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					                 Screening,
Definition:

• Screening is a unit operation that separates
  materials in and/or on water (found in different
  sizes) from water and from entering water
  treatment facilities and mains.

• The unit involved is called a screen.
Where are Screens Located Within Treatment
            Plants or Systems
Legend
 Sluice Gate
Bar Screen
Motor
Waste Sludge
Pump
Belt Filter Press
 Classification of Screens


– Opening size [Coarse, Medium and Fine]
– Configuration [Bar Screens and Mesh
  Screens]
– Method used to clean the entrapped materials
  (manually, mechanically, raked or water-jet
  cleaned)
– Fixed or moving screen surface.
               Classifications
• Coarse Bar Racks

– remove coarse debris (twigs, branches, rags,
  etc)
- Spacing         coarse 2 – 6 in
                  medium 0.8 – 2in

• Fine Screens
- 3/8 to ½ in. (up to 10 mm or less,
- book < 6 mm (1/4 in.))
         Types of Screens

- hand clean coarse screens     •
- mechanically cleaned bar screens   •
         Types of Screens
1. Moving •
2. Fixed •
                  Types of Screens
There are many types of screens that can be used in water and
wastewater treatment processes of which:

1 - Bar or rack screens: Bar screens composed of parallel bars. Bars
usually vertical or inclined
2 - Band screens: Consists of a perforated belt passes over an upper and
lower roller
3 - Perforated plate screen: Consists of a fixed band of perforated
screens
4 - Wing screens: It has radial vanes which rotate on a horizontal axis
5 - Disk screens: Circular perforated disk with or without supporting bars
6 - Grating screens: Consists of two sets of parallel bars.
7 - Mesh screens: Mesh screens composed of a fabric with mesh size
depend on floating and suspending matter.
                     Bar Screens
•   Design Criteria of a Bar Screen :
•   Approach Velocity
•   Optimum Velocity : 0.6 m/s (through the screen opening)
•   Maximum Velocity : 0.75 - 1.0 m/s (to prevent entrapped
    materials being forced through the bars).
•         Minimum Velocity : 0.4 m/s to prevent deposition of
    solids.
•         Typical Range : 0.6 - 1.0 m/s
•   Headloss :
•   hL = 0.05 - 0.15 m for drinking water.
    hL = 0.10 - 0.40 m for sewage (wastewater)
•   Cleaning Mechanism: Manual Mechanical
•   Angle of Inclination (10-60 )
•   Maximum head loss (cm)
           Band or Belt Screens

• Flexible woven wire mesh screens normally
  installed for a river supply.
  – Consists of sections of perforated mild steel plates
    connected together in a form of a band which is
    revolved by an electric motor.
  – Water passes inward through the screens and
    solid matter is washed off by high pressure water
    jets directed from inside of the screen.
Disk Screens and Drum Screens

Similar in principle to band screens, differing only
  in the form of the moving screen.
   – Rotating metallic disc - partially immersed.
   – Solid caught in the screen are taken to the top, where
     they are scrapped by the moving screen.
   – Diameter : 2 to 5 m
   – Speed 0.05 m/s
   – Hollow drum.
   – One end of the drum is closed.
   – Water enters through the other end and passes out
     through the perforation.
   – Water jet is used for cleaning
          Disposal of Screenings
• Screenings is the waste materials collected from screens.
  Screenings should be properly disposed. Various methods of
  screening disposal were used such as:

      - burning,
      - burying,
      - digestion,
      - dumping into large bodies of water,
      - and shredding and returning it to wastewater collection or treatment
         system.


• Inland burying is efficient in small treatment plants, while
  burning is best for medium and large treatment plants. Other
  methods cause problems and may need subsequent
  treatment. Digestion is used for large systems and in
  combination with the treatment of the organic portion of
  municipal solid waste.
         Design of Coarse screens
• - factors to consider
• - clear openings between bars: opening needed - typically
  less than 2 ins., at 22 - 45 o incline
•      - location :installed ahead of grit chambers, may also be
  installed ahead of equalization tanks, if present.
• - approach velocity:
• at least 0.4 m/s to prevent deposition of solids, and should
  not exceed 0.9 m/s at peak flow rate
•      - head loss through screens - limited to 150 mm (6 in.)
• - screens handling and disposal - quantity of screenings
  depends on the type of waste water, geographic location,
  screen size and weather
•              - screenings - vary from 3.5 - 80 m3/106 m3,
  about 80% moisture and density of 960 kg/m3
•      - controls - operation cycle about 15 minutes for
  mechanically raked screens
     Fixed Screens: Bar Screens
• Bar racks (also called bar screens) are composed of
  larger bars spaced at 25 to 80 mm apart. The
  arrangement shown in the figure is normally used for
  shoreline intakes of water by a treatment plant.
• The rack is used to exclude large objects;

• the traveling screen following it is used to remove
  smaller objects such as leaves, twigs, small fish, and
  other materials that pass through the rack.

