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Gas-Phase Biofiltration 427

refers to the amount of time some unit of influent air would take to pass through the
empty biofilter bed space. In general, this is expressed as
 V 
EBRT =  b  (1)
 A f 
3
3
3
where V is the volume of the biofilter bed (m or ft ) and A is the airflow rate (m /h
b f
or cfm). The EBRT is always larger than the true residence time of the air passing
through the biofiltration system. This is because the solid-support medium occupies a
significant amount of the total area in the bed. The EBRT should not be used as a true
measure of treatment time because of the highly variable nature of the solid-support
material. The “true bed residence time” (TBRT) can be expressed as

 V × M 
TBRT =  b p  (2)
 A f 
where M is the medium porosity. Medium porosity can be anywhere from 20% to 80%
p
depending on the intraparticle (space within individual particles) and interparticle
(space between different particles) porosity. Porosity can be defined as
 V 
M =  s  (3)
p
 V 
ss
where V is the volume of a given space and V is the volume of solid-support material.
s ss
The porosity of a biofiltration medium can be determined via a simple displacement
experiment in a volumetric cylinder or via more sophisticated methods such as gas
chromatography and the use of inert gas flow through experiments (13).
The EBRT or TBRT are usually analogous values and are directly related to the
performance of the biofiltration unit. Industrial biofilters have TBRTs that can be as
short as 15 s and as long as over 1 min (14). These times are usually a function of the
design of the system relative to the concentration and formulation of the contaminants
in the airstream. More recalcitrant, less water soluble, and so on, compounds require
longer residence times. The longer the EBRT or TBRT, the better the removal of the
biofilter. However, the airflow rate at most facilities is dictated by air-change rates in
buildings or by the process from which the air is derived. Thus, as a designer of biofil-
ter systems, one’s only method of changing the EBRT or TBRT is to manipulate the size
of the biofiltration unit. This may appear to be fairly simple; however, cost and space
may not make this an easy proposition.
When evaluating the levels of contaminants to be treated, the most utilized measurement
is volumetric mass loading (VL). Volumetric loading is defined as
 A × C 
I
f
VL =   (4)
 V f 
3
where C is the concentration of influent (g/m ). Typically the range of VL is 10–160
I
3
g/m /h. Although loading is important in assessing a biofilter’s needs in terms of size,