Technology And Mechanism Of Low-Carbon Biological Nitrogen Removal Based On Counter Diffusion Of Electron Donor And Acceptor

According to the traditional biological nitrogen removal theory,carbon sources were essential electron donors for biological denitrification.Due to the scarcity of carbon sources,adding external carbon sources to low-carbon source sewage has become essential for ensuring nitrogen removal performance.Although numerous new biological nitrogen removal methods have been developed,the majority of them were still in the laboratory or pilot research stages because of their challenging operating environments.For many large-scale urban sewage treatment plants,the traditional biological nitrogen removal theory has remained the preferred method.Various engineering practices showed that traditional carbon consumption was 5 mg BOD/mg NO₃^--N,which was significantly higher than the 2.86 mg BOD/mg NO₃^--N（theoretical carbon consumption value）.However,the production of excess sludge yield quickly increased with the addition of external carbon sources.This phenomenon demonstrated that increasing nitrogen removal performance by adding additional carbon sources led to inadequate denitrification’s"targeted regulation"of the additional carbon sources.It is crucial to figure out how to improve denitrification’s"targeted regulation"of the additional carbon sources in order to lower the carbon consumption for biological nitrogen removal.The authors reviewed the effects of electron donor and acceptor on the biological nitrogen removal process and reviewed the influence of matrix diffusion direction on the microenvironment.It found that the diffusion direction of the electron donor and acceptor could affect the biological nitrogen removal process by the regulated microenvironment.Carbon,nitrogen,and oxygen all diffused in the same direction in traditional sewage treatment systems,and their concentrations were high on the perimeter of the sludge floc while they were low inside.As a result,outside the floc,the high concentration of carbon sources affected the nitrification process,and many carbon sources entered the aerobic oxidation pathway rather than the denitrification pathway.It was one of the main reasons why the actual carbon consumption of biological nitrogen removal was much higher than the theoretical value.Based on this,a low-carbon biological nitrogen removal technology based on counter diffusion of electron donor and acceptor was proposed.The counter diffusion of the electron donor and acceptor could regulate the microenvironment to improve denitrification"targeted regulation"of carbon sources and reduce carbon consumption.The main contents are as follows:（1）Carbon/nitrate counter-diffusion deep nitrogen removal technology and low-carbon consumption mechanismFor deep nitrogen removal of biologically treated wastewater,external carbon sources were added to the biofilm,and nitrate was diffused from the sewage to the biofilm,forming a carbon/nitrate counter-diffusion deep nitrogen removal system.Using comparative experiments,investigate the nitrate removal performance and carbon-saving effect,and study the effects of carbon source type,C/N,HRT,and nitrate load rate.The microbial community composition and function of the surface and internal biofilms were analyzed to explore the mechanism of denitrification enhancement,denitrification"targeted regulation",and carbon-saving in the carbon/nitrate counter-diffusion system.The TN,NO₃^--N,and COD concentrations of the biologically treated wastewater were 29.80±1.3,29.48±1.26,and 100.26±4.19 mg/L,respectively.COD/TN was 3.36.In the system of carbon/nitrate counter-diffusion and carbon/nitrate co-diffusion（control system）,the effluent TN were 5.60±1.62 and 10.96±1.54 mg/L,the effluent NH₄⁺-N was less than 1 mg/L,the effluent COD was lower than the detection limit,and the denitrification target efficiency of carbon sources was 82.09%and 72.11%,respectively.In addition,under different carbon source types（sodium acetate,glucose,and PCL）,C/N（5.0,4.5,4.0,and 3.5）,HRT（2.00,1.75,1.50,and 1.25 hr）,or nitrogen loading rate（1.0,0.8,0.6,and 0.4 g NO₃^--N/（L·d））,the denitrification performance and the organic matter removal performance of the carbon/nitrate counter-diffusion system were better than those of the control system.Carbon/nitrate counter-diffusion could also effectively alleviate the nitrite accumulation caused by C/N restriction.Carbon/nitrate counter-diffusion could reduce sludge yield,increase the denitrification target efficiency of carbon sources,and reduce carbon consumption.Carbon/nitrate counter-diffusion could regulate the microenvironment,leading to highly enriched denitrifying bacteria in