Feasibility Study On Efficient Removal Of Nitrate From Water By Methane-based Membrane Bioreactor

Biological method to remove nitrate in wastewater is the most widely used wastewater treatment method.However,the current biological method mainly uses organic matter as the carbon source and electron donor to promote the growth of functional microorganisms and drive the reduction of NO₃^-,which increases the operating cost of sewage treatment and may cause secondary pollution of the water body.The greenhouse gas methane produced by the anaerobic digestion of sludge in sewage treatment plants is a clean and cheap electron donor.If used as a carbon source,its low solubility will not cause secondary pollution to water bodies,and at the same time,it can reduce the greenhouse effect.And it plays a positive role in the carbon source and energy recovery in the sewage treatment process.Based on this,this study uses methane as the electron donor to drive nitrate reduction,and uses bubble-free hollow fiber membrane gas supply to construct a membrane biofilm reactor to improve the methane gas-liquid mass transfer efficiency;nitrate-containing sewage is the research object.The effectiveness of this reactor to remove nitrate nitrogen and the utilization rate of methane were investigated.On this basis,through in-situ batch experiments,microbial community structure and functional gene analysis,the key factors that restrict the nitrate removal efficiency of this reactor are analyzed,and the theoretical basis and practical support are provided for the realization of high-efficiency and low-carbon nitrate removal processes.First,build a methane matrix membrane biofilm reactor（CH₄-MBf R）to analyze the effects of water velocity and methane partial pressure on methane mass transfer,and select appropriate mass transfer conditions to enrich functional microorganisms.When the methane partial pressure is 0.05 MPa,the gas-liquid mass transfer coefficient of methane is about 0.229 h^-1 at a water velocity of 0.6m/h to 3.4 m/h.When the water velocity is 2.6 m/h,as the methane partial pressure increases from 0.05 Mpa to 0.14 MPa,the gas-liquid mass transfer coefficient of methane increases from 0.22 h^-1 to 0.51 h^-1.Considering the methane mass transfer efficiency and microbial film formation,the circulating water speed is 2.6 m/h,and the methane partial pressure is 0.11 Mpa.At this time,the methane gas-liquid mass transfer coefficient is 0.42 h^-1.Under these conditions,the microorganisms are enriched.After 62 days,the nitrate nitrogen removal efficiency of the reactor can reach 200 mg NO₃^--N/L·d.In addition,a large amount of volatile acid is detected in the reactor during the enrichment period,and the highest volatile acid content It was 938.6 mg/L,mainly acetic acid and propionic acid,and its concentration reached 835.6 mg/L and propionic acid 103.0 mg/L,respectively.In order to investigate the efficiency of this reactor for the treatment of nitrate wastewater,under the condition of 200 mg NO₃^--N/L in the influent,the hydraulic retention time（HRT）was gradually shortened from 2 d to 0.5 d,corresponding to the removal rate of nitrate nitrogen They were 99.8 mg/L·d,199.8 mg/L·d and 320.0mg/L·d.Under the condition of HRT of 1 d,the nitrate nitrogen concentration of the influent water was increased to 400 mg/L and 600 mg/L successively,and it was found that the highest nitrate reduction rate in this reactor could reach 398.2 mg NO₃^--N/L·d.A small amount of volatile acid was also detected at each stage of the reactor operation,which was less than 90 mg/L.Through COD carbon balance calculation,it is found that the utilization rate of CH₄-MBf R to methane is as high as 84.8%,indicating that hollow fiber membrane aeration can reduce the waste caused by the escape of methane in the form of bubbles;at the same time,up to91.6%of methane energy is used by microorganisms.It is used for nitrate reduction.In order to further explore the key factors affecting the efficiency of CH₄-MBf R nitrate removal,four sets of in-situ batch experiments were carried out and the microbial community structure and functional genes in the reactor were analyzed.The batch biochemical reaction mechanism,functional microorganisms（heterotrophic denitrifying bacteria,acetogenic bacteria,methanotrophic bacteria）and the existence of functional genes（nar G,nap A,nir K,nir S,pmo A）together reveal that this reactor removes nitrate nitrogen In this study,the nitrate bioreduction process driven by methane as an electron donor is completed by heterotrophic denitrification mediated by volatile acid as an intermediate product,which means that the amount of volatile acid generated in CH4-MBf R can be used as a regulatory nitrate The key factor of removal efficiency.