| Recently,with the emergence of various new types of pollutants,toxic compounds in urban domestic sewage and industrial wastewater has profound negative impacts on sewage treatment plants.Perfluorooctanoic acid?PFOA?is one of the typical perfluorinated compounds?PFCs?widely found in water bodies.Due to its environmental persistency,difficulty of degradation,bioaccumulativity,endocrinal disruptiveness and other biological toxicities,PFOA has become one of the issues of environmental research.Currently,sewage treatment plants are widely regarded as important sources and destinations of perfluorinated compounds.Aerobic granular sludge is an aggregate formed by microorganisms entangled with each other under specific conditions.It not only exhibits good tolerance to toxic organic compounds,but also has diverse microbial species inside with assorted metabolic pathways.Moreover,it offers possibility for the gradual degradation of persistent organic compounds.However,to date,studies on the removal efficiency,environmental behavior of PFOA and its impact on the performance in granular sludge system have not been conducted.Therefore,this present study firstly investigated the effects of PFOA on the cultivation of aerobic granular sludge in wastewater,then explored its removal efficiency,environmental behavior and mechanism of interactions with aerobic granular sludge.Finally,based on the above research results,the effects of different concentrations of PFOA on the properties of granular sludge system,relevant mechanisms of these effects,and the relationship between changes in microbiota structure and decontamination performance were studied deeply and systematically.Thus,this work provides the necessary theoretical basis and guidelines for effective treatment of sewage containing perfluorinated organic pollutants.The main findings thus include:?1?The presence of PFOA slows the granulation process of aerobic sludge.The time of complete granulation in the group R1?42 d?was delayed by 14 d compared with the control group R0?28 d?.The particle diameter of granular sludge formed in the presence of PFOA was smaller and the size distribution was uneven.The biological activity and sedimentation ability of the sludge declined.The extracellular protein?PN?content and protein/polysaccharide?PN/PS?ratio of the sludge in the granulation process both markedly increased,and were positively correlated with surface characteristics of sludge,indicating that PN plays a crucial role at promoting sludge accumulation and particle formation in this study.Formation of sludge granules is beneficial for improving the reactor's tolerance to toxic substances,and the presence of PFOA delays the stabilization and upgrade of granulation process in aerobic sludge.The presence of PFOA changes the composition and structure of the microbiota in the granular sludge,and overall slows the enrichment and growth of the functional microbes associated with the granulation process of aerobic sludge.?2?After different concentrations?0.1,0.5,1.0 mg/L?of PFOA was added to the aerobic granular sludge system,the mass balance calculation of PFOA in the aqueous phase and mud phase indicated that the system removal rate of various concentrations of PFOA was around 35%.However,aerobic granular sludge had almost no biodegradative effect on PFOA,and removal in the aqueous phase was mainly achieved by adsorption.Fourier-transform infrared spectroscopy?FTIR?and X-ray photoelectron spectroscopy?XPS?analyses showed that the adsorbed species mainly include hydroxyl groups,amides of the proteins,polysaccharides and phosphates.After adsorption of PFOA,the content of N element in the granular sludge increased by 1.61%,and the contents of C and O elements decreased by 0.3%and 2.28%,respectively.The organic state of F element remained unchanged,indicating that the carbon-containing groups?-C-C,-C-O,-C=O,-COOH?and nitrogen-containing groups?-NH2,-NH4+?were involved in adsorption of PFOA.The kinetic and thermodynamic properties of adsorption revealed that aerobic granular sludge exhibits better adsorption performance for PFOA than the common flocculent sludge.The equilibrium adsorption capacity?0.14 mg/g?and the initial adsorption rate?0.06 mg/min?were 2 times and 6 times those of common flocculent sludge,respectively.The adsorption of PFOA by aerobic granular sludge is mainly based on uneven multi-layer adsorption and a certain degree of electrostatic adsorption.?3?The effects of different exposure concentrations of PFOA?0,0.1,0.5 and 1.0 mg/L?on the properties of aerobic granular sludge system were investigated.The results showed that PFOA exposure negatively affected flocculation and sedimentation properties of the granular sludge,leading to increases of system effluent ESS and sludge SVI.The magnitude of increases was positively correlated with exposure concentrations of PFOA.The appearance and microstructure of granular sludge changed simultaneously.The stress response in appearance was characterized by outgrowth of white plush substances.The microscopic effect was showed in decreased diversity of biological species,the internal pores enlarged into cavities,and the cellular contours became blurred.Long-term exposure to PFOA environments led to gradual increases of total EPS,LB-EPS and TB-EPS in granular sludge with increasing PFOA concentrations.The increase of LB-EPS in the outer layer was significantly greater than that of TB-EPS in the inner layer.Meanwhile,PN and PS contents in different types of EPS also showed similar changes as described above,indicating that the presence of PFOA plays an important role in facilitating the production of EPS.The fluorescence intensity of soluble microbial products in the EPS samples was significantly quenched with increases of PFOA exposure concentration,indicating higher concentrations of PFOA has strong inhibitory effect on metabolic activity of microorganisms in the granular sludge.By comparative analysis with the control group,it was found that exposures to PFOA at various concentrations resulted in reduced decontamination efficiency of the reactors,and the magnitude of decrease was positively correlated with the concentrations of PFOA.Lower concentrations?0.1 and 0.5 mg/L?of PFOA exposure did not noticeably affect the final removal efficiency of the pollutants.Higher concentration?1.0 mg/L?of PFOA exposure resulted in significant decreases in removal efficiency of all the pollutants.Particularly,the removal rates of TN and SOP decreased markedly to 80.01±2.47%and55.07±2.46%,respectively.The typical periodic changes of nitrogen and phosphorus elements and microbial internal polymers in the reactors suggest that PFOA inhibits the conversion of NO2--N to NO3--N and significantly inhibits the activity of NOB,thus decreases the removal rate of TN.The higher the PFOA exposure concentration,the more significant inhibitory effects on the anabolism of microbial polymers?PHA,glycogen?and the activity of polyphosphate-accumulating organism?PAO?,eventually leading to decline of the effectiveness of phosphorus removal.?4?High-throughput sequencing results showed the presence of PFOA resulted in decrease of the diversity and complexity of the microbiota in aerobic granular sludge,with more simplified microbiota structure at higher concentrations of exposure.Further analysis of each sample at phylogenetic level indicated that low concentrations of PFOA?0.1,0.5mg/L?inhibits the growth of NOB and microbes related to phosphorus removal.However,it had little effect on AOB and denitrifying bacteria.High concentration of PFOA?1.0mg/L?inhibited all types of microbes related to nitrogen and phosphorus removal,and significantly decreased the relative abundance of these microbes,consistent with the macroscopic effects of decontamination functions of the reactors.Thiothrix and Chloronema were the main strains causing sludge bulking in this study,and different exposure concentrations of PFOA were linked to different types of bacteria that cause sludge bulking.Thiothrix had highest relative abundance?61.61%?in the presence of 1.0mg/L PFOA.Chloronema had highest relative abundance?23.16%?in the presence of 0.5mg/L PFOA.This indicated that the dominant microbial species causing sludge bulking in R2 and R3 reactors were Chloronema and Thiothrix.Analysis of the FISH results directly reflected the degrees of inhibition on various functional microbes by different concentrations of PFOA,and confirmed that changes in amount of functional nitrogen and phosphorus removal microorganisms led to decline of the decontamination performance of the systems.This finding was consistent with the results from high-throughput sequencing. |
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