Study On Performances Of A New Type Biotrickling Filter For NO_x Removal And Characterization Of Extracellular Polymeric Substances

The biological removal of NO_x from contaminated gas streams by biofilters is efficient low-cost NO_x abatement technologies. In China NO_x are mainly emitted from coal-fired power plants. Gas exiting the scrubbers is typically with temperatures between 50 and 60°C. Additionally, the flue gas discharged from coal-fired power plants is estimated to contain typically 3%⁸%（v/v） O2, which had negative influence on traditional denitrifying process. In order to eliminate these drawbacks in biofilters, the technology of novel aerobic denitrifying bacterial, which were cultured and inoculated in biotrickling filter to ascertain oxygen effects on NO_x removal under thermophilic condition, is meaningful. In the previous study of our group, an aerobic denitrifier Chelatococcus daeguensis TAD1 was isolated from the biofilm of a field biotrickling filter and its aerobic denitrification activity was evaluated under various conditions in batch reactor experiments at 50°C. In the present study, a new type filler biofilter applying Chelatococcus daeguensis TAD1 was established. This novel type biofilter was furthermore used to treat NO from simulated flue gas under thermophilic aerobic conditions.First, we investigated the nitrogen removal efficiency and N₂O emission characteristics of the novel thermophilic aerobic denitrifying bacterium, Chelatococcus daeguensis TAD1, under different C/N ratios and p H values in a batch reactor. Nitrogen removal efficiency and N₂O emissions were dramatically influenced by C/N ratio and p H. Moreover, multifactor analysis of variance suggested that these two factors also had significant interaction effects on N₂O emissions. The optimum C/N ratio was determined to be 8（where p H was set to 7）, at which a very high nitrogen removal efficiency（> 99%） was achieved. Under these conditions, N₂O emissions were only 34.43 μg/L and the N₂O emission factor was 0.046%, which could offer a promising new microbial resource for nitrogen removal and reduction of greenhouse gas emissions during wastewater treatment.The extraction and quantitative analysis of extracellular polymeric substances（EPS） have been frequently reported in studies of activated sludge. However, little is currently known about the characteristics of EPS during the nitrogen removal process by thermophilic bacteria. We investigated the EPS produced by a thermophilic denitrifying bacterium Chelatococcus daeguensis TAD1 during nitrate removal in batch culture. The effects of growth phase, type of carbon source, p H, temperature, and the C/N ratio on the EPS produced by the denitrifying bacterium TAD1 were examined. The protein content of the EPS increased from 6.3 to 89.8 mg/g dry cell weight with an increase in cultivation time from 4 to 12 h, but remained almost unchanged thereafter; total EPS showed a similar pattern. Regarding the carbon source, the amount of EPS produced by TAD1 decreased in the following order: sodium succinate, sodium acetate, sodium citrate, then glucose. Temperature and the C/N ratio exerted a less obvious effect on EPS production. Varying the p H of the growth medium from 5 to 9 changed the EPS and its components in complex ways. We also found a strong correlation between the total EPS content and the nitrate removal efficiency. Therefore, EPS should be taken into consideration when analyzing pollutant removal by microorganisms.Packing material is the adhesion matrix for the microbial growth, and its nature decides the growth and distribution of microbial biofilm. We investigated the surface modification method to improve the surface properties of the filler to provide more favorable growth environment for microorganism. The microbial cells are covered in negative charges under natural conditions, while the common used packing material surface is also usually in negative charges, which might cause some negative effect for micro-adsorption on the filler. Based on this theory, we selected the common used ceramic filler as the matrix by coating metal cation（Fe3+） to make the surface of ceramic with positive charge group composition. We discovered that the optimum modification condition was the calcination time of 4 h at the temperature of 500°C. Under this condition, the shape and particle size of ceramsite did not change after modification, but the density increased by 17% and porosity increased by about 15%, and the isoelectric point PI increased to 8.5, while the surface p H value reduced to 3.46. The surface of p H is much lower than the isoelectric point, to ensure that its surface is electropositive. The surface physical and chemical properties of the modified ceramsite filler were excellent and it showed strong adsorption and immobilization for nitrogen removal bacteria. The start-up time was shortened in the biotricking filter applying the modified ceramic, and the biomass was high.The new type biofilter, in which the fillers were the suspended carriers combined modified ceramsite could be quickly started up by inoculating the thermophilic denitrifying bacterium C. daeguensis TAD1. The NO concentration in the inlet stream ranged from 200 mg/m3 to 2000 mg/m3 during the operation, and inlet loading ranged from 8.2-164 g/（m3·h）. The whole operation period was divided into four phases according to the empty bed residence time（EBRT）. The EBRT of phases I, II, III and IV were 88 s（9-43 d）, 44 s（44-61 d）, 66 s（62-79 d） and 132 s（80-97 d）, respectively. An average NO removal efficiency of 90% was achieved during the whole operation period, and the elimination capacity increased linearly with the increase in NO inlet loading and the maximum elimination capacity reached 146.9 g/（m3·h）. No clogging was observed, although there was a high biomass concentration in the biofilter bed. The remarkable performance in terms of NO removal could be attributed to the rich bacterial communities. The microbial community structure in the biofilm was investigated by high throughput sequencing analysis（16S r RNA Mi Seq sequencing）. The experimental results showed that the microbial community structure of the biofilm was very rich in diversity, with the most abundant bacterial class of the Alphaproteobacteria, which accounted for 36.5% of the total bacteria, followed by Gammaproteobacteria（30.7%） and Clostridia（27.5%）. It was worthwhile to mention that the dominant species in the suspended biofilter biofilm were all common denitrifying bacteria including Rhizobiales（inoculated microbe）, Rhodospirillales, Enterobacteriales and Pseudomonadales, which accounted for 19.4%, 17%, 21.6% and 7%, respectively. The inoculated strain TAD1 belonged to Alphaproteobacteria class. Because high-throughput 16 S r RNA gene paired-end sequencing has improved resolution of bacterial community analysis, 16 S r RNA gene sequencing of these bacteria could provide more functional and phylogenetic information about the bacterial communities. In the long running period of 97 days, no clogging phenomenon was observed in the biofilter, which indicated that it overcame the disadvantage of traditional fixed biofilters in long-term operation. The results presented here demonstrate the feasibility of the new type biotrickling filter for the thermophilic removal of NO_x from gas streams.As EPS compose the intercellular space of microbial aggregates and form the structure and architecture of the biofilm matrix, therefore, extensive investigation is required to realize EPS-based implementation in field processes. This study investigates the EPS in biofilms of Chelatococcus daeguensis TAD1 established in a suspended biofilter for nitric oxide（NO） removal under thermophilic conditions. Polysaccharide was the main EPS component under all experimental operation conditions of the aerobic biofilter, although the EPS contents and components varied under different operating conditions. As the concentration of the inlet NO varied from 200 to 2000 mg/m3, the EPS and protein contents generally increased. At the highest inlet concentration（2000 mg/m3）, the EPS and protein contents reached 0.118 mg/g and 0.055 mg/g respectively（representing increases of 7.3% and 35% respectively over the inlet concentration of 200 mg/m3）. In contrast, the polysaccharide content was quite stable against inlet NO concentration. Decreasing the EBRT increased the EPS and polysaccharide contents, but exerted little effect on the protein content. Varying the p H of the circulating fluid from 4 to 8 changed the EPS and its components in complex ways. We also found a strong correlation between the total EPS content and the NO removal efficiency. Therefore, it is possible to take EPS into consideration for biofilter control.