Flux restoration backwash pressure

Pressure, Flux, Frequency, and Duration of Backwash Backwashing conditions such as pressure, flux, frequency, and duration, in practice, are usually obtained by trial and error. Kennedy et al. (1998) studied the backwash conditions in order to maximize the net flux per filtration cycle. They found that increasing the backwash to filtration pressure ratio (Ph/Pf) above 2.5 did not result in any significant increase in flux restoration within the range of backwash pressures tested (0.2—1.6 bars) (Fig. 6.15). Their conclusion that applying high backwash pressures (8 limes the filtration pressure) cannot restore irreversible flux decline was in agreement with membrane suppliers recommendations. 

Increasing the backwash duration from 0.5 to 1 min had almost negligible effect on flux restoration. However, when the backwash duration was increased to 2 min, a significant effect on flux restoration was observed for aU backwash pressures tested. Increasing the backwash pressure increases the water consumption, which reduces the system recovery. It is, therefore, important to optimize the backwash pressure and duration to achieve as high a degree of flux restoration and system recovery as possible. 

Net flux serves as a tool to determine the optimal backwashing conditions since it reflects the operation efficiency by taking into account water production and consumption, including the time spent for backwashing. Kennedy et al. (1998) observed that, for all backwash pressures and durations tested, the net flux increased up to a pressure ratio of 2.5, then decreased, even though flux restoration increased (Fig. 6.15, right). The reason is that above a pressure ratio of 2.5, the flux decline that was recovered by backwashing was small compared to the quantity of water consumed to realize the flux restoration. 

Figure 6.15 (a) Effect of backwash pressure on flux restoration and water consumption, and (b)... 

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