| Membrane bioreactor(MBR)has become one of the most promising technologies in the field of wastewater treatment due to its excellent effluent quality,low sludge yield and compact device structure.However,membrane fouling during long-term operation is the main obstacle to large-scale commercial application.Compared with physical,chemical and biological cleaning methods,the method of controlling membrane fouling by electric field has the advantages of good cleaning effect,stable operation process,no additional chemicals and no secondary pollution.This is a clean and efficient method for in situ membrane fouling mitigation.However,the additional electric field consumes additional energy and increases the operating cost of the system.Microbial fuel cell(MFC)can release electrons through anodic oxidation of organic compounds(electronic donors)in wastewater,and then generate electricity.Using the electric energy generated by MFC to obtain the electric field in MBR and constructing the electric field MBR system can not only effectively alleviate membrane fouling and reduce the operation cost of MBR,but also promote the practical application process of MFC.The main research contents and results were as follows:(1)Preparation and characterization of conductive microfiltration membranes.Cu-nanowires(Cu-NWs)were blended into polyvinylidene fluoride(PVDF)casting solution to prepare conductive microfiltration membranes.Compared with the commercial PVDF membranes,the contact angle of the prepared membrane(35% Cu-NWs content based on the casting solution)decreased by 19.7% and the pure water flux increased by 40.0%.Cu-NWs established an excellent conductive network in the active layer of the membrane,which provided more active sites for oxidation-reduction reaction(ORR)and enhanced its catalytic oxidation-reduction ability.Cu-NWs embedded in the active layer of the membrane could reduce the adsorption of protein molecules in bovine serum albumin(BSA)solution on the membrane surface and decreased the thickness of the protein contaminated layer by 18.5%.After cleaning,the thickness of the contaminated layer on the Cu-NWs conductive microfiltration membrane surface decreased by 49.0%.(2)Preliminary study on the operation characteristics of spontaneous electric field membrane bioreactor(SEF-MBR).A batch SEF-MBR was constructed using the 5% Cu-NWs conductive microfiltration membrane as both cathode material and filter medium,graphite felt as anode,proton exchange membrane(PEM)as separating material between anaerobic and aerobic chambers.The SEF-MBR was used to treat synthetic wastewater.The results showed that the chemical energy in simulated domestic sewage was converted into electrical energy by the metabolism of electrochemical active bacteria in anaerobic chamber.The spontaneous electric field intensity and output power density were up to 0.077 V/cm and 307.7 m W/m2.The coulombic efficiency(CE)was 28.8±3.5%.The SEF-MBR obtained removal rates of94.5±0.4% for chemical oxygen demand(COD),99.9±0.1% for ammonia nitrogen(NH4+-N),78.5±4.3% for total nitrogen(TN)and 86.6±4.2% for total phosphorus(TP).After more than two months of operation,the flux attenuation rate of Cu-NWs conductive microfiltrationmembrane was 90.2%,and that of PVDF membrane was 93.0%.The thickness of the contaminated layer on the Cu-NWs conductive microfiltration membrane surface about 80.0μm,which was 55.3% lower than that on the PVDF surface.The adsorption biomass,EPS concentration and pollutant biovolume on the Cu-NWs conductive microfiltration membrane surface were 37.3%,31.6% and 24.6% lower than those on PVDF membrane,respectively.