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A report from the AgTech Centre

Researchers tackle the challenge of controlling moisture in biofilters

Biofilters can provide livestock producers with a sound method of managing odour, but researchers are discovering that controlling moisture levels is presenting a challenge.

Biofilters function with the help of micro-organisms, says Peter Llewellyn, an Engineer-in-Training with Alberta Agriculture, Food and Rural Development (AAFRD). In the biofilter, a layer of organic material provides the environment for the micro-organisms, which break down the odour as it passes through the filter. Moisture, however, is essential in keeping the bugs healthy.

"When there is no moisture in the biofilter, the micro-organisms die," says Llewellyn. "When they die, they are unable to fulfill the job of consuming the odourous compounds and reducing odour which is why this is a concern for us."

Odour management has become a key issue in livestock operations and biofilters are proving to be an effective way for producers to manage odour from livestock ventilation systems. Under controlled conditions, Llewellyn says biofilters have reduced odour by as much as 90 percent.

Researchers tried several methods to keep the biofilter material moist. One option was an oscillating sprinkler similar to what many people use on their lawns. Unfortunately, this method not only watered the biofilter, but also the side of the barn and the ground around it. In the drip line method, water was gradually added to the biofilter from just below the top of the media. Llewellyn says they may experiment with using a fine mist near the fan, which would distribute moisture and keep the biofilter more uniformly damp. Even if a successful method of watering is found, he says, there is no way to manage the effect of the seasons.

"In summer, everything dries out so much faster than in winter," says Llewellyn. "In the winter the ventilated air still moves through the biofilter, but only at about one third of the rate of summer."

Another way to control moisture is to use different materials that would trap and hold the water. White wood chips and bark mulch have been tested. So far, says Llewellyn, bark mulch appears superior.

"The bark mulch has a higher ability to retain water," says Llewellyn. "The mulch also contains more micro-organisms, so it also does a better job of reducing odour."

Researchers in Ontario are working with porous foam blocks to trap moisture, but there is concern about disposal once the foam has deteriorated. White wood chips and bark mulch for example can be composted. Llewellyn says future material testing may include adding fine-grain compost to bark mulch to see how moisture levels are affected.

Monitoring moisture levels within a porous material in order to regulate the moisture level is another challenge, says Llewellyn. Getting an accurate reading of moisture levels within the biofilters has researchers trying to adapt existing measurement technology. Commercial sensors are being used to gauge moisture levels, but sensors don't always record the necessary information.

"We are having a difficult time getting them to measure the moisture level within the biofilter rather than the level in the air," says Llewellyn. "As a result we end up measuring the moisture levels back in the laboratory, which is very labour intensive."

Llewellyn's research into biofilters and controlling moisture levels is ongoing, but he hopes to have some practical results for producers soon. Biofilters provide a cost-effective way of managing odour. AAFRD testing has determined the installation cost to be between $0.40 and $1.00 per litre per second ($0.20 to $0.50 per cubic foot per minute). When managed properly, biofilters can have a life span of three to five years.

The AgTech Centre, located in Lethbridge, Alta., is part of the Alberta Agriculture, Food and Rural Development Agricultural Engineering Branch. Its mandate includes all aspects of agricultural sustainability.

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