Study On New Microalgae Biofilm Culture Technology Coupled With Wastewater Treatment And Biofilm Formation Mechanism

Microalgae have attracted extensive attention in biodiesel,wastewater treatment,food health care,medicine and other fields due to their advantages of short generation cycle,no competition with crops for cultivated land and rich in lipid,protein and polysaccharide.Especially in the current situation of accelerating urbanization and deepening industrialization,the combination of microalgae cultivation and pollution control can not only deal with wastewater and waste gas generated by human activities,but also accumulate lipid,protein and polysaccharide by microalgae themselves,so as to realize the purose of recycling waste resources.However,there are some problems in the cultivation of microalgae in wastewater,for example,the growth of microalgae is easily affected by environmental factors,and the concentration of microalgae biomass in suspension is low and difficult to harvest.Therefore,three kinds of nutrient-rich microalgae were firstly studied in this work to explore the feasibility of their cultivation in actual wastewater,so as to screen out the microalgae species with strong pollution resisitance and environmental adaptability.Then,aiming at the shortcomings of traditional biofilm reactors,a new type of microalgae biofilm reactor was constructed and its key operating parameters were optimized,and the biofilm system was utilized to realize efficient accumulation of microalgae biomss and wastewater treatment.Finally,the extended Derjaguin Landau Verwey Overbeek（XDLVO）theory was used to analyze the interaction between microalgae cells,as well as microalgae cell and the carrier material in the process of microalgae biofilm membrane formation,so as to reveal the mechanism of microalgae biofilm membrane formation.The main contents and conclusions of this study are as follows:（1）Firstly,Chlorella vulgaris（C.vulgaris）,Scenedesmus obliquus（S.obliquus）and Spirulina platensis（S.platensis）were selected as the research objects,and the feasibility of their cultivation in actual wastewater was investigated.The relative abundance of these three kinds of microalgae was analyzed during culture,it was found that C.vulgaris grew better than S.obliquus,and S.platensis died out after a period of culture in actual wastewater when considering the temperature and light photoperiod vary on diurnal and seasonal scales.In addition,whether aerated or not,the bioreactors achieved higher accumulation of microalgae biomass and more effective removal of pollutants in summer and autumn conditions.Furthermore,the kinetic model was used to describe the growth process of microbial biomass,it was found that the maximum specific growth rate was 0.07-0.46 d^-1.Compared with the aeration condition,the maximum specific growth rate in the same season under the non-aeration condition was lower,indicating that aeration was beneficial to the biomass growth of the system.（2）A four-channel flat plate microalgae biofilm reactor was developed using walnut shell as carrier material,and the key operating conditions（light intensity,walnut shell size,CO₂concentration）were optimized by single factor experiments.The results showed that when the walnut shell size was 0.8-1.2 mm,the light intensity was 150μmol m² s^-1,and CO₂concentration was 2%,the biomass yield of C.vulgaris and S.obliquus reached the maximum value,97.45 and 70.49 g m^-2,respectively.Elemental analysis showed that the addition of external CO₂ and walnut shell increased the content of organic and inorganic carbon in microalgae culture,and thus increased the storage of carbon materials（lipid and polysaccharide）in microalgae cells.At the same time,organic carbon in the leachate of walnut shell may change the metabolism mode of microalgae from photoautotrophic to mixotropic type.Polyphenols in the walnut shell leachate can reduce microalgae cell damage caused by lipid oxidation products and prevent access of free radicals to lipid molecules,thus contributing to the accumulation of lipid in microalgae cells.In addition,the changes of Algal organic matter（AOM）components on the surface of microalgae cells cultured in walnut shell leachate before and after culture were studied.The content of protein and polysaccharide in AOM on the surface of microalgae cells cultured in walnut cultured in walnut shell leachate increased,which was conducive to the adhesion of microalgae cells on walnut shell surface and the formation of microalgae biofilm.（3）The microalgae biofilm reactor was used to culture microalgae in wastewater and compared with suspension culture.It was found that after the 8-days cultivation,the microalgae biomass yield and productivity of biofilm systems were significantly higher than that of suspension systems.Among them,the highest microalgae biomass yield and productivity of72.87 g m^-2 and 9.11 g m^-2 d^-1 were achieved in S.obliquus biofilm system.The highest lipid yield and polysaccharide yield of 2.77 and 2.96 g m^-2 d^-1 were also achieved in S.obliquus biofilm system,while the highest protein yield of 2.66 m^-2 d^-1 was achieved in C.vulgaris&S.obliquus biofilm system.The physicochemical parameters of microalgae biodiesel were then analyzed.It was found that microalgae biofilm system had little influence on the performance of microalgae biodiesel,and which met the standards of Europe（EN）and the American Society for Testing and Materials（ASTM）standard well.In addition,compared with microalgae suspension systems,the removal rate of COD,TN,and TP by microalgae biofilm systems has little difference.Among them,the removal rate of COD,TN,and TP by microalgae biofilm systems reached 65.8-74.6%,74.1-83.6%,and 76.9-87.7%.In this study,the Photobiotreatment（Ph BT）model was used to describe the removal process of TN and TP in the microalgae biofilm systems and suspension systems,and the maximum specific removal rates of TN and TP were1.70-1.97 d^-1 and 0.80-1.58 d^-1,respectively.（4）In this study,XDLVO theory was used to analyze the interaction between microalgae cells,as well as microalgae cell and the carrier material during the cultivation of microalgae biofilm.It was found that with the development of microalgae biofilm,the total interation between microalgae cells changed from repulsion to attraction,or the energy barrier gradually disappeared,which would lead to the aggregation of microalgae cells.At the same time,the interfacial energy barrier between microalgae cell and walnut shell was gradually reduced or eliminated,which was beneficial for microalgae cells to form stable biofilm on the surface of the carrier material.