Influence of released air on effective backwashing length in dead-end hollow fiber membrane system

Abstract

To obtain a better backwashing strategy, 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 on effects of released air from echoes. 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 and dissolution. The recombination process 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. The mathematical model predicted the volume of final remaining air that depended on the instantaneous pressure difference, backwashing flux, backwashing duration, membrane surface area, water viscosity and temperature. Experimental verification of ultrasonic waveforms was consistent with the model prediction. 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. These studies were expected to have implications for the design and operation of hollow fiber membrane backwashing.