A biofilter is currentlyin use at the Idaho asphalt plant ro control odors (Fig 1)
Biofiltration is being used by Idaho Asphalt Supply Inc.`s Blackfoot, Ida., asphalt plant to control odors.
The odors come from off-gas emissions from the blending of raw asphalt and polymer additives.
Biofiltration shows promise for being an effective tool to control odorous plant emissions for the asphalt industry. Eliminating odors allows the placement of asphalt facilities in less-remote settings. Such placement will reduce transportation costs since plants can be located closer to population centers where asphalt materials are needed.
The development of the biofiltration technology in this asphalt application was a joint effort by Idaho Asphalt Supply Inc. (IAS) and the Idaho National Engineering & Environmental Laboratory (Ineel).
In 1994-95, IAS was faced with a growing number of complaints from local citizens, county commissioners, and the state environmental compliance agency regarding foul odors. To address these concerns, IAS installed an odor-suppression system consisting of three separate granular-activated carbon (GAC) beds to treat the plant`s primary emission sources.
The primary sources are:
- Vented headspace gases from storage tanks of raw asphalt cement
- Finished asphalt products (raw asphalt mixed with assorted additives)
- The blending operation, which produces polymer-modified asphalt.
Plant management considered the blending operation to be the biggest contributor to the plant`s foul odors. Emissions from the storage tanks are continuous, whereas the blending process is a periodic batch operation.
Although the GAC odor-suppression system initially worked well, all three beds quickly became overloaded and ineffective as a result of the high emission loads. Furthermore, the carbon bed receiving emissions from the asphalt-polymer-blending process ignited, creating a safety hazard.
Scientists and engineers in the Ineel Biotechnologies Group teamed with IAS to solve the odor problem using biofiltration. Although biofiltration has been used extensively in other industries as a safe, effective, environmentally benign technology for odor control,1 no documented accounts exist describing its use for treating asphalt-plant odors.
In response to IAS`s urgency to address its odor problem and the need for a fire-resistant technology, the team designed, built, and tested a full-scale biofilter (Fig. 1) for treating emissions from the raw asphalt-blending unit.
After the first full season of operation in 1997, the team concluded that biofiltration was effective for treating the emissions from the polymer-blending operation. IAS is currently evaluating biofiltration for treating emissions from the other plant sources as well.
Biofiltration technology
In biofiltration, emissions are passed through a bed of porous, biologically active material, such as peat, soil, or compost. Microorganisms in the active material oxidize odorous compounds to sulfate, carbon dioxide, and water.1 In this case, primarily hydrogen sulfide (H2S) and hydrocarbons were oxidized.
System components include:
- Bed material
- A gas-tight container to hold the bed
- A gas-distribution system to evenly distribute the emission gases across the cross-sectional area of the bed
- Equipment to precondition the gas stream to appropriate temperatures and relative humidity levels prior to entering the biofilter (Fig. 2)
- Equipment to monitor bed-medium moisture and temperature, inlet-gas moisture and temperature, and inlet and outlet pollutant concentrations.
Amendments to the bed medium are often used to improve gas-flow characteristics, replenish nutrients, and neutralize acidic byproducts. Amendments include items such as bark chips, slow-release fertilizers, or crushed limestone.
Biofiltration offers several advantages. It requires lower capital and operating costs than conventional technologies.2 It also completely degrades (oxidizes) odorous compounds, which eliminates the need for costly waste treatment or GAC regeneration.2 In addition, the technology`s passive operation (that is, no high temperatures or pressures) offers increased safety.
Design and construction of IAS biofilter unit
The biofilter unit at IAS`s Blackfoot plant (Fig. 1) was constructed in the spring and summer of 1996 using readily available, inexpensive commercial-grade materials.
The unit is rectangular and accommodates 1,536 cu ft of bed material.
The floor and three walls are constructed of reinforced concrete and coated with epoxy to resist corrosion from acidic byproducts. The remaining wall and roof are constructed of removable plate steel, which allows easy access for manipulating the bed material.
