Study On Membrane Fouling Characteristics And Mechanism During The Process Of Positive Osmosis Treatment Of Algae-containing Water

Forward osmosis(FO)has recently been an emerging water treatment technology,which described as low energy consumption and low operation.However the membrane fouling is still a bottleneck problem towards the wide-spread application of forward osmosis.To address this problem,this study constructes the forward osmosis system focusing on the membrane fouling characteristics during the treatment of algae-rich water.The Chlorella vulgaris was used as the model algae for these experiments.The fouling effects were subsequently evaluated by membrane flux decline,reversibility analysis of membrane fouling.And the scanning electron microscopy(SEM),the total reflection-fourier transform infrared spectroscopy(ATR-FTIR)and the fluorescence microscope were also used to contrastively analyze the membrane fouled by the original algae,algal cells or EOM.Moreover the mechanism of membrane fouling were analyzed using XDLVO theory and fluorescence labeling and confocal laser scanning microscopy(CLSM).The experiment compared the membrane flux of EOM,algal cells and algae by forward osmosis and then the reversibility of membrane fouling were researched.The results showed that the membrane flux decline were 13.9%,11.4%and 27.6%for EOM,algal cells and algae,respectively.And the original algae caused more severe membrane flux decline than the sum of algal cells and EOM.Meanwhile,the irreversible membrane fouling of them had the same varying tendency.And it revealed that there was a synergy between EOM and algal cells for membrane fouled by the original algae.Based on the above research,the effects of chemical condition(Calcium ion,Ca2+)and other coexistence components(humic acid and kaolin clays)in water on membrane fouling caused by the algae were further investigated.The results found when the 10mmol/L Ca2+ was added,the membrane flux decreased by 19.6%and the irreversible membrane fouling increased by 14.0%.This is due to reducing the electrostatic repulsion between foulants and the membrane surface by charge neutralization and bridging between membrane and foulants for Ca2+,and then the membrane fouling was aggravated.After adding Kaolin clays of 100mg/L,the membrane flux decreased by 18.5%and the membrane flux recovery rate increased by 3.3%during the 6h working time.It attributed to EOM being adsorbed by Kaolin clays,and then on the one hand,the formation of thick and dense fouling layer on the membrane surface by crosslinking action increased the flux decline.And on the otherhand,the probability of EOM adsorbing on the algal-cells and membrane surface were reduced,therefore the membrane irreversible fouling was alleviated.When 10mg/L humic acid was added,due to the adsorption of humic acid on EOM accelerating the formation of EOM fouling layer,the membrane flux decreased by 18.5%and the flux recovery decreased by 6.5%.The XDLVO theory analysis showed that they could reduce the free energy of adhesion of membrane-foulant and the free energy of free energy of foulant-foulant in varying degrees,and reduced the repulsion between them,therefore it promoted the formation and development of membrane fouling.And CLSM image analysis o membrane fouling showed that the thickness of the fouling layer was increased from 8.8μm to 13μm,11.8μm and 10.0μm when adding 10mmol/L Ca2",100mg/L kaolin clays and 10mg/L humic acid respectively.In addition,not only increased the thickness of fouling layer and the content of protein,polysaccharide and algal cells,but also changed the distribution of algal cells,which the algal cells were wrapped in fouling layer in the presence of Ca2+.The addition of kaolin clays only increased the content of foulants,but did not change foulants’ distribution in the fouling layer.And when the humic acid was added,the protein and polysaccharide were increased and the number of algal cells decreased in the fouling layer.