Study On Harvesting Of Microalgae In Urban Sewage By Magnetic Flocculation

In the context of increasing energy demand,microalgae,regarded as one of the promising raw materials for biodiesel,has been attracting much attention.The use of urban sewage to culture microalgae can not only realize the recycling of sewage,but also reduce the cost of sewage treatment and biodiesel production.However,due to the small size,scattered distribution and low content of microalgae,the high harvesting cost becomes a bottleneck restricting the development of the whole process.The existing microalgae harvesting methods are unable to meet the requirements of efficiency and economical demands.In this study,therefore,magnetic flocculation was used to harvest Chlorella and S.obliquus cultured in municipal wastewater.Nano-Fe₃O₄ was prepared using co-precipitation method and coated with Polyethyleneimine?PEI?by high-speed stirring,so that demands of harvesting efficiency and cost can both be met.Through the single factor experiment,the optimal conditions were determined that molecular weight of PEI was 70,000 and mass ratio of PEI to nano-Fe₃O₄ was 1.The magnetic materials before and after coated were characterized by SEM,TEM,XRD,FT-IR,VSM and Zeta potential.They were spherical,and their size were relatively uniform,at about 10²0 nm.The materials before and after coat ing were superparamagnetic;saturation magnetic strength was 73.09 emu/g and 69.21 emu/g respectively;Zeta potential was-20.8 mV and 32.7 mV respectively.The prepared magnetic material was used to harvest Chlorella and S.obliquus in the sewage,and the harvest conditions were optimized by single factor and response surface experiments.In the single factor experiment,the effects of dosage,flocculation time,stirring speed and stirring time on the harvesting efficiency were investigated.The harvesting efficiencies of Chlorella and S.obliquus reached?93.73±0.36?%and?94.78±1.48?%,respectively.For Chlorella,after optimization using response surface experiment,optimal parameters were as follows:dosage of magnetic nanomaterials was 20 ml/L,flocculation time was 20 min,stirring speed was 800 r/min,and stirring time was3 min.Under such conditions,harvesting efficiency was?98.92±0.41?%.For S.obliquus,after optimization using response surface experiment,optimal parameters were as follows:dosage of magnetic nanomaterials was 15.5 ml/L,flocculation time was 15 min,stirring speed was 730 r/min,and stirring time was3 min.Under these conditions,harvesting efficiency was?98.45±0.35?%.Based on optimized results above,the mechanism of magnetic flocculation is studied from three perspectives:Zeta potential,microscopic morphology of the algae and structure of protein and polysaccharide,and the result is that the harvesting process is mainly about charge attraction.After harvesting microalgae,nano-Fe₃O₄ can be recovered by ultrasonication and re-coated with PEI for reuse,reducing the cost.Meanwhile,with this method,oil content of the microalgae and oil quality of the biodiesel does not reduce.This paper compares with former studies,analyzes the magnetic flocculation method established in this paper fro m the aspects of technical feasibility and economic feasibility,and finally discusses the industrialization prospect of this method.