Backpulsing

R. Sondhi and R. Bhave, Role of Backpulsing in Fouling Minimization in Crossflow Filtration with Ceramic Membranes, 7. Membr. Sci. 186, 41 (2001). 

For the conditions in Fig. 9, improvements of 35-50% in the permeate flux were observed when a secondary membrane was used, owing to a reduction in the protein fouling of the primary membrane. Little or no flux recovery was observed with each backpulse, as might be expected from the relatively low resistance of the yeast layer and the irreversible nature of the protein fouling. The flux continuously declined with time owing to irreversible fouling, though the rate of decline was reduced by the SMY. 

In practice, air pressure or pumps can be used for generating backpulses. Air pressure may be preferred since pumps sometimes generate particles which can possibly plug membrane pores on the permeate side [Norton Co., 1984]. The backflushing system... 

Figure 5.12 Details of a backflushing system using piston backpulses [Lahiere and Goodboy, 1993]...

For the reason of possible blocking of the membrane pores on the permeate side, only permeate or other prefiltered cleaning agent should be used in the backpulses. 

Mores W.D. and Davis R.H., Direct observation of membrane cleaning via rapid backpulsing. Desalination 146 2002 135-140. 

Backflushing can remove the fouling layer both at the membrane surface and within the membrane. The forward filtration time and the duration of the backpulse need to be optimized since permeate is lost to the feed side during the backpulse. A schematic is shown in Figure 9.19. See Ref. [17] for additional details. 

Davis, R. H., Cross-flow microfiltration with backpulsing. Membrane Separations in Biotechnology, 2nd edn, W. K. Wang, ed. (New York Marcel-Dekker Pub. Co., 2000). 

Minimization of Flux Decline With Backpulse or Backwash... 

A number of operating parameters need to be studied to optimize the overall filtration performance. Critical among these are the cross-flow velocity, transmembrane pressure, pore diameter, or MWCO and concentration of the retained species at the end of a batch operation or steady state concentration in continuous filtration. This latter parameter can be related to the recovery of product in the permeate or retentate. Other important operating variables are temperature (and hence viscosity), pH, backpulse or backwash, and pretreatment. 

Membrane Fouling. Pretreatment of the membrane or feed solution prior to filtration may be desirable within allowable limits. The various treatment options are discussed in Sec. 6.3. At the start of a filtration run, the solute or solids concentration is relatively small and progressively builds as the permeate is removed from the system. If a substantial flux decline is observed at low solids concentration, membrane fouling aspects are believed to be important. A flux decrease with an increase in solids concentration is largely due to concentration polarization and can be minimized through efficient fluid hydrodynamics and/or backpulsing. PH221123)... 

Table 11 shows the performance of polymeric and ceramic filters for the separation and concentration of yeast and E-coli suspensions. The ceramic filters, due to their superior mechanical resistance, can be backpulsed to reduce flux decline during concentration. This is illustrated in Fig. 19 for the filtration of yeast suspension with 0.45 pm microporous cellulose triacetate membrane. Polymeric membranes can be backwashed at pressures up to about 3 bar. The data in Fig. 20 show the flux improvement with backpulsing using 0.2 pm microporous alumina membrane. ... 

Figure 20. Effect of backpulsing on flax stability with 0.2 mm microporous alumina membrane.

Backpulse This is achieved by rapid (typically lasting a fraction of a second) application of periodic counterpressure on the permeate side, typically with the help of an automatic time switch or a microprocessor, to push back a specific (as low as possible) permeate volume in the opposite direction. It is used in many CFF applications (especially with ceramic membranes) as an effective technique to disrupt, reduce or destroy the concentration-polarization boundary layer. Backpulsing also helps to minimize particle/gel infiltration into the microporous structure. Typical backpulse frequencies (cycle times) are in the range of 3 to 10 minute. ... 

Backwash This is similar to the backpulse technique but is less intensive in terms of the pressures applied across the membrane to dislodge particles/gels from the membrane surface. Backpulse is typically carried out at pressures exceeding 4 bar and often in the range 6 to 10 bar, whereas a backwash is carried out at lower pressures (e.g., 2 to 3 bar). 

Liquid Backpulse Solid membranes are backwashed by forcing permeate backward through the membrane. Frequent pulsing seems to be the key. 

Membrane Cleaning Using Chemical Agents and Backpulsing.369... 

Hydrophobic and hydrophilic membranes Control of operating parameters (critical flux) Rinsing water quality Cleaning agents and chemical cleaning Backpulsing... 

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