1. Screening Jae K. (Jim) Park, Professor
Dept. of Civil and Environmental Engineering
University of Wisconsin-Madison

2. Screening
Objective
To remove large objects such as rags, paper, plastics, metals, and the like. These objects, if not removed, may damage the pumping and sludge removal equipment, hang-over weirs, and block valves, nozzles, channels, pipelines, and appurtenances
First unit operation used at wastewater treatment plants.
Type of screens
Coarse screens: used primarily as protective devices, e.g., bar racks (or screens), coarse woven-wire screens, and comminutors
Fine screens: openings of < 2.3~6 mm (< 0.1~0.25 inch); used to provide pretreatment or primary treatment

3. Coarse Screens

4. Coarse Screens
Coarse Screen
Manual screening removal
Too wide

5. Coarse Screens
Coarse Screen

6. Coarse Screens

7. Screenings

8. Screen

9. Screen

10. Coarse Screens Type Location Description
Bar racks or Ahead of pumps and May be manually or mechanically
bar screens grit removal facilities cleaned.
Coarse Behind racks or ahead These are flat-, basket-, cage, or
woven wire of trickling filters disk-type screens used to remove
media screens relatively smaller particles. Cleaned
by removing from the channel.
Openings vary from 3 to 20 mm.
Comminutor Used in conjunction Grinders that cut up the materials
retained over screens. Provisions
to bypass the comminutor is always
made.

11. Screw Pumps
To pump the raw sewage from wet wells to the head of the wastewater treatment plant for gravity flow during the treatment.

12. Fine Screens Moving Screens
Fixed or moving screens
20~35% SS and BOD5 removal; grease removal, increased DO
Moving Screens
Type Description
Band screens Consist of an endless perforated band which passes over
upper and lower rollers. A brush may be installed to
remove the material retained over the screen. Water jet is
also used to flush the debris.
Wing or shovel Consist of circular perforated radial vans that slowly rotate
screens on a horizontal axis. The vans scoop through the channel.
Strainers or Consist of a rotating cylinder that has screen covering the
Drum screens circumferential area of the drum. Openings vary from
0.02 to 3 mm.

13. Screening
A device with openings of uniform size to retain coarse solids found in wastewater
Consists of parallel bars, rods/wires, grating, wire mesh, or perforated plate with circular or rectangular shape
Bar racks/screens: used to protect pumps, valves, pipelines, and other appurtenances from damage or clogging

14. Typical Screening Devices
Inclined
fixed screen
Rotary drum
screen
Centrifugal
screen
Rotary disk
screen

15. Channel depths: >100 ft
Bar Screens
Width: 2 to 14 ft
Channel depths: >100 ft
Through Flow Design
Dual Flow Design
Most commonly used at medium- and large-size wastewater treatment plants

16. Bar Screens

17. Frontloader Bar Screen
A back-cleaned unit, which enables the rake to catch the captured material and keep it from dropping back into the flow.
A back-cleaned unit, which enables the rake to catch the captured material and keep it from dropping back into the flow.
The teeth of the rake extend through the bars and raise the material on the upstream side.
Can be used in channels up to 4.5 feet deep and 4 feet wide with bar spacing down to 1/2" clear openings.
The headroom required for the unit is only 8 feet.

18. Aqua Guard® Bar/Filter Screen
A continuous, self-cleaning device that utilizes a filter-rake combination to remove floating and suspended materials from a moving stream.
Available in widths of 1.5 to 15 ft. and discharge heights of 5 to 50 ft. with power requirements of 0.5 to 3 HP.
The standard angle of inclination: 75º (60º and 85º are also available)
Removal of particles as small as 1 mm to debris larger than 200 mm

19. Curved Bar Screen Operates through a 90 degree circular action
The horizontal and vertical position control by proximity switches
Hydraulic operation & fully automatic control (0~60 min. timer)
Efficient cleaning by variable operation timing
No damage by trapped material
Maximum screening area
Hot-dip galvanized or stainless steel finish

20. Climber® Screen
Removes solids from the bar screen by means of a precision gear-driven cleaning rake running in guide rails.
Designed to eliminate moving parts below the water line - low maintenance time and expense
Width: 18" to over 30’
Lift: 2' to over 125',
Bar spacing: 1/4" to 6”
Equipped with overload protection device

21. Step Screen®

22. Step Screen®

23. Wash Press Vacuum Conveyor
Designed for effective washing of organic material from screenings
Vacuum Conveyor
Designed for long transport distances, i.e. up to 15 meters (50 feet) vertically and/or up to 50 meters (165 feet) horizontally

24. Rotamat® Fine Screen
Removes, transports, compacts, and dewaters screenings.
Simple operation and low equipment costs by combining four procedures into one piece of equipment
Used for screening raw wastewater, septage, and sludges.
All stainless steel construction
Only one moving part
No metal-to-metal wearing parts
One machine with a single motor to screen, wash, compact, and dewater the debris from wastewater.

