Effect Of Low Driving Pressure And Flushing On The Performance Of Gravity-Driven Ultrafiltration Membrane

The shortage of water resources is a global challenge for humanity,particularly in providing safe and adequate drinking water in remote rural areas of developing countries.Thus,research on water treatment technologies has become very critical.For decades,membrane treatment technologies have been vital in producing safe drinking water from contaminated surface water.In particular,gravity-driven membrane（GDM）ultrafiltration（UF）systems have received increasing attention and are also used in various applications due to the advantages of not requiring any backwashing,low energy consumption,and relatively simple maintenance.However,during the filtration process of GDM systems,some substances（natural organic matter,microorganisms and colloids,etc.）in the feed water inevitably accumulate on the membrane surface by adsorption,causing membrane pore blockage and filter cake layer formation,resulting in a dramatic decrease in flux.After a long-term operation,membrane fouling will reduce its effectiveness and sustainability,thus reducing its applicability in practical applications.Pressure is an essential element of the GDM filtration process,which affects the rate of biofilm formation as well as the quality of the effluent water due to different pressures,the reasons for which were not given and explained in detail in previous studies.In addition,the decrease in flux caused by membrane pollution is hard to recover due to the lack of control strategies（backwashing,chemical cleaning,etc.）.Regular flushing of GDM systems can effectively mitigate the flux degradation caused by concentration polarization while maintaining the excellent performance of the membranes themselves.This thesis first investigates the effect of low pressure（50 mbar）and high pressure（150mbar）on the performance of gravity-driven membrane（GDM）systems for purifying surface water.The results showed that the lesser driving pressure was conducive to alleviating membrane fouling in the GDM system.The effluent quality gradually improved throughout the operation,with a significantly higher removal efficiency of UV₂₅₄.Compared to groups（150mbar）,groups under a pressure of 50 mbar have higher normalized permeability,better effluent quality,and can form thinner biofilms.At low driving pressure（50 mbar）,the GDM system（PES）showed an 11.91%increase in UV₂₅₄removal compared to the corresponding high driving pressure,forming biofilms with lower potential for removal of disinfection by-products（DBPs）and humic substances.The distribution of bacterial communities forming biofilms differed between the two GDM systems at 50 and 150 mbar,which could be responsible for the decrease in humic substances and UV₂₅₄.Secondly,this paper investigates the effect of flushing on GDM process performance at a head pressure of 50 cm.By testing the water quality indicators such as UV₂₅₄,DBPs,EEM,and characterization of organisms in the water effluent,it was found that the periodic flushing system effectively removed contaminants from the water,and the thickness of biofilm was significantly reduced with better effluent quality.In particular,the UV₂₅₄removal rate increased considerably with GDM operation.This result is consistent with the disinfection by-product generation potential（DBPs-FP）.Therefore,periodic flushing can reduce biofilm resistance and increase the flux.In addition,from the fouling model,the fouling blockage mechanism is related to the structure of the membrane itself,independent of the flushing method.This paper focuses on optimizing GDM system operation and investigates the water purification efficiency under different pressure and cleaning methods,providing theoretical support for the practical application of GDM.