Wednesday, October 29, 2008

Fouling amelioration - (1) Hydrodynamic

Exploiting the hydrodynamic is one of the mayor approach in order to achieve the higher operation permeability of the membrane. In most of the submerged membrane, air bubble scouring is always used to promote the hydraulic turbulence in the vicinity of the membrane. The bubble especially coarse bubble promote the scouring efect to the fouling layer at the membrane surface. How ever, concerning the energy for aeration and to prevent the damage of the over scouring that can break the floc of the sludge, the air scouring is only can be applied up to certain extent.

The role of bubbles is to provide direct shear, induce secondary flow of liquid and to move the membrane (in case of hollow fiber) [9 in CBB1]. This approach has several disadvantages [CBB1]:
  1. The shear forces experienced by the membranes are relatively weak so that only modes fluxes can be used.
  2. Increasing the bubble flow reaches a "plateau" in term of flux improvement and this make "turn up" to higher throughput difficult.
  3. It is difficult to achieve effective bubble distribution and a significant portion of the aeration energy will have little impact.

In these reasons, applying the new method in exploiting the hydrodynamic to improve the operational flux is necessary.

Many new methods had been applied in order improve the filtration performance of the membrane; novel membrane module, ultrasonic, addition of porous and suspended membrane carrier, external configuration, and advance aeration system.

Vibrating membrane
The method of moving the membrane as attempt to control the concentration polarization and fouling in membrane filtration has been proposed over last two decades; vibratory shear enhanced process (VSEP) for pressure driven membrane process, vibrating membrane in pervaporation, and rotating submerged membrane by Huber Co.

The application of the vibration submerged microfiltration hollow fiber membrane system was reported by Genkin et al., [2006]. The strategy to vibrate the membrane is by connecting the membrane cassette to rotated wheel with amplitudo of 4 cm as shown in Figure 1. By this method, it is possible to vibrate the membrane cassette up to 10 Hz. For further improvement of shear rate at the vicinnity of membrane, vanes was inserted next to the cassette. The system is tested using 5 g/l yiest solution using 4.1011 m-1. The system was evaluated in term of normalized critical flux improvement using 2-10 LMH flux step. 

Figure 1. Design of Vibrating membrane


Critical flux of up to 80 LMH are atainable in the vibrating frequency of 10Hz. The obtained flux is significantly higher than what can be achieved with either bubbling or crossflow (Typically 30-40 LMH). Further modification in addition of Vanes provide an approximately double enhancement of critical flux and enable achievement of critical flux of 100LMH at 8.3Hz and 130LMH at 10Hz. such high values of critical flux have not been achieved using traditional static membrane techniques and suggest that non-fouling operation is a possibility. The intellegent manipulation of hydrodynamic condition can produce dramatic improvement in critical flux.

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Figure 2. The Oscillation MBR

The another method in order to improve the hydrodynamic properties of the mixed liquor in the vicinity of the membrane is by membrane oscillation; cross oscillation and vertical oscillation [Figure 2]. When testing the membrane system by oscillation frequency of 0.5 Hz resulting laminar velocity of 0.2 m/s, TMP 3.41 Psi, in 1.8-2 g/l MLSS concentration, S. C. Low and his co-worket (2005) found that these approach can maintain the more than 76% of initial flux or even higher for vertical oscillation of 90% during the 5 hours of filtration test.

Local aerator
The smart design of module gives the possibility to localize the aeration. It creats the higher turbulence with less air and energy. Special aeration device was designed by F. I. Hay et al. (2008) to appropriately clean the membrane surface with effective utilization of air [Figure 3]. The air hole 1 mm from the thin pipe was attached on the surface of the module. Air introduced from the top end of the pipes hence effectively cleaned the membrane surface. This specific arrangement of the aeration device enabled efficient utilization of the applied air solely for cleaning purpose by minimizing escape of air bubbles to the sourounding media. With intermittent aeration (Intensity 2.5 l/min, duration 1 min on per 30 min) using this device with periodic chemical cleaning allowed stable operation with no increase in TMP for 2 months of operation.

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Figure 3. Local aerator for intensive fouling prevention F. I. Hay et al. (2008).