• The arrangement then protects the pumping station
  that lifts this water to the treatment plant.
                    Bar Screens
• Coarse screens or bar racks (< 2.5 inch openings) :
• (1) removes large objects, rags, debris ;
• (2) protects downstream pumps, valves, pipelines ;
• (3) cleaning may be accomplished manually or
  mechanically ;
• (4) mechanically cleaned bar racks (5/8 inch - 1-3/4 inch)
  typically used instead of coarse manually cleaned screens
  ;
• (5) bar rack is inclined to facilitate , cleaning ;
• (6) approach velocities should ensure self-cleaning, but not
  dislodge solids ;
• (7) typical design : maximum velocity of 2.5 ft/sec through
  bar rack opening.
       Microstrainer (Fine Filter)

Microstrainer have been used to remove
  suspended solids from raw water
containing high concentrations of algae .

It consists of a fine fabric or screen wound around
   a metal drum
HEAD LOSSES IN SCREENS
   AND BAR RACKS

                H



v       V
•   Head loss calculations determine the hydraulic head requirements for screen
    installation :

                                   2                 
                                    V  v2
                                1                    
                      H  H L                      
                                C  2g                
                                                     
                                                     




C =0.7 for clean and 0.6 for clogged : empirical discharge coefficient to account
   for turbulence and eddy losses

V : velocity of flow through openings in rack (m/s)

v : approach velocity in upstream channel (m/s)

g : acceleration due to gravity (m/s2)
    Applying Bernuli before and after
              the screen
•




• Or

         V12=V22 + 2g(h2 – h1)
HEAD Loss IN MICROSTRAINERS
                   Example

• A bar screen measuring 2 m by 5 m of surficial
  flow area is used to protect the pump in a
  shoreline intake of a water treatment plant. The
  plant is drawing raw water from the river at a
  rate of 8 m3/sec . The bar width is 20 mm and
  the bar spacing is 70 mm. If the screen is 30%
  clogged, calculate the head loss through the
  screen. Assume Cd = 0.60.
• Solution:

• For screens used in
  shoreline intakes, the
  velocity of approach is
  practically zero. Thus,
• From the previous figure,
  the number of spacings is
  equal to one more than
  the number of bars. Let x
  number of bars,
                Example
• Design a bar screen chamber through
  which maximum, average, and minimum
  rates of flow are respectively 15, 7.5, and
  3.0 cfs; the screen is such that there is
  one more space than there are bars; the
  outlet is controlled by a proportional weir
  such that depth of flow is directly
  proportional to rate of flow ?.
  Solution
• A schematic diagram of the proposed screen is given
  in previous slide. A number of assumption need to be
  made by the designer engineer concerning screen
  incline angel, a, flow velocity through screen, Vs,
  number of bars, Nb, bar's diameter, db, and space
  between bars, Sb. In this case we assume;

   α = 30o,            Vs = 1.0 fps,           Nb = 20
              db = 0.25 inch,      Sb = 1.0 in
• Width of the screen, Ws, is equal to
   Ws = Number of Spaces x Space Distance
       = (20 +1) x 1 = 21 inch = 1.75 ft
Width of screen chamber, Wc, is equal to
              Wc = Width of screen + Bars thickness
                = 21 + (20 x 0.25) = 26 inch = 2.17 ft
Length of the screen, Ls, is equal to
             Depth of the screen chamber, dc
      Ls =
             Sinus on angle α
Flow through screen = Area available for flow x flow
  velocity
               Q = A x Vc
               A = Wc x dc
               Q = Qmax = 15 cfs
               Vc = 1.5 fps
              Qmax
      dc =                  = 4.61 ft
               Wc x Vc
Maximum depth of the screen chamber, dc, is
         maximum dc = dc + (0.67 to 1.00) = 5.28 ft
Length of the screen Ls = dc/sin α = 9.22 ft
Maximum flow velocity through the screen chamber, Vc,
  is
                    Qmax
Maximum Vc =                      = 1.62 fps
                  Wc x maxdc

             = acceptable
         Comminutors

Shredding devices (communitor
 or grinder) : shreds material to
       1/4 inch - 3/8 inch.
• Comminutors are devices used in water and
  wastewater treatment either in combination with
  screens or independently with the aim of
  chopping the oversized suspended and/or
  floating material found in water and wastewater
  or escaping the screens before entering the
  treatment facilities and altering its operation.

• Comminutors consist of two sets of cutters one
  is fixed while the other is moving. The distance
  between the two sets equal to the size of
  chopped material required.
• Comminution technology has been evolving quite
  rapidly in response to the increasing burden entrained
  solids have placed on treatment facilities.

• More advanced devices have been developed in rapid
  succession. The result has been an exciting and fluid
  race between the leading manufacturers to develop
  the best size reduction device.

• The latest grinder innovations to be introduced have
  coupled the power of twin shaft grinding with higher
  flow capabilities and screw screening systems. Here’s
  a rundown on the past and present state of the art in
  wastewater solids reduction
          Comminutor Design

For comminutor design, environmental engineer
 or designer need to supply manufacturer with
 the size of suspended and floating materials
 present in water to be treated and that after
 treatment along with its density and hydraulic
 and organic loadings. Accordingly manufacturer
 decides on the equipment needed to achieve
 the objective.

				
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posted:3/20/2013
language:English
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