Page 113 - Water and wastewater engineering

3-10 WATER AND WASTEWATER ENGINEERING

TABLE 3-5
Typical rack and screen characteristics
Type Remarks
Coarse screens
Bar racks Clear opening 2 to 8 cm between bars
Steel bars 1 to 2 cm in diameter
Inclined or vertical; if inclined, angle is 20 to 30 from vertical
Trash racks Clear opening 8 to 10 cm between bars
Steel bars 2 to 4 cm in diameter
Inclined or vertical; if inclined, angle is 20 to 30 from vertical
Fine screens Clear opening 0.5 to 1 cm between elements
Mechanically cleaned on either a continuous or intermittent schedule
Inclined or vertical; equipment supplier specifies angle
Provide head space or other means to raise the screen entirely
out of the water for inspection and maintenance
Adapted from Foellmi, 2005, and Kawamura, 2000.

Gates. Sluice gates may be used on either the interior or exterior of the tower. Historically, gate
valves have been preferred because the other valves become fouled with debris.

Coarse Screens. Also known as bar racks, these screens are provided to prevent leaves, sticks,
and other large pieces of debris from entering the tower. They are located as shown in Figure 3-3 .
Rack and screen characteristics are summarized in Table 3-5 . Bar rack screenings are taken to a
licensed landfill.
To maintain the desired inlet design velocity, the design of the port must take the area
occupied by the bars into account.

Example 3-2. Revise the area estimate made in Example 3-1 to take into account the area occu-
pied by a bar rack with a clear opening of 4 cm and a bar 1 cm in width. Assume that the opening
is a square.

Solution. The arrangement of a bar and opening is shown in the sketch below.

1 cm
4 cm

Individual
bars