Mechanisms

Biofiltration combines the mechanism of adsorption, the washing effect of water (for scrubbing), and oxidation. Soils and compost have porosity and surface areas similar to those of activated carbon and other synthetic adsorbents. Soil and compost also have a microbial population of more than 1 billion antiomycetes (microorganism resembling bacteria and fungi) per gram (Alexander 1977). These microbes oxidize organic compounds to carbon dioxide and water. The oxidation continuously renews the soil beds adsorption capacity (see Figure 5.22.2).

Another distinction is that the moisture in the waste gas stream increases the adsorption capacity of water-soluble gases and is beneficial for the microbial oxidation on which the removal efficiency of biofilters depends. Conversely, the moisture adsorbed by synthetic adsorbents reduces their air contaminant adsorption capacity and removal efficiency. In addition, biofilter beds also adsorb and oxidize volatile inorganic compounds (VICs) to form calcium salts.

Gases in air flowing through soil pores adsorb onto or, as in GC, partition out on the pore surfaces so that VOCs remain in the soil longer than the carrier air. Soil-gas partition coefficients indicate the relative strengths of retention. The coefficients increase with VOC molecular weight and the number of oxygen, nitrogen, and sulfur functional groups in the VOC molecules. In dry soils, the coefficients for VOCs have been reported from 1 for methane to 100,000 for octane. However, under moist conditions, because of the water-soluble nature, the soil-gas partition coefficient for octane is probably a thousand, and acetalde-hyde is around several thousand (Bohn 1992).

The biofilter's capacity to control air contaminants depends on the simultaneous operation of both adsorption and regeneration processes. Thus, overloading the system through excessive air flow rates can affect the biofilter's removal efficiency so that adsorption rates are lower than the rates at which chemicals pass through the filter. Once all adsorption sites are occupied, removal efficiency diminishes rapidly.

A second limiting factor is the microbial regeneration rate of the adsorbed chemical, which must equal or exceed the adsorption rate. Toxic chemicals can interfere with microbial processes until a bacterial population develops that can metabolize the toxic chemical. Biofilter bed acidity also reduces the removal efficiency because the environment for soil bacterial is inhospitable. In most cases

FIG. 5.22.1 Soil bed. A biofilter consists of a bed of soil or compost, beneath which is a network of perforated pipe. Contaminated air flows through the pipe and out the many holes in the sides of the pipe (enlarged detail), thereby being distributed throughout the bed.

of biofilter failure, the limiting factor is filter overloading rather than microbiological processes because of the great diversity and number of soil bacteria.

After start up, biofilter beds require an adaptation time for the microbes to adapt to a new air contaminant input and to reach steady state. For rapidly biodegradable compounds, the adaptation time is no more than several hours. As the biodegradability decreases, the adaptation time can take weeks. After start up, the bed is resistant to shock load effects. Table 5.22.1 summarizes the biodegradabil-ity of various gases.

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Organic Gardeners Composting

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