Chemical backwashing

The trend in the use of deep bed filters in water treatment is to eliminate conventional flocculators and sedimentation tanks, and to employ the filter as a flocculation reactor for direct filtration of low turbidity waters. The constraints of batch operation can be removed by using one of the available continuous filters which provide continuous backwashing of a portion of the medium. Such systems include moving bed filters, radial flow filters, or traveling backwash filters. Further development of continuous deep bed filters is likely. Besides clarification of Hquids, which is the most frequent use, deep bed filters can also be used to concentrate soflds into a much smaller volume of backwash, or even to wash the soflds by using a different Hquid for the backwash. Deep bed filtration has a much more limited use in the chemical industry than cake filtration (see Water, Industrial water treatment Water, Municipal WATERTREATiffiNT Water Water, pollution and Water, reuse). 

Separate ketdes and backwash towers are frequendy used to convert ion-exchange resins from one ionic form to another prior to packaging, and to cleanse the resin of chemicals used in the functionalization reactions. Excess water is removed from the resin prior to packaging by a vacuum drain. Both straight line filters and towers or columns are used for this purpose. 

A typical physical-chemical treatment system incorporates three "dual" medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Improper backwash. Blowoff of resin from the vessel during the backwash step can occur if too high a backwash flow rate is used. This flow rate is temperature dependent and must be regulated accordingly. Also, adequate time must be allotted for backwashing to insure a clean bed prior to chemical injection. 

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. 

The next stage is elution, in which the captured ions are displaced from their sites when a suitable chemical solution, called the eluant, is flushed through the column. With the completion of this operation, the resin sites are restored to their original state and the beads are stated to be regenerated. The new pregnant eluate is either stored or is administered a precipitating treatment the column is subjected to a further backwash to remove its residual eluant. It is then fit to go on stream once more. 

The chemical constituents of the backwash water (which may be acidified) and of the various eluants (recycling, new and special) are specific to the problem of displacing the captured ions from the resin and reactivating its sites. The final stage of IX, namely, the recovery in a solid form of the values from the eluting solution, is effected by chemical methods which yield the desired precipitate. This is settled out, filtered and dried for despatch. 

However, because both acids and bases are used in cleaning, it may be possible to store these chemicals in a stirred buffer tank to allow the solution to neutralise. This solution may then may be slowly added to the concentrate waste to dilute it and allow for a safe disposal (Mauguin and Corsin 2005). Note that the volume of this discharge is much lower than both the concentrate and filter backwash discharges. 

Pretreatment is required to remove suspended solids from the concentrate and to reduce the likelihood of precipitates forming in the injection zone. Solids may be present in the concentrate, especially when filter backwash is blended with the concentrate prior to injection. Pretreatment costs include filtration of the solids and chemicals for pH adjustment (Mickley 2006). 

In the early 1900 s, the rapid sand filter was introduced, together with chemical coagulation and periodic backwashing of the filter. Effective size of the rapid sand filter media might vary from 0.3S to 1.0 mm typical size is 0.5 mm. This type of filter has demonstrated the ability to remove effectively applied suspended solids of 5 to 10 ppm at flow rates of 2 to 3 gal/min per square foot of bed area. 

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... 

---