Page 459 - Air Pollution Control Engineering

11_chap_wang.qxd 05/05/2004 5:19 pm Page 431
Gas-Phase Biofiltration 431

phase to the liquid phase. These are the aqueous solubility of the chemical and the rate
of microbial metabolism in the biofilm.
Because the degradation of target compounds always occurs in the liquid phase,
biofilters must maintain a hospitable environment for the microbes present in the
biofilm. Generally, biofilters operate at a neutral pH of 6–8. However, some applica-
tions require low pH systems (pH of approx 2), such as the use of Thiobacillus species
to oxidize hydrogen sulfide and other reduced-sulfur compounds. At neutral pHs,
numerous genera have been identified in operational biofilters, including Pseudomonas,
Alcaligenes, Xanthomonas, and several others. Although these organisms have been
implicated in biofilter operation, there is likely to be a consortium active in a successful
biofilter working together to degrade the chemicals of interest.
It is generally accepted that many types of microorganism contribute to the overall
degradation of the chemicals in the system. This includes bacteria, protozoa, and
fungi. Although microbial metabolism is required for destruction of the target chem-
ical, too much metabolism can lead to biomass overgrowth and subsequent clogging
because of the biofilter bed. To compound this issue, filamentous fungi can cause sig-
nificant decreases in performance with only modest increases in growth because of
their highly filamentous nature. Thus, when considering the growth of these systems,
it is desirable to achieve a balance among chemical input, microbial growth, and
microbial death. The sum of this would be a constant microbial population that could
be maintained consistently over a relatively long period of time.
There has been some debate regarding the effectiveness of inoculating biofiltration
units with microorganisms. It is safe to say that synthetic media require some sort of
microbial inoculum. However, natural media may or may not require such inoculum.
The capabilities of the indigenous microorganisms should be evaluated at bench/pilot
scale to determine if they possess the required metabolic capabilities. Should the nec-
essary organisms be present, classical microbial ecology theory suggests that the
microbes most adapted (fastest degraders or most capably of surviving in the system)
will outcompete those less adapted. Although inoculating may not harm a biofilter sys-
tem, it may be a waste of time and resources. Conversely, inoculating synthetic media
with specially selected microbes (from a laboratory enrichment for example) may
significantly increase degradation rates. This inoculum may not grow in the system
at a steady-state level and may lead to an overgrowth in the system and subsequent
operational problems.

3.4. Chemical Considerations
It has been shown that malodorous gases often contain a rich “cocktail” of chemical
species (5). Such typical compounds include hydrogen sulfide (H S), mercaptans,
2
volatile organic and inorganic compounds (VOCs and VICs), volatile fatty acids, aromatic
and aliphatic compounds, and chlorinated hydrocarbons. These gases can obviously
pose an environmental threat in addition to their unpleasant odor. Therefore, the chemical
nature of these compounds is important when choosing a biofiltration option, if possible.
This section discusses the most important issues to take into account when examining
the pollutant one is trying to abate.