Backwashing membrane

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. 

A typical UF pilot plant has been used in this study. Examples of application for these membranes can be found in the literature [40, 58]. The UF unit woks in deadend mode (2.5 m h ) and it can be operated in filtration, backwash and chemically enhanced backwash (CEB) modes as described in the literature for similar UF systems [40]. The specifications of the hollow fiber UF modules and the operating conditions are summarized in Table 5. 

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

Air Bacfiflush A configuration unique to microfiltration feeds the process stream on the shell side of a capillary module with the permeate exiting the tube side. The device is run as an intermittent deadend filter. Every few minutes, the permeate side is pressurized with air. First displacing the liquid permeate, a blast of air pushed backward through the membrane pushes off the layer of accumulated solids. The membrane skin contacts the process stream, and while being backwashed, the air simultaneously expands the capillary and membrane pores slightly. This momentary expansion facilitates the removal of imbedded particles. 

Similar to filter backwash, the concentrate from these membranes requires treatment before it can be disposed of with the membrane concentrate. However, the total amount of solids produced after the treatment of filter backwash can be 60-80% greater than MF and UF concentrate due to the addition of coagulants prior to the granular media filters (Bergman 2007). 

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]. 

Nanofiltration membranes are used to remove hardness from drinking water [4,5]. They may also be used to remove other unwanted dissolved species, even the partial removal of nitrates from ground water. It was recently shown that RO and NF membranes may be backwashed by direct osmotic pressure to clean membrane surfaces, a simple and very beneficial technique [6, 7]. 

Ultrafiltration and microfiltration can be backwashed occasionally to remove accumulated solids from membranes. UF and MF membranes may be used to remove micrometer-sized and upper suspended particles, namely bacteria, algae, and so on, they can also be used to remove Guardia and Cryptosporidium, as well as most viruses found in surface water. In fact, the solid layer ( cake ) adhering to the membranes in the latter two techniques acts like a dynamic membrane [8, 9], removing smaller particles even at colloidal and virus levels. 

Sagiv, A. and Semiat, R. (2005) Backwash of RO spiral wound membranes. Desalination, 179, 1-9. 

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... 

Hollow fiber membrane modules can be backwashed to remove foulants whereas tubular and most spiral configurations cannot be backwashed. Backwashing of traditional spiral-wound modules would break the glue lines holding the membrane leaves together or cause blistering and delamination of the membrane from the backing in both spiral and tubular modules (TriSep Corporation has recently developed a back-washable, spiral-wound module (SpiraSep—US patent 6,755,970), that is used in immersed systems see below). 

Hollow Fiber Small footprint Low capital cost Can be backwashed Can easily be integrity tested High membrane area per unit volume (high packing density) Plugging of fiber (inside-out feed) Bridging of fiber bundle (outside-in feed) Difficult to clean... 

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