Environmental Engineering : Treatment of raw water

By Aditya Kumar|Updated : September 8th, 2021

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Treatment of Water

 

 

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Theory of Sedimentation

·     Stokes Law

(a)

 byjusexamprep for d < 0.1 mm.

 Where,

 byjusexamprep The velocity of the settlement of particle in m/s.

 byjusexamprep The diameter of the particle in the meter.

 byjusexamprep SP gravity of the particle

 byjusexamprep 

 byjusexamprep Kinematic viscosity of water in m2/sec.

(b)

byjusexamprep 

 byjusexamprep 

 byjusexamprep 

(c)

byjusexamprep 

 Where,

 T = Temperature of water in oC

 byjusexamprep is in mm/sec.

 d is in mm.

(d) byjusexamprep 

(e) byjusexamprep 

Sedimentation Tank : Surface Over flow Rate

(a) byjusexamprep 

 byjusexamprep 12000 to 18000 lit/m2 day for plain sedimentation.

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

 

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(b) Velocity of flow, byjusexamprep 

(c) Time of horizontal flow, or Detention time

 byjusexamprep 

(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) byjusexamprep Efficiency

 byjusexamprep 

 where,

 byjusexamprep of 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 byjusexamprep 

 

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(h) Detention time ‘t’

 byjusexamprep for a rectangular tank.

 byjusexamprep for circular tank

 where,

 d = Dia. of the tank

 H = Vertical depth of wall or side water depth

(i) byjusexamprep

(j) Scour velocity, byjusexamprep 

 Where,

 byjusexamprep 0.04 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

·     Alum byjusexamprep 

 byjusexamprep 

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

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

 to give 0.234 gm of Al(OH)3 ppt.

 byjusexamprep 

·   Copperas byjusexamprep 

 byjusexamprep 

·    Chlorinated Copperas : byjusexamprep 

 byjusexamprep

·    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.

byjusexamprep Dynamic viscosity (N-s/m2).

Flocculation

·    Velocity gradient,

      byjusexamprep 

·     byjusexamprep 

·   Detention time is 10 to 30 minutes.

·    byjusexamprep 

·    Number of particle collision byjusexamprep 

·       byjusexamprep 

·      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

·     byjusexamprep 

where,

N = Number of unit required

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

·   byjusexamprep 

·    Sand layer depth is 60 to 90 cm.

·    byjusexamprep 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

·     byjusexamprep 

·    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.

byjusexamprep Approach velocity or filtration velocity in m/s.

byjusexamprep Depth of filter in meter

byjusexamprep Shape factor (for non-spherical particle)

byjusexamprep The diameter of sand particles in the meter.

byjusexamprep Accelerations due to gravity in m/s2.

byjusexamprep Porosity

byjusexamprep Newton’s dray coefficient.

byjusexamprep Mass friction of sand particle of dia d.

Rose Equation, byjusexamprep 

 

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

·   byjusexamprep 

 Where,

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

byjusexamprep Unit weight of meter in kN/m3.

byjusexamprep Depth of filter bed in the meter.

byjusexamprep Submerged unit weight of sand in bed of depth ‘D’

·     byjusexamprep 

·     byjusexamprep

Where,

byjusexamprep Depth of expanded/fluidized bed in the meter.

byjusexamprep The porosity of the expanded fluidized bed.

·    byjusexamprep 

·    byjusexamprep 

Where,

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

·  byjusexamprep 

·   byjusexamprep 

where,

byjusexamprep The porosity of the expanded bed

byjusexamprep Backwash velocity in m/s

byjusexamprep Setting velocity in m/s.

 

Disinfection or Sterilization

(i) Treatment with Ozone

 byjusexamprep 

(ii) Disinfecting Action of Chlorine

 byjusexamprep

 

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(iii) Doses of Chlorine

byjusexamprep

(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) Super chlorination

 (vii) Dechlorination

 

byjusexamprep

 

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

byjusexamprep

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 byjusexamprepwhere,

byjusexamprep calcium hardness removed in mg/lit (expressed as byjusexamprep 

byjusexamprep Magnesium hardness removed in mg/lit (expressed as vbyjusexamprep

byjusexamprep Lime added in mg/lit (expressed as byjusexamprep

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

 byjusexamprep

Demineralization Process for Removing Hardness

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

Tolerance Limit

 

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