3. Introduction
Sedimentation is the process of allowing particles in suspension
in water to settle out of the suspension under the effect of
gravity. The particles that settle out from the suspension become
sediment, and in water treatment is known as sludge. When a
thick layer of sediment continues to settle, this is known as
consolidation. When consolidation of sediment, or sludge, is
assisted by mechanical means then this is known as thickening.
In water treatment sedimentation might be used to reduce the concentration of particles in
suspension before the application of coagulation, to reduce the amount of coagulating
chemicals needed, or after coagulation and, possibly, flocculation. When sedimentation is
applied after coagulation, its purpose is usually to reduce the concentration of solids in
suspension so that the subsequent filtration can function most effectively.
4. THEORY
Standard texts should be referred to (for the time being) for the theory of sedimentation. It is relevant to
note that development of the theory is based on the initial assumption the particles to be removed from
suspension are spherical and solid, whereas in practice they are irregular in shape, exist as a wide range in
size and can be permeable to flow of water. The challenge for researchers is to develop the theory to take
account of these realities. The theory reflects two basic forms of sedimentation: unhindered and hindered
settling.
When the concentration of particles in suspension is small, approx < 1% v/v, the particles settle with
negligible impact on each other. As particle concentration increases beyond about 1% then adjacent
particles progressively affect, i.e. hinder, the behaviour of each other. When particle concentration exceeds
about 8% v/v the suspension is in the fully hindered regime (see method of concentration measurement
below). The properties of the suspension are now a function of the volumetric concentration of the
particles in the suspension, as well as dependent on other properties of the particles. With increasing
particle concentration the degree of contact between particles progressively increases
When the particle concentration in the suspension is about 18% v/v then there is a transition in how the
suspension appears. For concentrations greater than about 18% v/v, the upper interface between the
water and suspension should be distinct. For concentrations less than about 18%, the interface is diffuse,
increasingly so with smaller concentrations.
5. THEORY
When the particle concentration is about 26% v/v adjacent particles become in
permanent contact with each other such that the suspension enters the
thickening regime.
Hindered settling in water treatment exploits suspensions with concentrations
between about 12 and 22% v/v. A suspension in this range is called a floc
blanket, which is actually a fluidised bed.
Method of particle volume concentration: In water treatment the particles, flocs,
produced by coagulation are readily deformed. Therefore measurement of
volumetric concentration has to be a compromise. A method used for more than
40 years is to collect a sample of the suspension to the top mark in a graduated
cylinder, e.g. 1L, which is then left undisturbed for 30 minutes. The proportion of
the liquid volume occupied by the settled particles after 30 minutes is taken as
the particle concentration in the suspension.
6. Types of Sedimentation Tanks
1. BASED ON METHODS OF OPERATION
a. Fill and draw type tank
b. Continuous flow type tank
2. Based on shape
a. Circular tank
b. Rectangular tank
c. Hopper bottom tank
3. Based on location
a. Primary tank
b. Secondary tank
7. Fill and Draw Type Sedimentation Tank
In case of fill and draw type sedimentation tank, water from inlet is stored for some time. The time may be 24
hours. In that time, the suspended particles are settled at the bottom of the tank. After 24 hours, the water is
discharged through outlet. Then settled particle are removed. This removal action requires 6-12 hours. So, one
complete action of sedimentation requires 30-40 hours in case of fill and draw type sedimentation tank.
Continuous Flow Type Sedimentation Tank
In this case, water is not allowed to rest. Flow always takes place but with a very small velocity. During this
flow, suspended particles are settle at the bottom of the tank. The flow may be either in horizontal direction or
vertical direction.
Horizontal flow type sedimentation tank
These tanks generally in rectangular shape. They have more length twice its width. Because they need to flow
more distance to settle all suspended particles. The maximum permissible velocity in this case is 0.3m/sec.
Vertical flow type sedimentation tank
The vertical flow type sedimentations tanks are generally in circular shape and flow takes place in vertical
direction. Hopper bottom is provided at the bottom of the tank to dispose the collected sludge.
Types of Sedimentation Tanks based on
Methods of Operation
8. Types of Sedimentation Tanks based on Shape
Circular Tank
Circular sedimentation tanks are preferred for continuous vertical flow type sedimentation tanks. In this case
influent is sent through central pipe of the tank and radial flow takes place. Mechanical sludge scrappers are
provided to collect the sludge and collected sludge is carried through sludge pipe provided at the bottom. But
circular tanks are uneconomical as compared to rectangular tanks but they have high clarification efficiency.
Rectangular Tank
Rectangular sedimentation tanks are mostly preferred sedimentation tanks and are used widely. The flow
takes place in horizontal direction that is length wise in rectangular tanks. Sometimes baffle walls are
provided for rectangular tank to prevent short circuiting. Maintenance costs are low in case of rectangular
sedimentation tanks. They are also suitable for large capacity plants.
Hopper Bottom Tank
In case of hopper bottom tank, a deflector box is located at the top which deflects the influent coming from
central pipe to downwards. Sludge is collected at the bottom and it is disposed through sludge pump.
9. Types of Sedimentation Tanks based on Location
Primary Sedimentation Tank
Primary sedimentation tank is a normal sedimentation tank in which water is stored at rest for
some time and sludge collected at bottom and oily matter collected at top are removed. After
primary sedimentation process the wastewater is discharged into aerobic filter where activated
sludge process take place.
Secondary Sedimentation Tank
After activated sludge process the wastewater enters secondary sedimentation tank in which
suspended particles contains microbes are removed and are reflected towards aerobic filter to
maintain high microbe concentration in aerobic filter.
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