the surface and interior biofilms,which increased the space utilization of the carrier.Further,functional divisions formed at different depths of biofilms due to the relative dominance of denitrifying and organic-degrading bacteria in internal and surface biofilms,respectively.Besides,the abundance of denitrification metabolism and denitrification functional enzymes in the carbon/nitrate counter-diffusion system was higher than those in the control system.All of these phenomena were conducive to improving denitrification performance.Denitrifying bacteria that are significantly concentrated in the carbon/nitrate counter-diffuse system,weakening other heterotrophic microorganisms.The range and capability of the carbon source diffusion were decreased by dosing the carbon source from inside the biofilm,due to decreased sodium acetate’s effective diffusion coefficient.The two phenomena were beneficial to improve the ability of denitrification"targeted regulation",and in favor of reducing carbon consumption.The microenvironment formed by the carbon/nitrate counter-diffusion could strengthen the deep nitrogen removal of biologically treated wastewater with low-carbon consumption.（2）Carbon/oxygen counter-diffusion SND nitrogen removal technology and low-carbon consumption mechanismFor treating low-carbon source sewage,low-carbon source sewage was added to the biofilm,and the oxygen molecules was diffused from the sewage to the biofilm,forming the carbon/oxygen counter-diffusion SND nitrogen removal system.Using comparative experiments,investigate the nitrogen removal perfomance and carbon-saving effect,and study the effects of carbon source concentration and DO concentration.The microbial community composition and function were analyzed to explore the nitrogen removal pathway,denitrification"targeted regulation",and carbon-saving in the carbon/oxygen counter-diffusion SND nitrogen removal system.The research object was the domestic wastewater of the school dormitory area with low-carbon sources.Its TN,NH₄⁺-N,and COD concentration were 31.73±1.25,30.62±1.24,and 121.39±4.96 mg/L,respectively.COD/TN was 3.83.Under low DO condition（DO<1 mg/L）,for the carbon/oxygen counter-diffusion system and carbon/oxygen co-diffusion system（control system）,the effluent TN was 7.73±1.95 and16.08±2.52 mg/L,the effluent NH₄⁺-N was 3.12±0.93 and 7.49±1.81 mg/L,and the denitrification target efficiency of carbon sources was 67.45%and 45.73%,respectively.Under higher DO conditions（DO>2 mg/L）,the effluent NH₄⁺-N of the two systems was less than 2 mg/L.Thus,complete nitration was achieved.However,carbon/oxygen counter-diffusion improved denitrification performance.Under low DO conditions（DO<1 mg/L）,the carbon/oxygen counter-diffusion system had better nitrification and denitrification performance than the control system with different carbon source concentrations.Under different DO concentrations,compared with the control system,the carbon/oxygen counter-diffusion could improve the total nitrogen removal effect,reduce sludge yield,improve the denitrification target efficiency of carbon sources,and reduce carbon consumption.Carbon/oxygen counter-diffusion could regulate the microenvironment,resulting in the formation of high oxygen concentrations and low carbon sources outside the biofilm.This condition could reduce aerobic oxidation of carbon sources and improve the nitrification"targeted regulation"of oxygen.Thus,the autotrophic nitrification process proceeded smoothly.At the same time,the high carbon source concentration and low oxygen concentrations were formed inside the biofilm.This condition could improve the denitrification"targeted regulation"of carbon sources.And it was beneficial to improve the denitrification performance.In addition,nitrogen removal microorganisms such as AOB,denitrifying bacteria,heterotrophic aerobic denitrifying bacteria,and comammox bacteria were enriched,which was conducive to simultaneously nitrification and denitrification.Moreover,it enriched the nitrogen removal pathway,which was conducive to forming a low-carbon consumption nitrogen removal pathway.In a low-DO environment,ammonia nitrogen oxidation was dominated by a more energy-efficient complex nitrification pathway induced by a single microorganism（comammox bacterial）.Furthermore,in the carbon/oxygen counter-diffusion system,complex nitrification pathway was more dominant,and the abundance of KEGG modules and functional enzymes related to nitrogen metabolism was higher,which was conducive to biological nitrogen removal.The microenvironment formed by carbon/oxygen counter-diffusion could enhance the biological nitrogen removal perofomance of low-carbon source sewage with low-carbon consumption.