Bar Screens in Sewage Treatment: Clear Spacing, Screen Slag Volume and the Design Checks That Matter

Pollutants in sewage generally appear in three forms: suspended, including floating; colloidal; and dissolved. Physical treatment targets the first two - the suspended solids and part of the colloids that would otherwise block pump impellers and pipeline valves and add load to everything downstream. That is why physical sewage treatment is usually called solid-liquid separation. Looked at by principle, the job splits two ways: sometimes the water is constrained and solids are removed as it flows, and sometimes the solids are constrained and the water is allowed to flow away from them. The bar screen, or grate, belongs to the second family. It is the most important auxiliary equipment in a sewage pumping station, built from a group of parallel bars set obliquely at the inlet of the pump station's collecting tank.

Classifying Bar Screens

By shape there are two types, plane and curved. A plane bar screen is composed of bars and a frame, while curved screens split further into fixed curved and rotating drum designs. By clear spacing between bars, three types are recognized: the coarse bar screen at 50-100 mm, the medium screen at 10-40 mm, and the fine screen at 1.5-10 mm. Both plane and curved screens can be made in any of those three spacings. Because the bar screen is such an important physical-treatment facility, newly designed plants generally install two screens - coarse and medium - or even three, coarse, medium and fine together.

By slag-cleaning method there are manual and mechanical designs. Manual cleaning suits small plants. Once the slag amount rises above 0.2 m3/d, a mechanically cleaned screen should be used, simply to improve the working and sanitary conditions of the staff.

Placement and Clear Spacing

A bar screen must be set before the sewage treatment system or before the pump. The amount of suspended solids and floating matter it intercepts - collectively screen slag - varies widely with the chosen bar spacing and the nature of the sewage, so spacing requirements are worth stating exactly. Before the sewage treatment system, the clear spacing should be 16-25 mm where mechanical removal is used and 25-40 mm where removal is manual. Before the pump, spacing is determined by the pump requirements. If the clear spacing of the screen before the pump is not more than 20 mm, a second screen before the treatment system may be omitted.

The quantity of screen slag tracks the clear spacing. At 16-25 mm the slag amount is 0.10-0.05 m3/103 m3 of sewage; at 25-40 mm it is 0.03-0.01 m3/103 m3. A working platform is then set behind the screen, generally 0.5 m above the highest water level upstream of the screen. For manually cleaned screens the platform length along the flow direction is not less than 1.2 m; for mechanically cleaned screens it is not less than 1.5 m, and the passages on both sides should be at least 0.7 m wide.

The Design Checks

The design content of a bar screen covers dimension calculation, hydraulic calculation, screen slag amount calculation and the selection of slag-cleaning machinery. A handful of variables do most of the work:

• Bar screen width depends on bar width, clear spacing and the number of gaps between bars. Bar width s is generally taken as 0.01 m; coarse screens use e = 50-100 mm, medium screens e = 10-40 mm and fine screens e = 1.5-10 mm.
• Flow velocity through the screen should be 0.6-1.0 m/s. Too slow and solids settle out in the channel; too fast and captured material is driven through.
• Head loss through the screen combines a calculated head loss with the gravitational constant of 9.81 m/s2 and a blockage coefficient k, generally 3, representing how much the loss grows once the screen fouls. The resistance coefficient depends on the cross-sectional shape of the bars; for a rectangular section it is taken as 2.42.
• Tank geometry. To avoid water surging before the screen, the bottom of the tank behind it is lowered by the head loss as compensation. Total tank height adds the upstream channel height and a freeboard, generally 0.3 m; total tank length combines the upstream channel height with the lengths of the gradually widening inlet section, the inlet channel width and the gradually shrinking section joining the screen tank to the outlet channel.
• Daily screen slag amount is calculated from the slag produced per 10 m3 of sewage, taken as 0.1-0.01, with coarse screens using the small value, fine screens the large value and medium screens the middle value, together with the maximum design flow and the total variation coefficient of the sewage.

Why the Numbers Matter

Bar screens look like the simplest item in a treatment plant, and in a sense they are. But the spacing decision propagates: it sets how much slag the plant must handle, how long the platform has to be, and whether a second screen is needed at all. Getting the clear spacing, velocity and head-loss allowance right at the design stage is far cheaper than discovering, after commissioning, that the screen either passes what it should catch or surcharges the channel in front of it.