Allen Boyette, senior engineer and manager of the Cary, North Carolina office of E&A Environmental Consultants, Inc., tackled the topic, “Building Longevity Into Biofilter Performance,” at BioCycle’s National Conference in Albuquerque in May, 1999. After years of designing and troubleshooting biofilters, Boyette has many practical insights into what it takes to extend their life.
The first area of focus is the media. “There is a long list of things to consider when figuring out how long a biofilter media lasts,” he says. “First is the temperature. The higher it is, the more active the biological process, and thus the actual media is going to break down more quickly. The amount of dust and particulate matter in the air stream is another factor. If you have a lot of dust blowing through a porous media, you start to fill up all the pore spaces faster than the microorganisms can break it down. So the higher the dust concentration, the shorter the media life.”
As pore spaces fill, the pressure drop builds. “It’s not that the biofilter media or the microorganisms quit working,” he adds. “It’s that the air handling system can’t physically get the air through the media because the pressure drop is too high.”
In terms of temperature, Boyette provides the example of a biosolids composting facility in Cape May, New Jersey, where the blowers have the ability to run in both negative and positive modes. “In the negative mode, the air is pulled down through the compost pile, directed through a header and then to the biofilter. That type of air stream typically is hotter — it is 130° to 140°F and has a very high ammonia concentration.
You need to look at whether that air stream should be sent straight to the biofilter and what impact that will have over time versus putting it through a pretreatment system, such as a spray chamber, to cool it down and remove some of the ammonia and dust particles. If the composting facility is at a wastewater treatment plant, treated effluent can be used to cool the air and then recycled back to the head of the treatment plant. In short, different kinds of air streams mean that different design considerations are necessary to optimize how long the biofilter is going to last and how well it is going to work.”
Media selection is another major factor. There are two general media bases — soil and compost. “On a size basis, the compost based filters are much better,” says Boyette. “They can be operated at either 30 or 45 second retention times whereas the soil based biofilters have a two to three minute residence time. Therefore, compost beds are smaller and a little bit cheaper to construct. For media life expectancy, however, compost with wood chips is organic, so it is going to break down. There is going to be a gradual increase in head loss over time whereas the soil base is a little less organic so the media is going to last longer. The big question is the operating cost. Compost based media is typically a lot fluffier, and has a lower head loss than soil. The lower head loss means less electricity usage thus lower costs.”
Boyette adds that a goal is to operate the biofilter as close to 100 percent humidity as possible. “Keeping it wet is a key factor in biofilter performance. I approach this by hitting it from the bottom and saturating the air coming in, and then doing surface irrigation on top. Fifty to 55 percent moisture is a good target range for a compost-based biofilter media. Whenever I go to facilities having problems with their biofilters, nine times out of ten the media is dried out.” By Nora Goldstein.