| Conventional drinking water treatment processes have faced several obstacles that are severely affected by water pollution and shortage,which makes it difficult to produce potable water effectively.Low-pressure membrane filtration,which includes ultrafiltration?UF?and microfiltration?MF?,is one of the most promising treatment technologies for improving water quality.Periodic hydraulic backwashing is a necessity for the routine operation of UF/MF membranes,while more attention has been directed toward membrane fouling and cleaning.In the current work,we critically investigate the influence of air,liquid and solid on the backwashing effect and its mechanism in the process of water treatment.Firstly,the selective permeation of backwashing solution was investigated in dead-end hollow fiber membrane system.Laser bijection sensing?LBS?technology was imported to detect the position of the cleaning agent inside the fiber lumen.The limited effective backwashing distance revealed by LBS could be attributed to the outflow flux of backwashing solution in axial direction,which dissipates the driving force?pressure?of the cleaning solution further to the longer distance along the hollow fibers.Inspired by the instantaneous pressure jump in backwashing process,an intermittent backwashing mode was employed and the experimental setup was also upgraded to provide a more accurate monitoring of the cleaning solution of long membrane fibers.The results showed that the intermittent backwashing mode outperformed the continuous backwashing mode by reaching a longer fiber length in a short washing period.Secondly,effects of released air on the effective backwashing length of dead-end hollow fiber membranes were investigated in this study.Ultrasonic spectrum and AGU-Vallen Wavelet software were imported to ensure a more intuitive understanding.Modeling and experimental results in this study showed that the air accumulation at the most distant membrane point was a recombination process which contained air release as the pressure type changed from inside-outside to outside-inside and air dissolution in the outside-inside pressure condition.Moreover,longer fiber was more prone to the air accumulation in the most distant membrane lumen.To overcome the effects brought by released air and maintain backwashing effectiveness,a novel method was developed by adding a segment of gas filtration membrane at the end of the hollow fiber which had finally extended the effective backwashing length.Thirdly,hysteresis phenomenon in backwashing process was experimentally investigated in this study with a dead-end hollow fiber membrane.The out-of-step changes in TMP and flux is just the real hysteresis phenomenon.Methods of visualization and ultrasonic spectrum analysis were implemented to better comprehend the hysteresis phenomenon.The membrane resistance and membrane fouling are two factors that cause the hysteresis phenomenon.Moreover,fouling level and fiber length were determined as two key factors that affect hysteresis phenomenon by the data and model derivation.Pressure evaluation index“?bw”is proposed to quantify the result of TMP vs time.The result indicated that once the fouling degree was known,the maximum TMP inside fiber lumen was also determined.More serious the fouling,the longer time and more energy were required for backwashing and there is a linear relationship between fouling level and“?bw”.Once the backwashing flow cannot provide an effective backwashing to the longer fiber,the fouling level will be the decisive factor affecting the hysteresis phenomenon.Generally,longer membranes do not have to deliberately delay the backwashing duration but just need to overcome the resistance of fouling layer. |
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