Water treatment is any method that improves the water quality To remove color, turbidity, dissolved solids, suspended solids, objectionable taste and odor, excess hardness, salinity, toxic materials, various microorganism.

Theory of Sedimentation

It is the removal of suspended particles by gravitational settling. Sedimentation tanks designed to reduce the velocity of flow so as to reduce turbulence sedimentation is based on Stokes law. There are two types of sedimentation
1) Plain Sedimentation- Also called type 1 sedimentation, based on discrete settling where no coagulants are added
2) Coagulation Sedimentation- Also called type 2 sedimentation, based on colloidal settling where chemicals are added to form flocs.

  • Settling Velocity using Stokes Law- The settling velocity of a discrete particle having d diameter can be computed using Stoke law. (a) \:V_{s'} = \frac{g}{18}(G-1)\frac{d^{2}}{v}

Valid for d < 0.1 mm.


Vs’ = Velocity of settlement of particle in m/s.,

d= Diameter of the particle in meter.

G= Specific gravity of the particle

ν= kinematic viscosity of water in m2/sec

 (b) \:V_{s} = [\frac{\frac{4}{3}gd \:(G-1)}{C_{d}}]^{\frac{1}{2}}

 R_{e}> 10^{4}

 And\: if \:R_{e}< 0.5^{4}, C_{D}=\frac{24}{R_{e}}

 And\: if \: 0.5 \leq R_{e} \leq 10,000

 C_{D} = \frac{24}{R_{e}}+\frac{3}{\sqrt{R_{e}}}+0.34

 (c)\: V_{s}= 418\: (G-1) d^{2} \left(\frac{3T\:+\:70}{100}\right)

For d<0.1mm

Where, T= temperature of water in ° C, is in mm/sec. d is in mm.

 (d)\: V_{s}=1.8\sqrt{gd\: (G-1)}\: For\: d>0.1mm

 (e)\: V_{s}= 418\: (G-1) d \left(\frac{3T\:+\:70}{100}\right)\: For\: 0.1<d<1\: mm

Types of sedimentation tanks

a) Quiescent Type (Fill and draw-type) – Tank is filled with incoming water and allowed to rest, generally for 24hr. 6-8 hr required for removal of sludge. The tank is designed to treat maximum daily demand equals 1.8 times of average daily demand.

b) Continuous flow type- It is designed to achieve an ideal condition of equal velocity at all points. Depending on type of flow these may be of the following 3 types- horizontal, circular, and vertical flow.

Horizontal Flow (Rectangular SedimentationTank)


  • Settling of particles is same as in case of Quiescent type of equal depth
  • Flow is horizontal and steady and settling velocity is uniform
  • The concentration of suspended particles is the same at all height
  • Particles are removed when they reach bottom of the settling zone

Important terms in calculations

(a) Overflow rate- Defined as settling velocity of that particle if introduced at the topmost point at the inlet will reach a bottom-most point at the outlet

Overflow rate, Vs = Discharge/Surface area


Vs= 12000 to 18000 lit/m2/ day for plain sedimentation, and Vs= 24000 to 30,000 lit/m2/day for sedimentation with coagulation.

(b) Velocity of flow, Vf= Q/ BH

(c) Time of horizontal flow, T= L/Vf ==>> LBH/Q

(d) Time of falling through height ‘H’ , T= H/Vs ==>> LBH/Q

(e) Detention time, td= L/Vf ==>> H/Vs

  • 4 to 8 hr→ For plain sedimentation
  • 2 to 4 hr → For sedimentation with coagulation

(f) Percentage lighter particle- P1=(Vs/Vo)*100

Where, P1= % of lighter particles (with setting velocity (Vsf)′ is less than Vs) which shall be removed in an ideal settling basin.

(g) Percentage of particle removed  P_{r} = (1-x_{s})\: +\: \displaystyle\sum\limits \frac{V_{s'}}{V_{s}}.\triangle X

Where xs is a fraction of particles that have settling velocities less than Vs. it means (1-Xs) is a fraction of those particles which have settling velocity more than Vs.

(h) Detention time ‘t’ for circular tank  t = \frac{d^{2}(0.011d \:+\: 0.785H )}{Q}

Where d= dia of the tank,
H= Vertical depth of wall


It is also called sedimentation with coagulation. Coagulation is added to neutralize the –ve charge on colloidal particles and allow them to coagulate. Coagulant forms the precipitate known as a flock.

Chemical used for Coagulation

1) Alum

  • Alum reacts with bicarbonates to form Al(OH)3 as a ppt.
  • The addition of Alum imparts permanent hardness because it converts bicarbonate into sulfate.
  • The addition of alum produces CO2 which forms carbonic acid which decreases pH and hence corrosiveness is increased, hence in order to increase the pH alkali may be required such as lime Ca(OH)2 or Soda Na2CO3.

2) Copperas

  • Also called ferrous sulfate. Used with lime in order to increase the pH.
  • The effective pH range is more than 8.5. Since value is more than the PLD for pH hence rarely used for drinking water treatment.

3) Chlorinated Copperas

  • When chlorine is added with iron slats Ferric Sulfate and Ferric Chlorides is formed
  • Iron salts are more commonly used in treating sewage water, whereas alum is used treating the raw water
  • Imparts bad taste to water
  • Produces heavy flocks
  • Ferric sulfate + Hydrated lime = Ferric hydroxide ppt

4) Sodium Aluminate

  • It reacts with permanent hardness present in water and gives ppt of Calcium aluminate. Since it removes permanent hardness hence it is often used treating boiler feed water
  • The method is costlier as compare to Alumtreatment



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