerobic digesters, and bioscrubber technologies include a wide range of methodologies that are applicable to a diverse spectrum of air-quality applications. Each device type has advantages and limitations. These will be described below.

This chapter also describes recent breakthroughs in bioreactor technology and the combination bioreactors with other air pollution abatement technologies. These breakthroughs include the use of surfactants to expand the range of organic compounds that can be treated with bioreactor methods and breakthroughs that utilize biological organisms in temperature and pH ranges not typically used in biological abatement of air pollution.

After defining the basic vocabularies associated with these methodologies and outlining the fundamental requirements for their success in air-quality applications, this chapter will explain some of the ways these technologies can be combined with conventional air-quality remediation methods to further optimize cost and performance.

Bioremediation

The processes are described as follows.

  • 1. Biofilter: An in-ground or above-ground filter bed through which moist air at ambient temperature containing water soluble volatile organic compounds or sulfur-containing compounds like hydrogen sulfide (collectively air pollutants) pass. The bed is made from porous organic or inorganic media that act as a substrate for bacteria or fungi. The air pollutants are removed from the air by transferring to moisture surrounding the microorganisms which are sequestered to the media bed.
  • 2. Biotrickling filter: Similar in design to trickling filters used in the treatment of wastewater but differing in application because these devices treat air laden with pollutants described in the biofilter definition above rather than pollutants dissolved in a liquid. Biotrickling filters are typically round vessels with cone-shaped bottoms and no top. A packing bed typically between 5 and 15 ft deep that is made from plastic media with approximately 95% open area is supported above the cone-shaped bottom section of the tank. The packing bed remains moist through recirculated spray of liquid collected in the cone-shaped bottom section of the tank. The sprayed liquid trickles over the packing bed. Air laden with pollutants is passed through the moist packing bed in a countercurrent direction (compared with the flow of the trickled liquid). The air is cleaned as it passes through the packing bed when pollutants are transferred from the air to moist biological slime attached to the packing material. The pollutants are chemically changed by biological organisms in the slime. Solids are periodically removed from the bottom of the cone-shaped section of the tank and dried prior to disposal.
  • 3. Aerobic digester: A vessel containing aerated liquid with suspended biological organisms that chemically change pollutants described in the biofilter definition above. The liquid can contain free-floating neutral buoyancy porous material that act as substrate for biological organisms. Solids are periodically removed from the bottom of the tank and dried prior to disposal.
  • 4. Bioscrubber: Typically, round tanks with lids that contain a packing bed and recirculated liquid system like the design described in biotrickling filter above. Air laden with pollutants described in the biofilter section above is passed through the moist packing bed in a concurrent or countercurrent direction (compared with the trickling liquid). The pollutants are chemically changed by biological organisms in the slime.
  • 5. Biofilter technology: Biofilters use biologic colonies that reside on a supporting substrate (biomass) and are selected for their ability to produce enzymes that reduce absorbed organic pollutants to less hazardous or less volatile forms. The biofilter itself is a combination of adsorber (the medium on which the bacteria colonize provides an adsorption surface) and absorber (the moist biofilm on the medium surface absorbs the contaminants).

Biofilters are considered by some to be green technology, that is, environmentally friendly. The organic chemical action that occurs within a biofilter is often more complex than that of inorganic chemisorption systems. Biofilters, however, can significantly reduce or eliminate the transportation, storage, handling, and use costs of oxidizing and neutralizing chemicals common to competing wet chemical technology.

Typical Applications and Uses

Biofilters are often used to control the emissions of water-soluble or condensable hydrocarbons (such as alcohols), phenols, aldehydes such as formaldehyde, odorous mercaptans, organic acids, and similar compounds. They are used to control emissions from aerosol propellant manufacture and filling operations, meat processing and packing processes, pharmaceutical manufacture (fermenter emissions), and fish and other food processing sources.

Candidate pollutants that can be controlled by biofilters, in general, must be water soluble because the biodegradation occurs in the moist biofilm layer supported in the biofilter. Aliphatic hydrocarbons are generally more easily degraded than aromatic hydrocarbons. Halogenated hydrocarbons show an increased resistance to this method as their halogen content increases, although some exceptions exist.

A typical biofilter is shown in cutaway format in Figure 3.1. The basic components consist of a humidification system to produce a saturated gas stream (to the lower left), a substrate to support the biomass, a containing vessel, and some means (such as a fan, upper right) to move the gases through the biofilter.

FIGURE 3.1

Biofilter. (Monsanto Enviro-Chem Systems, Inc.)

Biofilters have also been used to control the emissions of propane and hexane from the filling of aerosol cans. These systems can be built into the ground, so the containing vessel becomes the surrounding earth. Buried distribution pipes introduce the contaminated gas beneath the biomass and its support. The gases percolate and diffuse through the biomass layer.

Some meat packing facilities ventilate their meat processing devices (cookers, etc.) into biofilters for odor control. More intense odors are controlled using packed towers, tray towers, fluidized bed scrubbers, and varieties of spray-type devices where oxidizing chemicals are used. These devices can be followed by biofilters, however, wherein the latter act as polishers to remove residual pollutants.

