Study On The Performance And Mechanism Of Loach On Enhancing Efficiency,Reducing Carbon And Mitigating Clogging In Vertical Flow Constructed Wetland

Constructed wetlands,as a new type of treatment process for wastewater,have been widely used in wastewater treatment,particularly in non-point source pollution control in rural areas,due to their advantages such as low operational cost,diverse structure,stable effect and high ecological benefits.However,during the actual operation of the constructed wetland,high pollutant loads in wastewater will stimulate the metabolism of microorganisms,resulting in increased emissions of greenhouse gases（GHGs）during the operation of the constructed wetland,which greatly reduces the ecology of the constructed wetland.At the same time,excessive accumulation of extracellular polymeric substances（EPS）secreted by substrate layer microorganisms during the operation led to clogging of constructed wetlands,which significantly reduced the effectiveness of wastewater treatment and operation stability of the system.In view of the problems existing in the operation of constructed wetlands,most of the previous studies started from the optimization of substrate,microorganisms and plants,while ignoring the role of wetland animals and their synergies of plants and microorganisms.Based on the concept of ecosystem integrity,this study established a loach-plant-substrate-microbial system to discuss the direct effects of loach feeding and excretion,as well as the synergistic effects with plants and microorganisms on the water,sediment and gas emissions within the system,and to clarify the mechanism of this system on increasing efficiency and reducing carbon and alleviating congestion.It could provide theoretical support for the construction and popularization of constructed wetlands with high efficiency and low discharge.The main experimental conclusions are as follows:（1）Loach-plant-substrate-microbial system not only enhanced the wastewater treatment performance of constructed wetland,but also effectively reduced GHG emissions.The removal efficiency of chemical oxygen demand（COD）,total phosphorus（TP）and total nitrogen（TN）in the loach systems were higher than that in non-loach system,reaching 59.16%,35.98%and 40.96%,respectively.The emission fluxes of CH₄and N₂O in the loach system were significantly reduced,thus reducing the global warming potential（GWP）of the system and GWP required for unit pollutant removal.（2）The bioturbation of loach promoted the migration and transformation of carbon,nitrogen and phosphorus in sediment.Loach bioturbation promoted oxygen transport and sediment re-contact and utilization,increased soluble organic carbon（DOC）content in the system,and promoted nitrogen conversion and phosphorus accumulation in the system.The results of excitation-emission parallel factor analysis（EEM-PARAFAC）showed that the humic acid content in the loach system was higher,indicating that loach promoted the metabolic process of microorganisms,which was conducive to the transformation and removal of pollutants in sediment.（3）Loach effectively reduced the content of clogging in the substrate layer and improved the hydraulic performance in the system.The direct feeding effect of loach and the indirect synergistic effect with microorganisms could metabolize and utilize the excess EPS in the system,effectively reduced the contents of polysaccharides and proteins in EPS,increased the proportion of humic acid and nucleic acid,and reduced the accumulation of system clogging.EEM-PARAFAC also confirmed that loach promoted microbial metabolism and produced more new EPS,which was conducive to the transformation and decomposition of macromolecular proteins in the system.Loach bioturbation improved the hydraulic characteristics of the constructed wetland,increased the porosity and permeability coefficient of the substance layer,increased the actual and theoretical hydraulic retention time（HRT）of the system and the connectivity between different layers,and reduced the"dead water"area of the system,which was conducive to the improvement of the pollution removal effect and long-term stable operation of the constructed wetland.（4）The effects of loach on microbial enzyme activity and community structure achieved the goals of increasing efficiency and reducing carbon and alleviating blockage.Loach increased the relative abundance of Proteobacteria,Chlorobiaceae and Comamonadaceae,which was conducive to carbon and nitrogen transformation and cycling in the system.Loach could increase the activities of dehydrogenase（DHA）and fluorescein diacetate（FDA）and the relative abundance of Bacteroidota in the sand and gravel layer,which was conducive to the hydrolysis of organic matter and excess clogging matters in the metabolic system of microorganisms.Loach increased the relative abundance of denitrifying microorganisms such as Aeromonadaceae,Rhodocyclaceae and Prolixibacteraceae in the gravel layer,which enhanced the denitrification process of the system and was conducive to the transformation of pollutants and the degradation of clogging matters.Redundancy analysis（RDA）showed that loach effectively improved the wastewater treatment effect and reduced CH₄ emission by promoting oxygen migration and microbial-driven pollutant transformation.By providing sufficient bioavailable carbon sources,loach promoted the complete denitrification process of the system and reduced the emission of N₂O.In addition,by increasing the contents of dissolved oxygen（DO）in sand layer and DOC and NO₃^--N in gravel layer,loach improved the metabolic rate of microorganisms in the system,promoted the transformation and degradation of EPS,thus reducing the accumulation of EPS and improving the hydraulic performance of the system.