Odor Control of Food Processing Operations by Air CleaningTechnologies (Biofiltration)1. INTRODUCTIONAs environmental awareness increases among the general public so do environmental restrictions among regulators. Odors emitted from food processing plants are becoming more of a concern as food plants and residential areas grow closer in proximiy. The objectionable odors 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. Odor thresholds are subjective among neighbors of processing plants and for this reason standard methods have been approved for testing odors.
Odor Control of Food Processing Operations by Air Cleaning
Technologies (Biofiltration)
1. INTRODUCTION
As environmental awareness increases among the general public so do environmental restrictions among regulators. Odors emitted from food processing plants are becoming more of a concern as food plants and residential areas grow closer in proximiy. The objectionable odors 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. Odor thresholds are subjective among neighbors of processing plants and for this reason standard methods have been approved for testing odors.
The accepted method for quantifying these odors is the use of an odor 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 odors. These results then determine thresholds for the general public.
Several technologies have been developed or improved recently to control odors from various industries. The best method for controlling odorous compounds is by eliminating the odor 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 odorous 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 odor 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 odorous compounds and VOCs are listed in
Table I and must meet the following criteria to be effective in the food processing industry:
• stable and predictable performance in removal of odorous compounds from the contaminated air stream,
• easily adaptable for applications experiencing large variations of air-flow rates and odor control intensity,
• no or low generation of environmentally harmful chemical by-products,
• low generation of solid and/or liquid waste materials, and
• low equipment costs and low operational and maintenance requirement.
Table I — Comparison of Biofilters to Other Odor Control Methods
Operating Cost Material Cost Maintenance Cost Pollution Risk Removes All Odors Removal Efficiency
Water Stripping High Low Low Yes No Low
Incineration High High High Yes Yes High
Carbon Sorption High High High No Yes High
Chemical Scrubbing Low High High Yes Yes High
Biofilters Low Low Low No Yes High
Zeager Bros. Inc., (1992)
Before an odor control/removal method can be selected, odor control objectives must be considered. The source of the odor 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 odorous compounds.
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2. BIOFILTRATION
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 odors 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 odorous compounds.
2.1 Bioscrubbers
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.
2.2 Biofilters
2.2.1 Bulk Media Filters
The vast majority of the current biofilter applications associated with odor 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 odorous 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 odors 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 odor control in wastewater treatment operations located on food processing sites.
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Table II — Design Parameters for Bulk Media Biofilters
Parameters Value Range
Bed depth 3-4 ft.
Loading rates 3-5 scfm/ft2 SA
Dwell time 30-60 sec.
Oxygen/VOC 100:1 ppm
Moisture — soil media compost 10-25 %
20-40 %
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 $10,000. 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 odorous 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.
2.2.2 New and Emerging Biofiltration Technologies
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.
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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.
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