Biofilters

Systems ECS Staff 07/29/2021

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 air 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. In addition, we’ve measured the scrubbing efficiency of many biofilters; we know what makes them tick.

Design Theory and Operation

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 to varying environmental conditions such as snow and rain, hot and cold; under average to best management practices biofilters remain effective.

Design Specifications

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 Time40 – 90 seconds
Media temperature10° – 50° C
Active media depth36” – 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 then 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 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).

 

Maintenance

Even though biofilters are quite resilient to varying environmental conditions there are
few things an operator should periodically check.

MoistureThe 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.
TemperatureThe 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 DensificationThe 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 InspectionThe 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.