Research On A New Technology For Harvesting Microalgae

Due to their high value nutritional compouds, microalgae show a broad application prospect in many industries. Limited by the tiny size and low cell concentration, developing a cost-efficient and efficient technology for harvesting microalgae is a great challenge. Therefore, this thesis aims at preparing a cheap, non-toxic and environmentally friendly microalgae flocculant, namely Cationic Starch Flocculant (CSF). Taking CSF degree of substitution (DS) as the target, the quadratics rotary orthogonal combination design was used to optimize the preparation condition of CSF. The prepared CSF was applied to flocculate Scenedesmus obliqnus. Under the optimum flocculation condition, the correlative properties of CSF flocculation for harvesting microalgae were explored. Meanwhile, CSF was compared with the traditional flocculant for harvesting microalgae in order to study the feasibility of this technology. The conclusions go as follows:1. By the investigation of CFS preparation, the optimum condition was reaction temperature at 84 ℃,54%(w/w) of etherifying agent,14%(w/w) of NaOH and 23%(w/w) of moisture. Under this condition, the optimal CSF degree of substitution was 0.35.2. The change of the starch structure during the etherification reaction was confirmed by infrared spectroscopy and environmental scanning electron microscope and the prepared CSF was confirmed to be the target product.3. Taking flocculation efficiency as the target, the optimum flocculation condition was as follows:stirring time 10 min, stirring rate 200 r/min, sedimentation time 20min. Under the optimum condition, Scenedesmus Dimorphus could be collected efficiently by CSF and the flocculation efficiency could reach above 97% in 30 minutes with the optimal cationic starch dosage (30 mg per gram microalgal dry weight).4. Microalgae secreted a mass of AOM (Algae Organic Matter) during the growth which could bond to CFS competitively with microalgae in the process of CFS flocculation for microalgae. The presence of AOM resulted in an increased consumption of CFS. Eliminating the interference of AOM in the microalgae solution, the optimal ratio of CFS consumption and microalgae biomass (dry weight) decreased when the microalgae biomass concentration increased. The optimal harvest phase was stationary phases.5. The safety and economy of CSF was assessed and the investigation indicated that CFS did not affect the physiological activity of microalgae. The flocculation cost for harvesting 1 kilogram microalgae was 0.2 yuan by using the prepared CSF (DS=0.32), which meet the demand of economy in microalgae biodiesel.6. The study of zeta-potential change during the CFS flocculation proved that the mechanism of CSF flocculation for microalgae included bonding bridging followed by neutralization.7. Taking flocculation efficiency, economic cost and environmental friendliness into account, CFS had major advantages over alum and chitosan.8. In conclusion, CFS flocculation for harvesting microalgae is an efficient, environmental and economical technology for microalgae harvesting.