ODOR BIOFILTERS
BACKGROUND
Wastewater treatment facilities are among the industries that have the potential to produce foul odors. Historically in the United States we have relied on collecting and then treating these vapors “chemically.” This effort not only involves the air ducts and oxidizing reactors, but also handling and dispensing these oxidizing chemicals many of which can be quite hazardous (such as high concentrations of chlorine, and potassium permanganate). In many installations two or more chemicals are utilized at different stages as just one oxidizer will not oxidize all of the odor causing compounds alone. Even then, after this two or multiple stage oxidation, odorous chemical compounds may still be discharged untreated!
An alternative to “chemical neutralization or oxidation” is utilizing microbes to oxidize these odors in specially designed biological treatment process units. This eliminates the purchase of the chemicals and the handling of them in concentrated solutions. Another advantage of the “odor biofilter” is that they are usually lower initial capital costs to construct compared to many other types of odor control. This “odor biofilter” alternative has been utilized extensively in Europe, where they were pioneered since the 1920’s.
Odor
biofilters have been shown to remove phenol, toluene, ketones, ethylbenzene,
aldehydes, amines, aniline, aldehydes, and aliphatic hydrocarbons. These
compounds are usually released from the wastewater in the treatment facility’s
Preliminary treatment process (screening and grit removal), Primary
sedimentation when the wastewater leaves the process cascading over the weirs
into the launders, in the aerated channels, and in the aeration basins. Most
often the aeration basin air is not scrubbed unless there are compelling reasons
to do so, as the volume of air to be treated is immense. Activated sludge
facilities which fully nitrify are less likely to scrub or treat the air from
their aeration basins than other facilities, as the odors resulting from the
process are usually bearable by most neighbors. Several facilities are able to
reduce their odor complaints by just treating the foul air from their
Preliminary and Primary Treatment processes only.
BASICS
of DESIGN
The odor biofilter has the following basic constructed components: 1) the
containment 2) the biofilter media, 3) the humidifier and wetting systems, and
4) biofilter air diffusion system, and the air ducts and fans.
1) The containment may be a concrete structure, an elevated wooden, fiberglass, or steel structure, or even an earthen-fabric-lined containment, depending on the facility, the type of compounds to be removed from the air and the volume of foul air to be treated. The bottom of the containments are usually sloped (suggest 2% slope) toward a drain pipe where the excess irrigation water, rainwater, and leachate from the media is collected and conveyed back to the treatment process train for treatment. The picture at the lower left shows the containment structure with the diffusion piping and the gravel installed. The porous filter fabric will be laid next on top of the gravel, and then the media will be placed above the fabric. Some structures feature a steel or FRP plate supporting the media off of the floor. The foul air enters this "plenum" and then passes thru very small holes in the plate and passes upward through the media.
2) The media may be as
simple as grains of sand, soil mixtures, compost mixed with soil, granular
carbon, or other type of media that will support the microorganisms in oxidizing
the particular odors in the air to be treated. The media is rarely deeper than
three feet (one meter) or so in depth. As we increase the depth of the media, it
requires more horsepower to push the air through the media, and that increase
both installation and operational costs.
The media when installed in the bed must create very small void spaces for the foul air to pass upward through the media. An example of what NOT to use would be “simple clay soil.” Clay soil would compact and prohibit the easy passage of air upward through it. Root stock, medium and small bark, and organic compost are examples of media that would allow the easy passage of air. In order to control “compaction” many engineered medias employ bark to help keep the media from becoming compacted.
Many of the different types of media must be replaced after three to five years of operation. The organic compost portions over time break down supplying the microbes with micro-nutrients, and cellular nutrients, and as such must eventually be replaced. Engineered solutions usually provide for multiple beds, so that at least one at a time may be removed from service and the media replaced. The media is most often replaced utilizing a very small tractor with a front end bucket to remove the spent media. The new media is usually installed by conveyor belts or is blown in so that it is NOT compacted during installation.
Some medias contain more large sand particles than others. I know of one installation that oxidizes odors from a pair of trickling filters that is strictly sand. The media has yet to be replaced. They have impact irrigation sprinklers on the top of the media, and insure that the media is kept moist. The treatment plant is now surrounded by upscale business offices, and an airport. I could not detect any odor at any time, as I walked completely around it on a no wind, summer afternoon.
3)
Microbes rely on water to sustain their metabolic process (life). As such, it is
absolutely critical that the foul (or polluted) air to be treated is moist, and
that the biofilter media is moist. (Remember that the foul air enters the
oxidation bed at the bottom, so it will usually dry out there first, hidden from
view.) In order to insure that the air is moist for those facilities in hot,
dry climates, humidifiers are usually installed in the foul air ducting leading
to the biofilter. This may be as simple as a ring of fine droplet water sprays
installed in the ducting. The media can be kept moist by installing drip
irrigation or even impact irrigation spray heads on top of the media. Many of
the engineered beds have two, three, or more drip irrigation systems installed
at different depths of the media. (Please see the picture to the lower left.)
