CE 548

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• CE 548
• Physical Unit Operations

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Introduction

• Physical unit operations operations used for the
  treatment of wastewater in which change is
  brought about by means of the application of
  physical forces.
• Physical unit operations are a major part of
  treatment systems.

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Screening

• First unit operation encountered is wastewater
  treatment plants.
• A screen is a device with generally uniform
  openings that is used to retain coarse solids.
• Screens may be divided into coarse (.25-6) and
  fine (lt.25) screens. Microscreens (lt50?m) are
  also available can be used to remove fine solids
  from treated effluents.
• The material that is retained by the screens is
  know as screenings. Screenings are typically
  hauled to a landfill or may be buried on site or
  incinerated depending upon the plant.

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Screening

• Coarse Screens (Bar Racks)
• Coarse Screens (Bar Racks) Composed or parallel
  bars or rods with openings of 0.25-6 and are
  used to protect pumps, valves from clogging by
  large objects and rags.
• Bar racks may be hand cleaned or mechanically
  cleaned (T5-2 and F5-3). The bars are mounted at
  some angle to the horizontal and the allowable
  head loss is 6 or less.
• Design of Bar Racks Considerations should
  include location ( upstream of grit chamber),
  approach velocity (1.25-3fps) clear openings
  between bars or mesh size, headloss through the
  screens (lt6), screenings and controls. Use 2
  units or 1 unit and a by-pass.
• Head-loss across bar screens can be represented
  by (equation 5-1)

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Screening

• Fine Screens
• Fine Screens. Composed of wires, grating, wire
  mesh, or perforated plates with openings.
• The application range from primary treatment to
  the removal of residual SS solids from biological
  treatment processes. Fine screens may static,
  rotary drum or step type. F5-4, p.322 and T5-4,
  p.323.
• Design of Fine Screens Fine screens would be
  preceded by coarse screens. Use at least 2 sized
  to handle peak flow.
• Head-loss across fine screens can be obtained
  from manufacturers rating tables or calculated
  using Eq. (5-2)

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Screening

• Microscreens
• Composed of filter fabrics with openings of 10
  35 ?m and are fitted on a drum periphery.
• The principle applications for microscreens are
  to remove suspended solids from secondary
  effluent and from stabilization-pond effluent.
• Typical suspended solids removal range from 10 to
  80 percent with an average of 55 percent.
• Problems encountered with microscreens include
  incomplete solids removal and inability to handle
  solids fluctuations.
• Table 5-6 gives typical design information for
  microscreens.

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Screening

• Screenings
• Screenings are the material retained on bar racks
  and screens.
• The quantity of screenings collected depends on
  the screen openings.
• Screenings retained on coarse screens consists
  of debris such as rocks, branches, leaves, paper,
  plastics, rags, etc. Typical data on the
  characteristics and quantity of coarse screenings
  are shown in Table 5-7.
• Screenings retained on fine screens include
  small rags, plastic material of various types,
  razor blades, grit, food waste, feces, etc. Fine
  screenings must be handled and disposed of
  properly because it contains putrescible matter
  and substantial grease and scum. Typical data on
  the characteristics and quantity of fine
  screenings are shown in Table 5-8.

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Flow Equalization

• Flow equalization
• Flow equalization is the damping of flow
  variations to achieve a consistent flowrate. It a
  method used to
• overcome the operational problems caused by flow
  rate variations
• improve performance of downstream processes
• reduce the size and cost of downstream treatment
  facilities
• Advantages (benefits)
• enhancement of biological treat ? shock loading ?
  dilution of inhibition
• improved effluent quality ? better sedimentation
  due to constant loading
• improved filter performance
• Attractive option for upgrading performance of
  overloaded plants.

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Flow Equalization

• Flow equalization
• Types
• On-line achieves both flow and organic damping
• Off-line achieves mainly flow damping
• Volume requirements (example 5-2)

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Grit Removal

• Grit is sand, gravel, cinders, eggshells, seeds,
  coffee grounds and other heavy solid material
  that is much heavier than the organics in
  wastewater. Specific gravity (sg) of
  approximately 2.7 but can be as low as 1.3 versus
  an organic sg of 1.
• Grit is predominantly inert and relatively dry
  and highly variable in composition ranging from
  13-65 moisture with a volatile content of 1-56.
  It has a bulk density of 100lb/ft3.
• Normally, grit chambers are designed to remove
  all grit that will be retained on a
  0.21-mm-diameter (65 mesh) screen, and some
  chambers are based on 0.15 mm-diameter (100
  mesh).
• Grit chambers are typically located after the bar
  racks but before the primary sedimentation tanks.

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Grit Removal

• The function of the grit chambers include
• Protect moving mechanical equipment from abrasion
  and wear.
• reduce formation of heavy deposits in pipelines.
• reduce the frequency of digester cleaning caused
  by excessive accumulation of grit.
• Types of grit chambers horizontal flow, aerated
  or vortex.
• Rectangular Horizontal-flow grit chamber
• Designed to maintain a velocity of 1.0 ft/s and
  provide sufficient time for grit particles to
  settle.
• Organic particles will be carried out at this
  velocity.
• Designed to remove grit particles that will be
  retained on a 65-mesh screen (0.21 mm dia).

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Grit Removal

• Aerated Grit Chambers
• Because some organic matter is settled with grit,
  grit washing is usually done in order to clean
  the grit before disposal
• Aerated grit is used in order to eliminate the
  need for grit washing.
• Typical design parameters are given in Table 5
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• The velocity governs the size of particle to be
  removed.
• if velocity is too great, grit will be carried
  out
• if velocity is too small, organic will be removed
  with grit
• velocity is controlled by quantity of air.

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Grit Removal

• Vortex-type Grit Chambers, Two Types
• PISTA unit  
• Wastewater enters and exits tangentially
• Rotating turbine maintains constant flow velocity
  and promotes separation of organics from grit.
• Grit settles by gravity into the hopper.

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Grit Removal

• Teacup unit
• Flow enters tangentially at the top of unit and a
  free vortex is generated.
• Effluent exits the center of the top of the unit
  from a rotating cylinder.
• Grit settles by gravity to the bottom
• Organics including those separated by centrifugal
  forces exit with effluent.

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Primary Sedimentation

• Objective to remove readily settleable solids
  and floating material.
• Can be used as the principal treatment to provide
  for the removal of
• settleable solids
• free oil and grease and other floating material
• a portion of the organic load
• When used ahead of biological treatment, their
  function is to reduce the load on biological
  units.
• Primary sedimentation removes 50 70 ss and 25
  40 BOD.

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Primary Sedimentation

• Design consideration
• Detention time Important since wastewater
  solids are heterogeneous in nature and
  susceptible to flocculation.
• Normally detention time vary from 1-1/2 to 2-1/2
  hours.
• Temperature effect is limited, however, at lower
  temp settling is retarded (Figure 5 48).
• Surface-Loading rates
• Sedimentation tanks are normally designed on the
  basis of surface-loading (overflow rate)
  expressed as gal/ft2.d. (Q/Asurface)
• overflow rate must be set low enough to ensure
  satisfactory performance at peak rates

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Primary Sedimentation

• Design consideration
• Scour velocity
• In sedimentation basins, horizontal velocities
  should be kept low so that settling particles are
  not scoured from the bottom of the basin.
• Critical velocity is given by

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Primary Sedimentation

• Sedimentation tank performance
• BOD and TSS removal Typical performance data
  for the removal of BOD and TSS are presented on
  Fig. 5-46. The figure is derived using the
  following relationship