Study On Biochar Enhancement Mechanism In Anammox Nitrogen Removal Process

Anaerobic ammonium oxidation（Anammox）has become a sustainable biological treatment technology to achieve high-efficiency and low-energy-consumption nitrogen removal of ammonia-containing wastewater.However,the residual nitrate in Anammox products limits the further improvement of its nitrogen removal efficiency.Existing research often uses organic carbon sources such as methanol as electron donors to strengthen the denitrification process to achieve further removal of nitrate.However,anaerobic ammonium oxidation bacteria（An AOB）are extremely sensitive to organic carbon sources,and improper dosing will reduce the nitrogen removal efficiency of the system.In recent years,biochar has been widely used to promote electron transfer between microorganisms due to its strong electron gain and loss ability,and some studies have shown that biochar can be used as an electron donor to enhance denitrification nitrogen removal.However,in the Anammox system,whether biochar can achieve synergistic and efficient nitrogen removal by Anammox and denitrification through enhanced nitrate reduction and its potential mechanism remain to be studied.Therefore,this study investigated the effects of biochar pyrolysis temperature（300°C,500°C,700°C）and dosage（3g/L,5g/L,10g/L）on the nitrogen removal performance of Anammox,and by changing the influent matrix,combined with the characterization of biochar and the analysis of the succession characteristics of functional flora,revealing the process mechanism of biochar to enhance the nitrogen removal efficiency of Anammox,and the following research results were obtained:（1）The nitrogen removal performance of anammox was significantly affected by the pyrolysis temperature of biochar.With the decrease of pyrolysis temperature,the total nitrogen removal rate of anammox reaction increased by 14.2%,which was mainly due to the rapid reduction of nitrate,whose reduction rate increased from 0.07 to 0.32mg/L·h^-1.The correlation analysis showed that the promotion effect of biochar on the nitrogen removal efficiency of Anammox was negatively correlated with the electrical conductivity of biochar（r=-0.62）,and significantly positively correlated with the electron donating ability of biochar（r=0.95）,the main reason for this was the biochar prepared at low temperature（300°C）has strong electron donating ability(EDC=2.64mmole^-g^-1),which can provide more electrons for nitrate reduction.On the basis of finding that 300℃is the optimal pyrolysis temperature,the effect of biochar dosage on the nitrogen removal performance of Anammox was investigated.The results showed that with the increase of dosage,the total nitrogen removal rate of Anammox reaction increased by 11.3%.（2）When the substrate conditions（NH₄⁺:NO₂^-=1:1）satisfy the Anammox reaction,the system takes Anammox as the main nitrogen removal pathway.Biochar promotes complete/partial denitrification of nitrate generated by Anammox,and cooperates with Anammox to achieve high-efficiency nitrogen removal.Compared with the control group,the total nitrogen removal rate is increased by about 10%.When the substrate conditions（NH₄⁺:NO₃^-=1:1）do not meet the Anammox reaction,the reduction of ammonium is limited by the reduction rate of nitrate.Biochar can rapidly reduce nitrate to nitrite by promoting partial denitrification reaction.At this time,the ability of An AOB to compete to regenerate nitrite is stronger than that of denitrifying bacteria,The generated nitrite is preferentially combined with ammonium for the Anammox reaction.Compared with the control group,the total nitrogen removal rate is increased by about 39.2%.When only nitrite or nitrate exists in the influent matrix（NO₂^-or NO₃^-）,the Anammox reaction stops,and complete denitrification is the main nitrogen removal pathway.The addition of biochar doubled the reduction rates of nitrate and nitrite.In conclusion,biochar can effectively improve the reaction nitrogen removal efficiency by adjusting the nitrogen removal pathway of Anammox.（3）The phenolic hydroxyl groups（-OH）and ether groups（C-O-C）in the oxygen-containing functional groups on the surface of biochar acted as strong electron-donating groups and weak electron-donating groups,respectively,to donate electrons for the reduction of nitrate,or transfer electrons from organic matter to quinone groups through the action of anaerobic microorganisms,and the reduced quinone groups further contribute electrons to nitrate reduction.The specific performance is that the activity of the electron transport system in the biochar group is enhanced,so that the electron transfer amount of the biochar group reaches 1.56mmole^-L^-1,which is twice that of the control group.The results showed that the promoting effect of biochar on the nitrogen removal performance of Anammox had nothing to do with its adsorption capacity and dissolved organic matter,and was mainly determined by the oxygen-containing functional groups and EPS on the surface of biochar.（4）The abundances of the two main functional bacteria Candidatus Kuenenia and Pseudomonas involved in Anammox and denitrification reactions in the biochar group were higher than those in the control group,and at the molecular level,the addition of biochar enhanced the expression of functional proteins such as Nar G,Nir S,Nor BC,and Nos Z involved in the reduction of nitrate.The results showed that biochar could improve the nitrogen removal performance of Anammox reaction by regulating the population structure of microorganisms and enhancing the expression of microbial functional proteins.