Two-stage biofiltration for treatment of POTW off-gases (Hydrogen sulfide)

Recent efforts have been made to reduce releases of air toxics and smog precursors from wastewater treatment plants. Hydrogen 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 was designed and installed at the Ojai Valley Sanitary District wastewater treatment plant. The first stage was an enclosed system with a medium of small, inert, porous stones. It was called an acid gas biofilter. The second stage was 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 was inert and because the second stage was protected from acidification by removal of H₂S in the first stage.; As a result of the observed flow heterogeneity, a two-dimensional steady-state computational fluid dynamics model was developed to simulate flow through a biofilter. This modeling effort demonstrated that the optimal design depends on the permeability of the medium, the operational flowrate, and biofilter design. In very low permeability medium, the flow had low flow heterogeneity regardless of the design. With highly permeable medium, the design was significant. Several different design options were compared. Choice of optimal design depended on the operational Reynolds number, however, the design with top, side inlet and bottom, same side outlet had the lowest flow heterogeneity of the five designs over a wide range of Reynolds number. This modeling effort also demonstrated that removal efficiency decreases as flow heterogeneity increases.