Page 466 - Air Pollution Control Engineering

11_chap_wang.qxd 05/05/2004 5:19 pm Page 438
438 Gregory T. Kleinheinz and Phillip C Wright

commission authorized an odor survey that determined the main source of odors to be
from the headworks and biosolids dewatering area.
Two types of vapor-phase odor-control technology were given serious consideration:
wet chemical scrubbing and biofiltration. Wet chemical scrubbing is very effective in
removing ammonia, hydrogen sulfide, and organic-related odors. However, the major
challenge in the design of a wet chemical scrubber is minimization of chemical use and
cost. Multistage systems accomplish this best, but these systems are still slaves to sto-
ichiometry. Although effective, operation and maintenance (O&M) costs are high as
well as capital costs. Biofiltration was also considered. Because our objective was a
reliable low O&M cost system, biofiltration was a viable alternative.
Two vendor-offered biofilter systems, each with proprietary media and guaranteed
performance, were considered. Vendor “A” offered a combination of a proprietary
mixture of organic material (estimated 3- to 5-yr life) installed over ceramic balls.
The estimated capital cost was $675,000 to $1,125,000 plus engineering, installation,
3
and ducting. Media replacement cost was $75/yd . Vendor “B” offered a specially
engineered compost media with a 5-yr guarantee and 12–20% the overall size as a
3
typical biofilter. Their media replacement cost was $200/yd . Neither option was
desirable to the client.
Rather than proceed with a “turnkey” vendor-supplied biofilter system, the client
chose to characterize the odor constituents in the airstream and to pilot test a biofilter
to demonstrate the system’s performance. Lava rock was selected as the media because
of its potential for long life, thus significantly reducing O&M expenditures.
3
After over 4 mo of operation, the 56-ft pilot-scale biofilter showed excellent per-
formance. There was no visible degradation of the solid support and biomass levels
were consistent, which indicated that lava rock would likely be an effective long-
term solid support. Although there were data collected on VOCs, H S, and NH there
2 3
was also a more subjective “smell test” performed by local residents, commissioners,
and other interested parties. All of these tests demonstrated that the biofilter was
effective in eliminating a significant portion of the objectionable odors in the
airstream. Whereas the ammonia, hydrogen sulfide, and VOCs were chosen for mon-
itoring, it was impossible to determine what portion of the total odor these chemicals
actually contribute. Because extensive air analysis work indicated that there were
hundreds of chemicals in the airstream, the client chose also to conduct subjective
tests for odor removal. Because each chemical in the complex airstream has a dif-
ferent dispersion rate in air (odor threshold), it would be nearly impossible to char-
acterize the removal of each of these chemicals from the pilot-scale system. Because
each person defines “odor” differently, the client thought it was important to gain
input on the pilot-scale system from local residents (who initially complained of the
odors) and from the commissioners who will decide on funding for a full-scale sys-
tem. All residents who smell-tested the system agreed that it significantly reduced
the odors from the airstream.
As a result of the success of the pilot-scale biofilter, the cost of a 45,000-cfm chemical
scrubber was compared to a lava-rock-based biofilter. The biofilter was to be constructed
in two existing unused 100-ft-diameter steel tanks with an existing concrete floor/foun-
dation and aluminum cover. The estimated and actual costs are compared next.