| Remarkable achievements have been received in water environment improvement and ecological restoration in recent years,the deterioration trend of surface water is alleviated.However,water pollution control is still arduous.The high content of N pollutant in the tail water after the treatment of urban sewage treatment plant has become the outstanding problem of improving surface water environment quality.Constructed wetland has good ecological effect,which can promote advanced treatment of water quality and water ecological restoration.If the constructed wetland and urban sewage treatment plant can be combined in function,a new sewage treatment technology system of treatment plant and constructed wetland?technical system of environmental engineering and ecological engineering technology?can be constructed based on the municipal sewage treatment technology.This technical system can further promote the environmental protection of surface water and the sustainable development of water ecology.There are still some problems in the treatment of tail water in urban sewage plant by constructed wetland,such as covering a large area,low efficiency in low temperature operation,improving effect and stability of denitrification.Therefore,optimizing the key process parameters of constructed wetland,identifing the influencing factors of nitrogen transformation function gene enrichment and regulating the nitrogen removal efficiency of constructed wetland are the practice demand and hot spot of theoretical research of deeply treatment of constructed wetland for the tail water of urban sewage plant.In this study,the influences of hydraulic load,C/N value,air-water ratio and temperature on nitrogen removal efficiency were investigated by constructing indoor horizontal subsurface flow constructed wetland simulation system.In order to clarify the micro ecological characteristics of nitrogen transformation,high throughput sequencing,molecular ecological network analyses and QPCR technology were used to analyze the internal microbial community structure under different conditions and the change of microbial functional genes for nitrogen transformation.A horizontal subsurface flow constructed wetland engineering was constructed to deeply treat the tail water of urban sewage plant based on the results of the simulation test.Continuous monitoring of treatment effect was also carried out.The main conclusions are as follows:?1?The research on removal efficiency of pollutant and microbial characteristics under different hydraulic loads show that the main microorganism group of horizontal undercurrent constructed wetland includes Proteobacteria,Firmicutes,Chloroflexi,Bacteroidetes and the dominant bacteria are Trichococcus,Geobacter.The molecular ecological network analyses showed that the interaction between species was more and the community structure was more stable at 0.6 m3/m2·d.When the hydraulic load was 0.6 m3/m2·d,the abundances of total bacterial 16 S rRNA,archaea 16 S rRNA,nitrification function gene?AOA?AOB?nxrA?and denitrification function gene?narG?nirK?nirS?nosZ?reached peak and the removal rate of NH4+-N and TN were the highest.The hydraulic load can greatly affect nitration process.When the hydraulic load was 0.50.6 m3/m2·d,the NH4+-N concentration of water outlet was less than 2.0 mg/L,the removal rate was more than 70%,which meet the class V water standard of environmental quality standard for surface water.When the hydraulic load was 0.7 m3/m2·d,the NH4+-N concentration of water outlet increased.Therefore,when the treatment target was the class V water standard,he hydraulic load should be 0.50.6 m3/m2·d.If there is higher requirement for water quality or larger available land area,the hydraulic load should be less 0.4 m3/m2·d.?2?The research on removal efficiency of pollutant and microbial characteristics under different C/N values show that the C/N values have little effect on the species composition of the microbial community and mainly affected the relative abundance of the dominant flora.The molecular ecological network analyses showed that when the C/N value was 6,the interaction between species was more and the community structure was more stable..The abundance of nitrogen conversion function genes firstly increased and then decreased with C/N value in the range of 4-8.When the C/N value was 6-7,the abundance reached peak.The NH4+-N concentration of water outlet was less than 2.0 mg/L and the removal rate was more than 78%,which meet the requirements of class V water standard.When the C/N value was less than 6,the abundance of AOA,nxrA significantly decreased,the abundance of narG?nirK?nirS?nosZ began to decrease,NO3--N and NO2--N accumulated in different degrees,and the removal rate of TN also decreased.Therefore,carbon source may be added to horizontal subsurface flow constructed wetland for reaching class V water standard.In order to balance the nitrification and denitrification capacity and ensure the economic and efficient operation of the system,the C/N value of water inlet should be 6-7.?3?Intermittent aeration significantly changed the structure of microbial community and increased the abundance of and nitrogen transformation function genes and the purification efficiency of pollutions.Absolute dominant phylums are Proteobacteria,Actinobacteria,the relative abundance of Firmicutes significantly decreased,and Nitrospirae and Acidobacteria start to enrichment.Nitrospira and Nakamurella become the dominant genus.The molecular ecological network analyses showed that proper aeration increased the interaction between species and the complexity of the network structure.When the ratio of air and water was 2-6,The function genes of nitrification and denitrification enriched and increased.The outlet water meets the class IV water standard.When the ratio gas and water was 6,the NH4+-N concentration of water outlet was 0.84 mg/L,and the removal efficiency was 89.73%.Compared with no aeration,NH4+-N removal rate increased by8.98%.When the ratio of gas and water ranged from 8 to 10,the gene abundances of narG?nirK? nirS?nosZ decreased,the denitrification process was limited,the concentration of NH4+-N increased,the accumulation of NO3--N increased,and the removal efficiency of TN decreased.Therefore,when intermittent aeration was used to strengthen oxygen supply in horizontal subsurface flow constructed wetland,in order to improve purification efficiency and save operation energy consumption,the best ratio of gas and water should be reasonably determined in the range of 2-6.?4?The research on removal efficiency of pollutant and microbial characteristics under different temperatures show that the temperature change can significantly affect the relative abundance of dominant bacteria in the range of 248 ?.The molecular ecological network analyses showed that low temperature decreased the interaction between microorganisms and the complexity of the network structure.In the range of 20-16 ?,the influence of temperature on nitrification and denitrification process was not significant.When the temperature was 16 ?,the nitrification process can also be successfully carried out.The NH4+-N concentration of water outlet was 1.66 mg/L and the removal rate was 80.13%,which could reach the class V water standard.When the temperature was in the range of 12-8 ?,the abundance of total bacterial 16 S rRNA,archaea 16 S rRNA and nitrification function genes?AOA,nxrA?significantly decreased.However,the abundance of denitrification function genes?narG?nirK?nirS?nosZ?did not change significantly.The nitrification process was obviously inhibited and the removal efficiency of NH4+-N significantly decreased.Therefore,when the temperature of tail water was lower than 12 ?,the measures of system heat preservation and temperature increase or strengthening should be considered to maintain or improve the efficiency of horizontal subsurface flow constructed wetland.?5?The horizontal subsurface flow constructed wetland was constructed as a core of the project of “suspended constructed wetland + horizontal subsurface constructed wetland + surface flow constructed wetland”,which was used for advanced treatment of the tail water from the southeast sewage treatment plant.The continuous monitoring for one year shown that in the mature and stable period of system from June to September,the removal rates of COD,NH4+-N,TN,TP and SS were 74.79%,80.90%,71.12%,78.39% and 91.90%,respectively.The water outlet can reached the class V water standard.The removal rate of SS by suspended constructed wetland was the highest?80.24%?,which could effectively prevent the blocking of subsurface flow wetland.The contribution rate for of removing NH4+-N,TN and TP by horizontal subsurface flow constructed wetland were 79.20%,64.64% and 81.71%,respectively.Surface flow constructed wetland could further remove TN and the contribution rate of removing TN was 23.00%.The construction cost of the demonstration project was 920 RMB per ton water,the operation cost was only 0.15 RMB per ton water.The water quality was obviously improved,and the ecological restoration effect was remarkable,which can provide technical support for the advanced treatment of tail water of urban sewage plant and the ecological restoration of urban river. |
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