Principles of sedimentation

Sedimentation

• Most suspended particles in water have a specific gravity greater than 1.0. 
• In still (stationary or standing) water the impurities tend to settle under gravity. 
• They remain in suspension in normal raw water supplies because of turbulence.
• When turbulence is reduced by offering storage to water the impurities settle to the bottom of the tank and can be removed.

This is the principle behind sedimentation.

• The basin in which this is carried out is called settling tank or sedimentation basin or clarifier.
• The theoretical average time for which water is detained is called detention period.

Theory of sedimentation

• The settlement of a particle in water is opposed by the following forces:
• Velocity of flow which carries particle horizontally
• Greater flow cross-section implies lesser velocity causing easy settling of particles
• Viscosity of water varies inversely with temperature. Warm water is less viscous and hence offers less resistance to settling. However, this factor cannot be controlled in treatment plants and hence is ignored
• Particles with higher specific gravity are more likely to settle. 
• Size and shape also affect settling rate. Mathematically, weight and volume of a spherically shaped particle vary with the cube and square of the diameter of the particle. Hence shape and size also affect settling rates and consequently, very small particles settle very slowly.
• Settling velocity of a particle is given by STOKE's LAW.

Sedimentation tanks

• Clarification of polluted waters is carried out by the process of sedimentation

Factors affecting sedimentation are:

• Velocity of flow
• Shape and size of particles
• Velocity of flow can be reduced by increasing length of travel
• Detaining particles for longer time in sedimentation basins
• Shape and size of particles can be altered by adding coagulants however, this is not done in plain sedimentation
• Rectangular tanks (long and narrow) with horizontal flow are preferred to circular tanks with spiral flow or horizontal radial flow
• Plain sedimentation tank may remove upto 70% of suspended impurities in water

Surface Overflow Rate (SOR) is the velocity at which the tank is designed to operate
As the particles settle (down) the liquid rises up
As long as settling velocity of particles is greater than the velocity with which the liquid rises up, the particles settle down and can be removed regardless of depth of tank.

PARTICLE REMOVAL IS INDEPENDENT OF DEPTH OF SEDIMENTATION

IN CASE OF HORIZONTAL SEDIMENTATION TANK, PARTICLE REMOVAL IS DEPENDENT ONLY ON OVERFLOW RATE

DISPLACEMENT EFFICIENCY = FLOW THROUGH PERIOD / DETENTION PERIOD

• Sedimentation is a water treatment process during which water is held still in a chamber called sedimentation tank or settling chamber due to which suspended solids sink to the bottom due to the force of gravity forming a sediment. Hence this process is called sedimentation.

PURPOSE OF SEDIMENTATION

• Removal of coarse dispersed phase
• Removal of coagulated and flocculated impurities
• Removal of precipitated impurities after chemical treatment
• Settling of sludge after treatment processes like activated sludge process and trickling filters

PRINCIPLE OF SEDIMENTATION

• Suspended solids in water that have a specific gravity greater than that of water tend to settle down by gravity as soon as turbulence is retarded by offering storage
• Inorganic suspended solids have a specific gravity of around 2.65 and organic suspended solids have a specific gravity of around 1.64
• Lighter particles take a longer time to settle
• The phenomenon of particles settling at the bottom of sedimentation tank is called hydraulic subsidence
• Each particle has its own hydraulic settling value that is responsible for hydraulic subsidence
• The basin in which flow is retarded is called sedimentation basin and the theoretical time for which water is detained is called detention period

TYPES OF SEDIMENTATION

• Plain sedimentation is the process of causing solids and impurities in raw water to settle naturally by the force of gravity. This method is cheap and used in all filtration and purification systems of water.
• Sedimentation by using clarifier and contact is carried out by mixing chemicals with water using rotating paddles for two hours per day and suspended particles are removed from the bottom of the tank.
• In case of chemically assisted sedimentation or clarification, chemicals are added to water by mixing suspended solids along with other impurities to stick together and form floc that settles at the bottom of the basin. 
• Chemically assisted horizontal sedimentation is commonly used.

ASSUMPTIONS FOR SEDIMENTATION

• Water flows through a tank as an irregular flow
• The intention of sedimentation is to create conditions in which flow is uniform for a long period enabling maximum amount of floc to form and settle before water reaches end of tank.

PARAMETERS INFLUENCING SEDIMENTATION

• Size, shape and weight of particle
• Viscosity and temperature of water
• Detention period
• Effective depth of tank
• Area of tank
• Surface overflow rate
• Velocity of flow and
• Design of outlet

Sedimentation tanks may operate:

• Intermittently or
• Continuously

Intermittent type sedimentation tanks operate by bringing water to complete rest
Continuous type sedimentation tanks operate by simply reducing the velocity of flow of water

Sedimentation tanks may be RECTANGULAR or CIRCULAR in plan

• Long narrow rectangular tanks with horizontal flow are preferred to circular tanks with radial or spiral flow
• RECTANGULAR sedimentation basins are SIMPLEST in design. In such tanks water flows horizontally through a long tank. Such sedimentation basins are found in large scale water treatment plants. 
• The only disadvantage being requirement of large area of land.