screening in wastewater treatment

1. SCREENING
Presenter:
Kiran Shrestha,
MSEnE010
Tutor:
Assoc. Prof. Shukra Raj Paudel
Department of Civil Engineering
IOE, Tribhuvan University
MSc in Environmental
Engineering
2024-01-21

2. Objectives of the Presentation
We will be able to learn the following things at the end of the
presentation.
 Process of screening
 Types of screens
 Head loss through screens
 Disposal of screening
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3. Presentation Outline
 Screening definition
 Features of screening
 Types of screening
 Location and housing of screens
 Hydraulics of screens
 Velocity at screens
 Headloss in screen
 Disposal of screening
 Design of screen
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4. SCREENING
• Screening is the first unit operation
in a wastewater treatment plant
• A screen is a device with the
openings, generally of uniform size,
used to retain suspended or floating
coarse materials (piece of cloths,
wood, leaves, plastics, rags, papers)
found in the influent wastewater
• These materials must be removed at
the beginning of treatment process,
otherwise could damage subsequent
process equipment eg pumps, valves
etc
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5. FEATURES OF SCREENS
• The screening element may consist
any of the following
 Parallel bars or rods,
 gratings,
 wire meshes,
 perforated plates
• The openings are usually
rectangular or circular shape
however can be of any shape
• Depending upon size of opening ,
the screens may be coarse, medium
or fine
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6. TYPES OF SCREENS
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7. COARSE SCREENS
• Coarse screens , usually bar screens
or bar racks and sometimes used in
conjuction with communiting
devices have clear openings ranging
from 6 – 150mm.
• They serve more as protective
devices
• A bar screen is composed of vertical
or inclined bars spaced at equal
intervals with relatively large
openings of approx. 15 to 40mm
across the channel through which
wastewater flows
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8. COARSE SCREENS : BAR RACKS
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9. COARSE SCREEN : CLEANING
• Bar screens are usually hand cleaned and
sometimes provided with mechanical
devices
• These cleaning devices are rakes which
periodically sweep the entire screen
removing the solids for further
processing and disposal . Some
mechanical cleaners utilize chains/cables
to remove the rakes through screen
opening. Screenings are raked to the
platform with perforations which permits
water back to system
• Mechanically cleaned racks are
generally erected vertically
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10. MEDIUM SCREENS
• Medium bar screen have clear opening of about 12mm
• These mechanically raked units are used before all the pumps or
treatment units. The bars used for screening are rectangular in cross
section usually about 10mm x 50mm are placed with larger
dimension parallel to the flow. A weir on the side of screen may be
used as overflow bypass.
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11. FINE SCREENS
• Fine screens are mechanically
cleaned devices using perforated
plates, woven wire cloth or closely
spaced bars with clear opening
typically less than 6mm(commonly
available in the opening size ranging
from 0.0035 to 6mm
• Fine screens are generally used for
the pre treatment of industrial wastes
to remove materials which tend to
produce excessive scum or foam.
• Fine screens are not normally suitable
for sewage because of the clogging
possibilities
• Fine screens may be of the drum type
or disc type mechanically cleaned and
continuously operated.
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12. COMMINUTING DEVICES(GRINDER)
• Comminuting device is a
mechanically cleaned screen that
shred the coarse solid materials and
return these materials into the
wastewater flow
• These devices cut the retained solids,
thus enabling them to pas along the
sewage. The solids from the
comminuting devices may lead to
more scums in the digester
• They are recommended for smaller
sized sewage treatment plants of up to
1MLD
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13. LOCATION AND HOUSING OF SCREENS
• Screening devices are usually located where they are readily
accessible as it requires frequent inspection and maintenance. Where
screens are placed in the deep pits or channels , it is necessary to
provide sufficiently wide approaches from the top and ample
working space for easy access and maintenance.
• The screen chamber to house the screening equipment depends on
the type of equipment and the climatic conditions. Screen house
(chamber) can be omitted for the hand cleaned screens if the climate
are not severe
• Mechanically cleaned screens generally need suitable housing to
protect and ensure proper operation. Ventilation of the house is
necessary to prevent moisture accumulation and corrosive
atmosphere
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14. HYDRAULICS OF SCREEN
• Screens are provided to remove the materials which would
impede the flow in treatment plant. hence continuous
cleaning arrangement can keep the interference of solid
materials to minimum
• On other hand periodic cleaning arrangement may cause
surges of high flow after cleaning
• Usually the base of the screen is placed a few centimetres
below the invert of the base channel and the grade of the
influent conduit is steepened immediately preceding the screen
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15. VELOCITY THROUGH THE SCREENS
• The velocity through the screen should be such that maximum
amount of screenings are retained without deposition
• Velocities of 0.6m/s to 1.2m/s through the open area for the peak
flows have been used satisfactorily. The velocity should not be less
than 0.3m/s to prevent the deposition of solids
• A velocity of 0.8m/s is appropriate for the considerable amounts of
storm water while preventing grit decomposition at the bottom of
the screen
• A straight channel succeeding the screen assures good velocity
distribution across the screen and maximum effectiveness of the
device
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16. HEADLOSS THROUGH THE SCREENS
• Headloss depends on the quantity and nature of screening
accumulated
• Headloss can be calculated using the following formula
h=0.0729(V^2 – v^2 )
Where, h is the headloss in meters
V is the velocity through the screens in m/s
v is the velocity before the screen in m/s
• The value of h is usually 0.15m and should not exceed 03m for the
clogged hand screen
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17. HEADLOSS THROUGH THE SCREENS
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18. HEADLOSS THROUGH THE SCREENS
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19. DISPOSAL OF SCREENING
• The quantity of screening varies depending on the type of screen as
well as sewer system type and its characteristics. Quantity of
screening removed by bar screen mostly ranges between 0.0035m3
to 0.0375m3 / 1000m3 of wastewater treated with an approximate
typical value of 0.015m3/1000m3 of wastewater.
• Screening is usually disposed off along with the municipal waste on
the sanitary landfill . It can also be brought back to the wastewater
after passing through grinders or disintegrator . Other option
includes incinerating or burning at the plant site
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