Enhancement Of Microalgae Biofilm Formation And Growth Regulation

Microalgae can use carbon dioxide in the flue gas of power plant as carbon sources and nitrogen and phosphorus in waste water as nutrients,as well as light energy as energy sources,to produce raw energy materials such as oil and high-value-added products such as astaxanthin through photosynthesis.In these cases,the process of microalgae utilization can be effectively integrated into bioenergy production and waste disposal.Microalgae biofilm is a microecological community formed through the proliferation and growth of microalgae cells on the surface of the carrier.The driving force in such a process was mainly attributed to the van der Waals force,electrostatic force,and the adhesion of extracellular secretions.Comparing with suspension cultivation systems for microalgae,biofilm cultivation systems features a couple of advantages in regard to efficiency and handing.This technology can also significantly reduce the requirement of water,thereby leading to a low energy consumption with a high biomass concentration during the process of microalgae harvesting.Notably,the growth of microalgae biofilm contains a series of complex energy transfer and biochemical conversion processes.The coupling effects of these various processes are mainly embodies in the following stages,such as the movement of microalgae cells in suspension towards the surface of the carrier,the initial adsorption and growth of microalgae cells,and the maturity of the biofilm.Nowadays,the study of microalgae biofilm is at the primarily stage.As the corresponding action mechanism is unclear,the regulation methods for biofilm formation,as well as the strengthening means for the subsequent biofilm growth,are still in short of pertinence.The poor adhesion stability and the long formation period are also the key problems that hinders the development of this technology.On account of this,it is of great significance to study the characteristics of microalgae biofilm and the biofilm formation mechanism on the surface of the carrier.Besides,this also helpful to explore the effective process enhancement methods and enhance the yield of microalgae biomass.Based on the biofilm cultivation systems for microalgae,the corresponding action mechanism and characteristics reflected in each stage of biofilm formation and growth were analysed step by step in this study.Firstly,in terms of the movement of microalgae cells in suspension towards the surface of the carrier,the sedimentation rate of microalgae cells is generally slow and the biofilm formation period is quite long.To solve this problem,an algae-algae co-flocculation system combined with zeta potential regulation method was proposed according to the law of Stock.Such a method not only significantly accelerated the settling velocity of microalgae cells but also greatly shortened the formation period of microalgae biofilm.Due to the three-dimensional pore structure of the copolymer,this work also made a basis for the study on the porous biofilm with a small mass transfer resistance.Additionally,to solve the problem reflected in the initial adsorption stage of microalgae cells such as weak interaction force and poor attachment performance,the mechanism and characteristics of microalgae cell adhesion on the surface of the carrier were comparatively analysed by using the DLVO theory.Thereafter,a series of microgroove carriers with different sizes and shapes were developed to study the effects of flow field and the gas-liquid interface behaviour over the surface of carrier during the process of biofilm formation.Take into consideration of the bubbles formed during the photosynthetic growth of microalgae,the effects of bubble behaviour on microalgae biofilm formation was also studied.The growth of microalgae biofilm as well as the formation of loose and porous microstructure in biofilm could be effectively induced through the regulation of bubble behaviours.Finally,based on the theoretical basis of the two-dimensional carrier established in the previous studies,a three-dimensional multilayer grid carrier was made by using three-dimensional printing technology.The effects of geometric parameters and surface characteristic,as well as the gas-liquid two-phase behaviour,on the biofilm formation and growth was assessed.Such a carrier can make a full use of vertical space,thereby significantly improving the mass transfer process of biofilm formation.Besides,this technology was applied to a new and efficient integrated bioreactor for microalgae biofilm cultivation and