Research On Algae Water Separation Based On Mixed Dead End/cross-flow Forward Osmosis System

With the intensification of the energy crisis,energy microalgae as the most promising alternative to fossil fuels the third generation of biomass energy has caused wide public concern.However,the energy consumption in the separation algae from water accounts for 20-30% of the total energy consumption in the process of microalgae energy conversion,which has become a major technical bottleneck in microalgae energy conversion.Forward osmosis is an emerging membrane filtration technology,possessing low energy consumption and low membrane fouling.But traditional cross-flow forward osmosis system limits the algae moisture content,because the algal broth needs circulation and pipelines need to hold a certain volume of algal liquid.To solve this problem,this study proposed a noval hybrid dead-end/cross-flow forward osmosis system.The effects of algal properties(algae species,algal culture p H),draw solution properties(draw solution types,draw solution concentration)and hydraulic conditions(stirring speed,cross-flow velocity)on the separation performance were studied.And the morphology and chemical composition of the cake layer before and after hydraulic cleaning were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy under optimal operating conditions.In order to further reduce energy consumption,a gravity sediment-forward osmosis combined process was proposed for separation algae from water.The results showed that:(1)The hybrid dead-end/cross-flow forward osmosis system could improve the drive force by increasing the draw solution concentration,thus greatly reducing the biomass moisture content.When the draw solution concentration was 5 M,the biomass moisture content could reach 71.82%.Under this condition,the initial water flux was 15.55 LMH,water flux was 0.98 when permeat volume is 210 ml and flux recovery rate was 97%.(2)After separation,all the algal biomass was trapped on the membrane surface,forming a thick filter cake layer.After hydraulic washing,the surface morphology of was completely recovered,and fourier transform infrared spectroscopy showed that only a small amount of polysaccharides adsorbed on the membrane surface.(3)The energy consumption of gravity sediment-forward osmosis combined process was 60% lower than that of forward osmosis.The biomass moisture content obtained was similar,and the flux recovery was above 90%.However,the flux reduction of gravity sedimentation-forward osmosis combined process was larger than forward osmosis,they were 0.76 and 0.94,respectively,when permeat volume is 210 ml.(4)Algal cell broth could generate serious flux reduction.Since no organic matter adsorption,the flux recovery rate could still reach 100% after three cycles.When bound extracellular organics added,the compactness and hydrophobic adsorption of the cake layer were enhanced,and the separation performance was decreased.When the bound extracellular organic matter and the dissolved extracellular organic matter exist simultaneously,the algae matter zeta potential decreased,and the sedimentation decreased,so that the cake layer was more porous,which improves the separationperformance.The membrane fouling mechanism was cake layer fouling and concentration polarization,which played a role in different stages of separation due to the different sedimentation rates caused by organic matter.