| Dynamic membrane bioreactor?DMBR?is an emerging membrane bioreactor?MBR?using dynamic membrane?DM?instead of microfiltration or ultrafiltration membrane,which has the advantages of low cost,low energy consumption,high flux and easy cleaning,ect.,causing extensive attention from wastewater treatment field.However,compared with the conventional MBR,DMBR uses coarse pore materials as the supporting layer for creating a cake layer when the activated sludge flocs are rejected and attached on the surface of support material in the filtration process.The deposited cake layer can realize the rejection of the particulate matters.Thus,the stability of DM in change has been the main technical bottleneck limiting the wide application of the DMBR in engineering.Therefore,this study focused on the optimization and regulation of the DM formation process.Through theoretical analysis and experimental verification,it explores the theoretical and technical approaches to enhance the design,operation and treatment efficiency of DMBR,which is of great significance to the development of DMBR technology.In this study,nylon mesh was used as the support material to make dynamic membrane module constructing a DMBR system.Combined with batch filtration tests,interaction energy analysis of dynamic formation process using extended Derjaguin-Landau-Verwey-Overbeek?XDLVO?theory was carried out for different sludge suspensions with different properties in a DMBR,and the effects of sludge properties on the DM formation was also analyzed.Then,on the basis of the above results,hydrophobic powder activated carbon?PAC?as the additive was added into DMBR to develop the hybrid PAC-DMBR system with the aim of enhancement of wastewater treatment efficiency in the DMBR.Thus,the filtration performance,pollutants removal,activated sludge properties,physicochemical and microbial characteristics of DM layer in the PAC-DMBR system were investigated.Furthermore,the views of the underlying enhancement and stability mechanisms of DM formation by PAC addition were discussed,and the practical applicability of the hybrid DMBR process was evaluated.The following conclusions can be drawn:1.The XDLVO theory was used to analyze the interfacial interaction energy in the formation of dynamic membrane,which was quantitatively analyzed the interfacial interaction energy between membrane and sludge flocs as well as between approaching sludge flocs and the deposited sludge flocs within a short-range?nm level?during the DM formation process.Moreover,the effects and relative contributions of Lifshitz-van der Waals?LW?,electrostatic?EL?and acid-base?AB?to the DM formation was also evaluated.It was shown that AB interaction energy played a key role in the adhesion behavior of sludge flocs on the membrane surface during the initial DM formation stage and also in the cohesion behavior of sludge flocs during the DM maturation stage.Three kinds of sludge suspensions with different properties?sludge A with the acceptable flocculability but poor settleability and dewaterability;sludge B with the normal flocculability,settleability and dewaterability;sludge C with the poor flocculability but acceptable settleability and dewaterability?were used to carry out the DMBR batch filtration experiments.The results showed that sludge A was easy to adhere on the surface of membrane and cohere on the surface of the deposited sludge flocs,which was conducive to promoting the initial formation and maturation of dynamic membrane.On the contrary,sludge C was difficult to adhere on the membrane surface and cohere on the deposited sludge flocs surface,which had a negative effect on the initial DM formation and DM maturation.The normal sludge B has the moderate flocculability which resulted in the moderate level of the initial DM formation and maturation.XDLVO theory was applied to analyze the interfacial interaction energy of the three kinds of sludge suspensions,and the initial DM formation time and effluent turbidity after DM maturation was used to evaluate the state of DM formation.The findings proved that the analytical results were consistent with the experimental results,and the state of DM formation depended on the adhesion interaction energy of sludge flocs-membrane and cohesion interaction energy of sludge flocs-sludge flocs.2.Combined with the theoretical analysis of XDLVO,the control strategies of activated sludge properties and dynamic membrane performance were studied from the aspects of dynamic membrane formation,stable operation and good filtration performance.DMBR as the membrane bioreactor,the rational control of the operation parameters?such as microbial biomass,F/M,HRT,SRT and aeration intensity,etc.?was the first control factor,which could directly affect the sludge flocculability,settleability and dewaterability;Maintaining the dynamic membrane with relatively dense,stable structure and appropriate thickness was the second control factor,which needed to moderately maintain the sludge