Application Of Micro-Electric Field Coupling Constructed Wetland To Strengthen Treatment Of Domestic Sewage With Low C/N

With the rapid development of social economy and the improvement of people’s living standard,the discharge of domestic sewage of urban residents has gradually increased,and the environmental problems of water resources have been more widely concerned.Constructed wetland（CW）treatment technology has the advantages of good treatment effect,lower cost,easy management,while considering environmental sustainability and social acceptability,and is widely used in the field of watershed water environment management.However,traditional CWs are often limited by temperature,climate,and low C/N in domestic wastewater in practical applications,resulting in unstable results of nitrogen removal and phosphorus removal in wastewater.Studies have shown that suitable intensity of micro electric field can stimulate plant growth and promote microbial life metabolism.Therefore,we promote the popularization and application of CW technology by establishing a coupled micro-electric field system with CWs and mixing carbon-based materials to compensate for the lack of carbon sources for comprehensive research.In this study,biochar and petroleum coke were selected as mixed fillers for the CW,and CWs with coupled micro-electric field was constructed to investigate the effects of the mixed substrate and the micro-electric field on the long-term decontamination effect of the CW,and to determine the most suitable current density parameters;The study evaluated the long-term decontamination efficiency and N₂O emission characteristics of the artificial wetland system by mixing heavy metals under the optimum micro-electric field conditions;the role and contribution of substrate in the decontamination process of CWs were clarified by constructing substrate batch tests;We investigated the effects of micro-electric fields on the antioxidant system of plants and the activity of key enzymes for denitrification and phosphorus removal,and used qPCR to analyze the key pathways of carbon,nitrogen,phosphorus and sulfur metabolic cycles in CWs under micro-electric fields.The main findings are as follows:（1）Biochar and petroleum coke can improve the decontamination effectiveness of CWs for nitrogen、phosphorus and heavy metal pollutants,while reducing the flux of nitrous oxide N₂O emissions.The micro-electric fields with applied current densities of 0.02 m A/cm²,0.06m A/cm²,and 0.09 m A/cm²all enhanced the decontamination efficacy to some extent,with the best decontamination effect of the enhanced CWs at 0.06 m A/cm².Under optimal micro-electric field enhancement,the NH₄⁺-N,TN,and TP removals in CWs with mixed biochar were 97.63%,77.20%,86.60%,and the TN and TP removals were increased by 24.83%and20.30%,respectively.Nitrogen and phosphorus removal efficiency in the wetland system was significantly inhibited by the mixing of heavy metals in CWs,and N₂O emissions were also higher.（2）The results of substrate batch tests showed that biochar and petroleum coke contained rich functional group structures and had adsorption capacity for NH₄⁺-N and TP,and the micro-electric field enhanced the adsorption capacity of petroleum coke for phosphorus.DOM released from biochar and petroleum coke included protein-like,tryptophan-like,and two types of humic acid substances,with higher DOM release from biochar than petroleum coke and a gradual increase in humification in CWs operation.Micro-electric fields can increase the dissolution of DOM from biochar and petroleum coke,but did not change the DOM characteristics.（3）Heavy metals disrupted the balance of plant antioxidant system,while biochar and petroleum coke facilitated plant nutrient uptake and micro-electric fields did not cause stress to plant growth.Micro-electric fields significantly increased the abundance of functional genes for nitrification（amo A,hao）and denitrification（nir S,nir K,nor B,nos Z）and decreased the abundance of functional genes related to the metabolism and carbon cycling of polyphosphorus bacteria,significantly promoting the biological denitrification process and inhibiting the biological phosphorus removal as well as carbon fixation and degradation processes.The accumulation of heavy metals in biochar is toxic to microorganisms,and the micro-electric field can mitigate this toxic hazard.Micro-electric fields increased the activity of AMO during nitrification,with no significant effect on NAR and PST.