Study On The Mechanism Of Enhancing Nitrogen Removal Reactors In Treating Tail Water Of Constructed Rapid Infiltration System

Constructed rapid infiltration system（CRI）,as a new sewage biological treatment technology,has the advantages of less energy-intensive,low operation cost and simple process.It has good removal effects on ammonia nitrogen,COD and suspended solids（SS）in sewage and their removal rates can all reach more than 90%.Thus,CRI is widely used for the treatment of domestic sewage and slightly polluted surface water.However,due to the CRI lacks of denitrifying carbon source,anoxic environment and denitrifying bacteria in its lower part,and the dielectric particles have no adsorption capacity to NO₃^-which is easy to move out from the system along with water flow,the removal rate of total nitrogen（TN）is only 10%-30%.To reduce the TN concentration in tail water of CRI,researchers have achieved some results by changing filler structure,water feeding modes,achieving short-cut nitrification with inhibitors and adjusting dry-wet ratio,etc.However,due to the complexity of operation process,poor controllability of experiment and the micro-study of pollutant removal mechanism was relatively insufficient,and without consideration of using tail water as resource.Those methods were difficult to be popularized.Therefore,to overcome the limitations of conventional improving methods of CRI and reduce the TN concentration in tail water of CRI effectively,this study did the research from the following two aspects:（1）Adding the anoxic environment and nitrate residence time of CRI,that is,building a water-saturated simulated column of CRI,and using its advantages of simple operation,strong controllability,stable removal performance,and can provide suitable growth environment and electron acceptor for denitrifying bacteria,to treat tail water of CRI;（2）Using tail water of CRI as resources,that meant,constructing a chlorella vulgaris bioreactor,and using its advantages of low consumption,strong applicability,can assimilate pollutants such as nitrogen and phosphorus.Moreover,compared with the water-saturated CRI,the chlorella vulgaris bioreactor has the advantages of small footprint,low cost,and let the sewage to be used as resource,to purify the tail water of CRI.Meanwhile,in view of the characteristics of the CRI tail water and the importance of C/N ratio for affecting the performance of sewage biological treatment process,this study focuses on the effects of C/N ratio in influent on pollutants removal performance and biological characteristics of both water-saturated CRI and chlorella vulgaris system.Moreover,to obtain the optimum culturing conditions of chlorella vulgaris in tail water of CRI,the effects of trophic modes combined with C/N ratio on pollutants removal performance of chlorella vulgaris in synthetic tail water of CRI were studied firstly,then the characteristics of chlorella vulgaris biomass and growth kinetics were analyzed.On those basis,the removal efficiency of pollutants of chlorella vulgaris in sterilized（ultraviolet sterilization）tail water and unsterilized tail water of CRI were studied.Lastly,the advantages and disadvantages of water-saturated CRI and chlorella vulgaris system in treating tail water of CRI were analyzed to provide the experimental references for reducing TN concentration in tail water of CRI,culturing chlorella vulgaris in sewage with high nitrate nitrogen（49mg/L）and alleviating eutrophication of lacks and rivers.The results are as follows:（1）Through studying the effects of C/N ratio in influent on pollutant removal performance and microbial characteristics of water-saturated CRI found that:（1）Increasing the C/N ratio in influent could enhance the denitrifying ability of water-saturated CRI.When the C/N ratio was 5:1,the mean concentration of TN in effluent was only 14.74 mg/L,which could meet the first grade A standard for urban sewage discharge（GB18918-2002）（TN≤15 mg/L）;（2）The quantity of denitrifying bacteria in the filler of water-saturated CRI increased as a result of the higher C/N ratio,and the pollutants removal performance was enhanced as the quantity of denitrifying bacteria increasing;（3）Analysis of the bacterial biofilms that had formed in the filler of water-saturated CRI showed that the elevated C/N ratio had enhanced secretion of the total biofilm EPS,which led to improve biofilm formation and resistant ability.Thus,when the C/N ratio of influent was 5:1,water-saturated CRI presented a good removal performance of pollutants in tail water of CRI.（2）Through studying the effects of trophic modes combined with C/N ratio on pollutants removal performance on biological characteristics of chlorella vulgaris system to treat synthetic tail water of CRI,found that:（1）The mixotrophic mode was more conducive to enhance the nitrogen removal ability of chlorella vulgaris system.On day 9 of the experiments,the residual TN concentration of chlorella vulgaris system with initial C/N ratio of 3:1 and 5:1 were 20.86 mg/L and 19.51 mg/L,respectively;（2）The mixotrophic mode was beneficial to promote the proliferation of chlorella vulgaris,when the C/N ratio was less than 3:1,it could promote the accumulation of lipids,proteins and carbohydrates in chlorella vulgaris’cells.Therefore,considering the nitrogen removal performance and the growth status of chlorella vulgaris in synthetic tail water of CRI,and in view of the advantage of carbon sources saving,the culturing conditions of mixotrophic mode,C/N ratio in influent of 3:1,hydraulic retention time was 9 days would be chosen for chlorella vulgaris culturing in real tail water of CRI.（3）Under the culturing conditions obtained by last experiment,the removal efficiency of pollutants of chlorella vulgaris in sterilized（ultraviolet sterilization）tail water and unsterilized tail water of CRI was studied and found that:（1）Under the same culturing conditions,the removal efficiency of nitrate nitrogen and TN of chlorella vulgaris in unsterilized tail water was better than that in sterilized tail water,and with the average removal rates of 45.22%and 41.52%,respectively.However the chlorella vulgaris system adopted sterilized tail water as influent was more likely to accumulate nitrite nitrogen;（2）The mean removal rates of COD of chlorella vulgaris in both kinds of tail water of CRI were as high as 60%;（3）However,since the negative influence of the quality of tail water of CRI,the chlorella vulgaris in unsterilized tail water occurred the agglomeration,while the phenomena of growth retardation of chlorella vulgaris appeared in the system with sterilized tail water.Overall,compared with chlorella vulgaris system,water-saturated CRI presented the advantages of good removal effect of pollutants in tail water of CRI.When the C/N ratio in influent was 5:1,the mean concentration of TN and COD in effluent could meet the first grade A standard for urban sewage discharge（TN≤15 mg/L,COD≤50 mg/L）.Although,due to the negative influence of tail water of CRI,the growth status of chlorella vulgaris in both two systems were not good,compared with water-saturated CRI,chlorella vulgaris system still had the advantages of low carbon source consumption,small footprint,could use the tail water as resource and the removal rate of nitrate nitrogen in unsterilized tail water by chlorella vulgaris could reach up to 45%,which indicated that chlorella vulgaris system has good potential to treat sewage with high nitrate nitrogen（49 mg/L）.