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Water Treatment by Plain Sedimentation: Stoke's law

Plain sedimentation is the process of removing suspended matter from the water by storing it in tanks. All the suspended matters settle down and they can be filtered. Water can be purified by many methods like screening, aeration, sedimentation, filtration, disinfection, softening, fluoridation.

Water treatment is a process of improving the quality of water for desirable drinking and other purposes. The main objective of water treatment is to ensure consumer's health by treating the water and making it safe for human consumption.

Now let us see in detail about the method, Plain Sedimentation:-

If a specific gravity of the particle is greater than the specific gravity of the water i.e., 1, then the particle will settle down under the action of gravity. The velocity of the particle is calculated using stoke’s law.

Now, let us understand more about Stoke's law.

Stoke’s law

Stoke’s law is valid for viscous flow and only for the size of particles less than 1 mm i.e., d<1mm.

According to Stoke's law, the velocity of the particle(or velocity of settlement), Vs=g(Gs-1)d^2 / 18v


Vs: velocity of the particle

d: diameter of the particle

v: kinematic viscosity of the particle

If the flow is Turbulent (particle size greater than 1 mm), in this case, velocity of settlement is given by,


If the flow is transient (particles size is in between 0.1 mm and 1 mm), temperature (T) is also taken into account in transient flow. The velocity of settlement is given by,


Types of Sedimentation

Sedimentation can be of two types discrete and continuous.

Discrete flow type: Fill the water in a tank, keep it for a while, which is called detention time. Then the particle will settle and water will be removed out, then another batch of water will be filled into the tank. It is obsolete nowadays.

Continuous flow: Water flows continuously. It is of two types again, vertical flow tank and horizontal flow tank type.

  • Vertical flow tank type: It is applicable only if V0<Vs (V0- velocity of flow, Vs- velocity of settlement), then only the particle will settle, if the particle with Vs<Vo they will move to the overflowing water tank.

  • Horizontal flow tank type: Particles with Vs<Vo will also start to settle. Settling tanks is more important because it's widely used. In horizontal type settling tank partial removal is also possible, unlike vertical type settling tanks where only complete removal is possible.

The assumptions involved are-

  • The particle which reaches the bottom of the tank is removed.

  • The concentration of particles of each size is the same in the vertical cross-section, that implies, let us consider at the height of H/2, for 1 mm particles, half of the 1 mm particles distributed at above H/2 depth and the remaining half of 1 mm particles distributed at below H/2 depth.

Watch the below video and solve some numerical on plain sedimentation.

That's all for today's blog. Thank you!

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Nov 20, 2021

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