As environmental awareness increases among the general public so do environmental restrictions among regulators. Odours emitted from food processing plants are becoming more of a concern as food plants and residential areas grow closer in proximity. The objectionable odours in the food industry are generally a result of the physical processing of foods in which biological or chemical reactions form volatile organic compounds (VOC). These reactions are often precipitated by such processes as heating, drying, or smoking of foods. Odour thresholds are subjective among neighbors of processing plants and for this reason standard methods have been approved for testing odours.
The accepted method for quantifying these odours is the use of an odour panel and a testing protocol approved by the American Society for Testing and Materials (ASTM):E-679. Thresholds are determined by selecting random participants and exposing them to different concentrations of odours. These results then determine thresholds for the general public.
Several technologies have been developed or improved recently to control odours from various industries. The best method for controlling odourous compounds is by eliminating the odour before it is formed thus eliminating the need for further processing of the air. This is not an easy task. Many food manufacturers cannot control all of the volatiles released during processing. Most odourous compounds emitted from food plants are not a public health concern but can be considered a public nuisance and, therefore, are subject to local governmental regulations.
Methods for controlling odour include but are not limited to: biofilters, bioscrubbers, activated sludge scrubbers, trickling filter scrubbers, and reactive chemicals. This report summarizes some of these technologies and focuses on the use of biofilters and their applications in the food industry. Technologies for controlling odourous compounds and VOCs are listed in
Table I and must meet the following criteria to be effective in the food processing industry:
Table I -- Comparison of Biofilters to Other Odour Control Methods
|Operating Cost||Material Cost||Maintenance Cost||Pollution Risk||
Removes All Odours
Zeager Bros. Inc., (1992)
Before an odour control/removal method can be selected, odour control objectives must be considered. The source of the odour must be determined. An evaluation of the concentration of VOCs in the waste stream is also necessary. These can be measured using a flame ionization detector (FID) which measures the minimum, maximum, and averaged concentration of VOCs over an extended period of time. Gas chromatography and mass spectrophotometry can be used to identify each individual compound in the stream. The temperature, relative humidity, flow rate, and number of particulates in the gas stream are all variables which must be measured. Once these variables have been identified and measured, parameters can be set for the control of the VOCs and odourous compounds.
Biofiltration is usually applied to gas streams with less then 1000 parts per million (ppm). Most commercial applications are found to be in the range of 5 to 500 ppm. The filtration process is based on the principle that VOCs and odours can be biologically oxidized into carbon dioxide and water by naturally occurring microbes. Bioscrubbers are used to biologically oxidize organic and inorganic compounds in an air stream. Biofilters are best suited for use on air streams containing low concentrations of organic pollutants. The main component of a biofilter system is the filter media which can consists of compost, tree bark, peat, heather or soil, or a mixture of various materials on which microorganisms can grow and metabolize odourous compounds.
Bioscrubbers are used to biologically oxidize organic and inorganic compounds in an air stream. Two units are used in the bioscrubbing process. In the first part of the process, a scrubber is used to absorb soluble waste gases into water. The second process occurs in a basin unit where waste compounds are biological oxidized. Bioscrubbers are used when by-products from oxidation would harm biofilters. These compounds are usually acids produced from H2S and NH3. Types of bioscrubbers include, activated sludge scrubbers and trickling filter scrubbers. In activated sludge scrubbers, gaseous pollutants are absorbed by a solvent in a countercurrent packed column. The absorbing column contains a mixture of sludge and water. Biodegradation takes place in a sedimentation tank. Treated water is recycled back into the packed column for reuse. In trickling filter scrubbing, contaminated gases are absorbed into a liquid phase as in activated sludge scrubbing. The contaminated liquid is then spread over a packed column containing microbes that degrade the pollutants.
The problem that occurs most often with bioscrubbers is an inability to control the biomass growth. An overgrowth occurs when there is an influx of nutrients in the column, which creates a pressure drop in the basin. When nutrient levels are cut off, removal efficiency decreases.
Currently, research is being conducted in the area of trickling filters. There is a potential for the trickling filters to produce the best removal efficiencies once biomass growths can be successfully controlled.
The vast majority of the current biofilter applications associated with odour control in food processing operations are reported to be bulk media biofilters. Bulk media filters use the natural packing material without scientifically engineering it for uniformity and efficiency. Bulk media biofilters are usually built by excavating a hole, inserting a pipe for gas distribution and blowing odourous air through a layer of natural media (soil, compost, peat, wood chips, or a mixture). Whittle (1997) reviewed the use of these filters in food processing plants. He reported finding biofilters used as early as 1957 in California. He noted that hundreds of these were reportedly used in Europe and Japan during the 1960's although there was no detail presented on how many were associated with food processing operations.
