Technology Solutions - American Chemical Society

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Technology▼Solutions Environmentally friendly odor removal

© 2003 American Chemical Society

This allows the footprint of these reactors to be much, much smaller than in the past,” he says. To achieve a gas contact time that is comparable with that of a chemical scrubber (1.6–2.3 s), the researchers passed foul air through the biofilter at a high velocity and chose a packing material with a high surface area. “H2S removal in high-performance MARC DESHUSSES

The smell of rotten eggs generated by more than 50,000 wastewater treatment plants (WWTPs) around the world is a major problem. The odor is primarily due to hydrogen sulfide gas (H2S), but a complex blend of reduced sulfur compounds, amines, and low-molecular-weight carboxylic acids are also to blame. Although some WWTPs have no odor controls whatsoever, many use chemical scrubbers to remove H2S, which is considered to be the biggest odor nuisance. Chemical scrubbers typically use a caustic solution, which absorbs H2S, and often include chemicals, such as hypochlorite (bleach) or peroxide, to oxidize H2S to harmless and odorless sulfate/sulfite. In addition, byproducts such as toxic halomethanes are emitted in the process. Researchers at the University of California–Riverside have now found a way to convert existing chemical scrubbers into biological trickling filters for odor control (Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 6308–6312). The technology eliminates toxic chemicals; removes H2S and other compounds, such as ammonia, carbonyl sulfide, methyl mercaptans, and volatile organic compounds (VOCs); and could save an average wastewater treatment plant in the United States $10,000–50,000 per year per scrubber. Moreover, the researchers estimate that if 25–40% of the chemical scrubbers worldwide were converted to biotrickling filters, it would result in net energy and chemical savings of $0.25–2 billion per year. “The use of biotrickling filters for odor control is not a new idea,” emphasizes Marc Deshusses, a professor of chemical and environmental engineering, who along with postdoctoral student David Gabriel, found a way to optimize the approach. “For the first time, we showed that you could have effective biotreatment with a gas contact time that is very short.

Polyurethane foam cubes, shown here being wetted prior to installation in a bioscrubber, provide the ideal home for sulfide-oxidizing microbes. When foul air, such as that emitted by wastewater treatment plants, is passed over the foam packing material, microbes living on it convert smelly H2S into odorless sulfate.

biotrickling filters can be partially limited by external mass transfer. The unusually high air velocity in our biotrickling filter resulted in a higher gas–film mass transfer coefficient and outstanding H2S removal,” says Deshusses. In the past, successful treatment of H2S using biological scrubbers has required longer gas contact times (10–30 s), and therefore, biotreatment was thought to be impractical for controlling large-scale

H2S emissions, such as those from WWTPs. In a biotrickling filter, a biofilm is grown on some sort of packing material. In this case, the researchers found that open-pore polyurethane foam cubes provided the best performance because of their large surface area. Water is sprayed on top of the packing material and allowed to trickle down, which keeps the microbes wet. Foul air passes over the material, and pollutants are taken up by the microbes in the biofilm. In the system designed by Deshusses and Gabriel, secondary wastewater effluent is constantly pumped through the biofilter, supplying nutrients and microbes to the biofilm. “We have a mixture of microorganisms in the biofilm degrading sulfides, VOCs, and other compounds,” says Deshusses. Although the researchers have not characterized which microbes are doing the work, Thiobacillus species are known to oxidize sulfide to sulfate, using carbon dioxide as a carbon source, and are likely to be involved. In addition, because there is nitrate in the feed, anaerobic nitrate-reducing bacteria, which also oxidize sulfide, could be involved, speculates microbiologist Joseph Suflita of the University of Oklahoma. Biotrickling filters are undergoing testing at the Orange County Sanitation District (OCSD) in California, which spends about $3.5 million per year treating odor emissions from its two WWTPs. Thus far, a total of five chemical scrubbers at OCSD have been converted into bioscrubbers, says Deshusses. After almost two years of continuous operation, the bioscrubbers still have H2S removal efficiencies of more than 98%. “The interesting contribution here is that they can use the existing infrastructure of the wastewater treatment industry,” says Suflita. “I think it would work at all wastewater treatment plants,” he adds. —BRITT E. ERICKSON

JULY 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 251 A