Environmental Engineering : Treatment of raw water

By Aditya Kumar|Updated : September 8th, 2021

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Treatment of Water



Theory of Sedimentation

·     Stokes Law


  for d < 0.1 mm.


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

  The diameter of the particle in the meter.

  SP gravity of the particle


  Kinematic viscosity of water in m2/sec.








 T = Temperature of water in oC

  is in mm/sec.

 d is in mm.



Sedimentation Tank : Surface Over flow Rate


  12000 to 18000 lit/m2 day for plain sedimentation.

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



(b) Velocity of flow,  

(c) Time of horizontal flow, or Detention time


(d) Time of falling through height ‘H’


(e) Detention time,


    For plain sedimentation tank → 4 to 8 hours

    For sedimentation with coagulation → 2 to 4 hours


(f)  Efficiency



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

(g) % of particle removed


 where corresponds to  



(h) Detention time ‘t’

  for a rectangular tank.

  for circular tank


 d = Dia. of the tank

 H = Vertical depth of wall or side water depth


(j) Scour velocity,  


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

  Darcy Weisbach friction factor.

 = 0.025 to0.03 for settling tanks.


Chemicals used for Coagulation

·     Alum  


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

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

 to give 0.234 gm of Al(OH)3 ppt.


·   Copperas  


·    Chlorinated Copperas :  


·    Sodium Aluminate


 Mixing Basin



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).


·    Velocity gradient,



·   Detention time is 10 to 30 minutes.


·    Number of particle collision  





A. Slow Sand Filter

· Depth of filter is 2.5 to 3.5 m.

·  Plan area of the filter is 100 to 200 m2.



·    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.


B. Rapid Sand Filter



N = Number of unit required

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


·    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


·    4.5% of filtered water is used as a backwash.

·     30 min. used for backwash.


Hydraulics of Sand Gravity Filters



 Frictional head loss through the filter in the meter.

 Approach velocity or filtration velocity in m/s.

 Depth of filter in meter

 Shape factor (for non-spherical particle)

 The diameter of sand particles in the meter.

 Accelerations due to gravity in m/s2.


 Newton’s dray coefficient.

 Mass friction of sand particle of dia d.

Rose Equation,  


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



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

 Unit weight of meter in kN/m3.

 Depth of filter bed in the meter.

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




 Depth of expanded/fluidized bed in the meter.

 The porosity of the expanded fluidized bed.




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




 The porosity of the expanded bed

 Backwash velocity in m/s

 Setting velocity in m/s.


Disinfection or Sterilization

(i) Treatment with Ozone


(ii) Disinfecting Action of Chlorine





(iii) Doses of Chlorine

(iv) Forms in which chlorine is applied

 (a) Free chlorine

 (b) Hypochlorites & Bleaching Powder

 (c) Chloramines

 (d) Chlorine dioxide


Type of Chlorination

 (i) Plain chlorination

 (ii) Pre-chlorination

 (iii) Post-chlorination

 (iv) Double chlorination

 (v) Break point chlorination

 (vi) Super chlorination

 (vii) Dechlorination



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

Water Softening

Methods of Removing Temporary hardness

 (i) Boiling


 (ii) Addition of lime


Method of Removing Permanent Hardness

 (i) Lime-Soda Process


 (b) (i) 






 (g)  where,

 calcium hardness removed in mg/lit (expressed as  

 Magnesium hardness removed in mg/lit (expressed as v

 Lime added in mg/lit (expressed as 

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


Demineralization Process for Removing Hardness


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

Tolerance Limit



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Aditya Kumar
Aditya KumarMember since Feb 2021
B-Tech(Civil Engg) NIT Allahabad
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