Page 451 - Air Pollution Control Engineering

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Gas-Phase Biofiltration 423

Fig. 1. Schematic of conventional packed-bed biofilter.

usually prehumidified to help prevent bed dryout, percolates up through this packed
bed. Water sprays, or drip feeds (see Fig. 1), are positioned over/in the bed to add extra
moisture to also prevent dryout, to provide a source for pH control, or to supply
additional nutrients. The bed is run in a minimum liquid condition to reduce pressure
drop, avoid wastage, and reduce entrainment of bacteria and production of anaerobic
zones; that is, the interparticle space is largely air and the water phase is stationary on
the surface of the solid support. Microorganisms are fixed within a biofilm on the solid
support in this type of application. Airflow may be either upflow or downflow depend-
ing on the engineering at the site and results of pilot studies. Both airflow directions
have demonstrated successes and failures, with other factors being more critical to the
success of the system.
Trickle-bed reactors differ from conventional packed-bed biofilters in that the packing
material is often synthetic packing (see Fig. 2), such as tellerettes or Pall rings, and the
liquid feed into the column is much greater. The liquid phase, after trickling through
the column, passes into another tank to allow settling of solids and additional biodegra-
dation before being pumped back. The interparticle space is largely waterfilled, with the
waterphase flowing through the media. Microorganisms are fixed within a biofilm on
the solid support in this type of application. Airflow in this type of system is usually
upflow, or countercurrent to the water flow.
The third treatment method, bioscrubbing, involves absorption of the target species
into a liquid that is sprayed countercurrently to the gas flow in a tower contactor (see
Fig. 3). The liquid phase containing the target species is then pumped around to an
activated sludge tank (see Fig. 3) where the biodegradation occurs by using freely
suspended microbes. The liquid phase is then pumped back to the absorber tower’s
spray feed system.
A large number of disadvantages (summarized in Table 2) prevent the widespread
development of biological waste gas abatement, despite its advantages (see Table 6)
(3–5,6).