There are two different operational modes depending on which control parameter is kept constant out of flux and trans-membrane pressure (TMP).
In general vacuum pressure driven membranes run at constant flux mode while TMP is monitored to track the membrane fouling status. All membranes used in MBR with immersed membranes fall into this category. On the contrary, positive pressure driven membranes run at constant pressure mode, where flux (or permeate flow rate) is monitored to track membrane fouling status. Most membranes with tubular or plate and frame configurations fall into this category with few exceptions.
From the perspective of membrane fouling, there is an inherent benefit of running membrane system at a constant flux mode. As explained in the section for “critical flux concept”, cake layer formation is not only expedited at high flux, but also cake layer becomes more compact. Once cake layer is formed, it is not removed voluntarily. Therefore, avoiding excessively high flux in any given moment is very important for long-term operation, which constant flux mode can achieve easily.
In constant TMP mode with water circulation through membrane module, TMP is set to obtain enough initial flux that allows mid-term permeate requirement. Due to the high initial flux, particles in water are dragged toward membrane surface much faster than they are back transported, thereby more particles deposit per permeate volume than in constant flux modes. In addition, the deposit layer is compacted fast due to the high flux from the beginning of the filtration. If the particles are soft and compressible, cake layer compaction becomes more significant, which leads to a fast flux decline.
Fig. 1 compares constant flux, constant pressure, and modified constant pressure modes. In Fig. 1a., initial flux is kept low and maintained in the same level while TMP is allowed to go up. In Fig. 1b, initial TMP is kept high, which results in high initial flux compared to the constant flux mode. Flux decreases fast under this mode due to fast accumulation of cake layer. In Fig. 1c, modified constant pressure mode keeps initial TMP just enough to obtain target flux. If flux declines, TMP is raised slightly to obtain the target flux. Overall, this mode mimics constant flux mode.
Fig. 1. Conceptual diagram of three different modes of membrane operation
© Seong Hoon Yoon