Page 311 - Materials Chemistry, Second Edition

294 Practical Design Calculations for Groundwater and Soil Remediation

Sizing an IC engine device is based on the volumetric flow rate of the
VOC-laden air to be treated. One vendor reports that their IC engine unit
can handle up to 80 scfm of VOC-laden air, while the other reports that their
unit can accommodate 100 to 200 scfm of influent gas (depending on the
VOC concentrations) for every 300 in. of engine capacity [2]. Conservatively
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speaking, a typical IC engine should not handle more than 100 cfm of VOC-
laden air. For a higher flow rate, a treatment system with a few IC engines in
parallel would be needed.

Example 7.16: Determine the Number of IC Engines Needed

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
IC engines. Determine the number of IC engines needed for this project.

Solution:
The average off-gas flow rate is 200 scfm, and a typical IC engine can
only handle 100 scfm as the maximum. Therefore, a minimum of
two IC engines in parallel should be used in this project.

7.6 Soil Beds/Biofilters
In biofiltration, the VOC-laden air is vented through a biologically active soil
medium where VOCs are biodegraded. The temperature and moisture of the
air stream and biofilter bed are critical in design considerations.
Biofiltration is cost effective for large-volume air streams with relatively
low concentrations (<1,000 ppmV as methane). Maximum influent VOC
concentrations have been found to be 3,000–5,000 mg/m . For optimum
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efficiency, the waste air stream should be at 20°C–40°C and 95% relative
humidity. The filter material should be maintained at 40%–60% moisture
by weight and a pH between 7 and 8. Typical biofilter systems have been
designed to treat 1,000–150,000 m /h waste air, with the systems having
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10–2,000 m of filter media. The typical depth of biofilter media is 3–4 ft [2].
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The typical surface loading rate is 100 m /h of waste air stream per m filter
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2
cross-sectional area. The required cross-sectional area of the biofilter (A filter )
can be determined as:
Airflowrate
A biofilter = (7.27)
Surfaceloading rate