Biological elimination of H2S and NH3 from wastegases by biofilter packed with immobilized heterotrophic bacteria
Introduction
H2S and NH3 are irritating, smelly substances with very low odor thresholds: 1.1 ppb for H2S and 37 ppb for NH3 (Henry and Gehr, 1980). These two unwanted gases are usually liberated in industrial processes, including food preparation, livestock farming, leather manufacturing and wastewater treatment Eikum and Storhang, 1986, Ryer-Power, 1991, Yang and Allen, 1994, Chung et al., 1996a. Many technologies have been used to treat malodorous compounds from contaminated air. As regulatory measures move toward more stringent control of malodorous compounds, the demand for cost-efficient air pollution control technology will increase. Currently, biotreatments have drawn great attention, especially biofiltration, because they cost less than conventional methods and have comparable removal efficiency (Leson and Winer, 1991).
In treating exhaust gas, the selection of packing materials and inoculated microorganisms has a decisive effect on the biofilter operation. Recently, utilizing immobilized cells as packing materials for wastegas removal has been proved to be very promising Chung et al., 1996b, Chung et al., 1998. In the case of H2S removal, heterotrophic bacteria Pseudomonas putida CH11 performed better than autotrophic bacteria Thiobacillus thioparus CH11, while operating at low inlet H2S concentration (<20 ppm) over a long-term period Chung et al., 1996c, Chung et al., 1996d. For NH3 removal, heterotrophic bacteria Arthrobacter oxydans CH8, isolated from piggery wastewater, performs better than autotrophic bacteria Nitrosomonas europaea, especially for treating high concentrations of NH3 Chung et al., 1997, Chung and Huang, 1998. However, there have been no studies on the biological treatment of H2S and NH3 in an air stream simultaneously. The Taiwan EPA sets the ambient air standard at 0.1 and 1 ppm for H2S and NH3, respectively. To reach the current H2S/NH3 emission standards and forthcoming higher standards in the future, the outlet exhaust must satisfy the current legal standards. Thus, critical operating parameters of the biofilter need to be established as soon as possible.
The objective of this study was to determine the effectiveness of co-immobilized biofiltration technology on gas mixtures of H2S and NH3. In this study, a P. putida CH11 and A. oxydans CH8 co-immobilized biofilter were used to remove a H2S and NH3 gas mixture, where P. putida CH11 is effective in removing only H2S and A. oxydans CH8 is effective in eliminating only NH3 Chung et al., 1996c, Chung et al., 1996d. Various ratios of inlet H2S/NH3 gas mixtures were introduced into the biological system to investigate the removal efficiency, mechanism, metabolized products and kinetic parameters of the biofilter. In addition, enzyme kinetic theory was used to develop a model to estimate the maximum inlet concentration for practical application.
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Organism cultivation and medium preparation
The original pure-culture strains of heterotrophic ammonia oxidizer, A. oxydans CH8 and heterotrophic sulfur oxidizer P. putida CH11 were isolated from swine wastewater Chung et al., 1996c, Chung et al., 1997. Stock cultures were both grown in nutrient broth at 30°C. The nutrient broth contained yeast extract 5 g/l, tryptone 10 g/l, and dextrose 2 g/l. In all continuous experiments, the inflow medium was used and stored in the nutrient tank. The inflow medium contained glucose 0.2 g/l, KH2PO4
H2S/NH3 removal efficiency in continuous operation
The removal efficiencies for different ratios (e.g., 1:1, 1:2, and 2:1) of H2S/NH3 gas mixtures at various time are illustrated in Fig. 2. A ratio of 1:1 for inlet H2S/NH3 was used during the first 7-day period, then a ratio of 1:2 was used for the following 7-day period, and a ratio of 2:1 was used for the last 7-day period. During the operating period, the circulation solution with fresh medium was replaced at day 14. When H2S and NH3 were mixed in a ratio of 1:1, both the removal
Conclusions
The results of our experiments have indicated that the biofilter successfully handled the gas mixture of H2S and NH3 within the 5–65 ppm range, showing removal efficiencies greater than 96% under these conditions. H2S and NH3 at high concentrations were observed to be inhibitory substrates for H2S removal. H2S of moderate concentrations favored NH3 removal. Conversely, high H2S concentrations resulted in low removal efficiency. The phenomenon was explained by kinetic analysis and the related
Acknowledgements
Funding for this work was provided partially by the National Science Council, ROC.
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