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Fig. 5. Full-scale biofilter design from the above case study. (From G. Kleinheinz, unpublished.)

Based on these costs and the pilot-scale demonstration, the biofilter system was
selected. Figure 5 shows a cutaway view of the basic design. Each existing steel tank
was retrofitted to hold 4-ft-deep lava rock media. Stainless-steel grating was used to
support the rock. PVC piping was used to distribute the foul air throughout the tank
floor. A spray system using nonpotable water was used to keep the lava rock moist.
Approximately 20 gpm/unit was provided to keep the lava rock moist. All drainage was
collected and returned to the headworks for treatment. A chemical feed pump was pro-
vided to allow for the addition of nutrients if needed. The biofilter exhaust would exit
the biofilter through the hatch openings on the aluminum covers. At a flow rate of
approx 45,000 cfm, the units were sized to have an approximate empty bed residence
time of 1.4 min. The lava rock has a porosity of approximately 50% for an actual res-
idence time of approx 42 s. Based on pilot testing, this should allow for further air
handling capacity in the future if needed.
The pilot test results demonstrated that biofiltration was a capital-cost-competitive,
low-O&M-cost solution for effective odor control at this site. The biofilter provided the
added benefit (over chemical scrubbing) of not requiring the on-site storage of large
amounts of toxic chemicals. Test data allowed for a properly sized system specific to
the odor constituents, rather than force-fitting a vendor system to the site. The pilot test
allowed for local politicians and area residents to sample the air quality from the unit,
which allowed their buy-in to the technology.
The unit has operated for over 2 yr with virtually no odor complaints from local
residents. Given the relatively low cost of operation and the success in terms of public
relations and odor mitigation, this application of biofiltration has been a success.