Primary Mechanisms Used

Absorption is the primary mechanism for the movement of the contaminant from the gas phase into the biofilm. Biologic oxidation occurs using enzymes (called oxygenase). This is the key mechanism for the oxidation of the contaminant once it is absorbed. Enzymes in the bacteria strain act as catalysts to fix oxygen to the contaminant, thereby oxidizing the latter. Some bacteria strains fix other chemicals to the contaminant where a reducing reaction follows. They basically extract a portion of the contaminant, for example sulfur in a mercaptan odor, changing the odorous compound's structure.

For long-chain hydrocarbons, a stepwise cleaving process can occur. Over time, the secreted enzymes break the hydrocarbon chain into smaller components that eventually result in CO, and water. These processes occur naturally in the environment. In the biofilter, conditions are created and maintained to make these processes occur more rapidly.

The gases typically mix through diffusion because the gas velocities are exceptionally low (to reduce pressure drop as well). Impaction and interception are minor in a biofilter given the extremely low vapor velocities at which these devices operate.

Design Basics

In mechanical function, biofilters can be compared to packed towers. The bio- mass-support medium is the packing, and the biofilm is the absorbing liquid. In the case of biofilters, however, the biofilm is stationary. It is attached to the support medium. The gas, therefore, is caused to move slowly through the biomass so that the contaminant gas can diffuse over to the biofilm surface, and time is allowed for the gas to penetrate the biofilm surface. As a result, gas velocities are under 1-2 ft/s. Biofilters therefore are generally large devices.

They need not be, however, unsightly. Figure 3.4 shows an above-ground biofilter, the housing of which has been designed for function and appearance.

This design is built in modular components to reduce costs and speed installation time. The upper vessel is made from fiberglass-reinforced plastic and is sloped to allow for strength and draining of snow and rain. It sits on a lined concrete basin, which provides structural support and houses the gas distribution system.

Because the bacteria strains used are living organisms, they require a suitable living environment to survive. This usually results in a requirement of humidifying and sometimes heating or cooling the gas stream within a narrow operating window to suit the bacteria strain used. Inlet relative humidity is usually above 95% and the temperatures are 60oF-110oF. Reduced

FIGURE 3.4

Modular biofilter (Envirogen).

moisture can dry out the biomass and excessive temperatures can kill the bacteria. The pH is usually 6-8, although some bacteria strains can function at a pH of 4 to as high as 10. One disadvantage most bioscrubbers have over other technologies described here is the adjustment of pH in the beds. Some bacteria generate acidic byproducts that lower the native bed pH. The acidity could decompose some bed types. This problem gradually increases the differential pressure required to pass gas through the bed and then eventually destroys the bed structure.

The device also must be designed to be replenished. Access doors must be provided, but adequate pull space must also be provided because biofilters are often bulk loaded with biomass support material that is dumped into place and distributed. For this reason, above-ground biofilters often are configured with driveways next to them allowing for mechanical removal and replacement of the substrate into dump trucks or other hauling devices.

Operating Suggestions

It should be clear from the previous comments that biofilters must be operated within their thermal and humidity window. Care should be taken to provide a reliable supply of humidification water and supply a suitably insulated vessel if cold environments are to be encountered.

For hard water, the use of softened water in the humidification system may be advised to reduce nozzle plugging. If a packed-type humidification device is used, periodic checks should be made regarding the packing condition. The packed zone's pressure drop should be monitored, and the packing replaced if the pressure drop rises above the vendor's prescribed figure.

The condensate from the biofilter should be accumulated and, if recycled, excessively large solids sent through a strainer or filter to prevent nozzle plugging. If the humidification system is lost, the biofilter can be lost. Recycled condensate can also be pH adjusted to partially reduce the effects of acidic buildup in the bed caused by some bacteria.

It is not uncommon with biofilters to experiment with various bacterial cultures and substrates. In part, this may reveal the art side of the science. The reality is that certain bacterial cultures respond to specific pollutants. When a mixture of pollutants is present, problems can result. Patience is therefore an asset if one is trying to tackle a multiple pollutant stream.

It is suggested that the temperature of the post humidification section and the bed temperature should be monitored. The post humidification section should be at the wet bulb temperature or within 2°F-3°F thereof. This indicates near saturation. The bed temperature reflects the bacterial living conditions. The bacteria culture supplier will have a design range within which to operate.

Aside from the service accessibility issues and preconditioning requirements mentioned previously, the biofilter can be operated as any other absorber.

Biotrickling Filter Technology

This device utilizes the same biological mechanism described above in the biofilter section, so it will not be repeated here. This section will point out conditions where this technology has advantages and limitations when compared to biofilter technology.

