Powdered Activated Carbon-submerged Hollow Fiber Ultrafiltration Membrane Systems For Algae-rich Water Treatment:Performance And Membrane Fouling

The discharge of excess nutrients(mainly nitrogen,phosphorus,etc.)in natural water bodies causes abnormal reproduction and growth of various aquatic organisms and plants.This phenomenon is called water nutrition.These excessive nutrients are mainly from farmland fertilization,agricultural waste,domestic sewage and certain industrial wastewater.Many rivers and lakes in China have eutrophication problems.It is urgent to explore algae water treatment technology that is suitable for China's national conditions,with high efficiency,low investment and low operating cost.In this experiment,a laboratory-scale immersion ultrafiltration membrane system was established,in which two different powdered activated carbon(PAC)dosing methods were used: pre-deposited PAC onto the membrane surfaces or pre-mixed PAC to the bulk feed,were used to complete fouling experiments.The changes in water quality before and after the experiment,membrane fouling,and membrane cleaning were analyzed.This experiment studied the effect of PAC on fouling by algal solution during filtration process,EOM distribution and foulant removal.Additionally,the relationship between PAC particles and algal cell growth and the subsequent influence on fouling were also discussed.This research has mainly obtained the following four aspects of research results:(1)The addition of PAC by either mode improved the removal of extracellular organic matter(EOM)from the algal solution.However,changes in the amount of PAC dosing had different effects on pre-mixed and predeposited PAC: pre-mixed PAC mitigated algal fouling,whereas pre-deposited PAC exhibited the opposite effect.This is likely due to the differences in the initial arrangement of algal cells and PAC along the cake layer depths.(2)The pre-deposited PAC avoided contact between cells and membranes,but aggravated the deformation of the cells and hindered their back-transport to the bulk solution.Furthermore,although the effect of PAC on the EOM fouling was marginal,there were highly synergistic effects when cells and EOM were present together in the PAC pre-deposition mode.(3)Changes in the PAC dosing mode altered the PAC-membrane interactions.Such as physical cleaning easily removed both algal cells and pre-mixed PAC from the membrane surfaces,whereas physical and chemical cleanings were both ineffective for pre-deposited PAC.Owing to the porous fouling layer in the pre-mixed case favored the retention of rejected EOM in the concentrated solution owing to a high backward migration rate.Conversely,the dense fouling layer in the predeposited case impeded the backward diffusion of EOM,which caused the majority of the rejected EOM to accumulate on the surface and at the bottom of the fouling layer.During chemical cleaning Na Cl O oxidized the EOM that was attached on the PAC surface,it enhanced the PAC-membrane interaction,which resulted in some PAC still remaining on the chemically cleaned membrane surface.(4)Decreased algae growth rates were observed in the reactors with PAC as a result of changes in the illumination and the cake layer structure,which could further intensify the contrast between the effects of pre-deposited and pre-mixed PACs on fouling.