Basic Research On Flue Gas Treatment By Haloalkaliphilic Microorganisms

Sulfur dioxide (SO2) and nitrogen oxides (NOx) from flue gas are major pollutants in air. Physical and chemical methods used for flue gas treatment are very efficient. However, they generate secondary pollution as well as require a lot of energy inputs and erosion apparatus. Microorganisms are used to treat flue gas to solve those problems. However, there is sulfide inhibition in neutral systems. To eliminate sulfide inhibition and reduce costs, haloalkaliphilic microorganisms are used to treat flue gas in this study. Following researches were carried out.Firstly, composite carbons of lactate and ethanol with high COD/SO42-ratio (4.0) were used to start up haloalkaliphilic sulfate-reducing bioreactor, to investigate performance of bioreactor and bacterial community shifts under different COD/SO42-ratios and hydraulic retention times (HRTs). The process of sulfate reduction by haloalkaliphilic microorganisms was feasible and the optimum COD/SO42- ratio was still 4.0. To reduce cost, low COD/SO42" ratio should be used from start-up in the subsequent experiments. Compared with lactate, ethanol was a more appropriate electron donor for haloalkaliphilic sulfate-reducing bacteria (SRB). When ethanol was used as sole electron donor with COD/SO42- ratio of 4.0, the optimum HRT was 18 h with a sulfate removal efficiency of 97.8% and a sulfate removal rate of 6.26 kg/m3 d. There was no sulfide inhibition in haloalkaliphilic sulfate-reducing bioreactor. Based on denaturing gradient gel electrophoresis analysis of 16S rRNA, the major SRB was Desulfonatronovibrio sp., which was only detected at a COD/SO42- ratio of 4.0 using ethanol as an electron donor. This finding can be used as evidence for the fact that COD/SO42 ratio was indeed 4.0. Different HRTs had no significant effect on the band corresponding to this species. Activated sludge was sampled from the bottom of bioreactor for the enrichment of SRB. Desulfonatronum sp. and Desulfonatronovibrio sp. were enriched.Secondly, composite carbons of glucose, lactate, and ethanol with high COD/NO3-ratio (4.2) were used to start up haloalkaliphilic denitrification bioreactor, to investigate performance of bioreactor under different NO3/SO42- ratios. Haloalkaliphilic simultaneous desulfurization and denitrification was reported. When HRT was 24 h, the optimum NO3-/SO42-ratio was 3.0 with a nitrate removal rate of 6.0 kg/m3 d and a sulfate removal rate of 1.39 kg/m3 d, respectively. There was no sulfide inhibition in bioreactor. Lactate was a more appropriate electron donor for denitrifying bacteria (DB), while SRB preferred to utilize ethanol. Based on denaturing gradient gel electrophoresis analysis of 16S rRNA, the major SRB and DB were Desulfonatronovibrio sp. and Halomonas campisalis, respectively. Decrease in NO3/SO42- ratio led to obvious changes in bacterial community. Although the SRB became dominant, the population of DB also increased. This finding can be used as theoretical support for the process of simultaneous desulfurization and denitrification. Activated sludge was sampled from the bottom of bioreactor for the enrichment of DB. Halomonas campisalis was isolated.Thirdly, lactate, glucose, methanol, ethanol, formate, and acetate were used separately to start up different haloalkaliphilic sulfate-reducing bioreactors with a COD/SO42- ratio of 2.0 and a HRT of 24 h, to investigate the effect of different electron donors on the performance of haloalkaliphilic sulfate-reducing bioreactors. The optimum electron donor was found to be ethanol with highest sulfate loading (10.0 kg/m3 d) and sulfate removal rate (8.60 kg/m3 d). Microbial degradation patterns related to sulfate reduction of different electron donors were determined. Oxidation process of propionate was coupled with sulfate reduction process in the lactate-and glucose-fed bioreactor. Glucose can be directly utilized by haloalkaliphilic SRB. The pathway of ethanol to pyruvate and Wood-Ljungdahl pathway were found in the ethanol-and formate-fed bioreactor, respectively.Then, Illumina MiSeq paired-end sequencing of 16S rRNA gene was applied to investigate the bacterial communities from haloalkaliphilic bioreactors fed with lactate, glucose, ethanol, and formate, respectively. Bacterial community in the glucose-fed bioreactor showed the greatest richness and evenness. Hierarchical cluster analysis of bacterial communities at genus level shows that the lactate-ethanol group was separated from formate and glucose group. Only 758 OTUs or 11.4% of the total OTUs were shared by them. The dominant phylum in the shared OTUs was Proteobacteria with relative abundance of 51.6%. The highest relative abundance of SRB was found in the ethanol-fed bioreactor. Using this sequencing method, a syntrophic propionate-degrading sulfate-reducing bacterium was detected in the lactate-fed bioreactor; bacterium which can convert ethanol or formate into pyruvate was detected in the ethanol-and formate-fed bioreactor, respectively. Different types of SRB, sulfur-oxidizing bacteria, and sulfur-reducing bacteria were detected in all bioreactors, indicating that sulfur may be cycled among these microorganisms. The findings of this study gave theoretical support for microbial degradation patterns related to sulfate reduction of different electron donors.Finally, using model flue gas adsorption solutions with different nitrate concentrations and salinities as influent, respectively, lactate- and ethanol-fed bioreactor were used to investigate effects of nitrate and salinity on the performance of haloalkaliphilic sulfate-reducing bioreactors, respectively. It was found that when COD/SO42- and HRT were 2.35 and 24 h, respectively, the addition of nitrate did not affect the performance of sulfate-reducing bioreactor when nitrate concentration was below 1000 mg/L. This result indicated that the process of haloalkaliphilic simultaneous desulfurization and denitrification was feasible. When nitrate concentration reached 1500 mg/L, suflate reduction was obviously inhibited by nitrate. Activities of other microorganisms were also inhibited by nitrate. When lactate and propionate coexisted in bioreactor, lactate was first used by SRB. Through comparing the concentrations of sulfate and organics in three different sample connections (Up, middle, and down), it was found that concentration of sulfate reduced at the bottom of bioreactor accounted for 62.1% of total concentration of sulfate reduced in bioreactor, and concentration of lactate consumed at the bottom of bioreactor accounted for 63.6% of total concentration of lactate consumed in bioreactor. These results indicate that most of sulfate was reduced at the bottom of bioreactor and most of SRB gathered in this place. When electron donor was enough, nitrate inhibition can be solved by prolonging HRT. It was found that when COD/SO42- and HRT were 2.0 and 24 h, respectively, the maximum salinity for haloalkaliphilic microorganisms was 1.63 M Na+ and sulfate removal efficiency was kept above 90%. These results indicated that haloalkaliphilic microorganisms can reduce sulfate and nitrate simultaneously at high salinity with high absorption efficiency and low cost.