wastewater treatment.The effects of the types and concentrations of nitrogen and phosphorus contained in wastewater on the decontamination performance of this bioreactor and the growth of microalgae biofilm were studied.The main research results are as follows:?1?A method for microalgae moved from microalgae suspension to substrate surfaces rapidly was proposed.Two species,ankistrodesmus falcatus and scenedesmus obliquus was mixed in the ratio of 1:4,so that they could form to the porous microalgae-microalgae co-fluctuation.Subsequently,based on the electrostatic neutralization theory,the zeta potential of microalgae-microalgae co-fluctuation suspension were respectively adjusted to positively and negatively charged aggregates by pH control.In this way,the inoculation time of sedimentation was shortened from 4h to 1.5 min in microalgae photobioreactor.The multilayer porous structure of the microalgae-microalgae co-fluctuation reduced the mass transfer resistance of nutrients in biofilm,which could contributed to increase microalgae biofilm density and lipid content by 20.23%and 36.94%comparing to pure microalgae culture,respectively.?2?The attachment process and mechanism of microalgae cells on the substrate surface in the flowing microalgae suspension were investigated with DLVO theory,then the related attachment strength was discussed.The different size and shapes of microgrooves were designed and fabricated by soft lithography to optimize the attachment of microalgae cells on the substrate surface.The results indicated the action of coming flow on the gas-liquid of U-grooves would destroy the biofilm.In contrast,the V-grooves,creating vortex,not only enhanced the attachment strength,but promoted the substrate transfer,thus the initial attachment time of microalgae biofilm being shortened from 135 min to 50 min on the substrate with 45°V-grooves.?3?The effects of bubbles formed during the photosynthetic growth of microalgae on the biofilm microstructure was analysed in this work.In terms of the carrier surface modified by the PTFE emulsion with different concentrations,the bubble behaviours such as growth,breaking,disengaging and rising could lead to the formation of loose and porous microstructure in microalgae biofilm,thereby reducing the resistance of mass transfer.Besides,these processes would also improve the destabilization of nutrient solution and enhance the diffusion of dissolved oxygen.This is beneficial to reduce the inhibitory effect of end-products and increase the yield of microalgae biomass.When the culture liquid flow rate was 3.5 mL min^-1 and the inlet height was2.5 cm,the maximum biomass concentration of microalgae biofilm could reach 177.89g m^-2 on the surface of carrier treated by 1%PTFE emulsion?the contact angle was 64degree?,and the corresponding production rate could reach 9.88 g m^-2d^-1.?4?To further strengthen the growth of biofilm on the carrier and make the utmost of the reactor vertical space,a multi-layer grid is constructed by 3D printing.The most suitable biofilm carrier material and the best structural parameters of the multi-layer grid were determined through studying the impact of two phase behavior of generated bubble and culture on the growth of biofilm during the growth process of microalgae biofilm.The most suitable biofilm carrier was semi-transparent photosensitive resin for the growth of biofilms.With the carrier mesh size of hexahedral,pore size of 8 mm³ and layers number of 3,the dry weight of microalgae biofilms could reach 240.75g m^-2,which is 169.87%higher than the dry weight of biofilms on the monolayer grid and84.71%higher than the dry weight of biofilms on the flat surface.?5?An multi-layer grid reactor was designed for microalgae biofilm cultivation and wastewater treatment.The wastewater purification efficiency and the microalgae growth rate were both improved with the wastewater passing through the multi-layer grid.The purification efficiency of total nitrogen within 7 day was 103.92%higher than that of the plate carrier.And the biomass concentration of microalgae biofilm was 34.57%higher than that of the plate carrier.With ammonia nitrogen as the nitrogen source,the nitrogen purification efficiency was the highest.Through the absorption kinetics of nitrogen and phosphorus by microalgae,the reaction rate constant?k?and semi-satiation constant?Km?of microalgae to nitrogen and phosphorus were obtained as follows:for nitrogen absorption,k=0.75(mg L^-1 NH₄⁺-N)/(g m^-2 DW)/d,Km=289.11 mg L^-1;for phosphorus uptake k=0.63(mg L^-1 PO₄^3--P)/(g m^-2 DW)/d,Km=253.22 mg L^-1.