| Constructed wetlands(CWs)have been used for small towns and rural decentralized sewage treatment for its advantages of purifying effect,cost-efficient construction,operation and maintenance,etc.However,traditional CW requires large area,difficult to reach its stable performance in cold climate regarding to nitrogen removal,which limit its widely application.To solve these problems,this study investigated five pilot-scale hybrid flow CWs with different enhanced operation methods,such as taking effluent recycling and filling additional sustained carbon sources.In addition,a full-scale constructed system with a noval double-level subsurface flow constructions has been developed and operated for 600 days to assess its efficiency of nitrogen removal.The nitrogen removal was evaluated in terms of wetlands configuration,operation ways,temperature,substrate,and carbon source.To further reveal the related mechanism,the nitrogen removal Kinetics and the Multicausal Path Analysis have been assessed.The integrated optimized operation methods and application strategy for consctructed wetland in different areas especially in cold areas ha ve been proposed.The results of the pilot-scale hybrid flow CWs studies showed that all the five hybird flow wetland systems have a good removal efficiency of COD,SS and TP.The Nitrogen removal of the five systems varied.The system with a Vertical Flow(VF)wetland unit achieved an ammonia removal efficiency of 92%-98%,while the removal efficiency in the HF system was only 68.6%.Due to a lack of carbon sources,the Total Nitrogen(TN)removal efficiencies of the HF-HF system and the VF-HF system were 51.4% and 52.8%,while the systems with recycling operation and filling sustained carbon sources had much higher TN removal efficiency,in addition the recycling of the effluent could partly balance the change of the alkalinity of the system.The TN removal rates of the R(HF-VF),VF + C and R(HF-VF)+ C systems were 72.2%,82.6% and 96.1%,respectively.The bacterial community structure of the various wetland units was analyzed by using pyrosequencing.The wetland units with the same function and environment characteristics had high similarities of the microbial community structure.The Vertical Flow(VF)wetland units had a higher abundance of Ammonia-oxidizing bacteria(AOB)and Nitrite-oxidizing bacteria(NOB),while the Horizontal Subsurface flow(HF)wetland units had a higher abundance of the denitrifying bacteria.The microbial nitrogen removal pathway and the the purification function the microorganism in different wetland units has been proved by the microbial community analysis.A full-scale double-level subsurface flow wetland has been set up and operated in the Northeast of China.The study proved that the novel double-layer subsurface flow enable the constructed wetland system to run stably without frozen in the winter.The results showed high removal efficiency in the whole year so that the effluent could reach the First-A discharge standard stably.The ammonia removal had been limted in the cold winter becasuse of the low temperature and anoxic conditions.The denitrification was also affected by the low temperature,but the denitrification rates had been basically stable when the water temperature increased above 15?.According to analysis on changes in the concertrations of the ammonia,nitrate,and alkalinity,the lack of the available carbon sources was an an important limiting factor on TN removal,and the recycling operation could improve the ammonia removal rate the increase of DO,while the nitrate and total nitrogen purifyings had been slightly improved by the recycling operation.The contribution by harvesting the wetland plants for the nitrogen and phosphorus removal were 13.6% and 14.2% in the plants growth period,while the contribution rates in the whole year period were 7.1% and 6.0%.By the pyrosequencing analysis on the bacterial community of the wetland system,it has been shown that the samples from the Bioaugmented wetland unit in the different years had a high similarity of the microbial community structure.By bioaugmentation,the abundance of bacteria with feature of removing nitrogen was increased,and the purification capacity and stability of the wetland at low temperatures had been improved.The analysis of the nitrogen removal pathways and kinetics in the wetland systems suggested that the ammonia removal process in the Vertical Flow wetland systems follow the First-order kinetic.,while the substrate saturation kinetics model(Monod model)was suitble for describing the denitrification because of the limition on the wetland operating conditions and lack of the carbon sources.Multicausal Path Analysis for the nitrogen removal indicated that,the direct path coefficient between the mass-loadings and removal rates for ammonia-nitrogen was 0.771,while the direct path coefficient between the mass-loadings and removal rates for nitrate-nitrogen was 0.435.The effect of the mass-load on denitrification process was less than the nitrification process,and the ammonia and COD mass-load also had significant impact on the nitrate-nitrogen removal rate.For p H of the raw water alkalinity,the effect on nitrate-nitrogen removal rate was more than that on ammonia nitrogen removal rate.The water temperature had significant influence on both ammonia nitrogen removal and nitrate nitrogen removal,while the indirect effect of the water temperature through by effluent COD concentration had played an important role.Based on the analysis results,the quantitative evaluation system for the application of constructed wetland has been established The evaluation index of the various constructed wetland had been estimated for the Principal Components Analysis by using SPSS statistical software.The statistic analysis showed that the Horizontal Flow wetland obtained the highest comprehensive score with the balanced performance on purification effect,construction economy and operation convenient.While the hybird flow wetland system holds advantages when high discharge water quality is requireddue to its better performace on TP and TN removal,Based on the technology requirements for pollution prevention a nd treament in the watershed and the results of the experiment,the integrated application strategy of the consctructed wetland for different wastewater and different areas has been proposed to guide the design,construction,and operation of the CW engineerings. |
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