ENVIRONMENTAL ENGINEERING : Treatment of raw water Notes

By Deepanshu Rastogi|Updated : April 16th, 2021

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

Water Treatment Process

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Theory of Sedimentation

  •    Stokes Law

(a)

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  for d < 0.1 mm.

 Where,

  byjusexamprepThe velocity of the settlement of particle in m/s.

 byjusexamprep The diameter of the particle in the meter.

  byjusexamprepSP gravity of the particle

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 byjusexamprep Kinematic viscosity of water in m2/sec.

(b)

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(c)

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 Where,

 T = Temperature of water in oC

byjusexamprep  is in mm/sec. and d is in mm.

(d)  

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(e)

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Sedimentation Tank : Surface Over flow Rate

(a)byjusexamprep

  Vo =12000 to 18000 lit/m2 day for plain sedimentation.

  Vo =24000 to 30,000 lit/m2/day for sedimentation with coagulation.

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(b) Velocity of flow,  

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(c) Time of horizontal flow, or Detention time

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(d) Time of falling through height ‘H’

          byjusexamprep

(e) Detention time,

      byjusexamprep        

    For plain sedimentation tank → 4 to 8 hours

    For sedimentation with coagulation → 2 to 4 hours

 (f)  Efficiency (byjusexamprep)

  byjusexamprep

 where,

  byjusexamprepof lighter particles (with settling velocity (Vs )less than Vo  which shall be removed in an ideal settling basin.

(g) % of particle removed

  byjusexamprep

 where corresponds to VO

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(h) Detention time ‘t’

  for a rectangular tank

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  for circular tank

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 where,

 d = Dia. of the tank

 H = Vertical depth of wall or side water depth

(i) byjusexamprep

(j) Scour velocity byjusexamprep

 Where,

  byjusexamprep0.04 for uniform granular sand and 0.06 or more for non-uniform (interlocking) sticky material.

 byjusexamprep Darcy Weisbach friction factor.

 = 0.025 to0.03 for settling tanks.

 

Chemicals used for Coagulation

  •    Alumbyjusexamprep

  byjusexamprep

 666 gm of Alum reacts with 48 gm of byjusexamprep to  give 146 gm of  byjusexamprep

  1 gm of Alum reacts with 0.73 gm of byjusexamprep. or 0.45 gm of byjusexamprep

 to give 0.234 gm of Al(OH)3 ppt.

  byjusexamprep

  •  Copperas byjusexamprep

  byjusexamprep

  •   Chlorinated Copperas :byjusexamprep

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  • Sodium Aluminate

  byjusexamprep

 Mixing Basin

  byjusexamprep

 Where,

G’ = Temporal mean Velocity gradient (per second).

P = Power dissipated in watts i.e., N-m/s.

V = Volume of raw water to which P is applied in m3.

 Dynamic viscosity (N-s/m2).

Flocculation

  • Velocity gradient,

       byjusexamprep

  • byjusexamprep
  • Detention time is 10 to 30 minutes.
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  • Number of particle collisionbyjusexamprep
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  • byjusexamprep   

 Filtration

 A. Slow Sand Filter

  • Depth of filter is 2.5 to 3.5 m.
  • Plan area of the filter is 100 to 200 m2.
  • byjusexamprep
  •   byjusexamprep
  •   Design period = 10 years.
  • Depth of sand is 90 to 110 cm.
  •   Frequency of cleaning is 1 to 3 months
  •   Rate of filtration = 2400 to 4800 lit/m2/day or 100 to 200 lit/m2/hr.
  •   Efficiency of bacteria removal = 98 to 99%.
  •   It can not be used if turbidity > 50 ppm.
  •   It is designed for maximum daily demand.
  • byjusexamprep 

B. Rapid Sand Filter 

where,

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N = Number of unit required

Q = Plant capacity in million lit/day (MLD)

  • byjusexamprep
  • Sand layer depth is 60 to 90 cm.
  •   of sand is 0.35 to 0.55 mm.
  •   Depth of tank = 2.5 m to 3.5 m.
  • Area = 10 to 80 m2 each unit.
  •  Rate of filtration 3000 to 6000 lit/m2/hour (slow sand filter × 30)
  • Cross-sectional area of Manford = 2 × cross-sectional area of lateral.
  • Cross-sectional area of each lateral = 2 to 4 times cross-sectional area of perforations in it.
  • Total cross-sectional area of perforation = 0.2% of the total area of 1 filter bed
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  • 4.5% of filtered water is used as a backwash.
  • 30 min. used for backwash.

