Hydrogen sulfide and volatile organic compounds are often found together in waste air streams. This combination is difficult to treat by biofiltration because oxidation of the sulfide produces acid, reducing the pH in the biofilter. Rapid declines in pH can inhibit treatment of organic compounds. A two-stage biofilter, with the first stage operated at low pH and an inorganic support medium, and the second operated at neutral pH with an organic support can eliminate the problem. A pilot-scale facility was operated on this principle. Comparisons were made among two-stage treatment, single-stage low pH treatment, and single-stage uncontrolled treatment to determine which of the strategies was most effective. In two-stage treatment the first stage did provide protection for the second, allowing it to operate at neutral pH. Single-stage low pH treatment was effective at removing sulfide and many organic compounds, and may be sufficient for many applications.

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