Interfacial Fouling Behavior Of Extracellular Polymeric Substances In Microalgae Cultivation And Forward Osmosis Separation

Since the 1990s,due to the growth of world population and the improvement of human life quality,the global energy demand has increased dramatically.With the continuous exploitation and utilization of fossil fuels,a large amount of carbon dioxide has been emitted into the atmosphere,leading to global warming.Microalgae,as an autotrophic organism,can use photosynthesis for efficient carbon fixation.It can not only use CO₂ as a carbon source for photosynthesis,but also can absorb nitrogen,phosphorus and organic matter to promote its growth,and produce algae biomass with high economic value.However,microalgae dewatering is still one of the major bottlenecks in terms of energy and cost aspects,hindering further application of the microalgae biomass production.In recent years,forward osmosis（FO）has attracted wide attention in wastewater treatment and water reuse due to its advantages of low energy consumption and low pollution.It can efficiently separate algae from water and realize water reuse and algae seed harvest.However,during the growth process of microalgae,extracellular polymeric substances（EPS）will be released into the extracellular membrane,and this part of EPS will combine with algal cells to form biofilm,causing wall fouling and membrane pollution in the photobioreactor.These EPS components are complex,and play an important role in wall adhesion and membrane pollution.In this study,Chlorella vulgaris and Scenedesmus obliquus were used as experimental algae species for the investigation of the interaction between EPS and reactor surface.The research was carried out to explore the membrane pollution mechanism caused by s EPS and b EPS in the process of algae water separation,which based on FO membrane separation technology.The mechanism of EPS adhesion was further elucidated combined with XDLVO theory.It provided a theoretical basis for further improving the fouling resistance of photobioreactor and reducing membrane fouling of membrane bioreactor.The main research contents and conclusions were listed as follows:（1）The EPS composition analysis showed that there were significant differences in EPS composition between C.vulgaris,and S.obliquus.The zeta potential of extracellular polymers of S.obliquus was lower than that of C.vulgaris,and S.obliquus was more likely to agglomerate in the broth.Proteins were the main components of b EPS,whereas the soluble microbial products were the main components of s EPS by using three-dimensional fluorescence.The b EPS was mainly composed of lower molecular weight organics,whereas s EPS was mainly composed of medium and long chain organics.The main components of s EPS and b EPS of C.vulgaris were hydrophilic and hydrophobic,while the s EPS and b EPS of S.obliquus were hydrophobic.（2）The adhesion of b EPS was the most significant,and its pollution to the PTFE and PMMA was also the most serious.The hydrophobic PTFE was more easily contaminated by EPS and algal broth.The results showed that the adhesion of dissolved organic matter（DOC）of PTFE was b EPS>s EPS>Broth,whereas that of PMMA was b EPS>Broth>s EPS.The XDLVO calculation showed that the absolute value of adhesive free energy of PTFE was much higher than that of PMMA in both C.vulgaris and S.obliquus and the order of adhesion free energy of PTFE and PMMA is consistent with that of DOC.In addition,the cohesive free energy of C.vulgaris and S.obliquus was b EPS>Broth>s EPS,and the cohesive free energy was greater than the adhesive free energy,showing that the EPS of algae had the tendency of mutual adsorption.（3）The mechanism of EPS membrane fouling in the process of FO based algae water separation was further studied by using XDLVO theroy.The results showed that the b EPS resulted in the fastest flux decline and the most serious membrane foulings,followed by algae broth.For C.vulgaris,the flux decrease caused by algal cells was greater than that caused by s EPS,but for S.obliquus,the flux decrease caused by algal cells was less than that caused by s EPS.The absolute value of adhesive free energy of C.vulgaris was b EPS>Broth>algal cells>s EPS;the absolute value of adhesive free energy of S.obliquus was b EPS>Broth>s EPS>algal cells.This result was found consistent with the decreasing trend of water flux.The linear fitting between the interfacial free energy and the flux decline was higher than 95%,indicating that the XDLVO theory can be used to quantitatively characterize the interaction behavior of membrane pollutants at the molecular level.Therefore,the research on the interfacial adhesion behavior of EPS in the process of microalgae culture and FO microalgae separation can provide a theoretical basis for further selecting the reactor materials with pollution resistance and designing the low pollution forward osmosis membrane treatment process.