Devices of this type are freestanding tanks with cone-shaped bottoms. In many applications, the cone-shaped bottoms are buried in the ground. Because these devices have three to five times more packing bed depth than biofilters, they have a longer residence time. This additional residence time increases the interaction between the foul air and the biological media, and that increases the removal efficiency. Furthermore, because the properly maintained media bed used in biotrickling filters has more open space than the media in biofilters, there is less differential pressure required to move the air through the bed (even though the bed is several times deeper) and that saves electrical energy.

Biotrickling filters use recirculated liquid that bathes the entire bed in a uniform way. This provides the ability to regulate the pH of the process in a way that is difficult for biofilters.

Aerobic Digester Technology

There is much literature available on aerobic digester design and operation, so those details are not included here. This section focuses on the innovative applications of aerobic digestion in air quality applications.

Although relatively new as a component in air quality, this technology has been used for over 100 years in wastewater treatment. Pacific Rim Design & Development and other firms successfully integrate this technology with bioscrubber technology and other technologies to treat more effectively a foul air stream that contains mixed organic compounds. The bioscrubber treats the volatile organics with high solubility in waterlike alcohols, and the aerobic digester treats the larger, often less soluble organic compounds like terpenes (organic molecules associated with wood) that require longer residence time for destruction.

The range of compounds that can be treated by aerobic digestion is increased using carefully selected surfactants that increase the solubility of organic compounds without adversely affecting the microbial colony.

The addition of neutral density foam or other material increases the aerobic digester efficiency when it is utilizing biological organisms that prefer to form slimes or colonies. This material is not necessary for organisms with a preferred free-floating habitat.

Particulate material in a waste gas stream will clog most wet scrubbers. This problem is turned into an attribute when an aerobic digester is part of an integrated treatment process and the particulate has a density that is near neutral. As an example, Pacific Rim Design & Development developed a Triple Integrated Process to treat waste gas from engineered wood facilities. In this application, three processes were combined into a single vessel. The waste gas laden with sawdust, alcohols, formaldehyde, and larger organic compounds was first treated with mist that contained surfactants in a downward direction as it passed through a tube at the center of the reaction vessel. This first treatment cooled the gas, agglomerated the particulate material, and enhanced solubility of the larger organic compounds when they reached the aerobic digester. The agglomerated material was forced to drop into an aerobic digester at the bottom of the reaction vessel when the gas made a 180° turn upward into the space between the outer wall of the reaction vessel and the outside of the central tube within the reaction vessel. A bioscrubber in the shape of a ring between the inner tube and the outer wall of the reaction vessel is used to remove the more-water-soluble compounds. Recirculated liquid from the aerobic digester is used to provide moisture and nutrients to organisms in the bioscrubber. The recirculated liquid is filtered prior to being sprinkled over the bioscrubber. The filtered material can be dried and used as fuel in some boilers.

In an integrated system that includes a bioscrubber and aerobic digester, the large thermal mass of an aerobic digester can act as a heat sink to stabilize temperatures in the bioscrubber during shutdowns of short duration.

The heat is distributed to the bioscrubber through recirculated liquid from the bioscrubber.

Integrating more than one biological process into a single treatment has real advantages but also requires a careful study of the habitat requirements for each biological group. This process is most successful when all integrated processes are optimal at the same temperature and pH, and are symbiotic with respect to nutrients, growth patterns, and more.

Aerobic digester design must include a means of adding oxygen to the liquid in its sump. This can be done through the injection of air or oxygen into the sump in a way that promotes mixing. Aerobic digesters used as part of an integrated waste gas treatment process are contained within a vessel, so they must rely on mechanical aeration.

Engineering an integrated system requires a careful study of required residence times and sequences of treatment. This topic is more variable than space is available in this chapter.

Bioscrubber Technology

Just as the biotrickling filter is an enhancement over the biofilter, the bioscrubber is an enhancement over the biotrickling filter. One large difference is that bioscrubbers are contained within a closed vessel and the biofilters and trickling filters are open to the atmosphere. The closed design of the bioscrubber easily allows them to be part of an integrated treatment system. An integrated system can be sequential bioscrubbers, with similar or different microbial environments or an integration with other types of abatement equipment as described under the proceeding section on aerobic digesters. The ability of bioscrubbers to treat in sequential stages also provides them with the ability to treat a wider range of organic materials. This is particularly useful when a waste gas stream has pollutants that are not treated by the same type of organism. In these situations, each stage hosts an organism or group of organisms that effectively target one of several compounds in the waste gas stream.

Bioscrubbers generally have greater removal efficiency than biofilters and biotrickling filters because bioscrubbers have longer residence time for reactions between the waste gas and microbes. In standalone operations (not integrated with other air quality technology), the sump could act as an environment for additional biological treatment. However, the sump is limited by available oxygen in the liquid, so there is limited value in building a sump with more than 50 times the volume of liquid that is being recirculated to the packing bed.

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