Fouling Amelioration - (6) Hybrid MBR (Mixed Liquor & Membrane)

Fouling Amelioration - (5) Hybrid modification (Membrane & Hydrodynamic)

Fouling Amelioration - (4) Hybrid MBR (Mixed Liquor & Hydrodynamic)

The approach of fouling limitation technique not always solely deal with individual characteristic of the factors that determine the fouling (membrane, hydrodynamic, and mixed liquor properties). In many cases the fouling limitation method are dealing with more that one of those factors. The techniques that exploiting the mixed liquor properties as well as hydrodynamic are further discussed in this section.

1) Flexible suspended carrier

Addition of flexible suspended carrier was reported by Q. Yang et al., (2006). This method was proposed to reduce polarization layer and cake resistance which are the main contributor of the filtration resistance in MBR. The role of the suspended carrier are to improve the characteristic of activated sludge by adsorbing the small biological flocs and colloidal matter. On the other hand the movement of suspended carrier scours the membrane surface and reduce the cake layer.

) Vibrating membrane + Coagulant [G. Genkin et al. ,2006]

Fouling Amelioration - (3) Mixed Liquor Properties

Fouling Amelioration - (2) Membrane properties

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.

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.

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

Fouling Autopsy - Foulant Analysis

Fluorescent in situ hybridization (FISH)

FISH (Fluorescent in situ hybridization) is a cytogenetic technique that can be used to detect and localize the presence or absence of specific DNA sequences on chromosomes. It uses fluorescent probes that bind to only those parts of the chromosome with which they show a high degree of sequence similarity. Fluorescence microscopy can be used to find out where the fluorescent probe bound to the chromosome. FISH is often used for finding specific features in DNA. These features can be used in genetic counseling, medicine, and species identification. In foulant analysis this method is used to detect Gram negative bacteria with phylogenetic probes monitoring of a-, b- and c-Proteobacteria, of the main ammonia-oxidizing bacteria
(AOB) and nitrite-oxidizing bacteria of the Nitrobacter and Nitrospira genera.

FISH is independent from the culturability of the bacteria and it is increasingly applied as it allows the evaluation of the presence and distribution of specific micro-organisms in activated sludge flocs. Despite the recent increase of FISH applications to activated sludge, the literature is mainly concerned with the investigation of strains with specific characteristics or with the biomass selected in the most common applications (CASP and domestic wastewater treatment).

The Fouling Amelioration

The General Method

Numerous method have been developed and reported in order to limit the membrane fouling. Some of the outstanding results are already patented or even applied in industrial MBR. The aproaches for fouling limitation is very broad from selecting the suitable biological parameters to applying the pulsation of electrical current to membrane surface. It is very difficult to make the general thought that may covel all approach that has been proposed and tested.

The methods for addressing the fouling in MBR is presented in Figure 1 and Table 1. Figure 1 shows the overall scheme in addresing the fouling. In this scheme all approach for fouling limitation is being tried to be covered. This scheme is developed in the basis understanding that the fouling is result of interaction between the mixed liquor and membrane to form the fouling layer. From this definition there are three different components that participated in fouling; mixed liquor, membrane and foulant layer it self. In order to limit the membrane fouling, these component mush be exploited to certain extent.

The general strategy and method of fouling limitation is identified by the approach to exploit these factors and devided in five cluster: membrane properties (1), hydrodynamic (2), mixed liquor properties (3), and the combination among them (4 and 5). The summary of the methods that has been reported is shown in Table 1. The further discussion is given in the next sub-section of each cluster.

Figure 1: The schematic diagram of fouling limitation.

Table 1: Summary of fouling limitation methods.