According to the critical flux theory, membrane fouling does not occur at all as long as flux stays below the critical flux. In immersed membrane filtration, all membranes are supposed to run below critical flux in order to avoid fast membrane fouling. If the flux in a portion of membranes is above critical flux, membrane fouling occurs quickly as discussed here. To compensate the permeate loss from the fouled area, the flux in other areas must increase under constant flux mode. However, as the system size grows, it becomes trickier to maintain even flux rates in all membrane surfaces.
Fig. 1 shows an example with four membrane cassettes. Due to the pressure loss in the permeate pipe, effective TMP decreases as the distance of cassette from the pump increases. As a consequence, the flux of cassette 1 is the highest while that of cassette 4 is the lowest. Pressure loss mainly occurs in tees, joints, valves, and/or another connectors, but it can be significant even in straight pipe in poorly designed system.
Fig. 1. Effect of pressure loss in permeate pipe on effective trans-membrane pressure (TMP)
Other common cause of imbalanced permeate flow is the imbalanced scouring air flows among cassettes. It can also occur when pressure head losses differ among permeate pipes due to the similar reasons discussed here. One way of avoiding mal-distribution of permeate flow is using permeate flow meters in each module or cassettes. However, it is not always possible in full-scale system due to the budget constraint.
© Seong Hoon Yoon