The Mixtrophic Cultivation And Harvesting Of Microalgae And The Treatment Of Its Culture Medium By Membrane

As the depletion of fossil fuels and the severe global climate problems, governmentsand research institutions all over the world are making great effort to search for renewableenergy. Microalgae, as a most potential source for bioenergy, have been investigated formany years due to its short growth cycle, high lipid oil and protein content and arealproductivity. Until now, the cultivation of microalgae which contains high lipid content hasbeen investigated by many researchers. However, it still has some obstacles to prevent thebiodiesel industrialized production by microalgae. Some of these obstacles wereinvestigated in this work and the results were shown as followed.（1） In order to promote the growth rate of Chlorella pyrenoidosa, the corn strawhydrolyzate was added into the culture medium as the additional carbon source.The cell density reached6.751×107/ml when the initial reducing sugar concentration was0.3596g/L in the culture medium. The algae was cultivated under25±2°C and illuminated12hours a day at intensities of4000lxcm-2. The cell density increased to8.93×107/ml afterchanging the nitrogen source of the medium. The optimal concentration of carbamide was0.0025mol/L.（2） After10min of sedimentation, the flocculation efficiency reached97.4%when thepH of medium was adjusted to11.5, which means increasing the pH of medium couldcause autoflocculation.（3） Six flocculants were investigated in this work, most part of algae could beflocculated by FeCl₃, Al₂（SO₄）₃, Alum and chitosan whereas Ca（OH）₂and polyacrymidecould not. Almost similar results were obtained for flocculation by Alum and Al₂（SO₄）₃.When the dosage of them was0.3g/L, the highest flocculation efficiency was obtained butthe flocs were unstable and the viability of algal cells was also changed. The flocs formedby chitosan were the most stable one. The optimal dosage of chitosan was0.08g/L whenthe biomass concentration of algae was0.54g/L. High flocculation efficiency could beobserved with a short sedimentation time when the dosage of FeCl₃was0.1g/L and0.12g/L. However, high dosage of FeCl₃would damage the viability of algal cells. So theoptimal dosage of FeCl₃was0.1g/L. （4） The pH of medium after adding flocculants would affect the flocculation efficiency.The highest flocculation efficiency was observed when the pH was adjusted to9. Theflocculation efficiency of Alum increased from59.12%to92.8%when the pH of mediumwas adjusted to6. The flocculation efficiency of Al₂（SO₄）₃increased from56.85%to90.6%when the pH of medium was adjusted to5.（5） The relationship between algal biomass concentration and flocculant dosage ofdifferent flocculants were investigated in this work. The flocculant dosage required toachieve high flocculation efficiency increased with the increment of algal biomassconcentration.（6） Among the five UF membranes, GR82PE could observe high COD and conductivityretention with a relative high permeate flux which is suitable for industrial production. Thepermeate flux, COD and conductivity retention of GR82PE at4bar,20℃was189.66L/m²h,43%and4.25%, respectively. The flow rate of the experiment was480L/h.UF membranes only can remove some parts of the organic in the solution whereas the NFmembranes not only reject almost all organic matters but also can achieve relative highconductivity retention. The permeate flux, COD and conductivity retention of Alfa Laval-NF at15bar,20℃was95.52L/m²h,93.02%and58.94%, respectively. Compared toDesal5-DK, Alfa Laval-NF is suitable for this procedure as it has higher permeate flux.