Our bio-oxidation beds are designed with the layered, sub-surface irrigation
systems to reduce the potential for the lower levels from drying out, and to
reduce any tendency for the water to “channel itself” into underground streams
that create un-watered media volumes. On the enlarged picture, you will be able
to see one of these "layers" of subsurface irrigation pipes being covered with
more media. Some systems may also employ moisture sensors to turn the
irrigation systems on and off automatically, thereby insuring that the beds are
watered based on the seasonal differences, varying drying rates, etc. It must be
stressed that the media remain MOIST, not wet, not submerged, not saturated!
Just “MOIST!”
4) There are many ways in which to collect the foul air in our treatment facilities, and transport it to the odor bio-oxidation beds, and disperse the air evenly through the media. The major differences appear to be how different the approaches are in the dispersal of the air in the media. EXAMPLES: Some employ a plate which the media rests on top of, or others in which pea gravel surrounds the perforated pipe through which the air disperses and moves upward through the bed. (Please see the attached diagrams.)
I foresee that future bio-oxidation odor beds will be more complex, with advanced instrumentation and controls. With each passing week, we see better field instruments at lower costs. When sensors for moisture content, oxygen concentration, pH, air flow, and specific compounds like hydrogen sulfide are employed in the design and construction of these odor control beds, operators will have more control and knowledge of the process. If we were to then tie these field sensors into our computerized Supervisory Control and Data Acquisition (SCADA) Systems, we could track and control these parameters.
The
TREATMENT PROCESS
(Click picture to enlarge) Bio-oxidation and biofiltration are terms used in the literature to describe the conversion of odor causing compounds into non-odorous compounds by microbial action. This is accomplished by the media particles each possessing a film wrapping of water and microbes. As the foul air is passed upward through the media, odor causing compounds are adsorbed onto this film of water and microbes. The microbes then “eat” the compounds that they can oxidize, (which includes the odor causing compounds that we wish to remove from the air.) The soil, compost, other mediums contain bacteria which have a phenomenal ability to adapt, and transform themselves in such a manner to optimize their existence.
If your facility will operate in very cold temperatures, you may wish to consider additional heat for the foul air to be treated. The optimal air temperature is usually between 30 degrees C and 40 degrees C. Cold foul air passing through the media will chill the microbes, lower their metabolic process, and they will not oxidize the compounds at an optimal rate. Some northern facilities have automatic temperature control systems to adjust the air temperature.
Biological activity in bio-oxidation odor beds is usually optimized when the pH of the media and its biofilm maintain a pH of about 7. This is usually addressed during design, by considering what chemical compounds will be in the foul air. If there are concerns that the by-products will include chemicals that will make the biofilm go acidic or basic over time, oyster shells, or other such pH buffers will be incorporated in to the media to maintain a stable pH at about 7.
In summary then, the microbes oxidize the odor causing compounds adsorbed onto the moisture film into nutrients that they can use for energy, cellular material, and create by-products of water, carbon dioxide, other non-odor causing compounds like salts and elements. The media selected should contain the micronutrients and nutrients necessary for the microbes to sustain themselves and reproduce. It is important to emphasize that the compounds adsorbed onto the media are in fact “changed” and do NOT remain in their original chemical composition. (The chemical compounds do not permanently attach themselves to the media.) The media must also possess good sorption capacity so that the microbes can oxidize the compounds adsorbed onto the moisture film around the media particles.
OPERATIONAL TESTING
As part of our operational testing and commissioning tasks after construction was completed, we placed a smoke bomb in the ducting upstream of the bio-oxidation beds to test for short circuiting through the media and a uniformity of foul air passing through the media. Our first attempt was a failure, as the media removed 100% of the smoke from the small smoke bomb. After securing several larger bombs, we were able to get smoke to pass through the media. We found that our contractor had done an excellent job in installing the media as we did not have any short circuiting (like may occur if the smoke passed up easily between the media and the sidewalls of the containment structure). We were in fact able to document the uniformity of the air across the media surface. This test may also be employed on a yearly basis or so, to insure that your media is performing properly.
If you find short circuiting, try to determine the cause. Often is it nothing more than ‘air channels’ forming along the sides or by making cracks in the media through which the air passes through without treatment. I have heard of operators using small hand operated garden tillers to stir up the compacted media, and restore the media when it had dried out and cracks developed. They had surface irrigation, and were able to do this without destroying subsurface irrigation pipes and nozzles. (One of the facility’s I retired from has the multi-level, subsurface drip irrigation systems. Roto-tilling one of those beds would result in a mess of irrigation tubing wrapped around the tines of the tiller!) (The picture at the left is one of several odor biofilters at the Fairfield-Suisun SD treatment facility, Fairfield, CA. Click to enlarge it.)
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