Filterability of mixed liquor can be estimated using a stirred cell under positive pressure as shown in Fig. 1. Relative membrane fouling potential of mixed liquor can be estimated by measuring the time to obtain certain volume of permeate under a constant pressure condition. In the experiment performed by Tarnacki et al. (2005), membranes with 38 cm2 was used at 1 bar, 20 oC, and 400 rpm.
There are two ways of estimating the filterability of mixed liquor.
i) Filterability, L15 (LMH/bar), is estimated after 15 ml filtrate is obtained by dividing the flux by pressure as shown in equation (1). L15 cannot be normalized against wide range of pressure since flux is not proportional to the pressure due to the reason described here. Thus, it is the best practice to perform this filtration test at a fixed TMP every time it is performed, e.g. 0.5 bar.
ii) Fouling index, FI30, is calculated by dividing the flux at 30 minutes, J30, by initial water flux, Jw,0 (Eq.2). FI30 is affected by TMP for the same reason as L15. Therefore, FI30 should be measured at a fixed TMP every time the test is performed.
One drawback of this method is that cake layer compaction plays a bigger role than in actual MBR due to the high TMP. For better accuracy, TMP can be lowered to 0.1-0.2 bar that is more common For the same reason as L15, realistic TMP for MBR.
© Seong Hoon Yoon