screening.pptx

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 2

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 3

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 4

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 5

6. TYPES OF SCREENS 6

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 7

8. COARSE SCREENS : BAR RACKS 8

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 9

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. 10

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. 11

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 12

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 13

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 14

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 15

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 16

17. HEADLOSS THROUGH THE SCREENS 17

18. HEADLOSS THROUGH THE SCREENS 18

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 19

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23. THANK YOU 23