The first step in the treatment of sewage is to remove floating and suspended matter such as cloth, paper, kitchen refuse, pieces of wood, cork, hair, fiber, fecal solids etc by screening waste water.
The objective behind screening waste water is:
(i) To prevent clogging of sprinkler nozzles or the surface of trickling filters.
(ii) To protect pumping parts, siphons etc., from damage.
(iii) To improve the efficiency of the biological processes in the treatment of sewage, as the floating solids occupy excessive space which ultimately reduce the retention time for waste water.
(iv) To prevent floating matter in the receiving bodies of water.
This article takes a look at two vital factors that influence the quality of screening:
1) Geometry And Dimension
What is the geometry and dimension of the waste water treatment screen?
Screening waste water is accomplished by means of screens, having openings of uniform size, circular or rectangular in shape. The screening element is comprised of parallel bars, rods or wires, grating, wire mesh or perforated plate. When composed of parallel bars or rods, it is called a rack or bar screen and when made from wire mesh, perforated plate etc, it is called a screen. Screens may be further classified depending upon their sizes of openings as coarse, medium and fine. It is usual in sewage treatment to employ medium bar screens of opening 25 mm or more as a filter for waste water.
2) Screens And Sizes Of Openings
Screen Type = Racks
Class / Opening in mm
Coarse / More than 50;
Medium / 25 to 50;
Fine / Less than 25;
Medium / 6 to 9;
Fine / Less than 6;
In the bar screens, the racks or screens are constructed of flat iron bars set on edge across the channel through which sewage flows with a velocity of at least 0.45 m/sec. The bar screens that are used for screening wastewater, stop in the direction of flow, the angle with the horizontal being 30 to 60. This facilitates manual cleaning of screens by the upward stroke of the rake. Screens are allowed to drain off for some time on a perforated platform over the channel. Disposal may be through burial in trenches, incineration and disintegrating in shredders and returning to the sewage or passing to the sludge disposal plants.
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