25. Rotamat® Fine Screen Screens, washes, and dewaters with a single unit
Easily installs into open channels for reduced installation costs

26. Mechanically Cleaned Bar Screens

27. Band Screen (2 & 5 mm)

28. Band Screen (2 & 5 mm)
A low backwash pressure (~30 psi) required to positively clean the screen
ProPaPanel® (US Patent No ). This thick plate (9 mm or 3/8") screening media resist hair pinning, contain a tapered hole to prevent plugging and are made of a corrosion resistant urethane.

29. Screens/Racks Design guidelines Design factor
Velocity through screen/rack (m/sec) 0.3~ ~1
Bar size (mm)
Width 4~8 8~10
Depth 25~50 50~75
Clear spacing between bars (mm) 25~75 10~50
Slope from horizontal (°) 45~60 75~85
Allowable headloss, clogged (mm)
Max. headloss, clogged (mm)
Manually Mechanically cleaned cleaned

30. Screen Arrangement Settling of grit and organic particulates
Most common

31. Design Example
Two identical bar racks; mechanically cleaned,  = 75°; bar spacing (clear) = 2.5 cm; Qpeak = m3/sec; Qmax = m3/sec; Qave = m3/sec Design values Velocity through rack at Qpeak = 0.9 m/sec Velocity through rack at Qmax = 0.6 m/sec Velocity through rack at Qave = 0.4 m/sec Diameter of the conduit = 1.53 m; slope of the conduit = m/m; velocity at Qpeak = 0.88 m/sec; depth of flow in the conduit at Qpeak = 1.18 m A. Design bar racks 1. Compute bar spacings and dimensions of the bar rack chamber Use peak wet weather flow for the rack chamber design. a. Clear area through rack openings = Qpeak/vmax = m3/sec  0.9 m/sec = 1.47 m2

32. Design Example - continued
b. Clear width of the rack opening = A/d = 1.47 m2/1.18 m = 1.25 m
c. Provide 50 clear spacings at 25 mm
d. Total width of the rack chamber  25 mm  10-3 m/mm = 1.25 m
e. Total number of bars = 49
f. Provide bars with 10 mm width

33. Design Example - continued
g. Width of the chamber = 1.25 m + 49  10 mm  10-3 m/mm = 1.74 m 2. Calculate the efficiency coefficient Efficiency coeff. = = = 0.72
Clear opening
Width of the chamber
50  25 mm
1740 mm
3. Compute the actual depth of flow and velocity in the rack chamber at Qpeak
a. Energy equation
b. The chamber floor is horizontal; Z2 = 0; the invert of the incoming conduit = 8 cm above the reference datum; Ke = 0.3

34. Design Example - continued

35. Design Example - continued
c. Simplifying the previous equation: d d = 0 d. Solving the above equation by trial and error d2 = 1.28 m; v2 = m3/sec  (1.74 m  1.28 m) = 0.59 m/sec

36. Design Example - continued
4. Compute velocity v through the clear opening v = = = 0.83 m/sec < 0.9 m/sec May redesign with 48 or 49 clear openings. The width of the chamber will be reduced and higher velocity through the screen will be encountered. 5. Compute headloss through the bar rack 6. Compute the flow depth and velocity in the rack chamber below the rack
Flow m3/sec
Net area at the rack 1.25 m  1.28 m

37. Headloss Calculation Bar racks (clean or partially clogged)
where C = empirical discharge coefficient = 0.7;
V = velocity of flow through the openings, ft/sec;
v = approach velocity in upstream channel, ft/sec;
g = acceleration due to gravity, ft/sec2.
Fine screens
Q = discharge through screen, ft3/sec; and
A = effective open area of submerged screen, ft2.

38. Headloss Calculation - continued
Screens (clean)
where W = max. cross-sectional width of the bars facing the direction of flow, m;
b = min. clear spacing of bars, m;
hv = velocity head of the flow approaching the bars, m; and
 = bar shape factor.
Bar type 
Sharp-edged rectangular 2.42
Rectangular with semicircular upstream face 1.83
Circular 1.79
Rectangular with semicircular upstream/downstream face 1.67
Tear shape 0.76

39. Design Example - continued
a. Depth and velocity in the chamber
b. Simplifying this equation:
d d = 0
c. Solving the equation by trial and error, d3 = 1.25 m and v3 = 0.61 m/sec.
7. Compute the headloss through the rack at 50% clogging
a. At 50% clogging of the rack, the clear area through the rack is reduced to half and the headloss through the rack is obtained from the energy equation

40. Design Example - continuedb. c. d. e.
Assume outlet channel do not change. i.e., d3 = d3’ and v3 = v3’
Velocity through rack openings at 50% clogging

