Fouling backwashing

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. 

During backwash the larger, denser panicles will accumulate at the base and the particle size will decrease moving up the column. This distribution yields a good hydraulic flow pattern and resistance to fouling by suspended solids. 

If the unit becomes badly fouled with suspended matter (for example, after a pipe brake has introduced excessive suspended matter into the system) it must be taken out of service and cleaned. This is done with an extended backwash, possibly at higher flow rates. If this does not remove the dirt, the manhole should be opened and the resin agitated with an air lance. Non-ionic detergents can be used, but not at the same time as the air lance or the resulting froth will be impossible to control. 

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]. 

Vortex filters supplied by Sonitec, Inc., ("Vortisand " filters— Vortisand is a registered trademark of Sonitec, Inc., Holyoke, MA) come complete with a chemical cleaning system. This system is used every 3-8 backwash sequences to reduce filter media fouling... 

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... 

The presence of complexing agents may entail further cleaning Fouling phenomena Backwash frequency can be quite high depends on solid content of waste stream... 

At the present time, Cummins has decided to install a pretreatment system using dissolved air flotation as the primary treatment followed by an anthracite/sand backwashable filter and an activated carbon polishing filter. The effluent from the filters then goes to the RO unit. The RO will remove the last traces of the most soluble oils and organics, it is Cummins intention to have the effluent from the filters at a turbidity of less than kS JTU s. This should keep the RO units from fouling and will allow the RO to be used with minimal cleaning. 

The MF air backwash system did not induce major flux improvement as shown in Figure A1.23. However, a part of the fouling layer would have been removed since the rejection of DOC and some solids decreased after the backwash,... 

The design and operation of the custom-built rapid mixer are described elsewhere (3,4). An effective rapid-mix step rendered a flocculation step unnecessary and enabled the use of Fe(III) doses as high as 20.6 mg/L (60 mg/L as FeClj) without any fouling. Moreover, backwash intervals as high as 29 min were... 

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. 

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