| Acid rain is the chemical reaction between atmospheric pollutants (such as sulfides andnitrides) and water and oxygen in the air. Amount of acid rain in China have undergone analmost exponential increase over the past decade and become the world's third largest acidrain area after Europe and North America.Anthropogenic emissions of nitrogen oxides (NOx),which are mainly produced during the burning of fossil fuels, have negative effects onecosystems and contribute to the formation of acid rain. In China NOx are mainly emittedfrom coal-fred power plants. The biological removal of NOx from contaminated gas streamsby biofilters is efficient low-cost NOx abatement technologies with no second-pollution. Gasexiting the scrubbers typically exhibit temperatures between50and60℃. The bioreactorpacking materials must contain suitable concentrations of denitrifying bacteria within thistemperature range. Additionally, While the flue gas discharged from coal-fired power plantsis estimated to contain typically3%-8%(v/v) O2, which had negative effect on traditionaldenitrifying process. In order to eliminate these drawbacks in biofilters, the technology ofnovel aerobic denitrifying bacterial, which were cultured and inoculated in biotrickling filterto ascertain oxygen effects on NOx removal under thermophilic condition, is meaningful.In the present study, an aerobic denitrifier was isolated from the biofilm of an fieldbiotrickling filter and its aerobic denitrification activity was evaluated under variousconditions in batch reactor experiments at50℃. This bacterium was Gram negative, shortrod, with the size of0.78μm×1.27μm. Based on biochemical studies and16S rRNAsequencing analysis, the isolate was identified as Chelatococcus daeguensis strain TAD1. Atpresent, no C. daeguensis strain has been reported to be an aerobic denitrifier. The C.daeguensis TAD1was examined to determine the effects of different carbon sources, C/Nratios and dissolved oxygen concentration on aerobic denitrification activity at50℃. C.daeguensis TAD1efficiently removed nitrate using disodium succinate as the sole carbonsource. Nitrate was hardly reduced when glucose was used at50℃. The optimal C/N ratiowas9, giving a denitrification efficiency of96.1%and higher carbon concentrations did notinhibit cell growth and denitrification activity. C. daeguensis TAD1tolerated oxygen levelsabout5.1mg/L. By change of different nitrogen sources activity of nitrite reductase was determined that its high activity was only high in the appearance of nitrate reductase activity.The presence of ammonia nitrogen could improve the overall nitrogen removal efficiency ofstrain TAD1, but the level of increase is not large. The maximum specific growth rate μmaxand the saturation constant Ks in the Monod equation were further determined by changingcarbon concentration. The denitrification efficiency of C. daeguensis TAD1was higher thanthat of mesophilic bacterium.The development of a thermophilic biotrickling flter (BTF) system to inoculate a newlyisolated strain of Chelatococcus daeguensis TAD1for the effective treatment of nitric oxide(NO) is described. A bench-scale BTF was run under high concentrations of NO and8%O2in thermophilic aerobic environment. The inlet NO concentration fluctuated betweenapproximately133.9and669.6mg/m~3and kept on a steady NOx removal rate above80%inan oxygen stream of8%at50℃. The BTF system was able to consistently remove80–93.7%NO when the inlet NO was535.7mg/m~3in an oxygen stream of2–20%. The biologicalremoval efficiency of NO at50°C is higher than that at25°C, suggesting that the aerobicdenitrifer TAD1display well denitrification performance under thermophilic condition. Inthe first phase, relatively shorter EBRT (less than1.5min), the removal was a masstransfer-limited process. However, at a phase with longer EBRT bacteria, the reaction becamethe limiting step of the removal process. When EBRT continue to increase, capacity ofmicrobial denitrification reached largest, and chemical oxidation in the gas phase increaseswith EBRT has gradually increased. The optimal pH value of biotrickling filterwas wascontrolled at about7.5-8.0. Starvation for2,4and8days resulted in the re-acclimation timesof Chelatococcus daeguensis TAD1ranging between4and16hours. A longer recovery timethan that for weekend shutdown will be required when a longer starvation occurs. The resultspresented here demonstrate the feasibility of biotrickling flter for the thermophilic removal ofNOx from gas streams.To illustrate the process of NO aerobic denitrifying removal by biotrickling filter, adynamic model has been developed. Based on the mass component profile of NO at thegas–liquid interface, the model was used to depict the mass transfer reaction process of NO inbiotrickling filter, focusing on the concentration distribution of NO in the gas, liquid phases.Additionally, effects of operating parameters such as inlet NO concentration and empty-bed residence time on NO removal efficiency were evaluated through a sensitivity analysis of themodel.
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