Research On Bentonite Flocculation Harvesting And Immobilization Of Microalgae

It is widely known that microalgae have the advantages of being easy to reproduce,rich in lipids and without competing for land with agricultural crops.On the other hand,wastewater contains highly concentrated nutrients that can be used to produce microalgae,which can further be used to prepare biofuels.The utilization of microalgae could lead to an excellent sustainability of the coupling treatment of domestic wastewater and biofuel production with strong prospects for development,but the problems of a high harvesting cost together with an inefficient treatment of domestic wastewater would need to be solved in order to achieve efficient treatment of domestic wastewater.In this study,YZL02,a strain of microalgae,which had a healthy growth behaviour,was selected from the waters of He Bei Province and domesticated by the domestic sewage of Hutuo River for identification at first.And the cationic starch was prepared by starch modification,and the flocculant was prepared by composited cationic starch with organic clay bentonite,then the effects of composite flocculant compositing ratio,p H and dosage on the flocculation efficiency of microalgae were studied.Finally,The preservation effect and the ability to remove TN,TP and COD from domestic wastewater of the immobilized microalgae preparations were observed.The results are shown as follows:1.The YZL02 appeared to be growing well,with a cell density of 1.6×10⁷cells per ml at day 13 and an optical density value of 1.820 for OD₆₈₀,and for which was identified as Chlorella sp.2.The optimized conditions for the synthesis of cationic starch which confirmed by Fourier infrared spectroscopy were shown that the molar ratio of starch dehydrated glucose sugar unit,etherifying agent（CHPTAC）and Na OH was 1:1:1.5,with 70°C and 6h,respectively.3.The optimal flocculation conditions for the composite flocculant were shown were shown to be a bentonite to cationic starch mass ratio of 2:1 and a p H of 9;0.1 g of composite flocculant could harvest approximately 0.4 g of dry weight microalgae after 10minutes at these conditions,and the results also showed that the composite flocculant could work over a wide p H range.4.It was showed that the absolute values of zeta potential of the composite flocculant and flocs were between 0-6 with the state of rapid coagulation and aggregation more compact,and that the composite flocculant had a spatial configuration,which provided a larger surface area for the attachment of Chlorella by observation with particle size potential analyzer and scanning electron microscope.The ability of flocculant adsorption and bridging was enhanced by the bentonite from the analysis of flocculation mechanism.5.The best storage conditions of microalgae were obtained by physical adsorption and embedding with bentonite in the dark at 4℃.The activity of microalgae was still well after storage 60 days,and the chlorophyll content reached 9.8 g/ml after 10 days.6.The encapsulated microalgae which immobilized by the bentonite and sodium alginates could have a high mechanical strength and resistance to shear and other factors compared with immobilized by traditional agent.In the domestic wastewater treatment test,the removal rate of total nitrogen could reach at 92.5%,the removal rate of total phosphorus could reach at 88.5%,and the removal rate of COD could reach at 81%after 10 days of treatment.At these conditions above,the contents of nitrogen,phosphorus,and COD in the water body were 6.67,1.38,and 22 mg/L,respectively.Compared to the national level effluent indicator standards,TN and COD could be achieved the standards,but not TP.The performance in removing nutrients from the wastewater could be considered satisfactory.