Characteristics Of Microalgae Adhesion, Growth And CO₂Fixation In A Photobioreactor With Gas-liquid Separation Membrane

Microalgae has been regarded as the most potential renewable energy owing to itsadvantages of high photosynthetic efficiency, carbon neutrality, ease to cultivate, shortgrowth cycle, high lipid content, strong adaptability and so on. Because of these merits,the use of microalgae to realize the carbon bio-fixation and its energy-orientedutilization has become one of the hot research topics so far. In conventional, microalgaeare cultivated with suspension mode and such operation mode causes high cost, lowbiomass yield and large energy consumption of downstream processes, limiting thewidespread application of the microalgae. The microalgae adhered immobilizationcultivation method can improve the biomass per unit volume of photobioreactor andcompared with conventional suspension cultivation mode, the immobilized operationmode offers many benefits, such as high operation stability, large biomass density, andlow energy consumption of subsequent processing. Thus, it is critically important toinvestigate the principle of the microalgae adhesion on matrix surface, to explore theoptimum adhesion conditions, to design new photobioreactor and to analyze the biofilmgrowth and CO2uptake characteristics.To these objectives, in this thesis, Chlorella vulgaris was adopted for allexperiments. A planar photobioreactor was developed to study the adhesioncharacteristics of microalgae on matrix surface and the effects of the initial inoculation,roughness of the substrate, pH and flow condition of culture medium on adhesion rateand adhesion amount of microalgae cells on matrix surface were achieved. In addition,to couple adhesion cultivation with CO2bio-fixation, a biofilm photobioreactor with agas-liquid separation membrane was developed. With the newly-developedphotobioreactor, the growth of the microalgae biofilm and CO2removal rate wereinvestigated. The impacts of light intensity, CO2concentration, gas flow rate, nitrogenconcentration and flow rate of culture medium on microalgae biofilm growth and CO2fixation of the photobioreactor were also examined to optimize the operationalconditions.The main conclusions achieved in this work are as follows.①Adhesion characteristic of Chlorella vulgaris cells on a solid substrate surfacewas experimentally investigated based on a planar photobioreactor, in which differentroughness on the surface of glass slide was polished. The effects of surface roughness of substrate, inoculation amount of algal cells, pH value and flow rate of culture mediumon the adhesion characteristics of Chlorella vulgaris cells were respectively discussed.The results denoted that the adhesion densities of algal cells on any substrate surfaceincreased with time and then tended to be stable gradually. Under the experimentalconditions, with the increase of substrate surface roughness and inoculation amount ofalgal cells in culture medium, both the adhesion rate and adhesion density of Chlorellavulgaris cells were enhanced. Furthermore, it was found that the adhesion performanceof Chlorella vulgaris cells on the substrate surface achieve to the peak at the pH valueof medium of6or the liquid flow rate of0.65mL/min. The inoculation amount was themost important factor followed by the pH value of medium and the liquid flow rate.②A photobioreactor with gas-liquid separation membrane and a kind ofmembrane which is rough and hydrophobic were designed for microalgae adhesioncultivation. The membrane was mainly made by PTFE and glassfibre. The resultsdenoted that the PTFE-glassfibre membrane presents a characteristics of favourablestrength, biocompatibility and gas permeability，which could realize the separation ofgas and liquid, and microalgae could adhere and growth on it.③A gas-liquid separation membrane photobioreactor experimental system wasdesigned and constructed. Microalgae growth and CO2removal characteristics in thephotobioreactor with gas-liquid separation membrane under the light intensity of100μmol·m-2·s-1，2%（v/v）CO2,gas flow rate of10mL/min, the nitrogen concentrationin the culture medium of1.5g/L and culture medium flow rate of2mL/min, wereinvestigated respectively. According to the results, the final biomass after cultivation of7days reached97.48g·m-2, which is6.26-folds than the air condition one; the maxChlorella vulgaris biomass productivity came to13.54g·m-2·day-1after cultivation of3days, max CO2removal ratio was58.54%after cultivation of6days.④In examining the effects of operating parameters on the microalgae adhesiongrowth characteristics and CO2removal characteristics in the photobioreactor with agas-liquid separation membrane, it is observed that the variations of light intensity, flowrate of CO2, flow rate of liquid culture medium and initial nitrogen (NO3-)concentrations have a significant influence on the changes of biomass accumulation,accumulation rate and removal rate of CO2, but the influence on pH and nitrogen (NO3-)concentration of culture medium is less significant. For another, the variations ofconcentration of CO2have a larger influence on not only biomass accumulation, accumulation rate, removal rate of CO2but pH of culture medium, while nitrogen (NO3-)concentration of culture medium is not affected greatly.⑤For the microalgae adhered photobioreactor with gas-liquid separationmembrane, the optimum operational condition which makes the rate of biomassadhesion, the accumulation of biomass growth and the rate of CO2removal reach thehighest point was100μmol·m-2·s-1of illumination intensity,10mL/min gas flow ratewith2%（v/v） CO2and2mL/min liquid flow rate in which the concentration of NO3-is1.5g/L.