Removal Of Nitric Oxide Via Denitrification In A Hollow-fiber Membrane Bioreactor

Nitrogen oxides (NOx) are major atmospheric pollutants, which could contribute to environment issues and health risk. Traditional physical and chemical technologies for NOx removal, such as selective catalytic reduction, selective non-catalytic reduction, carbon reduction method, pulse corona induced plasma chemical process, adsorption and scrubbing, most of which are high cost and high energy consumption, have low removal efficiencies and produce secondary pollutants. Compared with those physical and chemical technologies, biological technology for the removal of NOx is an attractive alternative because of its lower energy consumption, less secondary pollutants and lower operating cost. Membrane bioreactors have been widely applied to the removal of volatile organic compounds, demonstrating good performance. Nevertheless, little was reported on the removal of NOx with membrane bioreactor.A polyvinylidene fluoride (PVDF) hollow-fiber membrane bioreactor was studied for its ability to treat nitric oxide (NO) via denitrification. The experiments were designed to optimize the varying operation parameters and evaluate the effects of inlet NO concentration, gas residence time. pH value, inlet sulfur dioxide concentration, and inlet oxygen concentration on the removal of NO. The transformation process and pathway of NO in the membrane bioreactor was explained and then the N balance was deseribed. The quality of effluent and membrane fouling were studied during the continuous operation of membrane biotractor. A denaturing gradient gel electrophoresis of polymerase chain reaction-amplified genes coding for16S rRNA was used to analyze and determine the changes in baeterial communities and dominant strains in the membrane bioreactor. The main experimental results are as follows:(1) The experiment was operated steadily for nine months. NO removal efficiencies and elimination capacities maintained at60-100%and10-83g/m3·d, respecttively. With an increase in gas How rate. COD values of the supernatant fluid and the effluent gradually decreased, and the membrane fouling cycle can be effectively prolonged. Products analysis was performed to suggest that NO in the vials was directly reduced to N2O and N2by the inoculum from the bioreactor. Furthermore, no NH4+, NO2-, or NO3-was produced in measurable amount during the removal of NO. These results suggested the existing of denitrification.(2) The removal efficiency increased with the increase of gas residence time, whereas the elimination capacity decreased. The removal efficiency decreased with an increase in inlet NO concentration, whereas the elimination capacity increased. Microbial communities of the membrane bioreactor were sensitive to the variation in pH value and alkalescency corresponding to an optimum pH value of8. The maximum elimination capacity was83g/m3-d with a removal efficiency of86%under the conditions of gas residence time of30s, inlet NO concentration of2680mg/m3and pH8. NO elimination capacity and removal efficiency were inversely proportional to the inlet oxygen concentration. The inlet oxygen concentration should be controlled at2%or less to ensure higher NO removal efficiency. In addition, low-concentration sulfur dioxide had no great influence on denitrification removal of NO in the membrane bioreactor.(3) The PCR-DGGE results showed that the major populations of the membrane bioreactor were Spirochaetes, a-Proteobacteria, γ-Proteobacteria and Bacteroidetes groups. It was concluded that denitrification was mainly caused by a-Proteobacteria.