Transitions - Sustainable Design: The Science of Sustainability and Green Engineering - page 143

Figure 3.15 Packed bed
biological control system to treat
volatile compounds. Air
containing gas-phase pollutants
( C G ) traverses porous media. The
soluble fraction of the volatilized
compounds in the airstream
partition into the biofilm ( C L )
according to Henry's law:
C L = C G /H , where H is the
Henry's law constant.
From D.A. Vallero, Fundamentals of Air
Pollution , 4th ed., Academic Press, San
Diego, CA, 2007; adapted from S. J.
Ergas and K. A. Kinney. Air and Waste
Management Association, “Biological
control systems,” in Air Pollution
Control Manual , 2nd ed., W. T. Davis,
Ed., Wiley, New York, 2000, pp. 55-65.

Waste stream (containing pollutants)

Biofilm

Irrigation

Gas
phase

Porous media

Media
particles

Treated
airstream

C L

Gas phase

C G Media

Biofilm

All three types of biological systems have relatively low operating costs since
they are operated near ambient temperature and pressure conditions. Power needs
are generally for air movement, and pressure drops are low ( 10 cm H 2 O per

meter of packed bed). Other costs include amendments (e.g., nutrients) and hu-
midification. Another advantage is the usually small amount of toxic by-products,
as well as low rates of emission of greenhouse gases (oxides of nitrogen and carbon
dioxide) compared to thermal systems.
Success is highly dependent on the degradability of the compounds present in
the airstream, their fugacity and the solubility needed to enter the biofilm (see
Fig. 3.16), and pollutant loading rates. Fugacity is the propensity of a compound
to be released from one physical stage to another. For example, Henry's Law states
that a substances potential to flee from liquid phase to the gas stage is a function
of its vapor pressure and aqueous solubility. Care must be taken in monitoring
porous media for incomplete biodegradation, the presence of substances that may
be toxic to the microbes, excessive concentrations of organic acids and alcohols,
and pH. The system should also be checked for shock and the presence of dust,
grease, or other substances that may clog the pore spaces of the media. 21

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