Backwashed

Rather than use a cloth, a granular medium consisting of layers of particulate solids on a support grid can be used. Downward fiow of the mixture causes the solid particles to be captured within the medium. Such deep-bed filters are used to remove small quantities of solids from large quantities of liquids. To release the solid particles captured within the bed, the flow is periodically reversed, causing the bed to expand and release the particles which have been captured. Around 3 percent of the throughput is needed for this backwashing. 

Procedure for filtering through a filtering crucible. The trap is used to prevent water from a water aspirator from backwashing into the suction flask. 

Deep Bed Filters. Deep bed filtration is fundamentally different from cake filtration both in principle and appHcation. The filter medium (Fig. 4) is a deep bed with pore size much greater than the particles it is meant to remove. No cake should form on the face of the medium. Particles penetrate into the medium where they separate due to gravity settling, diffusion, and inertial forces attachment to the medium is due to molecular and electrostatic forces. Sand is the most common medium and multimedia filters also use garnet and anthracite. The filtration process is cycHc, ie, when the bed is full of sohds and the pressure drop across the bed is excessive, the flow is intermpted and solids are backwashed from the bed, sometimes aided by air scouring or wash jets. 

To keep the frequency of backwash and the washwater demand down, and to prevent undesirable cake formation on the filter surface, deep bed filtration is appHed to very dilute suspensions of solids concentrations less than 0.1% by volume. 

Stratification of the particles making up the bed, caused by the fluidization (fines on top), is not desirable. The soflds holding capacity of the bed is best utilized if the filtration flow encounters progressively finer sand particles. This is achieved in upflow filters where the fluidization due to backwash produces the correct stratification in the bed. Unfortunately, the filtration flow and the backwash take place in the same direction the disadvantage is that the washwater goes to the clean side of the filter. 

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). 

Total submergence is used in the vacuum disk filter thickener (Eig. 13) in which the cake discharge, by backwashing with filtrate, occurs as each sector passes through the lowest point of the slurry tank. 

The pressure filter with tubular elements has also been used as a thickener, when the cake, backwashed by intermittent reverse flow, is redispersed by an agitator at the bottom of the vessel and discharged continuously as a slurry. In some cases the filter cake builds up to a critical thickness and then falls away without bio whack. 

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 distributor is frequently installed at the top of the column for use during backwash. It collects water evenly and prevents resin from escaping the column should unexpected surges develop in the water flow during backwash. Columns lacking an upper distributor or screen to prevent loss of resin should have an external system to prevent resin from being lost to the drain. It is referred to as a resin trap and may consist of a porous bag that fits over the outlet pipe or a tank designed to lower the linear velocity. Resin drops to the bottom of the tank and is returned to the column when convenient. 

Resias are seldom used oace and discarded. Whether the system is mn batchwise or ia columns, the resia must be periodically removed from service and regenerated. An exception is the use of a resia as a catalyst ia organic reactions. Each cycle consists of two principal steps, adsorption and regeneration, and one or more iatermediate steps, tinse and backwash. Eailure to use good practices results ia poor cycHc performance. 

At times, the process stream flow must be increased after the initial installation to satisfy production demands. Depending on the magnitude of the increase, the existing system may or may not be able to handle the added flow. If it can, regeneration frequency must increase. Adding more resin to the column is often considered an alternative to installing another column. Resin addition lessens the space for backwash and may be a cause for poor column performance. 

As manufactured, most resias have a Gaussian-Hke distributioa of particle size. Very few are as small as 0.3 mm or as large as 1.0 mm. Most are betweea 0.5—0.8 mm. A backwash before usiag aew resia is common practice to assure uniform flow during the adsorption and regeneration steps. The backwash eliminates air pockets that may have formed while filling the column and sorts the beads such that the smaller sizes are at the top of the bed and... 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

GAC may be used in fixed or moving beds and in downflow or upflow mode. Eixed beds are operated in downflow mode and as such, provide some amount of soflds filtration however, influent soflds concentration must be kept low (less than 5 mg/L suspended soflds) to prevent rapid plugging of the bed. Entered soflds are periodically removed by backwashing. Upflow beds are more tolerant of soflds because they are fluidized and expanded by the wastewater entering at the bottom. In moving beds, the flow is countercurrent and makeup, fresh carbon is added continuously at the top of the unit while an equal amount of spent carbon is removed from the bottom. 

Filtration is employed when the suspended soUds concentration is less than 100 mg/L and high effluent clarity is required. Finely dispersed suspended soUds require the addition of a coagulant prior to filtration. Filters most commonly used in wastewater treatment are a dual media (anthrafUt and sand) or a moving bed or continuous-backwash sand filter. Performance data for the tertiary filtration of municipal and industrial wastewater are shown in Table 10. 

In diatomaceous-earth filtration, the powdered filter aid is built upon a relatively loose septum to screen out suspended soHds. The filter becomes clogged, and pressure losses become excessive backwashing is then necessary. The smallest removable particle is 0.5—1 p.m (see Diatomite). 

Ion Exchange. Ion-exchange treatment (215,216) entails elution of wastewater through a suitable resin until the available sites for ion exchange become fully occupied and the contaminated ions appear in the outflow. Treatment is then stopped. The bed is backwashed and then regenerated using an... 

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