Biofiltration to control odors

New technology can help reduce nuisance complaints for the most odorous of feed manufacturing.

Biofiltration as an odor reduction technology is common in western Europe where many feed plants are in the middle of residential or commercial areas. However, modern biofiltration technology now is a far cry from the beds of evergreen fronds used two decades ago. Today in North America, advanced inorganic biofilter media are used for some of the smelliest feed industry assignments, including odor control at rendering plants which supply feed manufacturers with meat-and-bone meal, fatty acid blends, and other highly aromatic feedstuffs. This new technology used in these extreme odor conditions may help reduce nuisance complaints and preserve feed manufacturing sites in urban and suburban zones.

Odorous chemical compounds occur in feed raw materials, in intermediate feed products, and through decomposition processes that take place during storage, heat treatment, warehousing, and transport. Plant emissions, especially exhaust air and dust, typically carry the greatest concentrations of odors outside the plant. Typical release points for odor in feed plants, in order of importance, are:

  • Extrusion cooking processes;
  • Product drying and cooling processes;
  • Grinding and other physical process operations;
  • Raw material reception, storage, and handling;
  • Storage, handling, and transport points during processing; and
  • Fugitive' emissions of building and process air.

Contained emissions of particulate matter are largely associated with emissions from product dryers and coolers and grinding equipment. Fugitive emissions of particulate matter may arise from transfer of potentially dusty materials, including discharge into hoppers and onto conveyors, and delivery to storage silos and sheds. Also, material collected by bag filters may become re-entrained in the plant air or outside, if it is not securely contained and carefully handled. Chemicals in the dust may volatilize to cause odor, or the dust itself may be odorous.

Feed plant odors at the chemical level

The rendering industry handles particularly odorous materials and passes along a number of typically odorous chemical compounds in rendered products sold to feed plants. These chemical compounds include: Sulfides, disulfides, baldheads, amines, quinoline, pyrazines, and organic acids, as well as alcohols, ketones, aliphatic hydrocarbons, and various aromatic compounds.

However, not all of the smelliest materials come from the rendering plant. Oxidation of fats and oils in vegetable raw materials as well as fermentation and other decomposition chemical processes can contribute to feed plant odors. Nonetheless, animal co-products generate some of the most odorous chemical compounds in feed plants, including:

  • Aldehydes and lower fatty acids deriving from feed-grade fats;
  • Alkanes and other chemicals generated by organic residue carriers;
  • Propionic and formic acids introduced for preservation purposes;
  • Amines from fish meal and fish oil;
  • Aldehydes, esters, and other natural or synthetic aromatics added to improve feed attraction, palatability, or intake; and
  • Contributions from other components in raw materials, including total reduced sulfur compounds (TRS), dimethylsulfide (DMS), dimethyldisulfide (DMDS), methyl mercaptan (MM) and hydrogen sulfide (H2S).

Biofiltration has been successfully deployed for feed plant odor control as both a replacement and complementary technology. In either case, each odor control technology must be carefully monitored for its own contributions as an emissions source and its effects on the manufacturing process.

Odor control systems

Some feed plants, such as aquafeed plants in urban or suburban zones, may use an odor control system comprised of a scrubber. However, materials which are added to the scrubber for improved performance, such as acids, hypochlorite, sodium hydroxide, etc., often are odorous themselves. If the scrubber and mist eliminator are not properly managed, emissions of these materials may be released with the plume.

In other extreme situations, the odor control system may consist of a thermal oxidizer. Then some emissions are likely to include combustion releases from the fuel, including sulfur dioxide and nitrogen oxides. Meanwhile, carbon monoxide may be emitted if the combustion process is not carefully managed. Metals, volatile organic compounds, chlorides and fluorides, all of which may carry odors, may be emitted where waste or recovered oil is used in the combustion equipment.

For both scrubber and thermal oxidizer systems, a biofilter also may be part of the odor control strategy.

Ontario case history

At Dundas, Ontario, Rothsay Concentrates operates a major beef and dead stock rendering facility which experienced a decade-long odor problem. Rothsay is Canada's largest rendering business and is owned by Maple Leaf Foods, which also owns Shur Gain Feeds, Landmark Feeds of Manitoba, and the Manitoba-based Elite Swine business.

At Dundas, primary sources of high-intensity odors included non-condensables from batch cooker exhaust, and emissions from the screw press. Other sources of odor included dryers, centrifuges, tallow processing tanks, and perc pans that were open to the plant atmosphere, and received discharge from the batch cookers. Offal from the slaughter plant and dead stock also were odor sources, but normally not significant when they were processed without delay. However, the type and age of these raw materials were important, because older materials that had deteriorated resulted in more odor during cooking and pressing operations.

