Breeding Of Auto-flocculated Microalgae With High-lipid In Municipal Sewage And Optimization Of Flocculation Conditions

The use of high-lipid microalgae cultured in municipal sewage to produce biodiesel will help people explore a new way of municipal sewage treatment while mitigating the current state of energy scarcity.In the microalgae biodiesel process,the high cost of the microalgae harvesting is a major bottleneck in the industrialization of the process.At present,high-efficiency,energy-saving and economical harvesting of microalgae has not yet been achieved.In this study,therefore,auto-flocculation method was used to harvest high-lipid microalgae in munincipal sewage.Through natural breeding and mutagenesis breeding,the best No.1 algae was obtained from the municipal sewage cultule system with fast growth rate,good flocculation effect,high lipid content and good sewage purification ability.Investigating the lipid production characteristics of microalgae,the lipid content and lipid production of No.1 were 27.34% and 0.3272 g/L,respectively.The flocculation efficiency of No.1 was found to be 96.46% and the concentration factor was as high as 148.The decontamination characteristics of microalgae were investigated.The removal rates of COD,ammonia nitrogen,TN and TP were 71.65%,81.89%,74.64% and 80.16%,respectively.The conditions affecting the auto-flocculation effect of No.1 algae were investigated,and the flocculation conditions were optimized by single factor and multi-factor experiments.In the single factor experiment,the effects of algae concentration,flocculation time,p H value,temperature,light quality and calcium and magnesium ions on the flocculation efficiency were investigated.The main influence factors were optimized by response surface methodology,and the optimal flocculation conditions were obtained: the OD680 was 1.23,the flocculation time was 3.4 h,and the p H was 10.5.The optimum flocculation efficiency obtained under these conditions was 92.00%.Based on the optimization results of flocculation conditions,the causes of auto-flocculation of No.1 algae were analyzed from cells,molecules,charges and other layers.It was identified that the No.1 algae was Limnothrix sp.,and belonged to Cyanophyta.The auto-flocculation of Limnothrix,which is mainly caused by the net-sweeping effect and the adsorption bridging between the extracellular polymer and the calcium and magnesium ions in the sewage.Electrical neutralization does not work in the auto-flocculation process of Limnothrix.Under the blue light irradiation,the flocculent phenomenon that the Limnothrix flocculates into a cluster is due to the secretion of a large amount of EPS by the algae,and the polysaccharide of B-EPS plays a major role in algae flocculation.Experiments on the harvesting of Limnothrix by auto-flocculation method under outdoor conditions show that the outdoor conditions have no significant effect on the growth and lipid production of algae.The harvesting efficiency of Limnothrix reached 88.55%,and the algae had a good removal effect on nitrogen and phosphorus in sewage.By analyzing the composition of fatty acid methyl ester,the microalgae harvesting method established in this experiment can improve the quality and low temperature fluidity of biodiesel.In this paper,the technical and economic feasibility analysis of auto-flocculation method for harvesting highlipid microalgae was carried out,and the industrialization prospect of the method was discussed.