Filtration precoat

Filter aids have been considered already in Chapter 5, and filtration process equipment will be discussed in Chapter 11. Precoat fihration is the application of a filter aid on a mainly conventional filter under clean conditions ie. in the absence of suspended solids 

Other than the fiber aid, to form a highly porous and uniform filter cake. The suqiension to be filtered is then introduced onto the filter. Clarification by filtration results by the action of the filter aid trapping the suspended solids within the filter aid cake, by the techniques described in Section 6.1. Only a thin layer of cake is usually considered to be inq ortant in this operation. If the filtration equipment is continuous by nature, e.g. a rotary vacuiuu filter, a thick filter aid cake may be enployed as the top layer contaminated with the material filtered during the clarification can be scraped off and discarded. The layers below the surface are then exposed for fiuther filtration and, therefore, economically viable rates of filtration ensue. Alternatively, if the filtration is conducted in a batch vessel thinner filter aid cakes are usually employed, with more frequent cleaning required. 

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Use of ultrafiltration (UF) membranes is becoming increasingly popular for clarification of apple juice. AH particulate matter and cloud is removed, but enzymes pass through the membrane as part of the clarified juice. Thus pasteurization before UF treatment to inactivate enzymes prevents haze formation from enzymatic activity. Retention of flavor volatiles is lower than that using a rack-and-frame press, but higher than that using rotary vacuum precoat-filtration (21). 

Precoat Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perhte, or other shave-able inert sohds. Some trial and error is involved in selecting a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetration. During this selection process, relatively thin precoat beds of I to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be... 

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

They are made by many companies, and they enjoy their widest use for filter-aid precoat filtration. 

Wang, L.K., Diatomaceous earth precoat filtration, in Advanced Physicochemical Treatment Processes, Wang, L.K., Hung, Y.T., and Shammas, N.K., Eds., Humana Press, Totowa, NJ, 2006. 

Refining. After saccharification, the hydrolyzate is clarified by precoat filtration, or possibly membrane filtration, to remove traces of insoluble fat, protein, and starch. Treatment with powdered carbon, granular carbon, and/or ion-exchange resins is then used to remove residual trace impurities, color, and inorganic constituents. The refined hydrolyzate can be dried to a solid product, evaporated to a high dextrose syrup, or processed to crystalline monohydrate or anhydrous dextrose. A typical process for production of crystalline dextrose is shown in Figure 2(7). 

Cleasby and Logsdon, Granular Bed and Precoat Filtration, in Letterman (ed.), Water Quality and Treatment A Handbook of Community Water Supplies, 5th ed, McGraw-Hill, 1999. 

PRECOAT FILTRATION - DIATOMACEOUS EARTH FILTRATION. The mOSt economical method of filtering fruit juices with a high suspended solids content is with precoat filters. In these, the filter sheet is created by precoating a liquid-permeable filtering element with the filter aid. In addition to diatomaceous earth, perlite and cellulose are also used as filter aids in fruit juice production. 

Filtration Equipment Good filtration equipment and proper operation are essential if the filtration process is to be effective and economical. Modem-day bleaching earths should not, except in rare cases, require the use of filter aids such as diatomaceous earth. Rather, the clay is used as the precoat. According to Veldkamp (123), the two main objectives in precoat filtration are as follows ... 

Precoat Filtration Applications, Advantages, and Disadvantages Summary... 

Glossary of Diatomaceous Earth Filtration (Precoat Filtration)... 

Diatomaceous earth filtration (or precoat filtration), accomplishes particle removal by physically straining the solids out of the water. The thickness of the initial layer of diatomaceous earth (DE) filter medium is normally about 1/8-in. (3 mm), and the water... 

Diatomaceous earth filtration, also known as precoat filtration or diatomite filtration, relies on a layer of diatomaceous earth about 0.3-cm (1/8-in.) thick placed on a septum or filter element. The septums may be placed in pressure vessels or operated under vacuum in open vessels. A schematic diagram of a typical pressure system is shown in Fig. 2. 

Any substances that can enhance the filtration efficiency are termed a filter aid. Diatomaceous earth is the most common filter aid for the precoat filtration system. An efficient, economical filter aid must (a) have rigid, intricately shaped, porous, individual particles (b) form a highly permeable, stable, incompressible filter cake (c) remove even the finest solids at high rates of flow and (d) he as chemically inert and essentially insoluble in the Uquid being filtered. Commercial diatomaceous earth, such as Celite diatomite, meets these requirements due to the wide variety of intricately shaped particles and inert composition that makes it practically insoluble in all hut a few liquids. 

Terminal turbidity breakthrough is not generally observed because precoat filtration is dominantly a surface filtration process. 

GLOSSARY OF DIATOMACEOUS EARTH FILTRATION (PRECOAT FILTRATION) (1,5)... 

AWWA, Precoat Filtration. Manual M30, American Water Works Association, Denver, CO, 1995. 

J. L. Cleasby and G. S. Logsdon, Granular bed and precoat filtration. In Water Quality and Treatment, Chapter 8, (Letterman, R. D. ed.), American Water Works Association, Denver, CO. McGraw-Hill Book Co., New York, NY, pp. 8.1-8.99, 1999. 

E. R. Baumann, Precoat filtration. In Water Treatment Plant Design, Chapter 15 (R. L. Sanks, ed.). Ann Arbor Science, Ann Arbor, MI, 1979, pp. 313-370. 

Both insoluble Mn02 and Fe(OH)3 are then removed by one or more solid-water separation processes, such as sedimentation (1,14,28), dissolved air flotation (16,22), filtration (23,26), membrane filtration (20,21,25,26), electroflotation (17), physicochemical sequencing batch reactor (18,19), or precoat filtration (29,46). 

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