Backwashing importance

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. 

With some types of particles the porous metal tends to plug, but they can usually be backwashed or washed with a solvent or acid/alkali to remove the particles from within the metal pores. This is one reason why manufacturer s testing or plant testing can be important to the proper selection. Once the internal plug-gage has reached a point of reduction in flow-through capacity, it must be discarded. The actual cost of this type of cartridge is several times that of the non-metal-... 

Columns are designed to have a larger internal volume than the quantity of resin they will contain. The extra space is to provide the necessary volume for a fluidized bed during backwash. Most units are designed for the space above the resin bed (free-board) to be between 50 and 100% of the packed resin bed. Small columns are, on occasion, designed for one-use applications. Since backwashing is of no importance, there is a tendency to fill the unit with as much resin as possible. That practice can be hazardous, especially if the resin swells as a result of oxidative attack or through conversion from one ionic form to another. 

For filtration to be effective and efficient, the suspended solids must be able to penetrate the filter bed to a sufficient depth without clogging the filter, so the choice of filter media, effective grain size, and bed depth is important. Equally, good distribution of raw water, efficient collection of filtered water, and distribution of backwash water are other vitally important design factors. 

With some exceptions mentioned later, the first stage in the regeneration sequence of operations is backwashing with a controlled upflow of water to give typically 30-40% bed expansion. Backwashing a bed, where applicable, serves to effect the following important requirements ... 

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. 

An important requirement for the successful operation of a mixed bed is the careful separation of the strong base anion resin from the strong acid cation resin by backwash fluidization. This is followed by contact of each type of resin with its respective regenerant in a manner which minimizes the... 

The size and range (0.3-2 mm) of resin beads is an important consideration vis-a-vis performance. Uniform beads have been developed to produce stronger resins that resist attrition and ehminate cross-contamination of the resins during backwashing in mixed-bed systems. Uniform size is important since it imparts [10,15,16] ... 

The solvent which has been almost completely denuded of solute may be suitable for recycling directly back to the first extractor, for re-extraction of more solute. Alternatively, it may require treatment before re-use, e.g. an alkaline wash to remove impurities which have accumulated during its previous contact with acid aqueous phases. It is advisable for any treatment process to be continuous, as are the extraction and backwash processes, to preserve the continuity of operations. The cost of solvent can be an important item of process economics unless it is efficiently recovered. Systems are therefore chosen with low solubilities of solvent in the aqueous phases, and care should be taken to avoid entrainment of solvent, general spillages, etc. 

Hollow Fiber The hoUow-fiber configuration is the most common configuration for MF and UF membranes. The hollow fibers arc 0.5-1.5 mm (less than 5 mm) in diameter, and several thousands of hollow fibers are packed into a modtrle (Fig. 6.4). The most important merit of hollow-fiber membranes in water treatment applications is that extensive pretreatment is not needed as the membrane can be backwashed (back-flushed) automatically. Backwashing is carried out by changing the direction of flow of the permeate stream so that it flushes out the deposited particles that formed a cake layer on the membrane surface during the filtration cycle. 

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. 

Thus, if incomplete cake removal due to fouling decreases the effective area to 90%, this will increase local flux, Jetfeaive by 10% and increase R by 10%. The importance of this can be seen by inspection of Eq. (10.10) where the numerator is increased and the denominator is decreased. The net effect on cycle time would be more than a 20% reduction. The problem is exacerbated because a rise in local flux increases irreversible fouling making backwash even less effective. 

Backwashable cartridge filters have low solids-loading limits therefore, they have potentially short intervals between backwash cycles. It is important not to expose backwashable filters to differential pressures over approximately 170 kPa because the particles may become too deeply imbedded in the pores to be removed by backwashing. With proper maintenance and repeated backwashing, this type of filter may last up to 2 years. 

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