1. ENG421 (6abc) – Screens and Grit Removal
Screens and Bar Racks
Screening : design considerations
Screening : design example
Grit removal : design considerations
Grit removal : design example

2. General Water Treatment Technologies (Week 4)
Treatment technologies (unit operations and processes) used determined by what needs to be removed, inactivated or modified

3. Screens and Bar Racks (1 of 7)
Screening is first step in water treatment
Usually a simple device at surface water intake
Grades or separates particulate matter by size
Particulate matter may :
- interfere with proper working of pumps, pipes, or
mechanical equipment
- damage pumps, pipes, or mechanical equipment
Water intake points often located below surface of lake/river to minimise :
- intake of floating objects
- damage from ice
- shore vegetation

4. Screens and Bar Racks (2 of 7)
Suspended matter :
- sticks
- branches
- leaves
- plastics
- cloth
- paper
- dirt, soil, mud, fine particles causing turbidity
Removal via two steps :
1. large solids removed by screens (opening 0.8 – 100 mm)
coarse screens
medium screens
fine screens
2. separated suspended matter (called screenings)
disposed via :
comminution (grinding/blending)
incineration then landfill
composting for agricultural use

5. Screens and Bar Racks (3 of 7)
Types of screens
- Bar or rack screens (most common)
sets of parallel bars
predetermined spacing between bars
stationary or vibrating
may be self-cleaning

6. Screens and Bar Racks (4 of 7)
Types of screens (cont)
- Revolving screens (trommel screen)
revolving cylindrical frame surrounded by wire cloth
open at both ends

7. Screens and Bar Racks (5 of 7)
Types of screens (cont)
- Shaking screens
rectangular frame lined with wire cloth
may be used in conjunction with conveying system

8. Screens and Bar Racks (6 of 7)
Types of screens (cont)
- Vibrating screens
high capacity and high efficiency
mechanically or electrically powered

9. Screens and Bar Racks (7 of 7)
Types of screens (cont)
- Oscillating screens
low speed oscillation
often used with silk cloth

10. Screening Screens and racks strain out floating and suspended matter
material removed is larger in size than opening
Strainer
Rack : parallel bars or rods
Screen : wire mesh and perforated plates
coarse screens remove bulky material
fine screens remove smaller matter
leaves, twigs, fish
microscreens remove algae and plankton

11. Screening – design considerations (1 of 5)
factors to consider :
type of raw water
size of suspended and floating objects
location of intake entrance
location of screen
type of screen
area of screen
based on flow velocity and headloss during screening
cleaning arrangements
screenings (i.e. collected material) disposal

12. Screening – design considerations (2 of 5)
area of screen
flow velocity across screen
must avoid carry-through of retained material (aka wash off)
manually cleaned screens : max velocity m/s mechanically cleaned screens : max velocity 1.0 m/s
more reliable, more frequent, less build up of material
may be kept below 0.1 m/s to protect small fish
design flow rate : maximum hourly rate x 1.1
(1.1 factor allows for fluctuation and waste)
flow velocity in rack chamber
must avoid grit settling
0.4 – 0.75 m/s

13. Screening – design considerations (3 of 5)
area of screen
headloss across screen
function of open, blocked and submerged areas of screen
usually not significant compared with headloss across entire plant

14. Screening – design considerations (4 of 5)

15. Screening – design considerations (5 of 5)

16. Screening – design example (1 of 7)

17. Screening – design example (2 of 7)

18. Screening – design example (3 of 7)

19. Screening – design example (4 of 7)

20. Screening – design example (5 of 7)

21. Screening – design example (6 of 7)

22. Screening – design example (7 of 7)
Design Summary :
Typical values Design Value
Flow velocity in chamber (m/s) –
Flow velocity through the screen (m/s) –
Headloss (mm head) max

23. Grit Removal Grit includes : sand
heavy particulate objects, e.g. silt, gravel. Shells
Grit chambers
shallow
elongated basins (up to 20 m)
collects particles
> 0.2 mm diameter
specific gravity > 2.65
very low flow rate
enables high density particles to settle at bottom
also used in wastewater treatment

24. Grit Removal – design considerations (1 of 8)
factors to consider in grit chamber design :
size of grit to be collected
size of basin
shape of basin
number of basins
removal and disposal of grit
Located as close as possible to raw water intake
usually immediately after the screens

25. Grit Removal – design considerations (2 of 8)
Types of grit chambers :
- velocity controlled
uses horizontal flow pattern
low horizontal velocity gives sufficient time for particles to settle
increase theoretical design length by 50%
allows for inlet and outlet turbulences, and shock loads

26. Grit Removal – design considerations (3 of 8)
Types of grit chambers :
- velocity controlled (cont)

27. Grit Removal – design considerations (4 of 8)
Velocity Controlled Grit Removal – design data

28. Grit Removal – design considerations (5 of 8)
Velocity Controlled Grit Removal – ideal design

29. Grit Removal – design considerations (6 of 8)
Types of grit chambers (cont) :
- aerated
uses injection of compressed air to create turbulence
separates organic and inorganic solids
lighter organic material kept in suspension
heavier inorganic grit falls to bottom
due to rolling motion of water turbulence

30. Grit Removal – design considerations (7 of 8)
Aerated Grit Removal – design data

31. Grit Removal – design considerations (8 of 8)
Types of grit chambers (cont) :
- constant level short term sedimentation
circular and shallow
rapidly rotating scraping devices
grit and organics not separated
not common due to poor efficiency

32. Grit Removal – design example (1 of 4)

33. Grit Removal – design example (2 of 4)

34. Grit Removal – design example (3 of 4)

35. Grit Removal – design example (4 of 4)
Design Summary :
Typical values Design Value
cross flow velocity (m/s) –
surface loading rate (m/h) –
hydraulic retention time (min) –
total length to width ratio :1 – 8: :1
total length to depth ratio :1 – 10: :1

36. References
Droste, R.L., 1997, Theory and Practice of Water and Wastewater Treatment, John Wiley and Sons, New York (TD430D ), pages , 219 – 227
Hendricks, D., 2006, Water Treatment Unit Processes, CRC, New York (TD430H ) , pages , 201 – 231
Nemerow, N.L. et al, 2009, Environ Eng : Water, Wastewater, Soil and Ground, 6th ed., John Wiley and Sons, New York (TD430 .E ), pages
Qasim, S.R., 1999, Wastewater treatment plants : planning, design, and operation, Technomic, Lancaster, USA (TD746 .Q ), pages pages ,
Viessman, W. et al, 2009, Water Supply and Pollution Control, 8th ed., Pearson, Upper Saddle River, pages