Whittle noted that the beds of media are usually about 3-4 feet thick. The dwell time of the air being stripped of odours was reported to be typically in the 30-60 second range. The dwelling rates identified in his study were 3-5 ft3/min/ft2 of surface area. Other operating parameters are listed in Table II. Whittle noted many applications in the food industry. Most of these he identified were associated with odour control in wastewater treatment operations located on food processing sites.
Table II -- Design Parameters for Bulk Media Biofilters
|Bed depth||3-4 ft.|
|Loading rates||3-5 scfm/ft2 SA|
|Dwell time||30-60 sec.|
|Moisture -- soil media compost||10-25 %|
|Temperature||37 oC optimum|
|VOCs||< 500 mg/ft3|
Source: Whittle (1997)
The bulk media biofilters are traditionally designed by plant managers or consultants. They are usually inexpensive, costing less than £6,000. However, control parameters that influence performance such as moisture in the bed and uniformity of media are not able to be controlled in such bulk media biofilters. Inherent problems exist with bulk media filters. Maintenance procedures are usually not developed or adhered to as required. Therefore, as initial performance may be very good, subsequent deterioration in removal efficiencies will probably occur resulting in undesirable performance.
New biofilter systems have been designed to overcome the deficiencies discussed above and help to assume optimum performance over an extended period of time. These biofilter systems have uniform media distribution. Furthermore, installed controls and operating procedures help assure that removal efficiencies remain high. These systems can be designed to remove specific undesirable odourous compounds not easily achieved in the traditional bulk media biofilter. The design and engineering of these systems comes with an increase in costs. Several systems were identified that are either currently in use in food processing operations or could be used in food processing operations.
Newly designed and engineered biofiltration systems use the same principles as bulk media systems. The basic requirements for efficient and reliable media materials are adequate porosity for absorption, homogenous packing materials to provide uniform air flow distribution, adequate surface area and pH buffering capabilities, and the ability to support micro flora population. The media is generally about one meter thick. The contaminated gas is blown through the filter media which contains microbes to oxidize organic material into carbon dioxide and water.
Different chemical compounds require different residence times in the media. Alcohols biodegrade first, followed by ketones, straight chained alkanes, and aromatics, respectively.
Aromatics are the most stable requiring the longest retention time to be completely oxidized. Biofilters have successfully removed many different organic and inorganic compounds in gas streams. Removal efficiency is usually 99% for easily biodegradable compounds and 90 % or better for volatile organics. Flow rates and retention times are determined by the chemicals in the gas stream, the diffusion rate of the gas into the filter media, and the reaction time required for the biological degradation.
The influent air must be monitored and controlled for humidity and temperature. Mesophilic bacteria are most often used in the filters and require a temperature range of 50 to 105 o F with an optimum removal temperature of 100 F. Removal efficiency is directly related to the number of microorganisms available for degradation in the filter. The number of microorganisms is directly related to the moisture content of the environment. The relative humidity is crucial to the performance of a biofilter. Excessive moisture reduces mass transfer of hydrophobic substances by clogging the available pore space, thereby reducing surface availability. If a dry gas stream is introduced to the filter system, it will absorb most of the moisture from the media and kill the microbial population and this will decrease the removal efficiency. Most units are enclosed in a sealed container and the gas stream is brought to near saturation before entering the filter system.
The influent should be free of dust particles that could clog filtration and restrict air flow. In the food industry flow restrictions can be caused from condensing fats and oils from animal byproducts or dust carry-over from spray driers. Pretreatment scrubbers are incorporated into the system if gas streams have large particulate loading problems.
One of the limitations of the biofilter is that it has a high resistance of the packing material to the air flow when compared to incineration. An incineration process operating at 1600 F may take a fraction of a second to oxidize a given volume of gas while a biofiltration process would take 10-30 seconds. The problem with incineration is its high energy requirement. Flow streams with low concentrations of VOCs cannot supply enough fuel to maintain efficient removal levels. This leads to new and different odours formed as a byproduct of the reaction.
An aerobic biofilter has been developed by EG&G Corporation to biologically oxidize odours and VOCs into carbon dioxide and water. The Biocube Aerobic Biofilter can treat a wide range of VOCs and odours including but not limited to alcohols, hydrogen sulfide, methyl mercaptans, esters, and acetaldehydes. The Biocube Aerobic Biofilter is made up of stacks of modular interlocking sections called Biocube trays. The trays are filled with a bioactive medium containing a mixture of organic and inorganic material scientifically designed to support the growth of microorganisms. The Biocube medium is continuously replenished by microorganisms and does not become spent, which reduces the amount of waste materials that can be accumulated by other systems such as activated carbon.
