Gravity-driven Ultrafiltration Treating WWTP Secondary Effluent:Performance Optimization And Mechanisim

The water resources in China are abundant,but water resources per capita are inadequate.The amount of municipal sewage is large and the water quality is stable.The treatment and reuse of municipal sewage can effectively alleviate the contradiction between supply and demand of water shortage.Ultrafiltration technology shows certain advantages in wastewater recycling applications,such as high solid-liquid separation efficiency,little footprint,and high degree of automation.However,due to membrane fouling problems,traditional ultrafiltration technology has disadvantages such as auxiliary equipment,complicated operations,troublesome maintenance,and large consumption of chemical agents.In the long-term operation of the gravity-driven ultrafiltration（GDM）process,the flux can be stabilized without any backwashing and chemical cleaning,and it has the advantages of simple operation,low maintenance,and low energy consumption.However,in the practical application of GDM technology,its low flux level and insufficient removal of soluble organic pollutants restrict its popularization and application.This study explored the removal performance and flux stablization of GDM when treating WWTP secondary effluent When the secondary effluent was treated,the GDM process basically had little removal of organic pollutants such as DOC,UV₂₅₄,fluorescent organics and SMX.However,the GDM process had exercellent removal of turbidity and Mn,whose removal could reach 87.67%and 84.08%,respectively.The stable flux of GDM can reach 5.47 LMH.In view of the insufficient removal of GDM pollutants,granular activated carbon（GAC）filtration was used as the pretreatment process to explore the pollutant removal and the flux of GAC-enhanced GDM process.Microfiltration was used to pretreat the secondary effluent to remove partical matter and explore the performance of the MF+GDM process.The removal of DOC and UV₂₅₄ by GDM coupled with GAC could reach 68.10%and 80.71%respectively.The coupled system could significantly reduce the concentration of SMP,protein,humic and fulvic acid in the secondary effluent,and reduce the fluorescence intensity.It could effectively remove SMX with a removal of over 95%.It can strengthen the removal of Mn,which can reach more than 95%.After MF pretreatment,GDM had no obvious change in the removal efficiency of organic pollutants and turbidity,and the removal of Mn decreased.After MF and GAC pretreatment,the GDM flux decreased by 16.22%and 10.47%respectively.GAC effluent and secondary effluent were mixed in the ratio of 7:3 and 3:7 before entering GDM,and the flux is at the same level as the individual GDM flux.In view of the shortcoming of low flux level of GDM,the removal performance and flux of GDM in intermittent mode and continuous operation mode were investigated and compared.DOC,UV₂₅₄,fluorescent organic pollutants and turbidity of effluent from GDM in intermittent operation mode are similar to those of effluent from GDM in continuous operation mode.20/4 and 16/8 had better Mn removal,which can reach 90.81%and 84.76%respectively,while the removal of Mn for 12/12 is 65.78%,which is lower than continuous operation.Intermittent operation can increase GDM operating flux,and the GDM operating flux level has a linear relationship with the intermittent time in the cycle time that the flux level was higher with the intermittent time increasing.In addition,it was also found that intermittent operation promoted the formation of loose,rough,and heterogeneous bio-cake layer,which improved the permeability and reduced the resistance of the bio-cake layer.In the intermittent operation mode,the abundance of microorganisms which could alleviate membrane fouling was higher and polysaccharide and protein content,which showed better linear relationship with flux,decresed.Finally,the biological effects of GDM were inhibited and strengthened by boiling and dosing sludge,and the mechanism of GDM flux stabilization was studied.The mechanism of microbial activities to increase the operating flux of GDM was:on the one hand,microbial activities helped bio-cake layer form loose,rough,and heterogeneous structure,maintain the high permeability,and reduce the resistance of bio-cake layer.On the other hand,microbial activities reduced the pollutants in the bio-cake layer and alleviate the membrane fouling.This thesis explored the mechanism of GDM removal of pollutants,studied the mechanism of GDM membrane flux stabilization and proposed the use of GDM process to treat the actual secondary effluent.The results enrich the basic theoretical research of GDM,provide theoretical support and feasibility for the application of GDM in water treatment and reuse research and promote the popularization and application of GDM technology.