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Biofiltration for the removal of reduced sulfur gases from low concentration air streams Wani, Altaf Hussain


In the work reported here, media effectiveness, bioelimination rates and the operational features of aerobic biodegradation of reduced sulfur gases (hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyl disulfide) in compost, hog fuel, and the mixture of compost and hog fuel biofilters have been investigated. Specific consideration was given to the biofilter media characterization, the elucidation of phenomena occurring during the transient conditions in biofilters, and the evaluation of media effectiveness in removing reduced sulfur gases both singly and in mixtures to illustrate the inhibitory effects, i f any, of one contaminant on the removability of another. Biofilter media characterization identified the significant role of filter material C /N ratio on biofilter media degradation and the length of media useful life. Stage-wise first order kinetics proved appropriate in describing the mineralization of media carbon. Compost filter media was found to be easily degradable with a loss of 17% media carbon within 127 days of incubation with ambient air in comparison to 6 and 12% carbon loss in hog fuel and the mixture (50:50 compost:hog fuel) biofilter media, respectively. Media decomposition was significantly enhanced in the presence of reduced sulfur gases as a result of increased bioactivity by sulfuroxidizing bacteria and other microorganisms thereby decreasing the media half-life by more than 50%. Evaluation of the transient response of biofilters exposed to abrupt changes in contaminant concentration and/or waste airflow rate highlighted the key role of sorption/desorption process during the transient state operation. Biofilters recovered to their initial removal capacities rapidly and, in general, the steady states were re-established within 2 to 12 hours after perturbations occurred. Contaminant concentration spikes in the waste air stream demonstrated major substrate inhibition that occurred with short-term exposure of biofilters to methyl mercaptan concentrations of 158 ppmv, however, in case of hydrogen sulfide similar inhibitory effects were observed at concentrations above 615 ppmv. Biofilters were found to be capable of withstanding downtime periods with rapid recovery to full performance when starvation ceased, and the reacclimation times for the biological activity after different periods of non-use were significantly shorter than the initial startup times. A Michaelis Menten type kinetic model modified for plug flow behavior of biofilters, with the assumptions of steady state, negligible dispersion, and rapid contaminant transfer between phases very well described the bioelimination rates in the three biofilter media materials investigated. Hydrogen sulfide biodegradation was found to be independent of the coexistence of organic sulfur gases with the maximum bioelimination capacities of 136.1, 136.8 and 138.3 g m"3 h"1 in compost, hog fuel and the mixture biofilter, respectively; and there were no noticeable differences amongst the three biofilters in their capacities for the bioelimination of hydrogen sulfide. However, the bioremovability of dimethyl sulfide and dimethyl disulfide was significantly reduced in presence of other reduced sulfur gases, and the filter materials significantly varied in their capacities for the removal of methyl sulfides. The maximum elimination capacity of the compost, hog fuel and the mixture biofilter for dimethyl sulfide, as a single pollutant, was reduced by a factor of 1.4 to 2 from its initial values of 5, 3.8 and 4.6 g m⁻³ h⁻¹, respectively, with the co-supply of methyl mercaptan and dimethyl disulfide. But, the presence of hydrogen sulfide had no adverse effects on the biodegradation of dimethyl sulfide. Instead it slightly improved the dimethyl sulfide bioelimination. Dimethyl disulfide maximum 3 1 elimination rates of 16.9, 12.3 and 13.6 g m⁻ h⁻ , as a single contaminant, in compost, hog fuel and the mixture biofilter respectively, were significantly reduced to 10.8, 8.4 and 9.6 g m⁻³ h⁻¹ in presence of hydrogen sulfide, and to 7.5, 6.1 and 7.4 g m⁻h⁻ with the co-supply of dimethyl sulfide.

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