| The rapid development of urban industrialization and increase of human activities result in trichloroethylene (TCE) contamination in the groundwater situation getting worse. Given the universality and the complexity of its treatment, TCE contamination treatment has become an issue at home and abroad. Due to its high efficiency, complete degradation, low cost, and no secondary pollution, bioremediation technology has have become a new technology for TCE pollution control. Since the up-flow anaerobic sludge blanket (UASB) reactor has high biomass and microbial diversity, as compared with other anaerobic reactors, UASB reactor has uniqueness in the treatment of recalcitrant organic wastewater, in which contaminants could be degraded quickly by high biomass. However, there was fewer studies on the TCE biodegradation in the UASB reactor, as well as on the metabolic mechanism of TCE in anaerobic reactor.In this study, after reading a lot of relevant literature, TCE-degrading consortium were constructed in the UASB reactor. The methanogenic toxic effects of anaerobic granular sludge by TCE and granular sludge methanogenic activity recovery were investigated. Through acclimating and UASB reactor startup, TCE-degrading consortium were constructed in the UASB reactor, and the metabolic mechanism of TCE in UASB reactor was explored. In addition, the TCE removal efficiency was explored, and the changes of bacterial diversity and community structure and function were also studied by high-throughput sequencing under different operation parameters. Moreover, UASB reactor operating conditions were optimized by response surface methodology. The conclusions were as follows:TCE could inhibit the activities of methanogenic bacteria, and inhibition enhanced with the increase of TCE concentration. However, methanogens could gradually adapt to inhibition of TCE. The more the concentration of TCE was, the longer the methane bacteria need time to adapt. TCE had a strong inhibition on methanogens. The inhibition was moderate when TCE concentration was 10mg/L and 15mg/L, respectively, but the inhibition was severe when TCE concentration was higher than 20mg/L. When the TCE concentration was 5-15mg/L, anaerobic granular sludge methanogenic activity recovered, however, anaerobic granular sludge methanogenic activity did not recover, when the TCE concentration was more than 20mg/L. High concentration of TCE could cause the death of methanogens.Through cultivation and acclimation, TCE-degrading consortium was obtained successfully in the UASB reactor. Glucose and lactate served as carbon sources. When TCE concentration was 14.6-73mg/L, the TCE removal efficiencies was 85%-90%. The degradation products of TCE were cis-DCE, VC and ethene. The structure of microbial community was analyzed by 454 pyrosequencing. There were many kinds of fermentation bacteria (such as Petrimonas, Clostridium, Enterobacter and Lactococcus), TCE dechlorination strains (Dehalobacter and Geobacter), Methanosaeta (99.54%) that used acetate as sole carbon source and other types of archaea were in low abundance (<5%). The putative anaerobic metabolic process of TCE within the UASB reactor was as follows:various fermentative bacteria genera could ferment glucose and lactate, and generate H2; (ii) H2 was served as electron donor for TCE dechlorination by Dehalobacter and Geobacter; (iii) Syntrophobacter and Desulfovibrio could provide trace nutrients and maintain a strict anaerobic condition for dechlorinating bacteria, and (iv) acetate, generated from fermentation was used as carbon source for the growth of Methanosarcina.Under the following conditions:temperature of 30℃, the influent pH of 7.0±0.1, influent COD concentration of 3100mg/L and TCE concentration of 36.5mg/L, HRT reduced from 25h to 5h and TCE loading rate increased from 35.04mg/L/d to 175.2mg/L/d. The results demonstrated that TCE degradation efficiency was the highest at HRT of 25h (99.10%), but it was the lowest at HRT of 5h (83.99%). Because the shortened HRT resulted in the increase of TCE loading rate, the excess of TCE could not be completely degrade. In addition, the chance of microbes contacing with TCE was also reduced. Moreover, according to Illumina Miseq sequencing results, the class Dehalococcoidia in the phylum Chloroflexi was detected under different HRTs, in which there were many well-known Dehalococcoides sp. strains capable of complete dechlorination of TCE to ethene.The amounts of Lactococcus and Petrimonas were dominant under different conditions HRTs. In addition, relative abundance of Geobacter and Dehalobacter were decreased with the reduce of HRT. The results of RDA analysis showed that Lactococcus sp. was negatively correlated with TCE removal efficiency, while, the genera Clostridium, Desulfovibrio, Eubacterium, Petrimonas, Syntrophobacter and Thermanaerovibrio had a positive correlation with it.Under the following conditions:TCE concentration of 36.5mg/L, HRT of 15h, temperature of 30℃, and influent COD concentration of 3100mg/L, influent pH was enhanced gradually from 6.0 to 7.5. TCE degradation efficiencies grew with the growth of pH (6.0-7.5). TCE degradation efficiency was the highest at influent pH of 7.0 (96.76%), but it was the lowest at influent pH of 6.0 (79.33%). Furthermore, the TCE removal efficiency, when influent pH was less than 7.0, was significantly less than that when influent pH was more than 7.0, which indicated that partial acidic environment influenced greatly on the TCE degradation. The class Dehalococcoidia in the phylum Chloroflexi was not detected under different pHs, The amounts of Lactococcus and Petrimonas were dominant under different conditions pHs, because they were fermentative bacteria. In addition, relative abundance of Geobacter and Dehalobacter were increased with the enhancement of pH. However, when pH was 6.0, Dehalobacter was not detected and the relative abundance of Geobacter was low (0.81%), which could be the reason that TCE removal efficiency was the lowest under pH of 6.0. The results of RDA analysis showed that Lactococcus sp. was negatively correlated with TCE removal efficiency, while, the genera Propionicicella, Thermanaerovibrio, Syntrophobacter and Petrimonas had a positive correlation with it.Under the following conditions:TCE concentration of 36.5mg/L, HRT of 15h, the influent pH of 7.0±0.1, and influent COD concentration of 3100mg/L, temperature was gradually increased from 20℃ to 35℃. TCE removal efficiencies grew with the growth of temperature. TCE degradation efficiency was the highest at temperature of 30℃ (97.89%), but it was the lowest at temperature of 40℃ (77.90%). Microbial community, which could adapt to the enhancement of temperature, gradually dominated which improved the TCE removal. However, the activity of microbial community decreased under higher temperature so that TCE removal efficiency declined. Moreover, the class Dehalococcoidia in the phylum Chloroflexi was detected from 25℃ to 40℃. In addition, relative abundance of Geobacter and Dehalobacter were increased with the enhancement of temperature. However, when temperature was 40℃, Dehalobacter was not detected and the relative abundance of Geobacter was low (1.76%), which could be the reason that TCE removal efficiency was the lowest under temperature of 40℃. The results of RDA analysis showed that Lactococcus, Syntrophobacter, Desulfovibrio, Petrimonas, Clostridium, and Blvii28 wastewater-sludge group were negatively correlated with TCE removal efficiency, while, only the genera Thermanaerovibrio had a positive correlation with it.By using CCD in RSM, three variables (HRT, temperature and influent pH) were opted to optimize the operation of the UASB reactor. As shown by the model, the optimization operation conditions of UASB reactor were HRT under 19.44h, influent pH in 7.13 and temperature at 31.07℃. In addition, the TCE removal rate were predicted to 96.52%, respectively. The actual operation conditions were HRT for 20h, temperature is 32℃, and influent pH of 7.2, respectively. Then the average removal rate of TCE was 97.71%, which fit well with the predicted values. Moreover, the TCE removal efficiency improved by 8.36% as compared with that before optimization. |
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