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432 Gregory T. Kleinheinz and Phillip C Wright

Table 5
Comparison of Biodegradability of Various Chemicals
Rapidly degradable Slowly degradable Very slowly degradable
Alcohols Hydrocarbons Tricholorethylene
Aldehydes Phenols Trichlorethane
Ketones Methylene chloride Carbon tetrachloride
Esters Mercaptans Polyaromatic hydrocarbons (PAHs)
Ethers Hydrogen sulfide CS
2
Organic acids Nitroaromatics Monoterpenes
Amines
Source: ref. 5.

3.4.1. Biodegradability
It has been reported that not all VOCs (5), and indeed other classes of compounds,
are easily biodegradable. This results in incompatibility of the technology for all pollu-
tant chemicals. As environmental legislation becomes tighter, more novel and efficient
technologies for gas treatment will become necessary. The comparison of the relative
ease of biodegradation of a number of typical pollutants is presented in Table 5.
A number of research challenges exist to ensure the total removal of pollutants. The
“big picture” is how to modify existing bioreactors for the removal of major pollu-
tants. The problem, notably with recalcitrant compounds such as trichloroethylene and
polyaromatic hydrocarbons (PAHs), is that the size of the reactor to provide exit air of
an approved standard is often enormous. In the rare areas of high land availability, this
inefficient use of space is not a problem. Emerging technologies are being developed to
solve this problem, as discussed later in this chapter.
3.4.2. Solubility
In developing design considerations for biofilters, or assessing if biofiltration is an
appropriate treatment technology, there are numerous chemical considerations. One of the
most important chemical parameters is the aqueous solubility of the compound(s) of
interest. Because the biodegradation in biofiltration systems occurs in an aqueous
biofilm, it is critical that the chemical be able to partition into this phase. Once the
chemical is in the liquid phase, it is bioavailable, but not before. Chemical structure is
also an important parameter to consider because some structures are more susceptible
to biodegradation than others. Microbes can degrade chemicals at very different rates
(see Table 5). For highly water-soluble compounds, the rate of biodegradation in the
biofilm can be directly related to the rate of chemical movement from the air phase to
the aqueous phase. For compounds that are not very water soluble, the rate of diffusion
from the air phase to the liquid phase may limit biodegradation (26). It is desirable to
have the rates of biodegradation, and so on, be correlated to the residence time of air-
flow through the biofilter; that is, generally the more water-soluble the compound, the
more rapidly it is degraded in the biofilter and the shorter the residence time required.
Conversely, the less water-soluble compounds require longer residence times because
of the limiting effect of chemical diffusion. One additional consideration is the toxicity