Research On Chlorella Harvesting By Filtration With Uniform Shearing Vibration Membrane At High Shear Rate

Microalgae stand out in biomass energy because of its fast growth rate,small footprint,rich oil and biomass.The lack of efficient and low-cost microalgae harvesting technology is a major bottleneck restricting its wide application.Uniform shearing vibration membrane(USVM)system is a new type of membrane separation technology which can be used for microalgae harvesting.Its working principle is driving the membrane module to rotate at a constant speed through a servo motor with adjustable frequency,so that a stable and strong shear effect can be formed on the membrane surface,thus USVM can effectively alleviate membrane fouling during filtration.Compared with the traditional liner vibration membrane(LVM)system,USVM has a higher effective shear rate.Considering energy consumption and efficiency,this study selected a high shear rate with amplitude of 2 cm and vibration frequency of 5 Hz,and used Chlorella pyrenoidosa as the target algae species for the following studies:(1)The anti-fouling ability of USVM at high shear rate during long-term filtration of microalgae was investigated.Firstly,the critical flux experiment was carried out,and the operating flux of the experiment was determined to be 24L/(m~2·h).Then 24 h and 58 h continuous filtration experiments were carried out respectively.The transmembrane pressure(TMP)of the membrane both increased slowly and steadily,with increments of 2.4 KPa and 4.4 KPa.Scanning electron microscope(SEM)and Fourier Transform Infrared Spectroscopy(FTIR)were used to analyze the contaminated film before and after washing.It was found that the fouling membranes were effectively recovered after physical washing.The total fouling rates of the two membranes were 44.85%and 56.74%,indicating that the total membrane fouling did not increase significantly with time.The irreversible fouling rates of the two membranes were 10.20%and 10.54%,and there was no significant difference.Therefore,with the increase of time,the aggravated membrane fouling was mainly caused by the reversible fouling caused by algal cells and some extracellular organic matter(EOM)deposited on the membrane surface.By measuring the components of irreversible and reversible EOM pollution on the membrane and analyzing the Excitation-Emission-Matrix Spectra(EEM),it was found that the contents of TOC,protein,polysaccharide and humic acid did not increase significantly with the extension of filtration time.Therefore,it can be proved that the shear vibration membrane with high shear rate still has high anti-pollution ability in long-term filtration.(2)Although high shear rate can bring better anti-pollution effect,it is not clear whether it will affect the activity of Chlorella.Firstly,several commonly used activity determination methods were compared.The triphenyltetrazolium chloride(TTC)-dehydrogenase reduction method was used for subsequent research because of its advantages of simple operation,short time and accurate reflection of algal cell activity.The activity changes of Chlorella under high shear rate vibration and natural growth were compared.The results showed that compared with the Chlorella under natural growth conditions,the Chlorella under high shear rate vibration showed higher activity,and its activity had an upward trend.At the same time,the EOM components of the algae solution under the two conditions were measured and EEM analyzed,which reflected that the phenomenon of algae cell death and decline under natural growth conditions was more serious.Therefore,it can be proved that the high shear rate of vibration frequency 5 Hz and amplitude 2 cm not only did not reduce the activity of Chlorella,but also promoted the growth of Chlorella.(3)Since microalgae harvesting is a continuous concentration process,this study continued to explore the feasibility of USVM filtration and concentration of Chlorella under high shear rate.The algae solution was concentrated by 5 times using USVM.The results showed that the TMP of the membrane increased more and faster in the concentration experiment,and the membrane was seriously polluted.Through the analysis of the flux,pollution rate,SEM and FTIR of the polluted membrane before and after washing,the detection of microalgae content on the membrane,and the detection of reversible irreversible EOM on the membrane,it was found that the irreversible EOM on the membrane did not increase significantly,and there was no more serious humic acid pollution.The main body of membrane pollution was the reversible pollution caused by the deposition of a large number of microalgae on the membrane surface with the increase of algal concentration.In addition,the analysis of EOM in permeate and concentrated solutions showed that the membrane still had a certain interception effect on humic acid and protein under the condition of high frequency vibration.Therefore,it is feasible to concentrate microalgae by USVM.