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Wastewater from food processing or postharvest processing may contain debris, either suspended or floating on the surface. These coarse solids have to be removed at the very beginning of the wastewater treatment regimen. Screening of debris is sometimes considered as a "preliminary" treatment,

Screening Wastewater Treatment
Figure 3.3. A photo of an inclined screen.

not part of primary treatment of wastewater; however, the distinction is more semantics than anything else. Screening can be effective in the food industry to quickly and cheaply reduce the amount of relatively large solids (0.7 mm or larger) in the wastewater. The simplest type of screen is an inclined flow-through type of static screen with openings of about 1.5 mm to 6 mm for fine screens and 6 mm or larger for coarse screens (Fig.3.3). (There are also very fine screens with openings of 0.2 to 1.5 mm placed after the coarse or fine screens, which can reduce suspended solids to levels near those achieved by primary clarification/sedimentation.) In some


Imagens Dum Raio Para Pintar

Figure 3.4. A schematic diagram of a vacuum drum screen.


Figure 3.4. A schematic diagram of a vacuum drum screen.

cases, some sort of scrapping mechanism has to be implemented to avoid clogging of the screens.

Rotary drum screens may also be used in removing coarse solids from food and agricultural wastewater. They are composed of a drum that rotates along its axis, and the effluent enters through an opening at one end. Screened wastewater exits from the other end. The retained solids are washed out from the screen into a collector in the upper section of the drum by a spray of the wastewater (Fig. 3.4).

The screening media used in screens are generally of stainless steel with opening ranging from 0.7 mm to 1.5 mm. Some of the screening elements may consist of parallel bars or rods (this type of screen is called a bar rack), wires, grating, wire mesh, or perforated plates. Bar racks are used to protect pumps, values, pipes, and other devices from damages and clogging by large objects in the wastewater stream. Some bar racks are equipped with a mechanical scrapping accessory to clean the screen.

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