Deep bed filters

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. 

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. 

Fig. 5. Example of a deep bed filter, ie, a downflow gravity 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). 

In the precoat and body feed mode, filter aids allow appHcation of surface filtration to clarification of Hquids, ie, filtration of very dilute suspensions of less than 0.1% by volume, such as those normally treated by deep bed filters or centrifugal clarifiers. Filter aids are used in this mode with pressure filters. A precoat is first formed by passing a suspension of the filter aid through the filter. This is followed by filtration of the feed Hquid, which may have the filter aid mixed with it as body feed in order to improve the permeabiUty of the resulting cake. The proportion of the filter aid to be added as body feed is of the same order as the amount of contaminant soHds in the feed Hquid this limits the appHcation of such systems to low concentrations. Recovery and regeneration of filter aids from the cakes normally is not practiced except in a few very large installations where it might become economical. 

Clarification negligible cake cartridges precoat drums filter aid systems sand deep bed filters... 

Some typical disturbance patterns and control difficulties are summarized here. A detailed discussion is made in (1). Hydraulic disturbances are significant in amplitude. Diurnal variations as well as shock loads from rain storms or melting snow may cause major upsets. Significant disturbances also appear from internal sources like primary pumps, back-washing of deep bed filters or return sludge flow rate changes. The amplitudes are such, that quasi-stationary of linear control methods are seldom adequate. 

The effectiveness of deep-bed filters in removing suspended particles is measured by die value of die filter coefficient which in turn is related to the capture efficiency of a single characteristic grain of the bed. Capture efficiencies are evaluated in the present paper for nil cases of practical importance in which London forces and convective-diffusion serve to transport particles to the surface of a spherical collector immersed in a creeping How field. Gravitational forces are considered in some cases, but the general results apply mainly to submicron or neutrally buoyant particles suspended in a viscous fluid such as water. Results obtained by linearly superimposing the in-... 

Jonsson, L. (1997). Experiences of nitrogen and phosphorus removal in deep-bed filters at Henriksdal Sewage Works in Stockholm. Water Science Technol, Proc. 1997 Int. Conf. on Upgrading of Water and Wastewater Syst, May 25-28, Kalmir, Sweden, 37, 9, 193-200, Elsevier Science Ltd., Exeter, England. 

As a first approach, a deep bed filter will be used for gas cleaning, with sand as the filter media. Later, a more advanced high temperature filtration method will be employed. ... 

In deep-bed filtration (Fig. ID), particles are caught inside the filter medium. Examples of deep-bed filters are granular beds and some cartridge filters. Deep-bed filtration is used for dilute suspensions (<100 ppm) containing fine particles that are not easy to be removed by sedimentation or cake filtration. 

Deep-bed filters are employed for slurries with very dilute concentration less than 1000 ppm (parts per million by weight). The deep-bed has pores in which the fine particles are caught. Capture of particles in deep-beds depends upon transport mechanisms that carry the particles to the surface of the medium. As the deposit builds up, the permeability ultimately drops to a point where the bed must be regenerated or discarded. The deep beds are in the form of granular media (sand, crushed anthracite coal, garnet, usually backwashable)... 

Dilute materials that result in high resistance cakes are generally best handled in deep-bed filters or mixed with filter aids. 

For particle diameter <2 cm and >5 pm and solid concenttation 1 to 50%, consider settlers, filters, or centrifuges. For particle diameter <300 pm and solid feed concentration 0.01 to 20%, consider thickeners. Section 16.11.5.9. For particle diameter >20 pm and solid feed concentration greater than 50%, consider dryers. Section 16.11.5.5. For particle diameter 0.01 to 150 pm, consider deep bed filter. Section 16.11.5.13, or dissolved air flotation. Section 16.11.5.15. For particle diameter 0.6 to 40 pm and solids concenttation <0.1%, consider homogeneous separation via ultraflltration, Section 16.11.4.22. For particle diameters from 0.8 to 20 pm, consider using a filter aid to precoat on the filter medium. For example, use diatomous earth or perlite. A fine filter... 

Stirred tank crystallizers, see Section 16.11.4.6 and reactors. Sections 16.11.6.23 through 16.11.6.25. Liquid fluidized bed reactors. Section 16.11.6.26 hquid adsorption, Section 16.11.4.12 ion exchange, Section 16.11.4.13 backwash fixed-bed operations such as deep-bed filters. Section 16.11.5.13 liquid adsorbers. Section 16.11.4.12 and ion exchangers. Section 16.11.4.13. 

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