The floor is configured to facilitate drainage of accumulated water. A spray nozzle system, attached to the inside walls, can be used for water addition as necessary. The roof is pitched to facilitate condensate drainage to an inside gutter, which diverts water to the unit`s exterior.
Inside the unit`s shell is a gas-distribution manifold system and a bed of biologically active material. The manifold system is made of polyvinyl chloride (PVC) piping, covered in a layer of smooth sewer rock (1.5-in. nominal diameter). It is configured to evenly disperse incoming gases across the unit`s bottom surface and ultimately up through the bed.
Holes drilled in the piping, along regular intervals at the 4, 6, and 8
o`clock positions, allow emission gases to flow from the manifold upward through the overlying bed medium.
Above the manifold system is the bed medium, composed of a proprietary mixture of wooden materials, crushed limestone, and slow-release fertilizer.
For the biofilter`s first full operating season in 1997, emission gases flowed through the unit`s 1,536 cu ft bed at a rate of 250 cfm, yielding a 6.1-min empty bed-residence time. Prior to entering the biofilter, emission gases were cooled to 80-100° F. and humidified to 95-100% relative humidity (RH).
Unit operation
During the 1997 operating season (March to October), test data showed that the biofilter generally removed 40-100% of the incoming H2S (Fig. 3). Incoming H2S concentrations were variable. They ranged from a low of 16 ppm(vol) to a recorded high for one polymer-modified asphalt batch of nearly 30,000 ppm(vol).
These inlet concentrations translate into H2S loading rates up to approximately 1 lb H2S/cu yd of bed medium/hr.
During the 1997 operating season, the bed medium became acidified, and the pH of the unit`s effluent decreased to 2.0 as a result of complete H2S oxidation to sulfate (SO42-). This effluent consisted of condensate from the incoming emission stream and excess water added to the bed for moisture control. This effluent stream was pH-adjusted and recycled into other plant operations.
The low pH in the bed material was not a problem because the microorganisms degrading the H2S thrive in acidic conditions. Resistance to acids extends the life of the bed. Since longer bed use translates into lower operating cost, the bed medium installed in the biofilter in 1997 will continue to be used until the odor-removal efficiency drops considerably.
Removal of hydrocarbons by the biofilter was negligible, probably due to the acidified bed which was not conducive to the growth of hydrocarbon-degrading bacteria. From an odor-control standpoint, the lack of hydrocarbon removal was not a serious concern since H2S was felt to be the major cause of foul odors, and the biofilter proved effective in removing most of the H2S.
The biofilter`s sprinkler and gas-pretreatment systems were also evaluated. The two systems were effective in keeping the bed-medium`s moisture level at about 60% (water mass/wetted bed-medium mass), a level acceptable for bacterial growth and good biofilter operations.2 3
Inlet-gas temperature control was more challenging. The pretreatment system could not sufficiently maintain an 85° F. set point during mid-summer, daytime operations when polymers in a latex matrix were blended with raw asphalt. The large bed served, however, as a heat sink, moderating the high temperatures experienced in the inlet gas and providing a temperature environment suitable for sulfur oxidizing bacteria.
Tensiometers were used to measure water potential. These devices, commonly used in agricultural and horticulture applications, indicate the water content of the media by measuring the suction related to capillary forces.
In this case, bed drying was indicated by a higher tension (vacuum) reading. Ideal water content for the bed material is not well identified for biofilter operations. The extreme conditions are qualitatively easy to identify. If the material becomes too dry, the bacteria will go dormant or die. If the material is too wet, the pressure drop can become excessive and lead to excess water effluent.
Tensiometers proved useful for determining water-addition schedules and for identifying mechanical failures in water-feed lines and sump pumps. The system was generally operated at tensions between 10 and 15 millibars.
Effectiveness
Beyond the need for odor-control efficiency, IAS chose biofiltration for safety and ease of maintenance. Since the bed medium is kept in a very moist state and the inlet-emission gases are humidified, fire hazards are nonexistent.
Bed-medium samples taken at the start and end of the short 1996 operating season were analyzed per EPA`s toxic characteristic leachate procedure (TCLP) to determine if the medium exhibited a characteristic of hazardous waste under the Resource Conservation and Recovery Act (RCRA).