Operating efficiencies of 70%-90% are obtainable with a professionally designed unit with higher efficiencies available if extended residence times are economically feasible. These efficiencies, in the United States at least, are often less than the levels required by the regulatory authorities; therefore, biofilters are not as popular here as in other countries.

To be successful, a biofilter must be used under conditions that are conducive both to the viability of the biofilm and to the absorption of the contaminant. Typical biofilters are operated under 100°F and at 100% relative humidity. They usually operate using a preconditioning spray chamber or scrubber to ensure high humidity. Because the resistance to gas flow through a biofilter is significant, they are often exceptionally large devices. Sometimes, an entire field containing underground distribution pipes is used to provide an adequately large and stable biomass.

Biofilters are used in applications wherein the gas stream does not contain compounds that are toxic to the bacteria and does not contain high particulate loading, and where the gas stream temperature and humidity can be controlled within a range suitable for sustaining the bacteria colonies and where the concentration of pollutants is sufficiently low and sufficiently consistent so that the bacteria colony is not overwhelmed or starved. These conditions vary based on application and bacteria or enzyme selected. Table 3.1 is a list of popular pollutants that are treatable using biologic methods.

Operating Principles

Bacteria that produce enzymes suitable for the oxidation or reduction of the target pollutant are harnessed to do the work in biofilters. They represent millions of tiny catalytic oxidation sites that in most cases take oxygen in the gas stream and fix it to the pollutant to mineralize it (convert the pollutant to C02, water, and innocuous residuals). Some bacteria strains use their enzymes to cleave organic molecules or extract specific elements (such as sulfur), thereby changing the characteristics of the contaminant molecule.

TABLE 3.1

Common Pollutants Recognized as Biodegradable

Acetone

Heptane

Acrylonitrile

Hexane

Anthracene

Isopropyl acetate

Atrazine

Isopropyl alcohol

Benzene

Lindane

Benzoic acid

Methylene chloride

Benzopyrene

Methyl ethyl ketone

Butanol

Methyl methacrylate

Butylcellosolve

Naphthalene

Carbon tetrachloride

Nitroglycerine

Chlordane

Nonane

Chloroform

Octane

Chrysene

Pentachlorophenol

p-cresol

Phenol

DDT

PCB

Dichlorobenzene

Pyrene

Dichloroethane

Styrene

Dioxane

Tetrachloroethylene

Dioxin

Trichloroethylene

Dodecane

Trinitrotoluene (TNT)

Ethylbenzene

Vinyl chloride

Ethyl glycol

Xylene

Source: Derived from information from Microbac International, Bioremediation: A Desk Manual for the Environmental Professional, by Dennis Schneider and Robert Billingsley (Cahners Publishing), and from the Handbook of Bioremediation, by Robert S. Kerr (ed.), (Lewis Publishers).

Several firms have developed specific bacteria strains and/or enzymes tailored to the control of pollutants. If the gas stream can be conditioned to provide an environment wherein this bacteria strain or its enzymes can be sustained, the application is a candidate for biofiltration.

Figure 3.2 shows the basic components of an above-ground biofilter. It consists of a preconditioning and humidification chamber to raise the gas relative humidity to 100%, a gas distribution system or header, large vessel containing a mixture of organic material that both supports the bacteria colonies and provides food, the bacteria dosing system, and a condensate return system.

The mixture of organic material the bacteria in the biofilter adhere to is called biomass. This biomass may be cellulose or similar wood-based material, peat, carbon (or charcoal), straw, waste organic material, or plastic material (like scrubber packing) or mixtures thereof designed to support the bacteria colonies. Generally, a thin-wetted layer called a biofilm is formed throughout this medium, thereby extending the film's surface area (this is akin to the use

FIGURE 3.2

Biofilter components. (Monsanto Enviro-Chem Systems, Inc.) of packing in a packed tower). Because the bacteria usually enjoy a warm, moist environment, the humidification spray is used to prevent the biomass from drying out, thereby killing the bacteria. These reactions occur in a moist environment in the biofilm, therefore, the pollutant must be soluble in water and be absorbed.

The contaminant gas is absorbed into the moist biofilm and enzymes secreted by the bacteria reduce or oxidize the contaminant. Given adequate time, the hydrocarbons are converted to carbon dioxide and water vapor. In some cases, they are converted to methane gas, much as in a biologic water treatment system.

In some biofilters, the specific enzyme has been extracted remotely and a concentrated solution of that enzyme is used to coat a supporting medium (such as cellulose gauze). The enzyme fixes oxygen in the air to the hydrocarbon, thereby oxidizing it without depletion of the enzyme itself. In this way, the enzyme is considered to catalyze the oxidation of the contaminant. Figure 3.3 shows a compact gas cleaning device using an enzyme solution supported on a gauze-type medium.

FIGURE 3.3

CAP™ "Clean Air Plant" compact biofilter (SRE, Inc.).

 
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