Influence of selective permeation of backwashing solution on the cleaning effectiveness in hollow fiber system

The effectiveness of backwashing is strongly affected by selective permeation of backwashing solution along the fiber length in dead-end hollow fiber membrane system. This critical issue was systematically investigated with laser bijection sensing (LBS) technology to detect the position reached by 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. An instantaneous pressure jump of backwashing solution was measured in the first pressure sensor when the pump was opened on. The principle of pressure jumps in the fiber lumen and affecting factors were speculated with novel mathematical models. Pressure variation as well as modeling prediction showed that the pressure decrease inside fiber lumen was the fundamental reason for the ineffective backwashing for long membrane fibers. Inspired by the instantaneous pressure jump in backwashing process, an intermittent back-washing 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.