Screening is the first unit operation used at wastewater treatment plants. A screen is a device with openings for removing bigger suspended or floating matter in sewage which would otherwise damage equipment or interfere with satisfactory operation of treatment units. Screening removes objects such as rags, paper, plastics and metals to prevent damage and clogging of downstream equipment, piping and appurtenances. Some modern wastewater treatment plants use both coarse screens and fine screens.
The primary treatment incorporates unit operations for removal of floating and suspended solids from the wastewater. They are also referred as the physical unit operations. Screen is used to remove larger particles of floating and suspended matter by coarse screening. This is accomplished by a set of inclined parallel bars, fixed at certain distance apart in a channel. The screen can be of circular or rectangular opening. Industrial wastewater treatment plant may or may not need the screens. When packing of the product and cleaning of packing bottles/ containers is carried out, it is necessary to provide screens even for industrial wastewater treatment plant to separate labels, stopper, cardboard and other packing materials. The cross section of the screen chamber is always greater (about 200 to 300 %) than the incoming sewer. The length of this channel should be sufficiently long to prevent eddies around the screen.
Advantages
Manually cleaned screens require little or no equipment maintenance and provide a good alternative for smaller plants with few screenings. Mechanically cleaned screens tend to have lower labour costs than manually cleaned screens and offer the advantages of improved flow conditions and screening capture over manually cleaned screens.
Disadvantages
Manually cleaned screens require frequent raking to avoid clogging and high backwater levels that cause build-up of a solids mat on the screen. The increased raking frequency increases labour costs. Removal of this mat during cleaning may also cause flow surges that can reduce the solids-capture efficiency of downstream units. Mechanically cleaned screens are not subject to this problem, but they have high equipment maintenance costs.
Types of Screens
Screens can be broadly classified depending upon the opening size provided as coarse screen (bar screens) and fine screens. Based on the cleaning operation they are classified as manually cleaned screens or mechanically cleaned screens. Due to need of more and more compact treatment facilities many advancements in the screen design are coming up.
1) Coarse Screens
It is used primarily as a protective device and hence used as first treatment unit. Coarse screens also called racks, are usually bar screens, composed of vertical or inclined bars spaced at equal intervals across a channel through which sewage flows. Common type of these screens are bar racks (or bar screen), coarse woven-wire screens and comminutors. Bar screens are used ahead of the pumps and grit removal facility. Bar screens with relatively large openings of 75 to 150 mm are provided ahead of pumps, while those ahead of sedimentation tanks have smaller openings of 50 mm. Types of coarse screens include mechanically and manually cleaned bar screens, including trash racks.
Bar screens are usually hand cleaned and sometimes provided with mechanical devices. These cleaning devices are rakes which periodically sweep the entire screen removing the solids for further processing or disposal. Hand cleaned racks are set usually at an angle of 45° to the horizontal to increase the effective cleaning surface and also facilitate the raking operations. Mechanical cleaned racks are generally erected almost vertically. Such bar screens have openings 25% in excess of the cross section of the sewage channel.
Grinder or Comminutor
It is used in conjunction with coarse screens to grind or cut the screenings. They utilize cutting teeth (or shredding device) on a rotating or oscillating drum that passes through stationary combs (or disks). Object of large size are shredded when it will pass through the thin opening of size 0.6 to 1.0 cm. Provision of bye pass to this device should always be made.
2) Medium Screens
Medium screens have clear openings of 20 to 50 mm. Bar are usually 10 mm thick on the upstream side and taper slightly to the downstream side. The bars used for screens are rectangular in cross section usually about 10 x 50 mm, placed with larger dimension parallel to the flow.
3) Fine Screens
Fine screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth or very closely spaced bars with clear openings of less than 20 mm. Typical opening sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 in). Very fine screens with openings of 0.2 to 1.5 mm (0.01 to 0.06 in) placed after coarse or fine screens can reduce suspended solids to levels near those achieved by primary clarification. Fine screens are not normally suitable for sewage because of clogging possibilities.
Fine screens are also used to remove solids from primary effluent to reduce clogging problem of trickling filters. Various types of micro screens have been developed that are used to upgrade effluent quality from secondary treatment plant. Fine screen can be fixed or static wedge-wire type, drum type, step type and centrifugal screens. Fixed or static screens are permanently set in vertical, inclined or horizontal position and must be cleaned by rakes, teeth or brushes. Movable screens are cleaned continuously while in operation. Centrifugal screens utilize the rotating screens that separate effluent and solids are concentrated.
Velocity
The velocity of flow ahead of and through the screen varies and affects its operation. The lower the velocity through the screen, the greater is the amount of screenings that would be removed from sewage. However, the lower the velocity, the greater would be the amount of solids deposited in the channel. Hence, the design velocity should be such as to permit 100% removal of material of certain size without undue depositions. Velocities of 0.6 to 1.2 m/s through the open area for the peak flows have been used satisfactorily. Further, the velocity at low flows in the approach channel should not be less than 0.3 m/s to avoid deposition of solids.
Head loss
Head loss varies with the quantity and nature of screenings allowed to accumulate between cleanings. The head loss created by a clean screen may be calculated by considering the flow and the effective areas of screen openings, the latter being the sum of the vertical projections of the openings. The head loss through clean flat bar screens is calculated from the following formula.
where,
h = head loss in m
V = velocity through the screen in m/s
v = velocity before the screen in m/s
Another formula often used to determine the head loss through a bar rack is Kirschmer's equation.
where
h = head loss, m
K = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for rectangular bar with semicircle upstream, 1.79 for circular bar and 1.67 for rectangular bar with both upstream and downstream face as semi-circular).
W = maximum width of bar upstream of flow, m
b = minimum clear spacing between bars, m
hv = velocity head of flow approaching rack, m = v2/2g
θ = angle of inclination of rack with horizontal
The head loss through fine screen is given by
where,
h = head loss, m
Q = discharge, m3/s
C = coefficient of discharge (typical value 0.6)
A = effective submerged open area, m2
The quantity of screenings depends on the nature of the wastewater and the screen openings.
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