Backwashing fouled membranes

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

As noted in Section 6.1.2, in most applications the control of CP, and fouling, dictates the use of crossflow. However, for dilute feeds and low-pressure membranes it has been accepted that batch cycles of deadend operation with solids accumulation removed by periodic backwash requires potentially lower energy. Usually, deadend is at FF and the TM P cycles from a minimum to maximum or over a specified cycle time during the batch. If fouling occurs it is evident through a steady rise in TM Pmin or Rm. Occasional chemical cleaning may restore Rm. 

Membrane-deaning strategies are numerous and generally remain proprietary information. Physical deaning by relaxation or backwashing is used on a frequent basis but the efficiency tends to decrease with filtration time. As irreversible fouling accumulates on the surface, chemical cleanings of various intensities (i.e., cleaner concentration used) can be applied on a weekly to yearly basis [20]. 

Advantages Minimal filtration pretreatment Easier membrane cleaning Higher surface area means more filtration area per fiber True cross flow velocity minimizes concentration polarization and membrane fouling When no air is utilized for backwashing the less fiber movement leads to breakage of fewer fibers Can be created with a variety of inside fiber diamters... 

Smith et al. [80] studies the effect of backwash in maintaining the flow in membrane systems and concluded that the performance of this procedure, during the process of membrane filtration, can effectively remove most of the compounds responsible for the reversible fouling of the membrane, reducing the pressure of work and increasing the permeate flux. 

Development of high-frequency backwashed UF and MF processes for combating membrane fouling using high integrity hollow fibre membranes. 

Membrane filters are Memcor CMF. Trans-membrane pressure (TMP) and flux are maintained by regular - every 30 min - air-scour backwash. Eventually, TMP increases over time (normally about 30-35 days) due to particulate and biological fouling. When the TMP reaches 1.5 bar, the membranes are cleaned with chemicals using solutions of sodium hypochlorite and/or citric acid sequentially. The membrane flux is 44 Imh and the MF filtrate has a SDI value of 3 to 4 and turbidity 0.1 to 0.2. 

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