Contaminated air is first sent through a moisture integrater. This initial process humidifies and adjust the air temperature to 30 to 50 C. The air is then introduced to a stack of Biocube trays which can be configured in either a series, parallel, or combination of both. The filter media absorbs and oxidizes the contaminated air. Processed air is either released or directed through carbon adsorption beds to be polished.
The temperature required for efficient oxidation of contaminated air is the filter media is between 30 and 50 C. This temperature is easily obtained without excessive energy usage, thus reducing the operational costs. The number of Biocube trays required for effective oxidation of contaminated air is determined by the concentration of compounds in the air and the flow rate of the air stream.
The Biocube Biofilter has been successfully demonstrated for removal of gasoline hydrocarbons. Two prototypes have been installed at major oil companies for the treatment of soil ventilation off gas. Two other prototypes were installed at wastewater treatment plants for use in controlling H2S and odour emissions.
Clairtech has developed the BIOTON filter for the biofiltration of industrial waste gas purification by means of microorganisms. It was designed for the removal and degradation of odours, hydrocarbons, and acid-forming compounds. Waste gasses are collected and ducted to the BIOTON system. Before entering the filter itself, gasses may require pre-treatment: dusting, cooling, or humidification. The equipment required for this stage depends on the quality of the gas stream. A fan is used to propel the gas into the filter bed and through the layers of filter media. The media (micro selection) is prepared to the specific components of the gas stream. The BIOTON filter was designed to efficiently remove odour particles from food processing operations such as spray drying, brewing, and baking.
PPC Biofilter has installed its biofilter in different applications including McCormick spice company, Givion Roure fragrance company, and Bush Boat Allen flavor company. Scot Standefer of PPC Biofilter, says that in order to assess the quality of the waste stream, preliminary tests need to be conducted. After the source of odours have been identified, a filter media is specifically tailored to the compounds to be treated. A pilot system is then installed to evaluate the variables and make the proper adjustments for efficient removal of odourous compounds. Once all adjustments and recommendations have been considered, the biofilter system is installed. The biggest limitation is land area. The biofilter requires a large amount of space at a manufacturing plant. Some installations have been on the roofs of plants with less land area. The biofilter requires between 10 and 30 seconds to completely oxidize the odourous compound in the waste stream.
The current bakery oven exhaust EPA regulations require a system with 90 percent reduction efficiency for the VOCs in the exhaust. This generally restricts manufacturers to selecting catalytic incinerators that are expensive to purchase and operate. The cost of the equipment is thought to remain non-competitive in that there are only three manufacturers of suitable equipment.
The odours are released in the oven exhaust that contain water vapor, CO2 , other VOCs, and various combustion products. The VOCs are primarily ethanol produced by the yeast during the fermentation process. Processes that reduce the VOCs will also reduce the odour causing compounds. It should be noted that many individuals find the odour of fresh baked bread very desirable.
The baking industry believes they need a cost-effective method to reduce the VOCs as mandated by current EPA regulations. Therefore, they have solicited the aid of EPA in the use of a biofilter to reduce VOCs and odour. The biofilter was chosen because it is relatively inexpensive to purchase and maintain and has been shown to reduce toluene by 95 %. As ethanol is even more biodegradable than toluene, it is anticipated that efficiencies of greater than 95 % can be expected. A team comprised of the Food Manufacturing Coalition Systems Manager, bakery engineers, EPA researchers, University of Cincinnati researchers, and a technology firm called PRD Technologies are working together on this opportunity. They are currently modifying a biofilter that was built to more effectively remove the desired compounds. The biofilter will have an initial chamber to cool the oven exhaust from 350 oF to approximately 100 oF and to adjust relative humidity to help assume maximum performance. The pilot unit will be installed in an Interstate Brands Bakery in Cincinnati, Ohio during 1997. Optimism is high that this technology will work as planned and units could be available as early as 1998.
The smokehouse is commonly used to preserve meat products and to develop flavors and color. Smoking, drying, and cooking are accomplished in the smokehouse. One manufacturer of meat products noted that they had more than 200 smokehouses operating in the United States.
More than 200 different compounds have been isolated from wood smoke. The most important compounds have been identified as phenols, acids, alcohols, hydrocarbons, and carbonyl compounds. A number of the compounds contribute to the color, flavor, and curing of meat. The compounds that are released from the wood during smoking and are not deposited on, or reacted with the meat are discharged.
The emissions contain a number of odourous compounds. Electrical precipitators, afterburners, and water scrubbers have been used to control air pollutants and odours. High installation and operation costs cause the precipitators and afterburners not to be an acceptable technology. Wet scrubbers remove less than 80 % of the emissions and may not be as efficient as required by regulations or to prevent undesirable odours. This suggests that biofilters may have a market for reducing odours from meat smokehouses.