hydrophobicity and the appropriate hydraulic scour intensity on the membrane surface;Reducing dynamic membrane resistance,and maintaining high membrane flux while improving dynamic membrane rejection properties was the third control factor,which needed to modify the structure of dynamic membrane including optimization of sludge flocs size,cake layer thickness and pore size.Similar to the conventional MBR,the optimization of operation parameters could achieve the adjustment of the first factor and part of the second factor?such as hydraulic scour intensity on the membrane surface?,but limited achievement of third factor was done.Therefore,based on the knowledge that AB interaction energy was the determinant of the adhesion interaction and cohesion interaction energy,adding carrier with certain surface hydrophobicity into DMBR could realize the regulation of DM structure and performance.3.Hydrophobic powdered activated carbon?PAC?as the carrier was added into a DMBR to develop a hybrid PAC-DMBR.Through the long-term operation of two paralleled DMBRs,the DM formation process and physicochemical and microbial characteristics of DM layer in C-DMBR?without PAC addition?and PAC-DMBR3?with 3 g/L PAC dosage?were compared.It was indicated that PAC addition effectively shortened the time of DM formation,and DM layer with looser structure,more porosity and stronger incompressibility was in PAC-DMBR3 as compared with C-DMBR.Moreover,there were fewer residual pollutants on the mesh surface after air backwashing in PAC-DMBR3.Furthermore,PAC addition led to a significant increase in the sludge particle size and carbon content but a decrease in the EPS concentration in the DM layer for PAC-DMBR3.In addition,PAC addition positively changed the microbial community structure in the DM layer,which reduced the abundance of certain microbial phylum?such as Proteobacteria?regarding as pioneers in the colonization of the membrane surface,and increased the abundance of some phylum?such as Bacteroidetes,Chloroflexi and Firmicutes?that can degrade and transform organic matter.At the genus level,some specific microbial genera?such as Acinetobacter?with the function of biodegradation of complex organics were enriched.These findings indicated that PAC addition effectively modified the physicochemical and microbial characteristics of DM layer.4.The pollutants removal and process performances in PAC-DMBR3 and C-DMBR were compared under the long-term operation.It was found that PAC addition enhanced the removal of COD,UV254,NH3-N and color.Noteworthily,in DMBR process effluent turbidity was an important indicator to characterize the rejection properties and stability of DM layer in DMBR.PAC addition shortened the time of high turbidity effluent of every operation cycle,so that the turbidity in PAC-DMBR3 could decrease to near 0.1 NTU lower than the turbidity values detected in C-DMBR.Moreover,PAC addition effectively reduced the DM layer resistance and total filtration resistance,greatly improved the stable flux and prolong the stable operational time of the DMBR system.Furthermore,the activated sludge properties in reactors were analyzed and compared.The results indicated that PAC addition brought about the improvement in the sludge morphological structure,increase in the sludge size,enhancement of settleability,dewaterability,flocculability,decrease in the EPS concentration and variation in EPS organic component and molecular weight distribution.Moreover,Illumina Miseq high-throughput pyrosequencing results indicated that adding PAC improved the microbial diversity in the activated sludge,which promoted the enrichment of some genera?such as genera Acinetobacter,Comamonas,Flavobacterium and Pseudomonas?with the function of flocs formation,among which Acinetobacter could biodegrade some complex organics.5.The mechanisms of PAC-DMBR comprehensive interaction were discussed,preliminary views on three aspects were pointed out.Firstly,PAC particle with hydrophobicity could modify the balance relationships of interaction energy between membrane and sludge flocs as well as between approaching sludge flocs and the deposited sludge flocs,which promoted the initial DM formation and maturation,and improved the stability of DM layer.Secondly,biological PAC sludge was formed due to the adsorption of organic and effect of biological carrier by PAC particles,which resulted in the improvements of the microbial community diversity in the DM layer and activated sludge in the hybrid PAC-DMBR.Thirdly,PAC as a kind of component in the DM layer changed the cake layer structure that was formed by sludge adhesion and cohesion effects,which led to cake layer with more porous and incompressible,lower resistance and higher filtration flux.These mechanisms need to be further studied,so that the reliable theoretical information could be provided in the PAC-DMBR process or the process of strengthening DMBR with other carriers. 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