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A Breath of Fresh Air: Bacteria rid sewage of its stink

John Travis

Have you ever driven by a sewage-treatment plant and noticed a rotten-egg stink? Air-quality requirements force these plants to use devices called chemical scrubbers to eliminate malodorous hydrogen sulfide from the gases created by bacteria in sewage slurry. The process works, but it's expensive and depends on filtering gas through toxic chemicals such as lye and bleach.

photo

BLOCKING OUT GAS. Unlike the left tower, which uses chemicals, the tower on the right at this wastewater-treatment plant now uses bacteria-covered foam blocks (inset) to eliminate the hydrogen sulfide bubbling from treated sewage.

Deshusses/PNAS

Two researchers now argue that there's a practical, biological alternative to current odor-control systems. At a sewage-treatment plant in California, they've replaced several chemical scrubbers with ones using hydrogen sulfide–degrading bacteria and trickling water.

"It's a huge step forward compared to what people thought bacteria could do," says Marc Deshusses of the University of California, Riverside. "Our findings show that for hydrogen sulfide odor control at wastewater-treatment plants, you can convert chemical scrubbers to biological trickling filters and still have the same treatment capacity much cheaper and safer."

Sewage-plant operators "would really like to get rid of the chemicals because they are corrosive and dangerous," he adds. The changeover of the first scrubber proved so successful that the California treatment plant converted additional ones to biofilters.

"In my opinion, this is very exciting work. It takes biological air treatment to the next level," says Peter Gostomski of the University of Canterbury in Christchurch, New Zealand.

Instead of using chemicals such as sodium hydroxide, or lye, and the bleach sodium hypochlorite to degrade hydrogen sulfide, the new method sends the gas over a film of bacteria growing on polyurethane foam. The microbes convert the hydrogen sulfide into odorless hydrogen sulfate, which is carried away by water trickling over the foam.

Scientists have generally considered biofilters too inefficient to challenge chemical scrubbers. Researchers thought that to be degraded, hydrogen sulfide would need to be in contact with bacteria for 10 seconds or even longer, which would require an impractically large filter for a sewage-treatment plant.

However, Deshusses and David Gabriel, now at the Universitat Autònoma de Barcelona in Spain, optimized the surface area of their filter by filling the silo-shape scrubber with 4-centimeter-cubed, porous-foam blocks, which have nooks and crevices where bacteria can grow. They also forced the plant's gas through the scrubber at much higher velocities than anyone had tried before. This degraded the hydrogen sulfide in as little as 2 seconds.

Deshusses and Gabriel describe their work in the May 27 Proceedings of the National Academy of Sciences.

"Biological air-treatment systems have always been hindered by their size compared to chemical systems," says Gostomski. The new filters "can compete with chemical systems when space is at a premium."

Retrofitting a wastewater-treatment plant's scrubbers with a biofilter system is a relatively simple matter, say Deshusses and Gabriel. It should cost between $40,000 and $60,000 to convert most hydrogen sulfide chemical scrubbers, but plants can then save $30,000 a year in operating costs, such as the expense of the chemicals, say the researchers.

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Letters:

This article could have been a little less disingenuous. It would still have been a very good article if you had used "sulfuric acid" instead of "odorless hydrogen sulfate" and admitted that the process still required a little alkali to neutralize this waste stream that is "carried away by water trickling over the foam."

Jim Nottke
Pfafftown, NC

I would have liked it if the article had mentioned something more about where the bacteria for this were found. Do they occur naturally around us? Were they isolated from deep-sea hydrothermal vents?

Richard Fellows
Pleasanton, CA

The investigators used a mixture of naturally occurring bacteria from soil and other nonextreme environments.—J. Travis

References:

Gabriel, D., and M.A. Deshusses. 2003. Retrofitting existing chemical scrubbers to biotrickling filters for H2S emission control. Proceedings of the National Academy of Sciences 100(May 27):6308–6312. Abstract.

Further Readings:

Phillip, L. and M.A. Deshusses. 2003. Sulfur dioxide treatment from flue gases using a biotrickling filter-bioreactor system. Environmental Science and Technology 37(May 1):1978–1982. Abstract.

Sources:

Marc Deshusses
Department of Chemical and Process Engineering
University of Canterbury
Private Bag 4800
Christchurch
New Zealand

David Gabriel
Department of Chemical and Environmental Engineering
University of California, Riverside
Riverside, CA 92521

Peter Gostomski
Department of Chemical and Process Engineering
University of Canterbury
Private Bag 4800
Christchurch
New Zealand


From Science News, Volume 163, No. 19, May 10, 2003, p. 294.