| Constructed wetland（CW）have been widely used in wastewater treatment due to their advantages,such as good treatment performance,low energy consumption and operation costs,as well as high ecological benefit.However,during long-term operation,extracellular polymeric substance（EPS）secreted by microorganisms continuously accumulated on the surface of the substrate,which could lead to bio-clogging,thereby affecting the stable operation and performance of CW.Traditional prevention measures for bio-clogging（such as optimizing influent loadings,pretreatment for influent,etc.）,as well as the mitigation measures（such as adding chemical reagents,microbial agents,etc.）were generally limited by problems of high operating costs and large floor space.Coupling CW with bio-electrochemistry was considered to be an effective method to mitigate or prevent CW bio-clogging.In this study,the air-cathode with Ti O2/biochar photocatalytic was introduced into microbial fuel cell（MFC）to construct a photocatalytic fuel cell（PFC）,which could mitigate and prevent CW bio-clogging,and obtain electrical energy by enhancing the micro-electric field environment.In this study,the changes in pollutant removal performance and hydraulic conductivity（porosity and permeability coefficient）of bio-clogging mitigation groups（CW,CW-MFC,tandem CW-PFC and parallel CW-PFC）and bio-clogging prevention group（tandem CW-PFC）in the three processes of“stabilization stage,bio-clogging stage and bio-clogging mitigation stage”were studied.The content and composition of EPS,the characteristic of dissolved organic matter（DOM）in the effluent,as well as the microbial community structure diversity and potential metabolic functions in tandem CW-PFC were inspected,thus revealing the mechanism of PFC in mitigating and preventing bio-clogging of CW.The main results obtained were as follows:（1）CW-PFC pollutant removal and power generation performanceThe optimal preparation method of Ti O2/biochar air-photocathode was determined,and changing trend of pollutant removal and power generation performance of CW-PFC in the process of mitigating and preventing bio-clogging was analyzed.In this study,Ti O2/biochar photocatalytic materials were prepared by a modified ultrasound-based procedures method.The photocatalytic performance of the prepared materials was the highest when the ratio of biochar:tetrabutyl titanate:2-propanol was 0.50 g:1 m L:40m L.Compared with CW-MFC（2.98 m W/m3）,Ti O2/biochar catalyzed and accelerated oxygen reduction reaction（ORR）,thus leading to higher power generation in tandem CW-PFC（CW-3 and CW-5）,and the power density could respectively reach 90.78 and105.92 m W/m3.In addition,the removal efficiencies of NH4+–N and chemical oxygen demand（COD）in tandem CW-PFC were respectively 31.8±7.2%and 86.1±6.8%,higher than those in the control system（21.1±11.0%and 73.3±5.6%）.（2）Effects of PFC on bio-clogging mitigation in CWThe degradation effect of CW-PFC on EPS was clarified,and the influence of micro-electric field on the content and characteristics of bio-clogging matters and EPS was investigated.During the stable bio-clogging stage,the effective porosity decreased from 29.5%to 9.6%,and the permeability coefficient decreased from 0.034 to 0.008 cm/s in the sand layer of the bio-clogging mitigation system.After bio-clogging was mitigated in situ in tandem CW-PFC,the porosity of the sand layer could be restored to about62.5%of the initial porosity.The changes in the microstructure of the bio-clogging matters indicated that the micro-electric field formed in the tandem CW-PFC destroyed the dense structure of the bio-clogging matters and weakened the surface adhesion of cells.In addition,the micro-electric field inhibited the aggregation of EPS by increasing the electrostatic repulsion,and made the negatively charged particles（protein,polysaccharide,humic acid,etc.）move toward the anode to be degraded by microorganisms.Fluorescence-PARAFAC analysis showed that the effluent DOM of tandem CW-PFC had a high degree of humification,and its components are mainly aromatic protein-like substances,which further proved that EPS was degraded into low-molecular organic acids and protein-like substances.（3）The mechanism of PFC to mitigate and prevent the bio-clogging in CWThe structure and diversity of microbial community in the CW-PFC system,and the change of the potential metabolic functions were examined.The mechanism of CW-PFC system to mitigate and prevent bio-clogging was clarified.The micro-electric field generated by the tandem CW-PFC improved the electrostatic repulsion and surface adhesion between EPS and made the EPS directional migrate to the anode,beneficial to the deeper degradation of microorganisms.At the same time,the enrichment of Geobacter,Trichococcus,Anaerovorax,as well as the up-regulation of the abundances of carbohydrate metabolism and amino acid metabolism genes were promoted.As the most common EAB,Geobacter and Trichococcus perform significant roles in the extracellular electron transfer process and can decompose the refractory a high-molecular-weight of EPS for generating electricity.Anaerovorax is the dominant microorganism for the degradation of protein-related pollutants,which can help to accelerate the decomposition of EPS.In addition,the tandem CW-PFC system reduced the enrichment of biofilm formation microorganisms（Micropruina）.Therefore,the PFC achieved effective bio-clogging mitigation and prevention,improving the operational stability and service life of CW.In summary,the novel air-photocathode PFC system showed great potential in the mitigation and prevention of CW bio-clogging,and was expected to provide a new solution for the long-term stable operation of CW. |
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