(3)The influence of electrode spacing on the operation characteristics of SEF-MBR was studied.The baffle,instead of PEM,was used as separating material between anaerobic chamber and aerobic chamber.A continuous flow SEF-MBR system was constructed.When the distance between Cu-NWs conductive microfiltration membrane module and graphite felt was shortened from 4 cm to 2 cm,the membrane fouling rate decreased from 0.15 k Pa/d to0.08 k Pa/d,the maximum output power density increased from 2.38 m W/m2(35.30 m W/m3)to 2.90 m W/m2(42.95 m W/m3),the spontaneous electric field intensity increased from 0.70 m V/cm to 1.20 m V/cm,The concentrations of hydrogen peroxide and ·OH produced in aerobic tank increased from 0.18 mg/L and absence to 0.86 mg/L and 0.12 mg/L,respectively.Besides biodegradation,electrochemical and oxidants could oxidize COD,resulting in a higher COD removal rate.With the increase of spontaneous electric field intensity from 0.70 m V/cm to 1.20 m V/cm,the concentrations of oxidants produced in the aerobic tank increased,leading to an increase of the microorganism metabolism rate and a higher nitrification reaction rate,which was beneficial to the removal of tryptophan.The TN concentration in effluent decreased from 9.9±0.7 mg/L to 7.5±0.9 mg/L.Due to the stimulation of electric field,the ATP content of sludge in SEF-MBR was 12.8% higher than that in control MBR.The increase of microbial activity promoted the uptake of positive phosphorus by phosphorus accumulating bacteria and reduced the TP concentration.Due to the existence of spontaneous electric field in the system,the effluent complied with the criterion of Integrated Wastewater Discharge Standard(GB18918-2002,Class A of the First Grade Standard)in China.(4)Study on the influence of spontaneous electric field intensity on the sludge property.The sludge property is one of the main factors affecting membrane fouling.Different electric field intensity affects membrane fouling by changing sludge properties.The results showed that SEF-MBR 1 obtained the lowest growth rates of MLSS [0.07 g/(L·d)] and EPS [0.45mg/(g VSS·d)],which were 50.0% and 42.1% lower than that of control MBR,respectively.When the average spontaneous electric field intensity was 1.25 m V/cm,0.93 m V/cm and 0.65 m V/cm,the average concentrations of H2O2 and ·OH produced by aerobic tank were 0.90mg/L and 0.13 mg/L,0.54 mg/L and 0.08 mg/L,0.42 mg/L and 0.07 mg/L,respectively.The higher the electric field intensity is,the higher the oxidant concentration is,and the lower the MLSS concentration is,which is conducive to alleviating membrane fouling.At the end of the operation,the water content of SEF-MBR 1 was the lowest(91.84%),which was 6.4% lower than that of control MBR.The sludge volume of SEF-MBR 1 was 83.8% lower than that of control MBR.The specific gravity and activity of living cells in the activated sludge were increased by the spontaneous electric field.The specific gravity and ATP content ofSEF-MBR 1 were 98.6% and 0.69 mg/g VSS,respectively,which were 3.9% and 23.2%higher than those of control MBR.(5)Analysis of membrane fouling behavior in SEF-MBR.During 28 days operation,SEF-MBR produced relatively stable in-situ electric field intensity,hydrogen peroxide and·OH,which were 3.79 m V/cm,1.22 mg/L and 0.15 mg/L,respectively.Compared with the control MBR,particle size of activated sludge,EPS content and TMP growth rate decreased by 23.9%,52.8% and 43.8%,respectively.The formation and growth of biofouling layer on the membrane surface in SEF-MBR could be divided into three stages.Firstly,polysaccharides and proteins deposited on the membrane surface,adhered to microbial cells and gradually formed large aggregates.Then,EPS and microbial cells increase rapidly,especially the biovolume of β-D-glucopyranose polysaccharides,leading to the rapid growth of the thickness of the fouling layer.Finally,the growth rates of EPS and microbial cells slowed down,which reduced the growth rate of cake layer thickness.Abundant metal oxides or metal hydroxides were presented in the membrane fouling layer.The contents of Fe and Ca were higher.From the 6th day to the 10 th day,the growth rate of β-D-glucopyranose polysaccharides biovolume in the membrane fouling layer increased fastest.From the 10 th day to the 28 th day,the growth rate of total cells biovolume in the membrane fouling layer increased rapidly,while that of control MBR was protein.The biovolume of EPS was always larger than that of microbial cells.Therefore,EPS played an important role in the formation and growth of membrane fouling layer. |
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