Free drainage

There is no perfect method that can characterize sludge to predict membrane fouling propensity of the sludge. The methods described below have their own strengths and limitations. The correlation between the filterability test methods and actual membrane fouling rates must be checked case by case in order to find the best method correlated with membrane fouling in a given location.

1. Free drainage test

   Filterability of mixed liquor by membrane can be estimated by free drainage test as shown in Fig. 1. A filter paper, e.g. Whatman 42, can be folded into a funnel. After pour 50 ml mixed liquor to the funnel, filtrate volume is measured for a free determined time, e.g. 5 minutes, before the volume of filtrate is measured. The test conditions must be kept constant including the volume of mixed liquor, filtration time, etc. One downside is that maintaining surrounding temperature constant is tricky in this test unless room temperature is controlled.

   Although this method gives us an idea about the membrane fouling tendency by the mixed liquor used in the experiment, following limitations are apparent.

  • Unlike actual filtration in membrane tank, there is only very slight driving force to filter mixed liquor. Therefore cake compaction issue is much milder in the test than in the actual filtration.
  • No cross flow exists in the funnel unlike actual membrane filtration. Therefore, all particles and large enough macromolecules in mixed liquor affect the filterability by forming a cake layer. In crossflow filtration, large particles rarely form cake layer due to the back transport velocity from membrane surface in the crossflow condition as discussed here.
  • Particle sizes dictate the filterability by affecting the permeability of cake layer. In actual MBR, macromolecules such as SMP and EPS determine membrane fouling in general.
  • Filterability is affected by MLSS due to the lack of normalization against MLSS


2. Modified free drainage test (Thiemig, 2011)

   In order to eliminate or reduce the limitations of free drainage based filterability test, the mixed liquor poured in a Buchner funnel is mixed using a bench mixer with a flat blade as illustrated in Fig. 2. In addition, a larger amount of mixed liquor (500ml) is heated to 20 oC in water bath before it is poured into funnel in this method in order to reduce the temperature effect. Finally the time constant obtained is normalized by MLSS to reduce MLSS effect on filterability.   

   Experimental sequences are as follow

  1. Take a 1 Liter mixed liquor shortly before the test
  2. Insert a filter paper in the Buchner funnel (150 mm diameter, 0.6 micron pore size)
  3. Place an empty 250 ml mass cylinder under the funnel
  4. Clamp blade agitator at a height of 0.1 cm above the filter paper and mix at 40 rpm
  5. Mix the mixed liquor sample and temper a 500 ml sample in a water bath to 20oC
  6. Quickly pour the 500 ml sample into the funnel
  7. Activate a stopwatch when filtrate reaches 100 ml
  8. Stop the stopwatch when filtrate reaches 150 ml
  9. Measure MLSS as %
  10. Calculate Sludge Filterability Index (SFI) as follow

                                         SFI = delta_t (s) / MLSS (%)

materialsFig. 2. Setup and materials for measuring the sludge filtration index SFI (Thiemig, 2011)


© Seong Hoon Yoon