Biofilters are a well-established method for controlling odorous and volatile organic compound air emissions from composting operations. When designed and operated per Best Management Practices they routinely reduce VOC and odor emissions by more than 95%. ECS has designed and provided the air distribution, control, and monitoring systems for dozens of biofilters at large composting facilities and processing buildings such as MRF’s. In addition, we’ve measured and improved the scrubbing efficiency of many biofilters (we know what makes them tick).
Biofilters are a common means (thousands of installations worldwide) of scrubbing odors associated with exhaust air from composting and other processes that generate volatile compounds. They work based on the concept of an active biological film supported by a substrate – the same principal as composting. The volatile compounds are absorbed on to the surface of the media and bio-oxidized by the microbes present. As long as the media is appropriate, installed in a manner that does not allow the air flow to channel or short-circuit, and the moisture and temperature levels are maintained, biofilters work very efficiently to oxidize a broad spectrum of volatile chemicals at low part-per-million concentrations. The rule-of-thumb for biofilter efficiency is that a well maintained biofilter will provide a one log (factor of 10) reduction of most bio-oxidizable compounds. Biofilters are resilient and remain effective in varying environmental conditions such as snow, rain, hot and cold.
The specifications for a biofilter design depend on the volume and contents of the exhaust air, the climate, and the site’s sensitivity to odor generation. The following is a list of generalized design specifications.
|Residence time||40 – 90 seconds|
|Media temperature||10° – 50° C|
|Active media depth||36” – 66”|
|Media components||• 95-97% screened coarse resilient wood (ideally shredded root wood) chips sized 1” to 2”plus (discard the fines)|
• 3-5% stable compost (preferably made from the feedstocks themselves)
|Media moisture content||>50%|
|Max pressure drop through media||< 0.5” SP/foot of depth (once greater than this the media should be replaced)|
When building the biofilter, the media choice is very important to performance efficiency and longevity. A bed of relatively coarse stable media will provide more uniform flow, lower friction loss, and a longer lifetime than a bed finer degradable media. Only a very small amount of more degradable material, such as compost, should be added to otherwise coarse (with the fines removed) clean stable shredded wood. The primary reason for adding the compost to the media is to shorten the biological conditioning period required for the effective microbes to become established. This period is thought to typically take 4-8 weeks.
A critical operational factor for a biofilter is maintaining the correct moisture content in the filter media. The media must be maintained at greater than 45 percent moisture, so a higher design setpoint of 50% is chosen. If the media becomes too dry, microbiological activity will be suppressed and odorous gases will not be fully oxidized. Eventually channeling will occur as air moves through drier passages causing further localized drying and shrinkage of the media. Once this occurs, the media should be changed.
The exhaust air from a composting process is generally saturated (100% RH). This humidity maintains the moisture content in the majority of the media. The upper layer will often still dry due to evaporation, but this generally does not impact the overall performance. Imbedding a soaker hose near the top of the pile is a good means to both insure uniform moisture throughout the pile, and to provide a means of occasionally washing out soluble nitrates that can build up in the media (especially while composting biosolids). In hot and dry environments, adding surface irrigation is often required to keep the top layer moist.
Another important operational factor is the temperature of the biofilter media. An ideal range of media temperature is between 10° and 45° C. Biofilter efficiency begins to decline above 45° C and falls off quickly above 50° C. The media also volatilizes quickly at temperatures above 50° C (a bed may lose feet of height in a matter of months if high temperatures are maintained). Short term excursions in the 50’s° C range are generally acceptable so long as monthly average media temperatures are < 45° C. The compost aeration and control system needs to monitor and control the temperatures of exhaust air and biofilter media. Ideally, the system will automatically control the exhaust air temperature to an operator-chosen setpoint by adjusting the volume of dilution air. In some cases (especially when large volumes of pre-heated building air are mixed with process air) additional humidification is required to prevent dilution air from over-drying out the biofilter media.
The media bed and aeration floor must be constructed in manner that avoids short circuiting of the air. This is easily done with the correct density and particle size media built in a bed that extends past the edges of the aeration floor. Vertical walls tend to promote short circuiting; if they are necessary in the site design make sure that they are at least 2-4 feet back from the edges of the actively aerated floor (the higher the wall, the further the setback).
Even though biofilters are quite resilient to varying environmental conditions there are a few things an operator should periodically check.
|Moisture||The operator should take grab samples from the media once every two weeks to test for moisture content. If the media appears to be over drying, increase the irrigation frequency with a soaker hose.|
|Temperature||The operator should monitor the biofilter media temperature weekly (this temperature will be displayed on the operator PC in the automated control and monitoring software). The operator can vary make-up and exhaust damper control setpoints, as well as the relative settings of the supply and exhaust blowers, to control the temperature of this exhaust air. These settings are typically adjusted seasonally.|
|Pressure Drop/Media Densification||The operator should record the static pressure drop through the biofilter at a standardized operating condition (compost aeration process supplier should specify system setting during start-up that identify such a condition) once every six months to track densification in the media.|
|General Inspection||The useful life of the biofilter media depends on the material used and the operating conditions. Different types of coarse ground wood have varying resistance to breaking down. Also, higher temperatures tend to degrade biofilters more quickly. Generally the media is expected to last 1-3 years. Spent biofilter media is characterized by:|
• Cracking and channeling
• Breakthrough of contaminants (odors)
• Increased head loss (compaction and increased density)
• Shifts in media pH
Once the media is degraded, it should be removed and either added into the compost mix as an amendment or marketed as well-matured compost.
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