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426 Gregory T. Kleinheinz and Phillip C Wright

concentration (9). By lowering airflow rates, the biofilter has more contact time with the
chemicals and high degradation rates. The total loading on the system can be precisely
manipulated to achieve the highest degradation possible. Several companies are cur-
rently marketing systems that operate on this principle. In addition, the automotive paint
industry has used this adsorption/desorption technology to trap airborne pollutants and
send high concentrations to thermal oxidizers.

2.2.2. Mobilized-Bed Biofilters
As listed in Table 2, conventional technologies may be limited because of mass transfer
or mixing limitations. Three-phase fluidized (or mobilized) beds may be an alternative
to conventional packed-bed biofilter and absorber/scrubber/trickle-bed methods. They
have a number of inherent advantages for multiphase contacting, such as good inter-
phase mixing and heat and mass transfer performance. This contactor type also
removes the disadvantages of poor moisture and temperature control inherent in other
vapor-phase biofiltration systems.
There are some limited studies into this area (e.g., ref. 10); however, more work is
required before their widespread use is acceptable, particularly in relation to process
control and biological support matrices. Having said that, there are some industrial
examples of mobile-bed types of biofilters/bioscrubbers, such as the SC Bioreactor TM
system in the United Kingdom (Waterlink/Sutcliffe Croftshaw Ltd, Lancashire, UK).

2.2.3. Integrated/Train Processing
Some preliminary lab studies have been conducted which combine biological treatment
technologies into “treatment trains” for the treatment of complex waste streams contain-
ing chemicals with very different chemical properties (11,12). These systems combine the
benefits of other reactor systems such as liquid reactors or chemical catalytic reactors (i.e.,
fast degradation rates or the ability to degrade more complex species) with biofilters for
the removal of highly volatile compounds such as methanol and 2-propanol. By treating
systems with “treatment trains,” airstreams with over 10,000–15,000 ppmv of VOCs can
be successfully treated at > 95% efficiency.
As an example, one of the possibilities is to use catalytic combustion to partially
deconstruct the VOC molecules. Catalytic combustion is often not suitable alone, as the
by products are often toxic in themselves. Therefore, suitable downstream treatment is
important, and biofiltration offers a cost-effective route (12).

2.2.4. Extremophilic Systems
The operating window of many biofiltration systems is being widened by the appli-
cation of so-called extremophiles, which thrive under conditions that normal micro-
organisms may find intolerable. For example, temperatures of over 60–80ºC have
been demonstrated, as have extremes of pH (both high and low), tolerance to high
concentrations of pollutants, and extremely high salinity.

3. OPERATIONAL CONSIDERATIONS
3.1. General Operational Considerations

In order to understand biofilter operation, we must look at some important terminology
related to the operation of biofilters. The term “empty bed residence time”(EBRT)