In MBR, membrane deals with the activated sludge with various components. Membrane fouling inevitably occurs due to the interactions with the complex feed during the filtration. The course of membrane fouling under constant flux modes in immersed membrane filtration can be split by three stages (Cho, 2002; Zhang, 2006):
(i) Stage 1 – Initial adsorption
An initial short term rise of TMP occurs due to the adsorption of macromolecules on membrane surface, e.g. soluble microbial products (SMP) and extra-cellular polymeric substances (EPS). Since the adsorption is primarily driven by chemical and physical interaction between membrane and macromolecules, the initial membrane fouling occurs even at zero flux. Adsorptive fouling might be less with membranes with more hydrophilicity since the bound water molecules on membrane surface prevent macromolecules from directly contacting membrane surface.
In the example shown in Fig. 1, the initial TMP is around 5 kPa while the flux was 25.7 LMH, which can be translated to a permeability of ~5 LMH/kPa. Since the initial permeability of the membrane in clean water was ~15 LMH/kPa, permeability loss in the very beginning of the filtration is around 67%.
(ii) Stage 2 – Slow TMP rise
A long-term rise of TMP occurs mainly due to the continuous deposition of EPS and SMP. In addition, the gradual compaction of the deposit layer also contributes to the slow TMP rise. Since EPS and SMP have very low critical flux, they continuously deposit on membrane surface even at very low flux. The TMP rise can be either linear or weakly exponential. In Fig. 1, TMP increasing rate in this stage is calculated at 0.2 kPa/hr.
(iii) Stage 3 – Sudden TMP rise (or jump)
A sudden rise of TMP occurs, which is also known as the TMP jump. This characteristic TMP profile can hardly be ascribed to sudden changes in operational parameters such as feed properties, rapid cell lyses, and other environmental changes since it also occurs in lab-scale experiments of which operational parameters are rigorously controlled at a constant level.
The sudden TMP rise can be best explained by cake layer compaction (Chang, 2006; Park, 2006; Fane, 2007). Once pressure loss through the cake layer reaches a critical level, cake layer compaction starts from the bottom of the cake layer due to the cumulative nature of the downward force in the cake layer as explained here. As cake layer compaction proceeds, TMP must rise to compensate the permeability loss under constant flux mode, which in turn accelerates cake layer compaction. As a result, TMP rises exponentially as shown in Fig. 1.
All other theories explaining the sudden TMP rise are discussed here.
Fig. 1. Typical TMP rising pattern at 15 LMH in immersed membrane filtration with Yuasa’s flat sheet immersed membrane. MLSS = 12 g/L, 20 oC (unpublished data, Yoon).
© Seong Hoon Yoon