Table III, Table IV, and Table V show selected odour producing operations in food processing, selected uses of biofiltration for odour control in food processing, and the efficiency of a biofilter on the removal rates for various odourous compounds, respectively.
Table III -- Selected Odour Producing Operations in Food Processing
|Category||Odour Producing Process|
|Wastewater Pretreatment/Treatment of Food Processing Wastewater||
|Processed Foods - breaded chicken||Fryers|
|Mushroom Farm||Compost Pile Drainage|
|French Fry Processing||Fryers and Blanchers|
|Garlic and Onion Processing||Drying Operations|
Source: Carawan (1997)
Table IV -- Selected Uses of Biofiltration for Odour Control in Food Processing
|Source of Odours|
|Dissolved Air Flotation Skimming Tank Exhausts|
|Mushroom Compost Pile|
|Anaerobic Wastewater Treatment Discharge|
|Biosolids and Dissolved Air Flotation Skimming Dryer|
|Food Processing Residuals Composting|
|Wastewater Equalisation Tank|
Source: Carawan (1997)
Table V -- Removal Rates For Various Odourous Compounds In A Biofilter
|Odourous Compounds||Removal Rates|
Source: Zeager Bros. (1992)
The National Aeronautics and Space Administration (NASA) has several Centers with technologies and capabilities that would be relevant to the Food Manufacturing Coalition (FMC) odour control problem statement. The FMC has specified two areas of particular concern for this need. They are presented below with general information about the Centers active in these areas.
Based on preliminary database searching and interviews, no specific research activity was found related to packing materials for air filtration systems. However, most NASA Centers have some level of research activities in materials, composites, and processing innovations per year.
NASA has developed several technologies related to air filtration. Although NASA scientists have worked in the area of air filtration, most have not concentrated on the use of biofiltration. The Centers listed below are likely to have relevant experience and techniques that will benefit FMC in solving the odour control problem.
The biofilter has been shown to effectively control odours produced in food processing. Each specific application may require the development of a unique biofilter design. Numerous other opportunities exist for odour control in the food processing industry. In addition to being very effective, many of the biofilter technologies have low capital and operating costs.
Existing technologies have been identified as potential solutions to the food industry odour control problem. f the NASA activities identified, only the Stennis biofilter activity provides an adequate match to the industrial need described in this paper. Since that technology originated with a company and not as part of NASA's research activities, NASA's involvement would not be required in further research. However, off-the-shelf biofiltration technologies exist from several companies and specific designs can be provided through established air filtration system companies.
It is recommended that FMC explore solutions with the biofiltration companies listed above prior to launching on a research project for a new technology.
CET Engineering Services, Inc.
1240 North Mountain Road.
Harrisburg, PA 17112
P.O. Box 4030
Golden, CO 80901
Hillshire Farms and Kahn's
3241 Spring Grove Avenue
Cincinnati, OH 45225-1329
659 Vanmeter Street
Cincinnati, OH 45202
Interstate Brands Corporation
12 East Armour Boulevard
Kansas City, MO 64111
3000 East Marshall Ave.
Longview, TX 75601
The Pillsbury Company
Grand Metropolitan Food Sector
200 South 6th Street
Minneapolis, MN 55402
Zeager Bros., Inc.
4000 East Harrisburg Pike
Middletown, PA 17057
Cheremisinoff, Paul N. (1995). Reactive Chemicals in Odour Control. The National Environmental Journal. pp. 30-34.
Clairtech. Removal and Degradation of Odours, Hydrocarbons, and Acid-Forming Components. Biotin.
EG&G Biofiltration. Biocube Aerobic Biofilter - OFF - GAS. EM's Best. www.gnet.org.
Hall, Carl W. , A.W. Farral and A.L. Rippan.(1986). Encyclopedia of Food Engineering. Avi Publishing Co., Westport, CN.
Standefer, Scot. (1996). Evaluating Biofiltration. Environmental Technology. pp. 26-32.
Van Lith, Chris, Gero Leson, and Richard Michelsen (1997). Evaluating Design Options for Biofiolters. Journal of the Air & Waste Management Association. pp. 37-47.
Walsh, James L. (1996). What's That Smell. Industrial Wastewater. pp. 34-38
Wheelabrator Air Pollution Control. Fabric Filters. Fabric Filters Home Page. www.wapc.com.
Whittle, Thomas, and Jodi Quimby. Biological Filters for Odour Control. 27th Annual Conference, Environmental Engineering in the Food Processing Industry.
Zeager Bros. INC. Odour Remediation Using Biofiltration.
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