Recent efforts have been made to reduce releases of air toxics and smog precursors from wastewater treatment plants. Hydrgen sulfide is commonly the primary odor and is an important target for removal. Its oxidation, however, generates sulfuric acid and sometimes elemental sulfur, which can create substantial operational problems for biofilters. Declining pH may inhibit the organisms that degrade compounds other than hydrogen sulfide and may hasten aging of organic Biofilter media. A two‐stage biofilter has been designed and installed at the Ojai Valley Sanitary District wastewater treatment plant. The first stage is an enclosed system with a medium of small, inert, porous stones. It is called an acid gas biofilter. The second stage is a section of a traditional open biofilter filled with wood chips. The acid gas biofilter effectively removed H₂S and volatile organic compounds while causing much lower headloss than traditional biofilters. However, considerable flow heterogeneity in both the acid gas biofilter and the wood chip biofilter was observed. The two‐stage system presumably will have a longer bed life because the first stage bed is inert and because the second stage is protected from acidification by removal of H₂S in the first stage.