Biotreatment of H ₂ S- and NH ₃ -containing waste gases by co-immobilized cells biofilter

Gas mixture of H₂S and NH₃ in this study has been the focus in the research area concerning gases generated from the animal husbandry and the anaerobic wastewater lagoons used for their treatment. A specific microflora (mixture of Thiobacillus thioparus CH11 for H₂S and Nitrosomonas europaea for NH₃) was immobilized with Ca-alginate and packed inside a glass column to decompose H₂S and NH₃. The biofilter packed with co-immobilized cells was continuously supplied with H₂S and NH₃ gas mixtures of various ratios, and the removal efficiency, removal kinetics, and pressure drop in the biofilter was monitored. The results showed that the efficiency remained above 95% regardless of the ratios of H₂S and NH₃ used. The NH₃ concentration has little effect on H₂S removal efficiency, however, both high NH₃ and H₂S concentrations significantly suppress the NH₃ removal. Through product analysis, we found that controlling the inlet ratio of the H₂S/NH₃ could prevent the biofilter from acidification, and, therefore, enhance the operational stability. Conclusions from bioaerosol analysis and pressure drop in the biofilter suggest that the immobilized cell technique creates less environmental impact and improves pure culture operational stability. The criteria for the biofilter operation to meet the current H₂S and NH₃ emission standards were also established. To reach Taiwan's current ambient air standards of H₂S and NH₃ (0.1 and 1 ppm, respectively), the maximum inlet concentrations should not exceed 58 ppm for H₂S and 164 ppm for NH₃, and the residence time be kept at 72 s. (C) 2000 Elsevier Science Ltd.