Research On Technical Optimization And Membrane Fouling Control Of Microalgae Recovery By Air Flotation And Uniform Shearing Vibration Membrane

Microalgae is a kind of microorganism with both environmental and economic benefits.On the one hand,microalgae can use the nutrients in municipal wastewater for growth,which has a positive role in improving the ecological environment and reducing carbon emissions;on the other hand,microalgae metabolites can be used as raw materials for high value-added products such as biodiesel,pharmaceuticals,health products and cosmetics.However,its large-scale application still faces many technical and economic difficulties.Due to the tiny size and poor settling performance of microalgae,it is difficult and costly to harvest them.Therefore,the selection of suitable recovery technology is crucial to reduce the cost of microalgae production on a large scale.Among the existing microalgae harvesting technologies,air flotation and membrane separation technologies have their own advantages.Therefore,this study investigates the influencing factors of air flotation and membrane separation technologies to recover microalgae and the control methods of uniform shearing vibration membrane contamination respectively,and the specific research contents and results are as follows:(1)The bubble size is one of the important factors affecting air flotation,the smaller the bubble in a certain range the higher the microalgae recovery efficiency.With the increase of water depth,aeration rate and growth days,the recovery rate of microalgae showed an increasing trend.Under different temperature conditions,the best recovery of microalgae by air flotation was achieved at 25℃compared to 15℃and 30℃.The recovery rate of microalgae collected by the first air flotation and then incorporated into the original algal solution was significantly reduced,indicating that microalgae should be collected as soon as possible after air flotation to avoid incorporation into the algal solution again resulting in lower recovery rate.The study showed that the effect of bound extracellular organic matter(b EOM)on the recovery of microalgae by air flotation was greater than that of dissolved extracellular organic matter(d EOM).Air flotation recovered microalgae at different growth stages,and the heterotrophic stage was better than the autotrophic stage.In addition,the higher the proportion of hydrophobic microalgae,the higher the recovery rate of microalgae by air flotation.After air flotation,the proportion of high molecular weight(MW)material increased,while the proportion of low MW material decreased.The residual algal solution after air flotation was combined with membrane filtration,i.e.,air flotation was studied as a pretreatment unit for membrane filtration.Through short time pre-flotation,most of the microalgae cells and some EOM were collected by air-flotation,so the membrane contamination was significantly reduced,and the membrane flux decay was also significantly reduced,with the final flux of 51 L/(m~2·h)much higher than that of the non-pre-flotated microalgae solution(24 L/(m~2·h)).The results showed that the pre-air-floated microalgae had a positive effect on the subsequent membrane filtration,which could significantly improve the microalgae recovery efficiency.(2)Uniform shearing vibration membrane can further reduce membrane contamination by intermittent operation,and setting the intermittent time can reduce both reversible and irreversible contamination.Among them,the effect of filtering 9min with 1 min interval to reduce membrane contamination is better than filtering 54min with 6 min interval.more frequent intervals can further alleviate the concentration polarization phenomenon,so the membrane contamination is lighter.After the 12-h filtration experiment,the increment of transmembrane pressure(TMP)was only 0.6k Pa for 9 min intermittent 1 min filtration,which was 70%lower than continuous filtration,and the reversible and irreversible contamination rates of the membrane were reduced by 44.1%and 33.4%,respectively.Meanwhile,the algal cells attached to the membrane as well as the proteins,polysaccharides and humic acids in both reversible and irreversible EOM were reduced.(3)In terms of membrane cleaning,compared with physical cleaning,chemical cleaning method is more thorough in removing membrane contamination,and Na Cl O cleaning is better than Na OH cleaning.2%Na OH cleaning can restore membrane flux to 84.3%of new membrane,and 1%Na Cl O cleaning membrane recovery rate can reach92.6%.The total contamination rate of the 1%Na Cl O cleaned membrane was the lowest after 3 cycles of filtration experiments,which was 38.8%lower than that of pressurized water flushing,including 34.6%lower reversible contamination rate and46.0%lower irreversible contamination rate.Meanwhile,the contact angle of the membrane after 1%Na Cl O cleaning was closest to that of the new membrane,indicating that the membrane performance recovered well.