41. Design Example - continued
f. Simplifying this equation:
d’ d’ = 0
g. Solving this equation by trial and error: d’2 = 1.40 m and v’2 = 0.54 m
h. Velocity through rack openings = 2.114/1.4 m/sec = 1.51 m/sec
i. The headloss under 50% clogging:
h50 = 1.4 m m = 0.15 m
k. v50 = 2  v (0.83 m/sec) (#36) = 1.66 m/sec, v2 = 0.59 m/sec (#35)

42. Design Example - continued
7. Headloss increase due to 50% clogging = = m
Accurate control of the cleanup cycle and protection against surge load are vital.
A freeboard of > 0.5 m is required.
In this design, the bar screen is deep in the ground; thus, flooding and overflow is not a consideration.
B. Effluent structure
Previous calculations of flow depth and velocity were based on normal flow conditions. Due to a free fall into the wet well, the actual depth into the channel will be significantly smaller than the normal depths calculated earlier. Furthermore, the velocities through the screen will also be significantly larger.
Upstream v through Downstream Headloss
Conditions d2, m v2, m/s rack, m/s d3, m v3, m/s m
Clean rack
50% clogging

43. Design Example - continued
A proportional weir or Parshall flume is specially beneficial as a control section if a free fall is available on the downstream side.
Proportional weir is ideal because:
No converging or diverging sections are needed (compared to Parshall flume)
Raising of the floor may not be necessary (it prevents deposition of solid during low flows)
The velocity in the channel will remain fairly uniform even at lower flows
The head over can be calibrated for flow measurement

44. Design Example - continued
The flow through a proportional weir is given by:
where:
Q= flow through the proportional weir, m³/s
H= head over weir, m
Cd= coefficient of discharge (0.6~0.9), typically 0.6
L=length of the weir opening at a height H above the weir crest, m
g= acceleration due to gravity, 9.81 m/s²
Substituting the values, the previous equation is transformed into

45. Design Example - continued
To maintain a nearly uniform velocity under variable-flow condition, the depth of flow in the chamber must be proportional to the flow through the chamber or the head over the weir (H). This is achieved by keeping the factor [LH½] in previous equation constant.
To keep the bearings lubricated, the weir crest will be set 15 cm above the channel floor. The max. depth in the channel (d3) is 1.25 m. Therefore, the max. head over the proportional weir at peak design flow is 1.10 m ( m).
The length of the weir opening (L) at the max. head over the weir is calculated by:

46. Design Example - continued
Compute the geometric profile of the proportional weir
The factor [LH½] at different sections is kept constant:
Flow (m³/s)
Head over weir (m)
Weir length (m)
Flow depth (m)
Flow velocity (m)
Peak=1.321
1.10
0.27
1.25
0.61
Max=0.916
0.78
0.32
0.93
0.57
Ave= 0.441
0.37
0.47
0.52
0.49
0.220
0.19
0.67
0.34
0.152
0.13
0.79
0.28
0.31
0.050
0.05
1.27
0.20
0.14

47. Design Example - continued
Design details of the proportional weir
The ends of the weir are cut off at a height of 5 cm above the crest

48. Design Example - continued
Hydraulic profile through the bar rack at peak design flow when rack is clean and at 50% clogging

49. Design Example - continued
C. Quantity of screenings
From the figure at 2.5 cm clear spacing, the average amount of screenings produced at the design average (0.441 m3/sec) and peak (0.916 m3/sec) flows are 20 and 36 m3/106 m3 of the flow.
Average quantity of screenings
20 m3/106 m3  m3/sec
 3600 sec/hr  24 hr/day
= 0.76 m3/day
Max. quantity of screenings
36 m3/106 m3  m3/sec
= 1.37 m3/day
Range: 3.5~80 m3/106 m3
Average: 20 m3/106 m3 36

50. Design Details of the Bar Rack

51. Common Operating Problems
Obnoxious odors, flies, and other insects around the bar rack
Increase frequency and removal and disposal of screenings
Excessive screen clogging
Caused by unusual amount of debris, low velocity through the rack, or slow removal of debris
Identify the source causing excessive discharge of debris and stop it; provide a coarser rack, or reset the timer cycle or install level controller override
Excessive grit accumulation in the chamber
Caused by low velocity in the channel
Clean bottom regularly, reslope the bottom, rake the channel, or flush regularly with a hose
A jammed raking mechanism
Remove the obstruction immediately
A broken chain or cable
A defective remote control circuit or motor

52. Maintenance
Daily inspection: raking chain, sprocket, teeth, and other moving parts
Lubricate and adjust all moving parts
Perform routine maintenance
After dewatered, check for painting, cable, chain, or teeth, remove obstructions, and straighten bend bars
Screenings: odorous and attract flies and insects
The bar screen area should be thoroughly hosed off daily with chemical solution (chlorine or hydrogen peroxide)

53. Information Required for Bar Racks
Width and water depth in the channel
Clear spacing between bars
Velocity through screens
Type of cleaning equipment
Front-cleaned
Back-cleaned: does not jam easily due to obstruction at the base of the screen
Operation – intermittent by a timer or preset differential headloss across screen