URBANA, Ill. – Biofilters can dramatically reduce odors coming from livestock operations, clearing the air between farmers and their urban neighbors.
Moisture levels. According to University of Illinois research, the filter’s effectiveness can be improved by the choice of materials used, and by maintaining an appropriate moisture level in the filter.
“The biofilter process is fairly well established and has been used for years,” said Ted Funk, University of Illinois agricultural engineer.
With the right combination of organic material, odors can be reduced by as much as 80 percent, he said.
Funk and his colleagues are also working on an automated system that will maintain moisture content in the filter.
What’s a biofilter? A biofilter is a bed of wood chips or other organic material connected to a livestock building ventilation system or a manure storage with air ducts.
Exhaust fans push the air coming from the building or storage up through the organic material, and bacteria and fungi that grow there remove the odor-causing components by using them for food.
In their work, Funk and his colleagues used a biofilter to reduce odors coming from the headspace of an in-ground manure tank on a hog farm in western Illinois.
The test. Two filters using two different media were placed side by side. One filter used a debarking product (tree bark and dirt) and the other used wood chips and hay.
Air samples from each biofilter were collected and sent to an olfactory group at Iowa State University.
This panel of eight human “sniffers,” trained to judge the presence of odor, rated the overall odor strength of each sample.
“The debarking product worked better than the wood chips and hay to reduce ammonia,” said Funk. “However, both media were pretty effective in odor reduction on the average.”
For a biofilter to work properly, Funk said the organic material must remain moist or the organisms will die off and the air remains untreated.
Grab a sample. “The established method to monitor moisture content in the filter is to reach your hand in, grab a sample and see if it feels all right,” said Funk. “It’s kind of hard to get farmers excited about doing that.”
So Funk and his colleagues went back to the lab to develop an automatic system that would control moisture content.
“We wanted to produce a moisture-sensing technique that would give a readout of the moisture percentage in the filter,” said Funk, “and also trigger an electronic control to turn water on and re-wet the filter when it needs it.”
After testing a variety of moisture meters, they found that a relative humidity meter worked best at detecting a drop in moisture content.
Building a sensor. However, a relative humidity meter is essentially connected to a little tube and can only test one spot at a time. So Funk and his crew are building a much larger sensor – about 30-by-36 inches – out of a hog panel grid.
“We stack three grids together to form a capacitor. The top and the bottom grid are grounded and the one in the middle is energized at a high frequency,” said Funk.
“We bury it in our biofilter medium and determine the moisture content. When the moisture reading goes down below a certain level, the control circuit triggers a switch to turn the water on.
“Then the reading goes back up and the process repeats itself, irrigating the biofilter as needed,” added Funk.
“The farmer doesn’t have to worry about sticking his hand down in a dozen places to see if it’s wet enough.
The point. “The bottom line,” he concluded, “is if we can get this to work at a relatively low cost, there will be a lot more use of biofilters on hog farms. And that will greatly reduce the amount of odor emitted, which will make everyone happy.”
For more information on biofilters, read the Livestock Waste Facilities Handbook, published by MidWest Plan Service.
Section 3, “Outdoor Air Quality, Manure Management Systems,” offers good general information on how biofilters are made and work to reduce odor.
Get a copy. The book is available from MidWest Plan Service at www.mwpshq.org or by calling 1-800-562-3618.
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