Study On Aging Control And Mechanism Of Ultrafiltration Membrane By Chemical Cleaning Of Sodium Hypochlorite With Micro-nano Bubbles

Ultrafiltration technology is widely used in the reuse of sewage and wastewater and the advanced treatment of drinking water.It can effectively remove bacteria,pathogens,colloids,macromolecular pollutants and even some viruses in raw water.During membrane filtration,membrane fouling is unavoidable and chemical cleaning is an effective method for controlling membrane fouling.However,long-term chemical cleaning can cause irreversible degradation of membrane chemical stability and filtration performance（ie"membrane aging"）,shortening membrane service life,affecting stable operation of membranes,increasing operating costs,and restricting the further promotion and application of ultrafiltration membrane technology.Therefore,it is urgent to develop a green and efficient cleaning technology that can not only meet the needs of membrane cleaning,but also correctly control the aging process of membranes.Moreover,in recent years,with many advantages such as small size,long hydraulic retention time,high interfacial Zeta potential,and generation of active oxygen,micro-nano bubbles（MNBs）have gradually been used in water treatment.MNBs can not only strengthen the utilization of chemical agents,promote the degradation rate of pollutants,but also as a cleaning technology will not cause secondary pollution to the membrane,which also provides a new idea for controlling membrane aging.Based on this,this article mainly studied the aging control of ultrafiltration membranes by micro-nano bubbles intensified sodium hypochlorite chemical cleaning.First,in the experiment,the polyvinylidene fluoride（PVDF）flat ultrafiltration membrane was used as the experimental membrane,and the anionic polyacrylamide（APAM）was used as the characteristic pollutant.The flux recovery rate of micro-nano bubbles,0.005%-0.2%sodium hypochlorite solution single and combined cleaning on contaminated membranes was explored,and the microscopic characterization of the membrane before and after cleaning was carried out to verify the effect of the cleaning process on the microscopic characteristics of the membrane.Experiments showed that the membrane water flux recovery rate was only 0.12 during MNBs cleaning,while when cleaning with Na Cl O chemical agent,as the concentration of the agent increased,the cleaning effect improved until the Na Cl O concentration increased to 0.2%.MNBs strengthened 0.03%Na Cl O had the highest cleaning efficiency on the membrane,which would return to the original membrane flux within 2h.Through the microscopic characterization of the membrane before and after MNBs and Na Cl O cleaning,it was found that increasing the concentration of Na Cl O solution would accelerate the aging process of the membrane,but no higher membrane aging damage was found under the synergy of micro-nano bubbles,compared with the single cleaning of sodium hypochlorite.Secondly,the experiment explored the basic characteristics of MNBs in different concentrations of sodium hypochlorite solution,and analyzed the mechanism of MNBs in membrane cleaning.The average size of MNBs in pure water was about 261-405nm,and the Zeta potential of MNBs interface fluctuated between 3.5-9.1m V.Adding an appropriate amount of sodium hypochlorite could improve the stability of MNBs in water,and Cl O^-would preferentially adsorb at the gas-liquid interface,making the bubbles negatively charged.In the 0.03%sodium hypochlorite solution,MNBs showed excellent stability that he bubble size was in the range of 596-680nm during 3 hours of continuous operation,and the bubbles did not burst within one week after the process was stopped.In addition,the number of coalesced bubbles n was inversely proportional to the cleaning effect of MNBs synergistically with Na Cl O.Based on this,MNBs enhanced Na Cl O chemical cleaning was mainly due to its advantages of high mass transfer efficiency and strong stability.During membrane cleaning,Cl O^-at the gas-liquid interface and in the solution played the main role.However,when MNBs were unstable and coalesced easily,the size of the bubbles was much larger than the membrane pore size and would adhere to the membrane surface,which prevented Cl O^-ionic from cleaning the pollutants in the membrane pores.Finally,by immersing the membrane with Na Cl O and MNBs for a long time,the aging effects on the membrane including chemical stability and filtration performance were explored,and the interaction force between membrane-membrane and membrane-pollutants was analyzed based on XDLVO theory to further optimize the process parameters and establish the mechanism of MNBs strengthening Na Cl O to control membrane aging.The experimental results showed that MNBs did not find obvious aging phenomenon on the membrane,so as a green cleaning technology,MNBs could be used in membrane cleaning to control the membrane aging process.After 40 days of continuous immersion in 0.03%Na Cl O solution,the membrane matrix did not appear obvious damage.In addition,the mechanical properties of the aging membrane after static immersion in 0.03%Na Cl O,including tensile strength and elongation at break,were higher than the original membrane.The above results indicated that the aging behavior of 0.03%Na Cl O on the membrane could not be judged.Moreover,the degradation rate of the hydrophilic group of the membrane after 0.03%Na Cl O soaking was much lower than that of the membrane after 0.1%Na Cl O soaking,and the membrane still showed hydrophilicity in the later stage of the aging experiment.But under0.1%Na Cl O solution soaking,the hydrophilic group of the membrane was completely degraded and the contact angle was greater than 90°on the 10th day,indicating that the membrane was hydrophobic,and more,the roughness of the membrane was significantly reduced,membrane surface became smoother,and the mechanical properties of the membrane were severely damaged.Furthermore,after Na Cl O static soaking,the pure water flux of the aging membrane increased significantly,and the membrane resistance R_mcorrespondingly decreased.When filtering APAM pollutants,it was found that with the increase of membrane aging time,the rate of membrane flux decline and membrane irreversible pollution R_irrincreased significantly.Based on the XDLVO theory,it was found that the static immersion of Na Cl O solution weakened the non-polar characteristics of the membrane,converted the polarity of the electron donor of the PVDF membrane to the polarity of the electron acceptor,and increased the free energy of the interface of the aging membrane-APAM,which intensified the fouling behavior of the membrane,adversely affecting the control of membrane aging.However,in the analysis of the correlation between membrane fouling behavior and membrane-APAM interface free energy,it was found that the correlation was not obvious,which may be caused by the change of the chemical stability of the membrane under the chemical solution immersion.In summary,MNBs enhanced 0.03%Na Cl O chemical cleaning not only achieved the required cleaning effect,but also correctly controlled the aging process of the ultrafiltration membrane.Compared with the single chemical cleaning of Na Cl O,the cleaning process of MNBs enhanced Na Cl O not only shortened the cleaning time and significantly weakened the aging effect of the membrane,but also provided a theoretical basis for the application of the technology to engineering.