| In recent years,under the pressure of increasingly severe water resource scarcity and stricter sewage discharge standards,membrane bioreactors(MBR)have the advantages of good water treatment effect,small footprint,high treatment efficiency,and low residual sludge output.MBR has received extensive attention in the field of wastewater treatment and applications.Compared with the conventional activated sludge(CAS),MBR technology is an emerging technology that combines the advantages of high-efficiency membrane separation technology and activated sludge process in wastewater treatment.Although MBR technology has many advantages,membrane fouling seriously hinders the practical development of this technology.It is necessary to carry out research on the mechanism of membrane fouling in order to propose effective fouling prevention measures.In this study,a laboratory-scale submerged membrane bioreactor was started and operated to measure the sludge properties in the MBR under atypical conditions by observing the operation of the MBR,in order to explore the relationship between the change of sludge properties and membrane fouling.It also provides a control strategy for the corresponding membrane fouling of MBR in practical applications.Firstly,aiming at the foam fouling behavior in MBR,the interrelation between foam and membrane fouling is explored,and the foam fouling is explained by the ExtendedDerjiaguin-Verwey-Overbeek(XDLVO)interface thermodynamic theory and FloryHuggins theory High adhesion and high filtration resistance behavior of substances.At the same time,the membrane fouling behavior of bulk sludge during MBR operation was studied to reveal the inner mechanism of the formation of gel layer and filter cake layer,thus providing a new perspective for the study of membrane fouling control.Finally,according to the effect of sodium hypochlorite(NaClO)on typical pollutants(bovine serum albumin BSA)under the condition of membrane cleaning,to explore the influence of the spatial structure changes of protein pollutants on the membrane fouling dominated by the gel layer.The main findings are as follows:(1)By running MBR for two cycles under fine aeration and coarse aeration respectively,the study found that MBR is prone to produce a large amount of biological foam in the fine aeration mode,and in the foaming stage,the foulants have extremely high specific filtration resistance(SFR)and strong adhesion to the membrane surface.Chemical assays showed that the proteins and polysaccharides in the foaming samples were 6.4 and 5.4 times higher than those in the sludge supernatant samples,respectively.It is suggesting that the sludge system at the foaming period can produce large amounts of these foulants.The experimental results show that the foam foulants is related to the gel layer filtration process,and the Flory-Huggins theory is used to describe the chemical potential change during the gel filtration process to explain the extremely high SFR of foam.XDLVO theoretical analysis showed that the high interaction energy between the foam foulants and the membrane surface resulted in a strong adhesion ability.Finally,it was confirmed by 16 S rDNA gene sequencing that some foaming bacteria and filamentous bacteria in the sludge population may be the main causes of foam fouling,which provides theoretical support for the potential biological mechanism of foam fouling.This study provides a new explanation and basis for the mechanism study of foam fouling in MBR.(2)The MBR operating state and membrane fouling phenomenon in two periods of normal sludge and bulking sludge were recorded and compared.The study found that bulking sludge is easier to adhere to the surface of membrane modules than normal sludge,has higher filtration potential,lower dehydration and high viscosity,which will cause serious membrane fouling.A series of characterization measurements showed that the small particle size sludge formed by the bulking sludge acted as the skeleton of the fouling layer,resulting in a denser cake layer and improved filtration resistance during the filtration process.There are a large number of filamentous bacteria in the bulking sludge,which are intertwined and superimposed with the sludge particles in the filter cake layer,which further aggravates the membrane fouling.In addition,the filter cake layer produced by bulking sludge contains a large amount of extracellular polymeric substances(EPS),which is related to the formation of the gel layer.The study found that such foulants are released from the lysis of bulking sludge cells due to insufficient oxygen at the bottom after the formation of the filter cake layer.16 S rDNA technology further indicated that the bulking sludge had lower bacterial population abundance and activity at this time.Finally,this study uses Flory-Huggins theory and XDLVO theory to explain the reasons for the high filtration resistance caused by the gel layer formed by the bulking sludge and the strong adhesion with the membrane from the perspective of thermodynamics.This study provides new mechanistic insights into the fouling behavior of bulking sludge in MBR.(3)In order to explore the membrane cleaning mechanism,this paper studies the removal effect of sodium hypochlorite(NaClO)addition under different acid-base conditions on the removal of foulants with bovine serum albumin(BSA)as a model substance.It was found that under acidic conditions,the addition of NaClO would aggravate the membrane fouling behavior and in alkaline conditions,a short time(30min)reaction could reduce membrane fouling.The characterization shows that the addition of NaClO will affect the protein secondary structure of BSA,and there is a certain relationship between the content changes of α-helix and β-sheet and membrane fouling.The intermolecular hydrogen bonds contained in the β-sheets enhance the homogeneity of the gel layer,leading to further aggravation of membrane fouling.And the reason for the high filtration resistance of the gel layer composed of the protein polymer molecular skeleton is explained by the Flory-Huggins lattice theory.This study shows that NaClO will destroy the protein contaminant structure during the membrane cleaning process,there by aggravating/relieving membrane fouling.This provides a theoretical basis and guiding direction for subsequent researches related to membrane cleaning. |
PDF Full Text Request