 

Hydraulics of Sand Gravity Filters

 byjusexamprep 

 Where,

byjusexamprep Frictional head loss through the filter in the meter.

 byjusexamprepApproach velocity or filtration velocity in m/s.

byjusexamprep Depth of filter in meter

 byjusexamprepShape factor (for non-spherical particle)

byjusexamprep The diameter of sand particles in the meter.

 byjusexamprepAccelerations due to gravity in m/s2.

 byjusexamprepPorosity

byjusexamprep Newton’s dray coefficient.

 byjusexamprepMass friction of sand particle of dia d.

Rose Equation,  

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Hydraulic head loss and expansion of the filter during backwash of RSF

 Where, byjusexamprep

byjusexamprep= Head loss through the filter bed required to initiate expansion in the meter.

 byjusexamprepUnit weight of meter in kN/m3.

 byjusexamprepDepth of filter bed in the meter.

 byjusexamprepSubmerged unit weight of sand in bed of depth ‘D’

  •    byjusexamprep
  •    byjusexamprep

Where,

 byjusexamprepDepth of expanded/fluidized bed in the meter.

 byjusexamprepThe porosity of the expanded fluidized bed.

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Where,

f = mass fraction of sand of various sizes in the sand (as per sieve analysis0

  • byjusexamprep
  •  byjusexamprep

where,

 byjusexamprepThe porosity of the expanded bed

 byjusexamprepBackwash velocity in m/s

 byjusexamprepSetting velocity in m/s.

 

Disinfection or Sterilization

(i) Treatment with Ozone

  byjusexamprep

(ii) Disinfecting Action of Chlorine

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(iii) Doses of Chlorine

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(iv) Forms in which chlorine is applied

 (a) Free chlorine

 (b) Hypochlorites & Bleaching Powder

 (c) Chloramines

 (d) Chlorine dioxide

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Type of Chlorination

 (i) Plain chlorination

 (ii) Pre-chlorination

 (iii) Post-chlorination

 (iv) Double chlorination

 (v) Break point chlorination

 (vi) Online Classroom Program chlorination

 (vii) Dechlorination

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Test of Chlorine Residual

  1. Orthotoulidine test: color – matching method
  2. Arsenide ortho-toulodine test – when mineral present in the water sample, Also a color matching method.
  3. DPD and chlorotic test (Di-ethyl phenylene diamine): color making method.
  4. Starch iodide Test.

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Water Softening

Methods of Removing Temporary hardness

 (i) Boiling

  byjusexamprep

 (ii) Addition of lime

  byjusexamprep

Method of Removing Permanent Hardness

 (i) Lime-Soda Process

 (a) byjusexamprep

 (b) (i) byjusexamprep

      (ii)  byjusexamprep

 (c)  byjusexamprep

 (d)  byjusexamprep

 (e)  byjusexamprep

 (f)  byjusexamprep

 (g) byjusexamprep

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 where,

 byjusexamprepcalcium hardness removed in mg/lit (expressed as  CaCO3)

byjusexamprep Magnesium hardness removed in mg/lit (expressed as CaCO3)

 

 byjusexamprepLime added in mg/lit (expressed as  CaCO3)

Zeolite or Base Exchange or Cation-Exchange Process for Removing Hardness

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Demineralization Process for Removing Hardness

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Drinking water specification: IS: 10500, 1992 (Reaffirmed 1993)

Tolerance Limit

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Tolerance Limit

 

 

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