By 2002, Ontario's Ministry of Environment (MOE) wanted quick action on the concerns the Dundas plant had been receiving from the local community. The plant started with one scrubber, increasing to six as the treated odorous gas stream increased from 50,000 cfm to 250,000 cfm. Its odor control system main collection duct received input from multiple areas in two main buildings, with input from each area ranging from 60 cfm to 10,000 cfm. The scrubbers used 12% sodium hypochlorite--bleach--to remove odor, and potassium hydroxide to maintain the acid-base balance in the scrubbers.

"The scrubber system did its job for a long time, but increased residential development around the facility resulted in odor issues," recalls Amar Aulkh, Rothsay's chief engineer at Dundas. "Performance testing on the scrubber system conducted in 2002 showed that odor removal efficiency was below our 90% target.

"To solve the problem, we tested alternative scrubbers that utilized chlorine dioxide, ozone, chlorine dioxide and ozone simultaneously, and a neutralizer, but could not get the 90% removal efficiency we needed.

"At the same time," Mr. Aulkh continued, "we learned about biofiltration and incineration. We determined that incineration was a cost-prohibitive alternative for our operations... this technology relies on long-term dependence on fuels."

Biofiltration alternative

The Dundas plant's search for alternative odor control technology then focused on biofiltration. Using biofilter media supplied by Biorem Technologies of Guelph, Ontario, Mr. Aulkh and staff spent a month testing odor removal efficiencies from airstreams from different areas of the plant. He was told that the inorganic biofilter media had important differences compared to standard woodchip media.

"The other biofiltration technology providers claimed to achieve similar effectiveness with their own media," Mr. Aulkh recalled, "but their wood chips needed replacement every two or three years, which was a big job we wanted to avoid. Biorem claimed theirs would last 10 years.

"The pilot was important, because while we had visited four or five biofiltration installations and were impressed with the performance of all of them, you could stand on the filtration bed and not smell anything offensive. We didn't know if they would work for our plant."

Following the Dundas biofilter media pilot-testing program, the odor control project decision was finalized in November 2002. Construction of the final system began in January 2003, with the plant's consulting engineer Pinchin Environmental of Mississauga, Ontario, and Biorem engineering personnel on-site until completion in August. The multiple-port collection duct network from the original scrubber system was incorporated into the new 250,000 cfm biofiltration system.

"Biofiltration did considerably better than the scrubbers right away," chief engineer Aulkh reported, "and we quickly got odor removal efficiency over 90%."

Last year, in the Air and Waste Management Association's (AWMA) Odor Conference in Toronto, Pinchin consultants detailed performance of the odor control system at the Ontario rendering plant. Pinchin quoted engineering management at Rothsay, reporting that the plant had all but eliminated odor complaints by installing the biofiltration system that replaced a scrubber system that could no longer provide sufficient odor removal efficiencies.

Tests conducted soon after startup in August 2003, with triplicate samples collected on three occasions over a two week period, revealed an overall removal of greater than 94%. Preliminary results of MOE compliance testing that began last spring indicated average odor removal of 92%. More recent testing by a third-party consulting firm confirmed average removal efficiencies of 94%, with highs of 97%.

System design

The Dundas plant's new odor control system begins with a three-chamber preconditioning unit, where water from plant recycling and fresh water sources is sprayed into the gas stream at 500 gal/min in each chamber to achieve 98% relative humidity within the stream. The gas stream then moves to a mist eliminator for water drop removal, followed by an inlet plenum common to the two banks of three biofiltration cells.

Each of the six biofiltration cells contains a 5.5 ft bed of the BIOSORBENS inorganic media, and has a fan to draw the stream from the top of the bed and out the bottom to the outlet plenum. Individual cells can be isolated for work, if needed, with the other five still in operation. As part of the Dundas service contract, media samples are sent to Biorem every four months for checking on the effectiveness of the odor-removing microbes.

Each cell handles an equivalent amount of captured plant and process air, with each 150 hp fan working with a variable frequency drive (VFD) and controlled by a programmable logic controller (PLC).

"The VFDs are good to have," Dundas chief engineer Aulkh noted. "With the extremes of heat and cold we have, we can vary the horsepower accordingly, knocking it down to as low as 25% of capacity when there is no processing going on during a winter weekend.

"It is a good, environmentally friendly system that works," he added. "There are no chemicals. It is easy to operate--all automatic--with our stationary engineers having PLC panels for routine monitoring of temperatures, pressures, and flows. There haven't been any problems with the process. We've just had to clean out the mist eliminator and the preconditioning chamber sump every couple of months.

"We've seen a big difference since we installed the new system," Mr. Aulkh concluded. "This has helped us fulfill our environmental commitment', which includes assuring sound environmental management by requiring visible technology accountability at every level."

Rothsay has recently completed installation of a similar system for its Moorefield, Ontario, plant.

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