Study On Denitrification Process And Membrane Fouling Characteristics Of FB-MBR

In recent years,nitrate pollution in water has drawn continuous attention.Excessive nitrate accumulation will cause water eutrophication and pose a potential threat to human life and health.At present,biological denitrification technology plays a leading role in the field of nitrate removal in water body,but the traditional biological denitrification process generally has certain limitations.Therefore,it is urgent to find an efficient and economic biological denitrification technology.In addition,nitrate wastewater often contains a certain amount of salinity,which exhibits stresses on microbial community and brings challenges to biological denitrification process.In this study,a fixed biofilm membrane bioreactor(FB-MBR)process was established to explore the cultivation and enrichment of carrier biofilm during the process start-up period,investigate the biofilm denitrification efficacy and membrane fouling behavior characteristics under different carbon sources,and study the effects of different inorganic salt concentrations on system operation efficacy and membrane fouling behavior,in order to finally realize the stable and efficient operation and effective membrane fouling control of FB-MBR denitrification process.The main conclusions are as follows:(1)The carrier biofilm was cultured and enriched by rapid sludge discharge and gradually increasing the influent carbon and nitrogen loading.After 36 days,the start-up of FB-MBR process was successfully realized.In the whole process,the total nitrogen removal efficiency(NRE)and COD removal efficiency(CRE)remained above 95%and 80%.The biofilm gradually became thicker and firmly attached to the carrier with the color changing from light yellow to dark brown,and the biomass remaining at 1.525 gSS/g carrier.The results of three-dimensional fluorescence spectrum(3D-EEM)showed that the increase of tryptophan,aromatic proteins and humic acids in extracellular polymeric substances(EPS)promoted the formation of biofilm,and the release of protein fraction of soluble microbial product(SMPPN)aggravates membrane fouling process.The high-through sequencing results showed that the microbial community structure of the system exhibited dynamic succession,and the relative abundance of functional denitrifying microorganisms showed significant differences in all phases.Statistical analysis results showed that Acinetobacter,Flavobacterium and Pseudomonas closely correlated with SMPPN(rp was 0.835,0.834 and 0.892 respectively),which were the key bacteria to aggravate membrane fouling behavior during the start-up of the system.(2)The results of sequencing batch test showed that the specific denitrification rates of methanol,sodium acetate and butyric acid were significantly different.The denitrification rate of butyric acid system was the highest(10.87 mg/(gMLSS·d)),which was higher than those of methanol and sodium acetate(2.75,8.81 mg/(gMLSS·d)),but its NO2--N accumulation rate was also the highest.The increase in soluble hydrophobic proteins in the denitrification process of three carbon sources was the main factor leading to the increase of modified fouling index of soluble substance(MFIsol),of which the denitrification process using methanol as the electron donor is the most obvious.The lower total amount of EPS and the decrease of the relative hydrophobicity of EPS components resulted in a relative lower proportion of modified fouling index of solid substances(MFIss).The functional groups of membrane foulants causing cake layer resistance resulted from different carbon sources are similar.When sodium acetate was employed as an electron donor,the biofilm modified fouling index(MFI)showed the smallest.Thus,sodium acetate could be selected as the optimal carbon source for the system.(3)The denitrification performance of the FB-MBR system at lower salinity(0.5-10 g/L)was not significantly inhibited,whereas the denitrification efficacy of the FB-MBR system at higher salinity(15-30 g/L)decreased sharply.When the inorganic salinity reached 30 g/L,NRE decreased to 76.65%.When the salinity was in the range of 0.5-5 g/L,microorganisms increased the carrier biofilm biomass through secreting EPS;the continuous increase in salinity led to the biofilm abscission and the separation of plasma membrane,and further resulted in the sharp decrease of EPS content and the rapid increase of SMP content.Thus,membrane fouling rate(MFR)increased faster under higher salinity,and membrane pore resistance(RP)accounts for a higher proportion.3D-EEM results further showed that the release of tyrosine and tryptophan proteins in SMP was the key factor to aggravate the membrane fouling behavior of the system.