Webster Environmental Associates, Inc.

Gas Phase Treatment Biofilters

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Biofilters are an odor treatment technology that utilizes biological processes as the treatment mechanism. Biofilters are considered to be a “green” approach to odor control, because they utilize microorganisms in media to oxidize odor and air emission compounds to carbon dioxide, water, biomass, and other benign by-products such as chloride and sulfate. The by-products are either emitted in the outlet air, drained from the biofilter, or consumed by the microorganisms. The biological activity in a biofilter is similar to the activities performed by the microorganisms in activated-sludge secondary wastewater treatment processes.

All liquid treatment plant processes, pump stations, sludge thickening, sludge dewatering.

Typical biofilter sections are shown below. Biofilters work by routing odorous air through a porous filter media. The media represents the contact surface area, on which the microorganisms live, where the biological oxidation described above can take place. The key to effective biofilter operation is maintaining a healthy environment for these microorganisms to thrive in. The most important parameters for maintaining a healthy

environment is moisture content and pH. The microorganisms need water to remain active, and the presence of water effects the transfer of the contaminants from the air to the media. The desired microorganisms thrive at neutral to moderately acidic pH levels.

Biofilters are less control-dependent than chemical scrubbers, because the treatment system is more self-regulating. It is essentially a self-contained ecosystem, and is therefore likely to function longer, without excessive control. It is very important, however, to ensure moisture levels are controlled in the biofilter for it to function properly. Moisture content is assisted during dry periods by simple sprinkler systems and air humidification. The pH levels are often self-regulating within the ecosystem, and are assisted by proper choice of media. However, the reaction products in a biofilter treating hydrogen sulfide is sulfuric acid so preventing the formation of a very low pH is important and design should include corrosion protection on concrete and other materials.

Biofilters are typically good applications in dilute waste streams, like those typically found in wastewaters. The biggest drawback to this technology is the relatively large space requirement, compared to the technologies mentioned above. However, modular biofilter designs using synthetic, inorganic media require a much smaller footprint than the open-bed designs. Biofilters may also use more power than chemical scrubbers, but do not involve any chemical handling or storage.

Air streams with high concentrations of reduced sulfur compounds (RSC’s) such as mercaptans, dimethyl sulfide, dimethyl disulfide, diphenyl sulfide, carbonyl sulfide, and carbon disulfide can be treated in biofilters but the loading rates must be much lower than those required to treat H2S.

Biofilter media types include wood-chip/bark media, soil media, and inorganic synthetic media.
Wood-chip/bark media generally possess a large diversity and density of microorganisms, accepts moisture relatively well, has low initial costs, and is readily available. The normal lifetime for wood-chip/bark media is 2 – 4 years.

Soil media is a blended mix of soils, primarily sand-based. The primary advantage of soil media over wood-chip/bark media are their life expectancy. Soil has an estimated lifetime of over 30 years as a filter media. Soil is denser than wood-chip/bark media and therefore resists compaction, it resists acidification because of it’s inherent pH buffering properties, it is less difficult to rehydrate after drying out, and generally distributes the air more uniformly than wood-chip/bark media. The primary disadvantage is that it requires a smaller loading per square foot, and therefore may require a larger footprint and higher initial capital costs.

The inorganic synthetic media is newer to the market but well tested. It consists of strong, uniform sized gravel-like cores that do not compact as easily as organic media. This type of media may be used in the modular designs because it allows greater media depth and a smaller footprint. The cores are commonly coated with nutrient rich organic and inorganic adsorbents. The media typically comes with a 10 year life guarantee.

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