Performance And Mechanism Of A Novel Miniature Constructed Wetland For Purifying Low-polluted Wastewater In Mountainous Rural Areas

Sewage treatment in mountainous rural areas is related to the realization of the grand goals such as the protection of rural water resources and water environment,the improvement of rural living environment and the construction of beautiful villages.For mountainous rural areas with weak economic foundation,low willingness to participate,and strong spatial and temporal heterogeneity of water quality and quantity,the applicability and construction challenges of sewage treatment technology are much higher than municipal sewage,resulting in rural sewage treatment facilities in mountainous areas almost in a blank state.Compared with the centralized sewage treatment process of"high material consumption,high energy consumption and high carbon emission",constructed wetland technology has attracted much attention due to its advantages of low investment cost,low sludge production and easy maintenance.However,traditional constructed wetland technology is faced with the following problems:the substrate clogging causes obstacles to material and energy transmission in the bed,which induces the stress pressure of dissolved oxygen（DO）and organic matter deficiency,resulting in the weakening of the metabolic activity of aerobic functional bacteria and the reduction of aerobic enzymatic reaction rates.To address this key scientific issue,it is urgent to explore and develop a new type of constructed wetland technology with strong anti-clogging ability to enhance the resistance to organic load impact ability under complex water quality system,and to weaken the inhibition of DO and organic matter deficiency stress pressure on microbial metabolic activity in the bed.However,there are few reports on this aspect.Accordingly,this study proposes a miniature constructed wetland process with mini-sized,high-interface and strong mass transfer through the preparation of polymeric solid-phase slow-release carbon base and the construction of intermittent oxygen supply and combined gas-water in-situ remediation channels.Firstly,the removal performance of miniature constructed wetlands on nitrogen and phosphorus pollutants in rural sewage in mountainous areas under different operating parameters was investigated to reveal the construction principle of miniature constructed wetlands and the removal mechanism of conventional nitrogen and phosphorus pollutants.Secondly,the research on simultaneous control of conventional nitrogen and phosphorus pollutants and typical antibiotic sulfamethoxazole（SMX）in miniature constructed wetlands was carried out to clarify the succession law of dominant key microbial groups and the changes of microbial metabolic behavior in the bed under SMX stress,and to analyze the degradation path and mechanism of SMX in miniature constructed wetlands.On this basis,the low-carbon purification research of miniature constructed wetland demonstration project was carried out,and the purification performance,greenhouse gas emission flux and application prospect were systematically evaluated,which would provide technical support and scientific reference for the practical application of miniature constructed wetland.The main research results were as follows:（1）The polymer solid-phase slow-release carbon-based material with a slow-release carbon content of 132.61～148.27 mg COD·g^-1 was designed to realize the balance regulation of electron donor and acceptor in heterotrophic denitrification process under substrate deprivation stress,and enhance the ability to resist organic load fluctuation.An intermittent oxygen supply channel was innovatively constructed to break the anaerobic-anoxic(DO<0.5 mg·L^-1)dominated situation in the bed,and create an anaerobic-anoxic/aerobic alternating microenvironment(DO=0.2～3.2mg·L^-1),creating an ideal habitat for the enrichment of microorganisms such as simultaneous nitrification-denitrification and aerobic phosphorus removal.The gas-water combined in-situ repair with gas intensity of 8 L·（m²·s）^-1,water intensity of 6L·（m²·s）^-1 and frequency of 5 min every 6 d was carried out,which increased the porosity of the clogged substrate by 12.5%compared with that before repair.Through the orthogonal simulation optimization experiment,based on the nitrogen and phosphorus removal efficiency under different boundary conditions and the 3D-Smoother model,it was shown that DO of 3 mg·L^-1 and the hydraulic retention time of2 d were determined as the key operating parameters of the high-throughput miniature constructed wetland system.（2）The purification experiment of rural sewage in mountainous areas showed that during the stable operation period（108～138 d）,the efficiency of the miniature constructed wetland in removing chemical oxygen demand（COD）,ammonia nitrogen（NH₄⁺-N）,total nitrogen（TN）and total phosphorus（TP）increased by 13.7%,17.4%,16.0%and 10.5%,respectively,compared with the control group.The removal efficiencies of COD,Ammonia nitrogen（NH₄⁺-N）,Total nitrogen（TN）and Total phosphorus（TP）increased by 13.7%,17.4%,16.0%and 10.5%,respectively,compared with the control group,and were more resistant to organic load shock.Double fluorescence staining revealed that intermittent aeration coupled with air-water in situ remediation resulted in a large number of functional bacteria and a higher proportion of active functional bacteria in the miniature constructed wetland bed,with the fluorescence intensity of active functional bacteria reaching 71.6%,higher than that of the control group（40.3%）,accelerating the reduction and mineralization of protein-like organic matter and dissolved metabolites such as tyrosine and tryptophan.The prediction and analysis of metabolic pathways of functional bacteria revealed that carbohydrate metabolism（89.01%）,energy metabolism（4.71%）and metabolism of cofactors such as vitamins（4.33%）in the miniature constructed wetlands had obvious advantages,which were essential for accelerating the degradation and mineralization of carbon,nitrogen and lipid-like compounds.（3）During the 30～45 d period,the removal efficiencies of 200μg·L^-1 SMX by the miniature constructed wetland was more than 94%,and for medium and high concentrations of SMX(500～1000μg·L^-1)also showed strong resistance and control performance（>80%）,which indicated that the adaptability of microorganisms to SMX stress pressure was enhanced after 45 d of selective domestication.The main chemical reactions involved in the deconstruction of SMX were the breakage of the sulfonamide（N-S）bond,the-NH₂ acylation and hydroxylation on the benzene ring,and the substitution of the-SO₂ group into aniline analogues.In addition,under the stress of high SMX concentration(1000μg·L^-1),the microbial community reduced the biotoxicity effect of SMX by increasing the secretion of extracellular polymer(106.02mg·g^-1)to adsorb part of SMX and prevent its penetration into the intracellular body.With the increase of SMX concentration from 0μg·L^-1 to 1000μg·L^-1,the protein/polysaccharide value decreased from 5.51 to 3.6,which was mainly due to the activation of cell defense mechanism by SMX stress pressure and the increase of polysaccharide secretion to withstand environmental pressure changes.（4）The removal efficiencies of COD,NH₄⁺-N,TN and TP in miniature constructed wetland demonstration project were maintained above 70%under the impact of the fluctuation of organic load of rural sewage in mountainous areas.The effluent quality was stable and better than the discharge standard of the first criterion B,indicating that the miniature constructed wetland had the ability to resist the fluctuation of organic load of low-polluted water.During the stable operation phase,the CO₂,CH₄ and N₂O greenhouse gas emission fluxes and global warming potential contribution values of the miniature constructed wetland demonstration project were low,which were 172.35mg·m^-2·h^-1,0.092 mg·m^-2·h^-1,96.49μg·m^-2·h^-1 and 204.25 mg·eq-CO₂·m^-2·h^-1,respectively.This reflected the fact that the optimized substrate gradation could increase the alkalinity in the bed,allowing partial conversion of CO₂ to CO₃^2-and HCO₃^-,reducing CO₂ emission fluxes.In addition,intermittent oxygen supply helped to inhibit the activity of obligate strict anaerobic methanogens,promoting NH₄⁺-N oxidation,and reducing CH₄ emissions.Compared with other processes,the investment cost and operation and maintenance cost of miniature constructed wetland was lower.It had potential application prospects for controlling sewage in mountainous rural areas with strong spatial and temporal heterogeneity of water quality,improving the quality of water environment and helping to protect water resources.