Bacteria rid sewage of its stink. (A Breath of Fresh Air).

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.

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 Autonoma 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 an upcoming 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.