Pollutants in sewage generally exist in three forms: suspended (including floating) state, colloidal and dissolved state. The object of physical sewage treatment is mainly suspended solids and partial colloids that may block water pump impellers and pipeline valves and increase the load of subsequent treatment units. Therefore, physical sewage treatment is generally also called solid-liquid separation treatment of wastewater. From the principle, wastewater solid-liquid separation is mainly divided into two categories: one is that the wastewater is subjected to a certain restriction, and suspended solids are removed while flowing in the water; the other is that suspended solids are subjected to a certain restriction, and the wastewater flows to discard the suspended solids. The bar screen (grate) belongs to the latter. The bar screen is the most important auxiliary equipment in the sewage pumping station. The bar screen is generally composed of a group of parallel bars, obliquely placed at the inlet of the pump station collecting tank. [1]

By shape, bar screens can be divided into two types: plane and curved. The plane bar screen is composed of bars and a frame. The curved bar screen can be further divided into two types: fixed curved bar screen and rotating drum bar screen. By the clear spacing of the bars, they can be divided into three types: coarse bar screen (50-100 mm), medium bar screen (10-40 mm) and fine bar screen (1.5-10 mm). Both plane and curved bar screens can be made into coarse, medium and fine types. Because the bar screen is an important facility for physical treatment, newly designed sewage treatment plants generally use two bar screens of coarse and medium, or even three bar screens of coarse, medium and fine. By the slag cleaning method, bar screens can be divided into two types: manual slag cleaning and mechanical slag cleaning. Manual slag cleaning bar screens are suitable for small sewage treatment plants. When the slag amount is more than 0.2 m3/d, in order to improve the working and sanitary conditions of workers, mechanical slag cleaning bar screens should be used. [1]

A bar screen must be set before the sewage treatment system or before the water pump. The amount of suspended solids and floating matter (collectively referred to as screen slag) intercepted by the bar screen varies greatly depending on the selected bar spacing and the nature of the sewage. The clear spacing of the bar screen bars should meet the following requirements: before the sewage treatment system, 16-25 mm when mechanical removal is used, 25-40 mm when manual removal is used; before the pump, it should be determined according to the pump requirements. If the clear spacing of the bar screen before the pump is not more than 20 mm, a bar screen may not be set again before the sewage treatment system. The amount of screen slag is related to the clear spacing between bars: when the clear spacing is 16-25 mm, the slag amount is 0.10-0.05 m3/103 m3 sewage; when the clear spacing is 25-40 mm, the slag amount is 0.03-0.01 m3/103 m3 sewage. A working platform should be set behind the bar screen, generally 0.5 m higher than the highest water level upstream of the bar screen. For manually cleaned bar screens, the length of the working platform along the water flow direction is not less than 1.2 m; for mechanically cleaned bar screens, the length is not less than 1.5 m, and the width of the passages on both sides is not less than 0.7 m. [1]

The design content of the bar screen includes dimension calculation, hydraulic calculation, screen slag amount calculation and the selection of slag cleaning machinery, etc. Figure 1 is the schematic diagram of bar screen calculation. Among them, 1 is the bar and 2 is the operating platform.

Where: B - bar screen width, m;

s - bar width, m, generally take s = 0.01 m;

e - clear spacing between bars, m; coarse bar screen e = 50-100 mm, medium bar screen e = 10-40 mm, fine bar screen e = 1.5-10 mm;

n - number of gaps between bars;

Qmax - design maximum flow, m3/s;

alpha - bar screen installation inclination, degrees;

v - flow velocity through the screen, m/s, should be 0.6-1.0 m/s;

Where: h1 - head loss through the screen, m;

(2) Head loss through the screen

h0 - calculated head loss, m;

g - gravitational acceleration, 9.81 m/s2;

k - coefficient, the multiple by which the head loss increases after the bar screen is blocked by pollutants, generally k = 3;

xi - resistance coefficient, related to the cross-sectional shape of the bar.

When it is a rectangular cross-section, beta = 2.42. In order to avoid causing water surging before the screen, the bottom of the tank behind the screen is lowered by h1 as compensation, see Figure 1.

Where: H - total height of the bar screen tank, m;

h2 - freeboard of the channel before the screen, m, generally 0.3 m.

Where: L - total length of the bar screen tank, m;

H1 - height of the channel before the screen, m;

L1 - length of the gradually widening part of the inlet channel, m;

B1 - width of the inlet channel, m;

L2 - length of the gradually shrinking channel connecting the bar screen tank and the outlet channel.

(5) Daily screen slag amount calculation

Where: W - daily screen slag amount, m3/d;

W1 - screen slag amount per 10 m3 of sewage (m3/103 m3 sewage), take 0.1-0.01, coarse bar screen uses small value, fine bar screen uses large value, medium bar screen uses medium value;

Qmax - maximum design flow, m3/s;

K - total variation coefficient of sewage.