Biofilters offer low-cost, powerful control of hog barn odours

Biofilters are an emerging odour control technology with dramatic potential for low-cost, effective control of nuisance odours from hog barns, say engineers with Alberta Agriculture, Food and Rural Development (AAFRD).

"Biofilters may be a good option for producers looking for ways to reduce the potential of nuisance odours from their operations," says Peter Llewellyn, an Engineer in Training with AAFRD in Red Deer who has conducted practical trials of biofilter technology for the past two years. "Under control research conditions, odour reduction can be as high as 90 percent."

This type of experimental biofilter uses organic material such as compost or bark mulch as the filter medium, says Llewellyn. Odourous air is forced through the biofilters, where odourous compounds are absorbed into the organic material and broken down into harmless particles by naturally occurring bacteria.

"Odours are captured as air percolates up through the layer of organic material," explains Llewellyn. "The bacteria that live on the organic material do the actual work. To these bacteria, odourous compounds such as hydrogen sulfide and ammonia are food. They consume these compounds to sustain life."

The types of bacteria attracted to a biofilter depends on the type of organic material used, he says. Compost, bark mulch and wood chips are all very effective for capturing livestock odour.

The University of Minnesota has been a leader in applying this technology to agricultural operations in the U.S., says Llewellyn. AAFRD is building on the Minnesota work by adapting biofilter technology to the colder, drier climate of Western Canada.

"AAFRD biofilter design research focuses on the practical aspects of the livestock operation and is based on affordability, ease of management and use of existing fan capacity," says Llewellyn. "If a system is cost-effective, workable and reaches 75 percent odour reduction, we feel it's a good starting point."

Biofilters can be constructed easily using recycled wooden pallets, a sealed plywood duct, and aged wood chips, bark or landscaping material, he says. The technology requires an exhaust barn ventilation system, which is the most common ventilation system in Alberta hog barns.

The system used in AAFRD's research trials is a good example, he says. Biofilters were constructed by placing wooden pallets on the ground next to the barn and covering them with a fine plastic mesh. Organic material was piled up to 0.5 metres high on top of the pallets, which act as an air distribution system or plenum. Air from the barn's exhaust fans was directed to the biofilter via a ducting system built from plywood lined with a vapour barrier. Existing exhaust fans were used to push air into the plenum and up through the biofilter medium.

Depending on individual fan specifications, a biofilter should be half to two-thirds the square footage of a barn, says Llewellyn. Using a biofilter may marginally reduce airflow through the exhaust fan, but most ventilation systems should have enough capacity to maintain air quality throughout the seasons.

AAFRD trials determined an estimated installation cost of between $0.40 and $1.00 per litre per second ($0.20 to $0.50 per cubic foot per minute) of treated air. With proper management, the estimated life of a biofilter is three to five years before the organic material has to be changed.

A key limitation of biofilters is their potential to dry out, says Llewellyn. Bacteria do not function optimally in dry conditions and this could limit the effectiveness of the filter. "To address this problem, we are experimenting with a low pressure drip irrigation system that can be hooked up to a regular tap as needed."

This issue as well as optimizing the design and management of biofilters will be the focus of the research over the next two years.

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