RACKS AND SCREENS
Preliminary treatment of wastewater is the removal of debris and coarse biodegradables from wastewater with or without stabilizing wastewater by chemical addition.
The wastewater contains rags, grit, sticks, other large debris, and foreign objects. They make the operation process more complex and problematic.
Therefore, the method of removal of those objects before primary or subsequent treatment processes is known as the preliminary or pretreatment of wastewater.
Preliminary or pretreatment includes the following methods of treatment.
a) Screening and comminution
b) Grit removal
c) Pre-aeration
d) Equalization
e) pH control
f) Flotation
SCREENING DEVICES
The wastewater is fed into the sewer system either as gravity flow or by pumping into the treatment plant.
Screening is generally the first operation unit at wastewater treatment plants. These screening units include racks, coarse screens, and fine screens.
The purpose of using racks and screens in preliminary treatment is to remove large objects flowing with wastewater, for example, rags, plastics, paper, metals, dead animals, and the like. Removal of these large objects helps to protect pumps located downstream and prevent solids from fouling subsequent treatment facilities.
Racks and Screens
Coarse Screens
Coarse Screens are categorized as either bar racks (trash racks) or bar screens. The categorization depends on the spacing between the bars. Bar racks have a clear spacing of 5 to 10 cm, whereas bar screens typically have a clear spacing of 0.65 to 5 cm.
Both consist of a vertical arrangement of equally spaced parallel bars intended to trap coarse debris.
The debris captured on the bar screen depends on the bar spacing and the amount of debris caught on the screen.
Clear openings for manually cleaned screens between bars should be from 25 to 45 mm. Manually cleaned screens should be placed on a slope of 30-45 degrees to the horizontal, to help easy operations.
For manually, as well as for mechanically raked bar screens, the maximum velocities during peak flow periods should not exceed 0.76 m/s.
Hydraulic losses through bar racks are a function of approach (upstream) velocity, and the velocity through the bars (downstream), with a discharge coefficient.
Fine Screens
Fine screens are used more frequently in wastewater treatment plants for preliminary treatment or preliminary/primary treatment purposes.
Fine screens typically consist of wedge-wire, perforated plates, or closely spaced bars with openings 1.5 to 6.4 mm. Fine screens used for preliminary treatment are rotary or stationary-type units.
The clean water head loss through fine screens can be obtained from manufacturers’ rating tables or by computing using the common orifice equations.