Fine Screens

Screens, and particularly fine screens, were among the first wastewater treatment devices. At the turn of this century fine screens were often installed where wastewater was discharged into large rivers or wide tidal estuaries to remove unsightly floating matter and increase the efficiency and economy of chlorination.

Since the SS removal efficiency of fine screens is only 10 to 20% compared to the 60% or more achieved by sedimentation, fine screening is no longer an acceptable alternative to sedimentation. However, some industries use fine screens successfully to remove solids from waste of processes for meat packing, canning (causes excessive scum or foaming in digesters), wool, and textiles. When possible, fine screens should be installed at the source of the industrial waste.

Instead of sand filters, some wastewater treatment plants use fine screens to remove fine suspended matter from treatment plant effluent when it is discharged into streams that are likely to reach recreational areas.

Fine screens are also used in front of biological treatment units. The required net area of submerged openings is commonly 2 sq ft per mgd (0.186 m2 per 3785 m3 per day or 20,300 m3 per day per m2) for domestic waste and 3 sq ft per mgd (0.280 m2 per 3785 m3 per day or 13,500 m3 per day per m2) for combined waste. Fine screens are also used preceding trickling filters to reduce clogging of the distributor nozzles.

Design and Cleaning of Fine Screens

Because of the small size of the openings, fine screen cleaning must be continuous. Cleaning can be accomplished by brushes or scrapers, water, steam, or air jets forced through the openings from the back side. The efficiency of a fine screen depends on the fineness of the openings and the velocity of sewage flow through the openings. The most commonly used screening media are as follows:

Slotted perforated plates with 1/32 to 3/32-in- (0.8- to 2.4-mm-) wide slots. Where brushing or scraping is used, plates are more practicable than wire screens. Wire mesh with approximately 1/8- in openings Woven wire cloth with openings usually less than ]/i3 in. One type is made of bars that are wedge shaped in cross section, with the large end of the bar in the face of the screen. The wedge-shaped bars are set in slots of a series of U-bars to maintain spacing and hold the bars in place. The standard openings range from 1/100 to 1/ 4 in, and the slots are continuous. Wedge-shaped wire. The wire is pressed into a wedge-shaped section built into flat panels that have openings varying from 0.005 to 3/16 in. The screens are made of corrosion-resistant material, preferably stainless steel.

Revolving-Drum Screen

In this screen, a stainless-steel no. 12 to no. 20 woven-wire mesh is applied to a cylindrical frame. The cylinder rotates around its axes while it is between one-third and two-thirds submerged. Revolving drum screens require a fairly constant water level; therefore, a weir plate usually maintains water elevation.

Revolving-Drum Screen with Outward Flow

With this screen, the waste flow can approach the drum from a direction parallel to the revolving axis. The liquid flows into the interior of the drum at one end, passes through the filter media, and flows out at a right angle to the axis (see Figure 7.14.5). The solids, which are retained on the inside surface of the screen, are raised above the liquid level as the drum slowly rotates and are usually removed by a water spray.

FIG. 7.14.5 Revolving-drum screen with outward flow. A. Plan view; B. Side view; C. End view.

Revolving-Drum Screen with Inward Flow

With this screen, the waste flow approaches the drum perpendicular to its axis. The liquid passes through the screen and flows out at one end. The solids, which are retained on the outside surface of the drum, are raised above the liquid level as the drum rotates and are removed by brushes; scrapers; or backwashing with water, air, or steam.

A disadvantage of removing screenings by water spray is that the removed solids are mixed with large amounts of spray water.

Revolving-Vertical-Disk Screens

In operating principle, this screen is similar to the revolving drum screen except that instead of a drum, a slowly revolving disk screen is placed in the approach channel completely blocking the flow so that it must pass through the screen (see Figure 7.14.6). As the liquid passes through the screen, solids are retained, elevated above the water level, and flushed by a water spray to a trough.

This screen is not suitable to remove larger objects or excessive amounts of suspended matter; neither is it suitable to handle greasy, gummy, or sticky solids. It requires a constant water level, which is usually secured by using a weir. The screening medium can be 2- to 60-mesh stainless steel wire cloth. Screenings are mixed with large amounts of spray water.

Inclined Revolving-Disk Screens

This screen consists of a round, flat plate revolving on an axle inclined 10° to 25° from the vertical (see Figure 7.14.7). The disk consists of several bronze plates containing slots, generally 1/16 to 1/32 in (1.6 to 0.8 mm) wide. The waste flows through the lower two-thirds of the plates.

FIG. 7.14.6 Revolving-vertical-disk screen. A. Front view; B. Side view.
FIG. 7.14.7 Inclined revolving-disk screen.

As the plate rotates, retained solids are brought above the liquid level where brushes remove them for disposal.

Traveling-Water Screen

This screen has had limited use in sewage treatment to remove solids from plant effluent. This screen uses a series of tilted or inclined overlapping screen trays mounted on two strands of steel chain. The head wheel is motor-driven, moving screen trays out of the sewage for solids removal by jets of water, then returning the trays into the wastewater flow.

Endless Band Screen

This screen operates in basically the same way as the revolving-vertical-disk screen. Instead of a vertical revolving disk, a screen in the form of an endless band is installed in the approach channel and the incoming flow is forced to pass through it. This endless screen moves slowly around top and bottom drums while half or more of it is submerged.

Vibrating Screens

Vibrating screens are used in the food packing industry to remove grease and meat particles, eliminate manure, recover animal hair, remove feathers from poultry processing, and remove vegetable and fruit particles from canning waste.

Vibrating screens are flat and covered by fine stainless steel cloth of 20- to 100- or even 200-mesh supported by rubber-covered bars or stronger stainless steel, coarse-wire mesh. Vibration reduces the blinding and clogging of the fine screens. A manual or automatic spray washer with steam or detergents can reduce blinding and clogging and facilitate handling of greasy or sticky material.

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