The Study On The Improving Technology Of Nitrogen And Phosphorus Removal Efficiency In Biofiltration System By Different Electron Donors

Due to the influence of human activities and the excessive input of nitrogen and phosphorus in water,the eutrophication of lakes and rivers is becoming more and more serious.At present,the biofiltration system is widely used in the removal of nitrogen and phosphorus,which has the advantages of low cost,simple operation and high efficiency.The dissimilation reduction processes of nitrate include three important ways:denitrification,anaerobic ammoxidation and dissimilation reduction of nitrate to ammonium（DNRA）,in which denitrification and anaerobic ammoxidation convert nitrate nitrogen（NO₃^--N）to nitrogen,which are the most important ways of nitrogen removal;on the contrary,DNRA process converts NO₃^--N to ammonium nitrogen（NH₄⁺-N）,which is the way of nitrogen retention in water.Therefore,the intensity of the above three pathways will ultimately determine the net effect of nitrogen retention and removal in the biofiltration system.The type and richness of electron donors are the key factors to regulate the above pathways.In this study,simple carbon source sucrose,sodium acetate,refractory organic carbon source bagasse,rice straw and corncob,and inorganic electron donors ferrous sulfate and sodium sulfide were used as experimental materials.The effects of different electron donors in matrix on nitrogen and phosphorus removal in biofiltration system and the regulatory effects of C/N（DOC concentration/NO₃^--N concentration）on denitrification and DNRA were studied to explore the ways of nitrate removal.To find suitable electron donors for denitrifying biofiltration system to remove nitrogen and phosphorus.The main results are as follows:（1）The biofiltration systems were established and studied by introducing 7 different amounts of solid organic carbon sources（bagasse and corncob）in the matrix to test the nitrate removal efficiency and clarify the underlying mechanisms through metagenome.Also,liquid organic carbon source sodium acetate was added into sewage to further verify the effect of organic carbon amount on shift of DNRA and denitrification function.The results showed that high organic carbon in matrix could promote significantly NO₃^--N removal due to the reinforce of denitrification.High NO₃^--N removal efficiency with over60%was recorded in groups of high bagasse,high and medium corncob,regardless of SA addition.After sodium acetate addition,the abundance of functional genes（nir S,nap A,nor B and nor C）related to denitrification in the low organic carbon groups were significantly increased,which further promoted the removal of NO₃^--N in the low organic carbon groups,while the abundance of functional genes（nrf A and nrf H）mediating DNRA in the high organic carbon groups were significantly higher than that before sodium acetate addition,and increased the accumulation of NH₄⁺-N and nitrite nitrogen（NO₂^--N）in the high organic carbon groups.This indicated that high organic carbon combined different microorganisms,triggered the DNRA over denitrification.The switch from denitrification to DNRA occurred only in the case of the high solid organic carbon in matrix and high sodium acetate input in influent（the concentration of organic carbon is more than 80 mg/L）,especially high corncob groups.Compared to bagasse,corncob was the better suitable organic carbon source in matrix for denitrification due to slow and continuous release of organic carbon.Hence,in order to promote NO₃^--N removal and decline NH₄⁺-N accumulation in biofilters,it is necessary to screen suitable organic carbon source（mixed utilization of multiple carbon sources is recommended）and regulate its dosage（below 80mg/L）.Organic carbon addition regardless of matrix and influent both promoted the phosphorus removal efficiency,suggesting that organic carbon accelerated the process of biological phosphorus removal.（2）The sucrose,bagasse,rice straw,ferrous sulfate and sodium sulfide were selected as electron donors,and NO₃^--N,NH₄⁺-N,NO₂^--N and others parameters in influent and effluent and NO₃^--N reduction rate as well as the related functional genes in matrix were investigated by simulation experiment with different concentrations of NO₃^--N added.The results showed that the electron donor addition increased significantly the genes abundance（nir S and nir K）and rate related to denitrification,resulting in higher NO₃^--N removal efficiency（up to 68%±2%at the highest）in refractory organic carbon bagasse and rice straw groups.Easily degradable organic carbon sucrose may cause the enrichment of dissolved organic carbon（DOC）in a short time,the abundance of DNRA functional gene nrf A increased significantly,and the DNRA rate was higher than the denitrification rate,triggered the DNRA process,and leaded to the accumulation of NH₄⁺-N.Inorganic electron donors ferrous sulfate and sodium sulfide were not recommended due to poor denitrification gene abundance and rate as well as NO₃^--N removal efficiency.The decrease of C/N was beneficial to denitrification but restricted the occurrence of DNRA,finally reducing nitrogen retention capacity.Therefore,the regulation of C/N could play a decisive role in the switch of denitrification and DNRA.In addition,the addition of electron donors could improve the phosphorus removal efficiency,indicating that electron donors accelerated the process of biological phosphorus removal.In short,the addition of refractory natural organic carbon sources（bagasse,corncob,rice straw）to the matrix can significantly promote the removal efficiency of NO₃^--N（the removal efficiency is about 70%）.Inorganic electron donors（ferrous sulfate and sodium sulfide）were unsuitable due to poor denitrification gene abundance,low rate as well as low NO₃^--N removal efficiency.It is suggested that the solid detritus which releases DOC slowly and continuously in the matrix should be used as the main organic carbon source and the liquid organic carbon added in the influent should be used as the supplementary carbon source.Hence,in order to promote NO₃^--N removal and decline NH₄⁺-N accumulation in biofilters,it was very important to screen suitable electron donors（mixed utilization of multiple carbon sources was recommended）and regulate its dosage（below80 mg/L）.