Sedimentation in Water Treatment Process | Process and Design of Tank

What is sedimentation in water treatment tank and what are its design consideration>

Sedimentation tanks in water treatment are designed to reduce the velocity of water so as to permit suspended solids to settle out of the water by gravity. There are many different designs of tanks and most are empirical. No specific rules can be laid down and many contradictory results have been reported.

A sedimentation tank which may be very successful on one kind of water may perform poorly when dealing with a different kind of water. The success of a tank may be judged on its ability to maintain the claimed throughput and the agreed effluent water quality under adverse raw water quality conditions.

An effluent quality for suspended solids or turbidity of less than 5 mg/l and 5 NTU, respectively, would be acceptable to most designers. The number of suspended solids in a water, the nature of these solids, their shape and relative density, the extent of clarification required, the temperature of the water, the rate of flow that must be handled- all these influence the performance of a tank.

Therefore after performing laboratory tests using jar test apparatus on samples of raw water, the best guide to selecting a tank for a particular water is to research which type of tank has been successful before under similar conditions.

Plain Settling in sedimentation process in water treatment

In plain settling (or sedimentation), suspended solids in a water are permitted to settle out by gravity alone: no chemicals are used. For this purpose the water can be left to stand in a tank, although with continuous supply at least two such tanks have to be used alternately. Such fill-and-draw tanks are seldom used in modern plants, except for filter washwater recovery.Instead plain sedimentation tanks are designed for continuous throughput, the velocity of flow through the tank being sufficiently low to permit gravitational settlement of a portion of the suspended solids to occur.

The velocity with which a particle in water will fall under the action of gravity depends upon the horizontal flow velocity of the water, the size, relative density and shape of the particle and the temperature of the water. The theoretical velocity of falling spherical particles in slowly moving water V (mm/s), is given by

sedimentation in water treatment

where g = 9.81 m/s 2, r is the relative density of the particles, d is the diameter of the particles in mm and 7 is the kinematic viscosity of water in m2/s, which varies with the temperature of the water.

Maximum velocity to prevent bed uplift or scour

Apart from the settling rate in still water it is, of course, essential that once a particle has reached the base of the tank it shall not be resuspended by the velocity of flow of water over the bed.

Channel Velocity

sedimentation in water treatment

where r is the relative density, f is the friction factor in (4flvZ/2gd), ~ is in the range 0.04- 0.06 for sticky flocculent materials, and 0.10-0.25 for sand and g = 9.81 m/s 2.

Design of Sedimentation tanks in water treatment

For a tank of length l, water depth d and width w, let the inflow rate be Q ( – outflow)

sedimentation in water treatment

Let a particle of silt entering the tank to have a vertical falling speed of V. Then speed of horizontal flow = Q/wd and time of horizontal flow= lwd/Q
The time for falling distance d is d/V, and for the particle to reach the bottom before the water leaves the tank the time of fall must equal the time of horizontal flow, i.e.
d/V= lwd/Q
from which
V=Q/lw = Q/A

where A is the surface area of the tank. Q/A is known as the surface loading rate and is expressed as m3/h.m 2, m/h or mm/s. This is the process of sedimentation in water treatment.

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