Environmental Engineering

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• Environmental Engineering
• Lecture 6

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Sources of Drinking Water

• Rivers upland and lowland
• Lakes and reservoirs
• Groundwater aquifers
• Sea water (Desalination)

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WATER TREATMENT PROCESSES
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WATER TREATMENT PROCESSES
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Selection of Treatment Process
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PRE-TREATMENT OF WATER

• Steps required before standard treatment
  processes
• Screening
• Storage, equalization
• Aeration
• Chemical pre-treatment pre-chlorination

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PRE-TREATMENT OF WATERScreening

• Coarse Screening
• typically inclined bars of 25 mm diameter and 100
  mm spacing
• prevent large floating material
• Raking is facilitated by the inclination of the
  bars.
• Velocities are usually limited to about 0.5 mls
  through the screens
• Fine screens
• fine screens are fitted after the coarse screens
  if storage is not provided,
• If there is storage then fine screens are placed
  at the outlet of the storage tanks.
• typically mesh with openings about 6 mm diameter
• circular drum type or a traveling belt
• Micro screening.
• the mesh openings range from 20 to 40 µm.
• used only as the main (physical) treatment
  process for relatively uncontaminated waters.

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PRE-TREATMENT OF WATER Storage and Equalization

• Storage
• Serve as a safety line in the event of pollution.
  
• Also serve as reservoirs in time of low supply
• Should be equivalent to 7 to 10 days of the
  average water demand
• This period is good for settling and adequate to
  reduce most pathogens by exposure to daylight
• Storage time of about 12h is commonly used to
  reduce pumping costs and balance the demand
• Equalization
• Provide an 'equal' (or consistent) flow to the
  plant

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Equalization Example
Design the size of an equalization tank to
balance flow rates from a municipal wastewater as
given in columns (1) and (2)
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PRE-TREATMENT OF WATER Aeration

• Aeration is the supply of oxygen from the
  atmosphere to water to effect beneficial changes
  in the quality of the water.
• It is a common treatment process for groundwater
  and less common for surface waters. Aeration is
  used
• To release excess H2S gas which may cause
  undesirable tastes and odors.
• To release excess CO2 which may have corrosive
  tendencies on concrete materials.
• To increase the O2 content of water in the
  presence of undesirable tastes due to algae
  (fishy smell),
• To increase the O2 content of water which may
  have negative taste, color and stain properties
  due to the presence of iron and manganese in
  solution. The addition of oxygen assists the
  precipitation of iron and manganese.
• Aeration can be a simple mechanical process of
  spraying water into the air and allowing it to
  fall over a series of cascades (waterfalls),
  while absorbing or desorbing (stripping) oxygen
  in its journey.

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PRE-TREATMENT OF WATER Pre-chlorination

• Chemical pre-treatment is used to remove
  undesirable properties of water (bacteria, algae
  or excess color) is a more expensive process than
  chemical post-treatment.
• Pre-chlorination is used on low turbidity water
  with a high coliform count.
• The chlorine is injected into the water stream
  and over the period that it stays in the settling
  tanks,
• it oxidizes and precipitates iron and manganese.
• It also causes pathogenic kill and reduces color.
  
• Doses as much as 5 mg/l are used,
• Water authorities tend to use pre-chlorination at
  times of the year when the surface water supply
  is likely to be polluted from agricultural or
  industrial sources or when excess organic matter
  is transported

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PRIMARY-TREATMENT OF WATER

• Standard treatment is the set of unit processes
  that reduce color, turbidity and particulate
  impurities to acceptable levels
• Standard treatment consists of the following unit
  processes
• Sedimentation
• Coagulation and flocculation
• Sedimentation of flocculent particles
• Filtration

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PRIMARY-TREATMENT OF WATERSedimentation

• Sedimentation by definition is the solid-liquid
  separation using gravity settling to remove
  suspended solids' (Reynolds, 1982).
• In water treatment, sedimentation processes used
  are
• Type L to settle out discrete non-flocculent
  particles in a dilute suspension.
• Type n to settle out flocculent panicles in a
  dilute suspension
• Type L Settling tanks are of two types
• Rectangular
• length width ratio of 2 and a depth of the order
  of 1.5 to 6m.
• Circular
• Dimensions typically are 10 to 50 m in diameter
  and 2.5 to 6m in depth

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Rectangular Sedimentation Tank
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Circular Sedimentation Tank
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Sedimentation tank design

• Key parameters and typical values in the design
  of settling tanks
• Surface overflow rate
• 20-35 m3/day/m2
• Detention time
• 2-8 hours
• Weir overflow rate
• 150-300 m3/day/m

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Sedimentation tank design

• Stokes law for settling velocity

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Sedimentation tank design

• Stokes law for settling velocity

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Sedimentation tank design
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Sedimentation tank design
Surface overflow rate, Same for circular tanks
Surface area
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Sedimentation tank designexample
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Sedimentation tank designexample
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Sedimentation tank designexample
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