Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 142

from pump-and-treat systems, requires a comprehensive approach. Otherwise, we
are merely moving the pollutants to different locations or even making matters
worse by either rendering some contaminants more toxic or exposing receptors
to dangerous substances.
BIOLOGY IS THE GREEN ENGINEER'S FRIEND
With the foregoing attention to physics and chemistry in green design, we must
keep in mind that biology is invaluable in both active and passive treatment
systems. This is well known in water and soil cleanup. However, it applies to all
green technologies. For example, in recent decades air pollutants have been treated
microbially. Waste streams containing volatile organic compounds (VOCs) may
be treated with biological systems. These are similar to biological systems used
to treat wastewater, classified as three basic types: (1) biofilters; (2) biotrickling
filters; and (3) bioscrubbers.
Biofilms of microorganisms (bacteria and fungi) are grown on a porous medium
in biofilters and biotrickling systems. The air or other gas containing the VOCs
is passed through a biologically active medium, where the microbes break down
the compounds to simpler compounds, eventually to carbon dioxide (if aerobic),
methane (if anaerobic), and water. The major difference between biofiltration
and trickling systems is how the liquid interfaces with the microbes. The liquid
phase is stationary in a biofilter (see Fig. 3.15), but liquids move through the
porous medium of a biotrickling system (i.e., the liquid “trickles”).
A particularly green method of biofiltration uses compost as the porous
medium. Compost contains numerous species of beneficial microbes that are
already acclimated to organic wastes. Industrial compost biofilters have achieved
removal rates at the 99% level. Biofilters are also the most common method for
removing VOCs and odorous compounds from airstreams. In addition to a wide
array of volatile chain and aromatic organic compounds, biological systems have
successfully removed vapor-phase inorganics, such as ammonia, hydrogen sulfide,
and other sulfides, including carbon disulfide and mercaptans. The operational
key is the biofilm. The gas must interface with the film. In fact, this interface
may also occur without a liquid phase (see Fig. 3.16). According to Henry's law,
the compounds partition from the gas phase (in the carrier gas or airstream) to
the liquid phase (biofilm). Compost has been a particularly useful medium in
providing this partitioning.
The bioscrubber is a two-unit setup. The first unit is an adsorption unit.
This unit may be a spray tower, bubbling scrubber, or packed column. After this
unit, the airstream enters a bioreactor with a design quite similar to that of an
activated sludge system in a wastewater treatment facility. Bioscrubbers are much
less common than biofiltration systems in the United States. 20