Page 312 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 295

Example 7.17: Sizing Biofilters for Off-Gas Treatment
Referring to the remediation project described in Example 7.13, an off-gas
stream (Q = 200 scfm) containing 800 ppmV of xylenes is to be treated by
biofilters. Determine the size of the biofilters needed for this project.

Solution:
(a) The off-gas contains 800 ppmV of xylenes, which is equiva-
lent to 6,400 ppmV as methane (each xylene molecule contains
eight carbon atoms). This is beyond the typical range of <1,000
ppmV as methane. The maximum influent VOC concentra-
tions of 3,000–4,000 mg/m have been reported in the literature.
3
Although the xylenes concentration in this case (800 ppmV of
xylenes = 3,460 mg/m ) falls within the range, a dilution of off-
3
gas would be a conservative approach. The optimal influent
concentration should be determined from a pilot study. In this
example, let us dilute the off-gas four times; therefore, the influ-
ent flow rate to the biofilter becomes 800 scfm.
(b) The typical surface loading rate is 100 m /h of waste air stream
3
per m filter cross-sectional area. Let us convert 800 cfm to m /h
2
3
as:
Q = 800 ft /min = (800 ft /min)(60 min/h)(0.0283 m /ft )
3
3
3
3
= 1,360 m /h
3
Use Equation (7.27) to determine the required cross-sectional
area as:
3
Airflowrate 1,360 m/h
A biofilter = =
Surfaceloading rate 100 m/h/m 2
3
= 13.6 m 2 = 146 ft 2

Use a typical value of 4 ft as the depth of the biofilter.

Discussion:
If the biofilter is constructed in a cylindrical shape, the diameter of the
biofilter would be around 14 ft.