No TCLP constituents were detected. Therefore, when the bed medium is replaced, no special handling or treatment of the old medium is required. Plans are to use it as a soil additive for landscaping at the IAS plant site.
Safety is also realized, since the biofilter operates close to ambient conditions; that is, no extreme temperatures or pressures are present. Also, operating the unit creates no additional noise pollution. Finally, no exotic microorganisms are used, since exposure of the bed medium to odorous emissions enriched it with naturally occurring microorganisms which are capable of obtaining energy by metabolizing emission compounds.
The plant`s effectiveness at odor control was determined following the biofilter`s first full operational season (1997) by surveying the nearby residents. Approximately 400 residences, located within 2 miles of the plant and within the primary odor-plume pathway, were sent survey questionnaires. Questions included such things as residence location, length of residence, whether or not odors were perceived at the residence, and conditions associated with plant odors in 1996 before the biofilter was installed vs. conditions in 1997 after the biofilter was installed.
Nearly 50% of the questionnaires were returned. Table 1 summarizes the responses.
All portions of the targeted survey area (i.e., the area immediately surrounding the plant and southwest and northeast of the plant) were well represented.
The vast majority of the residents who responded (>92%) had lived in the area at least 4 years and were equipped to judge the odor conditions before and after the biofilter was installed.
Nearly one quarter of the respondents had never smelled the odors associated with the plant at their residences.
For those residents who claimed that they had smelled plant odors at their residences, roughly half felt odor conditions had improved after the 1997 biofilter installation. Approximately one-third felt conditions were unchanged and the remaining 15% felt odor conditions were worse.
Overall, these results were positively viewed. They indicated to IAS that although progress has been made in controlling plant odors, more work remained to totally eliminate the problem. Based on these encouraging results, IAS is currently evaluating biofiltration for treating emissions from the plant`s raw asphalt and finished product storage tanks.
Advantages of biofiltration
IAS`s experience with biofiltration for controlling plant odors has been positive, and its evaluation for controlling odors in all plant emissions continues. The following advantages have been noted by IAS and Ineel in using biofiltration:
- Lower capital and operating costs compared to other established odor-control technologies.
- Odorous compounds are degraded and not simply transferred to another medium.
- Thus far, spent bed medium does not have to be handled as hazardous waste. No characteristic hazardous waste constituents were detected in bed-medium samples that had been exposed to plant emissions.
- The biofilter operates in a passive nature and does not require or generate any extremely high temperatures or pressures and has no moving parts.
- Materials for constructing and operating the biofilter are readily available, commercial grade items.
- Provided the bed medium is kept moist, the biofilter poses no fire safety problems.
- The biofilter unit operates noiselessly as a result of its passive operation.
- The biofilter`s design and operation are simple. No microbiological innoculum is required since microbes indigenous to the bed medium become enriched and degrade odorous compounds.
- For the asphalt industry, biofiltration makes economic sense because it can help lower production costs. Operation of process plants in less remote settings reduces transportation costs.
Acknowledgments
Funding for this project was provided by Idaho Asphalt Supply Inc. Cooperative work between the Ineel and IAS was performed under DOE Idaho Operations Office Contract DE AC07 94ID13223. The authors thank Gero Leson for his biofiltration advice, especially those pertinent to the bed medium, and Robert Montgomery for his review of the manuscript.
References
- Leson, G., and Winer, A.M., "Biofiltration: An Innovative Air Pollution Control Technology For VOC Emissions," Journal of the Air & Waste Management Association, Vol. 41, No. 8, August 1991, pp. 1045-53.
- Vembu, K., and Walker, C.S., "Biofiltration Holds, VOCs, Odors at Bay." Environmental Protection, February 1995, pp. 27-30, 58.
- Devinney, J.S., "Monitoring Biofilters Used for Air Pollution," Practice Periodical of Hazardous, Toxic, and Radioactive Waste Management, Vol. 2, No. 2, 1998, pp. 78-85.
- Van Lith, C., et al., "Evaluating Design Options for Biofilters," Journal of the Air & Waste Management Association, Vol. 47, 1997, pp. 37-48.
